My POLINI THOR 250 Powered SKYMAX PPG Quad Project

Parts of this web site were last updated on January 11th 2018; thanks for visiting!
The most recent updates and modifications are at the bottom of this page.



This is Bruce Stenulson just after landing on 08-27-2017 after flying from South of Fairplay , flying over Georgia Pass, then returning over Hoosier Pass. This is my Polini THOR 250 powered PPG Quad, flying up to 15,029 feet ASL so far and flying cross-country flights of up to 67.4 miles.

I had flown my 'Bluehawk' PPG trike since 2013- primarily flying in Arizona's deserts at roughly 1800' ASL. That trike is set up with a HIRTH F33 engine which is rated to produce up to 28HP at sea level. The Bluehawk trike is a heavier build on a Flexfoil Kitebuggy under-carriage, built with 3/4" heavy walled ChroMoly steel tubing, with a dry weight of roughly 220 pounds. For flying from my 'home field' elevation of 9,940 feet, where a normally aspirated engine (without a turbocharger) produces roughly 70% of it's sea level capability due to the reduced air density, I wanted more power / thrust, and I wanted to fly in a substantially lighter weight rig.

So I set about choosing a stronger engine and a lighter weight quad airframe. I bought one from From there, I've been going over and optimizing all of the details in an effort to come up with a highly reliable PPG quad rig for my flying purposes. From my own perspective, many things needed modifications; This page offers details of the modifications I've made as well as tuning tips, information sources, and parts options & sources.

While doing my research, I came across James Weibe's excellent review of the POLINI THOR 250 engine, his installation in his BELITE ULTRA CUB, and the story of his cross-country flight from Kansas to Oshkosh, Wisconsin and back with only a couple of minor issues. (The link to this review is at the bottom of this page. )

With a power rating of up to 36.5 HP in a high reliability water cooled light weight engine, I decided I'd found the engine I was looking for.

The Water Cooled POLINI THOR 250 Dual Spark Engine

The Polini THOR 250 DUAL SPARK engine is provided with two spark plugs and two independent ignitions to ensure maximum security features in each flight condition. Liquid cooling system grants an extraordinary constancy of performance and more power at every RPM.

Polini THOR 250 DUAL SPARK engine has many more advanced solutions, including:

- The cylinder is die-cast in light alloy, it ensures a better thermal stability.

- The piston, gravity cast at high silicon content light alloy, reduces thermal expansion and the coupling slack. The design of the piston top is specific to optimize the compression ratio.

- It is provided with a balancing countershaft; this feature cancels the vibrations and guarantees such a driving comfort never felt during a flight and a longer life for the engine itself. The counter-rotating rollers decrease greatly the upsetting torque-steer for an extra comfort in flight.

- The centrifugal clutch is in oil bath with helicoid mechanical reduction.

- The electric starting is standard but to offer the maximum safety the engine is also equipped with manual starter with easy system thanks to the flash starter device that speeds up and simplifies the movements.

- It is equipped with the comfortable closed-circuit system for the recovery of fuel during transportation and the 12 V output for any use.

Small and compact THOR 250 DUAL SPARK engine has an extraordinary power to weight ratio.

Advanced technology and high performance of the new THOR 250 DUAL SPARK allow a better feeling of driving performance, making the flight more dynamic, responsive and sensitive, besides ensuring even greater stability.

The new THOR 250 DUAL SPARK engine adapts to different types of applications including one or two-seat trikes, powered hang gliders, one-seat small three-axis ultralight aircraft, and ULM motor-gliders

Polini THOR 250 DUAL SPARK is a 100% Made in Italy product.

Technical data Polini THOR 250

Engine: 2 stroke monocylinder, Liquid cooled

Displacement: 244 cc , Bore for stroke: 72 x 60

Power: 36 HP at 7500 R.P.M.

Cylinder: Aluminum with Gilnisil coating

Compression ratio: 11,5:1

Piston: Two chromium plated rings 1mm

Intake: Reed valve in the crankcase

Carburetor: PWK 28

Air filter: Air box

Ignition: Magneto and Electronic ignitions included on Dual Spark version

Battery charger included; Output power 80 W at 5500 RPM

Spark plug hood 5k Ohm resistance

Fuel type: Lead free petrol with 2% synthetic oil minimum ; 50:1 typically used after break-in

Gear reduction unit: Helical teeth in oil bath with 2,8 reduction ratio

Starting: (Electric starter optional) Pull start with self winding cable FLASH STARTER

Clutch: Centrifugal in oil bath

Muffler Expansion with oval silencer

Engine weight 18 Kg (19kg with electric starter) without radiator: =43# without radiator, ~54# complete installation

Propeller rotation: Clockwise.

THOR 250 Normal Operating Ranges

Water Temp 130 to 185 F; Max 194F

Cylinder Head Temp 130 to 185 F; Max 194F

EGT Operating Range Centigrade: 500c to 620c ; Max 650c

EGT Operating Range Farenheight: 932F to1148F ; Max 1202F

Best Efficiency EGT Operating Range: 1075F to1148F

Operating RPM Range ~1700 to 2200 RPM Idle, to 7500 Peak Power ; Red Line 8000 RPM

Fuel Efficiency ~3 litres/hr @ 5300 RPM ; ~8 litres/hr @ 7500 RPM (When jetted for sea level operation)


On August 14th, 2016 I flew from my launch point at just below 10,000 feet up to an altitude of 13,288 feet . The flight covered 29.5 miles and the total motor run time was 1 hour and 7 minutes. After landing, I measured the fuel used at 1.23 gallons for this flight. Engine RPM was between 7100 and 7170 during most of the climb. [118 main jet, EGT within the peak efficiency range: 1087 degrees F at 7100 RPM.]

Update Late 2017: I'm now flying the Mac Para Charger reflex wing; I have a 120 main jet installed. At full power & high RPM, I may use up to two gallons of fuel per hour. Cruising at reduced RPM uses substantially less fuel.

There is a good review at this PPG website:

The  Skymax  Quad

John Fetz had introduced me to Leon from Skycruiser Manufacturing while I was in Arizona in 2013. I had enjoyed meeting him & his wife. So after coming across a Youtube video of him test-flying one of the THOR 250 engines mounted on one of his PPG Quads, I gave him a call, and made a deal for him to do the installation of the THOR 250 Dual Spark engine onto a new SKYMAX Lowboy Quad.

Polini Watercooled 250

 Skymax Version

Also Available in Lowboy 

Now Using the Polini THOR 250 Engine which is water cooled for great durability 

Light Weight and Powerful

Complete units starting at $7,800.00

Above is a recent 2017 photo of Skycruiser's SKYMAX QUAD with the Polini THOR 250 engine; info, photo, and price  from their website as of November 2nd 2017. Contact Leon at Sky Cruiser Manufacturing:

FEATURES:Chrome-moly steel tubing used for the main Quad frame with an aluminum prop guard cage. Powder Coated ; 1" Solid Fiberglass Axles, fold down seat. Bare Quad weighs in at 58# before adding cage and motor installation. Low Attachment Points for easy inflation, good ground handling stability.


UPDATES: The photos and narrative below show some of the work I've done to fine-tune my PPG QUAD which I had ordered with it's water cooled POLINI THOR 250 Dual Spark engine installed. (The photo above is not one of  the Lowboy versions; it shows a different seat and fuel tank than mine; I like the extended fill neck fuel tank's looks.)

Above: My modified Skymax PPG Lowboy Quad with the Polini THOR 250 engine after many modifications as of August 11th, 2016.

In July of 2017 my rig's dry weight after modifications, ready for flying is checked at 135# with no fuel in the tank. Mounting the Apco Mayday Bi rescue parachute adds 9 pounds. Fully loaded with fuel and gear ready to fly, it's at roughly 180 pounds. With me in the pilot's seat with helmet and clothing, etc. my takeoff weight may be close to 365 pounds.

TECH NOTE: The minimum recommended operating temperature for this engine is at a water temp of 130 degrees F. While Polini now offers an optional 60 degree C (140 degrees F) thermostat (Polini part # 928830009 ) which mounts inside the cylinder head water hose fitting, it was not included with engines being shipped when I ordered my engine. With the rather large radiator, many flyers were reporting covering up to 2/3 of the radiator surface in order to get the operating water temperature to stay above 130 F degrees while flying. Adding the thermostat makes a lot of sense to me... flying with the engine below minimum temperature or flying with duct tape covering part of my radiator in hopes that I've covered the right amount of the radiator's cooling fin area doesn't work; I'm seeing H2O temperatures in flight that are much too low. UPDATE: I've now installed the thermostat- see details & photos below.

The good news: Belite now offers the Thermostat on their parts page- see the links at the bottom of this page. I've also recommended that Leon at Skycruiser Manufacturing add this item to their THOR 250 parts page. The link to their website / parts page is also listed at the bottom of this page.


Above: The Thermostat kit parts

Above: Coolant Drain plug with red fiber gasket on bottom of the water pump. 8mm wrench. (Once a catch container is in place, opening the reservoir cap allows the coolant to flow out freely. Save & reuse the coolant- there's just the right amount! (Fluid level in the reservoir should be down ~1" from the neck once the engine is run & coolant circulates to eliminate any air bubbles from the cooling system; this space in the reservoir allows for coolant expansion.

Above: Cylinder head top radiator hose & thermostat housing opened up (4mm Allen Wrench), ready for parts. (The original O-ring sits in a recess in the upper thermostat housing flange. Don't loose the top O-ring.)

Above: New lower O-Ring and lower aluminum adapter set in place; silicone grease is recommended on the O rings.

Above: Upper aluminum thermostat adapter set in place

Above: When properly assembled, there is no gap where the upper housing meets the head.

With the thermostat installed, once the water temperature is up & the thermostat is working, the water temp reading is holding at ~156 degrees F at full throttle on around test on a ~62 degree morning.

08-16 UPDATE: My THOR 250 setup NOW uses a Powerfin two blade propeller set with a 57-3/4" diameter. This is a ground-adjustable pitch propeller and each blade is adjusted individually. Pitch is adjusted to limit the THOR 250 engine's top end RPM to a maximum of 7500 engine RPM; I'm presently adjusted for ~7350 RPM max in flight.

When previously using the 56" diameter 3 blade GSC prop which Leon had supplied with this quad, I had to reduce the pitch on the three blade set to 9.1 degrees of pitch at the 75% of blade radius point, in order to get the engine top end RPM up to 7500. (I am using a digital angle gauge purchased from Harbor Freight for $30. Blade pitch is referenced to the center flat surface of the prop hub; that means that you set the gauge to zero against the face of the prop hub, then measure the angle of each blade at 25% in from the tip on the blade's back surface.) This pitch setting allows the THOR 250 to run at 7500 RPM at full throttle at 9940 ASL, but it was not really generating the thrust and speed that I need at this setting.

I've now switched to a two blade propeller setup using two of these blades in a different mid-part / hub; blade pitch setting will be steeper on the two blade setup. I'm now flying it at 12.5 degrees pitch, which does generate better thrust and airspeed. Presently for flying form ~10,000 feet ASL flying site: main jet is #118; Pilot Jet is #35 ; Air Screw is set to 7/8 to 1 turn out from bottom; Jet needle now has the clip in the second to the top slot of 5 for a leaner lower mid-range run. I'm running a 50:1 gas-oil mix, using 91 octane gasoline and Quicksilver synthetic mix oil.

TECH NOTE: GSC BLADE PITCH ADJUSTING: My technique for getting fine changes in blade pitch while making the adjustments: with all bolts loosened to no tension on the prop center hub and both pairs of prop blade mounting bolts, I'll simultaneously rock each blade forward and back within the prop hub while exerting the twisting force. A fine index mark on the base of each blade where it enters the prop hub helps me to see the slight changes in pitch angle while doing these adjustments. Once I have both blades at the identical angle, I snug down the hub bolts lightly, and push both blades fully FORWARD. The idea with this is to get the tracking of both prop blade tips as close to identical as possible (- within 1/16"). I'll do some final tweaking until they match as closely as possible. I use a measuring stick from the side cage frame tube to insure that both blade tips are tracking the same distance from that frame tube as I rotate the prop, recheck the pitch angle once more on all blades, and then tighten all bolts to the 100 foot-pound torque recommended by GSC.

Above: I added an enlarged metal instrument panel in order to mount the Fly Henry PPG Meter as well as the smaller water temperature gauge. ALL one might actually need beyond the water temperature gauge is an EGT gauge and a tachometer, but this PPG meter combines many additional capabilities. I also bought the fuel tank level probe so that I can have a fuel level display while on longer cross-country flights.

[NOTE: this PPGmeter malfunctioned / died after 38 hours of operating time; it lost the internal temperature reference, went into alarm mode with constant flashing messages, and would not disable / reset as it should. So I removed it and installed a TinyTach for RPM readout, and installed a separate digital EGT meter. 

On the left, I mount my Garmin GPS; it has topo maps loaded as well as offering an ongoing readout of altitude and relative ground speed. The PPG meter offers a readout of EGT, CHT, Engine RPM, Fuel Level, current flight duration,and hour meter. It stores minimum and maximum readings for later display. The EGT readout is especially helpful in establishing the optimum jetting for full throttle running.

[10-2017 UPDATE NOTE: I now use a newer Garmin GPSmap64s.]

On the right is the ignition control box for this Dual Ignition Polini engine. The top toggle switch is the master on/off switch; the green LED at the top right glows when this switch is turned on. The bottom center switch is the electric start push button momentary switch. The two red push button switches are used to test the ignitions by disabling each of them temporarily and separately. The right is for the magneto ignition; this normally OPEN switch is pushed to short that ignition to engine ground, thereby killing that ignition spark. The left red push button switch is a normally CLOSED push button switch; when it is pushed, the secondary electronic ignition is opened, isolated from it's 12V power source, killing that ignition spark. The engine will run on either ignition separately. It runs very smoothly and reliably with both ignitions active- especially noticeable when doing test runs at the full 7500 RPM.

To kill the engine in a conventional aircraft installation, the top master toggle switch is toggled to the left, killing both ignitions. However, when flying a PPG, we typically have our hands full of the control line / brake toggles while landing, so reaching for the toggle switch without disturbing the canopy isn't reasonable- it's simply not safe. So there needs to be a way to kill the engine with a kill switch on the throttle handle while making a landing approach, or whenever the pilot wants to immediately shut down the engine.

Note: My implementation of this dual ignition kill function from the throttle grip is being done with a DPDT momentary push button switch & some extra wiring running into the throttle handle.

I accessed the engine control box's wiring harness connections for the magneto ignition [pale blue wire in the photo below= hot & black = ground], located back near the engine, to add in the wires for the kill switch connections (coming from the Normally Open contact section of the DPDT Momentary push button switch which I added into the throttle's handle.)

To do the kill switch wiring for the secondary electronic ignition, it's necessary to interrupt the 12 volt supply to that ignition. That 12 volt supply comes into the ignition wiring unit on the violet wire with the black stripe shown in this photo below. The heavier pair of Red & Black wires runs inside the sleeving over the throttle cable down to the second Normally Closed set of contacts of the momentary push button switch in the end of the throttle grip.

Below: The under-side wires use the COMMON and N.O. (Normally Open) contacts to ground the magneto ignition when the switch pushed. The top heavier wires use the COMMON and N.C. (Normally Closed) contacts to OPEN the 12 volt power to the electronic ignition when the switch is pushed.

Above & Below: The new dual ignition kill switch mounted in a plastic end cap, and the final finished throttle.

Above: This photo shows the place where I drilled a hole to install the EGT sensing probe for the Fly Henry PPG Meter. It requires dismounting the exhaust system from the engine to do this installation. From my perspective, having an accurate EGT display is the key to being able to optimize the carburetor jetting & adjustments- to KNOW that the engine is running at the optimum temperature across the full power range. That's my approach to knowing that I can rely on my engine at all times.

The CHT thermocouple that comes with the Fly Henry PPG Meter is the type that has a ring mount that goes in under the spark plug- it's easy to install.

I was a bit challenged with the sensor lead lengths being short from the PPG Meter on the control panel back to the Engine's location on this PPG Quad installation- the EGT probe wire just barely made it! Their web site says that the leads have since been extended another 6", which will be handy for trike / quad setups.

Above: HANG LOOPS - Position and lengths During my hang test to define where I needed the main hang loops to be connected to this PPG Quad's lower frame for proper balance with me as the pilot, I found that I needed to locate the loops as shown in the photo above. I also found that the length of the hang loops as provided by Skycruiser were long enough so that, when hooked in, the trimmers on the risers on my Mac Para Muse 3 XL wing would interfere with the hang guide loops on the top frame. I've had a trimmer latch hang up on my BLUEHAWK trike in the past, & with the combined weight of the heavy trike plus pilot, I could not get it free without landing. It was a go-around with a lot of opposite side brake held in to compensate in order to land & free up the trimmer.

Since then, I'm quite fond of rigging a low hang point setup where the wing's trimmers are clear BELOW the riser guide rings. That makes them more accessible for possible in-flight trimming adjustments, while also keeping them where they can not hang up on the upper frame guide rings.

Also shown in this photo are the extension loops I fabricated for extending the bridles from my APCO Mayday 18 Bi reserve parachute. An identical pair of 24" long extensions were sewn up from 18.5 kN (4047#) 1" tubular climbing webbing, with a center stitched section where three layers overlap for 4". (Stitching also extends beyond this center overlapped section.) I machine sew these with a heavy black bonded polyester thread that's specified for this application- it's really strong thread that's also very UV exposure stable.

Above: My reserve parachute is mounted to the lower left from the seat; the container is strapped to the PPG quad's frame in 5 places to keep it solidly & safely in position. The bridles are run up along the frame structure, held in place with 18# cable ties to keep the bridles & their extensions stowed out of the way. The near-side bridle's un-deployed extra length is overlapped & stowed along the upper frame tube, held in place with light cable ties. The bridles (with their necessary extensions) also run down through the riser guide rings on the upper frame members, and are always connected to the same caribiners where the wing riser connection loops also attach to the low hang loops. In this setup, if it's necessary to toss the reserve / rescue parachute, the same PPG Quad hang angle will be preserved, an the quad will come down on it's wheels- rear wheels touching down first.

PWK 28 CARB: Jetting, Adjustment, Tuning

Above: Keihin PWK 28 carburetor and the air box / silencer setup. The upper brass screw is the idle speed adjustment. The lower brass adjusting screw is the idle & low speed AIR MIX screw. Adjusting this screw out CCW increases the amount of air / leans the low speed mixture.

The main jet is accessible through the larger aluminum bottom cap. A 126 main jet was supplied in this carb when the engine came from Polini, which is appropriate for sea level & low altitude operation. The prop you have and it's pitch determine how it loads this powerful motor. (With too much prop pitch, the engine can not run up to full RPM, regardless of the mixture.) The main idea is to prop a motor so that, at wide open throttle, the prop load limits the maximum RPM of the THOR 250 engine to 7500 RPM after the engine is broken in. More pitch can be used on an adjustable pitch prop at lower altitudes, where the engine can produce more power in the denser air.

UPDATED INFO: The stock pilot jet that came installed was a #45; the idle & low end mixture was richer than I wanted at my higher altitude. I next tried a #40 pilot jet, then a #35. I Tried a #32 for quite a while, but it acted lean on cold morning starts, so I have switched back to the #35. This should be ideal for running at 10,000 feet & above. I've also moved the jet needle's clip to the 2nd from the top of 5 slots, for a leaner run & quicker pickup coming up off idle. Air screw is set from 7/7 to 1 full turn out from bottom.

Above: the PPGMeter shows CHT on top left, EGT on lower left, flight / engine run on top right, and present RPM on the lower right.   Unfortunately, the PPGmeter malfunctioned after 38 hours of flight time and had to be removed.

TECH NOTE: The throttle cable must travel to move the slide up 28mm in order to open the slide completely on a 28mm throat carburetor - to give you maximum air/fuel flow for correct top end tuning and maximum power output from this motor. When I received my SKYMAX quad, the throttle cable setup was only opening the slide about 23 to 24 mm- not what was needed. The photo below shows the modification I made to the AVID throttle handle /housing / cable adjustment locking device to get maximum travel from fully closed to fully open.

Above: Throttle cable length rough adjustment is primarily done at the throttle lever. To be able to get this cable lock's allen set screw set where I wanted it, I drilled the added hole in the side of the throttle grip housing so that the allen wrench could be inserted through this hole to lock the cable at the desired length. The threaded adjuster on the carburetor's top housing has only limited adjustment range- not enough for what was needed here.

Above: A 4AH 12V AGM battery handles the starting job for this engine nicely. The hand pull rope flash starter is also included. The manual includes the instructions for it.

Above: A closer look at the lower end attachment of the new 8000# rated low hang loops in their position on the lower frame tubes.

Above is a view of the right side rigging showing the new 22.5" length low hang loops; this brings the risers down through the guide loops far enough for the metal trimmer buckles to be completely clear below the hang loops when the trimmers are set all the way out for more forward speed and wind penetration. Their lower ends are looped around the lower frame tube straddling the frame tube bolt just under the seat; this bolt will keep this loop from sliding forward or back on the frame to keep the hang angle consistent. You can also see my modified pack which fits within the forward lower frame in front of the seat.

Above: After only 5 months on the engine (while having the tarp cover over it to keep it out of the sun & weather) I was doing an inspection and noticed the start of these cracks in the rubber boot that connects between the carburetor inlet and the airbox / silencer. This photo clearly shows two lines of stress about 1/8" apart running around the circumference of the rubber boot. When I removed the airbox & it's connecting boot from the carburetor, I discovered that the carburetor inlet has a short "velocity stack" mounted to the outside of the inlet throat. And further, I discovered that this velocity stack had a raised rib around it's outer circumference- just exactly where the rubber connecting boot developed the stress cracks. Upon examining the interior of the mating rubber boot surface, however, I found that there was NO RECESS molded into the rubber boot to match the rib on the carburetor's velocity stack... No wonder the rubber was showing stress crack failure so soon into it's service life!

Above are the results of my work to remedy this issue. I first removed the 'velocity stack' ring from the carburetor, then proceeded to remove the offending raised ridge around it's outer surface. I used a Dremel belt and disk sander unit to do this, ending up with a smooth outer surface which matches the interior of the rubber boot.

Finding a listing for a replacement rubber part online for this Polini engine was elusive; while getting a new rubber boot would take time, I decided to do a reinforcing repair to the original one. (This is my already proven technique; I've used it before on similar parts on my Hirth F33 engine, which have since been in service for a long time.)

I had been told in the past by the owner of an auto parts store that the Permatex BLACK silicone adhesive / sealant was the strongest and most oil and fuel-resistant product to use on a molded rubber part like this. It's shown in it's package. The other tool you see in the photo above is a fly tier's bobbin loaded with a spool of kevlar fly tying thread. This is incredibly strong, tough thread; if you tried to break it by hand bare-handed, you would cut into your hands before ever breaking this thread- it's that tough. (You can find it on Ebay in an assortment of colors, or buy it at a store that sells fly tying materials & tools. The bobbin makes handling the thread far easier than it would be to work with the spool of thread alone.

The first step is to remove all traces of the 2 stroke mix oil from all surfaces and any cracks in the rubber connecting boot. For this job, I use an automotive spray 'Brake and Electrical Parts Cleaner'. All traces of oil must be eliminated so that the black silicone adhesive can bond strongly to the rubber. Once it's clean and dry, the first application of the adhesive is worked thoroughly into any checks and cracks to fill them completely, and then a coating is applied to the outer surface in the stressed area.

Next, while the adhesive is freshly applied and wet, the kevlar thread is wrapped around the entire stressed rubber area, criss-crossing back and forth to create a weave that crosses all of the stressed area and adding the desired strength. This is the area where the clamp will be reinstalled later. Once the thread has been wrapped on adequately, more of the Permatex black silicone is applied over it to end up with the thread being completely wetted and covered with a thin smooth layer of the adhesive. Make sure the inner surface is smooth- wipe away excess, then set the part aside in a warm place to let the silicone cure out for about 24 hours.

The resulting reinforcing repair is what you see in the photo above; all cracks filled & bonded with a layer of kevlar thread which results in the part being stronger than the original un-treated rubber part. It's ready for re-assembly in one day.

Above: I also noted fine cracks beginning to form in the engine-to-carburetor rubber boot - likely from the weight of the carburetor and airbox / silencer continuously pulling down on this rubber part. Again, this was seen within about 5 months of the factory assembly of this engine. This was something that I wanted to modify.

[You can see in the photo that I had installed a rubber tube stand-off and cable tie support from the upper front edge of the air box to the PPG frame to minimize movement & secure the assembly forward well clear of the prop, but more modification in the carb / airbox mounting was needed.]

My approach was to remove this rubber part, thoroughly clean it, and add the Permatex black adhesive sealant and the reinforcing thread wrap to the outside where the stress affects were found.

Taking the weight off this rubber mount part was my next project. I decided to simply add the stainless steel wire support link shown in this photo between the carburetor's top cover mounting screw and one of the mounting screws form the electronic ignition module's mounting plate. I used two electrical ring connectors on each end of the wire to provide the mounting rings which the screws pass through, then adjusted the length so that the rubber carb mounting boot was no longer being asked to carry the weight of the carburetor and air box / silencer. The result, as seen in the photo above, is effective- the SS wire is adequate to handle the task with a minimum of added weight, and easily removable / replaceable for normal carb servicing.

You'll also note in the photo that the throttle cable boot at the top of the PWK 28 carburetor was already cracking and failing after only ~5 months; I've temporarily wrapped it with some tape, but plan to do the kevlar thread & silicone sealant treatment to this part to when I next pull the carburetor top cover.

So Polini may need to improve the quality of their rubber carburetor mounting parts. Maybe they are already in the process of doing so- I hope so. But now I know to keep watching these areas during my pre-flight inspections.

Above: UPDATE: This is a rather disturbing discovery. The motor mounts which came with my engine are failing after only 15 months in service (and relatively limited flying time.) I have now replaced these with a different through-bolt bushing setup ! With the stock upper motor mounts, there is nothing to stop the engine from leaning way out of position if these upper motor mounts fail!

These are the parts of the replacement through-bolt motor mount setup.

New through-bolt upper mounts installed.

Here's a pilot's-eye view looking up at the Mac Para Muse 3 wing in flight in Colorado's incredibly blue sky.

[NOTE: In September of 2016 I ordered the new MacPara Charger reflex airfoil wing in the 31 square meter size. At my wibng loading in this thin high altitude air, it cruises at 37 MPH in full reflex trim- great for the long cross country flights I enjoy. The wing tip steering is ]

My Strobe Light can be quickly mounted just above the radiator, using nylon tie wraps. I use an independent Lithium-Ion Battery to power mine; it can optionally be wired into the 12V system. Operation with a strobe visible from 3 miles allows extended flight operations 1/2 hour before sunrise and 1/2 hour after sunset. It also makes this PPG more visible to low flying aircraft.

On two occasions my left rear wheel has bound up on the axle while doing ground run tests- it stopped this Quad from rolling rudely and abruptly. This would have been bad news if I had the wing in the air at the time. I first removed some excess powder coat paint from all four axles after finding that the inner sleeve of the bearing was not sliding over it, but that did not cure the problem. The wheel bearings needed to be shimmed away from the inner stop plate.

So I went to my local True Value hardware store and bought 8 "Machine Bushings", 5/8" inside diameter in 18 gauge thickness. I place one on the inner & one on the outer side of each wheel. I use a large 5/8" washer on the outside of all of this as an outer wheel 'safety stop flange' in the unlikely event of a wheel bearing failure. The new NyLock wheel nuts are set in close with just a slight bit of clearance, where the outer safety keeper washer can still be freely turned with my fingers. All wheels are turning freely & smoothly now, & should not be any cause for further concern.

After ~16 months, the lightweight Lawnpro 13" tires were all developing checks and slow air leaks. A temporary remedy is to add tire sealant to these tires. It's working for now- they are all holding pressure.

I have now installed a pair of replacement 15x6.50-6 smooth ribbed tires on the rear (similar to the tire shown below). These are a tougher 4-ply garden tire, weighing only 5.5 pounds each. They should be more 'cactus-resistant' than the light 13" Lawnpro tires.

Above : I also have a full set of  MARATHON FLAT FREE Tires for really nasty cactus & goat head conditions; the goat heads can't bother these tires!

Above: Waterproof Fabric Cover; designed for a boat up to 16' long with a wide beam. This fabric cover is wide enough to cover this PPG Quad on the trailer nicely, including covering the rescue parachute while it is mounted on the quad. 1 long Nylon strap with 5 quick-release buckles keeps it in place.

I had never been happy with the radical difference in the angle of the prop's rotation compared to the way the prop guard cage was installed on this Skymax by Skycruiser. And when I installed a 60" prop to do a test-run , I found that there was only 2-3/8" of clearance between the prop tips and the upper cage support tubes. This was far too close to run safely!  So I stripped off the netting and disassembled the cage support members. I had to grind away welds on the split ring clamps in order to do this modification, and had to drill out and re-tap a snapped off allen head bolt fragment in one of the clamps during the disassembly process.

I then cut down the upper cage support members to the required length, did the saddle notch, drilled the pass-through holes for the stainless steel clamps, and re-assembled the support arms and cage. The photo below shows the result of this modification. Now the tips of the prop on the upper part if it's travel are well clear of the support frame members. (Unfortunately, the tips of the blades actually travel below the bottom of the quad's frame under the fuel tank, leaving them very vulnerable to damage form ground debris and rocks kicked up by re wheels; even the shorter 56" blades on the GSC prop set took some significant damage from rock strikes during ground running tests.)

I also wanted to add some safety netting to the lower corners of the prop guard cage (like the lower right area where the throttle grip could pass through into the prop if it was accidentally dropped, or the area just behind where the rescue parachute is mounted) I've also been noting a comparatively poorer glide on this quad than what I recall of the glide on the Bluehawk trike; there's seemingly more drag. Part of this is having the wide stance of all 4 of the wheels of the quad out in the wind versus the sleeker single front wheel and narrower rear axle length of the trike. But I'm also thinking that all of those MANY lines in all of that close-weave netting on the upper areas of the prop guard cage have to be generating a significant amount of drag, compared to the Bluehawk's strong weed-eater cord  monofilament gridding.

So I pulled of the woven netting again, and went to work installing a lower drag netting grid with more coverage area, yet (hopefully) a significant amount less drag. This photo below shows the result. This is .065" commercial grade weed eater cord- tough & resilient. The process of installing it is one of drilling and deburring all of the holes, then threading through the cord, tensioning it, and tieing it off. It's done in several sections. We'll see how it feels in the air- this grid now closes off those lower areas nicely, too.

In the photo above you'll also note the two blade 56" GSC prop. I had ordered the 2 blade center yoke from GSC. I've added the stainless steel protection tape to each blade's leading edge from the very tip on in 9". This should protect these edges nicely.

[Note that the tips of this 56" GSC prop are even with the bottom of the Quad's frame; this has left them vulnerable to damage from ground debris / rocks, etc. The motor was originally mounted about 4" below the center of the prop guard cage. I modified this later to remount the motor 4" higher. Photos and details are included later on this page.]

Above is one of the cage edge line support fixtures I made from tough and flexible plastic.

Above and Below: I've mounted a 58" Powerfin Propeller for testing. This is the earlier style hub

I set the initial pitch at 12.5 degrees (measured at 75% of prop radius out from the center.) A test run here at 10,290 feet ASL in 59 degree dry air gives a peak RPM of 7400 to 7420 RPM- very close to the ideal of 7500 RPM for max H.P. output.

This propeller is running smoothly and providing good thrust; it's a good match for the THO% 250 engine flying at this altitude.



Adding An Extended Prop & Line Guard Hoop with 7" Stand-Offs To The Prop Guard Cage


Above: since the THOR 250 engine has the centrifugal clutch on the prop drive, when the engine is shut down the propeller has a lot of inertia and easily continues to windmill. During a landing, when laying the wing down after roll out & stopping the quad, the lines could easily get wrapped around the engine's output shaft- the prop cage did not extend far enough to protect the long prop blades and keep the lines from dropping between. The solution: Add an extension to the prop guard cage. Net added weight : only 3.1 pounds!

Above: after shaping the hoop using a 1/2" EMT conduit bender and my homemade Hip Bender the six extra stand-offs were fabricated and welded to the extension hoop. This is the result after painting, ready for installation.

The extension guard hoop is mounted in place to the original aluminum cage with 8 stainless steel worm drive clamps. This photo shows that the 58" propeller is now protected within the extended cage. It has good clearance.


Prop blade end clearance is over 1.6" minimum.

A rock kicked up by a wheel on the quad caused this damage to one GSC maple blade's tip. I filled it with slow setting epoxy, shaped it carefully, and re-varnished the blade before adding the stainless steel tape to the leading edges of all blades.& I have repaired other blade rock damage since this; when flying from the irregular ground of South Park's short grass prairie, this is an ongoing hazard. I have put in a lot of time smoothing ground and picking up loose surface rocks to minimize the possibility of a rock getting kicked up into the prop, but it does happen. Johnny Fetz showed me some good techniques for prop repair; that info has come in handy on several occasions.

 I decided to remove the pull starter; I never use it, so that's 1.1 pounds that I don't need to carry. The internal pawls degrade from vibration during normal flight, and require ongoing replacement to continue to function properly.& The electric starter works well, although I'v had to do maintenance work on it too periodically;& but this engine starts very easily with it.


After completing these modifications, I have flown many& flights over South Park, starting from& one of two launch points: one at ~9680' MSL and another& at 9944 feet MSL, and eventually climbing up to a maximum of & ~15,029 feet ASL according to my Garmin GPS. Photos from this and subsequent flights are now being combined on the PHOTO GALLERY page- CLICK HERE to jump to that page.

Above: The rear extension of the prop guard cage now has the tough blue line netting installation completed.


 Flying longer cross-country flights with a PPG leaves a pilot gripping the hand throttle fairly tightly for extended periods of time- often with your hands held up in a high position while also holding the control line handles. For me (& a lot of others I suspect) this can lead to tired / aching hands, reduced circulation, and colder hands during cool weather / extreme altitude flying.

I can adjust the trimmers on my MACPARA Muse3 wing for close to hands-off flying as far as the control lines are concerned; when let go, they're easily in reach right against the pulleys, but when I'm cruising cross-country, I can have my hands off the control lines for extended periods of time. This allows me to work with my cameras and GPS controls while flying. But up to now, that throttle control has always been tightly in my grip... so I decided to design a simple device to use as a 'Cruise Control' - to keep the throttle cable pulled in the desired amount for extended periods of time, whether at full-power climb when heading for the higher altitudes, or at an intermediate setting to maintain a given altitude in 'cruise' mode. The photos below show what I came up with.

The device is a type of variable-effect  stepped shim which, in use, is slid into the throttle housing straddling the cable to keep the throttle lever from being able to be pulled back up by the return spring within the carburetor. The material I had on hand was a piece of Basswood 1/4" x 1/2" x ~1.5" long. I cut a slot up the center from one end for about 3/4"; the slot is wide enough to straddle the throttle cable without hanging up on it. A hole was drilled in the opposite end where a ~6" long section of spectra 200# fishing line is tied; the other end is tied to the heavy wiring running from my throttle's kill switch. This line simply keeps the cruise control device from being dropped / lost.

When the device is slid all the way into the throttle housing, the throttle lever is held at close to full throttle position.

When the device is slid part way back out, a 'level cruise' RPM / throttle opening can be selected with a simple bit of trial and error of sliding the shim device either in or out.

When not in use, the device slips snugly inside the throttle grip, available for it's next use.

Being Prepared For An Engine Outage Over Heavily Timbered Terrain

When planning cross-country flights over the terrain we have here in the central Colorado Rockies, It's always good to give some thought to how to deal with unanticipated situations. One of those unlikely but possible situations which a PPG pilot might someday have to deal with is that of having an engine or wing malfunction while flying over timbered terrain.

As pilots, we do our ongoing maintenance and pre-flight checks to assure ourselves that our equipment is in serviceable condition, ready to perform reliably and continuously. But it also doesn't hurt to be prepared for a possible landing in less than favorable terrain. Being hung in a tree in your gear is a remote possibility; being able to get yourself down safely if you ever are is close to priceless!

Above is a "Self-Belaying Kit" which I will carry when on longer flights over the heavily timbered terrain. It consists of :

[1] A light weight climber's seat harness and Chest harness and 1 locking carribiener; I built this one from 4000# + 1" tubular climber's webbing and the heavy bonded polyester thread which I use to sew the low hang loops for my Quad. (The adjustable waist belt with the cam buckle to which the main leg loops are sewn just helps keep the leg slings in place; the main leg sling loops carry all of the load when in use. )

[2] A "Figure Eight" descending / rappelling device

[3] A 100 foot long length of 6mm quality climber's accessory cord

[4] A good pair of gloves for rope handling with leather palms

[5] A 4 foot section of 1" climber's webbing (Or a good quality Cam Buckle strap) & another carribeiner for use in fashioning an anchor point.

[6] 30' of strong 4mm utility cord

To use this gear if hung in a tree, etc., a stranded pilot (transformed then into being a climber) would first carefully get out a shorter section of rope and tie them self and their gear off to the tree- no need for a sudden unscheduled early decent! Next is to put on the climbing seat harness and rig the carribieners and figure eight as shown; the loose end of the rope in the above photo would be the one tied to or looped around a solid branch or part of the tree trunk, or tied to the webbing anchor point. Using the rope run through the figure eight more than once increases friction, and running this 6mm rope through doubled is fairly standard self-belay procedure; the center of the rope is looped over an anchor point attached to the tree so that, once on the ground, the rope can be retrieved from the anchor point for further use. [Mammut's 6mm cord has a breaking strength rateing of 1700 pounds. Sterling Rope's 6mm cord is rated at 1978#]

[A book or website on climbing ropes, rigging, and use and a bit of hands-on practice should further help in an understanding procedures for rappelling using a descender. Professional ski patrol personnel carry a similar kit for self-evacuation from a chair lift if the lift is shut down while they are on it; fire fighters carry similar gear with high temperature-resistant rope. A practice self-belay decent from a very modest height might not be a bad idea for those unfamiliar with rope & self-belay techniques. A Gym with a climbing wall and instructor might be a good place to familiarize yourself with the equipment and techniques; that info is beyond the scope of this web page.]

Above: REI gear. (I changed out to the buckle for my PG kiting use.) The figure eight chest harness helps keep you upright while doing a descent if your technique is less than perfect, and you may like the solider feel of rigging that way. REI sells both- I bought the set from them many years ago. I have used this climber's seat harness a lot for kiting my PG wing. The gear I'm carrying is lighter and more compact.

(Above & Below): My self rescue / rappelling kit as shown in the previous photo weighs 46 ounces, packed in this light weight mesh bag, ready for use. I have a converted day pack which is mounted to the forward lower frame of my Quad, where I carry this gear in easy reach. I can carry this kit, a GMRS radio, my cell phone, and a survival kit in the mounted pack and still have room for my wing's empty carrying bag and other items.

(Above): This is a photo of most of the main items included in my survival kit (28.75 ounces) which includes 2 Orion "Skyblaster II Alert signal flare launchers, a lightweight emergency bivy sack (3.4 ounces) and two mylar space blankets, two 30' lengths of 200# Spectra line, a LOUD signaling whistle, plus a small first aid kit including a roll of tough nylon tape, a chemical hand warmer, a fire starting kit, a mini 47 lumen LED flashlight & extra AAA battery, 3 protein bars, a light warm poly stocking cap, paper towels & TP, and a few other useful items. (An 8 oz. water bottle is not shown in this photo, but can be carried in the pack.)

I fly with the reserve parachute mounted and with these emergency survival and self-rescue kits always along because I'm inclined to fly over high rugged remote areas most days - that's where I live & fly- and I figure it's worth having along this 4.5 pounds of emergency gear. That's my personal approach - a part of prudent PPG flying in this beautiful and rugged part of Colorado's high country!

I carry a Midland GTX1000 GMRS radio which has up to 36 miles of line-of-sight range, while leaving a matching radio with someone back on the ground, and I leave my flight plan with them. If I do have to put down somewhere other than at my launch point, I'm fairly well prepared.

For photos taken during my flights, please CLICK HERE to visit the PHOTO GALLERY.

2017 Updates and Modifications

This update section was last updated on October 27th, 2017

Late in 2016, I had Andy McAvin ship me a new MacPara Charger 31 wing. This is a Reflex wing with a higher cruising speed when the trimmers are let out into the full reflex mode. In the thinner air here at altitude, with the ~380# total ready to fly wing  loading of the quad and me, I'm cruising at an average speed of 37 MPH. With trimmers pulled in fully for better climb angle, the speed is at 33 MPH. [In comparison, the 31m Muse 3 would cruise at 32 MPH when the trimmers were let out fully.]

 Really like the way that this wing launches and flies, and I find the wing tip steering very nice to work with. However, for use on my quad with it's riser guide rings on the upper frame, I had to do some modifications.

With the long travel trimmer adjustment range and the added wing tip steering, the Charger's risers are more complex and far bulkier than those on a simpler wing like the Muse 3 which I had previously been flying. The 2" I.D. riser guide rings provided with the Skymax quad (mounted on the quad's upper frame members to control the hang attitude)& were too tight a fit- the wing tip steering lines were regularly fouled / hung up and unusable, and other parts of the stock risers would also hang up in the guide rings, leading to problems and aborted takeoffs.

First, I fabricated a new set of low hang point loops that positioned the trimmer clamps where they were workable below the riser guide rings. Next, I fabricated a new set of larger 2-9/16" I.D. riser guide rings which offered more adequate clearance for the Charger's risers. I use the 2" guide rings to route the reserve parachute's risers separately, inside the upper frame members, and then down to the main carabiners on the low hang loops, keeping them clear of the wing's risers.


These modifications helped, but all of the extra D-rings and shock cord form the original rigging and routing of the Charger's wing tip steering lines running up through the middle of the riser bundle through the riser guide ring still left the wing tip steering toggles & lines fouled and unusable a large percentage of the time... so I decided next to try to remedy this aggravation.(This situation might not necessarily be an issue for a trike or quad rigged with a high hook-in setup, or for foot launch pilots.)

(The photo below shows the stock Charger risers through the 2 inch I.D. riser guide ring, with the stock wing tip steering line run up through the midst of the bundle.)

The photo below shows the original stock wing tip steering line rigging run through 6 D-rings with a lot of ~1/8" shock cord run back and forth everywhere.... when these risers are used through riser guide rings on a quad or trike setup using low hang point rigging, all the extra D-rings & shock cord result in wing tip steering line fouling and hangups far too often.

My solution: The risers come with two brake line pulleys sewn in place on the risers. On my quad with it's low hang point rigging, I need to use the upper pulley for the brake line. I use the upper brake toggle retaining magnet (the right one in the photo below) to park the brake / control line toggle while in flight. ( I use the wing tip steering all of the time while cruising; I only use the brake control lines during takeoffs and landings.)

This leaves the lower brake line pulleys unused, and so available to be re-positioned and re-purposed in a cleaner wing tip steering line setup. The idea is to end up with the wing tip steering line routed to bypass the riser guide ring completely, eliminating any of the fouling and hangups.

The photo above shows the 2nd lower brake pulley sewn in it's original position, down low and close to the trimmer clamp. I carefully pulled the stitching to remove it with it's webbing loop. Next, as shown in the photo below, I sewed it to the existing upper brake pulley's webbing loop with heavy duty bonded polyester thread; it's solidly in place. I then proceeded to remove all of the 6 un-needed D-rings and shock cord, and the old low eyelet through which the wing tip steering line had once been routed leaving nothing left for lines to hang up on. (I later found that the extra low position brake toggle parking magnet was hanging up in the riser guide ring, so I pulled it's stitching and removed it, too.)

The important thing to keep in mind if someone else decides to clean up their wing tip steering line routing for a quad /trike with riser guide rings and low hang point rigging is to only cut the thread carefully- do NOT damage the risers themselves! A seamstress's  'stitch puller' tool is very handy for this type of careful work.)

The photo below shows the finished result- the wing tip steering line toggle still parked on it's  magnet and snap fastener, the line run freely on the outside of the riser bundle up through it's own pulley. It performs flawlessly in flight now.

While flying my early flights with the Charger wing, I found that the torque from the Polini THOR 250's gear drive resulted in a left turn tendency when both trimmers were set to neutral. Since I wanted to be able to fly the full trimmer range during normal flying, I was looking for a way to compensate for this left turn tendency... after all, for long cross country cruising flights, it's really pleasant to have the rig set up for hands-off straight line flight for extended periods of time.

I noted in watching the wing that it only takes about 3" of inward deflection on the wing tip via a modest pressure on the wing tip steering line to compensate for that left turn tendency. On the Charger Riser control line setup, there is a short line which I have come to refer to this 495mm long " stC1 line" as a 'snubber' line connected between the C riser quick link / mallion and the wing tip steering line; by shortening it the right amount, it can also function as a 'torque compensator' device.

I first made up an adjustable parallel line with a 3/4" wide strap and cam buckle- similar to the regular trimmer cam buckle and strap setup. Once in flight, I pulled in the adjustable line until I had a hands-off straight line cruise. After the flight I determined that I needed a 'stC1' line that was 100mm shorter to keep that right wing tip pulled in the required ~3" to result in a straight ahead hands-off cruise.

So I made up a 395mm long line from 220 pound test Spectra line and installed it in place of the original 495mm green line. It's been performing as intended for many long cross country flights since then. You can see the short grey line in the photo below.



 I like the electric starter on my POLINI THOR 250 engine... but it does have it's own maintenance requirements. James Weibe, in his BELITE blog, wrote of having to repair a broken internal brush wire; nice to know.& I have had to do the same myself.. I ended up having to solder both brush wires to their terminals; it seems that the spot welding of the fine strand copper braid to the terminals results in a heat-induced fatigue of the copper. Since I did the soldering, I have not experienced any further issue with these brush wires.

The second starter issue deals with the starter's output shaft... the splined section is a separate piece of metal which is inserted into the starter armature's shaft, and crimped in place. It can work loose in service.... not an ideal design! The photo below shows the splined short shaft sitting inside the engine housing, visible when the starter was removed. The only good news is that it can't escape this area to get deeper inside the engine to cause more extensive damage. The bad news is that once it happens, you get to fix it.

 The photo below shows the recess in the end of the starter's armature shaft, and the insert shaft's smooth end laying along side just below to the right.

My solution was to first thoroughly clean both parts with spray 'Brake and Electrical & Parts Cleaner", to remove any oil , etc. I then put some thin CyA glue (super glue) into the armature shaft recess, inserted the smooth end of the splined shaft, and seated it to bottom solidly. Once the glue cured well, the starter has been back in service reliably for many long flights. The photo below shows the reassembled shaft.

The Polini manual starters are one of the more aggravating features of an otherwise fine performing engine; they really need to improve the design. It's made partially of plastic parts which self-destruct from vibration while the engine is running normally. While the 'pawl set' is relatively inexpensive, it's generally a too-frequent maintenance requirement if you are using only the manual starter. I had started my engine with the pull starter only 2 or 3 times, and left it mounted as a 'backup'.... but when I tested it, it would not engage. So I removed all of the pull starter components, and then glued a protective cover over the magneto flywheel opening where the manual starter had been mounted. This keeps debris from being able to get into the engine's magneto area.

 Below: The battery rides in a nice Velcro mounted nylon carrier, isolated from any frame vibration. On my Skymax, the wires would vibrate enough to eventually loosen the screw on one of the battery terminals... not so good a thing to have happen. So I added long cable ties around the battery and it's wires close to each terminal, so that the wires can not move. (I like to use lots of cable ties!)


The photo below shows damage done to one of the PowerFin prop blades by a rock strike during takeoff.. I repaired it, and the blade is back in service.





In this ~70% density air, with my loading on this 31 square meter wing,  the GARMIN GPS's recorded track data shows that I need to get rolling up to 28 to 29 MPH to lift off and fly. landing approaches are also typically at about 28 MPH, until I begin to flare for landing, when the glide speed reduces below that approach speed. This 29 MPH required speed for liftoff results in a need for a fair amount of usable ground to get up to liftoff speed- especially when taking off from soft ground.

A 380 pound rig rolling across natural short grass prairie at up to 30 MPH puts a lot of stress on the quad's structural members.

So the fiberglass axles and their steel mounting tubes on the quad's frame undergo a lot of stress when being used in these conditions, which results in some required ongoing monitoring / inspection and maintenance. Even trailering the Skymax quad on poorly paved and gravel roads takes it's own toll. Below I offer information on the items which I've had to rebuild, repair, and reinforce on my quad due to these structural stresses.


Upon returning home from the Lost Stirrup Fly-In in mid July this year, I found that welds had failed on both of the main upright frame member mounting points, as shown in the photos below. This attachment is formed by welding a larger I.D. sleeve tube to the underside of the main lower frame's lengthwise tubes on each side to support the entire upper frame members, engine installation, and prop guard cage. A major amount of weight is carried by these two MIG welds  which are holding everything above up in place... The load is from above, but these welds are trying to hold it all up by being welded to the underside of the lower main frame. These welds both failed after less than 50 hours of flying time, and some trailering to and from flying sites within a few miles of home.

An added consideration is that the location of these welds is not easily visible during a pre-flight or post-flight visual inspection; I was unaware of the developing problem until they broke loose completely. The resulting downward movement of the entire upper structure also caused damage to the prop guard cage structure; luckily the prop was not damaged.

 Instead of tearing the entire rig totally apart and packing in in a box to ship back to Skycruiser, I decided to repair and reinforce these weld failures myself. Yes, it required totally removing the engine and control system so that I could work on it properly; but I had the equipment here to do it effectively and quickly.

My approach was to first trim down the remaining excess lengths of the inner lower ends of the upright frame tube members. Relocate them to a position ON TOP of the lower lengthwise frame tube members required removing the motor mount cross members. Once re-positioned, I added sections of steel angle iron to the underside of the damaged lower frame tubes bridging over where the metal had broken away.

The next step (once the pieces were clamped in their new positions) was to drill and bolt through the new attachment points with high strength 3/8" bolts and nylon insert lock nuts. The result is shown in the photo below. The resulting structure is very solid, and the downward-bearing loads are now carried on top of the reinforced lower lengthwise frame members. I feel a lot better about flying this quad after this modification / repair. It's now also easy to check these through-bolt attachments during my pre-flight and post-flight  inspections.


 I noticed damage to the rear fiberglass axles at the point where they enter the rear axle carrier tube; the fiberglass was showing fiber breaks at the point where the end of the steel tube made contact with the fiberglass on the top surface. The original retainer pin hole had been drilled vertically with only 3/8" separation between the pin hole through the fiberglass axle and the contact point with the end of the tube where I could see the fiberglass was failing.

On closer inspection, I also discovered that both ends of the axle carrying tube had developed vibration fatigue cracks on the top surface along the edge of the mig weld which connects the axle carrier tube to the quad's framework above. The next three photos below show what to watch for during your inspections.

The position of these cracks indicate that they may be the result of vibrational down-load stress forces during flight; the axles with their wheels on the outer end must vibrate constantly during flight. Arc welding does induce stress into metals do to the heating being concentrated in a very small area; that's why steel tube built light aircraft frames are built using gas welding techniques, and heat stress relieving is done on welded areas during the build after each weld is completed. Arc welded joints need ongoing inspection and monitoring for the possible development of these stress cracks.

After doing an overlay weld with my wire feed welder  as shown in the photo above, I smoothed down the welds and repainted the area. I drilled new horizontally oriented holes for the axle locating bolts well away from the fiberglass axle load stress point at the end of the axle tube. I then chamfered the inner ends of the steel tube to minimize fiberglass surface fiber stress in those areas before installing new black fiberglass axles.

NOTE: I am now buying black 1" diameter round solid fiberglass sections for replacement axles from Max Gain Systems

(The black color may be more resistant to long term U.V. damage of the outer surface resins than light colored axles. I've had fibers raising from the surface of the original off-white axles in the past. I like the looks of the black axles.)



Below: While inspecting the quad, I noticed some deformation on the right end of the front axle carrier tube where it was welded to the upright piece. (This is rather rough ground I fly from, and the takeoff runs are much longer than when flying in denser air at lower elevations.  I'm likely close to 380 pounds ready-to-fly weight when I'm rolling at up to 30 MPH across the terrain to get up to flying speed. So for my purposes, I want tough axles under my rig. Being light weight is a lesser consideration, versus a need for structural strength and durability. )

After straightening, I welded on a close to full length piece of reinforcing steel angle iron (3/4" x 3/4" x 1/8" thick.) Added weight is modest, but the added strength is very welcome! I expect long service after this modification.

The photo below shows the rear end of my quad on the trailer with the 15x6.50-6 wheels mounted. I later also mounted two more of these tires on the front end. These larger diameter wider surface rear tires seem to roll up to liftoff speed more quickly than the 13" tires- they handle the short grass prairie irregularities somewhat better. (In the distance is Mount Silverheels at 13,882 ft. MSL.)

 The photo below shows how I have my rescue parachute mounted with 3/4" cam buckle straps (from ). The blue pack which rides between my knees carries assorted tools and gear. I now use my Garmin GPSmap64s tethered to the pack's outer pocket zipper pull ring where it's easily readable while flying- it's positioned similar to using a knee board.

The PPGmeterI had installed malfunctioned after about 38 hours of service, going into a constant error mesage flashing mode which could not be reset; this made it unusable. So I removed it. I installed a TinyTach to go with my separate H2O temp gauge. I now only temporarily mount an EGT gauge when I'm checking on a change in carburetor jetting. This water cooled engine runs very consistently and reliably, so if the H2O temp is within it's typical operating range of 157 to 161 degrees F, then all is OK.


Below is the DTC T100 Digital EGT Meter which I am now using; it's displaying 50 degrees F on a cool early November Colorado morning. I will only keep this gauge mounted when needed to check a change in carburetor jetting when I fly at lower elevations. It has a quick disconnect wiring connector. This meter has internal calibration and function selection programming capabilities via three internal switches; it's set up for 12V power supply now, and for reading a Type K thermocouple. I'm using the same thermocouple probe which I installed from the now defunct PPGmeter.

Here's a link to the meter only (with a blue LED display) for only $15.99 .You can also find a Type K thermocouple EGT probe on Ebay for under $15.00

Inexpensive DTC T100 EGT Gauge


SKYCRUISER MANUFACTURING:  parts are available. (Carb jets are not listed.)


Belite's Polini Engine parts page, with the thermostat offered for ~$45.25 + $8.00 Shipping

Flat-free "Cactus Country" Wheels by Marathon; ~$50 each; a smooth tread option is also available.

Links To On-Line Documentation And Related Info


CARB JETS : KEIHN PWK28 uses HEX main jets: 126 stock ; [118, 120, 122, 124 & 128 possibly useful on THOR 250]

10 piece pilot jet assortment kit - only $9.99 for all, free shipping from CA


APCO Mayday Bi Rescue Parachute: 47 m^2, 440# capacity

Rubber Anti-vibration Spacer 1" OD x 3/8" ID x 1.5" Thick (Item# X19-14) [For through-bolt upper motor mounts]

2 Pcs x AC 2A/250V 5A/120V 6 Pin DPDT Momentary Push Button Switch 6mm 1 NO 1 NC [For Dual Ignition Kill Switch]

James Weibe's THOR 250 Review: Polini Thor 250 Engine Review - The best ultralight engine we've seen

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