What's important? For starters, define the primary function: a float plane must be able to "get up on the step" and accelerate across the water to a speed adequate for the WING TO GENERATE ENOUGH LIFT to fly that plane off of the water (with good control response and stabilizer authority.) Adequate power / thrust from the engine / propeller combination used is only one necessary part of an entire HYDRODYNAMIC and AERODYNAMIC system; but putting on a bigger engine to compensate for a poor float setup on a plane that isn't working properly is definitely not the best approach......the brute force approach is crude, and it's results are the same. Keep in mind that half of the float's job is to allow the plane to get off of the water; the other half of the job is the LANDING; brute force wont help here.....
I'd like to digress : a brief discussion on CHOOSING A PLANE for float flying may be a good place to start. WING LOADING should be considered, along with the airfoil design on the plane. Control surface authority and adequate stabilizer design / area also need to be considered early on, as a float plane must also stabilize the added forward area of the floats. A plane that was designed to fly on wheels may be lacking in some basic areas when you consider adding the floats.
Wings with noticeable curvature in the rear half of the airfoil's top surface show greater tendencies towards boundary layer separation at decreased speeds or higher angles of attack; this is observed as the ailerons losing effectiveness, or "going soft". Airfoils which are designed with no curvature in the rear section of the airfoil are observed to display greater control effectiveness at lower airspeeds (at the same wing loadings.) A "straight line design" from about the center of the wing's top surface, through the trailing edge of the aileron, appears to have the least tendency to form separations or stall the control surfaces as the airspeed approaches the full stall. (All wings stall at some airspeed / wing loading combination in a given air density- having more effective control down closer to that stall speed is desirable.)
A LOW STALL SPEED and good control surface authority at just above stall speed are essential to consistent water flying success.
The bottom line: for good performance, wider chord wings (higher Reynolds Numbers) and lower wing loadings just work better for float planes! Larger wings and large, effective control surfaces are generally more likely capable of carrying and stabilizing the added weight and drag of floats easier, with less modification or customization necessary.
After a very unsettling first flight with my first float plane, a TELEMASTER 40 (with it's wing stretched to 78-1/2", equipped with 40% flaps, 60% ailerons), I added about 18 square inches of additional vertical stabilizer surface, in a triangular panel off the leading edge of the existing vertical stabilizer. The plane flew very well on floats after that, and also flew well without the floats, keeping the added stabilizer area in place. The Horizontal stabilizer had been stretched to match the stretched wing, and was adequate without further modifications.
I don't have any "magic formula" to offer you in calculating the possible required modifications to someone else's designs; I have several very successful float plane designs of my own, and I tend toward longer tail moments and larger movable control surfaces.
Want to really custom design to a specific aircraft? It gets a bit more involved, but not too bad; we simply start in the middle...
Without going into all of the possible intricate technicalities of float after body design and "stern post angle" effects, I'll give you a number: minus 8 degrees. So' "where's that???" you ask? Draw it out on your plans; you'll do a bunch of line drawing dy the time you have it all laid out right!
IF you're by chance trying to set up floats on a plane for which you don't have the plans, such as an ARF, all is not lost! Simply get a large piece of paper ( I do most of my drawing on Freezer Wrapping Paper) and trace the side view of the fuselage, adding in the wing airfoil shape, and propeller. By the time you've done this, you've caught up to those with the plans. (Leave enough room below the fuselage for drawing in the rough layout of the floats. On existing plans, this will either mean taping a set of float plans in the proper place below the fuselage, or taping on a blank piece of paper large enough to draw in the setup details.)
[ 1 ] DRAW IN THE WING CHORD LINE on your plans; this is a line from the center of the leading edge of the wing, through the center of the trailing edge of the wing. Extend this line both forward towards the nose, and back towards the tail. This os your "ZERO ANGLE OF ATTACK LINE" or wing incidence line, a critical setup starting point.
[ 2 ] LOCATE THE CENTER OF BALANCE on the plans; (this is generally at from 30% to 35% of the wing chord for most wings.) DRAW a perpendicular line (90 degrees) from the wing chord line, through the balance point, extending down below the bottom of the fuselage.
[ 3 ] Draw a third line, from the balance point, ANGLED BACK 8 degrees behind the perpendicular line. The STEP LOCATION will be along this line, at an appropriate distance below the fuselage. But there's obviously still more to do now.
[ 4 ] When a float plane is running "on step", building speed to eventually fly off of the water, in should be set up so that the wing is at a slightly positive angle of attack relative to the float's planing surface. If this angle is set properly, the wing will generate enough lift at about 150% of stall speed to lift the float plane off of the water with little or no up elevator input from the pilot. (This assumes that the wing incidence in relation to the horizontal stabilizer is set correctly.) For a nearly flat bottomed airfoil , a positive angle of attack of 2-1/2 degrees relative to the planing surface works well. For fully symmetrical wings, I use 4 degrees in the thinner air ; only a very slight touch on the elevator in needed to lift from the water here at around 10,000 feet elevation on my SKYFOX. the same setup handled effortlessly at the London Bridge Seaplane Classic at Lake Havasu, AZ, close to sea level.
DRAW another line: from the wing CHORD LINE, starting at the trailing edge of the wing, draw another line forward BELOW the CHORD line, down either 2-1/2 degrees or 4 degrees, depending on your wing's airfoil type. This is the PLANING SURFACE REFERENCE LINE; the planing surface of the floats must be set parallel to this line, at an appropriate distance below the fuselage, for "Hands Off" water flying performance. Again, knowing how it's accomplished, why settle for anything less?
[ 5 ] Next, we need to define the FLOAT TOP TO PROPELLER TIP CLEARANCE. For this value on all sizes of aircraft, the general recommendation is 2 inches. (Less may be workable with floats which don't create spray when trying to climb up onto the step.) Going any higher than this will only result in a LESS STABLE float plane for a given float tracking width setup. (Well designed floats don't create spray problems ; add spray rail on the insides of the float nose if necessary on floats which do create a lot of spray into the prop.
[ 6 ] From a point 2" below the propeller tip, draw a line PARALLEL TO THE "PLANING SURFACE REFERENCE LINE." This is the Float top reference line. If the float hull planing surface is parallel to the float's top surface, you've got it easy from here; those of you with a float plan pre-drawn can tape it in place, with the step on it's line, and the float top on it's line. If you\re designing from scratch, let's go on to the next step.
[ 7 ] The front tips of the floats should extend 1/3 of the prop diameter in front of the prop line; that would be 4" for a 12x6 prop on a .61 to . 75 sized 2 cycle engine, etc. From this, you can measure back to the step, finding the float's FOREBODY length; standard design practice calls for an after body length about equal to the forebody length for good water handling stability; on most of my designs, the afterbody reaches back from the step, 65% to 75% of the distance to the rudder hinge line. Andy Lennon referred to "Long Afterbody" float plans with an afterbody length of 130% of the forebody length; this might be appropriate on a plane with a short nose moment. The desired result is good pitch stability in the water handling- taxiing and maneuvering, as well as the takeoff and LANDING STABILITY!
The writings of Andy Lennon are THE BEST PLACE TO START on the subject of Hull and Float Design. In January, February, and March of 1991, RC Modeler Magazine ran his three part series on exactly that subject. An abbreviated version was included in a more recent article in Model Airplane News, on RC model design; issue and exact title aren't at my fingertips now. Ed Westwood's more recent article in Model Aviation magazine also covers a lot of good material.
I used these primary sources, among others, in designing the floats for which I sell plans and kits. Andy Lennon's Float Hull Design Data is the best I'm aware of; after building several sizes and watching them in action, I prefer the HULL WIDTH EVEN WIDER than he derived from the full scale data sources he used..
My narrow 40 size floats are 3-5/8" wide across the hull planing surface, and 35-1/2" long. My Wide body 40 size floats, for heavier planes with moderate fuselage lengths, are another 1/2" wider across the planeing surface- primarily to provide better action coming up on the step, and good planing action on takeoff. Added displacement volume is only a secondary consideration.
My 42" floats, originally designed for planes like the TELEMASTER 40, Sig Kadet Senior, and others with a similar fuselage length, are 4-3/8" wide at the step. With these floats, the plane was flown easily from a small lake at 10, 500 feet with an ASP on the nose; weight ready to fly was close to 9 pounds.
A flat bottomed float hull can work fairly well on lightly loaded, powerful float planes; a Keeled hull design, however, with triangular edge rails, comes up on step cleanly with virtually no spray, tracks straight even with hands off (If well aligned and solidly mounted), runs with less and less drag on step before liftoff, allowing greater airspeed, and can be built to handle everything from mirror smooth water to fairly rough wave conditions with ease! After coming to realize this, I said to myself, "why not work with the optimum hull design? Why settle for anything less?"
Sitting still, all the floats really are asked to do is to support the weight of the plane above; this is the static design requirement of having enough DISPLACEMENT VOLUME ; that's easy to calculate. But this has little to do with the actual DYNAMICS as the float plane begins to actually MOVE, acting against the water dynamically.
The objective is to overcome the DRAG that the water exerts, so that the floatplane can ACCELERATE to an AIRSPEED where the wing is generating enough LIFT to successfully FLY. An optimized float hull design, then , should be designed to work dynamically to progressively reduce the DRAG between the water's surface and the float hull in all phases of this dynamic interaction.
Maybe describing the "TASKS" that the floats must do in each phase of their operation is one of the ways to come to a complete understanding of the entire operation; In an optimized design, FORM FOLLOWS FUNCTION. and we're talking now about optimizing the form to progressively do ALL of these tasks in the best conceivable manner. (Some float designs may do one task or another adequately, but "Miss the Boat" on other critical tasks; an "Optimized" float design must do each and every one of the tasks well !!!)
Task 1: Staying afloat (hopefully upright!) Requires about 130% of "Neutral Buoyancy" Displacement, and adequate setup dimensions as far as float length and float tracking width to remain stable in all, including crosswind handling conditions.
Task 2: Maneuvering, Low speed; requires effective Water Rudder Design; This task requires the ability to steer effectively at speeds below where the air rudder is effective, and in windy conditions, without inhibiting the Drag Reduction Task later... see the water rudder design section for more insight on this subject.
Task 3: Maneuvering, moderate speed, in what is referred to as "Displacement mode", where the floatation provided by the float's displacement carries the weight of the floatplane. (Wing does not generate noticeable lift at this speed, except for the upwind half of the wing magically jumping into the air when turning crosswind... (grin)) In this mode, the tails of the floats sit deep in the water, possibly completely submerged; if float tails are narrowed to a very narrow wedge, they will allow water to be drawn in along the sides of a water rudder installed behind the tail, providing very effective steering. Squared off "boxy" float tails begin to develop "Cavitation" in this speed range, and water rudders located behind the tails on this type loose steering authority before the air rudder is really more than marginally effective. (This has led many to design water rudders which extend well below the bottom surface of the float tail... fine for this specific task, but a serious liability in later phases when drag reduction becomes the critical task....)
Task 4: TRANSITION from displacement mode to Planing Mode As full power is applied, the propeller exerts all of the thrust it can at low airspeeds. The water seems to be clinging to the floats, not wanting to let them plane out and free. The wing does very little to help in this phase; it's not generating any noticeable lift for quite a while yet! To get this task done well, the float bottom forward surface design becomes the most critical operating area; step height and shape are also at their most critical in this task, as is step location relative to the balance of the entire plane. Here's what has to happen: the bottom of the forward float hull, working with the thrust generated by the propeller, and working against the drag of the water, has to "climb up" onto the surface of the water, getting the water to shear away from the sides of the floats, and then out from under the afterbody of the floats, leaving only air underneath, behind the step. We're fighting our main battle with the capillary action of water here; this capillary action helps a well designed water rudder stay effective at increased speeds, as it loves to follow rounded surfaces and edges, clinging rather than letting go too easily. Sharp edges are best for float bottoms, and sharply defined edges well under 90 degrees are better, while rounded edges are bad news in this effort to break free of the water's hold. Sidecut is also very helpfull; that's where the top of the float is narrower than the planing surface, and as little as 5 degrees has proven to be very effective.. Triangle rails along the very outer edges provide a sharply defined edge, either "tunneling" water under the float hull bottom surface, or cleanly shearing it away from the sidewalls, enhancing the effectiveness of the float planing surface, and are arguably the most effective edge treatment we've seen.
Step design and location also comes heavily into play in this task, and the next. If the step is located too far forward, porpoising can be the result- pitch instability and a failure to plane stabily; you will have to be on the elevator stick all of the time. If the step is too tall, this can also result in less stability in many aspects of operation, especially in the transitions. If the step does not have a clean, sharp rear edge at the rear edge of the float hull's planing surface, it will be more difficult to get the afterbody of the floats well clear of the water, which is essential for the next task. Too low of a step could also result in difficulties in getting the float afterbody clear of the water, but we're talking about an average step height on 40 size floats at 1/2" working well on a good design; an average step height of 9/16" works well on my 60 size floats. NOTE: if the planing surface is too narrow, it can't be fixed with a taller step; adding the triangle rails will help marginal floats. Raw brute power may make it work when an adequate engine can't get the job done, but the real answer is to have adequate planing surface width to do the job right in the first place. Remember, you want to have good handling characteristics on the landings, too.
Task 5: Accelerating in planing mode to flying speed. In this mode, you need tracking stability, and progressively decreasing drag. If the drag can't be decreased enough to have the wing flying, generating lift and having good control surface authority, then the plane may not be able to get off the water easily (or at all.) If the floats don't track straight , drag is reinduced, and speed is lost again.
This is where the actual geometry of the float Hull's Planing Surface becomes the critical factor; too much angle in the area just in front of the step on a keeled hull will produce MORE DRAG than a shallow angle. Here again, Andy Lennon's specification of 5 degrees rise has been tested and proven to work well (170 degree center angle); when combined with the 1/4" triangular rails along the edges, the result is a planing surface which tracks well and rides higher and higher; even on mirror smooth water, the rail act to trap and entrain air in the area under the planing surface, further minimizing drag. A more V shaped hull will ride deeper in the water, will not entrain air, and will not progressively reduce drag as easily. Float hulls with a rise of 15 degrees from the keel (150 degree keel) on more heavily loaded floatplane setups can have problems getting up to a speed where the wing is generating the required lift to fly. Again, a larger, more powerful engine might supply enough additional thrust to overcome the extra drag if the float hull's shape isn't optimized.
In this task where minimizing drag is so crucial, we need to again look at Water Rudder Design. By the time the plane has come up on step, we've already aligned the plane into any wind (hopefully), and the air rudder is now taking over nicely. If the water rudder is still extending into the water when the floatplane is running on the planing surface, attempting to accelerate to flying speed, then your plane is paying a very heavy drag penalty; this alone can be more than enough to make the difference between flying or not! This is why I've come to believe that designing the water rudder and float body tail end to function effectively with the water rudder BEHIND the tail of the float is of major importance. After trying other approaches in my early float flying days, none of my water rudders now extend more than 1/2" below the bottom of the float tail. (More on this later.)
Task 6: Lifting off of the Water
Task 7: Landing on the water
Task 8: Decelerating and Transitioning back into Displacement mode
Task 9: Maneuvering back to the desired landing area