Flying boat design.
Flying boats or float planes? Gas or electric? Which are better?
Most model fliers who want to try flying off water fit floats to a well proven land plane, and that was the experience of this writer some years ago with gas models. Flying off water is a lot of fun.
A few basic rules apply to fitting a model with floats. The nose of the floats should protrude in front of the propeller by about half the prop diameter, the rear of the floats should be a halfway between the trailing edge of the wing and leading edge of the stab, and the step of the float should be just aft of the Centre of Gravity. When floats are added, more side surface is added to the model forward of the C of G than aft, so it is best to add some additional fin area to correct this unbalance. This can be in the form of a sub fin as used in many float planes, or multiple small fins attached to the upper and lower surfaces of the stab. Some models fly better with the C of G a little further forward when flying with floats. In this case, add weight under the nose of the floats. It is the furthest forward point, is out of view, and will automatically be removed when the floats are taken off to reinstall the wheels.
Gas or electric? If flying a gas model on floats, use an engine that has a very reliable idle. Nothing is more frustrating than having the engine die before being able to taxi back to shore. This is where electrics shine. But with electric models, remember that if a BEC is used and the battery reaches the point where the motor cuts out, the rescue boat will be necessary. By not using BEC, but a separate receiver battery, there is usually enough energy in the battery to taxi slowly back to shore, even if the flight has gone almost to the point of battery exhaustion.
To get back to flying, it is very important to understand about the angle of attack that the wing will assume when the model is planing on the step prior to lift off. It should be at about five or six degrees so that the model lifts of when a comfortable flying speed has been attained. With a land plane, either tail dragger or trike, we can use the elevator to control the attitude of the model during the take off run, and thus achieve the correct angle of attack for lift off. However, the floats of a float plane will plane on the water at an angle that is determined by the design, and in particular the angle of the planing surface which is the short portion just ahead of the step. A clue to getting this angle right is to look at the float plane when floating at rest. The wings should be at an angle of attack (chord line) of about five degrees when compared to the surface of the water. The chord line is not the bottom of the wing surface, but the line from the point of the leading edge to the trailing edge. This means that the bottom surface of an average model will be at a slightly positive angle to the water.
At the start of the take off run, with full up elevator and application of power slowly and smoothly, the model will plow for a little, then gradually rise up to plane on the floats. At this point, the back pressure on the elevator should be relaxed, but a very small amount maintained. Learn to observe how the bow line of the wave moves back from the nose of the floats towards the step. If the floats are set at the right angle, during this planing stage the wings will generate lift, and the model will gradually transition from being a boat to being a flying machine. It will gracefully lift off when the correct flying speed has been reached, but may require just a tad more back pressure on the elevator. If there is not enough angle of attack on the wings while on the step during the planing stage, they will not be generating lift, and the model is strictly a "marine vehicle." It may not achieve enough speed to lift off. If overpowered, the model will possibly achieve a very high speed on the step and eventually lift off, but quite suddenly. A well designed float plane or flying boat will require very little more power to take off from water than that required to take off from a paved runway. It is my experience that with a good float or hull design, it takes less power to get a model off water than off the average grass strip.
If the wings are at an excessive angle of attack when the model is on the step, it will want to lift off prematurely before a safe flying speed has been achieved, and there is danger of tip stall. On the other hand, if there is not enough incidence, there will not be enough angle of attack on the wing when the model is on the step. The pilot will try to overcome this by raising the nose of the model to get it off, but the heel of the float will dig into the water. The drag thus created will often prevent the model from reaching flying speed. To achieve the correct angle of attack while planing on the step with a float plane, it is necessary to adjust the length of the front or rear struts attaching the floats to the fuselage. When float tests are done on full scale planes like typical Cessnas, the trial flights are done with ground adjustable telescopic float struts until the best strut length is determined.
It is in this regard that flying boats are very different. There is no way that we can adjust the length of the float struts to get the correct angle of attack on the wings while the model is on the step. It is done on the drawing board by getting the incidence of the wing correct in relation to the planing surface of the hull. Full scale airfoils stall at much larger angles of attack than the airfoils on the models we fly. Some flying boats like the PBY Catalina had a huge amount of incidence. This needs to be reduced in a model. After many years of flying float planes, my first model of a flying boat was in 1993 with a Catalina. It flew very well from a hand launch, or off land using a dolly. But the first attempt at flying off water was hopeless. Naturally, one float tip is always in the water prior to starting the take off run. The obvious thing to do is use aileron to lift the dragging float out of the water and level the wings. But "Surprise!" When aileron was applied to level the wings, the model always turned in the direction of the tip that was dragging, and that was the opposite from what was desired. It was determined that adverse yaw was the culprit. The down going aileron, acting like a brake, produced far more drag than it did lift, and the model just turned in that direction. Differential aileron was incorporated and things improved, but there still seemed to be some adverse yaw, and in the process of getting on the step, the direction of the model was not very controllable. If the model did make it to planing on the step, it was in the air almost instantaneously at a very low speed, and this often resulted in tip stall. Excessive incidence was found to be the culprit. When incidence was decreased, the adverse yaw was reduced, aileron control was available as soon as the model turned into wind, and the wings could be levelled. The model planed on the step until adequate airspeed was built up for a safe controlled lift off. "Cheers!" For the many other lessons learned with that model of the Catalina, read the page on Multi Engine Electric models.
Landing is very easy. It is not necessary to line up with a narrow runway! Try to land at a slow speed, with the model in a nose high attitude, much as in a taildragger, but not necessarily in a full stall. If the model touches down while going too fast, the forward part of the floats or hull will hit the water first, and the model will skip or porpoise. Try to hold off just above the surface of the water almost as long as possible. As speed falls off, raise the nose slowly, just enough to prolong the float. When the correct landing attitude has been reached, stop moving the elevator stick back, and the heel of the floats (or the point of the secondary step in a flying boat) should touch the water at the same time as the main step. Maintain back pressure on the elevator stick as the model slows up and settles into the water. Insufficient back pressure on the elevator after landing, as on take off, can result in a model wanting to swerve. Is this called a "water loop?"
Some adequately powered models are able to take off in a very short distance. This is not the most scale like manoeuvre. The planing of a flying boat while on the step is pretty to watch, and a take off at part throttle is more scale like than blasting off in a short distance. Likewise, the best landings are made with a slow, flat, powered approach, flaring just above the surface of the water, holding off until speed is further diminished, then keeping the power on, even after touch down, so that the model planes on the surface for a distance before slowly settling into the water. "Touch and goes" are fun to do, but avoid "splash and goes." On a good "touch and go," let the model plane on the water for several seconds after touch down before adding power to accelerate and lift off again. It will teach you to do good landings. A "splash and go" is when the throttle is slammed open immediately on touch down. Quite possibly it was a heavy touch down that would have resulted in skipping, so the pilot is not learning anything about making a smooth controlled touch down.
So what is better, the float plane or flying boat? A float plane is probably easier in the area of take off, but a flying boat seems more forgiving when it comes to landing in windy or choppy conditions, and is less prone to flipping over than a float plane. This applies particularly to turning around after landing into the wind. Flying boats are very safe in the water in windy conditions, but when a float plane turns cross wind or down wind, it doesn't take too much wind under a wing or the tail to turn in on its back. Models without ailerons have of necessity a generous amount of dihedral and do not make good float planes or flying boats in anything but perfectly calm conditions. The slightest bit of cross wind means that they want to blow over sideways, or at least add weight to the float or tip float on the downwind side. The result is that the model wants to turn out of wind. A wing with minimum dihedral is best, and should of course in this case have ailerons.
When wind is present it will create waves. It is much easier to handle adverse conditions with a larger model than with a smaller one. While the MiniCat handles flight in windy conditions OK, the choppy water that comes with the wind makes for a bit of a challenge. Unlike most flying boats, the Catalina has the tip floats right at the end of those long wings. Any model of a Catalina is more demanding on pilot skill when operating in windy conditions. It is more prone than other models to digging in a tip float when landing. However, landings can be made with floats up, then lowered as speed diminishes on the step. I already have a request for a Mini Albatross using the same GWS Speed 400 motors as used in the Solent and MiniCat. With more conventional aspect ratio and tip float arrangement, a smaller model of the Albatross should be a safer option for conditions that are less than ideal. For flyers who want to operate safely in areas where wind is a problem, the bigger the flying boat the better. The Mars is tops, the larger 100 inch span Catalina next, then the Sealand Albatross and Solent.
Another question relates to flat bottom or "V" shaped floats or hull. The best seems to be a sharp "V" at the nose of the float or hull, transitioning to a very shallow "V" at the step. Rear of the step it is usually a shallow "V." A flat bottom float may plane very well on take off, but skips badly on landing and is very difficult to land smoothly on anything but the smoothest water.
Covering film, like monokote, does not stand up to repeated use under water. My preference is old fashioned nitrate dope to seal the hull, both inside and out. Then the hull is covered with silkspan (light weight tissue) applied with Nitrate dope. It can be painted with spray can Krylon or equivalent. Film covering is satisfactory down to a point just an inch or two above the water line.
An electric model has quite a bit of weight in the battery and motors. In the case of a bad crash, it is possible that there is not enough buoyancy in the model to keep it afloat until it is rescued. Some foam blocks in the fuselage and/or wing panels is like insurance. If it is there it is not likely needed, but don't be caught without it. The small plastic air sacks used in packing are an option. High wing float planes that tend to flip over when landing in windy conditions survive well with a foam wing!
The beam, or width of the hull, is important in a flying boat. If it is too narrow for the weight of the model, it will sink deep into the water. This makes it much more difficult to "get on the step". With a multi engine flying boat, the plowing that results during this phase may result in spray getting into the propellers. This reduces power considerably just when it is most needed. If a model is kept light, there is very little problem with getting on the step. The bottom surface of the hull forward of the step should be flared towards the edges to form a concave surface that reduces spray and actually adds lift as the water is displaced outwards from the centre line. An excellent reference for reading more on this and other associated topics is the chapter on Flying Boat Design in the book "R/C Model Airplane Design" by Andy Lennon published by Motorbooks International.
Good luck in this exciting department of model flying. Take a model with you next time you are camping at your favourite lake. Nothing beats flying off the smooth surface in the early hours of the morning, but keep it quiet! Go electric!