Flying off water.
What is different about it? Flying boats or float planes? Which are better?
Electric models are great for water flying. Gone are the days of gas flying when the engine got cold while idling on the approach, then coughing out in the middle of the lake after landing. But with electric models, remember that the speed control / BEC has a cut off point, so don’t fly until this point is reached. Land with enough “gas in the tank” as it were, to taxi to shore. Apart from this, batteries last longer if they are not regularly run down to the cut off point.
Most model fliers who want to try flying off water get some floats and attach them 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 typical model plane 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 halfway between the trailing edge of the wing and leading edge of the stab. 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. This can lead to some directional instability. It is recommended to add some additional fin area to correct this unbalance. This can be in the form of a sub fin under the tail of the model as used in many float planes, or small fins, usually four, attached to the upper and lower surfaces of the stab. Look at pictures of float planes and flying boats to see these. 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.
The first thing to learn in float flying is how to taxi. Without a water rudder, it may be difficult to turn a model on the water, especially when wind is blowing. It always helps to hold the control stick right back so the elevator is up. Small blasts of throttle should blow enough air over the rudder to make the model turn. If the model will not turn in the desired direction, try doing a 270 degree turn in the opposite direction. With a water rudder, it still helps to keep the elevator raised while taxi-ing. It lowers the tail and puts the water rudder further into the water. This applies especially when the model is fitted with an extension of the rudder below the bottom of the fuselage that serves as a water rudder. The bottom portion will not be in the water unless the elevator is up; stick right back.
To get back to flying, it is very important to understand about the angle of attack the wing should be at when the model planes on the step prior to lift off. It should be at about five or six degrees so that the model lifts off 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 the wing 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, and then gradually rise up to plane on the floats. This is called getting on the step. 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 sufficient flying speed has been reached, but may require just a tad more back pressure on the elevator. Take notice of the slight nose up attitude of the model when it lifts off smoothly. If there is not enough angle of attack on the wings while the model is on the step, they will not generate enough 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 will do so quite suddenly and climb abruptly. A well designed float plane or flying boat will require very little more power to gently take off from water than 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.
The important thing to remember during takeoff is the “attitude” of the model, slightly nose up. Full scale pilots talk about “the sweet spot,” that attitude in take off when the plane accelerates on the step and lifts off smoothly almost by itself. Take a good look at the attitude at the point of lift off, because this same attitude is what we will look for when it comes to landing. Read on.
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 angle of attack, there will not be enough lift. The pilot will try to overcome this by raising the nose of the model to get it off, but the heel of the floats may 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. These are adjusted 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.
Float planes are quite stable during takeoff because having two floats means that the wings stay level. But it is different in a flying boat with single hull and wing floats out under each wing. It is necessary to use ailerons to keep the wings level during takeoff so that one of the wing floats does not snag a wave and turn the model out of wind. But the key to keeping the wings level during takeoff is good aileron design and having a wing with very little dihedral. Any model with dihedral, float plane or flying boat, is extremely difficult to keep straight during takeoff. Any cross wind, or side gust, will lift the upwind wing and put more weight on the float on the downwind side of the model. This results in drag which turns the model out of wind. This is opposite to what is desired, and makes directional control impossible. Stay with models which have very little dihedral. This means that they should have ailerons.
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 tip float of a flying boat 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. The model turned out of wind and was uncontrollable. It was determined that adverse yaw was the culprit. The down going aileron, instead of lifting that wing out of the water, was acting like a brake, producing 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 instantly 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 can be very easy if done right. It is not necessary to line up with a narrow runway! But don’t get so undisciplined that you let the model land anywhere in the lake that it wants to go. You are the pilot in control. Be sure to land into wind, or if there is no wind, take off and land parallel to the shore line. Pick your landing path and stay with it. Close to shore is much easier for retrieval if something goes wrong, and don’t fly without a rescue boat and someone to assist.
Try to land at a slow speed, with the model in a slightly nose up attitude, 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. Worse still it may water loop while slowing down. Try to flare just above the surface of the water and keep from touching down immediately. As speed falls off, raise the nose slowly, just enough to hold off from touching the water. Maintain this until the nose is slightly raised and the model is in the same attitude as it was when at “The sweet spot” during takeoff. When the correct landing attitude has been reached, stop moving the elevator stick back, and the model should touchdown with the point of the step being the first part of the hull (of floats) to touch the water. 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 takeoff, can result in a model wanting to swerve. If not checked in time with up elevator and opposite rudder, this can result in that deadly “water loop.” This can also happen in full scale float planes and flying boats and usually has serious consequences. In wheel planes, when a plane goes out of control in a violent swerve at high speed during take off or landing, it is called a “ground loop.” In water planes we call it a “water loop.” In model flying it seems that I see more “water loops” than I do “ground loops.” They are more prone to happen when the water is smooth because there is no wind. In these conditions the speed on the water during take off and landing is higher than when flying into the wind.
Some adequately powered models are able to take off from water 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, fairly 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 quick "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 touchdown. The pilot is not learning anything about controlling the model while it is slowing down. Many landing accidents happen during the slowing down section of the landing, so it should be practiced on every touch and go, holding the elevator stick full back to keep the model going straight, and avoiding that dreaded water loop. If the model has been slowed down sufficiently before landing, there is no danger of it ballooning into the air again after touch down because of having up elevator.
The other thing to avoid in landing is doing a full stall such is when doing a three point landing in a tail dragger. It results in something akin to a “belly flop.” Certainly the model will land with a short run, but it is not a pretty thing to watch. As the model slows just above the water, wait for the slightly nose up “sweet spot” attitude. When it is reached, stop pulling the elevator back further, and wait for the model to touch down. As simple as that.
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 and taxi-ing in windy or choppy conditions. It 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, (especially high wing models,) it doesn't take too much wind under a wing or the tail to turn it on its back.
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 tends to skip if not landed smoothly. Flat bottom floats and hulls actually have a lot going for them, and the “V” hull in full scale planes was developed to reduce the danger of damage when landing is rough water. The material used for the bottom of the hull could be lighter than that used for flat hulls. One advantage of the flat bottom hull on a flying boat is that it is relatively easy to take off and land on grass. This is a consideration for modellers who don’t have a suitable water area nearby and want a flying boat that can be flown of the grass at their home field.as well as when they occasionally take a trip to the lake.
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. Nitrate dope is not readily available now as it once was, so clear brushing lacquer does as well; is just a little slower drying, taking about 30 minutes. For finishing, use colour spray can Krylon or equivalent fast drying lacquer. Film covering is satisfactory down to a point just an inch or two above the water line. If water gets into a wing of tail surface that is completely covered with film, it seems to take forever to dry out because film covering does not allow air to get in. It is often wise to cover the lower surface of the wing and horizontal tail surfaces with a covering like Litespan or CoverLite. These seem to allow the air to breath and thus dry out the wood.
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 tail section of a model gets a lot of spray. Make sure there are no openings where water can get into the wooden frame work. It takes very little water in the tail to move the C of G back a little. If your model becomes difficult to land smoothly at the end of a long float flying session, check the C of G. You may get a surprise. It is likely time to move the battery forward a little. If the bottom section of the rudder is used as a water rudder, it is wise to apply nitrate dope or clear lacquer to the lower tail section before covering. This is not necessary in other parts of the airframe except the hull.
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 thrust 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!