Float flying fun!

An RC pilot’s guide to flying off water

When most RC modelers decide to try flying off water the question arises, “So which is better: a floatplane or a flying boat?” A floatplane, where the fuselage is sitting up on two floats, is probably easier to control during takeoff, but a flying boat, where the lower section of the fuselage is shaped like a boat’s hull, seems more forgiving when it comes to landing and taxiing in windy or choppy conditions. A flying boat is also less prone to flipping over in rough water than a floatplane is. This applies particularly to turning around after landing into the wind. A flying boat is safe in the water in windy conditions, but when a floatplane turns out of the wind, it doesn’t take too much wind under a wing or the tail to flip it on its back, especially for light high-wing models. Because a flying boat has just a single hull, it needs floats attached under its wings out toward the wingtips. Also, some flying boats use sponsons attached to the sides of the fuselage, which look like stubby lower wings at the level of the waterline. For RC flying, however, sponsons don’t give sufficient buoyancy to keep the wingtips out of the water while taxiing crosswind.

For the most part, modelers just want to get some floats and attach them to their well-proven land plane, and that’s what I did when I first started with my gas-powered models. Today’s electric models are even better suited for water flying, and e-power systems make multiengine flying boats a breeze. Relatively speaking, electrics are more reliable than gas models, which can often have their engines die in the middle of the lake after landing. But remember, electric models have speed control and most include a low-voltage-cutoff function, so be sure to land with enough “gas in the tank” to taxi safely back to shore.

There are several models to choose from, from ARF flying boats to land planes that come with wheels and floats. Many are molded out of foam and these have the added advantage of having good flotation qualities. If they flip over in the water, they won’t sink, and they usually float high enough in the water that the radio and power equipment won’t get wet. Although most floatplanes seem to be high-wing designs, you should consider a low-wing design. When the wind comes up, high-wing floatplanes are the first to tip over while taxiing; low-wing planes are less affected by the wind.

Set up properly, a floatplane should have a slight nose-up pitch attitude when at rest.   Float Terminology When you decide to fly off water, you’ll need to learn a few aeronautical terms. The reference point for measuring the incidence of the wing is the “datum line” of the fuselage. The datum line runs horizontally through (or is parallel to) the horizontal stabilizer. “Wing incidence” is the angle between the wing chord line and the datum line of the fuselage. With flat-bottom airfoils, the “chord line” is not the lower surface of the wing; it is the straight line from the forwardmost point of the wing’s leading edge to the most rearward point of the trailing edge. In most flat-bottom airfoils, the lower surface of the wing is flat only from the main spar to the trailing edge, but from the spar forward, it sweeps up slightly. The chord line is usually about 2 degrees more than the flat surface aft of the spar. This angle of incidence is set and cannot be changed once the wing has been fitted to the fuselage. It is not to be confused with the wing’s angle of attack, which varies greatly during flight and depends mostly on flying speed. Add to the above terms two more that apply to floatplanes and flying boats: “beam” and “keel flat.” The “beam” for a single-hull flying boat is the width of the hull at its widest point, usually near the step; with a floatplane, the beam will be double the width of each float. The “keel flat” is considered to be the bottom surface of the hull or floats directly under the center of gravity. It is the area from directly under the wing’s leading edge back to the step. When the model flies at high speed on the water just prior to liftoff, it is what we call “on the step,” and it rides on this small keel-flat section of the floats or hull. If the model is blocked up on the workbench so that the fuselage datum line is parallel to the bench surface, the only part of the floats or hull touching the bench would be the bottom tip of the step. The keel flat should angle upward slightly, at about 2 degrees. Sometimes, however, it can also be set at 0 degrees, with the entire keel flat touching the bench. Getting the keel-flat setting correct for the floats is akin to checking the incidence for the wings. Forward of the leading edge of the wing, the keel gently sweeps up toward the nose.

FITTING FLOATS
If you decide to fit floats to your existing land plane, several basic rules apply:

• The nose of the floats should protrude in front of the nose of the plane by almost half the length of the propeller.

• The rear of the floats should be about midway between the trailing edge of the wing and leading edge of the stab.

• The float’s step should be aft of the model’s center of gravity, at about 45% of the wing’s mean aerodynamic chord.

Another question often asked is: “What’s better: floats with flat bottoms or ones that are V-shaped? Though flat-bottom floats are easier to build, the best combination seems to be a sharp V at the nose of the float, transitioning to a very shallow V at the step; aft of the step, it should remain a shallow V. Flat-bottom floats may fly better on takeoff, but they tend to skip if not landed smoothly. Actually, flat bottoms do have a lot going for them, but the V bottoms used with full-scale planes were developed to reduce the risk of damage while landing in rough water. The V bottoms cut through the waves, and the material used to build the bottom of V-shaped floats can be lighter than that used to build flat-bottom floats. With a flying-boat model, a flat-bottom hull has the advantage of being relatively easy to take off and land on grass fields—a consideration for modelers who don’t have a suitable water area nearby and want to fly off the grass at their home field all year-round.

When a model floatplane is at rest in the water, the pitch attitude at which it sits is determined by the angle at which the floats have been attached to the fuselage. The model should float in a slightly nose-up attitude. When floats are added to a plane, more side surface is added to the model forward of the center of gravity than aft. This can lead to some directional instability in flight, and some additional fin area can be added to correct this unbalance. This can be in the form of a subfin under the tail of the model, as used in many floatplanes, or small fins (usually four) attached to the upper and lower surfaces of the horizontal stabilizer about midway from the fuselage to the tip.

Some models fly better with their centers of gravity a little farther forward when flying with floats. In this case, add weight under the nose of the floats. It is the farthest-forward point, is out of view, and will automatically be removed when the floats are taken off to reinstall the wheels.

FLYING WITH FLOATS
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 the wind is blowing. It always helps to hold the control stick back so that the elevator is fully 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. Even with a water rudder, it helps to keep the elevator up while taxiing. It lowers the tail and puts the water rudder farther 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 lower portion may not be in the water unless the elevator is up with the stick right back.

Before we get into takeoffs, let’s discuss two flight terms: “attitude” and “angle of attack.” When the nose is pitched up, as in a climb, the model said to be in a “nose-up attitude”; when the nose is down, as in a dive, it is in a “nose-down attitude”; and level flight is “cruise attitude.” The “angle of attack” is the small angle between the chord line of the wing and the direction of airflow, which is a line parallel to the ground. The angle of attack might be 2 degrees in normal cruise at part throttle. At high speed, the angle of attack is lower so as not to produce too much lift, and may even be negative for a flat-bottom wing. If power is reduced a little so as to fly more slowly than normal cruise speed, there will be a loss of lift if the wing stays at the same angle of attack. To stay at the same altitude, lift is maintained by raising the nose slightly to increase the angle of attack to about 5 degrees; this is called “slow cruise.” If we slow down even more, you enter what is known as “slow flight”; to maintain altitude, the nose has to be raised more to increase the angle of attack to maybe 8 degrees.

As an aside, slow flight with a wing that is at too high of an angle of attack is often referred to as being “behind the curve.” Drag increases, and additional power is needed to keep the model from losing altitude. If the nose is raised more, the plane will likely enter a stall, where lift is lost and the model becomes uncontrollable. The stall does not come from lack of airspeed but because of too much angle of attack.

WATER TAKEOFFS
At the start of the takeoff run, apply and hold full up-elevator and slowly advance the throttle. With most electric models, full power isn’t necessary and may make for difficulties in control. The model will plow a little with the nose very high and then it will gradually rise in the water; this is called getting “on the step.” Now, reduce the back pressure on the elevator stick by about half. If some up-elevator is not maintained, the model may swerve violently and perform a water loop. If it does, pull back on the elevator. Learn to observe how the waves move back from the nose of the floats toward the keel-flat section. If the floats are set up at the proper angle, the wings will be at a medium angle of attack as the model gets up on step, and the model will gracefully lift off when sufficient flying speed has been reached.

Take note of the model’s attitude while running on step. The nose should be pitched up very slightly, also known as “the sweet spot.” If it does not lift off easily, it may require some back pressure on the elevator stick or an extra bit of power. If the nose is not pitched up slightly when the model is on the step, there won’t be enough angle of attack for the wings to generate enough lift for takeoff. If more up-elevator is applied than necessary, the nose may pitch up, putting the rear of the floats back into the water, creating more drag and slowing the model.

Without a slight nose-up attitude, the model will achieve a very high speed while on step and eventually become airborne only when it hits a wave big enough to throw it in the air. The speed will be so high that the model will start climbing abruptly. A well-designed floatplane or flying boat will require very little additional power to take off from water than from a paved runway. Actually, it has been my experience that a good float or hull design takes less power to get off the water than off the average grass runway.

To properly set up the model’s attitude (nose-up or nose-down), it may be necessary to adjust the length of the front or rear float struts attached to the fuselage. Flying boats are quite different in this regard. 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 keel flat.

A floatplane is quite stable during takeoff because the two floats keep the wings level. With a flying boat, however, there’s a single hull and floats under each wing, and it will be necessary to use ailerons to keep the wings level during takeoff. This helps prevent snagging a wave with a low wing float, which can then turn the model out of the wind. The key to keeping the flying boat’s wings level during takeoff is good aileron design and having very little wing dihedral. A model with significant dihedral, whether floatplane or flying boat, is extremely difficult to keep straight during takeoff. Any crosswind or side gust will lift the upwind wing and put more weight on the downwind float, which, again, results in drag that turns the model out of the wind. It’s best to stay with models that have very little dihedral but have ailerons, with a little differential mixed in.

WATER LANDINGS
If done correctly, landings can be easy; it isn’t necessary to line up with a narrow runway and land in a short distance. But don’t become undisciplined; don’t let your model land just anywhere on the lake. You’re the pilot in control, so be sure to land into wind, or if there is no wind, take off and land parallel to the shoreline. Pick your landing path and stay with it. Landing close to shore is much easier for retrieval
if something goes wrong. And don’t fly without a rescue boat and someone to assist.

A good landing starts with a good approach. A fairly flat approach with a small amount of power left on right through the landing is the way to go. The approach speed should be considerably less than the cruise speed, more akin a slow cruise. Start by closing the throttle, then applying just a click or two of power. Do not let the nose drop so that the model descends at a high speed, but apply some back pressure on the elevator stick so that the nose is only slightly down. This is where the elevator trim gets a good workout in full-scale flying, but with models, it’s easier just to hold some back pressure on the elevator stick.

Learn to judge your airspeed on the approach by the attitude of the model. The flight path is now downhill, so when the few extra degrees of angle attack are dialed in to compensate for the slower flight speed, the attitude of the model is only very slightly nose-down or even level. If the nose is too low, the speed will be too high; if the nose is too high, the speed will be too slow. Start the landing flare a few feet above the water, and keep the model just above the surface, leaving the power on. Gradually raise the nose to keep it in the air while the model slows until it is slightly nose-up in the sweet-spot attitude. Then stop moving the stick back farther and hold your breath; the model will land beautifully every time. Be patient in the flare; we are not in a hurry to touch down. It is essential to land at a fairly slow speed with the model in that slightly nose-up attitude so that the bottom of the step is the first part to touch the water.

As the model slows up after touchdown, it is important to keep the elevator stick back. Failure to keep the elevator up fully can result in a water loop. In model flying, it seems that I see more water loops than I do ground loops. They’re more prone to happen when the water is smooth because there is no wind, and in these conditions, the speed on the water during takeoff and landing is higher than when flying into a wind. Tail-dragger pilots, please note that we do not land on water at minimal speed in a full stall as done in three-point landings. Landings like these are akin to a belly flop. The sudden splash and slowing up are hard on the plane and not pretty to watch.

BY IVAN PETTIGREW

About the AuthorAlthough he would modestly disagree, Ivan Pettigrew is a legendary electric-model designer who has been involved with model and full-size airplanes his entire life. Now living in Chilliwack, British Columbia, he especially enjoys the serenity of flying electrics on nearby lakes. Of his many designs, Ivan notes, “I don’t aim to design scale models that will win static awards. My preference is to see them flying in a slow, scalelike manner, so that, in the air, they look like they could be the real thing.” You can find his plans at ivansplans.com.