Seaplanes and X-Plane 
Originally by Chuck Bodeen with edits for XP11 by yours truely 😉


Once a seaplane is in the water and released from contact with a dock it is subject to weathervaning which is the tendency of the plane to face the wind. Its the same physical principle that keeps an arrow going straight ahead. The strength of this effect depends upon how much of the plane is behind the center of buoyancy.



Wheeled airplanes tend to pivot on the main landing gear wheels. Tail wheel planes have more side area exposed to the wind behind the main gear and are more subject to weathervaning than planes with tricycle gear. For seaplanes, the pivot is around the center of buoyancy which varies according to the pitch attitude in the water. At rest in the water a seaplane acts like a tail dragger. As it starts to move forward you must hold the yoke full back to counter the moment produced by the engine thrust. This raises the nose of the floats and there may be just as much wind-exposed side area ahead as there is behind producing virtually no weathervane effect. If this progresses into a deep plow the weathervaning can even be reversed! Finally, when the floats are moving fast enough to plane or up on-the-step, the effect of side winds can be almost the same as a taildragger again. Because of weathervaning there are only two practical taxiing speeds: slow and on-step although plowing is sometimes useful in turning.

DHC-3 Otter_FLOAT_17

Getting on the step requires the nose to be lowered and staying there is no easy task. You must use just the right amount of back pressure on the stick. Too much or too little will increase the drag and reduce speed. For taxiing, the throttle has to be reduced after you are on the step. Continuing on step with full power you will eventually reach the speed where the plane will lift itself off the water and then fly like a regular airplane. Lack of proper elevator control on a step taxi or takeoff run can result in porpoising which is a pitch oscillating condition that can increase in magnitude if you do not reduce elevator back pressure. Otherwise you may need to reduce power and abort the takeoff.

You may think that smooth water would be the best. Actually the rough water associated with a nice headwind allows you to takeoff at a lower waterspeed which reduces the drag on the floats. The fact that you are cutting along across the tops of the waves also reduces water drag. Depending upon the design of the floats, it is usually not recommended that you rotate as the plane lifts off the water.

Landing is made difficult by lack of visual contact with features on the ground and is particularly troublesome when coming in on glassy, smooth water. On the other hand, hitting rough waves at high speed can damage the floats, so you should always use the slowest possible water speed. Usually the waves will be caused by the wind, so even without a windsock you should be able to determine the proper heading for final approach. Never land parallel to the wave fronts rocking the boat is not a good idea.
So after landing what do you do? Once again you are at the mercy of the wind and with no ground-gear friction or brakes to help you steer. Sailing is a technique that allows you to take advantage of weathervaning and get where you want to dock, even by going backward in the water! After landing in a wind of, say, 20 knots you can set the engine to idle, and use the flight controls to turn the nose toward the dock. Go past the dock and then use power and control to finish.



Docking can be rather tricky because in a real plane you will be out of the cockpit standing on one of the floats. With the engine off, you may need a paddle (standard equipment on seaplanes) if you need just a bit more propulsion, or you might have to use your foot to keep the floats from hitting the dock too hard. There is a lot more to learn including the combined effects of currents and wind.

DHC-3 Otter_FLOAT_33


Here is how the takeoff goes. Once in the water, water rudders are lowered (more about that later) and gear is raised followed by a slow taxi out to the end of the takeoff area and a turn into the correct heading. At this point be sure the brakes are off. In the water, the brakes activate an anchor that will tie you to the spot you drop it. Now raise the water rudders, lower the flaps one notch, set the elevator trim for takeoff, and advance to full pitch and throttle. Pull back on the yoke to reduce drag by raising the leading edge of the pontoons. At about 40 knots, push the yoke forward to get the tail of the pontoons out of the water and have the ship plane on-the-step. At 60 knots the plane will lift off the water by itself. Come to think of it, this is quite a bit like a takeoff for a tail dragger, X-Plane has a default wave height which causes the ripples in the altitude, but I made the takeoff tests with little wind and wave height set to 0.3





Real water rudders are usually placed at the rear of the pontoons or hull and are retractable so as not to be damaged or cause excessive drag during high speed operation. X-Plane has only one way to handle a water rudder. On the landing gear page, you can specify the longitudinal position, the area of the rudder, and the maximum angular movement. You have no control over the vertical position and X-Plane assumes that the water rudder is completely submerged if any part of the fuselage or floats is in the water.



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