Unlike landplane operations at airports, seaplane operations are often conducted on water areas at which other activities are permitted. Therefore, the seaplane pilot is constantly confronted with floating objects, some of which are almost submerged and difficult to see - swimmers, skiers, and a variety of watercraft. Before beginning the takeoff, it is advisable to taxi along the intended takeoff path to check for the presence of any hazardous objects or obstructions. Thorough scrutiny should be given to the area to assure not only that it is clear, but that it will remain clear throughout the takeoff. Operators of motorboats and sailboats often do not realize the hazard resulting from moving their vessels into the takeoff path of a seaplane.
To accelerate during takeoff in a landplane, propeller
thrust must overcome only the surface friction of the wheels and the increasing
aerodynamic drag. During a seaplane takeoff, however, hydrodynamic or water
drag becomes the major part of the forces resisting acceleration. This
resistance reaches its peak at a speed of about 27 knots, and just before
the floats or hull are placed into a planing attitude.
| The hydrodynamic forces at work during a seaplane
takeoff are shown in Fig. 15-9. The point of greatest resistance is referred
to as the "hump" because the increasing and decreasing effect of water
drag causes a hump in the resisting curve. After the hump is passed and
the seaplane is traveling on the step, water resistance decreases.
Several factors greatly increase the water drag or resistance; heavy loading of the aircraft, or glassy water conditions in which no air bubbles slide under the floats or hull, as they do during a choppy water condition. In extreme cases, the drag may exceed the available thrust and prevent the seaplane from becoming airborne. This is particularly true when operating in areas with high density altitudes (high elevation/high temperatures) where the engine cannot develop full rated power. For this reason the pilot should also practice takeoffs using only partial power to simulate the long takeoff run usually needed when operating at water areas where the density altitude is high and/or the seaplane is heavily loaded.
The seaplane takeoff may be divided into four distinct phases: (1) The "displacement" phase, (2) the "hump" or "plowing" phase, (3) the "planing" or "on the step" phase, and (4) the "lift off." The first three phases were previously described in the section on taxiing. The "lift off" is merely transferring support of the seaplane from the floats or hull to the wings by applying back elevator pressure. This results in the seaplane lifting off the water and becoming airborne.
To avoid porpoising during the takeoff run, it is important to maintain the proper pitch angles. Too much back elevator pressure during the planing or lift off phases will force the stern of the floats or hull deeper into the water, creating a strong resistance and appreciably retarding the takeoff. Conversely, insufficient back elevator pressure will cause the bows to remain in the water, which also results in excessive water drag. Experience will determine the best angle to maintain during takeoff for each seaplane, and if held at this angle, the seaplane will take off smoothly.
Because the seaplane is not supported on a solid surface and the float or one side of the hull can be forced deeper into the water, right aileron control is usually required to offset the effect of torque when full power is applied during takeoff.
The spray pattern for each particular seaplane should also be considered during takeoff. During acceleration the water is increasingly sprayed upward, outward, and rearward from the bow portion of the floats or hull, and on some seaplanes will be directed into the propeller, eventually causing erosion of the blades. This water spray is greater during the hump phase. The spray can be reduced during takeoff, however, by first increasing the planing speed about 10 knots, then opening the throttle as rapidly as practical. This method shortens the time that propellers are exposed to the spray. Again, the best technique must be learned through experience with each particular seaplane. Bear in mind that a rough water condition creates more spray than does smooth water.
Glassy water takeoffs in a low powered seaplane loaded to its maximum authorized weight presents a difficult, but not necessarily a dangerous, problem. Under these conditions the seaplane may assume a "plowing" or nose up position, but may not "unstick" or get "on the step" because of the adhesive action of smooth water; consequently, always plan ahead and consider the possibility of aborting the takeoff. Nonetheless, if these conditions are not too excessive, the takeoff can be accomplished using the following procedure.
After the bow has risen to the highest point in the plowing position with full back elevator pressure, it should be lowered by decreasing back elevator pressure. The bow will drop if the seaplane has attained enough speed to be on the verge of attaining the step position. After a few seconds, the bow will rise again. At the instant it starts to rise, the rebound should be caught by again applying firm back elevator pressure, and as soon as the bow reaches its maximum height, the entire routine should be repeated. After several repetitions, it will be noted that the bow attains greater height and that the speed is increasing. If the elevator control is then pushed well forward and held there, the seaplane will slowly flatten out "on the step" and the controls may then be eased back to the neutral position.
Whenever the water is glassy smooth, a takeoff can be made with less difficulty by making the takeoff run across the wakes created by motorboats. If boats are not operating in the area, it is possible to create wakes by taxiing the seaplane in a circle and then taking off across these self-made wakes.
On seaplanes with twin floats water drag can be reduced
by applying sufficient aileron pressure to raise the wing and lift one
float out of the water after the seaplane is on the step. By allowing the
seaplane to turn slightly in the direction the aileron is being held rather
than holding opposite rudder to maintain a straight course, considerable
aerodynamic drag can be eliminated, aiding acceleration and lift off. When
using this technique, great care must be exercised so as not to lift the
wing to the extent that the opposite wing strikes the water. Naturally,
this would result in serious consequences.
In most cases an experienced seaplane pilot can safely take off in rough water, but a beginner should not attempt to takeoff if the waves are high. Using the proper procedure during rough water operation lessens the abuse of the floats, as well as the entire seaplane.
During rough water takeoffs, the throttle should be opened to takeoff power just as the bow is rising on a wave. This prevents the bow from digging into the water and helps keep the spray from the propeller. Slightly more back elevator pressure should be applied to the elevator than on a smooth water takeoff. This raises the bow to a higher angle.
After planing has begun, the seaplane will bounce from one wave crest to the next, raising the nose higher with each bounce, and each successive wave will be struck with increasing severity. To correct this situation and to prevent a stall, smooth elevator pressures should be used to set up a fairly constant pitch attitude that will allow the aircraft to "skim" across each successive wave as speed increases. Remember, in waves, the length of the float is very important. It is important that control pressure be maintained to prevent the bow from being pushed under the water surface or "stubbing its toe," which could result in capsizing the seaplane. Fortunately, a takeoff in rough water is accomplished within a short time because if there is sufficient wind to make the water rough, the wind would also be strong enough to produce aerodynamic lift earlier and enable the seaplane to become airborne quickly.
With respect to water roughness, one condition that seaplane pilots should be aware of is the effect of a strong water current flowing against the wind. For example, if the velocity of the current is moving at 10 knots, and the wind is blowing at 15 knots, the relative velocity between the water and the wind is 25 knots. In other words, the waves will be as high as those produced in still water by a wind of 25 knots.
The advisability of canceling a proposed flight because of rough water depends upon the size of the seaplane, wing loading, power loading, and, most important, the pilot's ability. As a general rule, if the height of the waves from trough to crest is more than 20 percent of the length of the floats, takeoffs should not be attempted except by the most experienced and expert seaplane pilots.
Downwind takeoffs are possible, and at times preferable, if the wind velocity is light and normal takeoffs would involve clearing hazardous obstructions, or flying over congested areas before adequate altitude can be attained. The technique used for downwind takeoffs is almost identical to that used for upwind takeoffs. The only difference is that the elevator control should be held further aft, if possible. When downwind takeoffs are made, it should be kept in mind that more space is needed for the takeoff. If operating from a small body of water, an acceptable technique may be to begin the takeoff run while headed downwind, and then turning so as to complete the takeoff into the wind. This may be done by planing the seaplane while on a downwind heading then making a steep turn into the wind to complete the takeoff. Caution must be exercised when using this technique since wind and centrifugal force will be acting in the same direction and could result in the seaplane tipping over.
Crosswind takeoff techniques will be discussed later in the chapter.