Idling Position

When taxying with the engine idling or at a low RPM, the seaplane will remain in what is considered a displacement condition similar to being at rest on the water. This is the "idling" position. The recommended taxy speed is usually below 6 or 7 knots so that the propeller will not pick up water spray which causes serious erosion of the propeller blades. In calm or light wind conditions, the elevator control should be held full back to raise the seaplane's nose and further reduce the possibility of water spray on the propeller, and to improve overall manoeuvrability of the seaplane. This is particularly true if it is equipped with water rudders because more rudder area is kept in the water. Since seaplanes have no brakes, it is especially important to taxy at this slow speed in congested or confined areas because inertia forces build up rapidly, making the seaplane vulnerable to serious damage even in minor collisions.

ploughing position

When the power is increased significantly above idling, the seaplane will usually assume a nose-up or "ploughing" position. Most seaplane experts do not recommend the ploughing position for taxiing, except in rough water when it would be desirable to raise the propeller clear of the spray, or when turning the seaplane downwind during strong wind conditions. To attain this position, full power should be applied and the elevator control held in the full aft position. Seaplanes that have a high thrust line will tend to nose down upon application of power, in which case it is imperative that the elevator control be held in the full aft position. The "ploughing" position is brought about by the combination of the propeller slipstream striking the elevator and the hydrodynamic force of water exerted on the underside of the float's or hull's bow. After the planing position is attained, the power should be reduced to maintain the proper speed. If the water conditions are favourable and there is a long distance to travel, the seaplane may be taxied at high speed "on the step." This position is reached by accelerating the seaplane to the degree that it passes through the ploughing phase until the floats or hull are literally riding on the water in a level position.

planing position

Basically, the planing or step position is best attained by holding the elevator control full aft and advancing the throttle to full power. As the seaplane accelerates it will then gradually assume a nose-high pitch attitude, raising the bow of the float or hull and causing the weight of the seaplane to be transferred toward the aft portion of the float or hull. At the time the seaplane attains its highest pitch attitude, back pressure should be gradually relaxed, causing the weight to be transferred from the aft portion of the float or hull onto the step area. This can be compared to a speedboat's occupants moving forward in the boat to aid in attaining a planing attitude. In the seaplane we do essentially the same thing by use of aerodynamics (elevators). As a result of aerodynamic and hydrodynamic lifting, the seaplane is raised high in the water, allowing the floats or hull to ride on top of rather than in the water. The entire process of planing a seaplane is similar to that of water skiing.

The skier cannot make the transition from a submerged condition to that of being supported on the surface of the water unless a sufficiently high speed is attained and maintained. As further acceleration takes place the flight controls become more responsive just as in the landplane and elevator deflection must be reducing in order to hold the required planing/pitch attitude. This of course is accomplished by further relaxing back pressure, increasing forward pressure, or using forward elevator trim depending on the aircraft flight characteristics. Throughout the acceleration, the transfer of weight and the hydrodynamic lifting of the float or hull may be seen from the cockpit. When the seaplane is taxiing slowly, the water line is quite high on the floats or hull as compared to "on the step". At slow taxi speeds a small wake is created close to the bow of the float or hull and moves outward at a very shallow angle.

As acceleration commences, the wake starts to move from the bow aft toward the step area and the wake now turns into an outward spray pattern. As speed and lifting action increase, the spray pattern continues to move aft toward the step position and increases in intensity, i.e., slow speed spray may be approximately one foot outboard compared to about a 20-foot outboard spray at higher speed on the step position. Some seaplane pilots use the spray pattern as an additional visual reference in aiding them in determining when the seaplane has accelerated sufficiently to start easing it over onto the step. After the planing position has been attained, proper control pressures must be used to control the proper pitch attitude/trim angle. Usually this will be maintained with slight back pressure.

As for the amount of pressure to be held, the beginner will find a very "thin line" between easing off back pressure too much or too little. It can perhaps best be described as finding the "slippery spot" on the float or hull. Too much back pressure, acceleration rate decreases. Not enough back pressure or too much forward pressure also decreases acceleration rate. So that fine line or "slippery spot" is that position between not enough or too much back pressure. If one does not want to take off and just wants to continue to taxi on the step, a reduction in power is initiated at approximately the time the seaplane is eased over onto the step. Power requirements to maintain the proper speed with wind, load and current action will vary. More power will be required taxiing into the wind or an up-current or with a heavy load. However, 65 to 70 percent of maximum power can be used as a starting point. From either the ploughing or on-the-step position, if power is reduced to idle, the seaplane will decelerate quite rapidly and eventually assume the displacement or idle position.

Care must be taken to use proper flight control pressures during the deceleration phase because weight is now being transferred toward the bow and drag is increasing; hence, some aircraft have a nose-over tendency. This is of course controllable by proper use of the elevator controls.

turns

If water rudders have the proper amount of movement, a seaplane can be turned within a radius less than the span of the wing during calm conditions or a light breeze. It will be found that water rudders are usually more effective at slow speeds because they are acting in comparatively undisturbed water. At high speeds, however, the stern of the float churns the adjacent water, thereby causing the water rudder to become less efficient.

Furthermore, because of the high speed, the water's impact on the rudders may tend to force them to swing up or retract. Particular attention should always be given to the risks involved in making turns in a strong wind or at high speeds. Any seaplane will tend to weathervane into a strong wind if the controls are not positioned to prevent it. In a single-engine seaplane rudder should be applied as necessary to control the turn while aileron is held into the wind. On twin-engine seaplanes this tendency can be overcome through the use of differential power - using a higher setting on the upwind side.

The rate at which the seaplane turns when it weathervanes is directly proportional to the speed of the crosswind. When taxiing downwind or crosswind, the seaplane will swing into the wind as soon as the flight controls are neutralized or power is reduced. During a high speed taxiing turn, centrifugal forces tends to tip the seaplane toward the outside of the turn (see Figure 4). Simultaneously, when turning from a downwind heading to an upwind heading, the wind force striking the fuselage and the under side of the wing increases the tendency for the seaplane to lean to the outside of the turn. If an abrupt turn is made, the combination of these two forces may be sufficient to tip the seaplane to the extent that the outside wing drags in the water, and may even tip the seaplane onto its back (see Figure 5). Obviously, the further the seaplane tips, the greater will be the effect of wind, since more wing area on the windward side is exposed to the wind force.

Figure 4: Effect of wind and centrifugal force

When making a turn into the wind from a crosswind condition, the air-rudder may be neutralized and the seaplane allowed to weathervane into the wind. If taxiing directly downwind, a turn into the wind may be started by deflecting the air rudder in the same direction that the turn is desired. As soon as the seaplane begins to turn, the rudder should be neutralized; if the wind is strong, some opposite rudder may be needed during the turn. The amount of opposite rudder depends upon the rate at which the seaplane turns. The greater the amount of opposite rudder, the slower the rate of turn. Normally, the power should be reduced to idle when the turn begins because with the power on the left turning tendency of the seaplane may become excessive. Short bursts of power are best for turning in a small radius, but sustained excessive power causes a buildup of speed and a larger turning radius.

Figure 5: Effect of wind

The seaplane tends to use its centre of buoyancy (COB) as a pivot point wherever it may be. Centre of buoyancy moves laterally, as well as forward and aft. Each object in water has a point of centre of buoyancy. In a twin-float installation the effects of wind, power and flight controls are shared by the two floats and the average COB is free to move significantly. The centre of buoyancy (COB), or average point of support, moves aft when the seaplane is placed in a noseup or ploughing position (see Figure 3). This position exposes to the wind a considerable amount of float and fuselage side area forward of the centre of buoyancy. Therefore, when taxiing crosswind in this position, many seaplanes will show a tendency to turn downwind because of the wind force on the exposed area of the float and the fuselage. For this reason it is sometimes helpful to place the seaplane in a nose-up position when turning downwind, particularly if the wind velocity is high. Under high wind conditions, the throttle may be used as a turning device by increasing power to cause a nose-up position when turning downwind, and decreasing power to allow the seaplane to weathervane into the wind.

sailing

Occasions often arise when it is advisable to move the seaplane backward or to one side because wind or water conditions, or limited space make it impractical to attempt a turn (see Figure 6). In this situation, particularly if there is a significant wind, the seaplane can be "sailed" into a space which to an inexperienced pilot might seem extremely cramped. Even if the wind is calm and the space is inadequate for making a normal turn, a paddle (which should be part of every seaplane's equipment) may be used to propel the seaplane or to turn the nose in the desired direction.

In light wind conditions with the engine idling or shut down, the seaplane will naturally weathervane into the wind and then sail in the direction the tail is pointed (see Figure 7). With a stronger wind and a slight amount of power, the seaplane will usually sail downwind toward the side in which the nose is pointed. Rudder and aileron can be deflected to create drag on the appropriate side to control the direction of movement. Positioning the controls for the desired direction of motion in light or strong winds is illustrated in Figure 7. Lowering the wing flaps and opening the cabin doors will increase the air resistance and thus add to the effect of the wind; however, the effect of the air rudder may be reduced in this configuration.

Since water rudders have little or no effect in controlling direction while sailing, they should be lifted With the engine shut down, most flying boats will sail backward and toward the side to which the nose is pointed, much as a sailboat tacks, regardless of wind velocity because the hull does not provide as much keel (side area) as do floats in proportion to the side of the seaplane. To sail directly backward in a seaplane having a hull, the controls should be released and the wind allowed to steer the seaplane. Sailing is an essential part of seaplane operation. Since each type of seaplane has its own minor peculiarities, depending on the design of the floats or hull, it should be practiced until thorough familiarization with that particular type is gained.

During initial seaplane training, sailing should be practiced in large bodies of water such as lakes or bays, but sufficiently close to a prominent object in order to evaluate performance. Where there are strong tides or a rapidly flowing current, such as in rivers, care must be taken in observing the relative effect of both the wind and the water current. Often the force of the current will be equal to or greater than the force of the wind..

Figure 7: Sailing procedures

Before taxiing into a confined area, the effect of wind and the current should be considered carefully. Otherwise, the seaplane may be carried into obstructions with resulting damage to the wings, tail surfaces, floats, hull, or other parts of the seaplane. Generally, with a seaplane of average size and power at idle, a water current of 5 knots will more than offset a wind velocity of 25 knots. This means that the seaplane will move against the wind. When operating multiengine seaplanes, differential power can be used to aid in steering the seaplane along a desired course.

porpoising

Porpoising in a seaplane is much like the antics of a dolphin - a rhythmic pitching and heaving while in the water. Porpoising is a dynamic instability of the seaplane and may occur when the seaplane is moving across the water while on the step during takeoff or landing. It occurs when the angle between the float or hull, and the water surface exceeds the upper or lower limit of the seaplane's pitch angle. Improper use of the elevator, resulting in attaining too high or too low a pitch (trim angle) sets off a cyclic oscillation which steadily increases in amplitude unless the proper trim angle or pitch attitude is re-established. A seaplane will travel smoothly across the water while on the step, so long as the floats or hull remain within a moderately tolerant range of trim angles.

If the trim angle is held too low during planing, water pressure in the form of a small crest or wall is built up under the bow or forward part of the floats or hull. As the seaplane's forward speed is increased to a certain point, the bow of the floats or hull will no longer remain behind this crest and is abruptly forced upward as the seaplane rides over the crest. As the crest passes the step and on to the stern or aft portion of the floats or hull, the bow abruptly drops into a low position. This again builds a crest or wall of water in front of the bow resulting in another oscillation. Each oscillation becomes increasingly severe, and if not corrected will cause the seaplane to nose into the water, resulting in extensive damage or possible capsizing.

Porpoising can also cause a premature lift off with an extremely high angle of attack, resulting in a stall or being in the area of reverse command and unable to climb over obstructions. Porpoising will occur during the takeoff run if the trim angle is not properly controlled with proper elevator pressure just after passing through the "hump" speed, or when the highest trim angle before the planing attitude is attained; that is, if up-elevator is held too long and the angle reaches the upper limits. On the other hand, if the seaplane is nosed down too sharply, the lower trim range can be entered and will also result in porpoising.

Usually, porpoising does not start until a degree or two after the seaplane has passed into the critical trim angle range, and does not cease until a degree or two after the seaplane has passed out of the critical range. If porpoising does occur, it can be stopped by applying timely back pressure on the elevator control to prevent the bow of the floats or hull from digging into the water. The back pressure must be applied and maintained until porpoising is damped. If porpoising is not damped by the time the second oscillations occurs, it is recommended that the power be reduced to idle and elevator control held firmly back so the seaplane will settle onto the water with no further instability. The correct trim angle for takeoff, planing and landing applicable to each type of seaplane must be learned by the pilot and practiced until there is no doubt as to the proper angles for the various manoeuvres.

The upper and lower trim angles are established by the design of the aircraft; however, changing the seaplane's gross weight, wing flap position, and centre of gravity location will also change these limits. Increased weight increases the displacement of the floats or hull and raises the lower limit considerably. Extending the wing flaps frequently trims the seaplane to the lower limit at lower speeds, and may lower the upper limit at high speeds. A forward centre of gravity increases the possibility of high angle porpoising especially during landing.

skipping

Skipping is a form of instability which may occur when landing with excessive speed at a nose-up trim angle. This nose-up attitude places the seaplane at the upper trim limit of stability, and causes the seaplane to enter a cyclic oscillation when touching the water, resulting in the seaplane skipping across the surface. This action may be compared to "skipping" flat stones across the water. Skipping can also occur by crossing a boat wake while fast taxiing on the step or during a takeoff. Sometimes the new seaplane pilot will confuse a skip with a porpoise. Pilot's body feelings can quickly determine whether a skip or porpoise has been encountered. A skip will give the body vertical "G" forces, similar to bouncing a landplane. The porpoise is a rocking chair type forward and aft motion feeling. Correction for skipping is made by first increasing back pressure on the elevator control and adding sufficient power to prevent the floats from contacting the water. Then pressure on the elevator must be adjusted to attain the proper trim angle and the power gradually reduced to allow the seaplane to settle gently onto the water. Skipping will not continue increasing its oscillations, as in porpoising, because of the lack of forward thrust with reduced power.