User manual

Section Appendix A.2 Part B
component of lift sufficient to turn the aircraft. To maintain a heading it is therefore
important to keep the wings level when using rudder to turn there is a centrifugal force
tending to bank the aircraft away from the direction of turn and if allowed to occur this adds
a horizontal component of lift that counters the desired turn. This tendency must be
countered by a balancing control input (aileron slightly into the turn).
Control of the aircraft on the step is important in take off and landing phases c flight and
during high speed manoeuvring.
14 PORPOISING
This is a dynamic instability in pitch resulting in a series of oscillations, which tend to grow
larger, producing nose low pitch on water contact which worsens with each oscillation, hull
damage or destruction can result. Porpoising may occur just after climbing onto the step
during take off or high speed taxi or just before coming off the step after landing. A couple
of oscillations are common at these times and generally cause no trouble. Once on the
step the aircraft is more hydro dynamically stable than during transition to it so the
oscillations rarely continue and once off the step after landing the stability again increases.
The time where dangerous porpoising can begin is at high speed on the step just before
take off. Failure to maintain the correct pitch trim angle at this time will result in large
forces being applied to the hull through forward or rearward movement of the centre of
buoyancy. This may result in pitch change, further altering the hydrodynamic forces and
then worsening instability with progressively larger oscillations. In calm conditions this is
likely to be the pilots fault, but waves (even low waves of long wavelength) can induce the
problem. Incorrect or over loading and hot/high conditions also make porpoising more
likely.
Early on, porpoising can be arrested by returning the aircraft to the correct trim angle. As
porpoising worsens, more control input is required. So….
15 PORPOISING RECOVERY (GENERAL TECHNIQUES)
8) Return to best trim angle and hold that attitude.
9) As the bow reaches its most nose up point in the oscillation, add control back pressure
and then slowly lower the nose to the correct pitch angle as the aircraft "squats" into
the water nose high. Timing is critical. Then restabilise on the step.
10) Push the controls fore and aft to counter the pitch changes. Input must be slightly
ahead of the motion. This is difficult to time. Experience is essential, since timed badly
this can worsen the oscillations. (Do not use in PBY, instead go to No 4 below.)
11) Porpoising Abort. Close the throttles and draw back on the controls. Time this for
when the nose is rising. Let the aircraft come off the step.
WARNING: Abort early before the oscillations get severe.Do not try to fly out of porpoising
by applying power.
16 SKIPPING (& OCCLUSION OF THE STEP)
Contact with the water at higher than normal rate of descent, especially with a nose up
pitch attitude, can result in sudden immersion of the step and the area behind it. The
reaction to this is to force the aircraft back into the air when its forward speed can not
sustain flight. The generation of the forces to rebound the aircraft can be reduced by the
Vee shape of the hull and by air ducts at the step to allow trapped air out and smooth flow
of air back in to the area behind the step.
Less violent skips can occur when the aircraft contacts the water at higher than ideal
approach speed usually off a shallow approach and with a higher than ideal nose attitude.
Rev No: 02 Stichting Catalina PH-PBY
Page: 4 01 April, 2006