A review of porpoising instability of seaplanes

Abstract

A review has been made of the evidence on take-off and landing porpoising instability of seaplanes. The basic types of porpoising and their occurrence have been examined ; full-scale results have been correlated with model-scale and theoretical results. Porpoising instability has been divided into three basic types, (a) forebody, (b) forebody-afterbody, (c) step Lnstability. The first occurs during planing on the forebody only whenever the attitude decreases below a critical value. It is associated with a positive water pressure distribution over the forebody near the step ; there is no flow on the afterbody. The instability corresponds theoretically to that of a single planing surface. The second type occurs during planing on the front and rear steps whenever the attitude exceeds a critical value. It is associated with a positive water pressure distribution over the forebody and afterbody in tile neighbourhood of the steps only. There is no flow on the first 70 to 80 per cent of the afterbody. This porpoising corresponds to the theoretical case of two planing surfaces in tandem. The third type occurs when the water flow is not separated efficiently from the hull bottom at the main step. Large negative pressures alternate with positive pressures on the whole afterbody, the combination causing violent instability. Step instability is only present at high speeds but may occur down to quite low attitudes, and well below the stalling speed. Full-scale stability limits are measured in both steady and accelerated speeds. Under operational conditions a 2 deg amplitude porpoise has been chosen as the maximum permissible for safety. Three degrees of stable range are then defined : (i) the minimum stable range, corresponding to the limits given by undamped porpoising of any amplitude-- these limits are obtained from steady or accelerated speed tests ; (ii) the minimum stable range during steady speeds where limits are drawn to exclude porpoising of under 2 deg ; (iii) the operational stable range where limits are drawn to exclude porpoising of under 2 deg amplitude under accelerated conditions. The first is of predominantly research interest, the second is the operational case for zero acceleration (i.e., over load take-off), the third is of greatest operational importance. The stability limits are to some extent dependent on the degree of disturbance encountered, but once started, porpoising instability is independen t of disturbance. Step porpoising is particularly sensitive to disturbance ; in bad cases it often occurs at high speeds whenever the afterbody becomes even slightly immersed. A maximum value of disturbance should be laid down for design purposes. Model tests at steady speeds give the minimum stable range. At high speeds the Royal Aircraft Establishment range is probably smaller than the full-scale because of the disturbance used and represents the extreme case. R.A.E. model limits are 1 to 3 deg higher than the full-scale limits on the same seaplane, but are otherwise ill good qualitative agreement. The differences are probably due in part to the accumulated effect of differences in displacement, stalling angle and lift, damping, moment of inertia and radius of gyration, to differences in applied disturbance, and to scale effect. The first can be reduced by use of slipstream and care ill aerodynamic design ; the second by use of a laid down full-scale design disturbance. The theory of porpoising instability will give accurate results for forebody instability if accurate values of the derivatives are available. There is not as yet sufficient accurate generalised data for this and experimental determinations are as lengthy as measuring the actual limits.