Loads during a turn

It will be clear from the figures that the lift on the wings during the turn is greater than during straight flight; it is also very noticeable that the lift increases considerably with the angle of bank. This means that structural com­ponents, such as the wing spars, will have to carry loads considerably greater than those of straight flight.

Mathematically, W/L = cos в, or L = W/cos в

i. e. at 60° angle of bank, lift = 2W, stalling speed, 85 knots (44m/s) at 70° angle of bank, lift = 3W, stalling speed, 104 knots (53m/s)

Fig 8.5 Correct angles of bank Air speed 60 knots (31 m/s)

at 75° angle of bank, lift = 4W, stalling speed, 120 knots (62 m/s)

at 84° angle of bank, lift = 10W, stalling speed, 190 knots (98 m/s)

These figures mean that at these angles of bank, which are given to the nearest degree, the loads on the wing structure are 2, 3, 4, and 10 times respectively the loads of normal flight. This is simply our old friend g again, but in this instance it is certainly better to talk in terms of load than of g because the accelerations, and the corresponding loads, are in a horizontal plane while the initial weight is vertical; it is no longer a question of adding by simple arith­metic.

Whatever the angle of bank, the lift on the wings must be provided by CL. ypV2 . S. It follows, therefore, that the value of CL . ypV2 . S must be greater during a turn than during normal flight, and this must be achieved either by increasing the velocity or increasing the value of CL. Thus it follows that the stalling speed, which means the speed at the maximum value of CL, must go up in a turn; as before it will go up in proportion of the square root of the wing loading, and the stalling speeds corresponding to the various angles of bank are shown in the table assuming, as for the pull-out of a dive, a stalling speed in level flight of 60 knots (31 m/s). These are all fairly steep banks; for banks up to 45° or so the loads are not serious, there is no danger of blacking out, and the increase of stalling speed is quite small – even so, it needs watching if one is already flying or gliding anywhere near the normal stalling speed, and suddenly decides to turn. At steep angles of bank we have to contend not only with the considerable increase of stalling speeds but with all the same problems as arose with the pull-out, i. e. blacking out, injury to pilot and crew, and the possibility of structural failure in the aircraft. It may seem curious that the angle of bank should be the deciding factor, but it must be remembered that the angle of bank (provided it is the correct angle of bank) is itself dependent on the velocity and radius of the turn, and these are the factors that really matter. In the history of fighting aircraft the ability to out­turn an opponent has probably counted more than any other feature, and from this point of view the question of steeply banked turns is one of paramount importance. An aspect of this question which must not be forgotten is that of engine power; steep turns can only be accomplished if the engine is powerful enough to keep the aeroplane travelling at high speed and at large angles of attack, perhaps even at the stalling angle. The normal duties of the engine are to propel the aeroplane at high speed at small angles of attack, or low speed at large angles of attack, but not both at the same time. The need for extra power in steeply banked tight turns has resulted in a technique in which the pilot embarking on such a manoeuvre suddenly applies all the power available.