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Aviation History
1942
1942 - 0724.PDF
3i8 FLIGHT APRIL 2ND, 1942 TOPICAL AIRCRAFT PROBLEMS Fig. 24. German dive bombers Ju. 87 and Ju. 88. war, but the need has arisen for increasing considerably the bomb load and range This desire leads necessarily to a greater flying weight, such as the change-over from the Ju. 87 to the Ju. 88. If, now, one demands of heavier aircraft the same manoeuvrability, i.e., the same pull-out radius or the same time of turn, then the control forces are considerably increased. In Fig. 25 the control forces for a " standard'' aircraft of low weight are assumed equal to 1, and those for heavier aircraft referred to this. It was assumed that the heavier aircraft were geometric enlargements of the lighter, so that the similarity need only be applied to keeping the wing loading and main proportions constant. Flying speeds and pull-out radii were left unchanged. It is seen that an enlargement of the aircraft to three times size results in control column loads of six or eight times the "standard." This loading depends on certain characteristics of the aircraft, such as the position of the e.g. For the reasons stated, the provi sion of types of control surfaces which have great effectiveness com bined with low- control loads becomes import ant. With a nor mal non-balanced control surface there is, when the surface is moved, a force, P, as shown in the upper part of Fig. 26, which acts on the lever arm I from the op10 rrPk UK. Li ne K _1 U b a. E CON * K I UJ 0 \ —iy*y.y 0 1 2 GROSS 3 A WEIGHT G/GQ Fig. 25- Increase of control forces with in creased gross weight. P H = force on con trol column. G=gross weight. P Ho and G,, = control force and loaded weight of "standard" aircraft (taken = l). axis. The pilot thus has to overcome, on the control column, & moment P x I. The simplest way of reducing this moment is to set the hinge axis back, as shown at a in the lower part of Fig. 26. This reduces the lever arm of the control surface to /,. Another possibility is shown at b. This consists of hingeing to the trailing edge of the main control sur face a small balance flap, so linked up that when the con trol surface moves down the flap moves up. The control surface moment is then the difference between two moments, P,x/, and P„xl.,. Of these the main surface moment, P,xl,, is the product of a large force on a short lever arm, and the flap moment is the product of a small force on a long lever arm. Both forms of balance are cur rently used, and both have given good results. Both forms of balance really have the same basic object: NON-BALANCED ELEVATOR EjLEVATOR MOMENT MR*P I IP -+*• BALANCED ELEVATOR a) INSET BALANCE j MR' P. b) TRIMMING TAB M2«P111-P2.I2 Fig. 26. Aerodynamic balances tor reducing control forces. to place the resultant air force as close to the hinge axis as possible. However, the point of application of the air resultant varies both with the angle of incidence of the tailplane and with the elevator angle itself. Consequently, it is not possible to go as far as one would like. Particu larly at small elevator angles, small irregularities, or even .differences in surface roughness, can shift the point of appli cation of the air force on the control surface so far that, in the same aircraft type, one control will be normal and the other aerodynamically overbalanced. This will either cause a flicking outwards of the control surface or will start flutter. By further increasing aircraft size, the aerodynamic balance will not suffice, and it will be necessary to.go over to increasing the hand power of the pilot by some form of servo control. \_ Structural Strength The increase in flying speed imposes special requirements on the strength of the aircraft. This comes about by the fact that the accelerations to which the aircraft is sub jected during the pull-out become greater with increasing speed. If we take, for example, an aircraft which is glid-
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