Chapter 6

 

Chapter 6

Directional Control: Rudder controls, counteracting torque.

So far we have only considered longitudinal trim and the correct placing of weights to achieve it.

However, the question of movable controls will naturally occur. For those who wish to fit them, the simplest and most effective way is to have the hinged portions, such as the rudder, ailerons, elevators, etc., made separately, and then fastened to the main portion of the aerofoil by means of aluminium tabs in such a way that each control surface will remain fairly rigid in whatever position it may be placed.

However, from a purely performance point of view it is far better to leave all control surfaces definitely fixed and built integral with the model in a neutral position.

However well one may trim a job and mark the exact position of variable controls, the probability is that on a roughish landing the controls will all be put out of order!

Also, as these controls are very sensitive, especially on the elevator and rudder, it is a moot point whether they can be set accurately after each flight, and the chances are that in the heat of the moment and rush to make another flight these controls may be forgotten, with disastrous results.

Some form of semi-permanent adjustment is obviously necessary, and it is suggested that the "empennage," that is the stabiliser, elevators, fin and rudder, be made as a single unit, and detachable. This unit can be held in place by means of small wire hooks and rubber bands, and will take a knock safely. Not only that, but it is far easier for transport purposes, and transport amongst the aero-modelling fraternity is generally a very serious problem.

But to return to the adjustment of the tail unit. It will readily be seen that it is quite a simple matter to pack the elevator up slightly, either positive or negative, by cementing small strips of balsa underneath, and to very slightly offset the unit sideways to obtain a circling inclination to one side.

Now this cirling tendency must be only very slight, otherwise we shall have the model dipping a wing and spiralling to earth.

It is very important that these various semi-permanent adjustments be made for the glide only, so that when the motor runs out our model shall still be in complete stability and shall safely come to earth.

Of course, the turning tendency to the left is always present when the motor is running, owing to the torque or reaction of the airscrew, but this will be counteracted by offsetting the nose-block slightly to the right.

It is important that all superimposed adjustments while under power should be made to the nose-block, and the nose-block only. Not only should the direction of flight under power be adjusted this way, but also climbing abilities as well, by either giving a slight amount of upthrust if the model tends to be underpowered, or downthrust if the model should climb too steeply.

It will be realised that with the use of a gearbox to eliminate the initial burst of power there should not be excessive torque to counteract, but where a single-skein motor is used without a gearbox, then a certain amount of side- and downthrust is almost essential.

 
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