AIRBRAKES, SPOILERS AND FLAPS

As landing aids, split flaps are excellent Their advantage is that they create high drag with high lift coefficient, slowing the approach speed and steepening the glide angle, enabling better judgement of the point of touch down to be made. Operating loads on such surfaces are high at high airspeeds, and the modeller may well And the servo incapable of lowering the flaps fully if the model is not at the right speed (Fig. 13.6). As airbrakes, such flaps are usually beyond the servo’s power to operate when most needed.

On sailplanes, various types of spoilers and airbrakes are used. The spoiler is a simple hinged surface which can be raised on the upper wing surface to disrupt the airflow and create more drag. It also reduces the maximum lift coefficient and raises the stalling speed, which is a disadvantage. The nose-down pitching moment may also be quite severe. Such spoilers are usually somewhat ineffective as drag producers, especially if mounted too far aft on the wing. They are also usually a source of extra drag in normal flight, since there is some air leakage from inside the wing to the upper surface around the spoiler. Unless they are very carefully fitted, when shut, they break the aerofoil profile’s contours and may cause separation or, on a laminar flow wing, transition. Some of the same criticisms apply to air brakes, which are raised vertically by some form of‘parallel ruler’ type of linkage (Fig. 13.6c), either on one surface of the wing, or both. These are usually much more effective than spoilers, especially at high speeds. They may be used to prevent the airspeed in prolonged dives from exceeding safe limits. This is useful in bringing sailplanes down out of strong thermals. There are two types of such parallel action brakes. In the first, the brakes are directly above one another, and extend equally above and below the wing. Between them there is a gap in the wing through which the air can flow from bottom to top surface. The effect on drag is very powerful, and the air leakage also reduces Cl markedly. On closing the brakes, the gap is sealed as far as possible by the brakes themselves, but there is still usually some leakage. To avoid this, the two leaves of the brakes may be separated, so that there is never any gap through the wing, each brake component retracting into its own sealed box inside. These brakes are slightly less effective, but interfere less with flight when retracted. It is very common to use brakes which extend from only one side of the wing, the upper or lower. These, like

Parallel brakes

spoilers, may cause some pitching moments, requiring re-trimming.

To avoid die leakage and fitting problems of brakes which retract into the wing, several types of trailing edge air brakes are possible, the best being those which are self balancing and so require only small power from the servo. These may sometimes cause pitching oscillations of the model due to the formation of rotating vortices behind them, which also’ may strike the tailplane. The brakes should be placed outboard of the tailplane, or a ‘T’ or ‘V’ tail used to ensure that the surface is clear of the turbulent wake.

A very effective type of braking system which has become popular for sailplanes, is the so-called ‘crow’ or ‘butterfly’ mix. This normally requires sophisticated radio equipment

which permits the mixing of the ailerons and the flaps, together with the elevator.

When the action is required, the flaps, extending over the whole inboard section of the wing trailing edge from aileron ends to wing root, are arranged to go down hilly to 90 degrees or as close to this as possible. At the same time, both the ailerons move up together by IS to 25 degrees. The flaps create very high drag in this position and also at their outer end, adjacent to the ailerons, a strong vortex forms which adds yet more drag. The lift over, the flapped part of the wing remains high. The ailerons, because they are raised, remain effective as lateral controls. They effectively reverse the camber of the outer wing so the lift there is greatly reduced.

The braking effect is very powerful but there is usually a very strong nose-up pitch as the flaps go down. This requires the elevator to be coupled to the landing system to prevent the glider rearing up into a stall. Once the crow landing system is deployed, the model remains controllable for the final approach and touch down. A minor problem arises if the flaps, at their fully down position, make contact with the ground as the model lands. This can cause damage. Raising the flaps just before touchdown, or designing the model so that they cannot touch the ground in any position, will prevent trouble.

Parachute airbakes are excellent, providing they can be made reliable. The parachute itself may be housed in a special compartment in the fuselage, or following full-sized practice, in the bottom of the rudder. A light spring may be needed to ensure ejection of the brake ’chute. As a rule, such brakes are ‘one shot’ only, i. e. they cannot be retracted after deployment. Either the modeller must develop good judgement so that the parachute is never deployed at the wpong time, or some means of jettisoning it must be devised, so that an error can be retrieved. Full-sized sailplanes using such brakes usually possess other forms of air brakes as well, in case the parachute fails to deploy when needed, or in case the pilot inadvertently pulls the jettison handle instead of the deployment knob. The Polish ‘Zefir’ sailplane of 1958-62 possessed a brake parachute which could be retracted by means of a small hand winch, but the mechanical complications were considerable and the device was not used on later aircraft. Parachute brakes are a good choice for ‘flying wing’ models. Noel Falconer suggests a two point attachment for the brake ’chute, so that

deployment is more certain. One attachment may then be released to allow the parachute to trail, if an undershoot must be avoided.

Other forms of air brake are sometimes employed, including flaps sticking out of the side of the fuselage, split rudders which open like clam shells, and even broad wing struts which rotate through 90 degrees to give a braking effect. All these work to some extent but less effectively than the orthodox brakes described.