Landing Gears: Actions, retractile types, tricycles, floats and boatsTHE "undercart," as the flying folk call the landing gear, will probably cause the aero-modeller furiously to think. How to incorporate it in his model and at the same time make it strong enough to withstand landing shocks and look like the real thing!
It is only proposed here to deal with the functions of the model undercart, as the construction will be dealt with in a later chapter. But so important are the functions of this particular part that the writer has seen fit to devote a chapter to it alone.
The chief reasons for the undercart being there at all are, of course, to support the air-frame whilst stationary, at take-off, and to absorb the shocks on landing. Otherwise it is a nuisance, in so much that on most very efficient 'planes it is made to tuck away out of sight, as it offers a most serious resistance to the air whilst in flight.
Taking the prototypes first, the undercarriage legs are either spring, or hydraulically loaded. The latter type depends for its action on a piston in a cylinder pushing a certain amount of liquid through a small aperture. It will at once become apparent that on a small model we cannot use the latter method, but the spring method does offer possibilities. However, we must be very careful if we decide to use a sliding action of any sort, as a roughish landing may bend a leg, with the result that the sliding action is jammed.
Where it is decided to use this type, it is strongly urged that the builder, instead of the usual tube for the leg to slide in, uses a flexible tube, such as part of an expanding curtain rod or a Bowden outer covering.
On the other hand, of course, the whole leg can be a dummy, supported by a steel spring wire, or wires, attached to the fuselage in such a way that the legs as a unit spring slightly backwards and outwards to absorb the shock.
Probably the type that springs outwards is the easier to construct, and has a definite advantage over the type that swings backwards. It is this wise:
The landing wheels should be in front of our old friend the Centre of Gravity, otherwise the 'plane would tip over on its nose when stationary. But you will observe on the full-size aircraft that the point of contact with the ground is just behind, or actually in line with, the leading edge of the wing. Now if we adopt the spring-back method we may, on landing, get our line of contact with the ground pushed beneath the C. of G., or even slightly behind it, with the result that over goes the 'plane on to its nose.
Actually, of course, the positioning of the undercart is very important, but unfortunately for us it is fixed by the design we are copying.
For safety's sake it is advantageous if possible to have the undercarriage fairly well forward, so that the tail does not tend to come over on landing.
Unfortunately, however, for a take-off we want to get the tail up quickly, and so we must have the undercart fairly well back. There's a nice problem if ever there was. However, should the builder be prepared to sacrifice a certain amount of scale, it is suggested he can do a lot worse than by moving his wheels forward a little. This can easily be achieved by keeping the position correct where the undercart is attached to the fuselage or centre section-but the actual angle it makes with its fixing is altered slightly, so that the bottom of the legs are farther forward.
It must also be borne in mind that if we should be using a special airscrew to give us duration, and we have to exceed the original scale diameter, we shall have to extend the length of our undercarriage to prevent the tips of the blades hitting the ground. Here also it is advisable in nearly every case to deviate slightly from scale and add a little amount to the length of the legs, because, even when taking off on the smoothest of ground, the tail is likely to bump up and dig the propeller into the ground, or at least try to, with dire consequences for the propeller!
The undercart must be made sturdy, which introduces a certain amount of weight, but, as it happens, a certain amount of underslung weight adds enormously to the stability of our aircraft, and on the very small models of, say, 18 in. span and downwards, hardwood wheels to give weight will produce a very steady flight on a quiet day.
Retracting undercarts are quite possible and practical to the mechanically-minded builder, and are great fun to build. However, it must be borne in mind that wheels that retract inwards, like the Heston Phoenix and Hawker Hurricane, or outwards, like the Blackburn Skua and the Spitfire, do not affect the C. of G., but all undercarriages that retract backwards into the engine nacelles shift the centre of gravity in so doing, and therefore must have counter weights to keep the balance correct.
Retracting undercarts can either be made to retract manually, that is, a small knob or lever can be made to operate them, or else they can be either locked in the up or down position.
There is, of course, the automatic type, which can be worked either by the tension of the rubber motor slackening off as the turns die down (although this method is not advised) or they can be worked by a mechanical contrivance, as shown in Fig. 17, or a timing gear, as used for petrol model 'planes for switching off their ignition after a given length of run,' can be used.
Some of the most successful models that the writer has built have proved that the safest way of bringing a model down is to keep the legs retracted.
With a braced or solid underpart to the engine cowling, and a propeller made to easily knock out, the model will land on its " tummy " very satisfactorily, and end up by just sliding along the ground—and it will always finish the right way up. One model, in fact, was designed so that its legs were detachable, and were taken off for flying ! Of course, it necessitated a hand launch, but in flight it gave the exact appearance of a job in flight with the wheels fully retracted, and on all its many landings without an undercart it never came to grief.
The construction of models of old-time planes is very popular. This photo shows Mr. G. W. Day with his model of a 1911 Caudron.The construction of models of old-time planes is very popular. This photo shows Mr. G. W. Day with his model of a 1911 Caudron.
This, of course, leads up to the modern method of tricycle undercarts.
A tricycle undercart is the ideal landing gear for a model, because on landing the 'plane can be flown at its gliding angle on to the ground, the nose wheel taking the shock and the two rear wheels just taking the weight of the 'plane. Actually, this is the way a model comes down, as it has no pilot to flatten it out on landing.
The nose-wheel, too, helps to protect the airscrew from hitting the ground.
The chief difficulty is to get the nose-wheel to stand up to landing shocks, but as we probably want some extra weight in the nose here is an opportunity to use it to advantage. A parallel link motion can be used to give an up and down movement to the forks of the front wheel, or else they can be made of spring wire very firmly embedded to a strengthened underpart of the cowling. The pair of rear wheels, situated somewhere near the trailing edge, can be very lightly built, as they have very little load to carry.
Miles Kestrel sketch.Miles Kestrel sketch.
Here is Mr. H. J. Towner with his model of the Miles Kestrel Trainer. In centre the plane is shown in flight, and top is a sketch by the builder of his model.Here is Mr. H. J. Towner with his model of the Miles Kestrel Trainer. In centre the plane is shown in flight, and top is a sketch by the builder of his model.
Models have been fitted with "skis" for taking-off from snow, and these can easily be fitted, as'the springs from the undercart are still available, but where floats to take-off from water are required it is another proposition, as scale floats would be generally fitted with a pair of legs each and braced with thread, the whole being a fixture. The snag here is that they are apt to be wiped off on a landing, and are really not worth while from a flying point of view. However, amphibians and seaplanes with boat-shaped hulls are quite popular, the chief difficulty being to "unstick" these from the water. They require a lot of power to get them on to the "step" before rising, and it is just as well to fit an air vent between the top of the step and the outside of the hull, to prevent any suction occurring which would tend to hold the model down instead of letting it take off easily.