Chapter 7

 

Chapter 7

Wing Fixing: Rigidity, detachability.

THERE is another point directly affecting stability in flight, and that is the "rigidity" of all parts, and when designing wing fixings, tail fixings, undercarriage fixings and so on, this must be very carefully borne in mind.

We go to great lengths to get our trim exactly right after having studied - and, we hope, applied - the lessons learnt in the chapter on aerodynamics; and then if, during flight, the tail unit is not a fixture in the sense that it is rigid, and slight lifting of the elevator or turning of the rudder by a sudden gust occurs, all our good work is upset. Even should the model right itself in the air we do not know when the next disturbance will upset it again, and the whole flight is watched with our hearts in our mouths, wondering when the model will crash. Consistent flights mean consistent setting of the controls, and therefore no flapping part can be tolerated.

In considering the type of wing fixing it is as well to consider the problem of transport as well, as it may be advantageous to design our model to separate into more than the usual three parts - fuselage, wings and tail. Naturally, the size of the model also has a bearing on the subject, and here let us impress upon you the fact that most people build their models too small. Should storage space be a deciding factor, then we must perforce keep our size down, but even then, cleverly designed models that will take to pieces at home but will not fall to pieces in the air are a great asset; and it is suggested that builders should try, wherever possible, to keep to the S.M.A.E. formula of one inch to the foot for all normal models. Perhaps for the smaller prototypes of, say, 30 .ft. .span, we can build to a scale of one-and-a-half inches to the foot, which will give us a model with a span of 45 inches - a nice handy size, and something that will not want such a dead calm day to fly in!

Generally speaking, a high-wing design, that is, with the wings fixed on or near the top of the fuselage, is fairly straightforward, as the roots of the wings can be either hooked on the top of the cabin sides or else dowel pins pushed into paper tubes can be used, as shown in Fig. 14. These wings are then generally held by struts reaching from the bottom of the. cabin sides of the fuselage, supporting each wing panel perhaps half-way out from the root. These struts, of course, brace the whole wing and make it rigid. Should "these struts be attached to the base of the fuselage in the same way as the wings, by dowels and paper tubes or .by the use of press studs, the whole wing will knock off on a bad landing and perhaps save a bad crack-up.

Fig.14, Fig.15.Fig.14, Fig.15.

Press studs are quite suitable for small parts, and where no great strain is to be taken. Of course, heavier types are available, but generally they require such a pressure to snap them together that quite a lot of damage can easily be done to the surrounding parts if these are not purposely strengthened. It is often difficult to fix these fasteners firmly to the structure without their pulling out. As a matter of fact you will be surprised how strong they will be if you just "sew" the fastener into place with an ordinary needle and thread and a spot of cement or glue to make all firm, as shown in Fig. 15.

Where paper tubes and hardwood dowels are used, these should generally be about 1/8in. dia. up to, say, 1/4in. for the larger and heavier models. This 1/8in. dia. dowelling can easily be purchased at any wood store, and is usually made of birch.

In order to keep the wings firmly butted against the airframe, rubber bands are generally used. These form the easiest method of holding the wings into position, but also allow a slight movement should the wings come into contact with a solid object. The rubber bands can be fitted in two ways. Firstly, small steel wire hooks protrude from each wing root and are connected together by a rubber band in tension passing right through the cabin-top in such a way that the rubber band pulls both wing roots tight against the fuselage. This is no doubt the neatest method, but not too easy to fit on the flying-field.

The other method is to use external rubber bands, one on each leading and trailing edge, and each one separately attached to suitable steel hooks on the fuselage. If these are neatly fitted and the colour of the rubber band is chosen to harmonise with the adjacent colour they will not be too prominent; a very easy device is assured, and from experience gained in the flying-field we recommend this as the better method of the two.

These rubber bands are only used to keep the wings in position and to prevent the dowels from coming out of the paper tubes, and they do not take any of the lifting loads.

Fig.15a.Fig.15a.

The wing fixings of a low-wing model, a model whose wings are attached to the bottom of the fuselage, do really present a difficulty, because these wings are usually what are termed "cantilever" wings. A cantilever wing is one that is self-supporting and has no outside struts to support it.

Therefore we have to design a fitting that will be definitely rigid and will not allow wing flutter, but at the same time must be capable of withstanding normal landing shocks.

From a purely scale point of view the wings on a low-wing prototype are generally attached to a centre section which projects quite a distance either side of the fuselage, and from a transport point of view it may be advisable to make this centre section removable. If this be so, then our separate wing panels can be firmly plugged into the centre section and the centre section itself can be flexibly mounted to the. fuselage by means of vertically placed rubber bands inside the fuselage, and the centre section located by the arch formed under the fuselage to accommodate it. On any of the smaller powered prototypes this is quite a good method. Fig. 15a is a suggestion for a " Moth Minor " on these lines.

However, when a high-speed and high-powered job is under consideration the centre section is usually very well faired or streamlined into the fuselage, and here it is desirable as a rule to combine both together into one unit, and it is in this case that the wing fixing calls for a lot of thought.

Fig.16.Fig.16.

Should the dowels be rather long in order to get rigidity, the dowels may rip out quite a lot of framework on a bad landing. On the other hand, if they are short, in order to enable them to slip out quickly, then the wing will not be rigid and will probably have a variable dihedral and consequently an unstable flight. When building large models of, say, over four feet span, it is advisable to make detachable wing tips of about four or five inches down the span, and plugged in by whichever method is desired. This will help to save a wing in a crash.

Then there is a further method which the writer used successfully on the "Airspeed Envoy." This method consists of two aluminium "U" pieces attached to each side of the

centre section on the outer ribs. These "U" pieces fit over the root ribs of the wing panels, and balsa dowels are pushed right through the "U" pieces and through suitable holes in the wing panels. These dowels are made strong enough to hold all together rigidly, but in the event of a crash the dowels break and a fresh dowel is inserted, pushing out the broken pieces. These dowels or shear pins can be likened to a fuse in an electric circuit, or safety valve in a steam engine. No rubber bands are needed at all in this type of wing fixing.

It should be borne in mind that a certain amount of "spring" should be incorporated in each wing, otherwise it will be liable to snap. In order to determine how much spring to allow, a wing panel of, say, 20 inches from root to tip, when fully covered and tightened, should be able to " bow " or spring about one inch; if not, the construction is too heavy or not properly designed.

(There is a definite practical illustration of this in the building of tall factory chimneys. You may have noticed in a strong wind how two tall chimneys close together will slightly sway. One can see the two tops coming closer together, and then going apart again. This is allowed for in the construction, to prevent them snapping off if they were too rigid.)

Though not strictly to scale, the model shown in the top and middle photos might easily be thought to be a model of one of BritainThough not strictly to scale, the model shown in the top and middle photos might easily be thought to be a model of one of Britain's latest twin engined bombers! Designed and built by Mr. C. Rupert Moore, the model incorporates many novel features, is of most advanced design, and capable of a very fine performance. It is made as a scale model of an original design..

This model is to scale! It is of a Hawker Hurricane, and is powered with a 1/5 h.p. petrol engine. The photo was taken at Cranwell Aero-drome, and by careful positioning of the camera the cadets were introduced into the background to make a most realistic photo!.This model is to scale! It is of a Hawker Hurricane, and is powered with a 1/5 h.p. petrol engine. The photo was taken at Cranwell Aero-drome, and by careful positioning of the camera the cadets were introduced into the background to make a most realistic photo!.

 
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