SOME POINTS IN DESIGN.

Auckland Star, Volume LXIV, Issue 88, 15 April 1933, Page 3 (Supplement)

SOME POINTS IN DESIGN.

CLUB NOTES.

Last week I gave you the details of a small soaring glider, which has caused lots of fun and is of very simple construction. Now and again we like to leave the scientific side of model building and play a little. This glider gives us the opportunity. I showed one of these machines to a very sedate lady friend, and contrary to my expectations she did not sniff loudly and turn to more interesting matters, but was so overcome by awe at its gyrations, that she immediately decided that the one thing left in her life was to make one aud fly it. lam sorry I cannot tell you how she made a beautiful model right away and immediately put up a record with it, because she didn't. I gave her the very one she was. looking at and I trust she has had lots of fun with it. There is really something fascinating in seeing a well-balanced glider skim along, loop, recover and glide off steadily.

And now for something more serious. I have been requested to give a few general details on design of models. So many constructors like to make their own design machines, perhaps using one that has appeared in these columns as a starting off point, and carefully putting various theories into practice as they go along. Now, we know that a real aeroplane has controls, manipulated by the pilot, and so various forces may be brought into being to counteract outside interferences. A current of air (called a bump) is felt under one wing. The pilot corrects it with a movement of an aileron. The pilot can "open up" the engine at will. In a model it is obvious that all the controlling forces must be "biSlt in," and once the model is launched only correct design can save it from crashing should it encounter adverse air currents. Another thing that cannot be altered simply is the speed of the propeller during flight. It must start off with a small burst of speed and settle down to a steady pace, and die away as the rubber motive power runs out. Therefore, the correct propeller must be designed and fitted. If the propeller pulls the machine along too fast the model will go up and stall, then probahly nose-dive and crash. The correct propeller to use is one that pulls the model along at its best gliding speed. As the speed of the propeller gets slower as the rubber winds out the pitch of the propeller must be. such that it is correct during the time when the propeller is turning at its normal speed. Now how do we ascertain the normal flying speed? Just launch the model complete with rubber and propeller, but not wound up. The best gliding position of the wings must be found and the model timed over a distance. Work out the inches per second the model travels.

The list given below will help you to calculate what the forward speed in inches per second of any model should be, but first I had better explain the meaning of "wing-loading." If the weight of a mo4el is one ounce and it has a main lifting surface (do not take into consideration the tail surfaces) of one square foot the model, is said to have a wing-loading of one ounce to the square foot.

Models of various wing-loading will have the forward speed in inches per second.

Wing-Loading in'Ozs. —.20, .5, .75, 1, .1.5, 2. Forward Speed.—43.4, 67.2, 84, 96, 106, 135.

Now you know that a propeller will travel forward through the air according to its pitch and speed. I have given formulae for finding the pitch of propellers, and .must take it for granted that you know how to work this out. Don't forget to deduct about one-third from the amount of the theoretical pitch to allow for "slip."

Now to put these theories into practice. Suppose you have a model with a wing loading of .soz to the square foot. On looking at the table above you see that its correct flying speed is 67.2 m per second. Your propeller iB designed to have a theoretical pitch of 18in. Subtract one-third for slip, so that you have an effective pitch of 12in. Therefore this propeller must turn about 5i times per second to maintain the model in flight, or 330 revolutions per minute. Try. out the propeller with, various strands of- rubber until you get it, turning at just this speed. To be on the safe side make it turn just a shade faster than thijs. So put on a® a first experiment six strands of 1-8 rubber and give it 350 turns. Let the propeller go and time it. If it runs out before the minute is up take some rubber off, and vice versa.

Of course, you must make allowances as regards the slip of a propeller. A fast-turning propeller has more slip than a slow one. So that though I have stated that the usual slip or loss is onethird it may be a little less on a very slow-turning propeller or more on a fast o»e.

Don't forget that your propeller must balance exactly when a needle or thin wire is inserted in the central spindle hole. The propeller should be given a spin and it should slow up and stop in an absolutely horizontal position. It is only by attention to such details of design that the element of "fluking" is eliminated, so I would advise all model aeroplane builders to go more into the theory of aerodynamics and try to know the " why and wherefore" oi model flying.

A Jiighly successful outdoor meeting of the Auckland Model Aero Club was held on Saturday, April 1, in the crater, One Tree Hill. A light, but steady, breeze livened matters considerably, and fuselage models were the order of the day, some very fine and consistent times being recorded in this class. Results of the competitions held "were: —

Fuselage R.0.G.—1, Chinnery-Brown, 1.20, 1; R. Foster, 1.10, 2; G Perkins and H. Easton, 555, 3.

Scale Model H.L.—Won by E. Chin-nery-Brown, 14 2-ss. '

During the afternoon a most interesting talk was given by Captain E. Dickson, some time instructor at Sockburn, Ghristchurch. He illustrated his lecture with wartime photographs, war trophies,