Model Aeroplane Notes

Auckland Star, Volume LXIII, Issue 67, 19 March 1932, Page 3 (Supplement)

Model Aeroplane Notes

BY

The last week or so we have been talking about tho reasons why various parts of the aeroplane are made in particular ways. You may not have thought very much about the propeller of your bus, except how hard it was to carve, but I assure you there is a great deal of study ahead of you if you wish to learn "all about it." For those that want to go into the theory of propeller design d'eejJy there are text books to study. In this column I want to explain as simply as possible; some of the so-called mysteries of avrop-une construction, especially those past* most interesting to model builders. The propeller is ol'U'ii called "airscrew," and that name w really a very good one. Take an ordinary woodscrew and look at it carefully. At first you won't see much resemblance between it and an aeroplane propeller, but you will soon realise that the action is just the same. Push the end of the woodscrew in a piece of soft wood or even a piece of good hard soap. You will see that the sharp edges., on the thread of the screw form a continuous (spiral. Now, turn, the screw to the right. You will see it goes into the wood. The coarser tho thread, or in other words, the greater distance there is between the outstanding/sharp edges, the further it will penetrate at each turn. So we see that the PITCH or forward travel per revolution is by the coarseness or fineness of the thread. Now for the reason why it goes in at all, Well, the thread with its sharp edges cuts into the wood and pushes ba<ck against it as the screw is turned. This pushes or THRUSTS the woodscrew ahead into the wood. It is very important to remember this for exactly the same thiug happens in an airscrew. To make it simpler still, you will see that it is just the same action as a boy riding a scooter. He pushes back with one leg against the ground and obtains the thrust to propel himself forward. Now, there is one thing that you will readily see. A woodscrew moves forward exactly the' amount of its pitch at every, turn, because there can be no SLIP in such a hard medium, Air, however, is very fluid and gives way and is pushed back when the propeller revolves. Thus we say when we stand behind an aeroplane just starting to taxi off that we are standing in the SLIP-STREAM of the propeller. I know you must all have grabbed your liats in the blast of a propeller slip-stream. The model aeroplane propeller is really a portion of a screw, but is made as it is, in order to present a greater surface to the air, because the air is so fluid. Now, though the slip is very great when a machine is just taking off, itwill be seem that when in ilight the airscrew is also moving ahead very fast and becomes quite efficient. In a heavy model with a fast revving propeller easily as much as 50 per cent of the work it does is lost in SLIP. By careful workmanship and good design this slip is lowered greatly on present-day models. Firstly, if you iook at a propeller you will see that the BLAD.fr ANGLE alters from a coarse angle at the centre, or hub, to ft ,fin2r angle at the tips. It will be readily understood that the tips gravel much faster than the centre. Therefore, the angle

must be less there in order to get the pitch the same all the way along. Secondly, the section of the blade makes a great difference; You remember that a cambered wing section exerted more lift than a flat one. Well, a propeller is really a pair of wings t>et at an angle and being rotated. Therefore by giving the propeller blade the right shape it exerts more THRUST. Thirdly, balance your propeller. Not only make it exactly the same weight each side of the centre hole, but see that the PITCH is exactly the same. Now, I know I liave only touched very lightly on this subject aiifl will return to it again later, but in the meantime if any model aircraftsman wants a knotty problem explained just send along an addressed stamped envelope to Propeller, c/o "Auckland Star," and I will be happy to help if I can. Club Notes. Auckland. —Going strong here. We have lots of new members and room for more. Indoor meetings will soon be starting again, and we hope to see the 3m 20s record held by R. Foster beaten. Foster is now a senior and holds the position of recording officer. Ashburton. —Twin pusher flights with an "Arrow" were as follows: The best 1255, others of 92, 77, 21, GO. Official rules are being forwarded the members. "Christchurch Star" Club. —A keen organiser and loyal and enthusiastic members are making the "Star" Club a success. In spite of weather conditions many "fine flights are recorded.