Model Aeroplane Notes

Auckland Star, Volume LXIV, Issue 147, 24 June 1933, Page 3 (Supplement)

Model Aeroplane Notes

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As promised last week I am replying to one or two queries from studiously inclined readers who want to know a little of the "why and wherefore" of model aeroplane design. A Wanganui reader puts some really difficult questions, and if the answer is not quite as full as ho likes, I hope he will realise that the questions asked would take volumes to answer if gone into really thoroughly.

Question I.—What is the meaning of "monocoque"? Answer: This is a word describing a particular type of fuselage construction, in which the skin or covering takes all the strain and no longerons in their accepted sense are used. The ordinary Moth 'plane has the main longerons running fore and aft, and the doped fabric which covers in the fuselage is used only for streamlining and keeping out damp, etc. A monocoque fuselage may employ any number of stringers running fore and aft, but relies for its strength mainly on the covering material. It may be plywood or metal. Question 2.—What is the effect of sweepback in wings? Answer: When the main wing of an aeroplane is swept back, that is the tips are further to the rear than the centre, it makes the machine more stable, both longitudinally and laterally. The former condition is easily proved by the fact that when carried to extremes, such as in the "pteradactlyl" no tail assembly is necessary. Try it out yourself with a paper model cut from drawing paper. Sweepback also improves the lateral or sideways balance of an aeroplane just as a dihedral angle does. However, it should be watched that this sweepback is not overdone, for it lessens the actual life developed by the wing.

Question 3. —What effect has the placing of the tailplane above or below the level of the main wing in a monoplane? Answer: This depends greatly on the general design of the machine, and has been the cause of no little thought on the part of designers recentl}'. The detrimental l effect from a >vrongly positioned tailplane (horizontal fin) is seen mostly in the low, thick winged monoplane, where the tailplane has been fount! to be operating in very disturbed air, cftie to the flow of air from the main wing forming' eddies behind it and "blanketing" the tail.

This is usually most noticeable when coming in to land or when the aeroplane ;s in a stalling position. Supposing that the tailplane is operating in reasonably undisturbed air, its actual position whether high, midway or low on the rear of the fuselage does not have much effect. On some flying boats the tailplane is raised very high, but this is to keep that portion out of the water ■spray. One point, however, must not be forgotten. The angle of the tailplane to the line of "thrust has a tremendous effect, but I am not dealing with this here.

Question 4.—What is 'the difference between the Clark Y section with the leading edge above the trailing edge and che wings section that has a flat underside? What is the effect of raising the leading edge, and what relative traction of the camber should it be raised? Answer: Firstly, the flat undersided wing section is not a true aerofoil section at all. It is used mostly on single surface wings for ease of construction. It is inefficient as it causes air eddies and does not exert the lift of a properly designed wing section. The Clark Y section is an old and popular aerofoil best suited for reasonably fast or heavy' machines. For models of really lightweight construction that fly very slowly a section with a large concave on the underside is more efficient. Raising the front or leading edge of the wing gives greater angle of attack and generally speaking increases the lift of the machine. It should be raised l-16th of an inch for every inch of chord in a model. This means that if you have a wing with a chord of three inches, the leading edge should be 3-lCth of an inch

higher than the' trailing edge. Its camber too has an cffect. Remember that the actual chord line runs from the extreme entry edge to the trailing edge so that a wing lying Hat on a fuselage may have a slight angle of incidence because of the fact. Draw it out on a sheet of paper and you will see more clearly what I mean.

Question 5. —How can the centre of gravity of a model be determined during designing, as is found in some plana of American 'planes? Answer: This is practically impossible to most amateurs, as a very high degree of technical knowledge is necessary to so do. The usual way is to get the knowledge by actual construction, and it is really marvellous how close an experienced model builder can get to it, by making calculations and alterations as he goes along.

Question 6.—Knowing the centre of gravity, how can the correct wing position be found before the model is built, without the detrimental addition of lead weights, ete. Answer: This question is hardly answerable in its present form. If you knew the centre of gravity you must have known the wing position, because the weight of the wing and its position on the 'plane must be taken into consideration when finding the centre of gravity. We know before we start building that the correct position for the wing is to have the centre of lift exactly over the centre of gravity. Now, the centre of lift can be taken as onethird of the chord back from the leading edge. So that if you have a wing with a three-inch chord, its leading edge will be exactly one inch in front of the centre of gravity. I presume that when you refer to the use of lead weights for balancing, you are referring mostly to scale model construction, where the wing is necessarily in a permanent position and not adjustable, as in most ordinary flying models. I referred to this in my notes some time back very exhaustively, but I can repeat here that it can be taken for granted that in all scale models the nose must be made a little overweight, for the power (rubber) is distributed over the length of the machine, and not bunched up in front like the engine of a real 'plane. To get over this,- make your tail gear very, very light, remembering that a quarter ounce saved in the tail is equivalent to three-quarter ounce in the nose, or even more. The use of a motor stick enables you to keep the weight—and useful weight, too —forward and allows the constiuctor to make his fuselage and tail gear very light. «

L.T. (Te Aroha). wants to know, liow to bend the ribs foF ilie Grey monoplane. Well, these are actually carved from sheet balsa, not bent. Tliin balsa can be bent by soaking it in water and then pressing it slowly into shape on a hot object like the round of an iron or even the outside of a kettle. I have often used an electric light bulb for this purpose, but in this case it is better not to more than merely dampen the balsa, or you may have some broken glass about. B.A.S. (Auckland) would like a tracing of the Dornier Do.X. What kindhearted reader will copy out this plan, for our friend, and send them to me to forward him? F.M. (Kohukohu) wants to knowwhy a propeller blade has a constantly changing angle and where the angle is greatest. The angle of a propeller blade is greatest at the hub, and lessens constantly towards the tips. When you consider that blade angle and diameter have an effect oil pitch you will see that as every part of the propeller must do 6quaf work, the angle must be'greater where the diameter is less. if the angle was the same at the tip, this portion of the propeller, having a much greater distance to travel, would theoretically travel much further forward in one revolution than the centre. Of course, being of a solid nature, it can't but it forms great eddies, and is generally inefficient.