MODEL AEROPLANE NOTES.

Dominion, Volume 31, Issue 72, 18 December 1937, Page 18

MODEL AEROPLANE NOTES.

-BY

'AILERON’

Single-bladed Propellers. The latest development to attract model builders is the single-bladed propeller. At first sight there seems little to commend their use, but when the question is worked out properly the advantages are quite clear.

In the first place an ordinary two or three-bladed propeller of the fairly lowpitch type now being used is not quite as efficient as it might be. Each blade makes the air behind it swirl considerably, and as the other blade strikes this “burble” it does not get the chance to work as well as it might. Considering that very few ordinary propellers are more than 70 per cent, efficient this further loss is a serious matter.

On the other hand in the single-bladed type the blade has plenty of room to clear the burble it created on the last revolution and is thug always working to the best efficiency in undisturbed air. OE course,'the blade area must be large to compensate for having but one blade instead of two.

There is another purely practical consideration. It is quite easy for a builder to make a two-bladed propeller with small differences between the blades, differences of area or pitch. Moreover, the propeller shaft is liable to get bent in crashes, and this may result in the propeller not spinning true, or one blade presenting a greater angle to the air than the other.

These disadvantages are not present in the single-bladed type as even with a bent shaft all the propeller does is to alter its “angle of attack” slightly. The construction of .single-bladers is quite simple; half an ordinary propeller is carved and a balance weight added on the opposite side. This weight may bo sheet lead cemented to a short projecting stub of the propeller or else sheet lead rolled round a short piece of projecting rod. It is best not to have the weight too close to the shaft or too much will he required for balancing purposes. Incidentally, ordinary propellers with one blade broken may be modified in this way to suit smaller modelsI—a 1 —a handy way of making use of the broken juropellers that every modelbuilder seems to collect. 'There is no question of the practical efficiency of single-bladers. This year’s Moffett Trophy was won by a model equipped with one, and the latest long flight in New Zealand, R. Lynch’s 44.5 3-5 record, was made with a single-binder model. One last point —a single-bladed propeller is less likely .to get hooked up in telegraph wires than tho usual type. If should be pointed out that light models should not be fitted with single binders, and that extreme care should be exercised when experimenting with them on petrol models. In the latter case a propeller which came to pieces while the engine was running might cause considerable damage.