More on over & under propping.

Over propping (or having a prop of too steep a pitch or great a diameter to be more precise) is a common problem that many aeromodellers encounter without realizing what the problem is. We have set out to offer some basic guidelines to help the pilot to gain a better understanding of what over propping is and how to overcome it.

Below are a couple of diagrams to illustrate what is happening to the airflow over the prop blades under different circumstances. The sketch on the left is showing a good healthy & clean airflow in front and behind the blade, of which this is offering the greatest forward thrust possible for the job required.

The sketch on the right shows what is happening to the airflow over the blade during an over propped situation. The propeller blade is being driven through the air in an attempt to pull its load (the Aeroplane) through the air at a speed greater than the prop/engine combination has the capability of doing. 
There are several factors that govern this situation:- 
1) the engine may be underpowered for the prop it is trying to drive (this will result in too much fuel being used)
2) the prop may be too small a diameter therefore it never overcomes the aircraft drag (under propped)
3)The prop pitch may be too steep or the diameter too great resulting in similarities to 1) above.

The prop blade is churning away at the air with little effect as it has lost most of its purchase on the air. The most active part of the blade is the front surface, after the high point (exactly the same as an Aeroplane wing) after all "the propeller is only a rotating wing"!!!

To identify this problem is quite simple, the easiest way to spot it is with the aircraft in the air. Fly the plane at full throttle, and then as you ease back on the throttle stick, to about half or less, if the plane seems to actually leap forward i.e. accelerate to a slightly quicker but noticeable pace, then you are under propped as in 2) above.

What you have witnessed is the fact that by slowing the prop, it has now regained its purchase efficiently on the air through which it is passing and in turn this has resulted in the plane being pulled more effectively through the air at that speed.

In situations 1) & 3) above, the prop may well not be cavitating as described in the right hand sketch as the prop will be creating enough  load on the engine to prevent that from happening. When this is the situation, the throttling back test will simply reveal very little difference in engine sound between full & half throttle while the plane seems hardly to lose any speed. You may also notice a fair vapour trail behind the aircraft as well as heavy fuel consumption.

In this case, the prop diameter my be too great, or the pitch too steep. Try a shallower pitch prop or a smaller diameter. When doing these tests, don't be afraid to reduce diameter by a fair amount "say 1" (25mm)" or to de-pitch down by two inches (50mm) as this will give a far more discernable result.

During these tests, it will be very necessary to re-set up your engine's fuel air mixture between props as each one will offer a different engine load. It is also a good idea to set the mixture slightly on the rich side as it will lean out in flight (you do not want a dead stick on take off).

It is always good if you can to refer to the engine manufacturers guidelines for which prop the engine is capable of using. This is not always possible to do when the engine is one that has been lying around for sometime.

In situation 2) above, The prop is too small a diameter or has too steep a pitch for the aircraft while the engine has enough power to do what it likes with the prop. In this case the prop needs to be changed in diameter or pitch to match the power of the engine. When this is the case, the throttle back test will reveal the noticeable drop in engine speed until about half throttle"ish" and then the aircraft will do its leap forward as described earlier.