"Unloading a prop"
 

I am not sure as to what it means to 'unload' or 'load' a prop, so first let's start with the basics.


When one selects a prop, there are always two numbers to look at.... For instance, in my OS .40 FX engine, I would use a 10x6 prop as per recommendation by the company. The 10, represents the propeller diameter (right?) and the 6 represents (what?). Does the 6 have to do with the angle of attack (or pitch) of the blades?



....Ok, back to the original question of loading and unloading. I know that in order to provide thrust for the airplane there are two main ways this can be accomplished. (1) We can use a small prop and accelerate a small mass of air to a very high velocity. Conversely, we can (2) accelerate a large mass of air to a slower velocity using a much larger prop.

Does this have anything to do with loading and unloading the prop?

Many thanks,

RE: "Unloading a prop"
 

Typically, when someone is talking about "unloading" the prop, what they are referring to is that the engine will pick up additional RPM in the air over what the ground RPM. How much unload is dependent upon factors like engine timing, tuned exhaust, and aircraft drag. For instance, an exteme example of unloading would have been Formula One pylon ships that were powered by .40 sized engines with open exhaust. Needless to say, they were quite loud (at about 115 db). On the ground, they were prop'ed to run at about 22,000 RPM, but in the air they would unload to around 27,000 RPM. Most sport airplanes/engines unload far less, typically in the 1000-1500 range. Part of the reason that you set an engine rich of peak, so you have the fuel to allow the unload.

RE: "Unloading a prop"
 

yes. 10x6 means 10 inch diameter with a 6 inch pitch. pitch means that if the propeller (screw) makes one full rotation (360 degrees) with no slippage ( like a screw through wood, as opposed to water or air) that it will have moved 6 inches. The higher the pitch,( all other things beiing equal) the faster the airplane ( 8 inches in one rotation compared to 6 inches in one rotation. There is no free lunch so it is harder to turn a higher pitch or larger diameter prop, so low pitch or higher diameter props climb/ accelerate better,( like low gear in a car) but are limited to a lower top speed than more pitch/ smaller diameter. There are many other factors that affect speed/ climb rates and that is why there is no "perfect prop". It is as much trial and error as science, but fortunately, most of the trials have already been done, leaving you with just a few reccommended props for you motor.
Have fun flying!!
Mike Kosatka

RE: "Unloading a prop"
 

The prop is actually a wing. When the airplane moves through the air faster, the angle of attack of the prop wing decreases. So drag on the prop decreases and the engine can turn the prop faster. I think excessive unloading suggests that you are running too much prop for your engine/airplane. The generally tossed about figures are an increase of 500-1500 RPM.

RE: "Unloading a prop"
 

This subject has already been covered well, I just want to add that the so called correct prop for your engine doesn't take into account the airframe and flying style you prefer. For years I used 10x6s without question on my .40 sized sport planes. I switched to a 10x4 and everything about the plane I was flying improved greatly, and the all out speed difference wasn't very noticeable.

RE: "Unloading a prop"
 

I have heard or read that some high pitch fixed pitch racing props, particularly the ones on full scale pylon racers, will actually load as the airplane accelerates to a certain airspeed. The sound of the engine suddenly dropping rpm at 120 knots or so shortly after takeoff can be quite disconcerting to a pilot the first time he flys one of these planes. With high pitch props, the blades are in a partial stall at low airspeeds, as the airspeed increases, the blade's angle of attack to the airflow decreases and the blade unstalls pulling the engine rpm down as it gets a bite on the air.

RE: "Unloading a prop"
 

Yes it is true. My Sonerai engine will turn 3400rpm on the ground but drops to about 3100 at about 35mph. Very strange unless you expect it.

It happens because the prop is stalled until its gets some forward airspeed, effectivly reducing the angle of attack of the blades.