Big Props on Diesels ?

Good Day All,

I have recently been bitten by trimming large airscrews down and mounting them on my diesel engines. My question is what are the effects on an airship when VERY LARGE cut down airscrews are used ? So far, I have cut down a Master Airscrew "K" series 13x6 prop to an 11& 3/16ths., X 6 prop, and a Master Airscrew "K" series 12x6 prop down to an 11x6 prop. I ran both of these airscrews today on my PAW TBR .35 that was mounted to a RingMaster (yes, overkill...etc..).

It seemed to swing both airscrews fine on the ground, but I did notice it seemed to cause more vibration in the airship (especially with the cut down 13x6) than by using smaller airscrews such as a Zinger wood 11x7 W. I have also cut down a Zinger wood 10x6 to about an 8 & 14/16ths.. I plan on running that prop on a PAW TBR .19.

Another question, is it possible to have to big of airscrew for a given size of an airship ? What are the effects ?

Thanks guys,

If it turns Tree Trunks as props, it's gotta be a Diesel !

Dmacsimum Speed Limit.

The theoretical answer is easy. Never whack the ends off any airscrew! The practical answer is more complex. You, and I and many others cut down props with performance improvements, real or imagined. Some of these smart guys on this forum will give you the practical answer.

There is some elegance to prop design, and it is pretty much based on simple High School trig. However, the curves that result from that math are quite beautiful in their own right.

Design theory of airscrews is based on the simple proposition that the angle of attack to the relative wind the blade sees at every point from the hub to the tip should be the same. Or, more simply, a good airscrew has the tip flying at the same attack angle as the points near the hub.

Whacking the ends off props makes the prop tips fly at a higher angle of attack than elements nearer the hub. As an extreme example, imagine cutting an 11-6 down to a 7-6 and sticking it on a smaller engine. The angle of the tips is obscene compared to a true 7-6.

In combat and rat, we ran 9-8s cut down to 8 1/2-8s or 8-8s all the time. Never did figure why they worked well. So I tossed the text books and copied the winners.

You got me confused on that one, Larry. The pitch should remain the same unless you have got some goofy prop that has more pitch at one end than the other. Actually the pitch tells us how far forward the prop would pull the plane in one revolution if it were traveling through a solid medium (with no slippage). A 6 inch pitch prop, for instance, should pull the plane forward 6 inches in one revolution. But, since the portions of the prop farther away from the hub travel in a larger arc, the angle of attach becomes progressively flatter to keep the same amount of forward travel (or thrust) at every point on the prop. Of course, the real throust comes from the disk area, so the outer portions of the blade do more work than the inner portions, but the forward travel over one revolution still remains the same. Thus, if you took an 11 x 6 prop and a 13 x 6 that you cut down to 11 inches in diameter, they should, in theory, both measure the same at every station when measured on a pitch gauge.

The real problem (or advantage) in cutting down bigger props is that bigger props usually have wider blades and thicker airfoils. So, if you want a draggier, wider, paddle blade prop, cutting down a larger prop to the desired diameter might just do it. There is a lot of debate as to whether you should keep the tips square, or eliptical, or angled back or... I usually just cut them square. But you do have to balance the prop, and I have a feeling that this was your problem. Either it was out of balance, or one blade was wider or thicker than the other. This can cause problems as well.

Leonard --

The reason you were confused is that I answered from memory, not from thinking and doing math. You are quite correct.

For a 6" pitch airscrew, I calc:

3" from crankshaft centerline -- 18.56 degrees pitch

4" from centerline -- 13.81 degrees

5" from centerline -- 11.01 degrees

6" from centerline -- 9.16 degrees

-- regardless of total prop length, as you said. That is, 1" from the tip of a 12/6 is 11.01 degrees, and the tip of a 10/6 is the same 11.01 degrees. I didn't check these answers with any physical pieces, but they smell about right, I think.

Washout hub to tip would mildly affect the the numbers, but your answer is correct both theoretically and practically near as I can tell. Perhaps washout is what I remembered. Or perhaps I can't remember squat.