I didn't quite know which category to post this under, but here goes.

A fellow RC buddy I sometimes fly with told me that to compute the engine "load factor" of a particular prop you use the formula "Diameter to the 4th power times Pitch".

For example, for an 11 x 6 prop it would be 11 x 11 x 11 x 11 x 6 or 87846. For a 12 x 4 prop it would be 12 x 12 x12 x12 x 4 or 82944.

Does this really work for determining which prop to substitute for a "standard" prop such as a 10 x 7 for a .46 size engine. If so, what would be the acceptable load range for say an OS .46FX, or Thunder Tiger Pro .40, etc.? Or is this just hogwash?

I was told of a much simpler formula:

Double the length and add the pitch.

10x6= 26

I have applied this when trying to find the best prop for several different four strokes, from .52 to 1.20 size. There is some validity to it, because everytime I got too far beyond the "median" number, in either direction, the engine either bogged or overspeeded.

As an example, the "median" number for a YS 63 is 32, since a 13x6 is about the best general-purpose prop for that engine. It was "happy" with everything from a 12x8 to a 14x4. It did NOT like a 14x6, and wasn't "happy" with a 14x5.

So, yes, I think this is a useful thumbnail comparison factor.

Steve

This does give a very close approximation of the comparative loads but really it only works for props that have exactly the same blade shapes and if the true pitch is known. You won't get an accurate figure when comparing an APC 10x6 to an MA 11x4 but it'll be close enough for all practical purposes. Basically what it means is that if two props have the same load figure then the engine will spin them at the same revs.

I like the 2D+P formula and have used it for years. Seems to give a very fair indication of propeller loads. Easy, "do-it-in-your-head" math as well. Who wants to play with exponents at the field when it's 95 degrees?

Read that formula in a book "everything you wanted to know about r/c" back in the 70's. Works fine.

Has nothing to do with flight performance of a given prop though....

'Race

This is exactly the info I've been looking for. I've been looking at prop selections for my Supertigre S61 ABC and using that formula came up with the following. The instructions said to break it in on an 11x8 and here's how the rest of the props in my "collection" stack up against that size.

12x4 28
12x5 29
11x8 30
13x5 31
13x6 32
14x4 32

Looks like all these are relatively close.

Kurt

I also use the D x 2+P method. Works fine.

Rule of thumb--every change in load factor of 2 will increase/decrease engine RPM by about 300-500RPM.

For example:
Let's say you have a prop with a load factor of 46 and the engine spins 7500RPM

If you bump to a larger prop and increase the load factor to 48--you'll loose about 500RPM

If you drop down to a load factor of 44 you should see a gain in RPM of about 500RPM.

Give or take a bit. Just my observations at 6000' elevation on both glow and gas engines.

Twister,


I can agree to neither formula!

The 2D + P formula is way too simplistic.
It can make one think a 10x6 and an 11x4 offer the same load, which is pretty far from the truth...

...as Brian (downunder) found out a couple of months ago.

Your formula also gets pretty close results (less than 2.5% difference) for these two prop sizes; so it must also be simplistically wrong.

............and I have you to thank for that Dar. If it hadn't been for your suggestion to go from an 11x4 to a 10x6 to keep the load the same then neither of us would have found out about it .

So what formula do you use now?

Brian,


For props with a pitch/diameter value of between 40% and 80% I use the Reivers PropPower.

For other props I don't...