A while back I read in a book or magazine that to get a general idea of the load a prop will put on an engine you take [diameter x diameter x pitch]. But I am not sure my old brain registered that correctly. If that equation is right then the total number for a 14x6 would be 1176, while the total number for a 16x4 would be 1024. So the 16x4 would put less load on the engine? The same as a 20x10 has a number of 4000, while a 22x8 would be 3872 so the 22x8 would spin easier. Is this formula right or did I just get it mixed up, which seems to happen more and more as the years go by.

Your formula may be correct, but is good only for comparision among propellers, instead of for the load a prop will put on an engine.

The increment in diameter has a bigger effect on the load than the pitch.
The load increases with the cube of the diameter (D X D X D), and the square of the pitch (P X P).

http://www.hooked-on-rc-airplanes.co...ropellers.html

http://www.ampaviators.com/2007/05/15/propeller-basics/

http://en.wikipedia.org/wiki/Propeller

(Editted to add links)

i dont think a 14x6 has more load than a 16x4..theres no way...or a 20x10 over a 22x8

i have a ST 90 that likes a 14x6 prop, i cant imagine putting a 16x4 on it and be satisfied with the way it would spool up...i tried a 15x4 wideblade apc, just one time.

What are the units of load? Cubic inches? That's a volume, not a measure of energy???

It's only meant to be a rough guide to compare props of different pitch and diameter. It just compares the swept volume of the prop per revolution.
To give an indication of a comparison between the static thrusts at the same rpmyou'd multiply that number by the pitch (Force = rate of change of momentum)
To compare powers you'd then multiply THAT number by the achieved rpm
But it's all a bit hand wavy, because unless you're armed with the known speed of the model and handbooks of characteristic curves for the props, you're going to have to experiment a bit anyway.
I've just discovered one of the downsides to having larger models; the cost of the props. I've had a 16x6, a 16x8 and now an 18x 5 on my Edge540. and it's still not quite right.

go here http://mvvs.nl/MVVS/main_bestanden/M...pellerlist.htm and download the prop power calculator its an excellent tool. you will need ms excell

You can download a free program here
http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm
The "thrust" calculations seem to vary in accuracy.

It's not perfect because it does not use blade design. It only uses pitch and diameter but you can use the 'load' numbers to get an indication of whether you should expect more or less static rpm when switching from one prop to another.

Even after static running, the real test happens when you put the plane in the air because there's no other way to know if you like the way it works.

I HAVE FOUND IT!!!!!!

It is called Propeller Load Factor, and was developed by Dave Gierke.

The article was published in Model Airplane News in July 1999.

It has been reproduced in this newsletter, which includes graphics and tables:

http://dixieaeromasters.com/core/newsletters/Mar07.pdf

This other article written by Andy Lennon confirms the usefulness of the PLF:

http://findarticles.com/p/articles/m...6/ai_n8988659/

Your brain works perfectly, Poffit; thank you for bringing this article up.

I hear that. A 20 x 8 ProZinger ground strike brings tears to my eyes. I have dozens of 10, 11 and 13" APC's in my car and flight box in various pitches and one spare each of my 18 x 6 and 20 x 8 sizes.

Yep, thats the article I read. Now the question is, is that information useful for making decisions on buying different size props or not. I have a tach now and plan to experiment with an OS 91 with 14x6 and 16x4 props. According to the article, using the same brand and model of prop the engine should turn about the same rpm with each. We"ll see.

Aaaarrghhh!!!!
'Must be lots of wet weekends in Holland!

I do remember from the "old days" that we compared prop load for comparable prop sizing as double the diameter and add the pitch. So a 10 x 6 would = 26. A 10 x 8 =28 so a 11x4 would be a pretty equal replacement prop. So you could go up a few points and make it a 11 x 6 for a 28 and still be in range. Most motor manufacturers suggest a range and you can figure out just would would be approiate from that. The forumula came out of RCM and Clarence Lee I think.