Speaking of props, pitch verses lap time?

[Bob Reeves]

In my quest to find the holly grail of props for 4 strokes I've ran into some really weird results as far as pitch verses RPM != lap time. Can anyone explain why prop "A" that measures 6.5 pitch will fly the airplane at 5 second laps with the engine turning 8200 when prop "B" that measures 6 pitch will fly the same 5 second laps with the engine turning 7850. I've been calling it "efficiency" but has to be some magic in props as I have found many examples that simply defi logic if you only look at pitch.

Before you ask.. Same airplane, lines and prop diameter

[Allan Perret]

Props A & B in your example, same brand just repitched or two different brands ?

[Howard Rush]

Can anyone explain why prop "A" that measures 6.5 pitch will fly the airplane at 5 second laps with the engine turning 8200 when prop "B" that measures 6 pitch will fly the same 5 second laps with the engine turning 7850.

If anybody does, he's probably wrong.  As Chris Machin said, a propeller is not a screw going through cheese.  How they work is kinda complicated, but was figured out a long time ago.  You can calculate what a prop will do if you know the airplane's drag or the engine's power, but it's a lot easier just to try some props. 

[Bob Reeves]

Props A & B in your example, same brand just repitched or two different brands ?

Different brands.....

[Douglas Ames]

It's all about thrust output. Blade profiles, chord width, flat blade, under camber, etc. all factor into it. Efficiency is a good description. Remember De and Joe's great 10x6 prop test? Vast differences in thrust vs. same rpm/ engine/ model.

Bob, I realize this thread is about Stunt, but I believe here in 2012 the power exists with a 60% rule 4-stroke to compete in Profile Carrier. (ie- OS a-series) The challenge will be to find the right prop and mod the engine with an open pipe (<3" from c/l of head - high nitro rule) and ram air for the carb. I'd love to try it but my job situation sucks plus too many other projects.

[Bob Reeves]

It's all about thrust output. Blade profiles, chord width, flat blade, under camber, etc. all factor into it. Efficiency is a good description. Remember De and Joe's great 10x6 prop test? Vast differences in thrust vs. same rpm/ engine/ model.

Bob, I realize this thread is about Stunt, but I believe here in 2012 the power exists with a 60% rule 4-stroke to compete in Profile Carrier. (ie- OS a-series) The challenge will be to find the right prop and mod the engine with an open pipe (<3" from c/l of head - high nitro rule) and ram air for the carb. I'd love to try it but my job situation sucks plus too many other projects.

Ya, getting off the subject a bit.. I thought about this when I built the big FJ, I could legally fly it with a Saito 56 but with Saito only being able to turn about 10,000 verses the TT at 23,000... heaven knows what pitch prop that Saito would need to be swinging. Decided one couldn't get there from here and scraped the idea.

Kinda like my 3 cylinder diesel tractor verses a Chevy 350....

[Tim Wescott]

If anybody does, he's probably wrong.  As Chris Machin said, a propeller is not a screw going through cheese.

Three things that I can think of that are part of the story (but not all -- that would be wrong!) is first (and least) blade area, second the fact that the RPM you measure on the ground isn't the RPM in the sky, and third (and possibly most important), the zero-lift angle of attack of an airfoil (like, for instance, a prop blade) is never parallel to the "bottom flat" surface, and rarely parallel to a line through the center of the leading edge and trailing edges.

So if you're measuring prop pitch by the flat on the bottom of the prop, that's pretty much guaranteed to be less than the number of inches the prop will advance through the air when it is generating zero lift -- and it may well be a considerable under-estimate.  Because the prop will always be displaying some "slip", the blade area will make a significant difference to the force that the prop generates at a given speed/RPM combination -- and unless you measure the RPM in the air, you don't know what the true speed is anyway.

[Allan Perret]

Different brands.....

I think if you redo the test with same brand props but one of them repitched,  your results will more of what you were expecting to see with your first test.  IMOH, comparing 2 different brands is almost like apples and oranges.

[Dennis Adamisin]

You mentioned two brands, APC has generated similar results to yours for the same sizes.  Their electric props USED to be fairly thin with rather deep undercambered airfoil.  APC felt they needed to thicken the props from the hub and carried into the blade, they changed the airfoil, most noticeably by filling in most of the undercamber.

The old thin blade and new thick blade props pitch-out the same, & have the same blade shape. At a given RPM the thick props draw less power - however, that is because they are not doing as much work.  The thick props require 800-1000 RPM more to generate the same lap times as the old thin blade, and the power requirements pretty much level out.  I think the difference is in the airfoil, but other folks point out that the thin blades were probably flexing too - Remember those yellow Grish nylon props and how THEY flexed?

All of the above a rather windy way to say that no, I am not surprised at the differences you encountered between two brands.  Howard's suggestion (just fly it) is the most (ONLY?) practical & pragmatic means of comparing props.