I'm designing and building my own CF props, and I notice some are really thin and others have a wide chord? Is there rules of thumb to go by? Which ones are the most effecient?
Thanks,
--Rick

The best illustration that I can come up with is the contrast between a rubber power prop and (say) a pylon racer.

The rubber prop has large, usually fairly high pitch, blades intended to "move" a fairly large quantity of air at a low speed.

The other end of the scale is the "pylon racer" or "C/L speed" prop, very high a/r intended to move a smaller quantity of air very quickly indeed.

Each of these also illustrates the difference in drag and efficiency factors as well.

Within the rubber power group you will find the "paddles" on older and sport style models.

The modern competition models are using thin section, high a/r props that look not that dissimilar to over-grown pylon props.

Why is this? It is because the propellor efficiency has been recognised as an area that might yield the difference between first and nowhere in a competition.

If anyone knows of a good book on this subject (college grad level), please let me know. This is one area that I know I lack in and wouldn't mind hitting the books on.

When you change the blade chords on a prop with all else held fixed, the primary effect is to change the blade Cl. Wide chords produce low blade Cl's, and narrow blades produce high blade Cl's. Neither extreme is efficient. For best efficiency you want to operate somewhere in between, at the blade aifoil's best L/D point. This is in the range Cl = 0.4 - 0.7 for typical blade airfoils. Standard propeller design algorithms determine the blade chords required to achieve a specified blade Cl. There's one on Martin Hepperle's site.

There are a few approximate chord-scaling rules of thumb which are useful when sizing props:

chord ~ 1/Cl
chord ~ 1/rpm^2
chord ~ 1/diameter^3

Note the very large influence of the diameter. If you increase the diameter by 10%, the chords want to shrink by a factor of 1/1.1^3 = 0.75, or 25% narrower. The influence of pitch is implied by the dependence on rpm. Say you increase the pitch such that the rpm gets smaller by 10% (factor of 0.9). The chords then want to increase by a factor of 1/0.9^2 = 1.23, or 23% more. So bigger pitch demands wider blades and vice versa.

I appreciate your answer, Mark, although it wasn't my question... That's not why I'm posting.

I just wanted to say THANK YOU for participating here on RCU. I certainly know who you are, and am familiar with your outstanding work. I hope you'll stay with us, because there are aerodynamic issues raised here that would benefit greatly from your expertise.

I have a list, myself.

Oliver Wilson used to participate here, and we sure miss him.

Welcome!

I have had the unique opportunity to do some real research on this topic this past summer as an engineering research assistant at Global Aircraft Corp., here in Starkville, MS, for a reputable government agency. You all will be surprised to know, as I was, to learn how awefully inefficient our model propellers actually are. And this is nothing against model propeller manufacturers, please don't take this post like that, but most propeller twist distributions for model propellers are merely a theoretical one without any compensation for after-body flow adjustment or correct choice of airfoil. The most efficient model propeller we tested this summer, which unfortunately I am unable to name, only reached ~45%. As for the question at hand, it would be advantageous for the model propeller industry to make wider bladed props for the sole reason to increase their Reynolds number. This characteristic is the largest hindrance(sp?) on model propeller performance. Second is airfoil choice and thickness associated with it.... Hope this info helps... --CG

nIgHthAwK17,
I am designing my own, so please tell me more. How could I improve the design? What should the A/R be, if the avail props are too narrow? How thick? What effect would a higher Re be?
Thanks,
--Rick

Its really not so much a question of aspect ratio as it is the Reynolds number effect. If you don't know, as Reynolds number decreases, L/D goes to pot - and it doesn't matter what airfoil you use, it just happens. So, to increase your Reynolds number, increase your propeller station chord. Also, your airfoil choice will determine the optimal thickness ratio of the propeller cross section. As I said before, the twist distribution is the hardest thing to get correct, because in a perfect world, there would be a propeller designed specifically for every airplane. The reason for this is because every afterbody (fuselage) creates a different velocity profile through the propeller arc, hence affecting the angle of attack for every blade station along the blade. And the only two ways I know of to optimize and take into account the velocity profile are through CFD (computational fluid dynamics) or wind tunnel tests, both of which I assume you don't have access to.
To answer your question, I would need to know more about what size propeller we're talking about (diameter), what RPM you want to turn the propeller at, how much power your motor puts out at that RPM, and either what speed you want to fly at.

Nighthawk:

I just sent you a PM. Please email me directly at [email protected]

Suman