Boy I bet that got your attention!


I don't know about you, but I have shoeboxes (sorted by diameter) full of propellers of all shapes and manufacturers. Also when I first entered electrics (with the Brodak Electric Clown Arf), I bought lots o\f electric props----Graupner, APC, and Master Airscrew and a single Aeronaut . I finally ended up flying the APC, I am not sure exactly why, except they basically are the same form and are reasonably inexpensive.

So I haven't really tried a lot of props other than the APC, because up to now, they seemed to work reasonably well for me. They are pretty thin, and quite frankly, when I look to try a new one, they have an similar one with a different diameter and pitch.

I wonder if I walked myself into a local minimum--a term which describes an optimum point, at least for the path that I traveled--but with other better optimums that exist, but that I haven't really tried.

I just received the April Model Aviation where Dean P. has a nice article on airfoils. So now I wonder if there is some optimum airfoil for props. What is optimum? Well you like a good efficiency (thrust out at flying airspeed for power in from the motor), and also "authority". I'm not positive what authority is, but I guess to me it means as my airspeed drops, I want my constant rpm setup to supply as much extra thrust as possible.

So my question, for those who may know (aerodynamics is a black art to me), what sort or airfoil/prop shape is optimum, or at least pretty good. Is this a known quantity??? I know by limiting myself to flying airspeeds in the 50-55 mph range, that I am out of where most research is (except , maybe for Paul McCready).

Dean, how about some answers!

If you want the total information on Wing Sections I would suggest a book I have that answers about all of the questions.  Remember a prop is just a wing going in a circle.  Sounds like Control Line flying!

"Theory Of Wing Sections including a Summary of Airfoil Data" by Ira Abbott and Albert E. Von Doenhoff.

Included are point by point ordinates for most airfoils.  Great information for scale modelers also.

Clancy

Alan,
In some ways dialing a prop with electric is easier, well certainly more accurate if not easier.
Yes to get a real feel for what its doing, you need to maintain as much constant as possible, so adjusting lap times would be a good approach IMHO. However, that being said, If the lap times are close, it might be interesting as well, to explore how they manuever at the same setting.
with a glow engine, the needle setting is somewhat less predictable because the engine responds rpm wise to the load. In some of my experiences with glow, I have swapped two props that are just minutely different and left the needle the same.
Bottom line,, it does take time, and attention to detail to get any valid information comparatively with props.
Is it worth it,, I would say most definitely. I have had props that fly the plane at exactly the same lap times, and perform VASTLY different in maneuvering. I have had a setup that felt really good, and then experimenting found out that it wasn't as good as I thought it was when I bolted THE prop on there and flew it. BIG noticeable difference at the handle.
Is it worth YOUR time ? only you can decide, you may find that what you have is in fact the optimum,, however in my humble opinion, the act of experiementing, and trying different props is worth it if for no other reason than to increase your awareness of how the different parts of the system interact.

Alan,

I have also been looking at my prop draw and trying to find the "Hot Set Up" for electric. I've tried several different brands APC, Master Airscrew, Rev-up, TopFlite, and Zinger. I've tried to match all against the same lap time. The way I did it was to keep the RPM constant at 9000 and adjust the pitch and prop diameter. I used the APC E 12x5 (repitched from 12x6) as the base line. The thing that electric does that "wet power" doesn't is hold rpm. This allows an apples to apples comparison of the prop performance on a particular airplane. Now having said that you can get a very strict comparison but that may not be the real answer. Props are sensitive to speed (for us rpm) and a few hundred rpm difference can make a dog into a super star. So after getting the base line for all your selected props you need to then start to move the rpm and see if a prop that had a slow lap time can perform better at a higher rpm to hit the desired lap time. Some prop designs just need lower pitch and higher rpm to get into their "sweet spot". Problem is this takes lots of testing. The good news is that with electric you can set the timer for 1 min and pretty much see how it will pull quickly.

From the tests I did with my ship the only prop that improved on the performance of the APC E 12x 6 was the cut down (to 12") and repitch (to 4.8 to hold the 5.0 lap time) APC E 13x6.5. Buy cutting the length it gave a little more blade area that would hold in the wind a little better but it had a trade-off of higher amps (2950 vs 2450 for the APCE 12x5). I for the cooler weather I decided that the I wanted the extra margin in the battery pack (3850mah pack is installed). For the summer I will go back and test the 13x6.5  again.

Best,    Dennis

The paper by Adkins and Liebeck starts with Larrabee's and tweaks it a bit, as I remember.  Larrabee was a modeler and Larry Renger's professor and friend. 

The Abbot and Von Doenhoff book is noteworthy because Von Doenhoff is George Aldrich's cousin.  The airfoil data is at high Reynolds number, but there is some cool stuff there ne'ertheless.  I haven't looked for airfoils for awhile, so I'm out of date.  For our Reynolds number, you might start with http://www.ae.uiuc.edu/m-selig/ads/coord_database.html .  There's good stuff in Profilpolaren für den Modellflug (approx. title) by Dieter Althaus.  As I remember the Clark Y 6.9 therein looked like the bees' knees for a prop airfoil.  Then there's the issue of how to make an accurate airfoil that wee.   The APC guy can do it, and probably does, which may be why APC props are so good. 

Alan,
In some ways dialing a prop with electric is easier, well certainly more accurate if not easier.
Yes to get a real feel for what its doing, you need to maintain as much constant as possible, so adjusting lap times would be a good approach IMHO. However, that being said, If the lap times are close, it might be interesting as well, to explore how they manuever at the same setting.
with a glow engine, the needle setting is somewhat less predictable because the engine responds rpm wise to the load. In some of my experiences with glow, I have swapped two props that are just minutely different and left the needle the same.
Bottom line,, it does take time, and attention to detail to get any valid information comparatively with props.
Is it worth it,, I would say most definitely. I have had props that fly the plane at exactly the same lap times, and perform VASTLY different in maneuvering. I have had a setup that felt really good, and then experimenting found out that it wasn't as good as I thought it was when I bolted THE prop on there and flew it. BIG noticeable difference at the handle.
Is it worth YOUR time ? only you can decide, you may find that what you have is in fact the optimum,, however in my humble opinion, the act of experiementing, and trying different props is worth it if for no other reason than to increase your awareness of how the different parts of the system interact.

Mark,
I completely agree with you that electric does eliminate a lot of variables that you have with glow engines. After all, you can measure the motor stuff and eliminate a lot of BS. Just read all the stuff about stuffing in 75 engines in 40 size planes along with the prop comments to validate your statement! (sort of reminds me of the comments a few years back about 4 strokes where the laws of physics seem to be suspended when you swapped a heavy 4 stroke for a light 2 stroke engine!---cg didn't matter...etc.,  etc.....)

Here's another source I found that should give everyone something to chew on even if you only read  the sample chapter. The interesting thing is that it is pointed at model airplanes and not full sized aircraft (although the author does refer to full sized studies and real world examples).

http://www.supercoolprops.com/buybook.php

If you have time I suggest you read some of the other articles on this website, they are fascinating!! (especially if you are bit of science nerd!)

Bob