Prop tip shape and profile on clipped props

[Dennis Toth]

I tried this on the electric forum but it seems we don't have many prop tweaker there, although its about ECL it applies to IC setup as well. With the need for varying our diameter as a way to manage motor load (i.e. amps draw) I've noticed that the tip treatment seems to affect the prop performance for our E system more than the IC's. I have found for our E setups that you can tell how good a prop is working by the noise it generates. We have the advantage of being able to hear the prop noise that the IC guys can't. In an IC setup the engine will mask these prop inefficiencies by increasing rpm, we hold the set rpm and hear the wasted energy. The old rules for prop shapes were that square tips gave more line tension and drive, rounded tips took less power, angled tips were like rounded tips. My experience on E systems has been that the square tip pulls a lot more amps it is also much noisier, angle tip (like the APC shape) is a little softer up top but more efficient and pulls lower amps for the same pitch and diameter (diameter measured to the mid point of the angle).

I wondered about was how different shaping of the tip from leading to trailing edge impact the performance. I had always shaped the tip by simply rounding down from the top side with a smooth edge. Has anyone experimented with different shapes and what is the effect on flight performance and amps?

Best,                     DennisT

[Pat Johnston]

Dennis,
My buddies and I have done a lot of work on props like these.  Our work has stemmed from R/C Pylon Racing to C/L stunt.  Bottom line:  I'll put my modified wood props against any other prop in the world.  This will be sure to get some people fired up, I'm sure.  The tip shape we have found that works the best is one which looks like an elliptical section like a slender Spitfire wing.  The tips are very thin, leading edges pretty sharp and the trailing edges are rather sharp.  The peripheral of the tip is all sharp.  We tried one of these on Mark Scarburough's Electric Brodak P-40 and he noticed that the sound and smoothness was wonderful.  Said it was as quiet as any other prop he had tried.  Only problem was that it was not of the correct pitch to provide the right lap times.  I have made props which are in the area of 12 1/4" X 4 pitch and used them on our LA46's with great success.  What we have is a very efficient prop (low losses), a lot of blade diameter and overall effective disk area, which provides buckets of torque at the right lap times.  Talk about pull through the corners on hard maneuvers.
For those willing to develop the skills, and put in 1/2 to 1 hour on a prop, the results are totally worth it.  This effort is like having another 10-20% useable power.
Pat Johnston
Prop Mod Shop
Skunk Works

[Dennis Toth]

Pat,
Thanks for the input, would love to see some pictures of the larger stunt props. For electric Zinger makes a full line of wood pushers. Zingers have lots of pulling power even stock but the thick blades pull lots of amps. Some reshape could make the difference, then would be stiffer then the APCE props that we use now. I'm not sure if the stiffness is as big a deal as once thought but a good wood prop could be a great option.

Best,             DennisT

[Mark Scarborough]

There are a few electric dedicated props out there, wood ones that is,, go to esprit models website, and do a search for XOAR props,, they even have three bladed wood props.
The percieved issue with thin bladed electric props,, my THEORY is that we can get away with thinner blades, and a thinner hub for that matter, because our power is input to the prop in a smoother fashion, unlike a two stroke that beats on the prop forward and backwards 9000 times a minute! On my electric P-40, I used one of the 12x6 E props, thin blades,, in certain conditions, like heavy air, that prop would howl whenever I pulled a square corner,, now mind you this airplane was a TANK so it was really working a load into the prop. When we went to one of our modified props,, It really eliminated a lot of the noise especially in the corners. As Pat said, one of the really big things we do is sharpen the leading and trailing edges. One of MY pet "tricks" is to taper the blade thickness substatially in addition to sharpening the blades edges. Like Paul Walkers new thin wing,this reduces the load on the motor. My electric conversion props are thinner than my glow props,,
Thanks to Pat, I am a total prop junkie,, we get together at every contest and show off our latest find of the "perfect" prop blank,, we have some beauties that have the prettiest wood you can imagine,, such is out passion,,, sigh

[phil c]

The tip shape greatly affects the load generated by the prop.  APC has a pretty good program for efficient props, but the best tip shape appears to be an elliptical shape.  Many of the F2D flyers simply trim props to length to get a certain rpm.  I tried that, but reshaping the tip into a thinned down, elliptical shape reduced the noise and pulled the plane better.

Smooth counts too.  The surface should be smoothed with at least 1000 grit sand paper. Even better is to polish it with coarse rubbing compound.

Another thing that seems to work is an old idea from Hal Debolt.  Most props have a pretty flat rear face.  That is OK.  Reshape the tips so the airfoil section gradually fairs into a symmetrical section in the ellipitical tip.  Sharp edges are really good too.  Even on a stunter the tips are getting into the transonic range, so thin, symmetrical, sharp edges will have less drag.

[Dennis Toth]

Guys,
Thanks for the input, Phil you mentioned Hal Debolt, now I remember an article he wrote that showed a lot of what you described. As I recall Hal removed a lot of the material near the hub trailing edge and tapered it smoothly out to the 1/3 point as it doesn't add much to actually pulling the ship. I believe he shaped this area almost symmetrical then did something similar to what you recommend at the tips.

On tip shape one thing I found was that if you want to run the squared off tip that it should be tapered from the top face down to the edge for about 1/16" ish smoothly not rounded. This shape is what is on the Master Airscrew 3 blds I have run and is very quite and pulls well.

Yesterday I remembered doing some rework to the trailing edge of the APCE prop that I did in the past and applied it the the APCE 12x6Pusher that I want to use. The stock prop on my AXI 2826-10 was pulling 41 amps on a 4S1P TP 3900 pack. This pulled to much out of the pack so I had to reduce the diameter to 11 1/2" to get the amps down to 36.5 which pulled 78% from the pack (about as much as i could go without shortening battery life). I the mod I made was to sand off 1/8" of the tailing edge smoothly tapering from the hub to the 1/4 blade length then back smoothly to the 1/2 blade length. Its a smooth slice that thins the fat part of the blade cord not were it connects at the hub. Then I sanded out the undercamber in this area. I didn't touch the face or the tips. This mod reduced the amp load for the full length 12x6 to 35.4 amps. It does a little of what Hal discrib ed. I will next play on the tip with the idea you mentioned and see if I can reduce the load more.

Best,                      DennisT

[Dennis Toth]

Guys,
Flew the hub modified APCE 12x6P prop yesterday in 12+mph gusty winds and it worked great. It pulled through up top and held against windup as hoped. Looking back at my prop test data from last year on tractor 12x6 APCE props I did notice that the tractor had less undercamber near the hub the the pushers, I had done a mod on the tractor prop by simply sanding the back of the blade flat which also worked to reduce load.

Happy Holiday's and Merry Christmas.

Best,     DennisT