2 Blade Prop for BadAss 3520-650

[Ken Culbertson]

I am switching back to 2 blade props for most likely dumb reasons.  First, I can't afford the high-quality Carbon Fiber and second, I can't afford the high quality three blade spinners and finally, I have done some research on the differences between two and three blade props and I think I might like them better.  I went to them for ground clearance which is no longer an issue.  So, enough rationalization.  I get very little flight time each session so I would at least like to start with someone else success and go from there.

I need recommendations for a BA 3520-650 on a 6s 2800 Li-ion.  710 sq" 65 oz plane on 65' actual lines.  I use a Fiorotti timer and a lot of g-force boost.

My first choice would be a 12-5 BadAss but they only come in reverse rotation which I have tried and switched back.  Pitch wise I have been a 9800 RPM 6" pitch type on just about everything all my life.  Current prop is an 11-6 three blade MA.
Time to become more prop aware.  I can just barely clear a 13" without going to larger wheels which is an option.

Ken

PS:  I took my motor out of Endgame II today to test fit a new mount and improve cooling.  I discovered, much to my surprise that the 3520-790 that I ordered was actually a 650.  It performs well but gets a bit hot.  Should a 650 run hotter than a 790?  While I am doing all of this monkeying around with the nose on the plane would it be wise to swap the 3520-650 for a 3515-710 that is not being used much?  This may also partially explain why my 3515 in a similar size and weight  plane uses 600mah less to fly the pattern.

[Mark wood]

A 5 pitch is likely going to be slightly fine on 65' foot lines. A 6 pitch would be usable. For aerobatic use, we really want to be on the fast side of the efficiency curve and decelerate into the peak efficiency range rather than be slow of the efficiency peak and slow down away from it. In the first case as we slow down the efficiency increases and the thrust increases. In the latter case we slow down into lowered efficiency which results in thrust decaying as we slow down. This is what a variable pitch propeller does for us is to generally run the propeller as it optimum efficiency for a given speed, RPM combination. No, I don't know exactly what the best pitch is, only eyeballed.

From a pure efficiency perspective 2 blades are superior to 3 blades. There are a number of good reasons to run 3 blade propeller. You're running a 6 cylinder engine and the harmonics work against a two blade propeller. Three blade propellers balance better and run smoother, which is probably the best reason on the model scale for running them. Normally when it comes to performance we want fewer blades until the power loading per blade becomes too high then we have to increase the blade count. There's always a trade off and there are some three blade propellers that work better than some two blade propellers in specific installations. Dragging the ground is never good which is a forced configuration requirement.

The difference between 2 and 3 blades was one the things we tested on the Decathlons. One had a 3 blade MT and the other had a 2 blade Hartzell. The Hartzell definitely won that battle. How well a 3 blade Whirlwind would do in that arena would be nice to know. I know the difference between the MT and Whirlwind is also in favor of the Whirlwind which means that like the Master Airscrew, it is simply a not so good propeller. The test cases are rate of climb at Vy and altitude lost in a given aerobatic sequence.

There's one very important aspect in my decision tree and it is the trumping card. I am a died in the wool CAB. Cheap A$$ Bastard.

[Bill Schluckbier]

Just saw your new thread about Badass adding new prop sizes.  For electric (using a governed ESC) my experience has been that using slightly larger diameter props (say 1" larger than what would be customary) of relatively low pitch (say around 4" or 5") while being turned at high RPM seems to work best. This set-up seems to give you less speed variation in the wind with good vertical performance:

Modified brodak shark (.25 sized model)           :  11x4 at ~11,000 RPM
Modified brodak oriental (.40 size model)          :  12x4 at ~ 10,600 RPM
Brodak Leagacy             (.40 size model)          :  12x5 at ~ 10,500 RPM
Sig FAZER (really chunky wing, .40 side model) :  13x5 at ~ 10,200 RPM (I think, I would have to check)

I believe that when the motor unloads the wind milling low pitch prop generates more drag thus keeping the model from speeding up. The best way to find out is to simply buy a few props and test them out, they are not the same.  The wood props given their thicker blades appear to behave as if higher pitched than what their pitch number would suggest, APC and carbon props seem to behave more like what their pitch number would suggest.  I am assuming that the differences are due to  the thicker airfoil shapes common in wood props. I do not use an active timer and as I pilot I am not much of a threat to anyone, probably in the bottom half of the advanced group at a typical contest; keep that in mind when considering what I have written.

Good luck

[Matt Neumann]

I would like to see a 13 x 5 or a 13 x 4.  Even a 4.5 pitch.  Normal rotation.  Personally preferece.

[Bill Schluckbier]

I believe that XOAR makes a few, the prop on the FAZER was a 13x5 reverse rotation; I would assume the normal one is available as well.  I just checked their website and the do make 13" props in 4" and 5" pitch if you fly electric.