Trimming down props to go beyond square ??

[Bobmic]

I was looking for a way to increase the speed of my Sundowner without changing the motor.
I currently run a 770kv Power 32 on 5S and tested it with 10x10, 11x10 and 12x12.
With the 12x12 I flew only once for a short while to gather some data and the as expected the current was too high for the motor rating as it reached 70Amp with spikes going way above 80Amp. The 11x10 is just at the spec limits and the 10x10 is easily within the motor spec.
I was thinking of cutting down a 12x12 prop to something like 9.5x12 (beyond square) I know this is quite a bit but there are no 11x11 props and I am aware of the fact the this will be a wider prop but I can check to see if it doesn't draw too much current.
The question is why no one manufactures props beyond square - will I end up with a big boom... or does the prop starts to slip?

Thanks,
Bobby

[thehaze]

I'm no expert on props but my guess is that the maximum pitch is somehow related to the diameter and that's why you won't find a prop with a pitch number higher than the diameter.

I don't think trimming your prop is going to give you what you are looking for. If anything, it's going to make the power system less efficient.

[sihinch]

I run a 5.25 x 6.25 APC prop on my Nooner.

[Ededge2002]

http://www.soaringusa.com/AJ-Prop-10x23-F5D-Pylon-Blades.html

Most props do not go over square for several reasons.  The major one is that as pitch climbs the static thrust drops. This can lower take off and acceleration performance.  manufacturers cater to the masses and as soon as you try to step beyond that you can run into these troubles.

In the link I posted take a look at the angle of the prop hub to blade relationship!   These props are being made to race F5D pylon with the limiter. The slower turning larger prop has higher efficiency than the tiny 5x5 prop at crazy rpm. 

[Bobmic]

10x23...? Am I reading correctly is this a 23inch pitch?
So back to reality should I try to use a trimmed 12x12 blade to something like 10x12 or 9.5x12 without risking the plane?

[Ededge2002]

The only way you could cause damage is with imbalance.
Incorrect tip shape or any other factors will hurt efficiency but will not endanger the airframe itself.

Does xoar make the correct size?  They are quite good and I noticed a performance gain with one.

[wollins]

Bobby, you can totally cut down your prop to go square or over square, people do it all the time and so have I.  (and yes you saw correctly with the 10x23!) However as Ed pointed out, the danger lies in not balancing the prop after you cut it down.  (don't ask me how I know ... you may have seen my Swist F5D crash video?)

Colin

P.S. Incidentally there are a lot of oversquare props being made out there but most that I've seen are made for F5B (hotliners) and F5D (pylon racers) applications. Most of the ridiculously oversquare are "folders" for the F5B applications.

[Bobmic]

Thank you Colin - I will definitely balance the prop and will hope for it to work

Bobby