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        Discussion How does reducing prop diameter affect its pitch?

#1 BobRCnut Oct 29, 2010 09:45 PM

How does reducing prop diameter affect its pitch?
 
Because of the lack of a good selection of forward/reverse pitch props, I'm considering cutting down a pair of 8" forward/reverse pitch props to 6" to better suit the motors used. Since this will drastically affect the blade shape, I'm guessing the effective pitch of the props will also change, but how do I determine which way? Taking an inch off each blade will result in a "paddle-bladed" prop with wide tips. Since most props decrease in pitch as you go further out on the blade, the tips will now be at a greater pitch than the original tips. How will this affect the effective pitch of the now-shorter prop?

#2 rebell Oct 30, 2010 12:43 AM

Most model props are not true helical cut. So, cutting off the tips will alter its pitch in some way, but to say in what effect is not easy to say, each prop designer does it differently.

The big problem is what you already mentioned, you cut off the narrow tip to end with a wide tip and that will be at a different angle as to what a propper designed prop of that size will be. I once just cut the tip and the amp draw did not reduce as we thought is will, at the end the prop was wasted.

If I have to cut a prop I also reshape the tip to be closer to the original as possible. It is not just a quick field operation but you have to take your time doing it nicely.

#3 BobRCnut Oct 30, 2010 02:27 AM

Thanks for the input, rebell... I'm going to try it using templates to trim the blades. I'm trying to get away from a 5 x 3 3-blade, and cutting down an 8 x 4 to a 6 x 4 will (best guess) result in a bit more current, but I'm not pushing the motors hard anyway. I'll measure the comparative currents and post back with results just for reference. I'm doing this to avoid problems due to mounting a 3-blade prop on a 2-blade prop saver. GWS has promised some additional smaller forward/reverse 2-blade pairs, but I'm guessing it'll be a while. This is for a very small quadcopter running Hyperion 17 gram motors.

#4 JetPlaneFlyer Oct 30, 2010 02:32 AM

I take Rebel's point about props not being trully helical in pitch but providing you are 'trimming' it and not butchering it down to half it's original diameter I'd still say that 'as near as makes no difference' cutting down a prop wont effect it's pitch.

Take an 8" x 6" prop and cut it doen to 6" diameter and you have, near enough for our purposes, a 6" x 6" prop. The trimmed blades will have reduced aspect ratio so will be a little less efficient.

Steve

#5 Yak 52 Oct 30, 2010 05:07 AM

Nominal prop pitch is generally measured at 75% of the blade so the nominal pitch will certainly increase.

The problem is that a prop blade is also doing most of the work between 50-80% of the blade. The blade camber and thickness will have been optimised at this point.

The tip 20% of the blade, however will be optimised for best efficiency in spite of tip losses.

Chop off 25% and you will have the wrong shaped camber and too much thickness in the new 'tip' so you'll lose alot of efficiency. In other words the bit that was doing the work before is now the tip, with all the losses that means.

By chopping off the tip you obviously lose the lift from the bit you cut off, but you also mess up the efficiency of the part that made all the thrust.

That said, it will still make thrust, but the lack of efficiency will quite possibly obviate any other gains.

By all means try it, but be prepared to bin it!

#6 JetPlaneFlyer Oct 30, 2010 06:51 AM

Quote:

Originally Posted by Yak 52 (Post 16420104)
Nominal prop pitch is generally measured at 75% of the blade so the nominal pitch will certainly increase.

Assuming helical pitch then the pitch is the same at any point along the blade;)

#7 Yak 52 Oct 30, 2010 07:35 AM

Quote:

Originally Posted by JetPlaneFlyer (Post 16420319)
Assuming helical pitch then the pitch is the same at any point along the blade;)

I was assuming it wasn't helical :D

Your right though, Steve, pitch won't change much.

The diameter/pitch ratio will change ...

And the efficiency could suffer a lot!

#8 rick121x Nov 06, 2010 03:07 PM

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#9 rick121x Nov 06, 2010 03:21 PM

Yes the efficiency will suffer. A great deal of the design of excellent props goes into reducing the tip losses. Also, the hub to blade transition will occupy a greater percentage of the usable propeller shape - useful prop thrust area - just wasted.

And yes the prop will run at a higher RPM, so it is important that the motor has a KV rating that supports that function. And if you take care of that; since the motor will be operating at a higher RPM, you will probably want to use a lower pitch prop.

All of this is nicely taken care of using bench thrust testing stand - one of the finest tools I have constructed in my electric flying career. So many fuzzy problems seem have clear solutions with thrust, rpm, current and wattage information. ... I copied mine device from the one advertised by BP Hobbies.

And you don't even have to have the perfect prop for the testing process... getting data from similar, as well as larger and smaller props will allow you to make a very close approximation of the near-ideal setup. It is also entertaining fun

I have found that I shouldn't mess with the shape of high priced, high quality props. They are so good that any change is very likely to reduce the efficiency.

Richard, Good luck! :)


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