Author Topic: Prop rpm and pitch (Read 663 times)

Curare · « **on:** November 06, 2012, 07:18:11 PM »

Hey guys, after reading about Christ Rigotti's new stunter running a 740kv motor and 4.5" pitch prop, the gears in my head started crunching.

Knowing what I know (not much) that props around the 6" pitch mark were generally the order of the day for glow engines.

At a given rpm of say 10k, this works out to a rough speed of 55mph.

by my rough calcs, Christ's setup will be swinging a 12x4.5 at around 12k, for a speed of around 52mph.

Does this allow for the aircraft to re-accelerate better after a corner?

Is low pitch/high rpm better or is high pitch/low rpm better for stunt needs?

Dennis Toth · « **Reply #1 on:** November 07, 2012, 08:17:10 AM »

Curare,
This is an area that we in E power need to explore. We have a unique technology/power plant that allows us to eliminate the load variable that IC engines have as you change props/rpm. Our torque curve is pretty stable and flat increasing or decreasing as needed to hold the rpm. It seems that the high rpm low pitch would be beneficial from the re-acceleration point of view. However, with the higher rpm come prop precession effects. This is not as much of a problem with the lighter weight "e" props but as we get into the 13" diameter plus props may have an impact. Now that the NATs/Worlds and most of the contest season is over some our top fliers might have time to test along these lines and report the results.

Best, DennisT

Dennis Toth · « **Reply #2 on:** November 11, 2012, 07:35:51 AM »

Guys,
One aspect of the high rpm approach that I think we need to look at is the current draw. I did some quick tests a while back and it seems that at the higher rpm set for the same lap time the current draw was a bit higher for the same diameter prop. It seems the drag on the prop from the rpm increases more than the drag from increases in pitch. One solution might be to use a lower diameter with the lower pitch and higher rpm with a bit more blade area. Any one try this?

Best, DennisT

Curare · « **Reply #3 on:** November 11, 2012, 04:43:02 PM »

I'm unsure of the result of reducing diameter over increasing rpm, as to whether you'd end up better or worse off in terms of useable power and power absorbtion into the prop.

My limited understanding was that more diameter is better even if it's turning slightly slower, ala aspect ratio.

It may time to call in the prop gurus!

Dennis Toth · « **Reply #4 on:** November 12, 2012, 08:01:55 AM »

Curare,
Yes, the normal rule is larger diameter is better but one aspect that needs to be looked at is the impact on corners with the large diameter prop. On classic ships with smaller tail volume it can slow the rate of turn if it gets to large. A modern design can handle much more diameter. Prop weight is also a factor, more weight at higher rpm gives more pression nose in or out. Wood props are lighter and could be a benefit worth doing some work on.

Best, DennisT

Igor Burger · « **Reply #5 on:** November 14, 2012, 08:06:39 AM »

It needs some optimization. Larger prop at lower rpm leads to prop with P/D diameter close to 1. Such prop has better MAXIMAL efficiency compared to prop with P/D 0.5

But model slowed down to 1/2 of normal speed will push prop with P/D=1 to regime with stalled blades, so the thrust will be worse compared to the thrust of prop with P/D=0.5 and hi-rev prop with P/D=1/3 will be be even less efficient in it cruise speed but it will pull much better than those props before.

So it needs to find some compromise.

Author Topic: Prop rpm and pitch (Read 663 times)

Curare

Prop rpm and pitch

Dennis Toth

Re: Prop rpm and pitch

Dennis Toth

Re: Prop rpm and pitch

Curare

Re: Prop rpm and pitch

Dennis Toth

Re: Prop rpm and pitch

Igor Burger

Re: Prop rpm and pitch