Prerotator power required vs RRPM

Jazzenjohn

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I've been playing around with a new prerotator. I have test gear on it that shows how much power is required for different RRPM's. The data confirms the rough estimates most people use. Mine is a double chain reduction so it should be about 90% efficient. Here's what I get:

750 watts ~ 1 HP ~ 126 RRPM
1500 watts ~ 2 HP ~ 168 RRPM
2250 watts ~ 3 HP ~ 195 RRPM
3000 watts ~ 4 HP ~ 220 RRPM
 

Jazzenjohn

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23' Dragon Wings
 

Vance

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Nice Work John!

Nice Work John!

That is a higher rpm for the horsepower than I expected.

With the weak prerotator on The Predator I am lucky to see more than 100 rpm until I get rolling.

Once I am rolling it helps to accelerate the blades and I usually leave the prerotator engaged until 150 rotor RPM.

Thank you for sharing your results; you are always an inspiration to me with your thoughtful work and creative ideas.

Thank you, Vance
 

Jazzenjohn

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I have a double chain and sprocket reduction Vance. That is pretty efficient compared to many other drive systems. Also, my numbers are steady state, that is the power required to hold the blades steady at that RRPM. I had power spikes that were higher as it accelerated the blades, but that isn't really a useful number because it only describes how much excess power the system has and implies how quickly it could accelerate the blades. I have a knob I rotate to increase speed, so how quickly I turn it would also come into play. I took it up 6 steps on the dial, pausing to let the RRPM stabilize to roughly 2/3 max rpm of the motor while testing, then plotted it on a double logrithmic scale graph. It came out to have a nearly perfect straight line of data so I am comfortable extrapolating the numbers beyond 2 HP where I stopped the test.
 

multimike

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Your numbers seem right to me. My goped prerotator gets my 23' DW to 205 rrpm no wind and I estimate its horse power about 4 hp. It's probably not geared low enough to get to peak horsepower, hence it's doesn't reach 220. Nice job realizing that power vs. rrpm is not a linear effect.
 

GaryMac

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Would love to see some pictures of your setup... sounds very interesting.
 

WHY

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Jazzenjohn

John, great information. Am working with a "specialty starter company" (can't disclose name) about getting some custom work done on a 3 hp starter, am getting really good cooperation and assistance from them as long as I maintain privacy of name,( I thourghly understand this). Will be making this unit to fit down on mast like "goped" unit and running short splined sliding shaft up to bendix on head, what kind of "drive" attachment is used between most of the flex shafts and the bendix on the head ??

How far down on the mast would you say would be the best location for the motor ??

This unit will use a Planetary gear reduction as oppossed to the "spur" gear reduction on the Nippondenso or Hitachi units and will have a custom bracket to clamp to the mast.

Will also initially use a PWM controller on the input voltage, to see how this works out.

Since you have done some serious testing will appreciate any advice or recommendation you might have.

Tony
PS may later have custom "U" joint attached right on starter output.
 

Jazzenjohn

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I'm not a fan of having the prerotator unit mounted on the mast. It seems like an un-necessary complication to have a drive shaft for an ordinary prerotator. I'm always for the smallest lightest simplest solution. Most everything I do with gyros is toward the ultralight side.

A PWM speed control is, in my opinion, the best option for soft start on an electric pre. Coiled wires are simpler and cheaper, but don't seem like a realistic choice for batteries other than lead acid. I've switched to LiFe batteries and am happy with them so far. I haven't gone through a winter with them yet. I'm told the cold weather starting performance is poor, but that if you pay attention to battery temp it can be worked around.

Both spur and planetary gears provide high efficiency reduction so I like them both, spur gears are easier to source and have a fuller range of ratios. Planetaries are compact and strong.

I've been working with brushless motors lately. They are cheap, compact, and powerful. There is a nearly endless variety of them available. the ESC's are also comparatively plentiful and inexpensive. Drawbacks are they don't have as much torque at slow speeds, and sometimes "cog" unless they are sensored, and they tend to run at higher peak rpms which require greater reductions. I don't mind patting the blades once or twice to make the start up easier but some won't want to. I've been thinking about how to safely bring blades up to speed from a standstill with a brushless system that won't burn up the motor or controller but it's down the list of priorities right now.

Brushed motors have great low end torque, are more tolerant of the type of duty cycle spinning up blades is, and are generally inexpensive, but really good ones go up in price fast and the controllers seem to cost more.

If you're aiming at making something for the general gyro market than I think the ability to easily start up from a dead stop and driving the blades up to max rpm at a standstill on the runway suggests your on the right track with a good 12 volt brushed starter motor with integral gear reduction. Most bendix/ ring gears in use today are nearly or exactly the same ratio (Wunderlich) so optimizing the first stage of reduction should be straight forward.
 

Jazzenjohn

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Anything come of the specialty starter deal Tony? I've been eyeballing a Sky-Tec flyweight starter that appears to have both an integral bendix but may also have a first stage reduction and solenoid in one package for about $350 and 6.5 pounds. I haven't found out whether the Wunderlich ring gear most gyro pilots use is the same pitch and pressure angle as it or what the output RPM is to calculate an expected RRPM.
 

Jazzenjohn

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I like the direction you're going there, simple, lightweight, and compact!
 

Earthboundmisft

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Flip the motor over, put the rubber wheel on the other end.
Then mount the assy on a pivot where the wheel could start out close to the edge
of the ring gear, and move in towards the center as rpm builds.
A motorized screw jack could be the ratio controller...
 

Brent_Brown

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The new batteries are getting better. 40 volt 4AH. should be good to spin the blades up.
 

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Jazzenjohn

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It would be interesting to see inside that chainsaw... What motor and gearbox it is using. There are also 2 pole saws, a hedge trimmer,a leaf blower, and a string trimmer that all use the same batteries. I have been measuring the energy output for prerotations on my little unit and for a 140 RRPM prerotation I'm using about 700-800 milliamp hours of power at nominally 12 volts. That power pack should be good for at least 6 prerotations to 160 RRPM as I figure it. I don't think any inflight charging would be necessary for typical use. Carrying a charger for extended cross countries or for away flying is easy.
 

Jazzenjohn

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That's a pretty badass motor Brent. That motor is 9800 watts peak which is approximately 13 HP, if their numbers are correct. It will spin at 7800 rpm so you'll need a dual stage reduction. About 2-2.5 to 1 is about right going into a 12-1 Bendix. I'd like to see it spin some blades up... It would be a beast!
 
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