Prerotator power required vs RRPM

[Jazzenjohn]

The lowest (80 RRPM for 250 watts) and 2 highest numbers (280 RRPM/6000 Watts/ 8HP, and 330 RRPM/10,000 watts/ 13.4 HP.) are extrapolations from the middle data. 126-220 RRPM are from actual data logged testing. 23' Dragon Wing blades using a 2 stage chain reduction with estimated efficiency of 92%.

 

[XXavier]

Your data, fitted with a free on-line software [www.mycurvefit.com] and displayed as a log-log graph, so the curve is a nice straight line...

 

[Jazzenjohn]

Yes Exactly XXavier!, (I mentioned that in post #5). The data came out so well I have a high degree of confidence in the extrapolation of the higher numbers. What I'd like to know now is how will the data translate to longer and heavier blades like for 2 place machines. I would assume it would be a line raised above the single place line but with a steeper slope. I don't have a 2 place to test with at the moment. I have been thinking about making a 2 place machine for a while now, Maybe it is the right time...

 

[Alan_Cheatham]

The problem with using a straight line graph is at first glance one might think that power vs rpm is linear and so doesn't develop the proper understanding of how much additional power is necessary to achieve the higher rotor rpms.

Rotor power follows cube law, that is doubling rpm requires 8 times the horsepower, when graphed this way it's easier to understand why it's fairly easy to achieve the lower rpm of most prerotators but much more difficult as the rotor approaches flight rpm.

 

[Jazzenjohn]

The straight line graph with a log log scale as shown makes it much easier to use to figure out what RRPM you'll get with a motor of X watts, or how much power you'll need to get a target RRPM. Say you want to get 250 RRPM. Follow that number up to the line and then over to the watts required and it will be about 4200 Watts. If you're OK with 150 RRPM, then it'll be about 1100 Watts required.

You are welcome to use my results to graph it any way you want to Alan.

 

[XXavier]

The problem with using a straight line graph is at first glance one might think that power vs rpm is linear and so doesn't develop the proper understanding of how much additional power is necessary to achieve the higher rotor rpms.

Rotor power follows cube law, that is doubling rpm requires 8 times the horsepower, when graphed this way it's easier to understand why it's fairly easy to achieve the lower rpm of most prerotators but much more difficult as the rotor approaches flight rpm.

In the lower right of the image, you can see the best-fit polynomial, built with the five points power/RRPM given by Jazzenjohn:


The exponent isn't 3, but 2,82... Quite close...
Inserting the RRPM as x, you'll get the power in watt. Of course, always approximate, specially when extrapolated...

 

[Gyro28866]

I am assuming this is in 0 airflow and 0 incidence in the disk.
Several years ago, I was using a 18v Kobalt drill motor on my Wunderlick on my Bensen with a 23' disk.
In 0/0 I could achieve 110 rrpm. In a breeze (=<10mph), when the rrpm stabilized, I could start adding tilt and actually bring the stick to the full rear position and get up to about 175 rrpm.
I could pitch the drill motor overboard and gas the ole Mac, roll about 50' and be airborne.
The drill motor sure got some odd looks at the fly-ins.

 

[kmtck]

How about using high power Electric ducted fan units(EDF) at the rotor tips (similar to tip rockets) like these (https://hobbyking.com/en_us/dr-mad-...loy-edf-1250kv-3700w-8s-counter-rotating.html). The entire unit with battery and controller is lightweight and according to Dr. Ullman (https://cafe.foundation/blog/david-ullman-flying-on-multiple-tiny-motors/) gets 9lbs thrust. Maybe just buy two RC planes with everything already built in, strip the wings and tail feathers and mount fuselage on the rotor tips!

 

[Jazzenjohn]

kmtck That would work, but not at the blade tips, nearer the center. It was done by Dr. Taggart on his Gyrobee long ago. The advantage is that it doesn't produce any torque on the airframe. The disadvantage is it has a lower mechanical efficiency and has extra drag. https://www.rotaryforum.com/threads/ralphs-electric-prerotator-for-gyrobee.13945/

 

[Brian Jackson]

How about using high power Electric ducted fan units(EDF) at the rotor tips (similar to tip rockets) like these (https://hobbyking.com/en_us/dr-mad-...loy-edf-1250kv-3700w-8s-counter-rotating.html). The entire unit with battery and controller is lightweight and according to Dr. Ullman (https://cafe.foundation/blog/david-ullman-flying-on-multiple-tiny-motors/) gets 9lbs thrust. Maybe just buy two RC planes with everything already built in, strip the wings and tail feathers and mount fuselage on the rotor tips!

Ralph Taggart, designer of the GyroBee, experimented with using small RC motors & props near the root of the blades for prerotation. They would run until the tiny fuel tanks ran dry. I seem to recall it was successful, but don't remember if anything became of it, or to what degree of success. I just checked the GyroBee documentation and nothing was mentioned about it, so I think the photo and write-up that I am recalling was on his website (now offline). I couldn't find a photo on Google of it. Sorry.

The problem with tip propulsion is that energy (via fuel lines or electric wires) needs to run down the blade, making a structural blade modification necessary. It would best be designed with the blade as a whole rather than as an add-on, due in part to added tip weight and centrifugal loads. It's certainly doable because there are examples of such things.

EDIT: I see that John posted before I finished writing.

 

[marek218]

My rotor test bench motor 3000w spins 23' rotor 250rpm .

 

[dinoa]

Why not the EDF(s) near the blade straps, the blade as a duct and nozzle at tip.