This is the Power Required for Prerotation

Yo Dan
Just so happens I writing the Coriolis effect in PRA's new gyro pilot course.
This may help explain it.

Coriolis Effect, which is sometimes referred to as con-servation of angular momentum, might be compared to spinning skaters. When they extend their arms, their rotation slows down because the center of mass moves farther from the axis of rotation. When their arms are retracted, the rotation speeds up because the center of mass moves closer to the axis of rotation.

When a rotor blade flaps upward, the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place in order to conserve angular momentum. Conversely, when that blade flaps down-ward, its center of mass moves further from the axis of rotation and blade deceleration takes place. [Figure 3-5] Keep in mind that due to coning, a rotor blade will not flap below a plane passing through the rotor hub and perpendicular to the axis of rotation. The acceleration and deceleration actions of the rotor blades are absorbed by either dampers or the blade structure itself, depend-ing upon the design of the rotor system.

If the asymmetric airfoils have roughly similar drag when the bottom of the blade is approximately flat and it provides some lift, and when it is tipped down and provides no lift, doesn't that imply that for a variable pitch rotor on a jump takeoff gyro a symmetric blade would be more efficient? Especially with respect to the power required to prerotate? Or is the difference too small to bother with?

Is there a table somewhere with the approximate HP required to spin up regular rotor blades like Dragon wings or Sport rotors or Skywheels to different RPM's?

Jazzenjohn said:

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Is there a table somewhere with the approximate HP required to spin up regular rotor blades like Dragon wings or Sport rotors or Skywheels to different RPM's?

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I'd like to see that data too, John!

John, the difference at typical prerotation rpm, say up to 200 rpm, between zero lift pitch and flying pitch is too small to bother with.

Rotor profile power can be approximated by the use of this formula:

HP = 86 x 10^-15 x diameter^4 x RPM^3 x chord. Chord is inches and diameter is in feet.

10^-15 means the decimal point of 86 is moved 15 places to the left (86 with 13 zeros in front).

In longhand for a 23” rotor at 200 rpm with 7” chord, it would look like this:

.000000000000086 x 23 x 23 x 23 x 23 x 200 x 200 x 200 x 7 = 1.35 HP

Dragon Wings, because of the twist which places the blade tips at a slightly higher angle of attack, require another ~½ HP.

Thank you Chuck your the best!!
I'm keep'n this stuff, you guys make it so easy.

All_In said:

Thank you Chuck your the best!!
I'm keep'n this stuff, you guys make it so easy.

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I agree. Not only amazing knowledge of physics, history and science but also a rare talent for insight and debunking.

I am looking forward to your project with the web page John. When we have all of Chuck's old articles online for the membership to utilise it will be incredible.

My head spins when I think of all of the information Chuck has selflessly provided for the rotorcraft community. All still out there, no strings attached. If you can't figure how he came up with something he is always willing to explain the process.

.

It doesn’t take much of a spread sheet to do that little bit of arithmetic. My problem is that I use Excel so seldom that I forget the procedure and have to start off trying to make it add 2+2 and give the right answer.

Make entries in the yellow boxes and it will spit out the right answer.

It only calculates the profile drag power and drag due to pumping air will introduce increasingly greater error above ~200 rpm if the blades are not set for zero lift pitch.

Hi Chuck,

Ora and George did an experiment with 23' DW's. The power required above aprox. 150rrpm went up a great deal. Do you happen to know what their numbers were?

Phil.

Dennis, Chuck just made a clerical point. I read the reply and it wasn't an attack on you. Chuck gave the author credit, he just stated the figures were overly optimistic, not wrong.

I didn’t know they had taken measurements.

Years ago, Ernie built a rotor test stand using a junk automobile with the rear axle standing on end to run the rotor. I suggested he mount the axle on bearings so torque could be measured but he decided it wasn’t worth the trouble.

In any event, at 600 rpm on a 23’ rotor, the old stove bolt Chevy was huffing, puffing and blowing smoke. The rotor blade tips were warm to the touch from aerodynamic heating.

I think they took measurements up to 5 hp. George was getting some numbers for an electric prerotator. They used hydraulics to drive the rotor and recorded the presures it took to drive the rotor at various rpm. I think they found that up to 120 rrpm was relatively easy.

I assumed (didn't check) the sharp increase in power needed above 150rrpm was from the rotor starting to move a substantial quantity of air.

Phil

I just reread your post about the added AOA from the twist in DW's. You aproximate a 37% increase in needed power with the additional 2*. That's pretty substantial. You threw me when you said the power difference wouldn't be noticable at 200rrpm.

Phil

The increase in power between 150 and 300t/mn is 8 times more. Always (unless the blade twists). A starters car does not allow more than 3 hp, because it needs more 4000 electric watts. The battery can not.

All_In said:

...Coriolis Effect, which is sometimes referred to as con-servation of angular momentum, might be compared to spinning skaters. When they extend their arms, their rotation slows down because the center of mass moves farther from the axis of rotation. When their arms are retracted, the rotation speeds up because the center of mass moves closer to the axis of rotation.

When a rotor blade flaps upward, the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place in order to conserve angular momentum. Conversely, when that blade flaps down-ward, its center of mass moves further from the axis of rotation and blade deceleration takes place....

Click to expand...

Hi John,

I think you might be misrepresenting Coriolis force a bit. You might want to take a look at http://www.rotaryforum.com/forum/showthread.php?t=19495&highlight=Coriolis for a more expanded view on that matter.

Also, as I understand it, there is no Coriolis force acting on the rotor blades. They are flying around a perfect circle, which just happens to be tilted back by the blowback angle with respect to the rotor disk.

-- Chris.

Thank you Chris, I'll check it out.
It wasn't mine that quot came from the FAA Rotrocraft Flying Handbook, so opps FAA.

I feel like a welfair bludger ere.
Readn all this info i need, and i never even asked a question.

Thanx agin CB for the generosity of the substantial amount of information that seems to always be sitn on top of your head.

And Dennis, grow up.

Greg idea birdy, I'd like that data too.