Induced Thrust?

[stiletto01]

Hi Freaks

I have a few questions coming your way

I don't post often, so a little bit about where I'm at skill wise may help.you answer. I have flown coax, a mSR, and a mSRX. I bought a used mCPx before i developed a throttle hold reflex and promptly drilled it into the ground so my cp experience is limited.

Before I started this RC adventure, I used to assume that it was induced thrust alone that gave helis their abilities. That was until I started reading this aero forum and discovered that it's actually lift.

And that lift improves as the air moves thru the rotor system in forward flight.

so here's my first question...

Which rotor system produces more lift on a coaxle in a hover? The top rotor? or does the induced thrust of the top rotor increase the efficiency of the bottom rotor.

[Russell Harding]

My guess is that the lower rotor sees downward moving air and therefor lower relative angle of attack for the incoming air and produces less lift, if they were at the same rpm.

However, I'd also assume that if the lower rotor is producing less lift, it would also produce less drag/torque. To balance the torque, perhaps the lower rotor is turning at a higher rpm in hover.

Just thinking out loud. I don't know a lot about the specifics of coaxial helis.

[Russell Harding]

There is some discussion of lift vs thrust in this recent thread.

DISCLAIMER: there is a bit of bad information in there. Read it skeptically and research with other sources.

https://www.helifreak.com/showthread.php?t=594985

[stiletto01]

Originally Posted by stiletto01

Which rotor system produces more lift on a coaxle in a hover? The top rotor? or does the induced thrust of the top rotor increase the efficiency of the bottom rotor.

Oh no... my apologies. I never ment to use the words lift and thrust together. After 6 pages of that thread I can see where that goes.

I think the correct term may be induced flow. Also I don't think I should mix lift and efficiency either.

and after reading your post it made me think about AOA, counter torque, rotor rpm and OMG coaxes are complex.

I would have to think then, that the lift is synchronous between the two rotor systems.

I have another question.

When the mSRX is in FFF the nose pitches up and it porpoises like a sine wave. Is this caused by gyroscopic precession from the asymmetry of lift. or is it the AS3X having fun with us.

Can I expect this effect (the proposing ) to be present in a larger FP like the upcoming 200SRX

and does this effect, the pitching up, decrease with the higher head speeds of CP's.

[extrapilot]

It doesn’t go anywhere- the vast majority of the posts in that thread are nonsense.

There is no answer for your initial question. The problem is that the rotors influence each other, so it is not simple to segregate them in terms of which is producing more thrust, or is more power efficient, etc, and that is specific to a configuration. Mostly this is being done in CFD, where it is possible to look at forces in this way (i.e. lift/drag for a specific blade during its orbit). And, the current data suggests the upper rotor is more power efficient than is the lower for a conventional coax design in hover.

[Russell Harding]

High speed oscillations are are common occurrence and are generally gyro related.

[agrimm1]

Don't forget that the bottom rotor is not cutting thought a clean cross section of air, efficiency losses from that alone most likely mean that the top one is producing more lift.

[wlfk]

Experimentally you could imagine measuring the force applied by one of the rotorshafts in a stable hover, then work out from the weight of the aircraft the relative share carried by each rotor.