Strange behaviour on the wind

[dimachorny]

I have encountered a strange problem when our small rotorcraft (2 meters rotor diameter, rotates clockwise when looking from the top) is hovering while an oncoming wind (about 10 knots) is blowing onto the platform. The craft begins to roll counterclockwise (if looking at the craft from the tail end forward and the nose of the rotorcraft to the wind.), loses altitude and if we don't take the controls and intervene rapidly, the craft could be lost. It rolls always in the same direction relatively to the wind. It means if we rotate the helicopter 90 deg, lets say right side to the wind, now it will pitch to the nose (again counterclockwise to the wind). At first we had twisted blades, which we thought might be causing the problem. However, after replacement with an untwisted blades, we still have the same problem. By the way, if there is no wind, then there is no problem. The helicopter has Bell-Hiller stabilization flybar with the pedals.
Thank you.

 

[MiketheEngineer]

I am not much into rotocraft - but have done some flying...

The incoming wind is creating more lift on the advancing blade and less on the retreating blade... Thats why helicopters are so much "fun" to fly - everything is changing very rapidly.

BTW - what kind of rotocraft has only a 2m blade - a model or unmanned ??

 

[dimachorny]

I agree with you about the advancing and retreating blades, but the point is that the helicopter rolls to the side of greater lift (to the side of the advancing blade).
The helicopter is a model.

 

[Compositepro]

Have you considered the gyroscopic effect of the rotor? If the wind drag on the body increases as it rotates relative to the wind, then the wind will try to tilt the rotor. A gyroscope tilts at 90 degrees to the applied torque.

 

[MiketheEngineer]

Got it backwards - Sorry

Gyroscopic action may be the answer as suggested or if the blades are rather flexible - may be they are twisting into the wind and actaully creating less lift?? Seems like I have read something where the swash plate handles this. Maybe I am wrong - kind of out of my element here.

Sounds like an interesting problem.

Most small model helicopters have blades so rigid that they can fly upside down. Maybe yours is starting to behave like a big boy helicopter

 

[MikeHalloran]

Maybe the flybar needs adjusting. Not that I knew what a flybar was a few minutes ago, but I found this educational:

http://www.swashplate.co.uk/ehbg-v17/ch30.html

and less so, this:

http://www.rchelimag.com/pages/howto.php?howto=22&page=1

Mike Halloran
Pembroke Pines, FL, USA

 

[dimachorny]

I have good news. We understood and solved the problem. As you noted the problem had a gyroscopic nature and to solve it, we increased the mass and the size of the flybar and this change led to dramatic improvement of the heli's behavior.
Thanks to everybody.

 

[rbs222]

throw in a roll rate gyro you should be good.

 

[PersonalProfile]

I understand you have resolved your issue, though the cause of this incident may be of interest:

http://www.atsb.gov.au/publications/investigation_reports/2006/AAIR/pdf/aair200606570_001.pdf

Regards,
Lyle