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Bell AH-1W SuperCobra

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Re: Bell AH-1W SuperCobra

Postby Johan G » Wed Feb 10, 2021 5:00 pm

S&J wrote in Wed Feb 10, 2021 3:53 pm:Watch the series

Absolutely do watch the series. It is one of the best introductions to helicopter physics. And less than an hour in total to watch. :D
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Re: Bell AH-1W SuperCobra

Postby S&J » Wed Feb 10, 2021 5:06 pm

Thankyou
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Re: Bell AH-1W SuperCobra

Postby Johan G » Wed Feb 10, 2021 7:50 pm

amue wrote in Wed Feb 10, 2021 7:46 am:Assuming the following situation:
I'm in a hover (so all blades are equal advancing) and I push the cyclic forward and I don't touch the collective and pedals, then there shouldn't be any change in applied torque. And even if the applied torque changed, torque changes should only create a yaw tendency not a roll tendency. Where does the roll tendency, I see in the FDM model, should come from?

Edit: I wrote the below thinking of a helicopter in level flight.
Edit2: The vertical moment arm is between the tail and main rotor hubs, not the center of gravity and the tail rotor hub.

When you push the cyclic forward the angle of attack of the advancing blades will decrease and the angle of attack of the retreating blades will increase. While this will cause a pitch moment lowering the nose it will also raise the tail rotor in relation to the main rotor hub, in effect extending the vertical moment arm between the main and the tail rotor hubs, and increase the roll moment from the tail rotor. In addition as the angle of attack of the advancing blades decrease, so will their drag, and when the angle of attack of the retreating blades increase, so will their drag. This will cause a yaw moment.

The fact that the vertical moment arm between the main and tail tail rotor hubs changes when pitch changes have caused some accidents, in particular when suddenly dodging objects, for example power lines, while flying at very low altitudes. If you violently pitch up, the tail rotor will suddenly be way below the main rotor and add almost as much roll moment as it add anti-torque moment, and can easily tip the helicopter over at too low altitude to compensate.
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Re: Bell AH-1W SuperCobra

Postby S&J » Wed Feb 10, 2021 8:06 pm

there's no advancing and retreating blade in a static hover

And I've just read through all the above and I'm speechless.
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Re: Bell AH-1W SuperCobra

Postby Johan G » Wed Feb 10, 2021 8:35 pm

S&J wrote in Wed Feb 10, 2021 8:06 pm:there's no advancing and retreating blade in a static hover

Oops. Was thinking of level flight the whole time I wrote that. Thanks for spotting it. :oops:
Low-level flying — It's all fun and games till someone looses an engine. (Paraphrased from a YouTube video)
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Re: Bell AH-1W SuperCobra

Postby amue » Wed Feb 10, 2021 8:40 pm

Regarding "a helicopter rotor is not a gyro":
Respect! The Navy got it (as one of a few, it seems): https://www.cnatra.navy.mil/pubs/folder5/TH57/P-401.pdf
Page 109, section 406 "Phase lag versus gyroscopic precession"
The rotor system is not a gyro...
Both a gyro and a rotor are circular systems and respond to applied forces somewhat similarly, but through completely separate mechanisms. However, numerous writings and pilots use the terms interchangebly even though they are not the same thing.
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Re: Bell AH-1W SuperCobra

Postby S&J » Wed Feb 10, 2021 8:55 pm

Oh well that's proof then.
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Re: Bell AH-1W SuperCobra

Postby amue » Wed Feb 10, 2021 9:39 pm

Johan G wrote in Wed Feb 10, 2021 7:50 pm: While this will cause a pitch moment lowering the nose it will also raise the tail rotor higher above the center of gravity, in effect extending the vertical moment arm between the center of gravity and the tail rotor hub, and increase the roll moment of the tail rotor.

No. Roll, pitch and yaw are defined in reference to body fixed axes. And the vertical arm between cog and tail rotor doesn't change in the body fixed system, regardless of the aircrafts attitude.
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Re: Bell AH-1W SuperCobra

Postby amue » Wed Feb 10, 2021 9:40 pm

S&J wrote in Wed Feb 10, 2021 8:55 pm:Oh well that's proof then.

I can't tell if it's irony or not...
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Re: Bell AH-1W SuperCobra

Postby S&J » Wed Feb 10, 2021 9:49 pm

Sarcasm
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Re: Bell AH-1W SuperCobra

Postby amue » Thu Feb 11, 2021 8:59 am

S&J wrote in Wed Feb 10, 2021 9:49 pm:Sarcasm

Ok.
But then you takes the video you posted as proof. Because as the video's target audience are kids and average joe it of course will not contain any simplifications, etc... . Yes, I understand.
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Re: Bell AH-1W SuperCobra

Postby S&J » Thu Feb 11, 2021 9:17 am

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Re: Bell AH-1W SuperCobra

Postby S&J » Thu Feb 11, 2021 9:56 am

amue wrote in Wed Feb 10, 2021 9:39 pm:No. Roll, pitch and yaw are defined in reference to body fixed axes. And the vertical arm between cog and tail rotor doesn't change in the body fixed system, regardless of the aircrafts attitude.


So the reason it rolls is because with a sudden rotation of a nose downwards the rotor acts like a gyroscope and rotates the vehicle to the left.

Everything is 'coupled' in a helicopter
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Re: Bell AH-1W SuperCobra

Postby amue » Thu Feb 11, 2021 11:19 am

S&J wrote in Thu Feb 11, 2021 9:56 am:So the reason it rolls is because with a sudden rotation of a nose downwards the rotor acts like a gyroscope and rotates the vehicle to the left.

Wrong, wrong, wrong.

Ok, one last try, then I give up:
The first link you posted is rather good. It actually confirms my statement. The rotor is not a gyro, but it "does act like a gyro" to "an unbalanced aerodynamic lift on the disc".
By pushing the cyclic forward the blade pitch on the rotor blades on the left side of the helicopter are increased and on the right side decreased. Simple speaking that means the lift on the left side is increased while the lift on the right side is decreased. Now we have "an unbalanced aerodynamic lift on the disc".
Because of the phase lag of about 90 degrees (similar to but not the same as gyroscopic precession) the rotor disc is not tilted to the right but to the front. That means the 'gyro like effects' i.e. phase lag of about 90 degrees has already occured and the rotor disc tilt to the front is the end effect of the cyclic forward movement (that already includes the gyroscopic like effect). Therefore there is no additional moment to the left caused by the rotor disc tilting forward!
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Re: Bell AH-1W SuperCobra

Postby bell_pilot » Thu Feb 11, 2021 11:31 am

Swashplate delays pilot inputs to override phaselag. Cyclic inputs are transfered to the rotor as 90° input. Both of you are right.

Does fwd cyclic inputs rolls helicopter? I am not sure right now. I will check it. But considering hover, all forces are equal. Fwd cyclic changes only fwd thrust angle. While bodyframe doesnt change, there shouldnt be a rolling movement with fwd cyclic input.
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