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Is there any dissymetry of lift in the forward flight of the helicopter after applying the cyclic control?

Asked by: 1855 views Aerodynamics, Flight Instructor, General Aviation

Dear friends,

I was curious about the time varying loads on the helicopter blades in the forward flight. After reaching lots of books and documents, I came to know that some control (cyclic) is applied to take care of the dissymetry of lift in the forward flight. My question is, if the dissymetry of lift is eliminated by applying the cyclic control or by blade flapping (whatever), then why the load should vary with time for the 90 and 270 position of the blade? Can anybody explain this? Please let me know the justification. I am really curious about it. 

Thanks

Pratik

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1 Answers



  1. KDS on Apr 26, 2017

    Helicopter aerodynamics become quite complex. I’m not sure if I’ll hit the area of your question, but I’ll give it a shot.

    When the aircraft is hovering in still air, both sides are producing equal amounts of lift. The cyclic is used to hold the helicopter in position laterally and longitudinally.

    When the cyclic is moved forward, the angle of attack on the retreating blade is increased. Because of gyroscopic precession the lifting force that is applied on the retreating blade manifests 90 degrees later. That causes the rotor disk to fly up in back and down in front. With the lift vector change, the helicopter moves forward.

    As the air flows across the main rotor in horizontal flight, one side of the rotor will be flying into greater relative wind. To compensate for that the blade angle difference from one side to the other increases as the aircraft moves faster. The faster the flight, the greater the difference. Eventually, the retreating blade reaches a critical angle of attach and stalls. That is called retreating blade stall and it is one factor that limits the speed of a helicopter.

    You mentioned that you had read various publications, so you have probably already found this one, but just in case you haven’t, I’ll give you a link to the Helicopter Flying Handbook.

    https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/

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