Lockheed XH-51 experimental helicopter - GIF

[Altair]

Lockheed XH-51 experimental helicopter with a rigid rotor and retractable skid landing gear. Three variants were built in the early 1960s.




Previously:

Bell 214ST helicopter - photo
HESA Zafar 300 attack helicopter - photo
Piasecki X-49 SpeedHawk compound helicopter - GIF
Kamov Ka-22 compound helicopter - photo
Hughes XV-9 research helicopter - photo

[mlochala]

Looks fast. The action shots reminds me of the opening scenes of the old TV show Airwolf from the 80's.

[nova_robotics]

Looks fast. The action shots reminds me of the opening scenes of the old TV show Airwolf from the 80's.

You're right. It looks too fast. How do they get around the problem of losing lift on the retreating side of the rotor? Is that why there's a giant jet engine on the retreating side? I'm very interested in the details because I bet there's some super smart engineering behind this.

https://en.wikipedia.org/wiki/Dissymmetry_of_lift

Edit: Yup. It's waaaaay too fast. It had a top speed in level flight of 413 km/h, and 486 km/h in shallow descent. That's about 200 km/h faster than it should be able to go. Very interesting.

https://en.wikipedia.org/wiki/Lockheed_XH-51

[Frank S]

Not an aeronautical engineer but I can see a couple reason why the propulsion would be on the retreating side of the rotor. When a helicopter is hovering the air speed on both blades is equal causing both blade to be balanced in lift. when in forward flight the air speed of the retreating rotor would be less. Since the blades are fixed there is no cyclic to cause forward motion and the collective is a fixed angle.
to fly forward on this machine it needs thrust from the jet engine. being on the retreating side the suction of the intake air combined with the out flow of the thrust in my opinion increase the air flow on the retreating blade allowing it to have a closer resemblance to the air speed of the advancing blade. But this seems counter intuitive since lift is caused by higher pressure under the blade with a lower pressure above it due to the curvature and attack angle I would think the suction of the intake while it would speed up the air flow it would also create lower density in the air stream under the blade disrupting the laminar flow creating turbulence, Perhaps the thrust of the engine being many times greater than the velocity of the intake would be supplemental to this.
But as I said I am no aeronautical engineer