Hughes 369 Question

[klas]

A LONG time ago, I had an opportunity to fly a Hughes 369. I have been meaning to ask this question to many people since, but never got around to it so thought I'd do it here:

 

We were flying along & did a right hand turn. The turn was very steep & I swear the main rotors were almost perpendicular to the ground. During the turn, there was a sound I have never heard before. It sounded as if a chain would come off of a bike, but then kick back in, when you are riding your 10-speed. There was a higher speed whirl, then back to normal. Any one have any idea of what it could have been?

[Tenacious T]

maybe droop stop pounding

[apiaguy]

blade slap

[iChris]

We were flying along & did a right hand turn. The turn was very steep & I swear the main rotors were almost perpendicular to the ground. During the turn, there was a sound I have never heard before.

 

As Apiaguy stated above, that noise was most likely "Blade Slap". That distinctive noise you hear when one blade of the main rotor runs into the tip vortex left by a previous blade. Blade Slap is also known technically as Blade Vortex Interaction (BVI).

 

The condition is extremely noticeable during descents and rapid high G steep angle turns. The MD500's rotors turn about 492 RPM (top of the green). The blades travel along the same path and pass a given point in rapid succession. Given a five-blade rotor system turning 492 revolutions per minute each blade passes by a given point in the tip-path plane 41 times per second.

 

The two bladed R44 rotor system turns about 404 RPM. Each blade passes by a given point only 13 times per second.

 

The MD 500 has a Noticeably higher pitch slap than you maybe use to with a two bladed system.

 

Note: Since you were likely in 1G flight or more (loaded rotor) during that steep turn, droop stop pounding should not have been an issue.

Edited October 11, 2010 by iChris

[klas]

I don't know if it was blade slap - I have heard that often & it sounded nowhere like it. Maybe this would be a more accurate description - it sounded as if you missed a gear in a manual transmission in a car.... You know, you are driving along, switching gears, miss it & then since you are still pressing on the gas, the engine speed revs up noticeably for that brief moment until you get back in gear. Blade slap, at least the ones I have heard, sounds like slapping - this sounded as if you revved up the engine briefly.

[iChris]

I don't know if it was blade slap - I have heard that often & it sounded nowhere like it. Maybe this would be a more accurate description - it sounded as if you missed a gear in a manual transmission in a car.... You know, you are driving along, switching gears, miss it & then since you are still pressing on the gas, the engine speed revs up noticeably for that brief moment until you get back in gear. Blade slap, at least the ones I have heard, sounds like slapping - this sounded as if you revved up the engine briefly.

 

If the owner or pilot you were flying with didn't seem concerned, it may have not been a problem other than the high level of power (torque) they were using to stay level in that steep turn. Minus this steep turn, I assume the rest of the flight was normal.

 

Many things could have caused that noise:

 

Main transmission drive shaft

 

Tail rotor drive shaft

 

Tail rotor drive shaft damper

 

Oil cooler blower

 

Oil cooler blower belt

 

The list could go on. Refer back to that operator or pilot. I'm sure they would know of any issues with that MD500.

Edited October 11, 2010 by iChris