Robinson 44 Landing Skid 1

[drawoh]

Does anybody know the cantilever spring rate of the legs for a landing skid of a Robinson_44 helicopter?

I have been asked to sort out some vibration problems. It appears that our equipment is shock mounted to the helicopter on one side, and to the end of the landing skid on the other. I have vibration charts and I have a weight. I am trying to make sense of it.

I want to come up with an easier solution than "Buy a bigger helicopter."

I do not know if this is FAA approved.

JHG

 

[thruthefence]

I think if you pull out a copy of the R44 POH, you will find that the Mfg's stance, is that nothing is to be attached to the landing gear of the helicopter, without approval. Thus, you are not likely to get any factory help on this. The field approval route is possible, but the first thing they will look at is what the mfg says about such things. And also, Robinson is an outfit that had a bunch of stuff put into the TCDS that locks you into "approved behavior".

 

[EdBundy]

Hi
Heli vibration is a bid of mixed basket to sort out thanks to the nature of the beast. Perhaps you can share a bit more info on the problem, i.e. the mass of the equipment, are the connections to the skid rigid, what kind/size/layout the equipment is, etc. In my experience shock mounts are very specific in their application and may transfer vibration into the equipment they're supporting. Also, heli skids are usually very stiff compared to the rest of the structure, with natural frequencies higher than what the heli drivetrain can excite; so the vib is due to the combined structure. My comments may seem obvious but there's little info to go on here.
Oh and by the way, no I don't know what the skid stiffness is

Ed

 

[drawoh]

EdBundy,

I am trying to get our customer to share details with me! All I have are vibration charts and a YouTube video.

I have identified a vibration driven by the rotor, which consists of something bouncing up and down about its attachment to the side of the helicopter. I think one side of the pod is attached to the skid. It is possible the pod is cantilevered from the side of the helicopter somehow. Either arrangement would react to the blast of air from the rotor. It is acting at the right speed. When I rotated the vibration data by 45[°], one axis dropped off close to zero. This is consistent with the leg being loaded in tension and compression. There is no reason to assume that I am seeing a harmonic.

I am also seeing vibrations of around 0.2g at 120Hz, which seems a lot. Our system sits on shock mounts that ought to be isolating this stuff. This is the right speed for the engine. Possibly, our shock mounts are working properly on vibrations that are magnified by something.

JHG

 

[thruthefence]

There is a two-per rev vertical with an R44 that you are NOT going to get rid of. Just the nature of the beast. Bell 206, 205, 212, same deal. With some later designs eg, the 206L, elaborate damping systems isolate the M/R from the fuselage, smoothing the ride.

 

[drawoh]

thruthefence,

This is an external pod, so the problem is more serious than rotor vibraton transmitted through the fuselage.

I have photographs of the pod now, and it appears to be attached at one point to the fuselage, and at two points to the landing skid. I figure this thing is going to move if a gust of wind somehow manages to hit it.

JHG

 

[thruthefence]

Any chance of seeing a pic, or is it "eyes only"?

Not trying to get in anyone's business here, but FYI:

http://www.robinsonheli.com/srvclib/rchsn13.pdf

There's a note in the "FAA Approved" P.O.H. saying the same thing.

Chapter & verse, on request.

 

[drawoh]

thruthefence,

I should not release the photos.

Thank you for the link. That, probably, is exactly what I need.

JHG