Re Folding mast,

[loafar]

Now that I have everyone"s attention re Tall tails / Bunnings Aerospace etc.Need some advice on Folding mast"s.My ship wont fit into my shed no more.

[asra]

Modify your shed.I don"t like folding masts.Ross B

[loafar]

Maybe I just let the tyre"s down.

[Greg Mitchell]

Bunnings Aerospace.....LOL!

[WayneB]

Apart from the obvious advantages re storage and transportation advocates of folding masts claim benefits in terms of suppressing transfer of vibration.

[asra]

Modify your shed.I don"t like folding masts.Ross BDITO Ross.If you need to reduce the height then consider folding from the upper cheek plates, and disconnection of rods.

[Greg Mitchell]

Hi Wayne,I"m just going to have a stab at it. Simply an opinion based on what I read. And gyros I have flown in. I"m not sure its a simple as transfering vibrations with a folding mast.My understanding is, .....Mast is more effective as a single length beam, especially if it is round tube in reducing 2 per rev. Something to do with in plane resonance? The relationship between, that of the mast"s length and flexibility and the rotor, with respect to resonance. That and added complexity and weight usually. It is my view that a joint mid beam is bad engineering practice and therefore,

[Brian]

I agree, The mast is more effective as a single beam...but... my single seater has got a couple of big cheek plates half way down where the mast is cut in two and it still flies perfectly smooth ie. it hasn`t transferred any more vibration to the airframe or stick. The plates are 6mm 5083.I"m right out of my depth here on this topic but the thing that worries me with the above is the holes through the actual mast becoming elongated due to shake/ vibration, even if you have no shake as such, there is shake happening I assure you and even with checking the bolts tension, movement might well be happening as you have a hollow mast that may be getting a crushing effect happening.The fact that you say it flies perfectly smooth concerns me as well cause most gyros shake to some degree and when they dont shake, then some spot somewhere is absorbing the shake.I have been told of a plot who was flying along and suddenly the stick shake vanished, he landed immediately and found a very significant crack in the hub bar.Anyway, Sam and Mitch are giving good advice and may be able to expand on what I"m trying to say.

[Greg Mitchell]

I agree with you 100% there Browny. The mast does become more stiff.A stiff mast we know will exacerbate shake from aerodynamic excitation, which is the primary factor causing 2 per rev shake in a set of blades.The rotor gets a stronger hit from the airstream in the lateral position than the longitudial position resulting in 2 per rev shake.When the rotors in-plane resonant frequency is the same as its rotational speed it vibrates from the hits it recieves each revolution.A rotor will be resonant at some frequency higher than the excitation, IF that rotor is sufficiently stiff in-plane and is mounted on a flexible mast, then the rotor will not respond with a dramatic and noticeble 2 per rev shake.Cheek plating a folding mast as Browny says stiffens it and if shake is present other remedies need to be sought out. "Magic" Donuts/bushings, "Magic" Slider Heads, and spring loaded mast and or push rod struts all have been used in an effort to reduce stick shake.Chopper is right though there is always shake somewhere and we need to be vigalent.I have a short stiff mast, yet I have zero stick shake.However, if I push back in my seat, I can feel the slight bumping of the mast transmitted thru my butt and back.The double slider head reduces/takes that out and the whole control system is reprieved from that. Hope this makes sense.Mitch