Mast Construction Materials

[skyguynca]

Ok we all know the more rigid the mast is the more the 2/rev we feel the heavier we get. Some dampen the mast, some the head and some just live with it. Now I am starting a new gyro and plan on using probably 2 1" round tube 4130 or it just might be 1x2x.125 mast tubes...still haven't decided because of the nature of the installation. The two tubes, one from each side of the airframe to a "A" frame style, to the rotorhead. I want to dampen it a bit to help with 2/rev. Any suggestions?

[rehler]

I don't believe two 1" daimeter tubes will be near strong enough in the 1" direction, not ever solid 4130 steel. 1" is not enough effective depth for the vertical beam.

[Victor Duarte]

David,
why don't you consider to have a separate pylon hanging the fuselage, dampened by 3 or 4 focused dampers ?
cheers

[skyguynca]

I was talking drawn tubing not solid. I was thinking 1" dia with .035 wall going up to the head from each side of the airframe and then a piece of 1/2" .035 wall going from the firewall back and up to the head would support it nicely.......I could be wrong but the numbers seem to say yes........what do you think?

[rehler]

You didn't mention the third tube. All three tubes form a piramid and should work fine.

I didn't actually expect you to use a solid 1" rod. I just wanted to state that even if it were solid it would be too small a diameter to keep from bending - IF only the two tubes were used. But having 3 tubes would work well - 4 tubes would be even better.

If you use 1/2" tubes they need to be short or they could get bent. As you know, larger diameter with thinner wall is much stronger (to resist bending) than small diameter and thicker wall. The diameter needed depends on the unsuported length of the tube.

[skyguynca]

Oh incase I forgot to mention it, I think your gyro is great. I was at your website checking it out. The cyclic design and linkages are pretty inventive. I bet that machine climbs pretty sweet. The tractor I am doing won't be fast, climb will be decent but it will be a really nice machine to fly hot or cold. Totally enclosed for the winter w/heat and open for summer. I am still working the mast design trying to keep it simple. I may even use streamline tubing but at 2" size.....

[Victor Duarte]

David,
streamline tubing is great, if you can get a sufficient size, the max i found is 56mm large (2"), if you find larger, please tell.

Kent, how do you calculate the unsupported lenght ? have you a thumb rule ?
Thanks you.

[skyguynca]

2" for all purposes as a mast is fine for just one tube if supported atleast 3/4 of the way up, 2 tubes will be pretty strong, even for a 2 seater

[Doug Riley]

Without a slider head, pivoting mast unit or other form of AT LEAST lead-lag relief, you should NOT use a triangulated mast with a semi-rigid 2-blade rotor. The flexing of the Bensen-style cantilever mast is your vibration isolator. Make that mast rigid and you'll turn the whole machine into a paint shaker. That's dangerous as well as uncomfortable.

It's not necessary to "damp" the mast's movements since they're not resonant -- or at least they better NOT be. Let it flex!

[Victor Duarte]

Doug ,
that's interesting and i didn't see things from this point of view.
But how to do when you do another construction than a bensen ?
BTW the bending/stiffness balance is pretty hard to adjust if you don't want it to be resonant , or am i wrong ?

BELL made a lot of progresses by mounting the engine-mast-rotor on dampers (i mean elastomeric shock absorbers). What is wrong with that ?
Thank you.

[skyguynca]

True Doug. In my earlier post I also was thinking of using the standard 1x2x1/8 like a standard redunat mast like the Bensen, Bee, KBs but with one coming from each side with the 1"side faceing fwd and aft still as a "A" frame. For a single place it should be ok without any additional bracing.

[Doug Riley]

Victor, there may be a slight difference in the use of language here. Bell's rubber mounts are yet another solution (besides the slider, Bensen's flexible metal mast and RAF pivoting mast). However, I don't understand them to be "dampers" or "shock absorbers," in the strict sense of those words.

A shock absorber/damper is device that EXTRACTS energy from a moving system and dumps it overboard -- usually in the form of heat. By taking away energy, dampers prevent resonant behavior in systems that would otherwise be subject to it (such as the spring suspensions in cars). The Bell rubber mounts may do some of that, but rubber acts mostly as a SPRING. A spring STORES energy for a period of time, but doesn't get rid of the energy -- it simply puts the energy back INTO the system at a later time.

A spring reduces the magnitude of peak loads by storing a part of the peak energy and releasing it later, during what (we hope) is a "non-peak moment." The Bensen mast is also almost a pure spring; it stores the energy and gives almost 100% of it back to the system. This keeps the peaks of the rotor vibrations from passing into the frame.

As for resonance, our 1/rev and 2/rev rotor vibrations are pretty low in frequency -- five or ten cycles per second. A plain metal mast has a resonant frequency of thousands of cycles/sec. (tap it and it makes an audible musical note!). No problem there. You'd have to be careful with rubber mounts (like Bell). They could be soft enough to have a resonant frequency close to the rotor's vibes.

Industrial rubber mounts (such as Barry, Lord and so on) are sold with a specified resonant frequency. The ones we use on Rotax engines are around 15 Hz (cycles/sec) if I recall.

I can't see where spreading the legs of a redundant mast out at the bottom would accomplish anything helpful -- would it? The mast would be weaker in compression and sideways bending than one in which the tubes touch all the way down. Compression and sidweways bending are significant in rollovers and in rotor strikes in back of the gyro.

[skyguynca]

Doug, it is to give the pilot better visability and keep the mast from directly in front of him. I may still use it in one piece directly infront, just depends on how it all comes together. I am still doing alot of drawing and crunching numbers to make it all work. I am trying to make a simple to construct enclosed tractor gyro that can be built in numbers like the Bensen and its variants that will allow the average guy with average tools build one. I am staying away from welding and machining as much as possible. This will be a tractor, tricycle gear with removable canopy for all season flying.

[Victor Duarte]

Originally Posted by Doug Riley

Victor, there may be a slight difference in the use of language here. Bell's rubber mounts are yet another solution (besides the slider, Bensen's flexible metal mast and RAF pivoting mast). However, I don't understand them to be "dampers" or "shock absorbers," in the strict sense of those words.

Doug, i apologize , i was wrond in the words used, your explanation is definitely clear.

I agree with the "rubber mounts", i have examinated some with a freq of 7Hz, depeding on the SHORE number. i even found a moldable elastomer with several Shore choices possible. I'm looking for a product that could have a "spongious" behaviour.

This is a subject of interest. and your advise is helpfull.

I'm curious about the ideal solution for David's problem.

thank you

[darrellwittke]

Say Skyguy, just a thought, if you are willing to have a single mast in front of your gyro pilot, perhaps it could be made of 4130 chrome-moly? I don't know if you're aware but Chuck B. mentioned in some post if he builds another gyro he may use steel tubing for the mast. What this accomplishes (as I understand it) is a somewhat flexible means of acheiving a slider head. As you probaly know, as long as steel is not deformed it is very accepting of deflection and performs well as a spring. My understanding is an appropriate size of tubing would allow the mast to move in an elliptical pattern to compensate and smooth out the two-per-rev. shake in the best way possible.

I know you state you prefer to stay away from welding and machining but my thought is this hasn't been attempted yet (as far as I know) but seems relatively simple and could be worth the effort if it gives the smoothest stick on any gyro (as seems possible from Chuck's explanation as I understood it.)

Just a thought to make you aware if you hadn't seen that post. (And besides, with the speed with which you get stuff done, it'd be a way for lazy lurkers like me to see if the theory works in practice )

Can't wait to see the project website, best of luck.