Chromoly Mast Construction

Earthboundmisft

The 'FIXER'
Joined
Jun 1, 2006
Messages
2,110
Location
Satellite Beach, (Sea Park),(Original 'Lums Bums'
Aircraft
The 'MISSFIT' gyro, GRAND CHAMPION, Bensen Days 2010
Total Flight Time
700 F/W, 227 Gyro
A question to those qualified. Educate me regarding the qualities, good and bad, of the use of 2x2 4130 tubing for mast construction. The application will be a compressed two place tandem, with a single mast. Slider rotorhead. Spring gear. 15 gal. fuel. 300lb/170hp engine. Thank you, Mike.
 
Mike,
did you find some 2x2 tubing. I looked once and could not find it. I am going round
 
There's nothing categorically wrong with using 4130 square tubing. Aircraft-grade 4130 square is made by squishing 4130 drawn (seamless) round, so obviously the absence of cracking in the corners is important. Round tubing probably would be a better choice, since it's considerably cheaper and eliminates the corner-cracking question.

Steel has better fatigue qualities than aluminum alloys, in that there is a load level (called the endurance limit) below which the material will take an unlimited number of cycles and not break. Aluminum has no definite endurance limit. It will break even at a very low load level if enough millions of cycles are imposed on it.

Tervamaki (later Magni) has used steel masts since the late 60's.

The biggest weakener of masts, aluminum or steel, is drilling holes in them. By using proper clamp-on fittings instead of through holes, you can restore some 40% of the mast's bending strength. That's how much you otherwise lose when you drill. 4130 presents the additional possibility of welding on fittings instead of drilling.

Bensen used 6061-T6 extrusions because they are easy to build with and pretty widely available at low cost. 6061-T6 is a commercial-grade alloy that is not all that special.
 
Mike:

I wouldn't deprive you of the satisfaction of designing it yourself. Use 75,000 psi as tensile strength for 4130 N. Tensile is easy to calculate and large safety factors cost you very little in weight.

Beam strength, not tensile, is the critical factor with our simple "pole" masts. Consider that the rotor can generate steady thrust of some 2.5 G's and momentary thrusts of perhaps 4.0. Consider that the rotor can be angled back some 35 degrees aft to a vertical mast (like the Gyrobee's) or 25 degrees to a ten-degree raked mast (like Bensen's). Then figure the bending load in foot-pounds -- the setup is no different than a big vertical torque wrench.

Now the fun begins. Learn about a number called Section Modulus. It's a calculation based on the various dimensions of your beam's cross-section. You have to dig a bit, but all this stuff is now on the 'Net. Play with some wall thicknesses and the resulting Section Moduli.

It may help to know that a Bensen double 1x2 6061-T6 mast could take about a 600 lb. aft-bending load applied at the rotorhead if it had no holes drilled in it. Presumably, you would expect your steel mast to do as well or better.

Watch the weight, though. Steel weighs about 3.5 oz. per cubic inch, as against 1.5 for aluminum.

Have fun!
 
Doug, I have kicked this around for some time, and yes I need to run some numbers.
I am curious about the 4130 mast damping qualities, good or bad, verse the typical double 1x2
aluminum construction. I plan to turn the mast rearward to form the rear keel, with welded attachments for the remainder. I would like to deviate from the 'erector set' construction methods. I think the weight penalty could be minimized. Thanks, Mike.:yo:
 
I wouldn't deprive you of the satisfaction of designing it yourself.

Well said Doug.

Anyone contemplating designing their own aircraft, especially one that deviates from an established successful design, needs to be able to "do the numbers". Only sound engineering will make a sound aircraft and many times it's the small details that are the critical ones, such as stress concentrations at member attachments and fatigue resistance.

.
 
Mike, I think that you will find that you can use much thinner wall in the CroMo, other wise the weight will be an issue.
How much thinner? round tubing? I don't know, you need to speak to one of those aeronautical engineering types.
:)
 
Mike,

A useful tool is a free beam analysis program called Beamboy 2.2, just Google it to find a download site.

You said in your description that you "plan to turn the mast rearward to form the rear keel", from this I assume the mast is raked forward, if so and depending on the details of your design this can introduce additional bending and torsional loads compared to a more conventional gyro mast making analysis more critical.

Also, Ernie B. has stated that a slider head is ineffective on a flexible mast so keep that in mind.

.
 
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Mike, stiffness and damping are two different things. Neither aluminum alloy nor steel has any appreciable damping qualities. They both make nearly perfect springs (or tuning forks, if you prefer). That is, they store energy when bent within their elastic limit, and give just about 100% of it back when the load is released. OTOH, damping devices (like car shock absorbers) take the energy, turn it into heat and dump it overboard. I.e., they don't give it back.

Rotor vibes are very low in frequency; obviously a 1/rev is six-ish cycles per second, and a 2/rev is twice that. Well below the lowest frequency we can hear, and not likely to resonate in any mast that is strong enough to resist the bending loads we talked about. You can find the resonant frequency of a mast by clamping it at one end and giving it a whack, just like a big tuning fork. I haven't tried it recently, but I believe you'll get an audible note -- meaning its resonant frequency is many times higher than your rotor vibes.

Once you pick a tubing size and wall, you can check its spring constant to see how soft it is in bending, compared to a standard mast. Obtain one mast of each type. Clamp each one in a horizontal position, restrained at one end as a cantilever beam. Hang a known weight from the other end and measure how far it bends.

I haven't tried it, but I suspect that a 4130 mast with appropriate wall thickness and the more usual 2x2 x 1/8 6061-T6 will not be that different. That is, both will flex and isolate the rotor head a bit -- but remember that Bensen's flexing-mast system has never been ideal. It's simple and tolerable in small gyros, but even they can be made smoother with a more elaborate "soft mast" suspension or slider head. Such a suspension becomes just about mandatory with the 3/4 ton, 2-place flying tanks that many people seem to like.
 
Alan, I hope the engine, passenger and fuel weights can be managed in such a way that will allow me to have the mast straight or raked rearward like a traditional set-up. I plan to 'stack' the two seats, with the fuel tank strategicaly placed. No rear controls. I understand the bending moments imposed on other than reward raked masts. The fact that the slider doesn't work on a flexible mast is actually useful in this case. That suggests I could increase wall thickness for added safety.

Doug, actual tests of the mast material is a great idea. I wonder what would be an appropriate 'test weight'? This is the help I am looking for. Thanks guys.
 
Maybe the weight of a person. Again, the full-cantilever bending breaking strength of the usual aluminum gyro mast is a few hundred pounds.
 
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Run the numbers on 2x2x.065 wall 4130. Looks roughly comparable to 2x2x1/8 6061-T6, only about 1/3 heavier for the tubing alone. You might make some of that back by using welded fittings instead of bolts and thick brackets.

Don't take my word for it, though. I'd use round tubing in any case.
 
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