Mast wall thickness change at folding point?

Brian Jackson

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GyroBee Variant - Under Construction
This may be more of an academic question, but after entertaining an idea I thought I'd pose it to the knowledgeable folks here. For folding masts that use the heavier wall (3/16") single tube, is there a compelling reason the short upper portion cannot be reduced to the lighter 1/8" wall?

It's easier for me to discern airframe loads below the fold point than above it, so I don't fully grasp what kind of forces the upper mast is subjected to. Has this been done before?

Regards,
Brian Jackson

Edit to add: This is more for a GyroBee type design, not a heavier 2-place.
 
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I am not familiar with this particular application Brian.

As a rule of fabrication a bolted joint needs to be stronger than a straight tube.

If only the joint is stronger it creates a stress riser on either side of the joint so great care must be taken to manage the transition.

I suspect the reason for the heavier wall is to avoid having to make a complex transition.

In my opinion the answer to your question is the tube should be thicker on both sides of the joint.
 
Thank you, Vance. The thought came about after looking at common rotor cheek plate thicknesses, and also the staggered bolt pattern typical of many of the folding mast systems (to partially address stress runout). If you've seen Sport Copter's folding mast, the plates rigidly attach to the upper mast with staggered bolts which creates a sloped plate edge instead of a hard edge straight across the tube. The plates and upper tube are rigidly unitized. The pivot and lock bolt both go through the lower mast. But once you're past the hinge and bolt transition it seems like the heavy wall thickness of the upper tube could be reduced by 1/3. "Seems" being the operative word here :)
 
Cheek plates don’t have the leverage on them that the mast does Brian and don’t deal with forces in the same way.

In my opinion you are talking about very little weight savings and a requirement for better design and fabrication.

I feel a folding mast was a bad idea from a weight and strength standpoint.

I understand the desire for a folding mast and it appears it has been done successfully so if I was doing it I would copy an existing design exactly in case there was something subtle I didn't understand.

Hopefully someone with successful experience with a folding mast will post a more specific answer.
 
Hmm, why would you make it different?
But on the other hand - have you thought of folding it sideways?
Then you could fold the mast and keep the blades on, maybe.....?
At least, if you use an electric prerotator.

Cheers
Erik
 
Vance: I agree it's not worth the introduction of an unknown just for a small weight savings. That's why the question was academic rather than being seriously considered. I am doing a folding mast modeled after the Sport Copter design.

Erik: I actually considered a sideways folding mast but again... introduction of an unknown. Fun ideas to play with though.
 
Brian: A couple thoughts.

First, the wall thicknesses of the main frame tubes in a Bensen-syle frame are driven by two considerations, besides their basic load-bearing strength. First is the need for adequate bearing area where bolts and bolt holes pass through the tubes. Second is the simple fact that extrusions have a constant section. The upshot is that the walls are much thicker than they need to be along most of the span of each tube. The only places where the walls NEED to be 1/8" or 3/16" thick is in the vicinity of bolted joints. The holes both reduce cross-sectional area and act as stress raisers.

Second (and contrariwise), a Gyrobee, or similar design having a vertical mast, loads the mast in a different way than a Bensen/Brock. Bensen raked his mast aft, so that the mast is loaded in nearly perfect tension when the rotor is flying at a 10 deg. disk angle of attack. Thus the mast experiences relatively little bending load, at least in cruising flight. 'Bee-class craft, OTOH, intentionally ignore this strategy, partly to accommodate the tall shape of the Rotax fan-cooled, inline engine. As a result, a 'Bee mast is constantly loaded in bending in normal flight. In fact, some numbers I ran long ago showed that a plain 2x2x1/8 mast on a 'Bee would be subject to a constant fore-aft bending load that's a little closer to the limit than most of us would like. Hence the universal (AFAIK) use of either a double 1x2 mast or single 3/16 wall on vertical-mast gyros.

Bending, of course, is greatest near the fulcrom. The hinge of a foldable mast is something of a fulcrum, in that it is typically way stiffer than the tubing above and below it. Moreover, there are bots and holes at the hinge. So, for both reasons of bending strength and restoration of the local strength lost to bolts and bolt holes, the tubing on both sides of the hinge should have adequate wall thickness.

If there were an easy way to thin out the tubing wall in mid-span, away from the hinge and any other bolts, this could be done with no loss of safety factor, but it couldn't easily be done with simple extrusions. Don't go at the tubes with a belt sander!

Those of us who play with sailboats face the same issue with masts -- the cheapest way to make them is with aluminum extrusions, but if you do that and make the mast adequately strong at bending points and bolting points, it's way too heavy up top. But you live with it because building a tapered mast out of aluminum is enormously expensive for the few pounds saved.

This inability to adjust the amount of metal per foot in long-span frame members is one reason why built-up truss construction, as in Vance's Predator, is theoretically more weight-efficient than long extrusions. With very careful design, you can place just enough metal at each station to meet your safety factor, without needless weight. Again, though, the weight saving is modest and must be "weighed" against the much higher labor cost.
 
I get a bearing strength Fbry of 58,000 psi for T6-6061 that works out to 7250lbs force for four 1/4" bolts for 1/8" wall thickness. Clamping force would add to this. Shouldn't this be enough for a single place?
 
Yes, it is, and that was the point when talking about a Bensen. In fact, Ron Menzie mentioned to me years ago that 3/16" bolts would be fine, and that Bensen had called out 1/4" bolts at the key joints to make it less likely that "Strong-Arm Charlies" (his words) would strip the 3/16" nuts. Of course, the Bumblebee and its derivatives use 3/16" bolts and assume their builders are smart enough not to strip the nuts.

The real need for beefing-up in the case of a Gyrobee mast is the added bending load created by the long, vertical mast.

If you like numbers, calculate the section moduli of three hypothetical masts: (a) 2x2x.125 (b) double 1x2 x.125 and (c) 2x2x.1875 and check out their respective bending strengths. Don't forget to use 6061-T6's fatigue strength, not its one-time yield or ultimate tensile strength, and to allow for the weakening effect of the holes.

If the rotor thrust is, say, 550 lb. and the rotor flies tipped back at 2 deg disc AOA while the mast is vertical, then the backward pull at the top of the mast caused by a 'Bee mast's verticality is 550 x sin(12 deg). This load varies 2/rev, and also with maneuvering and with airspeed, which is why we need to use fatigue strength (in the teens kpsi, instead of the one-time number, about 45kpsi).
 
OK, if I got this right 14ksi fatigue strength for T6 6061 taken from this source https://www.osti.gov/servlets/purl/10157028/. For a Gyrobee with the weakest of the three masts 2X2X1/8. 550lbs* sin12deg=114lbs and maximum stress should be 7,434 psi with section modulus of 0.552in^4 and assumed mast cantilever 36". This should be adequate considering that varying rotor drag at 2/rev and not the 114lb rotor thrust component is causing most of the oscillation. I'm moving my next question about "how limber a mast " to. a new thread https://www.rotaryforum.com/forum/equipment-parts-components/rotor-blades/1131786-how-limber-a-mast
 
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