Hinges under stress - mechanical engineering type question

Thinking about a hinge like this (end view): ____(O)____ where "___"
is the plate, "( )" is the barrel, and "O" is the hinge pin. It would
be bolted through the plates to the surface below. The barrel sits
"on top" of the plates. Hope that makes sense.

It's in tension and/or compression (some torsion also), so obviously
the hinge pin needs to have a lot of shear strength. Clevis pins are
designed for this application, so it's a matter of doing the sums and
selecting a properly-sized one from the catalog.

But the *barrels* of the hinge are a weak point. A plain rolled
piano-type hinge would obviously just pull apart if the barrels aren't
connected back to the plate. What's the best way to prevent this?

Weld the barrels back to the plate so they are closed and can't pull
open? Makes sense - but how sure can you be of the tensile strength
of that weld? Build a test article and test it to destruction?

Or perhaps you could make the hinge by folding a strap around the
clevis pin, and back over itself. Then you have a double-thick plate
- extra weight. And tension would tend to pull the barrel out of
round and make the clevis want to center between the straps:
=====O==== rather than "on top": ____(O)____. Also, you'd have a
sharp reflex bend - potential crack site - where the barrel comes down
around the pin and then flattens out as the top layer of the two-layer
plate: __(

Ideas? I know somebody has to have already solved this. btw - assume
that the issues of properly bolting the hinge to the material below,
and properly heat -treating the assembly, have been solved. We're
just looking at the mechanics of the hinge barrel, unless I'm missing
something.

(Rich, yes, you know why I'm asking... ;-) )

Corrie "alllllways thinkin'..." B.

Most suppliers have the extruded piano hings that will not pull apart before
failure. I know Aircraft Spruce has them in 6 Ft lengths aluminum or
stainless steel.

"Corrie" wrote in message
om...
Thinking about a hinge like this (end view): ____(O)____ where "___"
is the plate, "( )" is the barrel, and "O" is the hinge pin. It would
be bolted through the plates to the surface below. The barrel sits
"on top" of the plates. Hope that makes sense.

It's in tension and/or compression (some torsion also), so obviously
the hinge pin needs to have a lot of shear strength. Clevis pins are
designed for this application, so it's a matter of doing the sums and
selecting a properly-sized one from the catalog.

But the *barrels* of the hinge are a weak point. A plain rolled
piano-type hinge would obviously just pull apart if the barrels aren't
connected back to the plate. What's the best way to prevent this?

Weld the barrels back to the plate so they are closed and can't pull
open? Makes sense - but how sure can you be of the tensile strength
of that weld? Build a test article and test it to destruction?

Or perhaps you could make the hinge by folding a strap around the
clevis pin, and back over itself. Then you have a double-thick plate
- extra weight. And tension would tend to pull the barrel out of
round and make the clevis want to center between the straps:
=====O==== rather than "on top": ____(O)____. Also, you'd have a
sharp reflex bend - potential crack site - where the barrel comes down
around the pin and then flattens out as the top layer of the two-layer
plate: __(

Ideas? I know somebody has to have already solved this. btw - assume
that the issues of properly bolting the hinge to the material below,
and properly heat -treating the assembly, have been solved. We're
just looking at the mechanics of the hinge barrel, unless I'm missing
something.

(Rich, yes, you know why I'm asking... ;-) )

Corrie "alllllways thinkin'..." B.

The Sisu 1A Maximum Airframe Load Factors: + 6 G, - 4.6 G
The Gross weight: 730 lb

And the wings are probably a lot higher aspect ratio than the ones your
considering. Long skiny/thin wings need stronger spars than short fat 'uns.

Just a WAG your probably looking at around 30,000 ft/lbs at the spar root for a
cantilevered wing? For a 6 inch deep spar that's around 33 of those little
hinge points - IIRC the numbers from the earlier thread correctly.

Of course getting 33 of them coplanar so they fold might take a bit of
length......

Then again with the ever reducing cost of CNC machine work you just might
consider one-of-a-kind units in a larger size.

I also remember seeing a post here a year or 2 ago about some extruded
graphite/carbon fiber hinges. I have no idea if they are even as strong as
aluminum (kind of doubt it) ones but it might be worth a quick internet search.

A 1500-lb aircraft pulling six g's puts a 9,000 lb
load on the spar whether the wingspan is 25 feet or 50 ft. Unless
I've forgotten something fundamental, you'd have 2250 lbs of tension

Either I'm mis-reading or you have forgotten something fundanental. The length
of the wing does have a very real effect on the spar attachment points.

A quick example. Pick up an 8 lb sledge hammer by the head at arms length. 8
lbs in your hand. An easy thing to do. Grab the handle half way and hold the
head out away from you again. Bit of a strain on the forarm muscles isn't it.
If you go to the gym on a regular basis try again at 3/4 of the handle.

Another visual aid is a gymnast on the rings trying to do an "iron cross".
Almost anybody can hold themselves up on the rings with your arms locked at
your side but as your "wing span" increases so does the strain on the upper
body muscles.

The same thing happens to spar attach fittings. My little 600 lb 16 ft span
canard has over 100,000 lbs on its little spar caps at the fuselage at 6 G's.
3 is about all it will ever see unless I bounce it real bad.


I hadn't considered laid-up carbon fittings.

Forget hand laid carbon. The end result is generaly no better than E glass.
Graphite takes considerable process control to take advantage of it's full
potential.

cal (BD5ER) wrote in message ...
A 1500-lb aircraft pulling six g's puts a 9,000 lb
load on the spar whether the wingspan is 25 feet or 50 ft. Unless
I've forgotten something fundamental, you'd have 2250 lbs of tension

Either I'm mis-reading or you have forgotten something fundanental. The length
of the wing does have a very real effect on the spar attachment points.

A quick example. Pick up an 8 lb sledge hammer by the head at arms length. snip
The same thing happens to spar attach fittings. My little 600 lb 16 ft span
canard has over 100,000 lbs on its little spar caps at the fuselage at 6 G's.
3 is about all it will ever see unless I bounce it real bad.



I haven't forgotten the bending moment. It's just that I don't think
it comes into play. For a given weight of airplane, the tensile force
on the bottom spar cap is the same for any length wing, because the
downward force creating the tension is acting through the airplane's
center of gravity. The moment arm from the cg to the hinge doesn't
change with span - only with the spanwise location of the hinge.

Yes, the balancing upward force acts through the wing's center of
lift, but remember - the lift force is actually a distributed load,
not a point load. If the span is longer, then the lift-per-unit-span
is smaller for a given weight. It cancels out. I'm willing to admit
that I'm in error - my aero structures and statics classes were a long
time ago. But I think I'm figuring things correctly.

I hadn't considered laid-up carbon fittings.

Forget hand laid carbon. The end result is generaly no better than E glass.
Graphite takes considerable process control to take advantage of it's full
potential.

Makes sense to me. Thanks for the advice. I'm strongly leaning
towards aluminum or steel - assuming I do this crazy thing at all,
mind you!