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stuart(at)harnerfarm.net
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 Posted: Fri Mar 25, 2016 1:50 pm Post subject: Hinges, additional thoughts |
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Guys,
The discussion about elevator hinges and how they are mounted got me to thinking.
When two surfaces are held together with a bolt, you torque that bolt to “preload” the joint. I know it took me a while to understand this and read about it somewhere. I looked in my copy of AC 43.13 but it was not there. Perhaps one of the Tony Bingelis books, Experimenter or Kitplanes, I just don’t remember for sure.
The rest of this is from my memory banks such as they are. And I am still not an engineer but this is fun to think about. J
Anyway, the point is that if a bolt exerts 10 pounds of compression force to the two plates, any load on the plates that is less than 10 pounds will not be “seen” by the bolt. Pull on one plate with 9 pounds of force and the other plate moves with it and the bolt is still stressed at 10 pounds. But if you put 11 pounds of force on one of the plates the bolt now experiences 11 pounds of stress. If the bolt can’t take 11 pounds, it either fails or stretches enough to allow the plates to move because there is no more force between them. Trouble begins when the bolt is either not torqued properly or the forces on it exceed its limits.
For our Kolb hinges, substitute stainless steel pop rivets for bolts. The principals of mechanical attachment of two plates still hold even if one of the plates has a radius and one does not. As long as the forces on the joint do not exceed the preload forces exerted by the rivets, there will be no movement of the joint. For all practical purposes the joint acts as a solid structure.
Now the question becomes what forces are seen by the rivet in question?
First one must determine the force exerted at the hinge pin and then multiply by the moment arm (distance from pin center to rivet center. Then divide by the number of rivets. I don’t have the data on how much force is applied to the hinge pin and I do not know the clamping force or limits of the rivets.
It would take some detailed analysis of the forces to see what is at the hinge pin and what direction they are going.
I must admit when I first saw a flat hinge riveted to a round tube I raised an eyebrow because it is not something you usually see. My Firefly was a quick build kit so Bryan had already installed the hinges with temporary aluminum rivets. After installing all the rivets on final assembly and seeing how well everything was tied together I forgot all about it.
Now, having said all of that, I think there is a way to get more strength out of that hinge to tube joint. Put an arc into the leaves of the hinge with a radius that matches the tube it will be connected to. This could probably be done with some pipe and a bench vice. Once the two surfaces have more contact area, you could then stagger the rivets into two rows. This would have the benefit of putting the holes farther apart from each other and out of alignment with the adjacent hole, thus distributing the stresses into the parts more evenly. It might even look a little nicer on the finished product. You would have to be careful to not get the rivet holes too close to the edges of the hinge leaf. If that were a problem you could go with a slightly larger hinge.
Another option would be to run a full length hinge from root to outer tip of the elevator. More strength, more rivets and a built in gap seal.
Downsides of all this are more complex, more work and possibly more weight.
Overall not a bad mental exercise and a good topic for those days at the hangar when the weather keeps you and your friends on the ground.
In practicality, since there are no known reports of failures relating to the elevator hinges it is fairly safe to say there is no reason to change the design.
Pre-flight inspection of all hinges on the aircraft control surfaces was one of the first things I learned as a student pilot. Eyeball every pin and safety device, wiggle, rotate and pull to check for tightness. I still do this on every flight (Thank you Mr. Thomas) so if there were ever a problem develop I am sure to catch it on the ground. Something like this should never be discovered at an annual inspection if you are doing proper pre-flights. So, if my Kolb ever develops a loose hinge to tube joint I might have to consider one of the options outlined above.
In the meantime:
Build (it was a lot of fun and easy)
Fly (still learning something every flight)
Enjoy!
Just my $.02 worth offered with a $.02 instant rebate.
Stuart
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victorbravo(at)sbcglobal.
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| Posted: Fri Mar 25, 2016 2:08 pm Post subject: Hinges, additional thoughts |
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THAT is precisely the type of discussion and communication that I was looking for when all of this started. THAT is the two-way exchange of information which should be encouraged on all type-specific forums. THAT is what should prevail over ruffled feathers and bruised egos.
Bravo.
Bill Berle
www.ezflaphandle.com - safety & performance upgrade for light aircraft
www.grantstar.net - winning proposals for non-profit and for-profit entities
--------------------------------------------
On Fri, 3/25/16, Stuart Harner <stuart(at)harnerfarm.net> wrote:
Guys, The discussion about elevator hinges and how they are mounted got me to thinking. When two surfaces are held
together with a bolt, you torque that bolt to “preload” the joint. I know it took me a while
to understand this and read about it somewhere. I looked in
my copy of AC 43.13 but it was not there. Perhaps one of the
Tony Bingelis books, Experimenter or Kitplanes, I just
don’t remember for sure. The rest of this is from my
memory banks such as they are. And I am still not an
engineer but this is fun to think about. J Anyway, the point is that if
a bolt exerts 10 pounds of compression force to the two
plates, any load on the plates that is less than 10 pounds
will not be “seen” by the bolt. Pull on one
plate with 9 pounds of force and the other plate moves with
it and the bolt is still stressed at 10 pounds. But if you
put 11 pounds of force on one of the plates the bolt now
experiences 11 pounds of stress. If the bolt can’t
take 11 pounds, it either fails or stretches enough to allow
the plates to move because there is no more force between
them. Trouble begins when the bolt is either not torqued
properly or the forces on it exceed its limits. For our Kolb hinges,
substitute stainless steel pop rivets for bolts. The
principals of mechanical attachment of two plates still hold
even if one of the plates has a radius and one does not. As
long as the forces on the joint do not exceed the preload
forces exerted by the rivets, there will be no movement of
the joint. For all practical purposes the joint acts as a
solid structure.
Now the question
becomes what forces are seen by the rivet in question? First one must determine the
force exerted at the hinge pin and then multiply by the
moment arm (distance from pin center to rivet center. Then
divide by the number of rivets. I don’t have the data
on how much force is applied to the hinge pin and I do not
know the clamping force or limits of the rivets. It would take some detailed
analysis of the forces to see what is at the hinge pin and
what direction they are going. I must admit when I first saw
a flat hinge riveted to a round tube I raised an eyebrow
because it is not something you usually see. My Firefly was
a quick build kit so Bryan had already installed the hinges
with temporary aluminum rivets. After installing all the
rivets on final assembly and seeing how well everything was
tied together I forgot all about it. Now, having said all of that,
I think there is a way to get more strength out of that
hinge to tube joint. Put an arc into the leaves of the hinge
with a radius that matches the tube it will be connected to.
This could probably be done with some pipe and a bench vice.
Once the two surfaces have more contact area, you could then
stagger the rivets into two rows. This would have the
benefit of putting the holes farther apart from each other
and out of alignment with the adjacent hole, thus
distributing the stresses into the parts more evenly. It
might even look a little nicer on the finished product. You
would have to be careful to not get the rivet holes too
close to the edges of the hinge leaf. If that were a problem
you could go with a slightly larger hinge. Another option would be to
run a full length hinge from root to outer tip of the
elevator. More strength, more rivets and a built in gap
seal. Downsides of all this are
more complex, more work and possibly more weight. Overall not a bad mental
exercise and a good topic for those days at the hangar when
the weather keeps you and your friends on the ground. In practicality, since there
are no known reports of failures relating to the elevator
hinges it is fairly safe to say there is no reason to change
the design.
Pre-flight
inspection of all hinges on the aircraft control surfaces
was one of the first things I learned as a student pilot.
Eyeball every pin and safety device, wiggle, rotate and pull
to check for tightness. I still do this on every
flight (Thank you Mr. Thomas) so if there were ever a
problem develop I am sure to catch it on the ground.
Something like this should never be discovered at an annual
inspection if you are doing proper pre-flights. So, if my
Kolb ever develops a loose hinge to tube joint I might have
to consider one of the options outlined above. In the meantime:Build (it was a lot of fun
and easy)Fly (still
learning something every flight)Enjoy! Just my $.02 worth
offered with a $.02 instant rebate. Stuart
| | - The Matronics Kolb-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?Kolb-List |
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