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williamtsullivan(at)att.n
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 Posted: Tue May 20, 2008 6:23 am Post subject: Firefly landing legs |
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Jack- What size and type of wheels and tires are you currently running? Also, regarding Jim Hauck's comments on steel legs- I thought factory legs were a type of aluminum spring. Pound for pound, aluminum is supposed to be stronger. With steel legs being lighter, wouldn't they be weaker? How about an opinion from an engineering standpoint.
Bill Sullivan
FS/KX/447
Windsor Locks, Ct.
[quote][b]
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jbhart(at)onlyinternet.ne
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| Posted: Tue May 20, 2008 5:49 pm Post subject: Firefly landing legs |
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Date: Tue, 20 May 2008 07:20:14 -0700 (PDT)
From: william sullivan <williamtsullivan(at)att.net>
Jack- What size and type of wheels and tires are you currently running? Also, regarding Jim Hauck's comments on steel legs- I thought factory legs were a type of aluminum spring. Pound for pound, aluminum is supposed to be stronger. With steel legs being lighter, wouldn't they be weaker? How about an opinion from an engineering standpoint.
................
Bill,
The wheels are 4" AZUSALITE - AZUS and the tire is CHENG SHIN 260 x 85 or
3.00 - 4 10x3.
Anything can be called a spring. But some materials well elastically deform
more than others before they take a permanent set and do not return to their
original shape.
"Pound for pound, aluminum is supposed to be stronger." What is important
is the difference in strength for a desirable given shape. In this case a
landing gear leg. If you made a pure iron leg the same shape as a pure
aluminum leg, the iron leg would be 2.85 times stronger and 2.91 heavier.
So if the aluminum leg weighed two pounds the iron leg would 5.83 pounds.
But now say you do not make them identical. It has been established
engineering wise, that when you try to bend a rod, that most bending load is
carried by the material closest to the rod's surface. If this is so then
why bother with the center at all? Thus one can use a round tube. If you
can throw away enough of the center, then the iron leg weight can approach
that of the solid aluminum leg but with some sacrifice in strength from the
original iron leg.
"With steel legs being lighter, wouldn't they be weaker?" To follow along
on the above, very little pure aluminum and iron is used today.
Metallurgists discovered how to add different impurities to both aluminum
and iron to boost their load carrying ability. Steel, which is much
stronger than iron, is formed by adding among other things carbon. In fact
7075-T6 aluminum is equal or stronger than annealed 4130 steel. BUT, if you
heat treat the 4130, it can be about three times stronger. This heat
treating gives steel the edge.
There is another metal property that must be considered. It is called the
modulus of elasticity. It describes how much the material will deflect for
a given load. In the case of the aluminum and iron leg example given above,
it means that the iron/steel leg will deflect about three times more than
the aluminum leg for the same load. Or the aluminum leg is stiffer. This
means that aluminum is less able to absorb impact and give back energy. In
some ways this is good. Bend the leg to absorb energy to save the cage etc.
By heat treating the steel you can increase its ability to absorb impact
energy and to return it. But there are some trade offs. The more springy
you make the leg the more likely the aircraft will want to bounce. This can
be countered some what by using large wheels with under inflated tires as a
dampening mechanism. Also, as the hardness of the steel is raised, the more
difficult it is to drill holes, etc. If the hardness is raised too high,
there is no possibility of failure bending. The steel becomes so brittle,
it will snap when it is overloaded.
And so trade offs must be made. I hope this helps you out.
Jack B. Hart FF004
Winchester, IN
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williamtsullivan(at)att.n
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| Posted: Tue May 20, 2008 6:26 pm Post subject: Firefly Landing Legs |
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Jack- Thanks for the definative explanation on the differences between aluminum and steel legs. There has been a lot of discussion in recent months, without getting back to basics.
do not archive
Bill Sullivan
FS/KX/447
Windsor Locks, Ct.
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jbhart(at)onlyinternet.ne
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| Posted: Wed May 21, 2008 4:52 am Post subject: Firefly landing legs |
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Bill,
"But there are some trade offs. The more springy you make the leg the more
likely the aircraft will want to bounce. This can be countered some what by
using large wheels with under inflated tires as a dampening mechanism."
I failed to mention that there is another option. That is add metal back to
the design or to use thicker wall tubing. If desired, this will stiffen the
leg, but it adds weigh and un needed strength.
For example carbon fiber wing spars for gliders, if designed to meet strength
requirements, are much to flexible. This is countered by adding in twice as
much or more carbon fiber to reduce flexure.
Jack B. Hart FF004
Winchester, IN
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