Now it's getting more interesting, and I've answered my own question.
The gear leg is 4130 rather than an aluminum alloy because the yield
strength of 4130 (80 ksi) is reached at a mere at 700 lb force (up) on the
nose wheel, and deflection at the nose wheel end of the leg for this force
is about 0.6 inch. These values are based on the measured values of 1.25 in
diameter and 0.125 in wall thickness, for a 20 in long cantilever beam (the
gear leg) fixed at one end (which isn't literally true but this assumption
is good enough for now). 7075 would come close with yield strength of 73
ksi but it would deflect three times as much at the same load for the same
cross section.
So, if the leg will take a permanent set at not much more than 700 lb (and
fail at less than 1000 lb), how can the safety cable break without damage to
the gear leg?
Something isn't right here and I can't see what it is, but these
calculations say that it takes a lot more force on the nose wheel to break
the safety cable than it takes to bend (or break) the nose gear leg.
Best regards,
Rob Housman
A070
----- Original Message -----
From: "Rob Housman" <robh@hyperion-ef.com>
<fillinger@ameritech.net>; <europa@avnet.co.uk>
Subject: RE: Trike Nose Leg Bungee Breaking.
> Agreed, on all points, and especially on the manual's pretense regarding
> flexing of the gear leg, which seems a bit unlikely. Not knowing either
the
> force to stretch the bungee enough to load the 2.5mm (3/32 inch) diameter
> safety cable, or the cable's tensile strength (and not having the nose
gear
> leg handy to make measurements) I'll make a guess that the breaking force
> for a single length of the cable is 1200lbs and the pivot point on the
gear
> leg is located at around 1/4 of the overall length (for a 3:1 mechanical
> advantage), which computes to 1600lbs at the wheel to break the cable with
> none of the load taken by the bungee (for a load shared by four parallel
> cables - but the calculated value is an approximation because it is
actually
> a single cable wrapped over the assembly). I'll go out on a limb here
> (because I'd rather guess than calculate) and say that a mere 1600lbs is
not
> going to flex that beefy gear leg by much, and definitely not enough for
the
> eye to perceive. Later I'll get out my (ancient) Mechanics of Materials
> textbook and do the calculation for the deflection of the gear leg under
> load.
>
> Now that I see how little it takes to break the safety cable it makes me
> wonder why the nose heavy Europa has a steel assembly instead of one
> fabricated from aluminum.
>
>
> Best regards,
>
> Rob Housman
> A070
>
> -----Original Message-----
> From: owner-europa@post.aviators.net
> Subject: RE: Trike Nose Leg Bungee Breaking.
>
> Hi! Rob.
> IMHO The main item to protect is the propeller and its attendant parent
> attaching point !
> In my case thank the Lord for the safety strap/cable. IMHO I would like to
> know the load at which the nose leg tube provides "flexing" ? and then
next
> I would like to know its yield load? Having bent mine DOWN with a
> "wheelbarrow" bounced landing! All these questions because I don't believe
> the theory that the leg is ACTUALLY providing sufficient "FLEXING" on its
> own even with moderately heavy landings.
> When a) and b) bounced it , it was with such force that the TAIL of the
nose
> wheel spat got a bash and also gravel rash from the runway ! { a) and b)
> shall remain nameless! But that's where I believe the rot started with my
> bungee}
> Regards
> Bob Harrison G-PTAG
>
>
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