Re: Question: Vertical speed on landing?

Date:         09 Mar 97 22:55:39 
From:         RydellSCB <RydellSCB@prodigy.net>
Organization: Prodigy Internet
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Brian A. Reynolds wrote:
>
> Kim Hackett wrote:
> >
> > >Can somebody please tell me what is the vertical landing speed
> > >limit for a B737 or B757 at MaxLandWeight (prior to any structural
> > damage)?
> >
> > For certification to FAR Part 25, I think the landing gear must pass a
> > limit drop test at 10 ft/second at its maximum landing weight and
> > 6 ft/sec at the maximum takeoff weight.  Reserve energy gear drop testing
> > is required at a higher sink rate, but I don't remember what it is.
>
> FAR 25.723(b)
> The landing gear may not fail in a test, demonstrating its reserve
> engergy absorption capacity, simulated a descent velocity of 12 fps at
> design landing weight, assuming airplane life not greater then the
> airplane weight acting during the landing impact.
>
> I didn't see any reference to max takeoff weight.

As the FAR's show, the vertical decent rate limit is governed
by max landing weight.  The 12 fps is taken as an ultimate (failure)
condition, and 10fps is limit (i.e. yielding).  I don't know
if gear is actually _tested_ to 12, I think only to 10.
(I've seen the drop test videos - what an explosive event!)
BTW, that rate applies to all jet transport airplanes.

For any sort of a max takeoff weight landing
situation, the airplane is already committed to flight and
would dump fuel to get the weight down to landing weight
before it tried to land again.  I don't
think _any_ airplane could land at more than 7 or 8 fps (approx)
at max takeoff.  If it did, you'd certainly have a gear
breakaway event (vertical breakaway) which the
gear support structure is required to survive without puncturing
the fuel tank.  At that point, you're talking about a
crash scenario anyway, which is governed by different requirements.
The landing gear becomes irrelevant, and the design is then goverened by
crash requirements for the gear supports, wing, fuselage,
engine struts (and fuse pins), etc.

By far the most punishing loads on the gear
during takeoff (short of breakaway) are RTO's.  (Seen those
videos, too - glowing brakes are amazing to watch...)

Stephen C. Baier