Re: B737 flaps

Date:         16 Jun 97 21:35:39 
From:         mikeh@zeta.org.au (Michael Hore)
Organization: Kralizec Internet, http://www.zeta.org.au/
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"duboille" <duboille-737@calva.net> writes:

>Who can explain me exactly the reason of the 20.000 ft limitation for flaps
>extension on B737 series.Many thanks in advance.

I don't have the exact information, but I can make a (hopefully)
informed guess, based on a bit of basic aerodynamics.

As you go higher, your stall speed increases in terms of
TAS (true airspeed) (it would stay constant in terms of IAS).

But also, as you go higher, the speed of sound *decreases*,
so your various Mach limits will also come down in terms
of TAS.

This means that the higher you get, the narrower the safe
speed range in which you can fly.  The exact numbers will
depend on aircraft type, weight, etc.  Now the 737 has a
rather low Mach number cruise (.74, I saw in another post).
Thus I would guess that above 20,000 ft, if you're flying
a safe margin above the stall, your margin below your
Mach limits isn't all that much.

Now extending the flaps would compromise the whole situation
- the flaps aren't designed to handle compressibility (Mach
number) effects, so you'd be violating the basic assumptions
under which they're designed, and thus your structural
integrity.

That's my guess, anyhow.

Cheers,  Mike.

----------------------------------------------------------
Mike Hore          mikeh@zeta.org.au
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