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I never really understood "coffin corner" until THIS post. I knew that
stall and critical mach converged. The issue about jet power/fuel flows
however, makes the picture complete (and was new for me). Interesting
bit of trivia (for a gasoline powered guy)
Dave
Jim Sower wrote:
Stall speed approaches Critical Mach at altitude. One
component of the "coffin corner" in the performance curves of
airliners. Idle fuel flow (which is a constant) approaches cruise
power fuel flow (which decreases with altitude). Minimum IAS
approaches Critical Mach.. These conditions can intersect, depending
on aircraft and weight, at some point in the early 40s. You're at idle
power, but also cruise power - you can't reduce power. You're at
critical Mach so you can't accelerate. If you want to go down, you
can't reduce power because you can't go past idle; you can't push the
nose over because you'll go transonic. You're stuck. You can shut
down an engine or two for a while - that will let you come down without
busting Mach. You might deploy a little spoiler or lower the landing
gear, but the former assures some transonic regions on the top of the
wing and the latter could cause some supersonic flow around the gear
with unpredictable results.
Sort of painting yourself into a corner ... Jim S.
cardmarc@charter.net
wrote:
Stall speed approaches TAS at altitude. I think that is one reason why the big boys use Mach numbers and stick shakers. Now I said more than I know. Maybe the military and heavy iron guys can chime in.
Marc
From: Jim Sower <canarder@frontiernet.net>
Date: 2005/01/05 Wed AM 07:10:04 GMT
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: IAS and Vne! Whoa!
Actually, I'm talking about flutter. VNE (for our homebuilt purposes)
seems to be that IAS at which the designer who almost certainly hasn't
gone into it too very deeply, begins to perceive that he is on the cusp
of being uncomfortably close to it. Flutter that is. It's on that page
in the POH that even SMELLS like it's been pulled out of someone's ass :o)
I think I'll get some sleep ... Jim S.
Jim Sower wrote:
Actually, I've been given to understand that *both* IAS and TAS are a
factor. I don't know exactly what the relationship is, just that they
both come into play.
But now I've told you more than I know ... Jim S.
John Slade wrote:
> Apparently Vne is NOT, it is a factor of True Airspeed!! That's
contrary to what I've read with respect to canard pushers, Ed.
As I understand it VNE for the Cozy, for example, is 220 mph IAS.
It's all to do with how many air molecules are hitting the airplane.
Someone tell me I'm wrong. ???
John
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