aerobubba(at)earthlink.ne
Guest
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 Posted: Fri Mar 30, 2007 6:13 am Post subject: envelopes and gauges, was TAS and Vne & Flutter at Altitude |
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The following is non-RV related techno-mumbo-jumbo, so go ahead and delete
now rather than scolding later if one isn't interested....
Hi Jim-
| Quote: | I was told that the MU-2 has an airspeed indicator that shows the critical
airspeed for that flight altitude. The critical airspeed is shown as a
moving
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| Quote: | (red?) line on the IAS indicator.
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The Vmo/Mmo pointer on such an airspeed indicator moves so as to respect
whatever the pertinent limit is, be it V or M. The pointer has diagonal
red and white stripes to differentiate it from a conventional fixed red
line, and is referred to as a "barber pole" in common parlance.
| Quote: | Another method I've heard of using is an indicated airspeed to a fairly
low
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| Quote: | altitude like 10,000' and a limiting Mach number above that altitude.
Part 23 identifies a maximum operating airspeed (Vmo) and a maximum
operating Mach number (Mmo) as airspeed limits. Where Vmo is an IAS? And
Mmo
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| Quote: | as a
fixed Mach number is a varying TAS? (Mach 1 being about 760 knots TAS at
sea
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| Quote: | level to about 660 knots TAS at 35,000?)
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Consider a graph of altitude (vert axis) vs IAS (horiz axis). We plot the
various airspeed limits on the graph to build an airspeed envelope. First,
we make a vertical line at Vs. Next we make another vertical line at Vne.
These two lines along with the zero alt line make up three side of our
airspeed operating envelope. Last we have Mmo, or max operating Mach.
Recall that Mach (the speed of sound) is dependant upon air temperature,
with higher temps yielding a higher speed of sound. If we were to plot
Mach on our graph, it would be a line that started out high on the left
(higher alt and lower speed) and angled down towards the right (lower
altitude and higher speed). If our limiting Mach is less than 1, it will
be represented by a line below the Mach line and with a lesser slope than
the Mach line. The crossover altitude where the Vne and Mmo lines meet
(upper right corner of the graph) is the altitude where the barber pole
starts to move towards a lower value. In the case of a turboprop with a
lower Mmo, that will be a relatively lower altitude, perhaps the low teens.
In the case of the average transport jet, that alt may be in the low to mid
20's. The upper left corner of the graph is where Mmo and Vs converge. In
the case of a high flying jet we are bumping up against Mmo, which implies
that we are transonic. Somewhere on the plane, like just above the
cockpit, there is most likely supersonic flow. If we go any faster, or
pull any g, we start to get shock waves on top the wing with flow
separation aft of that, resulting in a rumble in the airframe. Yet the air
is so thin that we are flying quite close to the critical angle of attack.
Any increase in angle of attack will also induce flow separation and
resultant bad things. It is entirely possible for a plane to have enough
power to climb to an altitude where the air is so thin that the plane could
stall and immediately overspeed in the recovery. This is, of course, how
that operating realm came to be known as "coffin corner".
| Quote: | BTW, I estimated a limiting Mach number of about 0.26 for 230 mph TAS at
10,000'. Anyone have a better number?
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| Quote: | Regards,
Jim Ayers
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glen matejcek
aerobubba(at)earthlink.net
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