In article ,
"John Carrier" writes:

Yesterday I wrote:
John, I have to respectfully disagree. While I don't have the
specifics for a 757 or 767, here's a list of the thrust/drag limits
(Which often exceed the published flight limits) for a number of
similarly performing transonic military aircraft. The data sources
are teh Standard Aircraft Characteristics for each aircraft, which
uses the same flight test data used to create the Pilot's Handbooks
and NATOPS.
All are Standard Day conditions

Sea Level 35,000' Notes
Vmax Vmax Mmax Vmax Vmax Mmax (Placard Limits, etc.)
KTAS KEAS KTAS KEAS
F-86H 600 600 0.91 545 304 0.94
B-47E 545 545 0.83 485 270 0.85 Lim. 425 KEAS/M 0.86
B-57B 521 521 0.79 475 262 0.83 Lim. 500 KEAS/M 0.83
A-3A 545 545 0.83 510 284 0.89
AV-8B 575 575 0.87 528 294 0.92
S-3A 430 430 0.65 443 324 0.72 Vmax is at 20,000'

Of all the examples, the S-3 comes eth closest to, say, an airliner,
with its fat body and high bypass engines. Even so, there isn't much
difference. The thing driving drag the most is Mach Number. (The drag
rise due to compressibility getting going) Since Mach 1 is about 85
Kts lower at 35,000' than it is at Sea Level, It's not too surprising
that you'll have more knots in hand at low altitudes.

The problem with your comparison is that it shows only SL and 35K
(tropopause) speeds. An examination of PsubS curves would show zero PsubS
increases slightly with altitude to a point (well below tropopause) and then
suffers the mach effect as you describe giving slightly slower speeds in the
stratosphere. The issue is engine efficiency versus transonic drag effects
and normally produces results as the S-3 illustrates. While most of my high
speed experience (approaching placard etc) is in supersonic aircraft
(different rules, different PsubS curves), I recall the A-4 exhibited a
similar behavior ... faster at mid altitudes than either very low or very
high.

Ah - that was a simplification on my part, to keep the table a
manageble, and understandable size. If you like, I could give you the
full PsubS curves for all of them, verified to be within 2%, but
that's not really relevant. Given the usual shpae of the thrust and
drag curves, peak Mach Number will occor at the Tropopause. While the
thrust is decreasing with altitude, the drag's decreasing too, and,
until the air temperature stabilizes, (The definition of the
Tropopause), the thrust decays more slowly. (Note that there are some
thrust curves that do bias the altitude where T-D=0 downward - High
Bypass Turbofans tend to have a lot of Ram Drag, and thus don't like
high Mach Numbers. - that's why I think the S-3 is the best match from
the data above. (And, in fact, it does show teh behavior that you
note - I listed Vmax for 20,000' in that case, rather than 35,000'.
The 35,000' numbers for the S-3A a 420 KTAS, 232 KEAS, Mach 0.73.

For all the others, it's Vmax in Kts is a Sea Level, Mmax is at
35,000. I'll be glad to send the Vmax graphs from the SAC Charts if
you like. All airplanes are different, of course, and for a transonic
jet, the thing that will drive what Vmax is more than anything else is
Mach Number. The drag rise can get pretty steep for many shapes above
Mach 0.7, depending on the wing sweep, airfoil thickness/chord ratio,
and the area distribution. This can lead to a situation where, as
altitude increases, the drag is, in fact, increasing faster than the
thrust. An A-4, with its moderate sweep, and fairly blunt body may
very well behave that way. When you were flying the A-4, did you ever
fly it without external tanks? Those will make a big difference on
something as small as a Scooter, especially in terms of the point
where the drag rise accors, adn the magnitude of the increase in drag.


I'd like to know who the brave soul was that pushed a B-47 120 knots over
its airframe limit ... funny structural things happen in such cases.

I don't think anyone ever did - 545 KTAS is the point where the thrust
curve and the drag curve cross at Sea Level. After all, teh original
question was about what was "theoretically possible", ignoring
airframe limits. If somebody were to really have taken one that fast,
they'd have had all sorts of lateral control problems - the 425 KEAS
limit was due to wing flex when the ailerons deflected, leading to no
roll control at 425 KEAS, and reversal at some point above that speed.
Bailing out wouldn't have been much of an option - the airflow over
the canopy would have had a local Mach Number of around 1.2-1.3, by my
calculations.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster