Maximum Speed of Airliner At Low Altitude

In article ,
"John Carrier" writes:

Top Posting fixed, to improve following teh context

"Peter Stickney" wrote in message
...
In article ,
"Mortimer Schnerd, RN" writes:
Ian MacLure wrote:
Operationally its irrlevant because the FAA limits speed below 10K
ft to 250Kts IIRC.
As to the actual max speed attainable at low altitude, I believe
you
have it correct.


Agreed. Thicker air equals lower speed. FAA equals even lower speed.

Not necessarily - thicker air also = much more thrust. It's all pretty
much a wash, with a transonic airplane. Usually the TAS in units/time
(mph, kts, km/hr) is higher, but the Mach Number's a bit lower. *The
Speed of Sound is proportional to absolute temperature. It's warmer
near the surface, so there's more mph/Mach Number.

Generally not true. Indicated airspeed top end is usually highest at low
altitude, but true airspeed capability will rise with increased altitude.
This all assumes no airframe structural limit, which is frequently below the
aircraft's capabilities in commercial designs.

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.


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

On Tue, 15 Jun 2004 17:22:32 -0400, (Peter
Stickney) wrote:

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

Amazing. Is the same true at an intermediate altitude, say 20,000 ft?

This reverses the WWII piston-engine experience, where the max
airspeed was always at higher altitudes (though falling off well below
35,000).

Indeed, one of the hopes for the jet engine was that speed would
increase (and fuel burn decrease) as the planes went higher and
higher. Whittle believed that. Or at least I believed that he believed
it. http://www.warbirdforum.com/whittle.htm

all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
The Piper Cub Forum www.pipercubforum.com
Viva Bush! weblog www.vivabush.org

Clip, clip clip ... is everybody happy?

Agreed. Thicker air equals lower speed. FAA equals even lower
speed.

Not necessarily - thicker air also = much more thrust. It's all pretty
much a wash, with a transonic airplane. Usually the TAS in units/time
(mph, kts, km/hr) is higher, but the Mach Number's a bit lower. *The
Speed of Sound is proportional to absolute temperature. It's warmer
near the surface, so there's more mph/Mach Number.

Generally not true. Indicated airspeed top end is usually highest at
low
altitude, but true airspeed capability will rise with increased
altitude.
This all assumes no airframe structural limit, which is frequently below
the
aircraft's capabilities in commercial designs.

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.

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.

R / John

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

In article ,
Cub Driver writes:
On Tue, 15 Jun 2004 17:22:32 -0400, (Peter
Stickney) wrote:

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

Amazing. Is the same true at an intermediate altitude, say 20,000 ft?

Pretty much so, depending on the shapes of the thrust and drag curves
of that particular airplane. I've got a longer, (and possibly
Caffeine-Deprived) reply to John Carrier's reply that goes into more
detail.

This reverses the WWII piston-engine experience, where the max
airspeed was always at higher altitudes (though falling off well below
35,000)

That's due to something a bit different. The piston engines on WW 2
airplanes are, of course, all supercharged, so that they produce their
maximum power at some point above Sea Level. This means that the
thrust produced by the propeller doesn't decay. Propeller Thrust is
((Engine Horsepower/True Airspeed) * Propeller Efficiency) 1,000 HP
is 1,000 HP, whether it's at Sea Level, or 20,000'. With a
controllable pitch propeller, the prop's going to vary its pitch so
that the force exerted on the air is equal to the torque by the engine
at some particular RPM. This gives the same thrust for a particular
airspeed regardless of altitude - until the engine is at a higher
altitude than the supercharger can deliver its full output, and power
(torque) drops off.
(Torque is proportional to Manifold Pressure.
On an airplane with a controllable prop, the prop controls RPM, and
the Throttle controls MAP (Torque). Constant speed props use
governers to automatically vary the prop pitch to match the RPM
commanded by the Pilot or Flight Engineer operating the engines. (Much
easier workload))

The drag decreases with the decrease in air pressure as you increase
altitude. So, if the power remains constant, and you've got less
drag, you can go faster. The rub is that since thrust is proportional
to Power / Speed, teh faster you go, the less thrust you have.
1 HP = 3.75 # of thrust at 100 mph, 1.88# of thrust at 200 MPH, 1.35#
of thrust at 300 MPH, 0.94# at 400 mph, and 0.75# at 500 mph.
This means that using a propeller to fly fast requires bucketloads of
horsepower, with the increases in engine size and fuel burn that go
with it. When you factor in teh decrease in propeller efficiency as
the airflow over teh prop goes supersonic, you can see that props
aren't very good for poing anywhere fast. But they do produce buckets
of excess thrust at low speed, which is good for takeoff adn climb
performance.


Indeed, one of the hopes for the jet engine was that speed would
increase (and fuel burn decrease) as the planes went higher and
higher. Whittle believed that. Or at least I believed that he believed
it. http://www.warbirdforum.com/whittle.htm

That's becasue a jet produces a fairly constant thrust across teh
speed range, instead of constant power. This means that teh faster
you go, the more power you have. Since the thrust of a jet decreases
more slowly with altitude than the drag is decreasing, you get a
higher top speed (discounting transonic effects) the higher you go.

Whittle wasn't teh only one who believed it - others did as well.
There's a report on the feasibility and tradeoffs of jet proulsion
from 1924 on the NACA Tech Reports server:
Jet propulsion for airplanes
Buckingham, Edgar , National Bureau of Standards (Washington, DC,
United States)
NACA Report 159, 18 pp. , 1924
This report is a description of a method of propelling airplanes by
the reaction of jet propulsion. Air is compressed and mixed with fuel
in a combustion chamber, where the mixture burns at constant
pressure. The combustion products issue through a nozzle, and the
reaction of that of the motor-driven air screw. The computations are
outlined and the results given by tables and curves. The relative fuel
consumption and weight of machinery for the jet, decrease as the
flying speed increases; but at 250 miles per hour the jet would still
take about four times as much fuel per thrust horsepower-hour as the
air screw, and the power plant would be heavier and much more
complicated. Propulsion by the reaction of a simple jet can not
compete with air screw propulsion at such flying speeds as are now in
prospect.

http://naca.larc.nasa.gov/reports/1924/naca-report-159/
Updated/Added to NTRS: 2003-08-19

So - the potentials were understood, and there were people working on
the problem for quite a while - notably Dr. Griffith of teh Royal
Aircraft Establishment. Griffith was big on producing complicated,
baroque designs that were well beyond the ability of anyone in the
1920s and 1930s to build - multispool contrarotating reverse-flow
axial compressors, for example. Whittle determined that things could
be much, much simpler, and came up with his simple centrifugal
designs. They weren't theoretically the most efficient, but they were
simple, and tolerant of off-design conditions. When Whittle first
presented his ideas to the RAF and the RAE, they consulted their tame
expert, Griffith, who advised them that Whittle's ideas were
impracticable. Whether this was due to personal jealousy, or Griffith
being unable to wrap his mind around the idea that the complicated
solutions to the problem that he was working on were unnecessary is
something I havent' been able to figure out.
With a bit more researchm, there could be a story, there.

Whittle adn von Ohain weren't the first to run Gas Turbines, btw. The
credit for that goes to Brown-Boverei Engineering in Switzerland, who
began building stationary Gas Turbines for use as industrial
powerplants in the 1920s. This didn't give them any insight into
aircraft Gas Turbines, however. Allis-Chalmers, which was
Brown-Boverei's licensee in the U.S. was a notable failure in the Jet
Race, completely dropping the ball on their homegrown turbofan
development begun in 1941, and in their licensed production of teh
DeHavilland Goblin as the J36 later in the War. Luckily, we had GE and
Westinghouse on the ball.

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

Just out of curiosity, what is the source for all your PsubS data?

A-4F always clean. TA-4J on occasion clean.

R / John

After some research concerning those aircraft that were decidedly subsonic
in level flight (no pushover from altitude to gain greater speed), it would
appear mach effect is the overriding concern. The last low altitude record
before the transition to high (F-100, with several ... F-86, F-4D ...
previous to that) were all done at the Salton Sea. Hi temp (higher TAS for
mach) and low altitude (-227 MSL), delayed transonic drag rise.

The PsubS bulge doesn't occur until you get into the cleraly supersonic
designs. Then it behooves a "low altitude" record to occur as high above
MSL as possible. Hence the sageburner and later Greenamyer efforts in the
high desert (less IAS, more TAS, 988 mph for Darryl ... great film by the
way).

Bottom line, in our running discussion, I now find your argument compelling.
I was incorrect.

R / John

Bottom line, in our running discussion, I now find your argument compelling.
I was incorrect.

Psst, John....Uhhh, this is RAM - all arguments / disagreements are required to
last indefinitely, with neither side budging an inch. You're breaking ALL the
rules!

D says hi and asks about you often.
v/r
Gordon

Krztalizer wrote:


Bottom line, in our running discussion, I now find your argument compelling.
I was incorrect.

Psst, John....Uhhh, this is RAM - all arguments / disagreements are required to
last indefinitely, with neither side budging an inch. You're breaking ALL the
rules!

Shouldn't we cut a corner off his membership card for committing such a flagrant
violation of protocol? ;-)

Getting back to Pete's point, was the MiG-17's top level speed altitude (usually
given as 13,000 feet) likely because of engine temp limits at lower altitude plus
the use of A/B up higher, or for the reasons you mention in this thread? The other
swept-wing subsonics sans A/B all seem to be fastest on the deck. I wonder if the
F-86D/K/L Sabre's top speed graph was similar to the MiG-17's, owing to the A/B --
Walt? I think the only F-86 graphs I have are for navy Furies and the F-86H.

Guy

John Carrier wrote:

After some research concerning those aircraft that were decidedly subsonic
in level flight (no pushover from altitude to gain greater speed), it would
appear mach effect is the overriding concern. The last low altitude record
before the transition to high (F-100, with several ... F-86, F-4D ...
previous to that) were all done at the Salton Sea. Hi temp (higher TAS for
mach) and low altitude (-227 MSL), delayed transonic drag rise.

snip

And in between the F-86 and F-100 records, ISTR the Brits took a Hunter to Libya
for the same reason.

Guy