Post by Cub DriverPost by Peter StickneySea 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.
Post by Cub DriverThis 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.
Post by Cub DriverIndeed, 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