Prop planes and the speed of sound
T. Walsh
impossible for a propellor driven plane to break the sound barrier? If
so, can anyone explain the reason? Also - is it true that planes are
designed in such a way as to prevent any part of their propellors from
moving faster than the speed of sound? Are the two (possible) facts
related? Any info or pointers to such are welcome.
Thanks.
T. Walsh
Fernando Paredes
From what i understand about propeller theory, T., you can't break
sound barrier in a propeller plane because the propeller can't
push/pull the plane that fast. something about not having enough air
mass to move, or something like that. hey, my prof didn't go into a
lot of detail, he just said you can't! and gave us some nifty formulas
to find range of a propeller airplane. hope this helps.
________________________________________________
FERNANDO PAREDES par...@seas.ucla.edu
SOCIETY OF LATINO ENGINEERS AND SCIENTISTS, UCLA
"It is far better to be feared than loved, if you cannot be both."
Machiavelli.
dr...@mit.edu
tow...@pilot.njin.net (T. Walsh) writes:
|> I hope this isn't an overasked question. Is it true that it is
|> impossible for a propellor driven plane to break the sound barrier? If
|> so, can anyone explain the reason? Also - is it true that planes are
|> designed in such a way as to prevent any part of their propellors from
|> moving faster than the speed of sound? Are the two (possible) facts
|> related? Any info or pointers to such are welcome.
I can't see why you couldn't build a supersonic prop-driven aircraft.
I can see many reasons why you wouldn't want to.
NASA (or was it NACA), built an X-plane with all-supersonic prop
blades quite a while ago. Besides being deafening on takeoff, it
reportedly was also inefficient and couldn't compete with jets.
Yes, modern prop blades are usually subsonic, but come close to the
speed of sound at the tips. The prop-fans were supersonic at the tips
(but perhaps only at altitude, I'm not sure). Large profile drag was
avoided by making the blades very thin at the tips.
Mark Drela First Law of Aviation:
MIT Aero & Astro "Takeoff is optional, landing is compulsory"
Don Morris
> speed of sound at the tips. The prop-fans were supersonic at the tips
> (but perhaps only at altitude, I'm not sure). Large profile drag was
> avoided by making the blades very thin at the tips.
I haven't seen anything on the prop-fans for some time. Why? Did
they not turn out as promised? Any enlightenment would be
appreciated.
Don Morris
Chris Miller
Traditional propeller designs create strong shocks and lots of shock
losses. NASA had a propfan technology program (that won the Collier
Trophy in 1987) that ended with propfan concepts from GE and
Pratt/Allison being flown on Boeing and Douglas aircraft. These high
speed propellers have high efficiency (~80%) at a Mach 0.8 cruise.
The propfan relative tip speeds are about Mach 1.15. The way we
avoided high shock losses was to put in a lot of sweep: ~30 deg.
forward until mid-span, then ~30 deg. aft to the tip. To have the
aircraft go faster than Mach 1.0 means that you'll get more losses
(and LOTS more noise).
Here's a couple of references:
Whitlow, J. B. Jr., and Sievers, G. K.: Fuel Savings Potential of the
NASA Advanced Turboprop Program, NASA TM 83736, presented at the
Aviation Fuel Conservation Symposium sponsored by the Federal Aviation
Administration, Washington, D. C., September 10-11, 1984.
Mikkelson, D. C.; Mitchell, G. A., and Bober, L. J.: Summary of Recent
NASA Propeller Research. NASA TM 83733, presented at the AGARD Fluid
Dynamics Panel Meeting on Aerodynamics and Acoustics of Propellers,
Toronto, Ontario, October 1-4, 1984.
--
--------------------------------------------------------------
Christopher J. Miller, MS 77-6 cmi...@lerc.nasa.gov, or
NASA Lewis Research Center tog...@roger.lerc.nasa.gov, or
21000 Brookpark Road chmi...@nas.nasa.gov
Cleveland, Ohio 44135 (216) 433-6179, -3918 FAX
--------------------------------------------------------------
R. Kyle Schmidt
>From: tow...@pilot.njin.net (T. Walsh)
>Subject: Prop planes and the speed of sound
>Date: 8 Nov 1994 10:10:21 -0600
A while bach, I saw a prototype of a plane call the MachBuster. This
was a very sleek, pusher prop design using a shaft drive V8 (driving a
multibladed fan, I believe). It then promptly dropped out of
existence, I have heard nothing more about it.
There was a blurb about it in a Popular Science/Mechanics Issue, but
about 4 years? ago.
As to props beyond the speed of sound, most props are designed to be
subsonic. The Harvard (we call it that here in Canada) also known as
the At- 6, or SNJ, had a constant speed prop, that when set to fine,
would break the sound barrier. This is what gives the Harvard its
distinctive noise.
Hope this helps.
------------------------------------------------------------------------------
R. Kyle Schmidt
Duct tape is like the Force: it has a light side, a dark side and it binds
the Universe Together!!
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Chris Miller
Don Morris <mor...@entropy.phy.ilstu.edu> wrote:
>I haven't seen anything on the prop-fans for some time. Why? Did
>they not turn out as promised? Any enlightenment would be
>appreciated.
Efficiency was very good, say ~20% better than the existing high-
bypass-ratio turbofan engines. But, noise was and is a problem and
without a cowl for acoustic treatment you can't do much. We also
heard that since fuel costs didn't go up as predicted (they went down
in real dollars) the airline customers weren't interested in new
propfan powered aircraft. Propfans couldn't be used to retrofit
existing commercial aircraft, and the predicted airline purchases were
to peak in 1996 or so and be near zero after 2000. Result: no market.
George Mantis
: In article <morris-1011...@mac113.phy.ilstu.edu>,
: Don Morris <mor...@entropy.phy.ilstu.edu> wrote:
: >I haven't seen anything on the prop-fans for some time. Why? Did
: >they not turn out as promised? Any enlightenment would be
: >appreciated.
: Efficiency was very good, say ~20% better than the existing high-
: bypass-ratio turbofan engines. But, noise was and is a problem and
: without a cowl for acoustic treatment you can't do much. We also
: heard that since fuel costs didn't go up as predicted (they went down
: in real dollars) the airline customers weren't interested in new
: propfan powered aircraft. Propfans couldn't be used to retrofit
: existing commercial aircraft, and the predicted airline purchases were
: to peak in 1996 or so and be near zero after 2000. Result: no market.
Pratt & Whitney is working on an engine called the ADP, which stands
for Advanced Ducted Propellor. It is esentially a prop-fan with a
duct, and (for now, at least) a standard PW4000 core. It has a 140"
fan diameter delivering about a 12:1 bypass ratio. Although the only
testing performed was on a technology demonstrator engine, run at idle
in one of NASA's large wind tunnels, the data indicate very low noise
and high efficiency as compared to a bill-of-material PW4000 turbofan
(no, that does NOT mean the PW4000 is noisy and inefficient :)
Thought you might like to hear about the work presently going on in
this field of propulsion. (Granted, I'm just a former Co-op who worked
in Engineering on the PW4000 for 7 months, but I did try and find out
as much as possible about what P&W was up to while I was there!)
__
________________/_|_ ******* * * * * * * * * * * * * * * * * * *
/____________________> George Mantis gm...@acs.bu.edu *
\ \ *****\_\******** * * * * * * * * * * * * * * * * * *
\__\
M.R.ROOTS
aircraft faster than the speed of sound. Having said that I as far as
I am aware it has never been done in level flight. In the 1950's NASA
Langley did a lot of research on supersonic propellers and aircraft
and produced some aircraft which were used in this research. I can't
remember the numbers of the aircraft, something like XH-88 I think,
but there were a series of them. I believe the best speeds attained
from these aircraft was somewhere in the region of 0.88 Mach with the
blade tip speed of around 1.48 Mach. Herein lies the main problem -
the propeller must travel much faster than the aircrft if thin blades
and fine pitch are used, which appears to be a more efficient method
of attaining high speed (The Russian Bear Tu -??? used coarse pitch,
however, and reaches around 0.85 mach). Once the propeller has
exceeded Mach 1 shock waves form and a lot of noise is generated, and
in fact one of the 1950's aircraft achieved supersonic flow across the
entir e propeller - the result was a very noisy aircraft that
apparently caused severe nausea in all onlookers! The problem is
basically getting the engine power to the propeller and converting
this to forward motion of the aircaft efficiently, adn so far this has
not proved successful with propeller aircraft.
On the second part of the question, propeller designers aim to keep
the propeller tip speed below mach 1 and if possible below about Mach
0.8 to reduce the noise. Once the tips or for that matter any part of
the propeller exceeds mach 1 shocks form and the associated "sonic
boom" is not ideal in todays noise conscious society.
That is my input on this subject, hope it helps.
Mark Roots
Clyde Smith-Stubbs
>> Also - is it true that planes are
>> designed in such a way as to prevent any part of their propellors from
>> moving faster than the speed of sound?
Most planes are designed so that propellor tips do not become
supersonic. The disadvantage is loss of efficiency and NOISE. Some
planes do have props that run supersonic - like a Cessna 180 on
takeoff - very noisy!
The so-called unducted fan engines that were being touted a couple of
years ago are essentially supersonic turboprop engines.
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cl...@hitech.com.au | P.O. Box 103, Alderley, | Fax: +61 7 300 5246
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David Soto
: I hope this isn't an overasked question. Is it true that it is
: impossible for a propellor driven plane to break the sound barrier?
False. Airplanes designed for subsonic flight don't do well in
supersonic and vice versa (regardles of prop or jet). So it's not that
one can't be designed, it's because it's not practical to create one.
All normal airplanes have subsonic proprellers (I'm exluding special
R&D crafts). For that fact, most (all) supersonic jets have subsonic
propellers too (the impellers in the engines that is). (The air has to
be slowed down to subsonic before being injected into the jet engine.)
So it is not that it is impossible, it is because it is difficult to
design a prop airplane that can fly efficiently in subsonic and
supersonic flight. Flying in the subsonic regime is required if you
want to be able to take-off and land regularly. ;-)
-ds
ERNESTO GRUNDSETH
: I hope this isn't an overasked question. Is it true that it is
There is a great article about breaking the Mach 1 Barrier, which
includes a very good explanation why a prop plan cannot break the
sound barrier, in "Air and Space" December 1990-1991, pages 58-72 by
Stephen Wilkinson.
In general, A prop plane can't break the barrier because can only
create thrust up til supersonic sppeds (of the plane). Thrust is lost
and creates a huge amount of drag and "becomes a flat plate" (pg66 of
the article).
This is the explanation the article gives. I would also encourage
anyone interested to read the whole article because it is quite
interesting.
Hope this clears this up
Ernesto Grundseth
er...@acpub.duke.edu
Goran Olsson, Plasma Physics, KTH
>The propfan relative tip speeds are about Mach 1.15. The way we
>avoided high shock losses was to put in a lot of sweep: ~30 deg.
>forward until mid-span, then ~30 deg. aft to the tip.
How is the sweep angle (minimum? optimum?) related to the Mach number?
(For props as well as wings.)
========================================================================
Goran Olsson, Plasma Physics, KTH, Stockholm, Sweden
ols...@plasma.kth.se
========================================================================
Raymond R. Cosner
turbine compressor blades both exceed the spped of sound at their tips
in some designs. In the case of helicopter blades, when that happens
the acoustics (environmental noise) goes way, way up.
--
Raymond R. Cosner
M.R.ROOTS
|> In article <941110191...@ferhino.dfrf.nasa.gov>, tog...@lerc.nasa.gov (Chris Miller) writes:
|> >The propfan relative tip speeds are about Mach 1.15. The way we
|> >avoided high shock losses was to put in a lot of sweep: ~30 deg.
|> >forward until mid-span, then ~30 deg. aft to the tip.
|>
|> How is the sweep angle (minimum? optimum?) related to the Mach number?
|> (For props as well as wings.)
Sweep angle is related to Mach number in so far as the local velocicy
component normal to the wing leading edge (or propeller) is the
important one. This component is thus reduced by increasing sweep
angle and the line of flight and spanwise components are higher. The
normal component is in effect the one that is generating the lift and
it experiences the local acceleration across the wing section
resulting in local shocks forming and these are reduced by
sweepback.
I hope this explains - if not email me and we'll take it from there.
Mark Roots