Re: Does the Lyulka AL-7 really have a supersonic compressor?
- From: Peter Stickney <p-stickney@xxxxxxxxxxxx>
- Date: Sun, 18 Jun 2006 11:54:26 -0400
Nele VII wrote:
Sorry for jumping in late, the press of Normal Life & all that...
There are many online sources, but mainly in Russian. Basically, they say
it was. However, Al-7 engine was quite a handful to operate, and is
primary reason of problems in Su-9 interceptors and one of the main
reasons of Su-11 interceptor failure due to the airfow sensitivity. Some
of the problems with the engine are described in English on the Intian Air
Force website that operated Su-7 fighter/bombers. Al-7F performed little
better in Su-7 because the intake was better "matched" than the one of
Su-9/11 (radar absence) and different aircraft requirement (mainly low/mid
op altitude).
For particular details, you should problably ask Pete Stickney :-)
Well, you've forced me to go look it up, as best as I can -
To work it back to basics - any continuous flow type air compressor
works by adding kinetic energy to the air (Speeding it up) and
converting that kinetic energy (Speed) to Potential Energy (Pressure)
by slowing the air down. In a centrifugal compressor, that happens by
feeding air in at the middle, using the rotating impeller to speed up the
air, and the diffuser section around the rim to slow the air down and
recover the pressure. With an Axial compressor, the work is done by banks
of compressor rotor (The turning part) and stator (Fixed to the outer case)
pairs. The more you can accelerate the air, the better - but things get
strange as you approach and exceed the speed of sound in that environment.
(The speed of sound in air depends on temperature - that will get important
later on).
One of the big advantages of Axial Flow compressors is that you can stack
them up, or cascade them. So that Stage 1 starts compressing the air,
Stage 2 compresses Stage 1's air a bit more, & so on.
One of the disadvantages is that you don't want to have too many stages -
the engine gets long and heavy, it takes a lot of power to turn the
compressor, and the requirement for precision-machined parts made out of
fairly exotic metals goes way up. So, the more compression you can get
from a single stage, the better.
The problem is, when you start getting pressure ratios of about 1.0 from a
single axial stage, the speed of the compressed air starts getting to be
supersonic. Supersonic air behaves a lot differently than subsonic air -
flow chokes in narrow passages, diffusers work differently, all sorts of
stuff.
In the U.S., the original work on supersonic compressors started in the late
1940s and early 1950s, with the NACA working with Westinghouse to dope it
out. In the Soviet Union, of course, the efforts were primarily by TsAGI.
both efforts led to usable compressors with supersonic stages by about
1953-54 or so. They're pretty much universal today.
Early engines with supersonic stages were a handful - The speed of sound
depends on temperature. As you compress the air, the temperature goes up,
and so does the speed of sound. At some point in the compressor cascade,
you'll have a stage where the airflow transitions from supersonic to
subsonic, with very different behavior. The problem is, that point is
always moving, due to external conditions like outside air temperature,
airspeed (Ram compression in the inlet plays a significant part about 'bout
Mach 0.7) and all that fun stuff.
So, in the mid '50s, you could build quite compact and efficient engines
(Sounds funny when referring to an AL-7 or a J75, but compared to what they
would have been without the supersonic stages, it's true), but they were
very sensitive to disturbed airflow, and changing engine and flight
conditions.
Nowadays, they're pretty much taken for granted.
<maury.markowitz@xxxxxxxxx> wrote in message
news:1150213931.322668.108950@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have seen the claim in several places that the Lyulka AL-7 has a
supersonic section for at least the first stage of its 9-stage
compressor. Is this really true?
The reason I ask is that I once received a lengthy e-mail suggesting
why it was so difficult to build such a thing (lots of stalls) and that
no one had done this before.
If it is true, are there other engines that also feature a supersonic
stage?
Maury
p.s. e-mail replies appreciated
--
Pete Stickney
Without data, all you have is an opinion
.
- References:
- Does the Lyulka AL-7 really have a supersonic compressor?
- From: maury . markowitz
- Re: Does the Lyulka AL-7 really have a supersonic compressor?
- From: Nele VII
- Does the Lyulka AL-7 really have a supersonic compressor?
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