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How supersonic inlets work

 
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pkent



Joined: 26 Feb 2013
Posts: 10

PostPosted: Sat Nov 24, 2018 11:15    Post subject: How supersonic inlets work Reply with quote

With reference to the presentation "How supersonic inlets work"
I have read it again looking for an answer to the question below.

On page 4 of Bob Abernethy's presentation
ref http://www.roadrunnersinternationale.com/pw_tales.htm
he says that the 22% increase in engine airflow that came with his compressor bleed invention "really helped Kelly's inlet performance".

Q. In what way did the increased engine flow help the inlet?

Thank you.
Pete
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wallan



Joined: 13 Jul 2003
Posts: 234
Location: UK

PostPosted: Sun Nov 25, 2018 10:13    Post subject: Reply with quote

I would suggest Pete Law would be able to help.
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admin02



Joined: 06 Aug 2003
Posts: 4

PostPosted: Tue Nov 27, 2018 10:08    Post subject: Re: How supersonic inlets work Reply with quote

pkent wrote:
Q. In what way did the increased engine flow help the inlet?


More air through the engine meant more air through the inlet and a corresponding increase in the inlet's thrust contribution.
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pkent



Joined: 26 Feb 2013
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PostPosted: Tue Dec 04, 2018 10:59    Post subject: Re: How supersonic inlets work Reply with quote

Thanks for replying. I don't understand though. I thought the thrust came from the pressure in the inlet so unless increased flow improves the pressure recovery there's no increase in pressure and thrust.

I was thinking it might be more along the lines of a better match for the inlet so less excess air to be accommodated and so less drag.

Q. Do you have a more detailed explanation for what happens?
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admin02



Joined: 06 Aug 2003
Posts: 4

PostPosted: Tue Dec 04, 2018 15:32    Post subject: Re: How supersonic inlets work Reply with quote

pkent wrote:
Q. Do you have a more detailed explanation for what happens?


Please see http://www.enginehistory.org/members/Convention/2014/Presentations/SR-71InletDesign.pdf

It is the best explanation I've ever seen.
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dpennings



Joined: 10 Dec 2016
Posts: 8

PostPosted: Mon Dec 10, 2018 15:31    Post subject: Reply with quote

as far as I understood, the increased mass flow helps against surging. This phenomen accours when the air velocity is to low compared to the blade velocity, it will cause a stall on the blades just like a stall on wings of an airplane.

Bleeding increases the mass flow and hence the velocity of the entering gas trough the first compressor stages. The energy of the bypass air flow ist not lost, because it gives trust. This mass flow also helped to cool the walls of the after burner by forming a layer on relative cool unburnt air along the wall.

I hope someone will understand my English Wink
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pkent



Joined: 26 Feb 2013
Posts: 10

PostPosted: Mon Dec 17, 2018 15:15    Post subject: Reply with quote

As in my first post, the statement I don't understand (despite reading the 'inlet design' article a few times) is "the increased engine airflow really helped the inlet performance".

Q. Very specifically. how was the inlet performance improved with higher airflow?

(Perhaps the inlet was designed for a higher airflow than the original engine was capable of supplying. just guessing.)
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kmccutcheon



Joined: 13 Jul 2003
Posts: 206
Location: Huntsville, Alabama USA

PostPosted: Thu Dec 20, 2018 10:18    Post subject: Reply with quote

pkent wrote:
(Perhaps the inlet was designed for a higher airflow than the original engine was capable of supplying. just guessing.)


I concur with your guess. I have carefully read Bob Abernathy’s account and also talked with Lockheed personnel who worked on the A-12 project. None of them worked on the program before the J58 was operational in the airframe, nor can they suggest anyone else to consult. During our conversations, I outlined what I think is a plausible scenario, and while they were unable to confirm it happened, they also could not argue that it didn’t. Here is what I think happened:

Pratt & Whitney Aircraft (P&WA) had committed to supply an engine (the J58) with a certain performance. Kelly Johnson’s team had designed the A-12’s engine installation, including the inlets, expecting this performance. P&WA’s computer modeling predicted the J58’s performance would fall short. If the J58 problems had not been cured, not only would the engine installation have to be redesigned to optimize the engine’s reduced performance, but aircraft performance would also fall short because a certain installed thrust was required to meet the aircraft performance goals. Bob Abernathy came up with a solution that allowed the engine to perform as promised. I interpret Abernathy’s presentation statement, "The increased airflow really helped Kelly's inlet performance" to mean that with the promised airflow, the inlet functioned as designed.
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Kimble D. McCutcheon
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pkent



Joined: 26 Feb 2013
Posts: 10

PostPosted: Sun Jan 13, 2019 15:32    Post subject: Reply with quote

A belated thank you Kimble. I have not checked the site for a few weeks. It was great that you know people with whom you can discuss your explanation.
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kmccutcheon



Joined: 13 Jul 2003
Posts: 206
Location: Huntsville, Alabama USA

PostPosted: Mon Jan 14, 2019 07:34    Post subject: Reply with quote

Glad to help.
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