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exhaust thrust

 
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aburhoe
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PostPosted: Wed Aug 05, 2009 10:31    Post subject: exhaust thrust Reply with quote

Hi all,

Iím looking for info regarding the amount of thrust obtained from the exhaust of unlimited racers, or any aircraft. It seems the phenomena is accepted to exist, but Iíve never seen any data that can correlate a specific mass of airflow through the engine with an amount of thrust.

Thanks!

Aaron
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wpearce



Joined: 08 Aug 2006
Posts: 37
Location: California, USA

PostPosted: Wed Aug 12, 2009 11:44    Post subject: Reply with quote

Hello Aaron,

Since no one else had responded I'll toss some info out.

I do not know the formula for calculation or how it is measured but, in "The Rolls-Royce Crecy" by Nahum, Foster-Pegg and Birch, it gives a comparison of the jet thrust from the Merlin vs the Crecy.

From the book, the Merlin is listed as 150 lbs of thrust which gave it 241 net hp from the exhaust. The Crecy had 350 lbs of thrust which gave it 865 net hp. These numbers are from straight exhaust, not with a turbo.

A racing Merlin uses larger stacks than a stock Merlin but also has 120+ even 130+ manifold pressure. I do know know what thrust is generated from a racing Merlin.

From "Reno for Gearheads", Graham White wrote: "It has even been argued that anything in excess of 100 inches does not produce any additional power to the propeller, however the aircraft will fly faster when running in excess of 100 inches. The argument presented is the fact jet thrust from the exhaust stacks is increased at the higher manifold pressure resulting in a higher air speed."

There is more out there, but I just can't remember where.
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Bill Pearce

www.oldmachinepress.com
www.bluethunderairracing.com
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aburhoe
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PostPosted: Thu Aug 13, 2009 19:41    Post subject: thrust Reply with quote

Hi Bill.

Everything that Iíve seen so far doesnít show calculation or measurement. Iím sure youíd be dying to do these types of experiments with your plane, right??

Thanks, Iíll pick up the book.

Aaron
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jschauer



Joined: 19 May 2004
Posts: 89
Location: Justin, Texas

PostPosted: Fri Aug 14, 2009 09:01    Post subject: Reply with quote

Somewhere in the NASA/NACA archives is a report on this, but I can't locate it. Perhaps one of you can remember how to get to the website?
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kmccutcheon



Joined: 13 Jul 2003
Posts: 200
Location: Huntsville, Alabama USA

PostPosted: Fri Aug 14, 2009 09:20    Post subject: Reply with quote

jschauer wrote:
Somewhere in the NASA/NACA archives is a report on this, but I can't locate it. Perhaps one of you can remember how to get to the website?


http://ntrs.nasa.gov/search.jsp

In the NACA collection, try searching for "exhaust thrust"
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Kimble D. McCutcheon
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wpearce



Joined: 08 Aug 2006
Posts: 37
Location: California, USA

PostPosted: Fri Aug 14, 2009 11:36    Post subject: Reply with quote

I remember seeing a few formulas but I can't recall where or if they really pertained to exhaust thrust. I'll see what I can scare up in the coming days.

As for the plane, it is not mine. I'm just a lowly crew-type-person but I appreciate you having a look. I'm not sure if it will be ready by Reno.
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Bill Pearce

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www.bluethunderairracing.com
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wpearce



Joined: 08 Aug 2006
Posts: 37
Location: California, USA

PostPosted: Fri Aug 14, 2009 11:46    Post subject: Reply with quote

I think I just found the NACA document...

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091844_1993091844.pdf
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Bill Pearce

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aburhoe
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PostPosted: Fri Aug 14, 2009 20:16    Post subject: whoo hoo!! Reply with quote

A gem of an article!

Thanks Bill, and all who responded.

Aaron
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