XF-84H Propulsion

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Basil

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Still one of the most interesting turboprop configuration because of the supersonic propeller in addition with afterburning trials - a well researched review:

 
From that report:

"But it would seem to be a poor tradeoff to give up 2036 horsepower in order to gain 1,357 pounds of jet thrust."

They actually cross over at about 490kt, above that the thrust becomes dominant.

HP_equiv_1357lb_thrust.png

If they were getting 80% efficiency out of the supersonic propellor, that would suggest that thrust could start to edge out the prop from the mid-400kt range. Since the fastest the XF-84H managed on the prop was about 450kt, the use of reheat to try to push it faster makes sense.
 
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Interesting calculation but I think the point where exhaust thrust becomes more dominant also changes with flight altitude. Rolls Royce had intersting charts for this when they developed the Crecy and compared exhaust thrust to shaft hp in regards to altitude.
I also wondered why the activation of the afterburner leads to such an amount of hp reduction. Perhaps additional backpressure downside of the turbine?
Btw, afaik the XF-84H was only a testbed for this propulsion system and not a planned production version of the F-84 line.
 
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My understanding is that the Navy had an interest in this program because of the ongoing concern by some that jet airplanes were not compatible with aircraft carriers. Another example of this concern was the funding of the Convair F2Y seaplane fighter.
 
It seems the turboprop version of the Convair B-60 would have used this type of staggered single-rotation transsonic / supersonic propellers driven by four Wright T35.
 

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Tailspin Turtle said:
My understanding is that the Navy had an interest in this program because of the ongoing concern by some that jet airplanes were not compatible with aircraft carriers. Another example of this concern was the funding of the Convair F2Y seaplane fighter.
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Until people figured out how to increase thrust quickly in a jet engine, the best way to add thrust now on approach to a carrier is a controllable pitch prop.
 
Basil said:
Interesting calculation but I think the point where exhaust thrust becomes more dominant also changes with flight altitude. Rolls Royce had intersting charts for this when they developed the Crecy and compared exhaust thrust to shaft hp in regards to altitude.
I also wondered why the activation of the afterburner leads to such an amount of hp reduction. Perhaps additional backpressure downside of the turbine?
Btw, afaik the XF-84H was only a testbed for this propulsion system and not a planned production version of the F-84 line.
Click to expand...
That puzzled me, too - until I remembered that scabbed-on "Partial Reheat" used on the last versions of the Brit Javelin interceptors. It was basically one step above a Field Expedient. To make it simple, instead of the Afterburner having it's own fuel pumps, it tapped fuel from the engine's combustor fuel pumps. This reduced fuel to the main gas generator, which meant that at lower altitudes, the core engine was starved of fuel, and produced less thrust. Until the jet had climbed to an altitude where the reduced fuel requirements meant that the fuel pumps had enough excess capacity to get 100% out of the core engine and supply the afterburner, there was a net loss of overall thrust.
Sounds like the same thing here.
 
P-STICKNEY said:
That puzzled me, too - until I remembered that scabbed-on "Partial Reheat" used on the last versions of the Brit Javelin interceptors. It was basically one step above a Field Expedient. To make it simple, instead of the Afterburner having it's own fuel pumps, it tapped fuel from the engine's combustor fuel pumps. This reduced fuel to the main gas generator, which meant that at lower altitudes, the core engine was starved of fuel, and produced less thrust. Until the jet had climbed to an altitude where the reduced fuel requirements meant that the fuel pumps had enough excess capacity to get 100% out of the core engine and supply the afterburner, there was a net loss of overall thrust.
Sounds like the same thing here.
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Yes, I also thought about the Javelin; could be the same reason. Btw this (Javelin afterburner) is such a typical British aviation thing - interesting ideas and concepts but not the money or not going the last mile to bring it to a successful end (like installing a larger or second fuel pump).
No offense to british engineering ;)
 
Basil said:
Yes, I also thought about the Javelin; could be the same reason. Btw this (Javelin afterburner) is such a typical British aviation thing - interesting ideas and concepts but not the money or not going the last mile to bring it to a successful end (like installing a larger or second fuel pump).
No offense to british engineering ;)
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The problem with British aviation engineering is that it is paid for by the British Treasury. And HMTreasury makes (insert ethnic stereotype of "never spends money" here) look like a wastrel!
 
Scott Kenny said:
The problem with British aviation engineering is that it is paid for by the British Treasury. And HMTreasury makes (insert ethnic stereotype of "never spends money" here) look like a wastrel!
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It was (is?) not only a thing in the British aviation industry but also in car industry, machinery industry, etc.; but we are straying from the topic.
 
Concur: My former small corner of UK Big_Pharma had an epic reluctance to spend money between 'flood-gate' purse openings.

The 'Not_Invented_Here' rule also applied: Unless the notion came from 'Off Site', either via a 'Development' lab or eg a Consultant, it was automatically beneath contempt.
Which meant local innovation had to be subtly incremental, sufficiently inexpensive and, above all else, stealthy...
Which usually meant me buying instrument connectivity makings from eg Maplin Electronics' catalogue, hand-wiring the needful...
{ Shakes head...}
 
Basil said:
It was (is?) not only a thing in the British aviation industry but also in car industry, machinery industry, etc.; but we are straying from the topic.
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I mean, when you're talking about aircraft development, some discussion of the fiscal environment is kinda necessary.
 

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