Blackbird's thrust question
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TurbineD
I received the following via email from Dr. Bob Abernethy this morning....
I received the following via email from Dr. Bob Abernethy this morning....
Bill,
At cruise conditions, Mach 3.2, about 80% of the air goes through the engine and 20% bypasses from the fourth stage into the afterburner thru 6 big pipes around the engine. The bypass air is in effect a partial ram jet and produces thrust. However at this condition the inlet produces most of the thrust and this inlet thrust is significantly increased by the bypass air. If all the air bypassed the engine it would be a pure ramjet.
My patent is dated October 1958 so I am amazed that the A12 Blackbird still holds all the speed and altitude records.
At cruise conditions, Mach 3.2, about 80% of the air goes through the engine and 20% bypasses from the fourth stage into the afterburner thru 6 big pipes around the engine. The bypass air is in effect a partial ram jet and produces thrust. However at this condition the inlet produces most of the thrust and this inlet thrust is significantly increased by the bypass air. If all the air bypassed the engine it would be a pure ramjet.
My patent is dated October 1958 so I am amazed that the A12 Blackbird still holds all the speed and altitude records.
You say toe-may-toe, I say toe-mah-toe
Very good technical thread despite the ad hominid attacks and such, unless they are directed to this old fighter pilot, heh heh. And I think we all learned a lot.
No doubt that the J-58 work helped in the design and performance of the F100 that powered my beloved Viper ten years later. So TNX to Dr Abernathy.
I wonder if we should call that sucker ( the F100) a "turbo-ramjet", as it bypassed a gob of air from the third comperssor stage via an annular duct back to the exhaust/burner ( versus 4th stage in the J-58). Certainly helped the overall thrust when in mil or burner power. Also allowed us a higher Mach than you would expect from a fixed inlet.
One pearl of wisdom from Dr Abernathy's article ( http://www.bobabernethy.com/pdfs/Nev...0of%20P&W3.pdf) has to do with the compressor blade flutter and deformation. So I lost a friend who liked to "run it out" after completing a test hop. Sure enough, one day he got a bit fast (I figure 800 knots indicated or so) and one of the compressor disks went boom and he didn't survive the ejection. Think it was the 4th stage one, not one of the fan ones up front.
All in all, one of the best technical threads I have seen.
No doubt that the J-58 work helped in the design and performance of the F100 that powered my beloved Viper ten years later. So TNX to Dr Abernathy.
I wonder if we should call that sucker ( the F100) a "turbo-ramjet", as it bypassed a gob of air from the third comperssor stage via an annular duct back to the exhaust/burner ( versus 4th stage in the J-58). Certainly helped the overall thrust when in mil or burner power. Also allowed us a higher Mach than you would expect from a fixed inlet.
One pearl of wisdom from Dr Abernathy's article ( http://www.bobabernethy.com/pdfs/Nev...0of%20P&W3.pdf) has to do with the compressor blade flutter and deformation. So I lost a friend who liked to "run it out" after completing a test hop. Sure enough, one day he got a bit fast (I figure 800 knots indicated or so) and one of the compressor disks went boom and he didn't survive the ejection. Think it was the 4th stage one, not one of the fan ones up front.
All in all, one of the best technical threads I have seen.
Last edited by gums; 5th Feb 2013 at 21:36.
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hi gums....
I have seen a working sectioned F-100 turning (under electric power) on a stand. It is a marvel.
Abernethy references an annular duct option for the J58, but concludes the cross section allows too much loss of pressure prior to entry into the AB. So he chose the "six big pipes".
I still regret taking such an energetic position v/v "Partial Ramjet"....
I have seen a working sectioned F-100 turning (under electric power) on a stand. It is a marvel.
Abernethy references an annular duct option for the J58, but concludes the cross section allows too much loss of pressure prior to entry into the AB. So he chose the "six big pipes".
I still regret taking such an energetic position v/v "Partial Ramjet"....
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about 80% of the air goes through the engine and 20% bypasses from the fourth stage into the afterburner
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Brian
If 80% goes through the engine (core), and the rest, 20% the bleeds, what is the percentage of air that bypasses the engine internals by transiting the area between the engine and case (nacelle)? Prior to the Engine intake?
If 80% goes through the engine (core), and the rest, 20% the bleeds, what is the percentage of air that bypasses the engine internals by transiting the area between the engine and case (nacelle)? Prior to the Engine intake?
Last edited by Lyman; 6th Feb 2013 at 13:36.
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Lyman,
Be careful with the basis of your percentages! 80% of the ENGINE flow goes through the core and 20% of the ENGINE flow through the bleed that feed back to the afterburner. The other bits (shock trap bleed, porous spike bleed etc) should/must be related to INTAKE flow
Be careful with the basis of your percentages! 80% of the ENGINE flow goes through the core and 20% of the ENGINE flow through the bleed that feed back to the afterburner. The other bits (shock trap bleed, porous spike bleed etc) should/must be related to INTAKE flow
Last edited by CliveL; 6th Feb 2013 at 14:14.
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Hi CliveL. Yes, i thought of that. Here is my problem. The Engine is a "recovery" bleed powerplant. That means that a portion of Engine intake is only temporarily removed, and later re-introduced. So far so good.
How is an accounting of the gaspath accomplished after the reintroduction? The gaspath is still entirely within the confines of the engine Core.
I think Brian has touched on it, but I am interested in the complete inventory of the power profile, by reference to Thrust, not mass. This is the fundamental area I have been trying (badly) to address, and the distillation of what two eminent people claim relative to this complex and amazing engine....
You claim that if related to Bleed recovery, there can be no claim of RamJet.
The inventor claims the opposite, that his Recovery Bleed system is a "Partial Ramjet".
On a technical forum, I woud expect explicit agreement. "Because I say so", has never been a satisfactory reply. I also do not understand the passion relative to this very very minor point.
Thanks for your reply, I remain a BIG FAN.......
How is an accounting of the gaspath accomplished after the reintroduction? The gaspath is still entirely within the confines of the engine Core.
I think Brian has touched on it, but I am interested in the complete inventory of the power profile, by reference to Thrust, not mass. This is the fundamental area I have been trying (badly) to address, and the distillation of what two eminent people claim relative to this complex and amazing engine....
You claim that if related to Bleed recovery, there can be no claim of RamJet.
The inventor claims the opposite, that his Recovery Bleed system is a "Partial Ramjet".
On a technical forum, I woud expect explicit agreement. "Because I say so", has never been a satisfactory reply. I also do not understand the passion relative to this very very minor point.
Thanks for your reply, I remain a BIG FAN.......
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Bill
You are mixing things up again! My earlier remarks were specifically directed at the flow that starts at the shock trap bleed and passes over the outside of the engine as cooling air. This is not in any way a ramjet.
The inventor's remarks relate to the internal bleed off the compressor returned to the engine aft of the turbine. Whether or not this can be regarded as a partial ramjet has been thrashed to death already in this thread.
You claim that if related to Bleed recovery, there can be no claim of RamJet.
The inventor claims the opposite, that his Recovery Bleed system is a "Partial Ramjet".
The inventor claims the opposite, that his Recovery Bleed system is a "Partial Ramjet".
The inventor's remarks relate to the internal bleed off the compressor returned to the engine aft of the turbine. Whether or not this can be regarded as a partial ramjet has been thrashed to death already in this thread.
Last edited by CliveL; 6th Feb 2013 at 14:50.
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Clive,
I've gone through your flow explanation and appreciate it.
I'd like to add, ref
From, for example, "Fast Jets - the history of reheat development at Derby" by Cyril Elliott "... in order to keep the mass flow constant, the area of the nozzle must increase with the sqrt of the jet temperature." (Since gas specific volume has increased significantly).
This book incidentally probably holds the world record for foldout size, 66" for the Adour reheat fuel system schematic.
I've gone through your flow explanation and appreciate it.
I'd like to add, ref
If I have it right, when you light up the afterburner keeping the primary jet exit area the same...
This book incidentally probably holds the world record for foldout size, 66" for the Adour reheat fuel system schematic.
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Hello Lyman,
Don't know if I can help but for an attempt can you be a bit more explicit?
Do you mean, for example, why/how do we get an increase in thrust as a result of the bleed flow becoming available at the afterburner?
Or maybe something else?
Don't know if I can help but for an attempt can you be a bit more explicit?
How is an accounting of the gaspath accomplished after the reintroduction?
Or maybe something else?
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Clive,
I've just noticed a Pj/Ps of 0.29 for the SR-71 at M3.2 (Peter Law's presentations on AEHS website). Can that tell us anything by itself on how the SR71 installation compared to Concorde?
I've just noticed a Pj/Ps of 0.29 for the SR-71 at M3.2 (Peter Law's presentations on AEHS website). Can that tell us anything by itself on how the SR71 installation compared to Concorde?
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Peter
Not by itself Peter, because as you point out the primary jet pipe area will be increased when you light up the afterburner and that will 'squeeze' the secondary flow. Concorde of course operated 'dry' in cruise. That same website (I think) gives the cooling air temperature at the nozzle and the primary jet nozzle pressure and temperature, so if one knew the fully open jet pipe area (can anyone help?) it should be possible to make some sort of comparison.
As it stands Ps/Pj is higher than a typical Concorde value (0.25) so there should be more cooling airflow, but this will be offset by that squeezing effect.
PS I left out the effect of increased Aj in my earlier explanation as I thought it would complicate things
- that's was why I was careful to specify primary jet area unchanged. I'm guessing that if Abernethy says cooling flow was increased when one lit afterburner than the temperature effect would outweigh the area change ....
I've just noticed a Pj/Ps [Ps/Pj?] of 0.29 for the SR-71 at M3.2 (Peter Law's presentations on AEHS website). Can that tell us anything by itself on how the SR71 installation compared to Concorde?
As it stands Ps/Pj is higher than a typical Concorde value (0.25) so there should be more cooling airflow, but this will be offset by that squeezing effect.
PS I left out the effect of increased Aj in my earlier explanation as I thought it would complicate things
- that's was why I was careful to specify primary jet area unchanged. I'm guessing that if Abernethy says cooling flow was increased when one lit afterburner than the temperature effect would outweigh the area change ....
Last edited by CliveL; 7th Feb 2013 at 07:24.
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New question
The increased airflow really helped Kelly’s inlet performance.
eg would it have been from upsizing the intake to handle the +22% engine flow and hence a greater area of rear-facing surfaces?
Recover Bleed Air Benefits Bleeds Open to Bleeds Closed
• Airflow Increase +22%
• Net Thrust Increase +19%
• Installed Thrust Increase +47%
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Where would the intake contribution to the +47% come from? ie not the fundamentals of where intake thrust comes from, but specifically what would have caused the change.
eg would it have been from upsizing the intake to handle the +22% engine flow and hence a greater area of rear-facing surfaces?
eg would it have been from upsizing the intake to handle the +22% engine flow and hence a greater area of rear-facing surfaces?
AS A GUESS, from the fact that it was an installed thrust improvement, it could have been that at Mach 3.0 the intake was actually too big for the engine without bleeds, so that they may have been spilling air through the forward bypass doors (to maintain the normal shock in its correct location). This air was spilled out sideways so no thrust recovery - just momentum drag. This could have been the case if they originally sized the intake assuming the engine would swallow more than it actually would without those bleeds or because the intake was actually sized by some other design case and they had to compromise a little bit. With the increased engine mass flow possible with the returned bleed open, they could keep the forward bypass doors closed.
One more question to add to the list .....
Last edited by CliveL; 8th Feb 2013 at 21:07. Reason: Change of explanation
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Clive/ Brian,
This is probably too-simplistic a viewpoint to have any merit for such a complex subject, but have to ask.
A lot of interest is always shown in the thrust contribution from the intake at high speeds. For a given installation it goes up with speed, eg for the SR-71 at M2 13%, at M3 54%.
If we now look at Concorde at M2 it's even higher at 63% than the SR-71 at M3.
Both installations were state-of-the-art for their respective design points.
Whilst I understand the actual numbers defining all the flows/thrusts, etc were very different I'm clinging to the idea that a comparison of different installations is perhaps valid using these percentages as they are ratios (non-dimensional).
Question. Is there some fundamental (simple?) reason why Concorde's intake thrust contribution is somewhat higher at a significantly lower speed?
Thanks.
This is probably too-simplistic a viewpoint to have any merit for such a complex subject, but have to ask.
A lot of interest is always shown in the thrust contribution from the intake at high speeds. For a given installation it goes up with speed, eg for the SR-71 at M2 13%, at M3 54%.
If we now look at Concorde at M2 it's even higher at 63% than the SR-71 at M3.
Both installations were state-of-the-art for their respective design points.
Whilst I understand the actual numbers defining all the flows/thrusts, etc were very different I'm clinging to the idea that a comparison of different installations is perhaps valid using these percentages as they are ratios (non-dimensional).
Question. Is there some fundamental (simple?) reason why Concorde's intake thrust contribution is somewhat higher at a significantly lower speed?
Thanks.
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Is there some fundamental (simple?) reason why Concorde's intake thrust contribution is somewhat higher at a significantly lower speed?
Because it was optimized for M2, and Blackbird for M3.