Intake momentum drag
Guest
Posts: n/a
Intake momentum drag
Hello everybody,
while going through my performance notes, I came across something that is not very clear > A JET A/C ON TAKE-OFF RUN SUFFERS FROM SLIGHT REDUCTION IN THRUST DUE TO INTAKE MOMENTUM DRAG
I can't figure out how it can be, is it the drag caused by the intakes of the engines that cause turbulent airflow and therefore a less efficient compression/combustion/detent ?????
Any help will be well appreciated ! - SF
while going through my performance notes, I came across something that is not very clear > A JET A/C ON TAKE-OFF RUN SUFFERS FROM SLIGHT REDUCTION IN THRUST DUE TO INTAKE MOMENTUM DRAG
I can't figure out how it can be, is it the drag caused by the intakes of the engines that cause turbulent airflow and therefore a less efficient compression/combustion/detent ?????
Any help will be well appreciated ! - SF
Guest
Posts: n/a
Erm..as I understand it intake momentum drag is also sometimes referred to as Ram Drag.
Consider a typical airliner jet intake, in a wind tunnel.
Immediately behind the intake 'lip' the intake forms a divergent passage ahead of the fan (i.e. it gets wider). This serves to decrease the speed of the incoming airflow and (by Bernoulli) increase the static pressure ahead of the fan, improving the compression performance of the compressor.
Or, if the aircraft were moving in still air, the airflow into the engine is actually accelerated by the divergent intake from stationary to produce the same effect (it is still 'slowed down' relative to the aeroplane's movement).
Broadly speaking, the change in momentum of the air into the engine due to the shape of the intake has to be due to a force applied to it. This force is what is apparent as a 'drag' on the engine.
This increase in static pressure ahead of the fan as the aircraft gains forward speed is also the reason why engine take-off thrust, if set using EPR (engine pressure ratio), must be set by a certain speed. Otherwise, the increasing pressure ahead of the engine (p2) means that actual numerical value of EPR will fall as the aircraft accelerates and this could lead to inaccurate EPR setting if left too late.
Not the clearest of explanations, I know, but I can't find my old Propulsion notes...
Feel free to correct/criticise/abuse y'all!
------------------
...proceeding below Decision Height with CAUTION...
Consider a typical airliner jet intake, in a wind tunnel.
Immediately behind the intake 'lip' the intake forms a divergent passage ahead of the fan (i.e. it gets wider). This serves to decrease the speed of the incoming airflow and (by Bernoulli) increase the static pressure ahead of the fan, improving the compression performance of the compressor.
Or, if the aircraft were moving in still air, the airflow into the engine is actually accelerated by the divergent intake from stationary to produce the same effect (it is still 'slowed down' relative to the aeroplane's movement).
Broadly speaking, the change in momentum of the air into the engine due to the shape of the intake has to be due to a force applied to it. This force is what is apparent as a 'drag' on the engine.
This increase in static pressure ahead of the fan as the aircraft gains forward speed is also the reason why engine take-off thrust, if set using EPR (engine pressure ratio), must be set by a certain speed. Otherwise, the increasing pressure ahead of the engine (p2) means that actual numerical value of EPR will fall as the aircraft accelerates and this could lead to inaccurate EPR setting if left too late.
Not the clearest of explanations, I know, but I can't find my old Propulsion notes...
Feel free to correct/criticise/abuse y'all!
------------------
...proceeding below Decision Height with CAUTION...
Guest
Posts: n/a
This question (IMD) crops up every so often.
To save you digging back this is what I said the last time and nobody flamed me for it then:
Warning! A physical description of intake momentum drag using only words and no diagrams is not for the fainthearted
All jet engines have intake momentum drag (IMD) because the air as it enters the intake is decelerated violently and redirected by the compressor into a radial flow. If you are an old centrifugal compressor guy (ah! ..Vampires/Meteors… then this is quite easy to visualise because the rotor on those compressors blocked the whole intake and the air had no option but to move out radially on its journey towards the combustion chambers.
With todays axial compressors it seems as if the air could just wiggle largely straight on through. In fact the first stage rotors are designed to grab the incoming air and deliver it at high radial velocity towards the waiting first row stators that act like a brick wall to this largely radial flow. This stator induced reduction in velocity results in a sudden pressure rise (Bernoulli). This now higher pressure but slower air slides off the stators into the path of the next row of rotors which add speed to it again only for the second row stators to “stop” it again. The process is repeated for as many stages as you use, upping the pressure each time.
The overall experience of air as it passes through a jet engine creates some drag forces (IMD being but one) and some thrust forces. The net thrust is just that. The sum of the thrust bits minus the sum of the drag bits.
Any help?
JF
To save you digging back this is what I said the last time and nobody flamed me for it then:
Warning! A physical description of intake momentum drag using only words and no diagrams is not for the fainthearted
All jet engines have intake momentum drag (IMD) because the air as it enters the intake is decelerated violently and redirected by the compressor into a radial flow. If you are an old centrifugal compressor guy (ah! ..Vampires/Meteors… then this is quite easy to visualise because the rotor on those compressors blocked the whole intake and the air had no option but to move out radially on its journey towards the combustion chambers.
With todays axial compressors it seems as if the air could just wiggle largely straight on through. In fact the first stage rotors are designed to grab the incoming air and deliver it at high radial velocity towards the waiting first row stators that act like a brick wall to this largely radial flow. This stator induced reduction in velocity results in a sudden pressure rise (Bernoulli). This now higher pressure but slower air slides off the stators into the path of the next row of rotors which add speed to it again only for the second row stators to “stop” it again. The process is repeated for as many stages as you use, upping the pressure each time.
The overall experience of air as it passes through a jet engine creates some drag forces (IMD being but one) and some thrust forces. The net thrust is just that. The sum of the thrust bits minus the sum of the drag bits.
Any help?
JF
Guest
Posts: n/a
Erm......may I suggest 'TINSTAAFL' (not that I'm biased in way, shape or form 
).
You don't get something for nothing.
There is no such thing as a free lunch.
There is always a cost in doing work.
Pressurising the airstream must have a cost. Screwing around with its pressure/density/temp. by playing around with the duct(s) through which it flows is going to result in an increased amount of drag.
I realise that what I've said is more of an idealogical statement than a 'proof'.
Think of it this way: Any change to a molecule's momentum ie speed/direction/mass requires an addition of energy. The only energy being added comes from the fuel being burnt.
This in turn is seen by a higher fuel flow for a given amount of thrust, or a lower thrust for a given amount of fuel, when compared to a situation where there is (for the sake of argument) no such thing as loss of energy due to a change in momentum.
Bit philosophical I know, but in accord with Newton et al...
--------
Now all I need is some smart arse relativitist or quantum theorist to come along...
[This message has been edited by Tinstaafl (edited 25 May 2001).]
).You don't get something for nothing.
There is no such thing as a free lunch.
There is always a cost in doing work.
Pressurising the airstream must have a cost. Screwing around with its pressure/density/temp. by playing around with the duct(s) through which it flows is going to result in an increased amount of drag.
I realise that what I've said is more of an idealogical statement than a 'proof'.
Think of it this way: Any change to a molecule's momentum ie speed/direction/mass requires an addition of energy. The only energy being added comes from the fuel being burnt.
This in turn is seen by a higher fuel flow for a given amount of thrust, or a lower thrust for a given amount of fuel, when compared to a situation where there is (for the sake of argument) no such thing as loss of energy due to a change in momentum.
Bit philosophical I know, but in accord with Newton et al...
--------
Now all I need is some smart arse relativitist or quantum theorist to come along...
[This message has been edited by Tinstaafl (edited 25 May 2001).]
Guest
Posts: n/a
Hey cool, thank you all for your input, it is a little bit clearer , however these (bloody) performance notes say that the Intake momentum drag is overcome by RAM effect as Jet A/C accelerates, thus the engine gaining back its performance ! so where has this Momentum drag gone ? why is the performance of the jet engine better ? Is there something obvious I missed ? SF
Guest
Posts: n/a
Superfly
Now I think we may be getting somewhere.
As I mentioned IMD is only one of many forces that act on a jet engine as a result of the air’s journey through the donk. A lot of these forces are very dependent on RPM and forward speed. IMD certainly gets bigger as RPM increases and also as the mass flow of the air approaching the first stage increases. But other factors can change whether the overall thrust of the engine drops or increases like the degree of “free compression” inside the intake. (I think if you ask WOK he would probably say that about 50% of his thrust in the cruise comes from the intake effects and not just the fuel burned)
So in direct response to you last post, may I suggest IMD has not gone anywhere, but other things have come into play/happened and the overall engine thrust/efficiency has increased because of these other factors.
JF
Now I think we may be getting somewhere.
As I mentioned IMD is only one of many forces that act on a jet engine as a result of the air’s journey through the donk. A lot of these forces are very dependent on RPM and forward speed. IMD certainly gets bigger as RPM increases and also as the mass flow of the air approaching the first stage increases. But other factors can change whether the overall thrust of the engine drops or increases like the degree of “free compression” inside the intake. (I think if you ask WOK he would probably say that about 50% of his thrust in the cruise comes from the intake effects and not just the fuel burned)
So in direct response to you last post, may I suggest IMD has not gone anywhere, but other things have come into play/happened and the overall engine thrust/efficiency has increased because of these other factors.
JF
Guest
Posts: n/a
Hi L337
I am not confident to assert anything about supersonics as my background is in low speed devices where IMD happened to be a critical directional stability issue, hence my interest in it. That said, re Concorde in the cruise, I too believe the supersonic outside air is decelerated through shocks by the intake to become subsonic by the time it gets to the rotating kit (much compression ensuing from this large drop in V thanks to Mr B)
So, I don’t see that the IMD component would be different from any high subsonic engine case, depending as it does on the force needed to deflect the mV of the airflow through 90 deg as it gets to the fan.
Cheers
JF
PS On a personal note do you feel threatened by the increasing use of composites?
JF
I am not confident to assert anything about supersonics as my background is in low speed devices where IMD happened to be a critical directional stability issue, hence my interest in it. That said, re Concorde in the cruise, I too believe the supersonic outside air is decelerated through shocks by the intake to become subsonic by the time it gets to the rotating kit (much compression ensuing from this large drop in V thanks to Mr B)
So, I don’t see that the IMD component would be different from any high subsonic engine case, depending as it does on the force needed to deflect the mV of the airflow through 90 deg as it gets to the fan.
Cheers
JF
PS On a personal note do you feel threatened by the increasing use of composites?
JF
Guest
Posts: n/a
I thought that I understood intake momentum drag and even the Concorde intake geometry.
But quite how 70% of the 'thrust' of the SR71 came from the intakes and only 30% from the engines themselves still baffles me. Anyone have a simple explanation??
But quite how 70% of the 'thrust' of the SR71 came from the intakes and only 30% from the engines themselves still baffles me. Anyone have a simple explanation??
Guest
Posts: n/a
JF
As you rightly say, on Concorde, at speeds above M1.3, variable position ramps, situated inside the engine intakes become active and are used to regulate the quantity and speed of the intake air arriving at the compressor. They do this by focusing a series of shock waves on the lower lip of the engine intake, through which the (supersonic) intake air is forced to travel, being decelerated and compressed during this process, before arriving at the compressor face at about M0.5.
Likewise, at the exhaust end, the exhaust gas has to be accelerated to very high speed to produce the required thrust, which is done by a system of primary and secondary nozzles forming an efficient convergent/divergent nozzle system.
One significant and rather unusual difference to bear in mind when considering supersonic airflow is that a convergent nozzle will slow and compress a supersonic airflow whereas a divergent nozzle will slow and compress a subsonic airflow.
The design of the intake and exhaust systems of a supersonic engine is critical, and plays a major part in the efficiency of the engine. Due to the efficiency of the design of the intake and exhaust systems on Concorde, at M2.0, roughly 25% of total thrust is produced by the intake system, with another 25% being produced by the exhaust system.
This leaves the core engine to produce only 50% of the total thrust required in cruise, well within its capabilities without requiring the use of reheat (with its attendant high fuel flow) and enables flight at M2.0 at sustainable fuel flow rates.
As you rightly say, on Concorde, at speeds above M1.3, variable position ramps, situated inside the engine intakes become active and are used to regulate the quantity and speed of the intake air arriving at the compressor. They do this by focusing a series of shock waves on the lower lip of the engine intake, through which the (supersonic) intake air is forced to travel, being decelerated and compressed during this process, before arriving at the compressor face at about M0.5.
Likewise, at the exhaust end, the exhaust gas has to be accelerated to very high speed to produce the required thrust, which is done by a system of primary and secondary nozzles forming an efficient convergent/divergent nozzle system.
One significant and rather unusual difference to bear in mind when considering supersonic airflow is that a convergent nozzle will slow and compress a supersonic airflow whereas a divergent nozzle will slow and compress a subsonic airflow.
The design of the intake and exhaust systems of a supersonic engine is critical, and plays a major part in the efficiency of the engine. Due to the efficiency of the design of the intake and exhaust systems on Concorde, at M2.0, roughly 25% of total thrust is produced by the intake system, with another 25% being produced by the exhaust system.
This leaves the core engine to produce only 50% of the total thrust required in cruise, well within its capabilities without requiring the use of reheat (with its attendant high fuel flow) and enables flight at M2.0 at sustainable fuel flow rates.
Guest
Posts: n/a
Hi Beagle
My copy of AP129 AL5 Dec55 (and even AP3456 at 2-1-3-1 Page 4) breaks down the forces produced inside a turbojet into thrust and drag elements. Both references show the compressor being the major engine contributor to thrust with only small amounts coming from the combustion cans and the diffuser.
I therefore reason that the amount of compression produced by one of Sir Frank’s lovelies is closely related to the thrust of same. Now add an intake on to the front of the donk and it seems reasonable to apportion the thrust “produced by the intake” and the thrust “produced by the engine” according to the amount of compression produced by each.
As to the numbers relevant to Concorde and Blackbird, I well remember Brian Trubshaw quoting “a bit more than half” the thrust in the cruise comes from the intake. Increase the speed of the aeroplane from M=2 to M=3.2 and that gives the intake a chance to really play the star role.
You might be interested in this quote from Paul Crickmore’s comprehensive article on the A12/YF-12/SR71 story as published in Wings of Fame Vol 8.
“The inlet system created internal pressures which reached 18 psi when operating at M 3.2 and 80,000ft, where the ambient pressure is only 0.4psi. This extremely large pressure differential led to a forward thrust vector which resulted in the forward inlet producing 54 per cent of the total thrust. A further 29 per cent was produced by the ejector, while the J58 engine contributed only 17 per cent of the total thrust”
Not quite the same numbers as you were quoting but yours would likely be correct when going a tad slower.
Regards
JF
My copy of AP129 AL5 Dec55 (and even AP3456 at 2-1-3-1 Page 4) breaks down the forces produced inside a turbojet into thrust and drag elements. Both references show the compressor being the major engine contributor to thrust with only small amounts coming from the combustion cans and the diffuser.
I therefore reason that the amount of compression produced by one of Sir Frank’s lovelies is closely related to the thrust of same. Now add an intake on to the front of the donk and it seems reasonable to apportion the thrust “produced by the intake” and the thrust “produced by the engine” according to the amount of compression produced by each.
As to the numbers relevant to Concorde and Blackbird, I well remember Brian Trubshaw quoting “a bit more than half” the thrust in the cruise comes from the intake. Increase the speed of the aeroplane from M=2 to M=3.2 and that gives the intake a chance to really play the star role.
You might be interested in this quote from Paul Crickmore’s comprehensive article on the A12/YF-12/SR71 story as published in Wings of Fame Vol 8.
“The inlet system created internal pressures which reached 18 psi when operating at M 3.2 and 80,000ft, where the ambient pressure is only 0.4psi. This extremely large pressure differential led to a forward thrust vector which resulted in the forward inlet producing 54 per cent of the total thrust. A further 29 per cent was produced by the ejector, while the J58 engine contributed only 17 per cent of the total thrust”
Not quite the same numbers as you were quoting but yours would likely be correct when going a tad slower.
Regards
JF
Guest
Posts: n/a
Thanks Bellerophon
I saw yours after I wrote mine to Beagle.
I think that between us we may have covered it for him. Only time will tell..........
I much appreciate your reply
JF
edited for sp. again
[This message has been edited by John Farley (edited 26 May 2001).]
I saw yours after I wrote mine to Beagle.
I think that between us we may have covered it for him. Only time will tell..........
I much appreciate your reply
JF
edited for sp. again
[This message has been edited by John Farley (edited 26 May 2001).]
Guest
Posts: n/a
JF and Bellerophon - many thanks for your replies! My memory of the SR71 figures was indeed incorrect - gleaned from an article 25 years ago. The figures I now have state that at M3.2, over 80% comes from the inlet - consistent with JF's figures. I vaguely remember that you have to 'integrate the total pressure envelope around the intake, engine and nozzle' to come up with that result. However, it still seems like black magic!
Amazing to think that aircraft such as the SR, Concorde (and who knows what else) have been doing this sort of thing for well over a quarter of a century now!
[This message has been edited by BEagle (edited 26 May 2001).]
Amazing to think that aircraft such as the SR, Concorde (and who knows what else) have been doing this sort of thing for well over a quarter of a century now!
[This message has been edited by BEagle (edited 26 May 2001).]
Guest
Posts: n/a
In the same way with a turbocharged engine, where with the turbo producing the majority of the power, you can shut down the reciprocating element and cruise on the turbo alone? I think not!
Doesn't the J58 merely enable the aircraft to achieve operation in a regime where the inlet magic and bypass ramjet elements start to work - just as a turbocharger uses the gas generation of the reciprocating engine to start its work when 'starting conditions' are met? Poor analogy, I guess - in fact probably anal-ology!!. But wasn't the Napier Nomad supposed to do something really clever and achieve excellent SFC by using a compound reciprocating/turbine system? We had one at my university - a great big lump of metal with a fiendishly complicated design which was rendered obsolete by far simpler turbojet and turboprop engines!
Doesn't the J58 merely enable the aircraft to achieve operation in a regime where the inlet magic and bypass ramjet elements start to work - just as a turbocharger uses the gas generation of the reciprocating engine to start its work when 'starting conditions' are met? Poor analogy, I guess - in fact probably anal-ology!!. But wasn't the Napier Nomad supposed to do something really clever and achieve excellent SFC by using a compound reciprocating/turbine system? We had one at my university - a great big lump of metal with a fiendishly complicated design which was rendered obsolete by far simpler turbojet and turboprop engines!
Guest
Posts: n/a
I've always thought of those turbo compound radials as the 'missing link' between piston engines & turbine.
Sort of a jet engine with a high frequency, intermittent ignition & low efficiency turbine.
If they could've left the valves open, kept the flame lit & improved the efficiency of the power recovery turbine it would pretty much be a turbo-prop.
---------
The Super Constellation: The best 3 engine aircraft Qantas ever operated.
Sort of a jet engine with a high frequency, intermittent ignition & low efficiency turbine.
If they could've left the valves open, kept the flame lit & improved the efficiency of the power recovery turbine it would pretty much be a turbo-prop.
---------
The Super Constellation: The best 3 engine aircraft Qantas ever operated.
doesn't look like a wink at all.
