variable exaust nozzle
11-23-2005, 08:33 AM
#26
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RE: variable exaust nozzle
Hank,
I think we are arguing two separate things ! (as is usually the case)
Sure, a Con di nozzle is a device to accelerate a flow up to Mach =1 at the throat and then supersonically accelerate it further in the divergent section, thus realising all of the available temperature energy and turning it into kinetic energy, after all thats what a nozzle does.
A variable area nozzle is purely a device for matching the plume pressure to the ambient static pressure for maximum efficiency. i.e. fully expanded.
If you underexpand your nozzle you fail to convert all of your energy into momentum.
If you overexpand your nozzle the nozzle walls will experience a pressure force in the drag sense , because the pressure of the jet is below ambient pressure.
I believe that is the thrust ( ho ho ho ) of the original thread. So the real answer is no, to mount this kind of device on a model aircraft would be next to useless.
Nick
I think we are arguing two separate things ! (as is usually the case)
Sure, a Con di nozzle is a device to accelerate a flow up to Mach =1 at the throat and then supersonically accelerate it further in the divergent section, thus realising all of the available temperature energy and turning it into kinetic energy, after all thats what a nozzle does.
A variable area nozzle is purely a device for matching the plume pressure to the ambient static pressure for maximum efficiency. i.e. fully expanded.
If you underexpand your nozzle you fail to convert all of your energy into momentum.
If you overexpand your nozzle the nozzle walls will experience a pressure force in the drag sense , because the pressure of the jet is below ambient pressure.
I believe that is the thrust ( ho ho ho ) of the original thread. So the real answer is no, to mount this kind of device on a model aircraft would be next to useless.
Nick
11-23-2005, 09:15 AM
#27
RE: variable exaust nozzle
OK Nick, i read you. We really were talking different things....i was arguing supersonic while you were in subsonic gas flows.
However i have to slightly disagree with you again with that this type of variable area nozzle would be next to useless, because of the following reason:
(As a basis we have exhaust velocities below supersonic local to consider....on full scale as well as our turbines)
Our turbines are always a compromise between thrust and running characteristics. Best example is a turbine a friend built a while back. It produced close 9kg static without a nozzle, and ran up from idle to full in a heartbeat. After adding a really really really "tight" nozzle, it would push an amazing 13 point some kilos, however runup was veeeeery timeconsuming and involved quite a rise in temp (constant rpm gave good temps). The nozzle that will find final use gives around 11kg...with decent spoolups.
Point is: no nozzle accellerates the air "zero", thus you only have the "clean thrust" available. Addind the " fixed variable area nozzle", accellerates the air (ideally) to mach 1.0 (dynamic pressure decreasing, i know...), ruining the running capabilities.
Now adding a variable area nozzle would be a great step in the right direction, because you can have the best runup times possible, with the highest thrust possible once operational rpm is reached (nozzle squeezes tight). However large this performance increase is depends on the individual engine....but i'm guessing you could gain about 10% thrust and decrease spooltimes by about 25% simply by adding a variable area nozzle on the available turbines...
Just a rough estimate....
However i have to slightly disagree with you again with that this type of variable area nozzle would be next to useless, because of the following reason:
(As a basis we have exhaust velocities below supersonic local to consider....on full scale as well as our turbines)
Our turbines are always a compromise between thrust and running characteristics. Best example is a turbine a friend built a while back. It produced close 9kg static without a nozzle, and ran up from idle to full in a heartbeat. After adding a really really really "tight" nozzle, it would push an amazing 13 point some kilos, however runup was veeeeery timeconsuming and involved quite a rise in temp (constant rpm gave good temps). The nozzle that will find final use gives around 11kg...with decent spoolups.
Point is: no nozzle accellerates the air "zero", thus you only have the "clean thrust" available. Addind the " fixed variable area nozzle", accellerates the air (ideally) to mach 1.0 (dynamic pressure decreasing, i know...), ruining the running capabilities.
Now adding a variable area nozzle would be a great step in the right direction, because you can have the best runup times possible, with the highest thrust possible once operational rpm is reached (nozzle squeezes tight). However large this performance increase is depends on the individual engine....but i'm guessing you could gain about 10% thrust and decrease spooltimes by about 25% simply by adding a variable area nozzle on the available turbines...
Just a rough estimate....
11-23-2005, 10:19 AM
#28
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RE: variable exaust nozzle
Hank,
yep yep I got you, I not really coming from that direction and had not thought about the run up conditon, I'm coming at it from a more of a full size perspective, ( by the way I was totally talking about supersonic flows , regarding matching the plume pressure )
Anyway, I read you with the nozzle compromise, and that would be indeed a very interest project, although you may well have to design your own control system . What is also of interest to me is that most of the model nozzles do not have a parallel section on the end of them and they will naturally tend to vena contrata inwards. This is bad in full size design as the final exit area (which is not the final geometric area) becomes a function of altitude , but for models with 'one' altitude is not so much of a problem.
Nick
yep yep I got you, I not really coming from that direction and had not thought about the run up conditon, I'm coming at it from a more of a full size perspective, ( by the way I was totally talking about supersonic flows , regarding matching the plume pressure )
Anyway, I read you with the nozzle compromise, and that would be indeed a very interest project, although you may well have to design your own control system . What is also of interest to me is that most of the model nozzles do not have a parallel section on the end of them and they will naturally tend to vena contrata inwards. This is bad in full size design as the final exit area (which is not the final geometric area) becomes a function of altitude , but for models with 'one' altitude is not so much of a problem.
Nick
11-23-2005, 02:09 PM
#29
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RE: variable exaust nozzle
Miniflyer --
i was not trying to just "pull" definitions from an encyclo. I gave the link which people can put to good use. indeed, i have gained a lot from the discussions here. simply put, the convergent-divergent nozzle as i understood it was to increase extraction of energy in the best way possible from the hot stream of gases. this i believe goes back to the bernoulli principle. pressure drops with an increase in velocity at the throat of the bernoulli tube but the opposite happens when it is allowed to expand where there's more volume.
similarly, if the hot gases are allowed to accelerate to supersonic at the throat of the convergent nozzle but instead of discharging it into the atmosphere, you pass it through a "divergent" section, it is further accelerated over the mach level. rockets, as stated by Nick, and quite correctly could use a variable nozzle to good effect but due to weight, the fixed nozzle works great too and is cheap.
the f-15 picture nozzle looks like a convergent-divergent section albeit variable....or?
why you'd want a mach+++ exhaust velocity on a model airplane engine is something else beyond me.
perhaps in this light: "....but i'm guessing you could gain about 10% thrust and decrease spooltimes by about 25% simply by adding a variable area nozzle on the available turbines..." may well be reason enough in model aircrafts. some engines may or may not benefit.
matching nozzle aperture and engine is certainly an issue with afterburners lit.....
at least that's my two cents
"A variable area nozzle is purely a device for matching the plume pressure to the ambient static pressure for maximum efficiency. i.e. fully expanded.
If you underexpand your nozzle you fail to convert all of your energy into momentum.
If you overexpand your nozzle the nozzle walls will experience a pressure force in the drag sense , because the pressure of the jet is below ambient pressure."
very clear and concise. i like that...
i was not trying to just "pull" definitions from an encyclo. I gave the link which people can put to good use. indeed, i have gained a lot from the discussions here. simply put, the convergent-divergent nozzle as i understood it was to increase extraction of energy in the best way possible from the hot stream of gases. this i believe goes back to the bernoulli principle. pressure drops with an increase in velocity at the throat of the bernoulli tube but the opposite happens when it is allowed to expand where there's more volume.
similarly, if the hot gases are allowed to accelerate to supersonic at the throat of the convergent nozzle but instead of discharging it into the atmosphere, you pass it through a "divergent" section, it is further accelerated over the mach level. rockets, as stated by Nick, and quite correctly could use a variable nozzle to good effect but due to weight, the fixed nozzle works great too and is cheap.
the f-15 picture nozzle looks like a convergent-divergent section albeit variable....or?
why you'd want a mach+++ exhaust velocity on a model airplane engine is something else beyond me.
perhaps in this light: "....but i'm guessing you could gain about 10% thrust and decrease spooltimes by about 25% simply by adding a variable area nozzle on the available turbines..." may well be reason enough in model aircrafts. some engines may or may not benefit.
matching nozzle aperture and engine is certainly an issue with afterburners lit.....
at least that's my two cents
"A variable area nozzle is purely a device for matching the plume pressure to the ambient static pressure for maximum efficiency. i.e. fully expanded.
If you underexpand your nozzle you fail to convert all of your energy into momentum.
If you overexpand your nozzle the nozzle walls will experience a pressure force in the drag sense , because the pressure of the jet is below ambient pressure."
very clear and concise. i like that...
11-23-2005, 06:04 PM
#30
RE: variable exaust nozzle
ORIGINAL: starwoes
why you'd want a mach+++ exhaust velocity on a model airplane engine is something else beyond me.
why you'd want a mach+++ exhaust velocity on a model airplane engine is something else beyond me.
The theory and work involved in creating a variable nozzle going beyond Mach 1 would be outrageous, though...and would have to be preceeded by a nozzle getting it up to 1.0 in the first place.
Be an interesting project...however first i need to get my other projects off the table, which may very well last a few more years at my current rate...
Hank
11-24-2005, 04:36 AM
#31
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RE: variable exaust nozzle
Unfortnatley there just is not the pressure to choke the flow at the throat in a model turbine.
To obtain sonic velocity at the throat you must drive you flow with a pressure greater than the critical pressure ratio, which is 1.89 . Therefore if ambient is 101325 then the total pressure just aft of the turbine must be 191504 Pa. Model turbines are just nowhere near that , or certainly the one I measured, (it was much nearer an NPR of 1.3)
So really the statement , "it would be alot of work", is on the right track ! , I think it would be more like a completely redesigned engine to obtain a choked nozzle !
We are starting to see model axial compressors in engines now, I wonder what NPR's these are operating at.
Nick
To obtain sonic velocity at the throat you must drive you flow with a pressure greater than the critical pressure ratio, which is 1.89 . Therefore if ambient is 101325 then the total pressure just aft of the turbine must be 191504 Pa. Model turbines are just nowhere near that , or certainly the one I measured, (it was much nearer an NPR of 1.3)
So really the statement , "it would be alot of work", is on the right track ! , I think it would be more like a completely redesigned engine to obtain a choked nozzle !
We are starting to see model axial compressors in engines now, I wonder what NPR's these are operating at.
Nick
12-03-2005, 10:40 AM
#32
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RE: variable exaust nozzle
WOW!!
this question started off benign enough but after reviewing several patents, i am intimately convinced that almost everyone is right regarding their different thinking on this subject. your particular discipline may determine your original line of thinking. i was convinced that it had a lot (or mostly) to do with speed based on what i was told by a respected instructor (and no the info wasn't free and also, u don't need airspeeed input into the engine...there are other ways of determining that based on the prior equation).
for instance: US patent #5,676,312
Quote: "In these known designs, the external, or controlled, flaps are controlled by known actuators such that pivoting movement of the controlled flaps is imparted to the follower flaps so as to change the geometry of the nozzle.
The known exhaust nozzles may change from converging-converging configurations used for subsonic aircraft speeds to converging-diverging configurations at supersonic speeds. In addition to changing the configuration, the cross-sectional area of the throat of the nozzle may also be regulated." end quote
And then again, i have read several patents that alluded to variable geometry for noise attenuation!!!! never even thunk that...i was thinking "military and speed" and who the heck gives a hoot about noise? then again, plume pressure has been raised several times including afterburner ops!! probably a whole lot more reasons for varying the nozzle that i am yet to uncover. learned a lot indeed. thanks for the thread.
SO! for your model application, if you intend it to be practical....then you need to determine what you need it for.
over and out....
this question started off benign enough but after reviewing several patents, i am intimately convinced that almost everyone is right regarding their different thinking on this subject. your particular discipline may determine your original line of thinking. i was convinced that it had a lot (or mostly) to do with speed based on what i was told by a respected instructor (and no the info wasn't free and also, u don't need airspeeed input into the engine...there are other ways of determining that based on the prior equation).
for instance: US patent #5,676,312
Quote: "In these known designs, the external, or controlled, flaps are controlled by known actuators such that pivoting movement of the controlled flaps is imparted to the follower flaps so as to change the geometry of the nozzle.
The known exhaust nozzles may change from converging-converging configurations used for subsonic aircraft speeds to converging-diverging configurations at supersonic speeds. In addition to changing the configuration, the cross-sectional area of the throat of the nozzle may also be regulated." end quote
And then again, i have read several patents that alluded to variable geometry for noise attenuation!!!! never even thunk that...i was thinking "military and speed" and who the heck gives a hoot about noise? then again, plume pressure has been raised several times including afterburner ops!! probably a whole lot more reasons for varying the nozzle that i am yet to uncover. learned a lot indeed. thanks for the thread.
SO! for your model application, if you intend it to be practical....then you need to determine what you need it for.
over and out....
12-03-2005, 05:28 PM
#33
RE: variable exaust nozzle
Here's a few a pics of my detailed Rozmus nozzle from Bob Fiorenze on my F-16.
The first pic is from Mike Cherry's Jet Book.
The second is a close up of the nozzle open.
The third is of the entire plane.
I could only tell a very minor difference on this D/F model with the nozzle opened or closed...
Kevin
The first pic is from Mike Cherry's Jet Book.
The second is a close up of the nozzle open.
The third is of the entire plane.
I could only tell a very minor difference on this D/F model with the nozzle opened or closed...
Kevin
03-16-2006, 11:13 PM
#38
Junior Member
RE: variable exaust nozzle
Anybody thought of using NiChrome 'Muscle wires?' They expand and contract in direct proportion to temperature OR electicity... Light, simple to control and available in various guages, with diferrent properties... I've even seen apps for 'butterfly wings'
