Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: agent 009 on December 02, 2005, 11:20:09 PM
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This might be a no brainer, but. I did read about DB 605 motor having exhaust exits designed for extra thrust. I thought about it & went, hmmm don't all of em point backwards, so creating thrust?
& does it make any difference if the stack is on the bottom like 109, or on top like Spit & Mustang?
I've looked pretty close & they all look fairly similiar in direction. Some angled a bit more "out" than others versus straight backwards.
Just curious.
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Well from what I understand the stacks on the spit (or hte hurricane can't remember) added +3 mph alone.
Maybe they only add thrust if they are constricted, and forced out at a higher speed? Or maybe if they take turns before releasing the exhaust it lessens the thrust? I don't know, but those are some ideas.
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Originally posted by Krusty
Well from what I understand the stacks on the spit (or hte hurricane can't remember) added +3 mph alone.
Maybe they only add thrust if they are constricted, and forced out at a higher speed? Or maybe if they take turns before releasing the exhaust it lessens the thrust? I don't know, but those are some ideas.
Well, there is a system called an 'Augmentor' that is designed to create a few pounds of jet-thrust from recip engine exhaust. Basicly is collects all the exhaust stacks to a single pipe pointed rearwards. Just past this common exhaust pipe is another tube of larger diameter, positioned a scant fraction of an inch from the end of the exhaust pipe. The fast moving, hot, exhaust gas from the common pipe draw cooler ambient air with it into this second tube, and this cooling mix of gases expands on its way down the tube, and when leaving the other end.
Like I said, it isn't much, only a couple pounds of jet thrust, but it helps. I dont know of any other designs that create jet thrust from recip exhaust, but that doesn't mean they dont exist either. I will say that short-stack exhaust, like what the Spit's seem to have (and not having poked under the cowl on a Supermarine product, I cant say for certain thats what they are) will not add much, if anything, in the way of jet-thrust.
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Originally posted by Tails
Well, there is a system called an 'Augmentor' that is designed to create a few pounds of jet-thrust from recip engine exhaust. Basicly is collects all the exhaust stacks to a single pipe pointed rearwards. Just past this common exhaust pipe is another tube of larger diameter, positioned a scant fraction of an inch from the end of the exhaust pipe. The fast moving, hot, exhaust gas from the common pipe draw cooler ambient air with it into this second tube, and this cooling mix of gases expands on its way down the tube, and when leaving the other end.
Like I said, it isn't much, only a couple pounds of jet thrust, but it helps. I dont know of any other designs that create jet thrust from recip exhaust, but that doesn't mean they dont exist either. I will say that short-stack exhaust, like what the Spit's seem to have (and not having poked under the cowl on a Supermarine product, I cant say for certain thats what they are) will not add much, if anything, in the way of jet-thrust.
They tested a Spit Vb in 1943 to see what could be done to increase speed. Initially the speed was 357 mph.
Replacing the regular' fishtail' exhaust with the multiple ejector exhausts gave an increase of 7 mph.
Removing the carb intake ice guard gave an extra 8 mph.
Changing the mirror from the flat rectangle, to the rounded cupped mirror added 3 mph.
Adding a whip aerial after removing the standard mast gave 1/2 mph.
Cutting the cartridge case link ejector chutes flush with the wing gave 1 mph.
Sealing all the cracks, rubbing down, painting and polishing the leading edge of the wing gave and extra 6 mph.
Polishing the rest of the aircraft gave an extra 3mph.
So they were able to increase the all out speed on that one Spit Vb from 357 to 385 1/2 mph.
Time to start bribing your crew chief :)
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Originally posted by Guppy35
They tested a Spit Vb in 1943 to see what could be done to increase speed. Initially the speed was 357 mph.
Replacing the regular' fishtail' exhaust with the multiple ejector exhausts gave an increase of 7 mph.
Removing the carb intake ice guard gave an extra 8 mph.
Changing the mirror from the flat rectangle, to the rounded cupped mirror added 3 mph.
Adding a whip aerial after removing the standard mast gave 1/2 mph.
Cutting the cartridge case link ejector chutes flush with the wing gave 1 mph.
Sealing all the cracks, rubbing down, painting and polishing the leading edge of the wing gave and extra 6 mph.
Polishing the rest of the aircraft gave an extra 3mph.
So they were able to increase the all out speed on that one Spit Vb from 357 to 385 1/2 mph.
Time to start bribing your crew chief :)
If a fishtail exhaust is a collector pipe, then I can see how they would get some extra speed going to the short-stacks. Aerodynamic cleaning for one, and a reduction in backpressure adding a horse or two to that Merlin for another.
Pretty amazing to see the kinds of effects one can have 'nickle and diming' their way to a cleaner aircraft, though :D
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I did read the 109 exhaust system was redesigned in 44. Wondered if this was what the reason was.
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Many aircraft were designed to have extra thrust from the exhaust pipes, it was just a clever and easy thing to do :)
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Originally posted by Tails
If a fishtail exhaust is a collector pipe, then I can see how they would get some extra speed going to the short-stacks. Aerodynamic cleaning for one, and a reduction in backpressure adding a horse or two to that Merlin for another.
The fishtail system had individual exhausts for all cylinders (one cylinder per one exhaust pipe) ie it was not a collector style. Early Spitfires had ejectors with two cylinders per one exhausts but that changed to one/one during the production of the Spitfire V. The DB 605A had one exhaust per one cylinder system.
gripen
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That must explain why the 190's have 2 slides for exhaust pipes instead of the tubes! Thanks :)
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Originally posted by frank3
Many aircraft were designed to have extra thrust from the exhaust pipes, it was just a clever and easy thing to do :)
Yep, and some manufacturers were slower to catch on than others, so it wasn't until 1943 that the A6M5 recieved the Sakae 21 radial engine, modified with individual ejector exhaust stubs for greater thrust augmentation.
Badboy
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Hi agent,
>This might be a no brainer, but. I did read about DB 605 motor having exhaust exits designed for extra thrust. I thought about it & went, hmmm don't all of em point backwards, so creating thrust?
You are right, using exhaust thrust was a standard feature on WW2 fighters. The total contribution of exhaust thrust to propulsive power could be as high as 20% at top speed.
Usually, short individual stacks gave higher thrust than longer stacks combining the exhaust of several cylinders. As a rule of thumb, radial engines with exhaust collectors generate only 50% of the exhaust thrust they'd have with individual exhaust stubs.
The augmentor mentioned here is rather unusual. Similar designs were fashionable in the late 1930s, and Focke-Wulf tried something like that on their fighters in 1943 without success, and apparently the British augmentor didn't make it into service either.
>does it make any difference if the stack is on the bottom like 109, or on top like Spit & Mustang?
No. The difference is simply due to the location of the cylinder heads, which are on top for the Allied aircraft and on the bottom for the German aircraft.
Regards,
Henning (HoHun)
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Originally posted by agent 009
I did read the 109 exhaust system was redesigned in 44. Wondered if this was what the reason was.
What's the source for this?.
Just curious, as I've read a pair of documents about improving the powerplant of the Me 109 in mid-43 and the exhaust systeme was in it, but nothing about it's final adoption.
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AFAIK the first plane to utilize exhaust thrust from an radial engine was the I-16 but probably even some WWI planes benefited from it. Here in Finland, the VL Humu got quite similar stacks as the late Zeros while the original Brewster B-239 did not utilize exhaust thrust.
I don't know if any true augmentation systems reached service during the WWII but after war at least the Convair 340 had such system, some of them might be still flying.
gripen
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Exhaust stacks where also modified to include flame dampeners so the
Aircraft are less visible by night and the pilots got better view since they are not blinded by bright flames spouting through the exhaust.
So not all exhaust stack modifications where done for speed, some where also for flame dampening.
Not sure if there was any sound engineering with it like to deflect the sound to extend backwards/upwards rather than forward/down, but i guess it did go that way anyways.
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The 1944 BMW801D2 output around 300lbs of exhaust thrust at Erhöhte Notleistung.
The Germans did the same thing and redesigned the exhaust system on the BMW-801 series begining in April 1943. The design changes were incorporated in June 1943 serial production.
All the best,
Crumpp
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Originally posted by gripen
AFAIK the first plane to utilize exhaust thrust from an radial engine was the I-16 but probably even some WWI planes benefited from it. Here in Finland, the VL Humu got quite similar stacks as the late Zeros while the original Brewster B-239 did not utilize exhaust thrust.
I don't know if any true augmentation systems reached service during the WWII but after war at least the Convair 340 had such system, some of them might be still flying.
gripen
I logged about 400 hours in the USN's C-131s. These were militarized Convair 340s. Indeed, they had an exhaust gas "thrust augmentation" system designed to supplement available power. The system functioned as advertised, but required more attention than the standard collector system.
My regards,
Widewing
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Originally posted by Guppy35
...Removing the carb intake ice guard gave an extra 8 mph....
Did Spitfire V suffer from carb ice? I thought it was fuel injected...
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Removing the carb intake ice guard gave an extra 8 mph....
That is probably not something you want to do in an aircraft. Especially one that is going to fly at any altitude.
All the best,
Crumpp
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Hi HoHun
Originally posted by HoHun
The total contribution of exhaust thrust to propulsive power could be as high as 20% at top speed.
I have the results of the all out level performance tests on a RR Merlin engine in flight, carried out in 1941, and showing just under 10% increase in horsepower from exhaust thrust, peaking at about 127hp at 20,000ft. But they were using a surprisingly large nozzle area in those tests, which were not optimised for thrust augmentation. Analysis of other test data reveals that reducing the nozzle area could double the exhaust thrust available. While the calculations show that double can be achieved, test results show slightly less, so I'm assuming that a 20% increase is a theoretical maximum due to the diminishing returns caused by the increased back pressure resulting from smaller nozzle sizes.
Unfortunately, it seems this type of data is limited.
Badboy
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Hi Badboy,
>I'm assuming that a 20% increase is a theoretical maximum
No, it's actual data from the Jumo 213A-engined Focke-Wulf Fw 190D-9.
The higher figure is not surprising, though: Propeller thrust drops with speed while exhaust thrust doesn't, so the contribution of exhaust thrust would be greater for the Dora which certainly was faster than the 1941 Merlin-powered test aircraft. (I have just learned that Hawker made very extensive flight tests with a Merlin XX engine, maybe your test was one of those?)
Additionally, one would have to check whether the contribution is expressed in relation to shaft power or to total thrust. The latter is more accurate and the basis for my figure, but the former is more natural if you're an engine designer :-) Using shaft power instead of thrust power would make the exhaust thrust share appear a bit smaller, of course.
As you pointed out, different engine designs can turn out different thrust figures depending on their characteristics. Apparently, the Jumo 213 turned out more thrust than the otherwise similar DB603, for example.
I haven't checked this, but it's my impression that the use of MW50 might lead to an over-proportional increase of exhaust thrust because it increases the mass flow beyond the amount necessary for combustion. (Does that make sense? Hm, it might be just the opposite :-)
Regards,
Henning (HoHun)
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Hi Porta. The source was a109 book I read long ago. I'm sorry I don't remember title, but at the time I didn't know I would be putting up a thread about it years later. Sorry.
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Thanks anyway :) .
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Are we talking here about actual thrust or reduced drag?
If what you are saying is true then you can taxi a propless plane just by its exhausts. I really doubt that since it means that the engine is increadibly inefficient. Too much energy goes to the exhausted gas instead of the drive shaft or to turning a supercharger.
The exhaust is a potentially large drag source. If not built right, the hot air comming out at different pressure and velocity than the surrounding airflow will create a lot of turbulance and disrupt the laminar flow. This is not "thrust" but reduced drag, only this (along with so called "cooling drag") is not usually counted with the parasitic drag. Or so I think.
btw, can someone please tell me the total piston volume of such an engine? this can be use to estimate the exhaust airflow at given RPM. Just for order of magnitude estimated to see if those thrust numbers makes any sense.
Bozon
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Hi Bozon,
>Are we talking here about actual thrust or reduced drag?
Actual thrust.
>If what you are saying is true then you can taxi a propless plane just by its exhausts.
Ignoring the practical difficulties, you can.
>I really doubt that since it means that the engine is increadibly inefficient. Too much energy goes to the exhausted gas instead of the drive shaft or to turning a supercharger.
Well, that's the way it was :-) The energy is not wasted, after all - it produces thrust. Thrust actually is more efficient than shaft power at very high speeds, but for climbing and cruising, a turbo-supercharger (which saw some use in WW2) or a power recovery turbine (introduced after WW2) is more efficient.
>btw, can someone please tell me the total piston volume of such an engine?
Total displacement:
Merlin 27 L
V-1710 28 L
DB605 35.8 L
BMW801 41.7 L
Regards,
Henning (HoHun)
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Ignoring the practical difficulties, you can.
My first thought was what quick way to lose a motor. Running around at full boost without prop would quickly overrev the engine.
All the best,
Crumpp
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ok, lets make a rough calculation. We assume that we are near sea level and not moving much:
R ~ 3000 rpm = 50 {1/s}
V = volume ~ 30L = 0.03 {m^3}
r = air mass density ~ 1.2 ~ 1 {kg/m^3}
so, the flow of air through the engine assuming sucktion every other piston cycle (4 beats engine). Compression is only done after the piston is filled so at every cycle the amout of air that goes in is half the engine volume of uncompressed air. The exhaust gas is only made of gas that passed through the cylinders. What bothers me is the very small mass per sec of air that goes through:
flux = 1/2*R*V*r = 1/2*50*0.03*1 = 0.75 {kg/s}
This is the airflow available to us. The thrust force is achieved by accelerating this given amount of air to the exhausts. Force is the change rate of momentum to the airflow. neglecting the airspeed coming in and setting the exhaust speed to be u {m/s}:
F = flux * u = 0.5 u
The number 300 lbs thrust was thrown above for the BMW801D2, so we'll use that as reference:
F = 300 lbs-force ~ 150 kg-force ~ 1500 N
So, if we use this amount of air to produce thrust F we need outflow speed of:
u = 2*F ~ 3000 {m/s}
I'd say that this is excessive beyond roundoffs and estimates.
0.75 kg per second is like trying to push yourself forward by throwing a milk carton backward once a second. At 300 lbs-force, Shaquil Oneil could fly by throwing milk cartons - he'd have to throw them like bullets though. Props and jet engines produce the thrust by accelerating huge amounts of air. Unless the exhaust process includes some other mechanism, like mixing with a lot of outside cool air and accelerating it by coverting thermal energy into flow momentum, I can't see how this is possible.
I'm still not sure that the numbers given are not "psaudo thrust" meaning less drag.
Bozon
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I would think 300lbs of thrust is just that.....
300 lbs of push as measured by BMW. We are talking about moving 41.7 liters of air 2700 times a minute. 112,590 liters is a pretty substantial volume.
All the best,
Crumpp
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Hi Bozon,
>I'm still not sure that the numbers given are not "psaudo thrust" meaning less drag.
I believe you did not add the mass of the fuel, which is ejected through the exhaust jet, too.
Using 41.7 L for the BMW801D instead of 30 L will also bring you a bit closer.
Then there are those 300 lbs(f), which I believe are inaccurate. A BMW801D chart repeatedly posted here indicates a maximum of 120 kp at 2700 rpm, which is a bit less. At sea level (if you insist on taxying ;-) it's just 73 kp at 2700 rpm.
There is absolutely no doubt that the experts of the era were talking about true thrust, not drag reduction. Von der Nüll graphed turbo-supercharger versus jet exhaust engines over speed and altitude, and Hooker actually pointed out that the measured thrust was a bit greater than the calculated thrust because of pulses in the exit velocity.
Regards,
Henning (HoHun)
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120 kp at 2700 rpm,
Hi Hohun,
120Kg is correct I believe for 1.42ata. Unless the charts are mislabeled from BMW. It should be around 150Kg for 1.58ata.
Now that particular report also has a different supercharger gearing from the BMW801E/S series too. That may very well effect it. Additionally the force of exhaust thrust is different without RAM effect.
Not that I am pretending to have done near the research into exhaust thrust as you have, just looking at the charts BMW provides for BMW801DV15 which have exhaust thrust curves included.
Is the Kp a typo? I don't see it on any of the charts.
All the best,
Crumpp
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good point about the mass of fuel.
Using AH fuel consumption rates, most planes are something like 200 GPH. This is about 0.3 kg/s and of the same order as the mass of air. That won't do.
40 or 30 litter, still doesn't make up for about 2 orders of manitude! Unless I have some units error or FUBARed something else, the outflow has to be several times the speed of sound. Simple outflow of the exhaust won't do. We need either another effect (mixing with cool air to extract thermal energy for example) or to check their definition of "thrust".
Do you have some online material or can post something I can read about this? If I have the time I'll have a look at the Aero-engineering faculty library.
Bozon
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Hi Crumpp,
>Is the Kp a typo? I don't see it on any of the charts.
No. kp is kilopond, it's to kg what lb(f) is to lb. If you're used to Imperial Measurements, you might have seen kp as kg(f).
(The kilogram is a unit of mass, of course, and we're talking about forces here.)
The kilopond is slightly anachronistic since it came into use only after WW2. Doubly anachronistic because it has already been replaced by the Newton as the unit of force :-)
Regards,
Henning (HoHun)
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Another thing I want to make sure. How much was the air compressed in the entrance to the piston? That should be something like the manifold pressure?
I neglected that compression.
Bozon
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No. kp is kilopond, it's to kg what lb(f) is to lb. If you're used to Imperial Measurements, you might have seen kp as kg(f).
Ahhh! Thank you. The BMW charts are listed in P (Kg) for abgasstrahtschub.
Why in the world would RAM or Manifold pressure effect exhaust thrust? You are compressing a given volume. Does not seem like it should matter what you compress 41.7 liters too, it is still 41.7 liters of volume.
Is it a function of the greater force of combustion??
I found a cool little chart that seems to convert it to PS gained. It says at 1.65ata, the exhaust thrust added the equivilent of 100PS to the output.
All the best,
Crumpp
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that would still be 40L but of more condensed air making it more mass. If it is only the manifold pressure then typical 60"hg is twice atmospheric pressure. Depending a little on temperature, it will add a factor of about 2. Still not enough.
I found a cool little chart that seems to convert it to PS gained. It says at 1.65ata, the exhaust thrust added the equivilent of 100PS to the output.
Does it say litteraly "the equivilent of"?
Bozon
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Does it say litteraly "the equivilent of"?
No and I completely misread the chart at the bottom. It is RAM effect over speed/manifold pressure/rpm. How completely embarrassing...
:o
I pulled the folder out when I replied last time. The report has multiple charts and this particular one has the graphs at the bottom for exhaust thrust, fuel consumption, etc.....
Right under the exhaust thrust graph is another line showing PS production. In tiny little letters it does say Lüfterleistung.
All the best,
Crumpp
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Hi Crumpp,
the relationship between power and force is simply P = F * v, so the power equivalent of the exhaust thrust depends on speed. There is no fixed conversion factor unless you fix airspeed.
(And using the above relationship, remember that the shaft power is not converted into propulsive power without loss :-)
Regards,
Henning (HoHun)
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the relationship between power and force is simply P = F * v, so the power equivalent of the exhaust thrust depends on speed. There is no fixed conversion factor unless you fix airspeed.
Ok that makes sense as to why it changes.
All the best,
Crumpp
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Originally posted by bozon
ok, lets make a rough calculation.
Ok, here is some data from a flight test at full throttle at 20k that you can use for your rough calculation:
Charge flow rate = 151 lb/min
Exhaust velocity = 1788 ft/s
That would give you about 140lb of thrust which isn’t going to make a 7000lb aircraft accelerate very quickly, but every little bit helps.
Badboy
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On page 193 of Mosquito by C. Martin Sharp & Michael J. F. Bowyer this note is made:
A more hopeful modification recognized at Hatfield from early tests, was the use of stub exhausts in place of the shrouded saxophone flame-damping type. Trials at Marham with DK336 fitted with open stubs indicated these gave too much glare at night, revealing the position of the aircraft and making night landings difficult. Closing the stub ends to oval section of slightly less area reduced the glare in tests on 25 November and increased the jet propulsion effect without reducing engine efficiency, giving a worthwhile net gain of from 10 to 13 m.p.h. depending on altitude. .... The shrouded exhausts - never popular with ground crews - were retained for dusk and dawn attacks, and others were fitted with oval stubs for day raids, as on reconnaissance aircraft.
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Originally posted by Badboy
Charge flow rate = 151 lb/min
Exhaust velocity = 1788 ft/s
That would give you about 140lb of thrust which isn’t going to make a 7000lb aircraft accelerate very quickly, but every little bit helps.
151 lb/min ~ 1.25 kg/s
close enough to what I assumed.
Exhaust velocity = 1788 ft/s ~ 600 m/s.
speed of sound at sea level is about 330 m/s. At 20k it would be even lower. The Exhaust velocity is supersonic ?! if we do accept that, it does give 140 lb.
Bozon
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Originally posted by bozon
that would still be 40L but of more condensed air making it more mass. If it is only the manifold pressure then typical 60"hg is twice atmospheric pressure. Depending a little on temperature, it will add a factor of about 2. Still not enough.
Are you accounting for the compression ratio of the cylinders? Usually about 6 or 7:1 on ww2 aero engines.
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Originally posted by bozon
that would still be 40L but of more condensed air making it more mass.
The working volume of the engine does not matter, it's only the mass flow (air and fuel) through engine and the velocity of the exhaust gases (pulses when these exit the ejector) which matters in the case of the plain ejector stacks.
In the case of the true augmentation system also the heat of the exhaust gases (and possibly cooling air) is used to generate thrust in addition to the exhaust thrust.
gripen
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Did you people consider the incredible amount of thermal expansion? Or is that considered to be added as the mass of fuel as HoHun suggested. I'd say that the mass of fuel cannot be added as a component since its effect to air component is not so straight-forward.
So how much does the air expand in one cycle of BMW engine if, say, -15 Celsius deg. air is sucked in the engine, fuel is added to mixture and detonated and the exhaust air is heated to what, 600 Celsius deg. at exhaust?
***
Mind you that shaping the exhaust stacks may also have quite much effect on the characteristics of the engine moving the torque and max HP to lower or higher rpm etc.
:)
-C+
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ok a bunch of remarks:
Are you accounting for the compression ratio of the cylinders? Usually about 6 or 7:1 on ww2 aero engines.
what goes on in the cylinder doesn't matter. What counts is the mass flow rate and that depends on the volume of air ENTERING the cylinders. Before compression. As gripen said.
In the case of the true augmentation system also the heat of the exhaust gases (and possibly cooling air) is used to generate thrust in addition to the exhaust thrust.
This is what I suspect, but WWII doesn't seem to have such a system, only simple ducts I think.
Did you people consider the incredible amount of thermal expansion? Or is that considered to be added as the mass of fuel as HoHun suggested. I'd say that the mass of fuel cannot be added as a component since its effect to air component is not so straight-forward.
If no cool air that was not in the cylinders is added, thermal expansion will only serve to accelerate the exhast gasses. What I did was just estimate the speed of exhasts gasses needed to produce the claimed thrust. I didn't care where did the energy come from or how it was accelerated. Supersonic (several mach) exhaust airflow doesn't seem to be realistic.
The fuel does matter as it is more mass being pushed backward at the speed of exhaust gas. It has a non negligible contribution in jet engines. However, in our case it is not greater than the air mass and doesn't explain the increadibly high speed needed.
Bozon
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bozon:
On your air density, I belive your forgot about the super charger? So if we change from 29" ambient to 65" would be your manifold inlet pressure , and hence your mass should be about twice as mutch.
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150 lbs dosn't sound like much until you see what it does at top speeds.
Lets say a 2000hp eng at 400mph TAS with a prop eff of 0.9 = 1690 lbs thrust
Now that 150 extra pounds is almost 10% of the total thrust.
HiTech
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yes HT, I neglected inlet compression as it's only a factor of around 2 and I was doing order of magnitude estimate. It still comes out very supersonic. gripen sent me an interesting NACA report and they really claim that the flow is supersonic with speeds of 1000 - 1800 ft/s. Then if I add the manifold pressure and corrected air density I fall into this range.
I'll have to read it through to see what the effect will be at the exit. I'm not very well versed in supersonic aerodynamics.
Bozon
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Hi Bozon,
Originally posted by bozon
I didn't care where did the energy come from or how it was accelerated. Supersonic (several mach) exhaust airflow doesn't seem to be realistic.
The nozzle velocity I quoted comes from a report by some very well known engineers, one of the authors was Dr Stanley Hooker, (look him up) of Rolls-Royce and the results come from flight tests, that have been validated by separate tests carried out by the RAE and Hawker Aircraft Co, on a Merlin engine installed in a Hurricane, and also in the high altitude test plant at Wright field USA. Those sources provide a large series of results and very comprehensive analysis. I have overlays of the graphs of those results and they all agree within very close limits. I also only quoted you a single value at one altitude, and the exhaust velocity varies significantly from sea level to 35k, peaking at 1901 ft/s at 30k, which might seem even more unrealistic to you, but the data has been so well corroborated and validated it is incontrovertible. Also, the exhaust ejector nozzles used in these tests were not optimised for thrust augmentation, so it is safe to assume, as others in this thread have pointed out, even more was possible.
Badboy
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Hi Badboy,
>one of the authors was Dr Stanley Hooker, (look him up) of Rolls-Royce
Maybe you already have this, but I just bought a copy and think some of the experts here would enjoy it:
"The Performance of a Supercharged Aero Engine" by Stanley Hooker, Harry Reed and Alan Yarker, ISBN 1872922112, ca. EUR 10 :-)
Regards,
Henning (HoHun)
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Originally posted by HoHun
Maybe you already have this, but I just bought a copy and think some of the experts here would enjoy it
Yep, picked up my copy from the Rolls Royce Heritage Trust about seven years ago, it is basically just a copy of the original report. Very good reading for anyone wishing to inform an aero engine performance model :)
Badboy
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After talking to someone with a better knoledge of supersonic flow, it appears that supersonic flow within the exhaust tube is plausible. Delicately depending on the shape of the tube it is possible to develop high speed flows without loosing a lot of the energy to shocks.
The piston itself, at ~3000 RPM, cannot be supersonic (unless it has a travel or a few meters), so the picture we get is relatively slow but hot, slightly compressed gas accelerating by expansion down the exhaust tubes up to supersonic speeds, and creating thrust. Thus utilizing both the energy invested in compressing it at the inlet and the residual thermal energy of the burning.
There will be however a shock wave at the exit of the tube where the fast gases meet the subsonic surrounding flow, the effect of which is hard to estimate. Perhaps this is one reason these engines are so noisy.
Thanks to the people contributing to the discussion and to gripen for the NACA report. It's always fun to learn new things.:aok
Bozon
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bozon:
How would the exaust temp effect things when
Speed of sound with an egt of 1300 deg is over 2000 fps.
HiTech
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No barrier break on the inside then, but how does it look as soon as it gets out into the totally frozen air at -40-50 celcius then??
Anyway it is a known fact that the design of the stubs could affect the speeds, and the exhaust design is quite a thing nowadays with racecars and bikes. (the turbine being at the other end if you see what I mean).
And HiTech: If you're in engines a friend of mine works on the development on Total Combustion Technology, called TCT. If you want some info I can find it, just ping it on this thread ;)
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Originally posted by hitech
How would the exaust temp effect things when
Speed of sound with an egt of 1300 deg is over 2000 fps.
1300 deg?! hmmm... THAT is my problem right there. I assumed much much lower temperatures and since the speed goes like the square root of T was not that significant. I estimated up to a factor of around 50% to change in speed of sound that agrees with temperatues of a few hundred degrees. Ignorance require too many assumptions to compensate for...
In that case, this is enough to solve the needed 1700 fps velocity in the tubes problem.
Thanks HT.
Bozon
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Originally posted by bozon
1300 deg?! hmmm... THAT is my problem right there.
The exhaust temp in the tests I've quoted are in the order of 1100 degrees, for which the speed of sound is just over 2400ft/s.
Badboy