exhaust stacks & thrust

[Widewing]

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

[Debonair]

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...

[Crumpp]

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

[Badboy]

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

[HoHun]

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)

[agent 009]

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.

[Porta]

Thanks anyway .

[bozon]

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

[HoHun]

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)

[Crumpp]

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

[bozon]

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

[Crumpp]

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

[HoHun]

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)

[Crumpp]

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

[bozon]

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