exhaust stacks & thrust

[HoHun]

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 Doubly anachronistic because it has already been replaced by the Newton as the unit of force :-)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)

[bozon]

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

[Crumpp]

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

[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.

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

[Crumpp]

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...



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

[HoHun]

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)

[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.

Ok that makes sense as to why it changes.

All the best,


Crumpp

[Badboy]

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

[Karnak]

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.

[bozon]

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

[justin_g]

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.

[gripen]

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

[Charge]

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+

[bozon]

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

[hitech]

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.