Water Injection Question

A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to give
more power for takeoff. I also came across this when reading about the P-47.
How does that system work? I presume it's different for pistons and jets.

Triple Delta

"DeltaDeltaDelta" wrote in message
...
A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to give
more power for takeoff. I also came across this when reading about the
P-47.
How does that system work? I presume it's different for pistons and jets.

Triple Delta



Doesn't this have something to do with methanol fuels and water? Supposedly
water increases HP in methanol.

DeltaDeltaDelta wrote:
A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to give
more power for takeoff. I also came across this when reading about the P-47.
How does that system work? I presume it's different for pistons and jets.

Triple Delta


Triple,

I am not sure about the jet physics. In the piston (large radial) world,
a water/alcohol mix is injected just downstream of the carburettor for
hight power operation. This has the effect of reducing combustion
chamber temperatures, thus allowing higher manifold pressures (i.e. HP)
without detonation. If you didn't inject water at these power settings,
the amount of excess fuel needed to keep the temps down would actually
flood the mixture, causing power loss.

The alcohol in the mix does virtually nothing to contribute to the power
output, it is there merely to keep the water fromm freezing at altitude.

Interestingly, Pratt & Whitney apparently stumbled upon this effect
while doing heavy rain water injestion tests in the early days.

Happy Flying!
Scott Skylane

Scott Skylane wrote:


Interestingly, Pratt & Whitney apparently stumbled upon this effect
while doing heavy rain water *injestion* tests in the early days.

Happy Flying!
Scott Skylane



Woops! Make that *ingestion*

Hmm...interesting. I always wondered about that MW50 (nethanol-water)
mixture used on some Luftwaffe WW2 fighters (most notably on the FW-190 and
derivatives)...Thanks for the info

Triple Delta

"Scott Skylane" wrote in message
...
Scott Skylane wrote:


Interestingly, Pratt & Whitney apparently stumbled upon this effect
while doing heavy rain water *injestion* tests in the early days.

Happy Flying!
Scott Skylane



Woops! Make that *ingestion*

Water injection cools the air both before and after combustion. The lower
temperatures allow more fuel to be burned (turbine) or allow more MP or
compression to be used without detonation (piston).

Mike
MU-2


"DeltaDeltaDelta" wrote in message
...
A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to give
more power for takeoff. I also came across this when reading about the
P-47.
How does that system work? I presume it's different for pistons and jets.

Triple Delta

"DeltaDeltaDelta" wrote in message
...
A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to give
more power for takeoff. I also came across this when reading about the
P-47.
How does that system work? I presume it's different for pistons and jets.

Triple Delta

3D.....See Rapoports answer for the technical explanation. Aircraft such as
the B-52 models prior to the G and the KC-135A used water injection takeoff
thrust if the weight and weather demanded it. It was a tremendous boost in
thrust in the B-52G and when the water ran out, a big deacceleration. As
you noticed, water injected takeoffs caused a lot of smoke.

JB

"Jim Baker" wrote in message
...

"DeltaDeltaDelta" wrote in message
...
A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to
give
more power for takeoff. I also came across this when reading about the
P-47.
How does that system work? I presume it's different for pistons and
jets.

Triple Delta

3D.....See Rapoports answer for the technical explanation. Aircraft such
as
the B-52 models prior to the G and the KC-135A used water injection
takeoff
thrust if the weight and weather demanded it. It was a tremendous boost
in
thrust in the B-52G and when the water ran out, a big deacceleration. As
you noticed, water injected takeoffs caused a lot of smoke.

JB

Should be "...Aircraft such as...prior to the H and the..."

JB

Water on the big round engines is called ADI. (Anti Detonation
Injection. It lowers the combustion temperature and accomplishes
exactly that.

A jet engine produces thrust by the amount of mass it can push out the
back end. By injecting water you have increased that mass and the
thrust increases without a cooresponding increase in internal
temperature. More fuel can be added also for the same internal
temperature withour the water.

The alcohol in the water is there to keep it from freezing and plain
water would accomplish the same thing as far as performance.

Water was/is only used on the older low bypass engines because their
low speed thrust is low and the metals used can't stand the
temperatures of modern engines. Modern engines have much better
low-speed thrust because of the high bypass fan and the ability to run
at a much higher temperature because of the metalurgy.



On Fri, 9 Jul 2004 16:42:42 -0700, "Jim Baker"
wrote:


"DeltaDeltaDelta" wrote in message
...
A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to give
more power for takeoff. I also came across this when reading about the
P-47.
How does that system work? I presume it's different for pistons and jets.

Triple Delta

3D.....See Rapoports answer for the technical explanation. Aircraft such as
the B-52 models prior to the G and the KC-135A used water injection takeoff
thrust if the weight and weather demanded it. It was a tremendous boost in
thrust in the B-52G and when the water ran out, a big deacceleration. As
you noticed, water injected takeoffs caused a lot of smoke.

JB

Geez Scott,
I've got 5 textbooks that cover water injection and they dont even come
close to explainin git as well as you did..way to go
"Scott Skylane" wrote in message
...
DeltaDeltaDelta wrote:
A couple of days ago, I saw a picture of a DC-8, i think, taking off,
trailing thick black smoke from the engines (they weren't on fire). The
description of the picture said that it was using water injection to
give
more power for takeoff. I also came across this when reading about the
P-47.
How does that system work? I presume it's different for pistons and
jets.

Triple Delta


Triple,

I am not sure about the jet physics. In the piston (large radial) world,
a water/alcohol mix is injected just downstream of the carburettor for
hight power operation. This has the effect of reducing combustion
chamber temperatures, thus allowing higher manifold pressures (i.e. HP)
without detonation. If you didn't inject water at these power settings,
the amount of excess fuel needed to keep the temps down would actually
flood the mixture, causing power loss.

The alcohol in the mix does virtually nothing to contribute to the power
output, it is there merely to keep the water fromm freezing at altitude.

Interestingly, Pratt & Whitney apparently stumbled upon this effect
while doing heavy rain water injestion tests in the early days.

Happy Flying!
Scott Skylane