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*

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

In article ,
Scott Skylane wrote:


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.

Scott,

Are you guys using ADI on the -6?

And congrats on making First Officer!!

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

Dale wrote:
/snip/
Scott,

Are you guys using ADI on the -6?

And congrats on making First Officer!!


Dale,

Yes, we use it, generally for takeoffs above about 85,000 lbs. If
anyone's interested the fuel burn for an R2800 CB-16 is as follows:

Max Dry power (53" M.P. 2800 RPM 1950 HP) 1850 lbs/hr

Max Wet power (59.5" 2800 RPM 2400 HP) 1550 lbs/hr

And thanks for the congrats!! It sure beats working for a living

Happy Flying!
Scott Skylane

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