Shaft-bow in Turbo-props

[WMUchickenhawk]

Today in systems class our instructor was telling us that one of the problems with some turpo-props is shaft-bow, she tried to describe it, but it didnt really make sense. She was also going to use on of out turbo prop models, but mx had left the cowling on. Could anyone here elaborate a little on what causes it? It sounds like the heat makes the shaft expand, or soften, but im not sure if this is correct.

 

[atlcrashpad]

Yup, it be da heat. MU2's are prone to shaft bow. You would have to rotate the prop after you landed to keep the shaft from bowing.

 

[FN FAL]

...It sounds like the heat makes the shaft expand, or soften, but im not sure if this is correct.

First, the heat makes it expand and harden, then after a while it softens, then after twirling your props for a bit you go out and have a cigarette.

 

[Diesel]

that's why you see garrett drivers turn the props after shutting down

pt6 and walters don't have that problem

 

[belchfire]

WMUchickenhawk...

Shaft bow should only be a problem if you
fail to follow the manufacturer's ground cooling
proceedures-common to have short taxies in
commuter/freight applications-there is another
reson for spinning the props in Garrets that
was documented in a trade pub of some sort
about 6 years ago...the MU-2 was the example.

Coking of the fuel nozzels. It was demonstrated
that if you reduced combustion chamber temp
by spinning the props within a minute or so of
shutdown you could significantly reduce the
buildups on the nozzels.

Are you in class at Regions Air??? Snicker!

FN, tormenting the fng's again! hehehe!

 

[j41driver]

Ahhhh....the lovely TPE331's.....

 

[MauleSkinner]

Today in systems class our instructor was telling us that one of the problems with some turpo-props is shaft-bow, she tried to describe it, but it didnt really make sense. She was also going to use on of out turbo prop models, but mx had left the cowling on. Could anyone here elaborate a little on what causes it? It sounds like the heat makes the shaft expand, or soften, but im not sure if this is correct.

Basically if left alone, the engines don't cool evenly. Heat rises, so you end up with warmer (very warm...remember, we're dealing with several hundred degrees C here) air on top of the engine and cooler on the bottom.

Since the metals at the top are also warmer, they tend to expand more than the metals at the bottom, resulting in an attempt to bow the engine.

PT6's and other "free turbine" engines aren't very susceptible to shaft bow because the run between the bearings that hold the shafts isn't very long. Garretts (MU-2's, B-100 King Airs, etc.) on the other hand, have one bearing at each end of the engine, which allows a little more flexibility, resulting in possible shaft bow. We're not talking about major warpage here, or even about something that's really visible, either...clearances between the turbine blades and housings are very small, so a movement of .020 inches will probably cause some binding.

If you let the engine cool down completely, the bow goes away, and you have no problems. If you are planning on starting the engine before it cools completely, you need to make sure that the engine spins freely. You do this basically in one of two ways...spin the prop to blow out residual heat, so that the temperature differential isn't so great, or turn the prop enough to flip the engine 180 degrees, so that the hot (expanded) part is now on the bottom cooling and shrinking, and the cool (not expanded) part is now on top, heating and expanding, resulting in "un-bowing" the engine.

This is just a "general" description...as belchfire said, follow manufacturer's instructions, and you shouldn't have a problem.

Fly safe!

David

 

[CA1900]

First, the heat makes it expand and harden, then after a while it softens, then after twirling your props for a bit you go out and have a cigarette.

:laugh:

:beer:

 

[belchfire]

Maulskinner-
thanks...I was trying to edit the uneven
cooling thing into my reply and for some
reason the system thought I was no
longer logged in...you saved me
from having to do it!

 

[WMUchickenhawk]

Thanks for all the info guys. I'm nowhere near flying a turbo-prop yet, but Im sure one awaits me.

 

[FN FAL]

:laugh:

:beer:

It's even funnier if you picture that bald guy from the movie "airplane" saying it.

 

[FN FAL]

FN, tormenting the fng's again! hehehe!

Yea, I used to have get out and spin the props on a B-100 king air once in a while. I didn't realize that there was a nozzle coking issue involved, thanks.

 

[TXDA2000]

I've also been told there is a wear issue associated with the way the components slide onto the shaft and interlock.

Some guys don't spin their Garretts, saying the whole theory is BS. If you've ever spun an engine with major shaft bow, you'll never forget it.

There was a Merlin IIIA on our field that you could do chin ups on the prop blades without turning over the motor.

 

[Diesel]

Simcom has a cutaway garett that you can spin and see how everything works.

Alll i can say is holy crap look at all those gears.

 

[FN FAL]

Simcom has a cutaway garett that you can spin and see how everything works.

Alll i can say is holy crap look at all those gears.

Kind of makes you respect the simpler and less efficient PT-6.

 

[FL420]

? for MauleSkinner

You do this basically in one of two ways...spin the prop to blow out residual heat, so that the temperature differential isn't so great,

Since the engine is geared, do you happen to know how many "blades" you should turn to ensure you have released the residual heat?

or turn the prop enough to flip the engine 180 degrees, so that the hot (expanded) part is now on the bottom cooling and shrinking, and the cool (not expanded) part is now on top, heating and expanding, resulting in "un-bowing" the engine.
David

Related question. How many "blades" to turn the shaft 180 degrees?

BTW. This isn't only a problem with Garrett turboprops. I was once furloughed for a few weeks when one of the IAE V2500 turbofans developed a bow in the rotor in our last remaining A320.

 

[MauleSkinner]

Since the engine is geared, do you happen to know how many "blades" you should turn to ensure you have released the residual heat?

Related question. How many "blades" to turn the shaft 180 degrees?

Question 1: Just spin it til you're tired

Question 2: I figured that out one time based on the gear ratio, but found it was just easier to spin it til I was tired

Fly safe!

David

 

[TXDA2000]

FL420,

There are several different gearbox ratios. Look into the intake and watch the impellor go round, the prop doesn't move a whole bunch.

 

[Tram]

Coking of the fuel nozzels. It was demonstrated
that if you reduced combustion chamber temp
by spinning the props within a minute or so of
shutdown you could significantly reduce the
buildups on the nozzels.

Doesn't the purge valve that opens on shutdown also help prevent coking?

You heard from AirTran yet?

 

[belchfire]

Yea, I used to have get out and spin the props on a B-100 king air once in a while. I didn't realize that there was a nozzle coking issue involved, thanks.

Er, well, that's what the magazine said.
Seemed reasonable to a (then) Aztruck
driver...they had these charts (how fast
the thing cooled off spinning vs. not
spinnning )and everything!

We had a restriction on starting-the EGT
had to be below 200C. I know that if you
were going to turn really quickly (less than
6 minutes on a passenger op) if you didn't
spin them you had to do a vent run that
could be continued into a start.

With in excess of 6000 hours (counting
both engines) around TPE's I never had
a shaft bow...there was this one time...
never mind!

Tram-ah yes, in a perfect world, where the
cops are British and the chefs are Italian and etc.
yes. But in the real world, where you see smoke from un-burnt (okay, partially burnt)kerosene wafting out of the intake and exhaust you know that all of the fuel didn't get used up!