Seaplane down off Miami Beach....

I'm wondering if fitting turbine engines on the old airframes
didn't pull something loose in the wing/mount.

Turbines run a whole lot smoother than the round Pratts that were on
originally. They don't have the power pulses that radial engines have.
I'd agree with the other posters that it sounds more like a
leaking/broken fuel fitting that went bad, ignited and led to
structural failure.

wrote in message
ups.com...
I'm wondering if fitting turbine engines on the old airframes
didn't pull something loose in the wing/mount.

Turbines run a whole lot smoother than the round Pratts that were on
originally. They don't have the power pulses that radial engines have.

I assume he was referring to the increased thrust that was probably obtained
with the turbine installation, which would create higher forces on the
structure transmitting that thrust to the airframe.

Of course, one would think that in a turbine retro-fit, that structure would
be upgraded to compensate. Hopefully, that's not actually the problem.

But I don't think Otis was suggesting that turbines would cause more fatigue
due to vibration than the original engines.

Pete

I assume he was referring to the increased thrust that was probably obtained
with the turbine installation, which would create higher forces on the
structure transmitting that thrust to the airframe.

I assumed the same. The round Pratts were 550hp engines, and the STC'd
PT6A-27 engines are flat-rated to 650hp. While the increased thrust
might add stress, my assumption was the weight reduction of the
turbines and their much smoother operation might nullify the power
increase as it relates to airframe stresses. It seemed a safe
assumption that that's what he was suggesting.

wrote in message
oups.com...
I assume he was referring to the increased thrust that was probably
obtained
with the turbine installation, which would create higher forces on the
structure transmitting that thrust to the airframe.

I assumed the same.

Curious you would introduce "run a whole lot smoother" and "power pulses"
then, if you thought those issues weren't relevant. Very odd. Even
stranger that those issues were the sum total of your rebuttal to his post.

The round Pratts were 550hp engines, and the STC'd
PT6A-27 engines are flat-rated to 650hp. While the increased thrust
might add stress, my assumption was the weight reduction of the
turbines and their much smoother operation might nullify the power
increase as it relates to airframe stresses. It seemed a safe
assumption that that's what he was suggesting.

I don't see how the smoothness of the operation of the engine relates.

As far as the weight reduction goes, if anything that would exacerbate the
problem, especially if that weight reduction is permitted to be moved over
to useful load. A heavier engine will dampen the initial acceleration (a
certain amount of the thrust is applied to accelerating the engine, rather
than the airframe to which it's attached), while a heavier airframe (ie
higher useful load) will allow higher forces to occur during that initial
acceleration.

Of course, once acceleration is relatively constant, the only real
difference is the difference in thrust, but again 100 more hp certainly
translates to more acceleration, and thus more force on the airframe.

All that said, as I mentioned before, I would expect certification of the
engine to take all of that into account.

Pete

Peter Duniho wrote:

A heavier engine will dampen the initial acceleration (a
certain amount of the thrust is applied to accelerating the engine, rather
than the airframe to which it's attached), while a heavier airframe (ie
higher useful load) will allow higher forces to occur during that initial
acceleration.

Really?

Just how elastic do you think the connections between the airframe and
the engine are? When it comes to acceleration, they better form pretty
much one piece, don't you think?

Gross weight is what we accelerate, not components.

Or are you speaking metaphysically?


Jack

"Jack" wrote in message
. net...
[...]
Just how elastic do you think the connections between the airframe and the
engine are? When it comes to acceleration, they better form pretty much
one piece, don't you think?

For the sake of this discussion, it doesn't really matter. The difference
in forces may be negligible, but *inasmuch as they might not be*, a lighter
engine doesn't help, it hurts.

Still, your comment about elasticity is irrelevant.

Gross weight is what we accelerate, not components.

The components are connected by structure designed for specific forces.

For example, I can add a one ounce weight to the back of my airplane with
some scotch tape, and it won't fall off, no matter how fast I accelerate.
But if I tried to pull the entire airplane by pulling on that one ounce
weight, the tape will fail, even at extremely low acceleration.

Pete

Curious you would introduce "run a whole lot smoother" and "power pulses"
then, if you thought those issues weren't relevant. Very odd. Even
stranger that those issues were the sum total of your rebuttal to his
post

Ya got me there Pete... nothing escapes your eagle-eyed gaze, eh?

Not really sure where I was going with that. On reflection, Otis' post
would suggest the higher power turbine conversion might have have an
adverse effect on the old Mallard airframe, even though as you
mentioned the STC should include reinforcing mounts, spars etc. to
allow for the increased stress on the airframe. If anything, I'd guess
the smoother running turbines should stress the airframe less than the
throbbing & vibrating radials in spite of their higher power.

(Disclaimer: I'm not an engineer, just an aero turbine analyst)

wrote:
I'm wondering if fitting turbine engines on the old airframes
didn't pull something loose in the wing/mount.

Turbines run a whole lot smoother than the round Pratts that were on
originally. They don't have the power pulses that radial engines have.

Not that they aren't subject to resonance issues. Remember the Electra?

Reports I saws this morning said that the NTSB said they
found a fatigue crack in the main spar, maybe the conversion
was not done well or the maintenance was not though enough.
I'll bet the fleet is grounded and they require immediate,
"before further flight" NDT inspections of the wings, etc.


--
James H. Macklin
ATP,CFI,A&P

--
Merry Christmas
Have a Safe and Happy New Year
Live Long and Prosper
Jim Macklin


"Ron Natalie" wrote in message
m...
| wrote:
| I'm wondering if fitting turbine engines on the old
airframes
| didn't pull something loose in the wing/mount.
|
| Turbines run a whole lot smoother than the round Pratts
that were on
| originally. They don't have the power pulses that radial
engines have.
|
| Not that they aren't subject to resonance issues.
Remember the Electra?

Jim Macklin wrote:
Reports I saws this morning said that the NTSB said they
found a fatigue crack in the main spar, maybe the conversion
was not done well or the maintenance was not though enough.
I'll bet the fleet is grounded and they require immediate,
"before further flight" NDT inspections of the wings, etc.

Chalk's has voluntarily grounded thier Mallards and is performing an exhaustive
test of the spars on one of them now. The NTSB rep voiced the opinion that age
alone would not be sufficient to cause this failure; some additional stress
would be required.

George Patterson
Coffee is only a way of stealing time that should by rights belong to
your slightly older self.