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Message
A while back there
was some discussion about slowing down our hot-rods as we enter the traffic
pattern. Several folks suggested flat-pitching the prop to decelerate
(something I have done myself). Several others said that was a bad thing
to do... harmful to the engine. I don't recall there having been any
matter-of-fact resolution to the issue.
Well, having just
read John Schwaner's really outstanding Sky Ranch Engineering Manual, I
am now an expert on horizontally-opposed aircraft engine dynamics (well, maybe
not an expert, but pretty well-informed!). Here's what John has to say
(paraphrased by me) about using the prop as a Jake-Brake (my
words).
Driving the engine
with the prop such that the RPM is higher than would be generated by the power
setting, i.e. pulling back the throttle and going to full-increase RPM at a
relatively high airspeed, has at least two negative
consequences:
> Piston
ring flutter - the design of the compression ring/piston groove presumes there
will always be a downward load on the ring to keep it seated, this load being
generated by positive pressure in the cylinder. Windmilling the engine
with a low power setting can reduce cylinder pressures such that the compression
rings are unloaded and begin to "flutter" in the ring grooves. This can
damage the lands of the grooves and damage the rings even to the extend of
breaking the rings. Does anyone doubt that would be a bad thing?
John says this isn't a significant issue in low airspeed/low RPM/low power
situations, such as pulling the power on short final, which, of course us
Lancair drivers don't do anyway, right?
>
Crankshaft counterweight detuning - this doesn't apply to us 4-cylinder
drivers, cuz we don't have counterweights. Don't know for sure about you
6-cylinder guys (I know the TSIO-540s in my Cessna 340 had
counterweights). If your engine has "tuned centrifugal pendulums,"
commonly referred to as crankshaft counterweights, pulling the power quickly or
operating at high RPM and low manifold pressure can "detune" your
counterweights. The counterweights are designed to balance torsional loads
and are optimized for the normal power range of the engine. The weights
are actually loosely mounted and can flop around if not loaded. One of the
big loads the weights are supposed to balance is the power stroke of each
cylinder. If there is little or no power in the power stroke and RPM is
still in the normal operating range, the counterweights suddenly become
unemployed (or underemployed, actually) and flop around with nothing to do
(sounds like some people I know). Again, not a good
thing.
My conclusion is,
since we all have compression rings to worry about (well, not the rotary guys),
the Jake-Brake idea is probably not a good solution. I guess we'll just
have to accept that we can't come screaming into the downwind at 175 knots
and expect to drop the rollers at the 180!
By the way, as
several others on the list have said, the Sky Ranch Engineering Manual is
an excellent resource. I highly recommend it!
Cheers!
Lee
Metcalfe
N320WH - 472
hrs. and holding
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