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From: "Metcalfe, Lee, AIR" <Lee.Metcalfe@jocoks.com>
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Subject: Flying Our Lancairs - Using the Prop as a Speedbrake
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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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<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>A while back there 
was some discussion about slowing down our hot-rods&nbsp;as we enter the traffic 
pattern.&nbsp; Several folks suggested flat-pitching the prop to decelerate 
(something I have done myself).&nbsp; Several others said that was a bad thing 
to do... harmful to the engine.&nbsp; I don't recall there having been any 
matter-of-fact resolution to the issue.</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>Well, having just 
read John Schwaner's really outstanding <U>Sky Ranch Engineering Manual</U>, I 
am now an expert on horizontally-opposed aircraft engine dynamics (well, maybe 
not an expert, but pretty well-informed!).&nbsp; Here's what John has to say 
(paraphrased by me) about using the prop as a Jake-Brake (my 
words).</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>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:</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>&gt;&nbsp; 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.&nbsp; 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.&nbsp; This can 
damage the lands of the grooves and damage the rings even to the extend of 
breaking the rings.&nbsp; Does anyone doubt that would be a bad thing?&nbsp; 
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?</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>&gt;&nbsp; 
Crankshaft counterweight detuning - this doesn't apply&nbsp;to us 4-cylinder 
drivers, cuz we don't have counterweights.&nbsp; Don't know for sure about you 
6-cylinder guys (I know the TSIO-540s in my Cessna 340 had 
counterweights).&nbsp; &nbsp;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.&nbsp; The counterweights are designed to balance torsional loads 
and are optimized for the normal power range of the engine.&nbsp; The weights 
are actually loosely mounted and can flop around if not loaded.&nbsp; One of the 
big loads the weights are supposed to balance is the power stroke of each 
cylinder.&nbsp; 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).&nbsp; Again, not a good 
thing.</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>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.&nbsp; I guess we'll just 
have to accept that we can't come screaming into the downwind at 175&nbsp;knots 
and expect to drop the rollers at the 180!</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>By the way, as 
several others on the list have said, the <U>Sky Ranch Engineering Manual</U> is 
an excellent resource.&nbsp; I highly recommend it!</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2>Cheers!</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial 
size=2></FONT></SPAN>&nbsp;</DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>Lee 
Metcalfe</FONT></SPAN></DIV>
<DIV><SPAN class=746454120-22112002><FONT face=Arial size=2>N320WH&nbsp;- 472 
hrs. and holding</FONT></SPAN></DIV></BODY></HTML>

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