Mailing List lml@lancaironline.net Message #16456
From: Metcalfe, Lee, AIR <Lee.Metcalfe@jocoks.com>
Sender: Marvin Kaye <marv@lancaironline.net>
Subject: Flying Our Lancairs - Using the Prop as a Speedbrake
Date: Fri, 22 Nov 2002 22:55:15 -0500
To: <lml>
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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