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Bill, I believe its because there is less load with the PSRU than with the automobile configuration. We use the automatic flex plate which in the automobile has all the damping/load of the automatic transmission fluid. The manual transmission flywheel probably weights close to 35 lbs, so has lots of inertia that slows down the engine acceleration a bit. Also, I suspect that some of our home made manifolds may have a bit more air leakage than the stock throttle body/manifold - it does not take much of an air leak at all to really rev the engine.
I am personally aware of two incidents where a 13B was fired up without a prop load and the throttle got advanced. One ended up with the bearings melting because the engine rev faster than the oil supply could supply pressure. The second incident ended up with the PSRU disintegrating and embedding bolts in the wall of the garage - it was estimated that the engine hit approx 13,000 rpm.
Whether you accept the rationale for the high RPMs with no load or not, Please do not disregard these cautions - it will rev faster than you can react.
My 1600 idle rpm throttle setting with the prop will give me 4500 rpm without the prop - that's the idle setting. So you can imagine just opening the throttle a little bit past idle would do without load
Ed A
----- Original Message ----- From: <bbradburry@allvantage.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Tuesday, October 25, 2005 8:44 AM
Subject: [FlyRotary] Re: Its a start!
Snip!
Running without a prop is a bit risky - overrev can happen suddenly.
Snip!
I don't understand how this is true...The engine has everything attached
that it has when it is installed in a car. It should run fine with just the
flywheel...adding the PSRU would be similar to turning the clutch and front
half of the tranny...????
Please explain.
Bill Bradburry
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