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ACRO E-mail Archive Thread: Prop Surging

Thread: Prop Surging

Message: Prop Surging

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From: "Bob Freeman" <Bob_Freeman at maxtor.com>

Date: Tue, 20 Feb 1996 20:13:19 UTC

Message:

  On the subject of  prop surging, I had a similar problem with my IO-540 
powered Acroduster II.

It usually ran fine,  but on occasion, I'd see a fluctuation in oil pressure 
(about 5 to 10 psi drop)  and rpm up to about 2850 rpm and then things 
would restabilize.  My first instinct was to overhaul the prop governor, 
which I did.  It did not fix the problem after being overhauled.  I then had 
the prop inspected to assure no problems there.  Still no fix.

We then checked the oil pressure relief valve to make sure that the ball 
had a good seating surface, again to no avail.  Next step was to go 
though all the oil fittings and eliminate all 90 degree turns, reducing all 
possible flow restrictions.  Still no improvement!  

Dick Demars suggested that a worn front bearing on the crankshaft could 
be 
causing oil pressure intended for the prop go elsewhere.   Before 
resulting to an engine  tear down,  I had the prop governor bench 
checked again and it 
worked perfectly.   

So the engine was torn down to replace the front crankshaft bearing, 
which as it turned out, appeared to have been "dinged" when the case 
halves were mated at the previous overhaul.  AHA! This must be the 
cause!  After reassembling the engine and flight testing, THE PROP IS 
STILL SURGING!!!   AHHHHH!    Back to square one.   

Next choice was to either bypass the inverted oil system to see if the 
problem was there, or swap prop governors.   I elected plan B, swap 
governors.  Dick Demars loaned me a governor from a rebuild project 
and it was installed on the Acroduster.   Low and behold, NO MORE 
PROP SURGE. 

Back to the governor shop.   Two governors that work perfectly on the 
bench. What's the problem?  Why doesn't it work properly on the 
airplane?    As it turns out, my original governor was the wrong governor 
for the engine.  It looks identical to the governor for an IO-540 but in fact 
should have been 
used on  an IO-360.  

The differences between the governors, as I understand it, were in the 
orifice diameter, counterweights and springs.   The governor was 
capable of doing the job about 99% of the time, but occasionally went 
unstable and couldn't keep up.   

Now that I have the right governor on the plane, it works great.  This is 
really a lesson for those buying used airplanes, especially homebuilts.  
I flew the airplane for about 7 years before this problem surfaced and it 
took a year and a half to trace to root cause.   I made the mistake of 
assuming that the part on the airplane was the right one.   Bad 
assumption.   Don't do anything major (like a major) until you're sure all 
the parts are correct.  The silver lining is that I now know my engine 
better than I would have otherwise and it's only got a few hours on it 
since bottom overhaul.  

Bob Freeman, VP,  Chapter 12