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Whirlwind 200AC RPM Issue

Youngman1

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Hello All

So I've been getting some great help working through my fuel venting and subsequent engine stoppage issue.

Unfortunately on both flights we have also noted something very unexpected going on with the RPM following the engine stoppage.

Setup
IO-360-A1-B6
Whirlwind 200AC
Saber Prop Extension
Rear Mounted- PCU5000X, P-540-036/AL-866.
Standard Hi Pressure Oil line from Governor to front of crankcase
Standard ACS Vernier Control

Series Of Events
  1. Full Power Climb 2700rpm/28.5" MP
  2. Set 2250rpm/ ~18"MP (level off)
  3. Further power reduction to ~13"MP/ 2250rpm
  4. Engine stops making power, noted by MP dropping to ~8" speed 105kts
  5. -RPM then quickly increases from 2200->2700rpm over about 2 seconds
  6. -Pull the RPM control to try and reduce RPM
  7. - RPM slowly reduces back to around 2400rpm then 2200rpm
  8. RPM then reduces along with airspeed during the dead stick landing

Oil Pressure was 87PSI before the engine stoppage, rose to about 90psi as the rpm increased and then returned to 87PSI. Oil pressure was then strong all the way through to roll out when the prop stops turning.

My understanding (which of course might be wrong) of how a counter weighted constant speed prop works is that with this event, there should have been no change in RPM as the governor continued to receive good oil pressure. I don't see how the propeller driving the engine opposed to the engine driving the propeller changes this when all the governor "knows" is RPM to it's input gear. The RPM Vernier control should be the only thing that can change RPM other than the loss of oil pressure which should see the prop drive course and reduce rpm due to the counterweights.

I've tried to engage with Whirlwind for some support working through this issue and have received the below response.

"I put your query to our technical director, and he said that in your scenario there are other factors in the mix. The propeller shouldn’t overspeed, but somewhat may be dependent on airspeed at the time and the pitching moment of the propeller. The pitching moment (blade twisting in the hub) has different pitching moments whether the propeller is under thrust load or wind-milling/braking"

I don't fully understand the answer given and would really appreciate some help trying to work out what the other factors involved could be here.

We've been thinking through this for a few weeks now and have checked the big things such as Governor spec, which we think is correct. The fact that the overspeed is so far away from what you would expect to happen made me think that there was some huge error/failure somewhere, but we just can't see anything obvious.


Thanks in advance for any advice.
 

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