![]() |
If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
|
#1
|
|||
|
|||
![]()
Piston props (that i have flown) don't feather during shutdown...
It's tough to diagnose a problem on a complex system like a feathering, prop without being hands on, and without knowing the details of the feathering mechanism of the model of prop involved, but my initial inclination is still that it is governor or hub, not engine.... Since then I took a quick look at the commanche prop hub, and they have a pair of locks that prevent the blades from feathering when the rpm is below a certain rpm... Those locks are stuck open or damaged... However, the blades still wouldn't feather as long as the engine has enough oil pressure for the piston to overcome the feathering spring... Since he specifically said he had oil pressure at idle I still am very suspicious that the prop governor has an internal leak after it gets hot... Denny Actually, it may just be a bad idle mixture setting. My engine when hot used to stall at idle (frequently rolling out with the throttle at idle). I didn't have a twin, but I can imagine that the prop would feather during such a sequence in a twin. |
#2
|
|||
|
|||
![]()
"Dennis O'Connor" wrote in message ...
Piston props (that i have flown) don't feather during shutdown... It's tough to diagnose a problem on a complex system like a feathering, prop without being hands on, and without knowing the details of the feathering mechanism of the model of prop involved, but my initial inclination is still that it is governor or hub, not engine.... Since then I took a quick look at the commanche prop hub, and they have a pair of locks that prevent the blades from feathering when the rpm is below a certain rpm... Those locks are stuck open or damaged... However, the blades still wouldn't feather as long as the engine has enough oil pressure for the piston to overcome the feathering spring... Since he specifically said he had oil pressure at idle I still am very suspicious that the prop governor has an internal leak after it gets hot... snip The last couple of years that I spent "in the business" I was seriously spoiled. All I had to do was walk 75 feet across the ramp to the prop shop w/attached governor overhaul facility. The gov shop had two machines, one kept with pristine "new" oil for checking/setting overhauled governors only, the other with "serviceable" oil, used for ops/function checking as-removed governors. I couldn't run one myself, but saw 'em used quite often. As you would expect, "hot" oil was used when operating the governors. Back to the PA30 in question, I'm pretty sure that the props installed will have compact hubs, opposed to steel hubs (with external blade clamps and external pitch stops). On these props, the locks are internal, working in the sorta the same fashion (weighted plungers w/springs), engaging a notch in piston rod (connects the piston to the yoke that twists the blades) at low/no rpm to prevent feathering on shut-down. As you've pointed out, a recip feathering on shut-down (or feathering causing the shut-down) is not a "normal" situation, neither is the lack of governed oil pressure at low rpm. In this situation, I would have gone next door and asked the prop boys if they could squeeze me in for a "dome job", stopped in the gov shop, look to see if anything was bolted onto the #2 gov machine and borrowed the gov pad plate. Then, with the engine "hot" woulda popped the nose bowl and the gov, installed the plate, used a differential compression tester to check the gov-to-prop circuit, plugged it, hooked a direct reading gage to the gov oil supply port and run that sucker noting the pressure supplied to the pad. Pretty simple then to carry the removed gov over and beg the gov dude to throw it on the machine, if the machine had been "warmed up" that day, would take about five minutes to wring the gov out. Depending on hangar space, would probably then yank the prop to get the dome pulled off and the locks looked at, addressing any other issues discovered as needed. E.T. to properly diagnose 2 hours skipping no aspects of "normal" propeller/govenor ops. Repair time/cost variable as to what was found. Have had various incidents over the years where a prop feathered on shut-down. 9 times out of 10, a very careful re-start and shut-down would result in normal ops. Often would happen again shortly thereafter. If it feathered "twice" the prop got repaired, end of story. BTW, I wouldn't bet that an engine would continue to run at a "normal" idle setting with the prop feathered, but then again I've never had the opportunity to try it... TC |
Thread Tools | |
Display Modes | |
|
|