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RPM Lags / Doesn't increase

plbarrett2004

plbarrett2004

Active Member
Hello all,

I've now seen some odd behavior by my prop/ governor on two occasions. The first time during some climb and range testing I noticed that if I was cruising with a lower RPM, say 1900, and low MP, say 20", that placing the condition lever full forward did not result in full RPM. As I would bring MP forward RPM would increase, but behind MP, and not to its full amount. To the extent that I would have to be careful not to go too far over square. I found that given a little time and airspeed variation the prop would eventually get back to normal RPM.

The second time today, I was cruising at 21" and 1900 RPM for a good bit. Then I started a descent and return to my home airport. It was a slow decent and I kept the cruise settings the same. Upon reaching pattern altitude, I brought the prop forward, but no real response. I played with bringing MP forward to "tow" the RPM with it, but no real success. At this point I was on downwind, so it was time for fuel pump, prop and mixture full forward, get setup for landing. But I was keeping an eye on the RPM. It still seemed low. As I landed and brought power forward for a touch and go, RPM was still lagging. I "think" I felt a difference, but in the moment that might have been my hypersensitivity. I quick glance at the gauge showed me going super high on MP while RPM was still way lower than it should be, despite the lever being full forward. Not wanting to run massively over square - I pulled the MP back and aborted the touch and go.

Quick triage shows all of the connections to be appropriate. Any thoughts on what could be causing this? I have been having slightly low oil pressure (detailed in another post) but I don't see a connection.

Attached is the engine data showing MP and RPM. Near the end what isn't show is when go full forward on the condition lever, but it occurs just prior to the MP going up.

Thanks!
 

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Hi Peter, I'm not sure if you have seen this, but unless your prop has specific RPM restrictions, there is nothing particularly bad about "oversquare" - RMP/100 and inches of MP are just numbers.

https://www.avweb.com/features_old/why-over-square-is-good/

In the graph you show does show a lot of variation over short periods of time - how confident are you in the sensors and the wiring to your engine management system?
 
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What sort of governor/prop?
My governor (PCU5000) won’t govern below 2000RPM regardless of how I try to achieve that. By the time you’re in the pattern at that low an MP it may as well be a fixed pitch. The governor then won’t start working until it gets enough RPM on go around and that would likely produce large lags. Especially if you are aggressive with the throttle.
What RPM do you check your governor at runup? What behavior does it exhibit?

I’ve never thought of trying that low an RPM in flight tbh. It’s not a turbine and not a condition lever.
 
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Thanks all.

Mickey - yes I have seen the article, and I'm not afraid to run oversquare. That said, there are limits to how far over square you can go - and I didn't want to find myself in a 29" and 1800 RPM situation. Lycoming has a chart that gives the limits for continuous operation for this engine, and the maximum MP at 1900 RPM is 25.5" MP. Chart attached.

The graph is a 2.2 hour flight so the variations aren't quite as severe as they maybe look. The variations at the end are attempts to bring RPM back up, and changes in power for the pattern and the aborted touch and go.

Richard - The prop is a Whirlwind 74 HRT, and the governor is a Jihostoj P-920-028/A. I do run up around 1800 RPM, and the prop goes coarse and returns to fine pitch as it should at that time. That said, after initial engine start, or after the prop has been pulled it does take 2000+ RPM for that initial prop cycle. Given what you say, and looking at the graph in a bit more detail, I suspect that this lag is normal for low prop rpm, and that in this instance I simply put myself in a situation where I didn't give it enough time to come up. And yes, I know it's not a turbine and a condition lever - just a mental slip from my time flying with a condition lever. :)
 

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Hi Peter
Yes I agree. I don't think you have enough MAP in order to get your RPM up into the governing range.
My RV10 requires 20" on the ground to get the RPM up to 2000RPM.
I went and had a look at my RV7 data - 700hrs and the same governor as you but with a Hartzell prop.
It would cycle as yours does at a lower RPM. that said, it still required 17-18" in order to get 1800RPM on the ground. I imagine yours would be similar.

As a separate question, Im interested in your decision to run such low RPMs.
Admittedly I had RPM restrictions on my Hartzell, but it had a definite cruise sweet spot around 2400 RPM. It would get slower above 2600 and was noticeably slower below 2300.

Cheers
 
And perhaps I've learned my plane doesn't like 1900 RPM! :D

I flew it tonight - investigating the oil pressure issue i have also posted. Still investigating that, but prop RPM performed as it should the entire flight. I didn't go below 2300 RPM.
 
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