MP decreasing when pulling the Prop Back?

Fearless Tower

Fearless Tower

Touchdown! Greaser!
Joined
Jan 1, 2010
Messages
16,473
Location
Norfolk, VA
Display Name
Display name:
Fearless Tower
Been troubleshooting an odd problem with my left engine (R-985) on the Twin Beech and running out of ideas, so I figure I'll throw this out here and see if anyone has any thoughts.

FWIW, the R-985 is a supercharged engine (has an an attached blower to boost MP). Unlike a turbo, it is direct drive with no waste gate.

Both engines are perfectly matched below about 6,500' MSL. As I climb above 6500' I have to start pushing the left throttle up past the right to keep MP matched. By 8000' I'm WOT on the left with MP dropping in the climb. For reference the right engine at WOT at the same altitude will give about 2" MP more than the left.

Now, the interesting thing I recently noted is that when I transition from climb to cruise up high, I lose MP on the left engine when I pull the prop back to cruise RPM. I've never seen that before in a constant speed prop. For example, if I level off at 11,000', I'll be a WOT on the left and 26.5" MP. As I pull the prop back from 2100 to 1800 RPM, the left MP will drop to 25" MP (right engine doesn't change with the prop at that altitude, so I end up pulling the right engine back to match.

I don't believe it is a gauge issue: The engines are perfectly synced and fuel burn between sides is within a gallon or two after a 3+ hour flight (for reference, cruise burn is about 22-23 GPH per engine).

We have been talking extensively with Covington and have not been able to find any cause. We soap checked the engine at the annual and found no induction leaks. Inspected the blower and it is in fine shape.

Any engine guys care to weigh in?
 
I'd definitely give it a compression test on all cylinders. If that checks out ok, then start looking at troubleshooting the blower.
 
coloradobluesky said:
I'd definitely give it a compression test on all cylinders. If that checks out ok, then start looking at troubleshooting the blower.
Click to expand...
We checked compressions - all perfect. Not sure what else to troubleshoot on the blower: we scoped both sides of the blower per Covington's recommendation. Both sides of the impellers look fine - no FOD damage.

It seems more like I'm not getting full fuel flow from the left engine up high, but not sure how that would relate to the MP drop when pulling the prop back.
 
Sensing line or gauge itself?
 
GRG55 said:
Sensing line or gauge itself?
Click to expand...
I don't think its the gauge - the fuel burn is virtually identical when the right engine is pulled back to match the left on the gauge and the props are clearly in sync.
 
Could you swap gauges to see if problem follows? At least you would take the gauge out of the equation..
 
Does the engine on the right and engine on the left pull the same as they used to before this problem occurred? Do you have away to test single engine performance? If I can make 75% power book, previously tested indicated airspeed (same density altitude) I am pretty sure my engine is performing the same as it did before. But its just a single engine. There might be a way to test the in flight performance of one engine. Like fly on one engine and see how fast it will go. You'd want to try it on known good engines and log the performance numbers and density altitude.
 
Last edited:
What is it that keeps the MP up on a supercharged engine when you pull back the RPMs (which proportionally reduces the speed of the supercharger)? If I just attached a blower to a crankshaft and increased the load on the crankshaft to reduce RPM, I would expect the blower also to have reduced output and thus a reduced MP. So given that your normal engine behavior does not show a reduced MP with reduced RPM and no throttle movement, I wonder if there is another piece of equipment involved that might be broken on the left side. (I know very little about superchargers. But I am something of an expert in overthinking problems and forgetting to check the simplest possible causes.)
 
Fearless Tower said:
It's a combined gauge. I suppose I could look at swapping gauge lines.
Click to expand...
If you can legally do that for as a temporary measure for a flight test with a mechanic's blessing, that would help you eliminate the obvious elephant in the room.
Let us know what you find, this one sounds interesting. I kinda agree with Ari that the #1 blower might be going if it cannot provide enough pressure at lower RPM.
I guess swapping the chargers is out of the question?
 
I'm certian you've checked this but mabe; WOT on the left isn't actually WOT, and the blower is compensating at lower altitude. Or, blower troubles.
 
bluerooster said:
I'm certian you've checked this but mabe; WOT on the left isn't actually WOT, and the blower is compensating at lower altitude. Or, blower troubles.
Click to expand...
I don't think we checked that yet, but I think that will be the next thing we do
 
bluerooster said:
I'm certian you've checked this but mabe; WOT on the left isn't actually WOT, and the blower is compensating at lower altitude. Or, blower troubles.
Click to expand...

If it's a direct drive uncontrolled SC, different throttle position would mean different MP even at takeoff. A wastegated charger setup would compensate, direct uncontrolled won't.
How is the SC driven? A slipping belt in car engines would cause symptoms like that.

Or the simplest thing: Is your air filter clean?
 
Sounds like the blower if your good up to 6,500 ft. You'd think a gauge issue would show at any altitude? Have you pulled the insulation wrap off of the blower to inspect housing for hairline cracks? Flange and fittings are the obviouse culprits.

One other thing to check against the right side, do you have fuel flow gauges? Most OEM flow gauges are not that accurate, but they will give trend information from what would be considered 'normal' indication at normal cruise power settings. If you are manually adjusting down the right side to meet the left, what is the fuel indication & calculated fuel burn vs actual, at the MP/RPM settings for target %HP BEFORE adjusting the right engine to the left?

FWIW, in cruise with my turbo'd engine, I'll pull the prop back 50-100rpm first in decent and the MP drops an inch, which is expected.
 
Reading this week the instructions for installing an Electronics International engine monitoring system. They say that some airplanes had a tiny hole--a deliberate leak--in the MP gauge line to discourage fuel or fumes working their way into the instrument. In a twin, one open hole and one plugged (or omitted when the line was replaced) would cause the OP's differential.

Go here: http://buy-ei.com/wp-content/uploads/2013/06/II-MVP-50.pdf

Scroll down to page 16.

Just a thought.
 
Dan Thomas said:
Reading this week the instructions for installing an Electronics International engine monitoring system. They say that some airplanes had a tiny hole--a deliberate leak--in the MP gauge line to discourage fuel or fumes working their way into the instrument. In a twin, one open hole and one plugged (or omitted when the line was replaced) would cause the OP's differential.

Go here: http://buy-ei.com/wp-content/uploads/2013/06/II-MVP-50.pdf

Scroll down to page 16.

Just a thought.
Click to expand...
Are you suggesting that such a scenario would result in a loss of power (or preventing from achieving full power at WOT? Or just a mismatch gauge indication?

At WOT, there is most definitely a power deferential between the two engines, and not just an issue with the indication.

I'm asking because I am no expert on induction systems.
 
and no waste gate, or intake governer that adjusts automatically (to prevent overboost???)
 
Fearless Tower said:
Are you suggesting that such a scenario would result in a loss of power (or preventing from achieving full power at WOT? Or just a mismatch gauge indication?

At WOT, there is most definitely a power deferential between the two engines, and not just an issue with the indication.

I'm asking because I am no expert on induction systems.
Click to expand...

It shouldn't make a significant difference in power. The side with the leak would be a tiny bit leaner, maybe not enough to notice. It would cause a difference between gauges.
 
Dan Thomas said:
It shouldn't make a significant difference in power. The side with the leak would be a tiny bit leaner, maybe not enough to notice. It would cause a difference between gauges.
Click to expand...
Sounds like that isn't my issue then. My left engine is definitely putting out less power than the right above 6500'.
 
I would suggest calling Rex at Tulsa aircraft engines. The Covington guys are good but Rex is better. I've dealt with both on the 1340 Pratt.

Have you noticed any difficulty in getting the engines synced or noticed any change in the engine note? If you have one engine making less power than the other with no abnormal indications you could have a cam going bad. the engine will start to sound like a twin slightly out of phase long before it quits on you. If you hear that sound and don't have performance degradation then you most likely have a main bearing going out.
 
IMG_2813.JPG This is the bearing we caught before failure because I could hear a change in the engine. It sounded like a twin out of sync as I previously described. There were no performance losses as there would have been with a cam going out but they sound the same.
 
Last edited:
Tarheelpilot said:
I would suggest calling Rex at Tulsa aircraft engines. The Covington guys are good but Rex is better. I've dealt with both on the 1340 Pratt.

Have you noticed any difficulty in getting the engines synced or noticed any change in the engine note? If you have one engine making less power than the other with no abnormal indications you could have a cam going bad. the engine will start to sound like a twin slightly out of phase long before it quits on you. If you hear that sound and don't have performance degradation then you most likely have a main bearing going out.
Click to expand...
Thanks. Will give Tulsa a call.

Haven't had any problems getting/keeping engines in sync. There is a power degradation, but only above 6500'. Everything is perfect down low.
 
You must log in or register to reply here.
Back
Top