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RPM high at idle throttle inflight

Raven31

Member
Situation: new build RV-8 with Superior IO-360M1S, Hartzell G2YRN/7605W-2X, PCU5000 governor.

No issues on the ground. Runup is good and prop cycles as it should. Engine will idle down to 600rpm.

In the air when I pull the throttle to idle I’m still at 1600 rpm. I can pull the prop control back and the rpm drops to flight idle. If I push it forward the rpm returns to 1600.

Just wondering if anyone has had this happen and if anyone has suggestions of what I should do/check?

I’ve talked to the guys at Aerosport Power, PCU guys and one Hartzell rep. No solutions as of yet.

Thanks in advanced.

Rob
 
Why do you think something is wrong? With prop control forward the prop will be in flat pitch, and the forward motion thru the air will drive it. You should notice an improvement in glide ratio when you pull the prop control back.
Edit: How much hockey is there in SC?
 
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That's how constant speed works, isn't it? At lower power settings, the air moving over the prop is what causes the prop RPM to stay higher at faster speed, but drop off as you approach the runway.

What's your MP like when on the ground at idle? Full throttle? How about both of those in the air?
 
That's how constant speed works, isn't it? At lower power settings, the air moving over the prop is what causes the prop RPM to stay higher at faster speed, but drop off as you approach the runway.

What's your MP like when on the ground at idle? Full throttle? How about both of those in the air?

Hey Brandon,

Yes, but this is happening at lower speeds like 70-80kts in testing. This requires me to carry power around the final turn.

This is abnormal from my 2 previous C/S RVs.

Rob
 
Why do you think something is wrong? With prop control forward the prop will be in flat pitch, and the forward motion thru the air will drive it. You should notice an improvement in glide ratio when you pull the prop control back.
Edit: How much hockey is there in SC?

Hi Bob,

I’m used to being able to go idle abeam the numbers and fly a constant airspeed around the final turn. If I try that in this plane I have a giant speed brake in front of me and I’d never make the runway. I end up carrying a lot of power to hold airspeed. Maybe it’s from the fat blades of this prop but I feel like something else is going on.

Yes, there is a huge improvement in glide with the prop control back. I don’t remember 1600 rpm at idle throttle, 70-80kts in my other RVs though.

Curious what anyone else with the same engine/prop combo see.

There is some hockey in SC, not as much as central NY where I grew up though. You’ve reminded me I need to update my profile. Just moved to CA a couple weeks ago. Maybe I’ll come visit.
 
Without a more detailed description, it sounds normal to me.

Not clear on why you think something's wrong?
 
Low pitch angle stop

Coupling the high idle speed and the fact that the a/c glides like a stone, it is most likely an unadjusted /mis-adusted low angle stop. Common rookie mistake.

Follow the approved data, of course - the manual. Generally, set the gov for 2700. Then set the low angle adj to pull the RPM down a tad to, say 2675. That will give full takeoff RPM after a few feet of roll and the best power off glide for the prop if the prop control is full forward. It will also reduce overspeed in case of rapid throttle movement.

Hum, couldn't find your prop in the Hartzell application catalog on line, but I assume that it is 74" by the numbers. That big prop should be a real stump puller, but also a big air brake, especially if it is set up too flat.

Ron
 
I have the same composite prop on my RV-14A. I don’t go full forward with the pitch until short final. Otherwise, it makes a great speed brake. It also allows you to significantly increase rate of descent if you find yourself a little high on the glideslope. :D
 
Coupling the high idle speed and the fact that the a/c glides like a stone, it is most likely an unadjusted /mis-adusted low angle stop. Common rookie mistake.

Follow the approved data, of course - the manual. Generally, set the gov for 2700. Then set the low angle adj to pull the RPM down a tad to, say 2675. That will give full takeoff RPM after a few feet of roll and the best power off glide for the prop if the prop control is full forward. It will also reduce overspeed in case of rapid throttle movement.

Hum, couldn't find your prop in the Hartzell application catalog on line, but I assume that it is 74" by the numbers. That big prop should be a real stump puller, but also a big air brake, especially if it is set up too flat.

Ron

Hey Ron,

Thanks for your response. I spoke with Kevin at Hartzell today and ran that by him and he agreed. Next step is to perform the static RPM check per the book like you said and make the adjustments to the low angle pitch stop.

Thanks again for your help I really appreciate it!

I’ll let you know how it goes.

Rob
 
Just wanted to follow up on this post. Issue has been resolved.

Ron was spot on with the Low Pitch Stop adjustment. I made several small adjustments and test flew until I finally had the result I was looking for.

Highly recommend folks keep an eye on MP and RPM during power reductions on your test flights with C/S props. At 4 inches of MP I was still at 2400 RPM until my speed reduced below 100kts.

Rob
 
There seems to be quite a few of comments implying that this is (was) normal...so now I’m wondering what is normal. If you loose power in flight will you now have a better glide ratio? I’ve flown a plane with an MT prop and MT governor that was setup from the factory...it behaved like an air brake when you pull power and wouldn’t drop rpm until you were below 100...I thought that is how they worked?
 
My -8 used to do that and I never thought twice about it, maybe I should have. But with power off and full flaps, I could do spectacular short approaches.

And when I did a wheel landing without bouncing after a power off short approach, that was a really good day!
 
Glide Ratio

There seems to be quite a few of comments implying that this is (was) normal...so now I’m wondering what is normal. If you loose power in flight will you now have a better glide ratio? I’ve flown a plane with an MT prop and MT governor that was setup from the factory...it behaved like an air brake when you pull power and wouldn’t drop rpm until you were below 100...I thought that is how they worked?

Yes, this will affect your engine out glide ratio IF you lose the ability for the governor to supply oil to the prop. The fine pitch stop that's being referred to is (on Hartzell propellers) an adjustable hard-stop on the propeller hub that is easily adjusted with a screw and jamb nut that is located on the nose of the hub. This stop physically limits how fine (flat) the propeller blade pitch can go, regardless of the governor setting or oil pressure supplied to the prop. I have no idea if MT props have adjustable fine-pitch stops.

The fine pitch stop can also reduce or prevent the tendency for engine over-rev when advancing the throttle on the initial take-off roll. I personally set my fine pitch stop so that engine will only reach 2650 RPM on a static run up. The prop will quickly reach governor speed (2700) very early in the takeoff roll.

Skylor
 
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