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Low RPM

FredMagare

Well Known Member
I have just conducted the first flight of my RV-9A. (Thank you Larry New for all your assistance!)

During runup, static RPM was low (about 2050 RPM). During takeoff and climb she was only producing 2300 RPM but airspeed and climb rate were acceptable, so the flight was continued. In level flight she reached 2400 RPM and 135KIAS at "full throttle".

I have two ideas about why this occurred, but I am seeking whatever thoughts or suggestions to remedy the low RPM issue.

What say you, Brother Aviators?

Specs:
0-320-D1A with Sensenich Fixed Pitch Prop (mid-pitch - neither climb nor cruise)
 
Are all your fairings installed? If so, I'd check the tach accuracy, timing, fuel flow, and MP to see if there is an engine related explanation. After that, it could be that you're over-propped.
 
What Kyle said....
The static RPM sounds pretty close (you didn't mention what the density altitude was at your airport... it does have an influence on static RPM), so I would guess that you do not yet have any of the gear fairings installed.
That will make a big difference.
 
My thinking on a fixed-pitch prop (or ground-adjustable prop) is to take it up to 7,500' or so, WOT, and see what you get with it leaned out for cruise. Don't be surprised if you're pushing 2,700 RPM in this situation and probably developing close to 75% power given the numbers you cite. Kind of a compromise between a cruise and climb setting, but you want a decent power setting for cruise. If your prop is ground adjustable, you can tweak it from there, but it sounds like you're close.
 
Sounds like you are over pitched? What brand Prop? Might talk to them, they usually have the solution if a it is a pitch problem.
IMO, As a general rule, If you are getting full throttle and appropriate fuel flow the engine should develop 2700 RPM full throttle at approximately 8000 feet.
 
Hmmm. Interesting input so far....

Prop is a Sensenich 70x79 (as recommended by Van's). I don't think it's overpitched but not sure.

Did not calculate density altitude. I'll have to do the math.

Only fairing attached was nose fairing (due to attached Nose Job stiffener) otherwise no main wheel fairings or wheel pants installed to increase drag. It seems to me that this would only affect max indicated airspeed and have little effect on prop RPM...?

I'm planning to check to make sure that I'm getting full-throw at the carburetor throttle arm (pretty sure that will be good).

Lastly, perhaps some other restriction in the intake maybe.... Planning to investigate that as well.

More thoughts are welcome. Please chime-in!
 
Only fairing attached was nose fairing (due to attached Nose Job stiffener) otherwise no main wheel fairings or wheel pants installed to increase drag. It seems to me that this would only affect max indicated airspeed and have little effect on prop RPM...?

Reducing drag will definitely increase your indicated airspeed and your RPM will increase as well. Think going downhill in your car.
 
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Hmmm. Interesting input so far....

Prop is a Sensenich 70x79 (as recommended by Van's). I don't think it's overpitched but not sure.

Did not calculate density altitude. I'll have to do the math.

Only fairing attached was nose fairing (due to attached Nose Job stiffener) otherwise no main wheel fairings or wheel pants installed to increase drag. It seems to me that this would only affect max indicated airspeed and have little effect on prop RPM...?

I'm planning to check to make sure that I'm getting full-throw at the carburetor throttle arm (pretty sure that will be good).

Lastly, perhaps some other restriction in the intake maybe.... Planning to investigate that as well.

More thoughts are welcome. Please chime-in!

I a way you just answered your question your self "no fairings, to increase drag" The more drag, the less speed. The less speed, the lower the RPM.

With a fixed pitch prop, max. speed will always be the speed at which the power output comes into balance with total drag value. If you change one of those constants it will change one of the others.

So if you had a constant speed prop. and you increased the RPM while at full throttle, that would increase the engine power output, which would make you go faster, with nothing else on the airplane being changed.

In your case, you can't change the engine RPM, but you can reduce the drag (install all the fairings). Doing so will make the airplane go faster. When it goes faster, the RPM will increase, which will then produce more power, which will then make it go even faster, which will..... well you get the idea. that will continue until the engine power output and the total drag find a balance point once again.

They fairings will have a more significant influence on a fixed pitch prop RV than one with a constant speed prop.

You said you have no main wheel fairings or wheel pants... I assume you mean main gear leg fairings or any of the wheel pants (just the nose leg fairing)? The bare main gear legs will actually be the major contributor of the total drag at this point.
 
Go fly it...

Like you I was a little concerned about the static RPM on my Sensenich metal prop - 2050 was all we could get on the ground. On the mainden flight the RPM at take-off felt a little low. In straight and level flight we were able to get it as high as 2800 RPM so I was quite concerned I would have to look at a prop change.

However... things have settled in nicely over the Phase 1 test period and while we're now getting 2090 Static RPM (possibly also helped by the installation of a Surefly SIM4N), the FT RPM is initially about 2690 RPM. If left it will eventually buiild to 2750 but it takes probably 30-40 seconds to slowly climb that high so I'm happy to live with that as I never use those kinds of settings in flight.

As the team have said above, let it settle in. Get some hours on it and then look at making any changes later once you've solidified the data.

And enjoy it.
 
I a way you just answered your question your self "no fairings, to increase drag" The more drag, the less speed. The less speed, the lower the RPM.

With a fixed pitch prop, max. speed will always be the speed at which the power output comes into balance with total drag value. If you change one of those constants it will change one of the others.

So if you had a constant speed prop. and you increased the RPM while at full throttle, that would increase the engine power output, which would make you go faster, with nothing else on the airplane being changed.

In your case, you can't change the engine RPM, but you can reduce the drag (install all the fairings). Doing so will make the airplane go faster. When it goes faster, the RPM will increase, which will then produce more power, which will then make it go even faster, which will..... well you get the idea. that will continue until the engine power output and the total drag find a balance point once again.

They fairings will have a more significant influence on a fixed pitch prop RV than one with a constant speed prop.

You said you have no main wheel fairings or wheel pants... I assume you mean main gear leg fairings or any of the wheel pants (just the nose leg fairing)? The bare main gear legs will actually be the major contributor of the total drag at this point.

Scott,

Yes, no main gear leg fairings or any wheel pants. I wanted to purposely increase drag so that I could carry higher power settings during the break-in. It would seem, that to increase the engine output, I would need to install at least some of the fairings...? Since the gear leg fairings have a greater reduction in drag than the wheel pants provide I will install those, fly, and gage the result. If it increases RPM in flight I will feel more comfortable about it all.

Again, thank you to all who have thrown their "two-cents" in.
 
Be advised the metal Sensenich prop for the O-320 has a 2600 rpm restriction due to harmonics that couldn't be engineered out the O-320 installation. The 70x79 should be a good prop for your -9. If you get 2600 rpm with nearly full throttle at 7500' DA you should have a good compromise between climb and cruise. But make sure all your fairings are installed and well-fitted.
 
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Here are the Lycoming power charts...
 

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Congrats on the first flight, Fred! It's a lot of fun to run through the test program. If you are not already, you may want to consider loading your engine data up on Savvyaviation.com to help you remember what you did, and to see any interesting trends. Best of luck!
 
Be advised the metal Sensenich prop for the O-320 has a 2600 rpm restriction due to harmonics that couldn't be engineered out the O-320 installation. The 70x79 should be a good prop for your -9. If you get 2600 rpm with nearly full throttle at 7500' DA you should have a good compromise between climb and cruise. But make sure all your fairings are installed and well-fitted.

Thank you, Sam. I have the limitation placarded on the panel. I will collect the parameters (temp) so that I can compute DA next time. The focus of our first flight was just to make sure it continued making airplane sounds and motions at altitude. :D
 
Congrats on the first flight, Fred! It's a lot of fun to run through the test program. If you are not already, you may want to consider loading your engine data up on Savvyaviation.com to help you remember what you did, and to see any interesting trends. Best of luck!

Thank you, Mickey. Fortunately, I now have an engine monitor that will allow me to do that although one of my CHT probes seems to be bad right now (it works initially then drops off-line). Already sent an email to JPI to get another.
 
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I think what you are seeing is to be expected at a lower altitude. What DA were you flying at when you had 130 KIAS? Try this same test at a DA around 8k. I have a ground adjustable sensenich and I see rpm a little bit higher than yours at SL but not by much, maybe 50 to 100 rpm. My pitch setting is more for a cruise prop setting though in my RV-8A.
 
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