RPM Problem

I have an RV-9A with a O-320 and a C/S prop in it. I was flying at 10,500 ft and started a decent. I pulled the throttle back but the Digital RPM gauge showed the rpms at redline. The engine was running well below redline as the manifold pressure was 15. At the time I contributed it to the turbulence i was encountering.
Today was the first flight I went on since the above incident. After 1000ft agl i brought the throttle back to obtain 2200 rpms, however: i kept bringing throttle back but but the RPM was still showing over 2400 and the manifold was at 12. The engine was definitely at close to idle. I advanced the throttle as to stay flying and the rpm was showing redline.
So I have an indication problem. I have a light speed ignition and a slick Mag. I did not build the airplane, so I was tracking down where the TACH gets its signal from. I confirmed it comes off the MAG by disconnecting the wire on the mag and running it. I had no RPM indications.
With that said I am not sure wether the MAG's output is bad or is it the TAC. I have an EII (Electronics International Inc) R-1 RPM indicator. Does anyone have any suggestions on which way to proceed? Oh, during sunup and mag check everything was fine.

Tim Crouch
Dues Paid 2016

It could be a loose connection on the P lead or the point that the Tach connects to the P lead, but my guess would be a problem with the Tach circuitry itself. You can test the continuity from the Tach all the way back to the mag. You are looking for the connector that taps on to the condensor, not the ground. However, I would think that a loose connection would show as a lower than indicated RPM, based upon my understanding of how they work. I suppose a bad condensor in the mag could confuse the tach. Maybe and EE that understands the electronics behind these digital RPM devices will chime in.

I don't know anything about your tach, but modern EMS' can take a P lead or a signal from an EI. If that option exists for you, you could consider picking up the signal from the Lightspeed. It's possible that the tach has a different circuit for that and will bypass the failed components.

Larry

Are you saying that you pull the PROP control back? Because it sounds like you are trying to control RPM with the throttle. If so, we have an altogether different problem. Have you flown CS props before?

It appears to me that you need a bit of education on C/S props... Read John Deakins article here: http://www.avweb.com/news/pelican/Pelicans-Perch-16-Those-Marvelous-Props-182082-1.html

aerhed said:

Are you saying that you pull the PROP control back? Because it sounds like you are trying to control RPM with the throttle. If so, we have an altogether different problem. Have you flown CS props before?

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Edited by author

Ok, in my haste to post and several background distractions my description has alot to be desired. I have 400 hrs in high performance c/s proped planes. So i will try and explain this again.
After climbout 1000ft agl i pull throttle back to where the manifold is at 22in then prop out till rpm slows to 2200. Whoo much better. The problem is that as i pull the throttle back to 22in on manifold the engine rpm is up around 2700 redline. It should have at least come down a couple of hundred rpm.
So i know the engine is no where near 2700. So i come back for landing pull throttle 1/3 to 1/2 way out and the Rpm is 2600 to redline and my manifold is showing 12 in. There is no way the engine is running that fast.
I removed the p lead off the mag and i had no engine rpm. So i know the rpm guage is using the mag not the lightspeed for its in put.
Anyway I think it is the indicator or a isolator in the circuit. I guess i will give EII a call.

Tim

Tim,
I'm not sure where to start....
The prop rpm is controlled by your prop governor in relation to the position of your prop control knob (or lever) - blue. The rpm that you're reading is prop rpm, not engine rpm. Do you understand this?

B Cunningham said:

Tim,
I'm not sure where to start....
The prop rpm is controlled by your prop governor in relation to the position of your prop control knob (or lever) - blue. The rpm that you're reading is prop rpm, not engine rpm. Do you understand this?

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Clearly you struggle to grasp the details yourself, so maybe you should stop attacking the O/P for not understanding his machine and how it operates. The only interface between the prop and the engine is 6 bolts and their bushings. They ALWAYS turn at the same speed.

Larry

TJCF16 said:

...
The problem is that as i pull the throttle back to 22in on manifold the engine rpm is up around 2700 redline. It should have at least come down a couple of hundred rpm.

Tim

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It depends on the range of pitch available to the prop, but most C/S setups can hold redline RPM in cruise flight at 22" MP. I think you'll need a larger reduction in power than that before the prop is up against the stops. Sounds to me like normal operation.

I don't have a lot of experience with CS props, just a handful of hours in a 182. However, I don't understand how you can get 12" of map at 2600 without being in a dive. That is idle level vacuum and the engine would have to be very lightly loaded. That is why I assumed the RPM indication was off. Can you help us understand this better.

Larry

BobTurner said:

It depends on the range of pitch available to the prop, but most C/S setups can hold redline RPM in cruise flight at 22" MP. I think you'll need a larger reduction in power than that before the prop is up against the stops. Sounds to me like normal operation.

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This all sounds like normal operation to me. Try testing your prop control in level flight. I think it's your descents that are keeping your prop RPM up.

Tim,

Per our phone conversation earlier today, please let us know what you find when you have had the chance to perform an in-flight mag check with the engine squared. The R-1 will always display the RPM from which it receives the highest signal. If your electronic ignition is supplying the highest RPM, it will display that. Same for your magneto. If the highest source drops 100 RPM or more, the R-1 automatically switches to the alternate source. The in-flight mag check should reveal if it's truly an indication error or not. At the very least, it will hopefully assist in isolating the problem.

Dave Arata
Sales/Support
Electronics International

Dave
Thanks i will try it Saturday Am. I am very curious to tryout your troubleshooting idea. I am also going to check and see if cycles between LEDs as i believe this will alert of possible mag failing. To all that have replied i think some are getting to hung up on the C/S prop. I have not even touched the prop lever it is in and prop on stops which the engine shiuld run just like a fixed pitch.

TJCF16 said:

Dave
To all that have replied i think some are getting to hung up on the C/S prop. I have not even touched the prop lever it is in and prop on stops which the engine shiuld run just like a fixed pitch.

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The reason 'some are getting hung up on the C/S prop' is because you keep posting incorrect statements, which we feel should be corrected. If your prop is correctly adjusted it will be on the fine pitch stops at the start of takeoff. But it won't run like a fixed pitch prop. If it did, by the time you had flying airspeed the engine would be over redline. Instead, the CS prop comes off the stops to a coarser pitch.

TJCF16 said:

To all that have replied i think some are getting to hung up on the C/S prop. I have not even touched the prop lever it is in and prop on stops which the engine shiuld run just like a fixed pitch.

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Actually in everyone's defense that has tried to help so far, the way you have worded your posts has been a bit confusing to me so maybe it is to everyone else.

You previously wrote "After climbout 1000ft agl i pull throttle back to where the manifold is at 22in then prop out till rpm slows to 2200. Whoo much better. The problem is that as i pull the throttle back to 22in on manifold the engine rpm is up around 2700 redline.

So first you pulled the throttle to 22"
Then you pulled prop control until the tach read 2200 RPM, but you pulled the throttle back to 22' and the RPM stayed at 2700? So which is it. The RPM reduced to 2200, or it stayed at 2700?

As for normal operation, a constant speed prop will not act like a fixed pitch with throttle movement if the prop control is set for high/maximum.

It will only do that after the blades have reach the high pitch stops. Reducing the Mp to 22" on an RV is not enough to cause the blades to hit the high pitch stops and then get a reduction in RPM. With the speed that RV's fly, there is still quite a bit of pitch rolled into the blades at 22".

What you are describing and expecting, may be normal with many slower certificated airplanes that have constant speed props, but with an RV it is not.

BTW, it is pretty easy to audibly hear even a small change in prop RPM so regardless of how you believe or agree it should work, you if think the RPM has changed but the tach shows that it has not, you should be hearing the change.