Fluctuating RPM in Cruise

TedR3

TedR3

Pre-takeoff checklist
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Ted
In level cruise flight, the rpm will creep up or down 100 rpm, requiring a throttle adjustment or re-trim. The rpm change is shown on the tach, which is digital and runs off of the mags.

The plane in question is a ’79 Archer II with the standard power plant/fixed pitch prop.

The throttle friction is cranked down, so the throttle is not slipping. There is no visible movement of the mixture.

Any thoughts on what might be causing this?

I’m guessing that something is effecting the mixture, and we should see a change on EGT gauge as this occurs.
 
Prop governor not governing well?

Induction air leak?

Tank vent problem that's creating vacuum then releasing?

Could be a bunch o' stuff.
 
Check the throttle cable for looseness in it's clamps. If it moves, you've found the problem.
 
Ron Levy said:
Yes, on planes with c/s props, but the OP noted his Archer has the standard f/p prop.
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Oops. Missed that. Just saw Archer II and thought those were C/S. Not much of a Piper aficionado here. Thanks.
 
Engine surging/pulsing or goes up for a while then comes down for a while? One thing to check is the coil in one of the mags collapsing. They're tricky because the problem only shows itself under load, there is no bench test that will uncover the problem. The way I find it is by using a loaner mag and swapping to see if the problem goes away. If it does, it's time for a mag.
 
denverpilot said:
Oops. Missed that. Just saw Archer II and thought those were C/S. Not much of a Piper aficionado here. Thanks.
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You have to go to a 235 Cherokee/Dakota or Arrow before you see a c/s prop (and even the first year or two of 235's had f/p props).
 
Henning said:
Engine surging/pulsing or goes up for a while then comes down for a while? One thing to check is the coil in one of the mags collapsing. They're tricky because the problem only shows itself under load, there is no bench test that will uncover the problem. The way I find it is by using a loaner mag and swapping to see if the problem goes away. If it does, it's time for a mag.
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The the coil problem I experienced happened shortly after takeoff, after the mag got good & hot, and there was no intermittent failure. It just died. Only after landing & cooldown would the mag fire again and run normal. This coil tested bad with an ohm meter on the bench

OP, what do the P-leads look like? 45 year old junk wiring?
 
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TedR3 said:
In level cruise flight, the rpm will creep up or down 100 rpm,
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How long before this starts happening? Right when you reach cruise? 10min after? hour?
TedR3 said:
requiring a throttle adjustment or re-trim.
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After the adjustment does it stay put?
TedR3 said:
The rpm change is shown on the tach, which is digital and runs off of the mags.
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Which Tach are you running?
Can you notice a change in power?



Henning said:
One thing to check is the coil in one of the mags collapsing. They're tricky because the problem only shows itself under load, there is no bench test that will uncover the problem.
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Im pretty sure you can test those conditions with equipment like a merc o tronic, but most people don't have one sitting around.
Henning said:
The way I find it is by using a loaner mag and swapping to see if the problem goes away. If it does, it's time for a mag.
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That works also, but if I remember correctly I thought they ran off both mags.
 
Need to ascertain here whether or not the OP thinks the engine is having momentary power drops, from any other symptoms. A simple wandering RPM on a fixed pitch prop with autopilot holding altitude is indicative of rising/falling air parcels, which happens on every flight.

If you have full engine monitoring it's a lot easier to answer this question, but I'm guessing that would be too much to ask for on a '79 Archer.
 
Not sure if it applies but many new airplane drivers have a huge problem finding a balance between trim & power.

In level flight the plane may hunt, especially in turbulance. It pitches up slightly climbing & slowing, the RPM drops slightly too, then the nose finally drops, it descends a little speeding up and the RPM increases. This is just how they work (non-constant speed props).

When I level off:

#1 Arrest the climb with elevator pressure, keep the power up.
#2 Let the airplane accelerate to near cruise speeds
#3 As the airplane reaches cruise simultaneously reduce throttle
#4 Trim for hands off

Eventually it becomes one fluid motion.
 
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bnt83 said:
The the coil problem I experienced happened shortly after takeoff, after the mag got good & hot, and there was no intermittent failure. It just died. Only after landing & cooldown would the mag fire again and run normal. This coil tested bad with an ohm meter on the bench

OP, what do the P-leads look like? 45 year old junk wiring?
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I was working on a Chieftain, left engine would surge on takeoff power, mag was bench tested good. Governor was replaced, same problem, fuel injection system was completely overhauled, same problem, I swapped mag between the engines, the problem swapped engines. Coils can fail in strange ways.
 
Coil, mag, induction air leak? Lots of things could cause this.

Mine did that until we traced it down to a leaky carb - sucking air in through the mixture valve (or whatever its called). Once that was repaired/replaced, no more problems. Also caused rough running at idle and the engine would diesel when trying to kill it with mixture. But in cruise, small fluxuations.
 
Thanks to all for your input.

I am not the pilot - it's a club plane and the issue was reported by another member on long trip.

The plane does not have an autopilot with altitude hold so that's not it.

I guess an inspection of the carb, air induction and mags is in order.
 
TedR3 said:
Thanks to all for your input.

I am not the pilot - it's a club plane and the issue was reported by another member on long trip.

The plane does not have an autopilot with altitude hold so that's not it.

I guess an inspection of the carb, air induction and mags is in order.
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Even more of an indication that the airplane or pilot is inducing porposing. Even slight increases or decreases in climb will affect RPM's.
 
TedR3 said:
Thanks to all for your input.

I am not the pilot - it's a club plane and the issue was reported by another member on long trip.

The plane does not have an autopilot with altitude hold so that's not it.

I guess an inspection of the carb, air induction and mags is in order.
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You need to fly it first before believing the report.
 
Geico266 said:
Even more of an indication that the airplane or pilot is inducing porposing. Even slight increases or decreases in climb will affect RPM's.
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And that's due to poor trim technique.

Dan
 
I would put my money on the problem being the pilot. It's amazing the amount if crap rental pilots report as broken that isn't.
 
Dan Thomas said:
And that's due to poor trim technique.

Dan
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It can also a result of long wavelength wind waves when flying relatively low. I flew home from DAY yesterday at 2,500' ducking under 45 kt headwinds. Even down low we bucked 20 to 25 knots for 3 hours.

For about 5 minutes I'd be doing 120 kts with me applying quite a bit of forward pressure on the yoke to keep from climbing @ 500 FPM or more.

This would be followed by about 5 minutes of doing 100 kts with me applying significant back pressure on the yoke to keep freom descending.

There was maybe a minute of straight and level 110 kt flight inbetwixt The gyrations.

Repeat, repeat, repeat. For three hours.

Note: I'm guessing on the times, could have been 10 minutes and not five...I'm not sure since I never timed it. I just flew home enjoyed the colors and tried to tolerate the 320 nm long roller coaster.
 
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timwinters said:
It can also a result of long wavelength wind waves when flying relatively low. I flew home from DAY yesterday at 2,500' ducking under 45 kt headwinds. Even down low we bucked 20 to 25 knots for 3 hours.

For about 5 minutes I'd be doing 120 kts with me applying quite a bit of forward pressure on the yoke to keep from climbing @ 500 FPM or more.

This would be followed by about 5 minutes of doing 100 kts with me applying significant back pressure on the yoke to keep freom descending.

There was maybe a minute of straight and level 110 kt flight inbetwixt The gyrations.

Repeat, repeat, repeat. For three hours.
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That pretty much describes our recent trip from SJT to PHX.
 
jesse said:
I would put my money on the problem being the pilot. It's amazing the amount if crap rental pilots report as broken that isn't.
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On the flip side, stuff that's broke but never reported. :)
 
bnt83 said:
On the flip side, stuff that's broke but never reported. :)
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Or stuff that's reported but always deferred by the rental place.

Rentals generally just suck. There's exceptions and good clubs that maintain things well, but generally...
 
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