RPM Drops and Recovery during flight

Sunday Puncher

Sunday Puncher

Filing Flight Plan
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Jul 10, 2024
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Ben Burns
Alright time for stump the chumps! First of all I am brand new here (and also to aviation) so thank you all for your time and energy, and ideas.

I have a 79' Grumman AA5A Cheetah with an 0320-E2G my issue is as follows. I have noticed lately a slow drop in RPM's during cruise flight. I am up leveled off, trimmed out cruising at lets say 2350 rpm and then the engine will slowly and smoothly drop down to 2290 or 2270 even.... I say slowly, it takes maybe 10-15 seconds or maybe even slightly longer for this to happen. I noticed first by a slow loss of altitude as I am now not trimmed for the reduced power. Started paying attention more, added power back in and resumed normal cruise... then some time later... maybe a minute or two it would "recover" and climb back up in RPM, so that now I was higher than before.

Through some experimentation, If I do not adjust throttle during this, it will follow roughly the same pattern as I just described, but recover to almost where it started (I say almost bc sometimes I swear it goes slightly higher than it was previsouly set at). It never runs rough, or coughs at all, its always a very smooth process. Usually once it happens it will continue to happen intermintenly for the rest of the flight. I have an engine moniter and it does not appear to be affecting the engine temps, or at least I have not noticed it.

It seems to happen after engine is warm and been running for a while but not always. More background on engine, it has one an electroair electronic mag on the right side of firewall and a traditional magneto on the left. During runup and mag test the traditional mag tests within spec and I get a very consistent 60-70 RPM drop. I have a digitial Tach so the exactness of the RPM's I have quoted are accurate. I do not know how many hours on are that mag either but could find out. I beleive the electroair was put on when the high compression cylinders were added and has a TT of around 350.

Other simple things I have checked throttle friction lock to see if it was slowing working its way out, it is not.

Any other questions about this please ask I will asnwer as best I can. I mention things about the mags because that seemed the most likely culprit.... but again I am very new. Thank you all in advance and I am currious to hear what you think.
 
Kinda sounds like normalish operation. If I set RPM it will eventually change as the airplane finds equalibrium, then trim to meet it. If there is any convection, updrafts, downdrafts, or even light to moderate turbulence forget it.

Even worse with passengers moving around.

The beauty of a constant speed prop shines here.
 
bnt83 said:
Kinda sounds like normalish operation. If I set RPM it will eventually change as the airplane finds equalibrium, then trim to meet it. If there is any convection, updrafts, downdrafts, or even light to moderate turbulence forget it. ...
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Agreed, this is the most likely explanation. With a fixed pitch prop, any time the airplane hits any updraft, downdraft, or pocket of air having slightly different temperature or density (which it does all the time), the engine will speed up or slow down slightly as the airplane descends or climbs. This is most easily noticeable in cruise flight when the airplane is perfectly trimmed.

Indeed, when flying single pilot, simply leaning forward or back a few inches can cause pitch/RPM oscillation like this.
 
Clip4 said:
Sounds like a tachometer issue or a dry cable.
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Electronic tach. No cable. A dry or sticky cable will make the tach erratic; it won't make it underread steadily. That would require slippage, such as the drive key on the cable worn off, and the RPM wouldn't recover after it fell.
Skyrys62 said:
Could a fuel vent cause this?
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The RPM would keep falling if the venting was plugged. Fuel flow would gradually diminish until the engine quit.

Gary M suggested trying the carb heat. Carb icing is way too common, and proper teaching and understanding of the physics of it is way too rare. Before you fly, check the METARS and see what the temperature and dewpoints are. They're in the METAR for good reasons, and carb ice is one of them. When they're close, ice is possible, even in the summer on a nice clear and hot day. Carb ice is not mainly a wintertime thing.

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Another couple of things to check: See if the mixture and carb heat controls are loose and shifting in flight.
 
My AA5B does the same thing. It does it even under low dew point conditions that will not produce carb ice, and in smooth air. Mixture control is not loose, the carb heat system operates as it should (and the door seals well against the air box). RPM can oscillate over a range of ca. 40 rpm. In high speed cruise I set it just below red line and let it vary as it wants. There's a few knot variation in air speed, and I have to pay attention to maintain altitude, but it's been benign for the 12 years that I've owned the plane.

The attached photos are from a flight at 7500 ft about a week ago showing the panel at both 2670 and 2660 rpm. I had trimmed the plane at about 2680 rpm/135 kt/8.9 gph, and it varied to as low as about 2650 rpm/133 kt/8.7 gph. I can live with this.
 

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How about the throttle friction lock? Could the lever/knob be sliding from vibration? You're able to "recover", which suggests the throttle closing slightly.
 
As described I suspect it’s the phugoid oscillations already mentioned. Picture a river with a rock in it and look at the ripples that set up downstream from the rock. Going up the ripple takes more power and slows you down. Going down the back side speeds you up. Same with air, especially that has gone over mountains, even far away. I see it almost constantly when flying back east from the west, especially between El Paso and maybe Ft Stockton (which I tend to hit mid-late afternoon). It can be so pronounced, especially in summer, that I need to ask for a block altitude because I can’t keep up with the downdrafts all the time. Its annoying but not dangerous.

Second possibility could be an ignition issue - most likely a spark plug rather than a mag. When you’re in the low RPM part of the cycle, try doing a mag check in the air. Watch all four EGTs and ensure they all rise but you’ll know immediately if a plug is bad because you’ll suddenly be running on three cylinders. I had a plug foul once and it caused intermittent problems but not as rhythmically as you describe.

I have the same setup as you for ignition. I doubt it’s the ElectroAir itself.

I’m still betting it’s the phugoid thing.

Add: I have a 180hp and with the ElectroAir at 7500ft I’m typically running 8 to 8.5 gph WOT. See if you can lean it back more than the 8.9 you mentioned.
 
I fly Cessna 172's mostly, but have had similar issues with altitude stability, just much less pronounced. My instructor made a big deal of perfect trim when cruising, and I attempt to achieve that. If I or my wife lean forward, that changes the CG just enough to cause a slight drop of the nose, the plane starts descending, speed and RPM rises. A new equilibrium becomes established, but when she sits back again, the reverse takes place.

Obviously, I blame all this on my wife, which is safe, since she does not read here. The reality is that every time that I reach behind the seat, or under it for a chart, a drink of water, of even adjust the squelch of a radio, trim is upset, and a change in speed, altitude, and RPM is a natural result.

Your plane is slicker than mine, so the acceleration is quicker, RPM changes larger, and altitude changes larger. That is why you get better MPG, so do not complain!

At lower altitudes, rising and falling air can set this off, so more often there, but at higher altitudes and thinner air, the excursions tend to be larger. I cruised at 7 to 11 thousand feet, and no matter how carefully I trimmed, I had to keep slight forward or aft alternating pressure on the yoke to avoid altitude changes.

OOPS, my wife just walked in and read over my shoulder, but definitely did not include the second paragraph in her scan!
 
Dan Thomas said:
Electronic tach. No cable. A dry or sticky cable will make the tach erratic; it won't make it underread steadily. That would require slippage, such as the drive key on the cable worn off, and the RPM wouldn't recover after it fell.

The RPM would keep falling if the venting was plugged. Fuel flow would gradually diminish until the engine quit.

Gary M suggested trying the carb heat. Carb icing is way too common, and proper teaching and understanding of the physics of it is way too rare. Before you fly, check the METARS and see what the temperature and dewpoints are. They're in the METAR for good reasons, and carb ice is one of them. When they're close, ice is possible, even in the summer on a nice clear and hot day. Carb ice is not mainly a wintertime thing.

View attachment 131155

Another couple of things to check: See if the mixture and carb heat controls are loose and shifting in flight.
Click to expand...

Next time it happens, take note of changes in fuel pressure. If it fluctuates, it could well be a partially blocked tank vent. Then switch tanks. If the problem goes away, it's pretty much certain.
 
Hang 4 said:
How about the throttle friction lock? Could the lever/knob be sliding from vibration? You're able to "recover", which suggests the throttle closing slightly.
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I was told to check this early by my CFI, he said he had never seen an issue with me failing to address this before so he doubted it, but said slow drop like that could be recession of the throttle. I did check and pay more attention and the throttle is not moving.
 
Dan Thomas said:
Electronic tach. No cable. A dry or sticky cable will make the tach erratic; it won't make it underread steadily. That would require slippage, such as the drive key on the cable worn off, and the RPM wouldn't recover after it fell.

The RPM would keep falling if the venting was plugged. Fuel flow would gradually diminish until the engine quit.

Gary M suggested trying the carb heat. Carb icing is way too common, and proper teaching and understanding of the physics of it is way too rare. Before you fly, check the METARS and see what the temperature and dewpoints are. They're in the METAR for good reasons, and carb ice is one of them. When they're close, ice is possible, even in the summer on a nice clear and hot day. Carb ice is not mainly a wintertime thing.

View attachment 131155

Another couple of things to check: See if the mixture and carb heat controls are loose and shifting in flight.
Click to expand...
Was also told this once too, and have ruled this out as well. I have experienced carb ice before and recognize how it shows up. Carb heat has normal operation during this if I use it etc.
 
Llewtrah381 said:
As described I suspect it’s the phugoid oscillations already mentioned. Picture a river with a rock in it and look at the ripples that set up downstream from the rock. Going up the ripple takes more power and slows you down. Going down the back side speeds you up. Same with air, especially that has gone over mountains, even far away. I see it almost constantly when flying back east from the west, especially between El Paso and maybe Ft Stockton (which I tend to hit mid-late afternoon). It can be so pronounced, especially in summer, that I need to ask for a block altitude because I can’t keep up with the downdrafts all the time. Its annoying but not dangerous.

Second possibility could be an ignition issue - most likely a spark plug rather than a mag. When you’re in the low RPM part of the cycle, try doing a mag check in the air. Watch all four EGTs and ensure they all rise but you’ll know immediately if a plug is bad because you’ll suddenly be running on three cylinders. I had a plug foul once and it caused intermittent problems but not as rhythmically as you describe.

I have the same setup as you for ignition. I doubt it’s the ElectroAir itself.

I’m still betting it’s the phugoid thing.

Add: I have a 180hp and with the ElectroAir at 7500ft I’m typically running 8 to 8.5 gph WOT. See if you can lean it back more than the 8.9 you mentioned.
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I didn't mention what I was leaned to in original post that was a commenter, but my GPH for a 7500 is about 6.8, in the 0320.

I do wonder about a plug issue and and I will try that mag check next time I see it.... Also very big and forgot to mention in original post but one other symptom of this is a "hunting" as my dad used to call it for RPM upon start up..... the RPM will fluctuate a lot for a while, in similar fashion to what it does in the air but much faster, I will see if it will let me post a video but I flew this morning, and on a hot start thought to take a quick video. (ok can't upload video, unsupported type, will keep trying to change it.)

This also started about the same time as the in air fluctuations. Which leads me to think it is not normal operation, and wind currents etc.... mainly since when it can happen in the air it can be in completely glass smooth flight.
 
chemgeek said:
Any chance you have a spark plug dropping out intermittently?
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Yes perhaps.... this is getting higher on my list of (that might be it) I am going to pull them and check condition etc.... at last annual I had a lot of lead buildup as all of that time was my training and I did not lean as much as I should have, but I have been much more diligent about leaning as I have become much more comfortable in the plane and the brain burden of flying is greatly reduced.
 
Sunday Puncher said:
Yes perhaps.... this is getting higher on my list of (that might be it) I am going to pull them and check condition etc.... at last annual I had a lot of lead buildup as all of that time was my training and I did not lean as much as I should have, but I have been much more diligent about leaning as I have become much more comfortable in the plane and the brain burden of flying is greatly reduced.
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I'd recommend bomb-testing the plugs after cleaning if your mechanic has the equipment. You might find a marginal, intermittent plug that way. If you properly lean, you can forestall lead fouling and keep plugs squeaky clean. Lean aggressively during taxi, and lean at cruise any time you are at 75% power or below. Religiously. This will get you book fuel consumption and will keep things clean. If you are leaning properly in taxi, you can't take off with the mixture leaned, because the engine will quit when you advance the throttle to takeoff power.
 
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