RPM upwards creep

[Pass your message]

Whilst training to do my IMC, I have noticed in the C152 I am flying a gradual, slow rise in the RPM,which affects trim/ altitude air speed etc. Is this common or just in this machine. I haven't noticed it before, but have always been VFR/VMC.Any ideas how I can control this?

[foxmoth]

Tighten the friction? If that is not holding it it may need maintenance.

[Genghis the Engineer]

What Foxmoth said.

G

[CJ Driver]

The C152 has a fixed pitch prop, which means that as the aircraft accelerates, the RPM will go up.

You're probably leveling off at top of climb, checking and setting the RPM, and settling into your IMC scan. Over the next few minutes, the mighty C152 continues to (modestly) accelerate by a knot or two. Result - the RPM has gone up, and you need to retrim slightly. Wait another couple of minutes, and it may have gone up a bit more.

A general problem with fixed pitch aircraft for IMC is that they are power/performance unstable. As they speed up, the engine RPM increases, and it produces more power, thus making it speed up some more. You compensate by reducing throttle. The aircraft slows down, so the RPM decreases, and it produces less power, thus slowing down more. You add power - repeat as necessary

Your instructor knows this of course, and this minor irritating additional workload is what makes the C152 a good IMC trainer

The practical solution is to set power slightly below max RPM, so that when the inevitable lift comes along and you start pushing forwards, the RPM rise stays below the red line. When the inevitable sink comes along later, and you need to pull a bit to maintain altitude, the RPM will reduce and the aircraft will slow down, but hopefully not too much. Providing you responded soon enough, and gently enough, to the first hint of lift or sink, the power will take care of itself. If you find you need to keep adjusting the power to maintain straight and level, it means your scan isn't keeping up, and you are in a form of pilot induced oscillation. Your instructor knows this too

[FlyingSportsman]

I noticed this too, and agree, tightening the throttle friction nut could perhaps be your problem. However, it may be the effect of 'windmilling'.

The change in RPM you are seeing could be due to the effect of the airflow turning the prop (windmilling) - the effect increases with speed. As you slow down you would expect the RPM to decrease from the value you set with the throttle and conversely as you speed up the RPM would inch upward on its own.

FS

[foxmoth]

Whilst this could be the case, the OP was only giving it as increasing RPM which indicates it is not increase due to speed change - and just a couple of knots would not give a noticeable RPM change anyway.

[BEagle]

Nope, Foxmoth, CJ Driver is absolutely correct. With a fixed pitch prop, RPM change with TAS will occur no matter how tight the throttle friction. More evident in some aircraft than it is in others.

Which is why the constant speed prop is such a boon when attempting to fly accurate speeds in spamcan puddlejumpers.

[foxmoth]

Beagle - I did not say that it would not change,but that it has only been noted as an upwards increase, if the speed change was the problem I would have expected him to notice an RPM reduction at times as well (such as when reducing from his too high speed). Also, CJ Driver said "the mighty C152 continues to (modestly) accelerate by a knot or two" and, whilst a speed change of 10kts or so would produce a noticeable change in RPM, it is unlikely you would notice the change over "a knot or two".
If it is a change due to incorrect level off technique I would have thought this would be picked up by his instructor.

[BackPacker]

A general problem with fixed pitch aircraft for IMC is that they are power/performance unstable. As they speed up, the engine RPM increases, and it produces more power, thus making it speed up some more. You compensate by reducing throttle.

Although I agree with the fact that a change in IAS leads to a change in RPM, I don't agree that that change in RPM actually causes a power change, and that power change subsequently leads to a further increase in speed. After all, we would all be flying at infinite warp speed after a few minutes if that were the case.

It would be interesting to do the following, in an aircraft supplied with a fuel flow meter and a fixed pitch prop. Set up a stable cruise, say 2350 rpm in level flight. In your average O320-powered spam can you'll be looking at about 100 knots and a fuel flow of 9 USG/hr (unleaned). Without touching the throttle at all, push the nose over and let the speed build to, say 130 knots. You will notice an RPM increase, but the fuel flow will hardly change. So the amount of fuel burned, and thus the power supplied, will hardly change. And you can do it the other way around too: Pull the nose up and let the speed reduce to 80 knots, without touching the throttle. RPM will decrease, but the fuel flow will hardly change.

Why will the fuel flow hardly change? Fuel flow in a carbureted engine is to a very large extent depending on the speed with which the air is rushing through the carburetors. And since you did not change the throttle position, the butterfly in the air inlet is still in the same position and will still restrict the air inflow. There will be a minor change though since at higher RPM the engine will "suck" a bit more air in. (If you had an MP gauge you would actually see a tiny drop.) But that suction change, and thus fuel flow, is at most linear with the RPM, and possibly even less than that.

So the %change in speed (due to pitch) will be higher than the %change in RPM/power delivered. So the %power change will not be able to overcome the %change in drag, and it's that drag that will slow you down to your initial speed. And that makes it a stable system, not a runaway unstable system like CJ describes. (Except possibly when you're at the back of the drag curve, but that's not a flight situation where you will want to be cruising, especially not in IMC.)

Tighten the friction? If that is not holding it it may need maintenance.

That's my thought too. I don't know specifically for the 152, but in a lot of aircraft the throttle cable is a Bowden cable which can only work in tension, not in compression. In order to make it a two-way and failsafe operation, the carbs are spring-loaded and are automatically pulled open. At the throttle assembly you pull on the cable, effectively pulling the carbs closed. Until you open the throttle at the throttle assembly, and then it's the springs on the carb assembly which open the carb butterfly.

When flying with constant power, it's the throttle friction (which is usually pilot-adjustable) which prevents the throttle assembly to open, against the pull of the springs on the carb.

(And of course this system has been chosen for safety. If the cable fails, the throttle defaults to wide open instead of idle.)