Turbo Normalised

[boofhead]

I have been flying a C182 with Lycoming turbonormalised 540. It gives 32 inches of boost.
I don't understand it well enough. Anyone who has experience on it, please comment.
On takeoff it is normal, 32 inches, 2400 rpm and 24 gph. In cruise it initially allows normal set up of, say 25 inches and 2300 rpm with fuel flows around 14 and the TIT close to max. But after flying for a while I see the rpm is down to 2100, even with the prop control full in. It acts like a fixed pitch propeller, and moving the throttle to do maneuvers causes the rpm to fluctuate, around 2000 to 2300 even with full increase.
I had a bit of trouble with rough running, could only keep the engine going by using a very low mixture setting, but the mechanic found dirty plugs and cleaned the injectors which seemed to fix it.
On approach, prop full in, the rpm when the speed is 60 and the throttle is set to around 15 inches, is 1100. Even a Cessna 150 rpm is not that slow.
Once on the ground again, a run up is normal. Performance seems normal, although acceleration on takeoff is nothing to write about. The rate of climb two up and half fuel is around 600 fpm, a little slow I thought. A go around is best descibed as Stately. But cruise is 125 IAS with 25 inches of boost.
However it is the rpm that concerns me. My mechanic says I just don't understand the turbo normalised engines. To get used to it. He says the Cirrus is worse, but I don't fly the Cirrus so that is no help.

[SNS3Guppy]

Boofhead,

Some discussion of the function of the throttle and propeller are in order, first. You can consider your engine to operate in two ranges, so far as the throttle goes. Barometric (or the normally aspirated range), and a boosted range. At sea level, where normal standard pressure is 29.92 inches...anything at 29.92 and below is barometric (on a standard day), and anything above that is in the boosted range.

The throttle is often misunderstood. Operators mistakenly often think they're somehow pushing more air and fuel into the engine by increasing throttle, when in fact they're doing somewhat the opposite. Think of your engine as a suction machine, or an air machine. You can compare it to a vacum cleaner. It sucks air in through the vacum hose, and blows it out through the exhuast. Your vacum hose is your aircraft induction.

You probably know that if you put your hand over a vacum cleaner hose, it makes a lot of noise, and the air pressure in the hose drops (as evidenced from the red ring on your hand, when you remove it). Because an engine is a suction machine, closing the throttle has the safe effect as putting your hand over a vacum cleaner hose; pressure drops. typically to about 13 inches or so at idle. Of course, even with the throttle closed, some air and fuel is still getting by the throttle plate...this is how the idle (and idle mixture) works.

If the throttle is opened in a normally aspirated engine, eventually you'll reach full barometric pressure...which again at sea level is 29.92 inches, and at five thousand feet is about 24-25 inches of manifold pressure. In fact, shut down the engine so it isn't "sucking" any more, and you'll read the barometric pressure on the manifold pressure gauge, where ever you are.

In a turbonormalized engine, the engine is capable of maintaining sea level manifold pressure (or slightly better, in many cases) up to the critical altitude (typically around 16,000 to 20,000')...then it starts dropping off. It does this by means of a turbocharger and wastegate system.

If you're operating the engine at less than the throttle setting necessary to maintain a boosted pressure (sea level barometric), then you're going to see manifold pressure drop off with a change in density altitude. Put into terms of your question, if you're operating the engine at 25" of manifold pressure, you're not using your turbocharger below 5,000'...you're operating in the strictly barometric range. If you climb, you'll see a change in manifold pressure unless you increase throttle and use the turbocharger to boost manifold pressure above barometric. This is normal.

That's the throttle. Nor for your propeller.

Your propeller will attempt to maintain a given RPM when it's in the "governing range." That is, the propeller governor has the ability to keep the propeller RPM constant, but not all the time. On the ground at idle, for example, the propeller blades are resting on their "low pitch stops", and the engine RPM isn't high enough to actuate the propeller...the propeller acts like a fixed pitch propeller. Once the engine is turning fast enough to enter the governing range, the propeller control may be used to reduce RPM, and the governor will maintain that RPM.

If you're maintaining 30 inches of manifold pressure and the propeller is holding 2500 RPM with the propeller control all the way forward, you can expect the engine to maintain 2,300 RPM once you retard the propeller control enough to achieve that setting. You can then climb and descend and the propeller RPM will hold that setting.

If you retard the manifold pressure enough, however, the propeller blades will no longer hold that setting. Once the throttle is reduced enough, there's not enough oomph or torque driving the propeller. Where ordinarily pushing up the throttle on a constant speed installation will cause an increase in blade angle to maintain the same RPM...pulling back the throttle will cause a decrease in propeller blade angle to maintain the same RPM. Eventually the blade angle will decrease enough that the propeller blades are resting on the low pitch stop...and can't decrease any more...and then the only thing that's going to happen is that the RPM will decrease. The propeller has run out of ability to maintain that airspeed, because it's reached it's mechanical limits.

If you're operating at low manifold pressure settings in cruise, you may be low enough that you can't maintain RPM (especially in a climb). The propeller reaches a point where the blade angle is limited by the low pitch stops, and can't decrease any more in an attempt to maintain RPM...then RPM simply decreases.

If you're operating at a low manifold pressure setting in cruise, and climb or fly from a cold area to a warm area or to an area of low pressure, then the manifold pressure will decrease, just like in a normally aspirated engine. In fact, the engine is acting as a normally aspirated engine because you've retarded the throttle far enough that you're not taking advantage of the turbocharger.

If your'e seeing higher than normal fuel flow, you're actually experiencing lower than normal fuel flow...because the fuel flow is measured as a pressure differential...a plugged injector makes for a leaner cylinder and hotter cylinder, but also shows as a higher fuel flow...something you might not see or recognize without a multi-point cylinder and exhaust temperature monitoring system.

On approach to landing, with low manifold pressures, you're going to see the propeller behaving as a fixed pitch propeller...again because the propeller blades are resting on the low pitch stops, and can't increase their angle any more. RPM will be a function of throttle setting, and airspeed. If throttle setting remains constant, then a higher airspeed will yield a higher RPM, and a slower airspeed, a lower RPM...because airspeed drives the propeller at low power settings.

[boofhead]

Thanks for that, Guppy. It is clear and helpful.
Even though the theory makes sense, no other engine I operate (from turbo charged 450 hp, through normal aspirated IO540 etc down to 60hp Rotax) loses so much rpm on final at normal speed. 1100 rpm still seems too slow, with manifold showing 15 inches and airspeed 60. I would be concerned if it was a normal engine, and nothing you have written makes me feel better about that.
The book for the airplane does not say anything about what to expect either. My mechanic says there are no rpm adjustments except for takeoff power, and it is good at that setting, showing 2400. Once the throttle is pulled back though, the prop goes straight to the fine pitch stop.
Has anyone flown one of these airplanes who can reassure me?

[SNS3Guppy]

The fine pitch stop is the low pitch stop. You're saying that when the propeller is pulled back, it remains on the low pitch (high RPM) stops?

If instead you're saying that the propeller goes to the coarse stops, or in other words the high pitch stops (coarse, or high RPM), that's another matter...the propeller shouldn't do that unless it's a full feathering propeller...and only then when the propeller is feathered. The Cessna 182 is not equipped with such a propeller.

What is the propeller RPM on the ground at 15" of manifold pressure?

Your mechanic is correct that you can't set the idle RPM through the propeller any more than you can do something to a fixed pitch propeller to vary idle RPM. The propeller is out of the governing range at low manifold pressure settings, and acts like a fixed pitch prop. Idle RPM is a function of fuel control, throttle ajustment, or for carbureted engines, carburetor adjustment. With the throttle above idle such as maintaining 15" of manifold pressure, idle adjustment is irrelevant, and there is no propeller adjustment.

On some airplanes, notably those with big Hamilton Standard propellers, there's a check done at barometric manifold pressure with the propeller fully retarded for a low RPM check...but not on the engine/propeller combination you're operating. It's a whole different ball of wax.

I don't know off the top of my head what RPM you ought to be seeing at the approach airspeed you're using, at the power setting you're using. Ive flown a T182 many times, but honestly don't think I've ever checked the RPM indication on final...as it's really irrelevant. You may have an indicator that's reading low, or it may actually be that low. The gauge you should be using at low power settings is the manifold pressure gauge, not the RPM gauge...because at that stage RPM is a function of throttle and airspeed...it's acting as a fixed pitch prop only, riding on the low pitch (fine) stops. There are much more important things to be looking at on final than the one gauge which gives you no information and describes something which is no longer in your control...the RPM gauge.

When the throttle is retarded below the governing range, the propeller should always go to the fine pitch, or high-RPM stop. This is the nature of the propeller you're using. Once that happens, RPM is strictly a matter of airspeed and throttle position. At low airspeeds, you're going to see a lower RPM.

Now that said, if you have an induction leak, you're going to see a higher manifold pressure than what that particular throttle setting might normally provide. In other words, if you have an induction leak, the manifold pressure may appear higher at 15" than what you're really providing with the throttle position. You may also see a rough engine, and variations in RPM at lower power settings, or variations in manifold pressure at higher power settings, as well as an inability to make full boosted manifold pressure...though not necessarily consistently.

[boofhead]

Interesting, but I don't think there is an induction leak. Operations are normal on the ground and on climb, even initial cruise, but after a while the prop lever must be set fully in to maintain 2300 rpm, which is recommended cruise. Throttle is set at this point to 25 inches, and fuel flow adjusted to 14 gph, giving a nice high TIT. No problem except for the prop selector needing to be fully in.
On descent, the rpm drops further, no matter the other settings or speed.
On final, with the prop fully in, a small amount of mixture leaning, and around 15 inches of manifold, the rpm is down to 1000-1100. It seems to be an accurate indication, this is the second rpm gauge fitted, and you can paractically see the blades.
If a go around is flown, the engine recovers quickly, although the rpm never gets to 2400, which it would if it were a normally aspirated engine.
I don't like not having control of the rpm. I can't complain about the power output, it seems to be adequate in that the airspeeds and climb rates are normal, or close to normal. I guess this is just a feature of the turbo normalisation.
I would like to know if anyone else has flown this type of setup in this airplane if they have seen similar low rpms, especially on final approach where I would have expected the airspeed to keep them higher, even if the throttle was fully closed.
I have had two occasions with this engine giving moderate vibration and a lack of power in flight, needing an emergency return to land. I think they were the loss of upper deck pressure, although the mechanic fixed them by cleaning the plugs and injectors and it is currently running smoothly. Don't know if this is associated with my suspicions or not, and again I am looking for what is considered normal for this engine and airplane so that I can decide if there is something wrong.

[SNS3Guppy]

Given that the RPM gauge has been replaced with no remedy, and setting aside the possibility of two faulty RPM gauges (and lacking a verification with a stroboscope or RPM meter), this leaves the more likely possibility that you have a governor problem, or a propeller problem.

What you're describing isn't a propeller resting on the low pitch stops, but a propeller artificially held off of them. This could be due to an internal problem, such as low nitrogen pressure in the propeller dome or a limiting mechanical problem which doesn't permit full blade travel to the low pitch stops. The most simple solution is a leaking prop dome, most typically through a filler schraeder valve.

If the propeller has been stored or the aircraft not flown for a time, airworthiness directives and concerns for inspection intervals may also apply, and even a new propeller can leak down. Check the nitrogen charge.

Another potential problem, of course, is in the propeller governor. Governors themselves seldom go bad...they're very simple arrangements, but they're still mechanical devices and still have potential to fail. A weak speeder spring, stuck pilot valve (debris in the pilot valve, for example), or failed flyweight attach pivot point can cause these problems...the catch being that often you'll have immediate feedback because the propeller control will be gritty, stiff, or immovable.

If indeed you're having problems controlling RPM or can't maintain RPM, then you have a serious problem and the aircraft shouldn't be flown until this is corrected.

[Von Klinkerhoffen]

Guppy , I believe this model of T182 has a 3 bladed McCauley , not a Hartzell Compact , boofhead ?? If so , this model of propeller has no dome pressure like a Hartzell . Governor oil pressure is used to increase blade pitch angle (low rpm) and the natural centifugal twisting moment of the blades (with a little help from a spring , I think) is used to decrease blade pitch angle (high rpm) . As Guppy has so rightly said , at any time the prop control is fully fwd (in) , the blades should be on their fine pitch stops and red line rpm , 2400 in this case , should be achievable on advancing the throttle . Listen to him , he knows his stuff ! :ok:

VK

[Tinstaafl]

Presuming the RPM is correct i.a.w. two separate gauge indications, something must be driving the blades to coarse pitch/low RPM. That would be oil pressure boosted & controlled within the CSU so what could be going on in there? Maybe the valve is misadjusted or has a worn seal allowing high pressure oil to get through to the hub?

[toolowtoofast]

at any MAP above about 18-19", the governor should start controlling the rpm. 2400 with prop at full fine sounds low for a TIO540, but I will take advice on that :).

It almost sounds like the governor is holding in a too coarse position hydraulically, and not letting the prop settle on the physical fine pitch stop - easily caused by poor adjustment of the propellor control teleflex. Once the MAP is insufficient to drive the propellor into CSU range though, the prop will sit on the physical stops no matter where the control is set. If you come in with 2200rpm set, but reduce MAP while S&L until it can no longer maintain 2200, it will just drop away with airspeed (if maintaining S&L though). As soon as you increase power again, if you haven't touched the prop control, it will re-govern at 2200rpm again.

What RPM can you achieve in a dive? would be interesting to see if it held 2400 or climbed through it. Remember the governor is controlled by flyweights inside it - driven by prop rpm. all you are doing is setting spring pressure when you adjust the prop lever, and if you accellerate it in a dive, it should still govern to the limited prop rpm

[SNS3Guppy]

The Type Certificate Data Sheet shows a McCauley B3D36C442/80VSB-1 propeller, at least for the T182T with a Lycoming TIO-540-Ak1A powerplant.

One should consider adjustment on the governor; if the minimum governing RPM is set to high, the governor will have too narrow a range and will easily slip out of the governing range at reduced power...resulting in the engine defaulting to the (in this case) low pitch stops.

Sludge buildup in the crankshaft sludge tube, oil passages, dome, or even governor assembly can result in sluggish operation or failure to operate throughout the full range.

A bypassing oil transfer bearing with a significant rate of leakage can cause problems, also. Whatever the case, it must be thoroughly investigated and addressed before flying the airplane again.

[boofhead]

I will be flying it again next week. I will see what I can learn. Thanks for all the advice here, it has shown me that there might be a problem, and I will pay a lot of attention to it.

[Tinstaafl]

You already know it's not right by a long shot after several flights with it repeating the behaviour. I'd be inclined to park it at the maintenance hangar now.

[411A]

Regarding the symtoms described...
The crankshaft oil transfer collar is most likely the cause, so have your mechanic remove the propellor and have a look.
It is very unwise to fly the airplane in its present condition.

[SNS3Guppy]

I will be flying it again next week. I will see what I can learn. Thanks for all the advice here, it has shown me that there might be a problem, and I will pay a lot of attention to it.


It would seem you missed the strong counsel to not fly it again until the problem is fixed.

[boofhead]

Follow up. The airplane has been with the shop for a week. Various things were done, including new plugs and a clean up of the fuel injectors. The prop was checked.
I flew it yesterday, and checked the prop myself, as well as the mixture and a few other things that might have had a bearing on what I was seeing, but it looked good.
During the flight it was perfectly normal, all instrument values within limits. On final around 80 knots with idle power the rpm was 1600 and at 65 knots it was 1300, quite respectable and acceptable.
So whatever was wrong with it now seems to have scampered off to infect some other person's airplane.
Thanks again for the help.