Throttle and Propeller handling at take-off?

[francesco.doenz]

I was surprised to see in a United States navy training film (at 2min 35 sec viewing time)

https://www.youtube.com/watch?v=3UZmEha ... e=youtu.be

that just after take-off the pilot retards the throttle BEFORE the prop? Is'nt it against the rules? And this is a teaching film!?
Many thanks for your inputs!

[Alfredson007]

No, that actually is the most common procedure. First you retard the throttle(MP), then rpm. When you ADD power, it's the another way round, first prop, then MP.

[francesco.doenz]

Sorry! thanks got it! Mixed it up! Some say prop lever always ahead of throttle lever...

[AKar]

There are no hard rules - except of course when given under the title "Limitations". However, there is a simple logic in this one. What you want to avoid is the situation where you attempt to produce very high power while restricting the RPM. This would be similar to driving a car slowly at high gear and pushing the gas pedal to the floor. You'll hear and feel she's not happy, and this is potentially harmful to the engine even if the car's engine can retard the ignition timing - something your antique engine in an airplane cannot do! Some high-performance airplanes, typically supercharged/turbocharged ones, may have a limitation on takeoff power given in maximum MP for continuous operation, and those require a throttle reduction soon after takeoff. It is most often sensible to bring the throttle back first and RPM next.

However, to underline this is not a strict, all-around rule (even if some want to think it is!), some airplanes are actually rated for something like 5 minutes at full throttle + full RPM for takeoff, and the maximum continuous is given as full throttle + reduced RPM. It of course makes a very little sense to first bring the power back to some self-invented setting (typically something like "squared" or equally arbitrary), then bring the RPM back, and then again increase the throttle back to full!

Most typically however the naturally aspirated airplanes are free of any power limitations after takeoff, making the sequence question a bit academic unless leveling off at lowish altitude, necessitating some reduction on the throttle. Most also operate happily at notably reduced RPM even at maximum sea level MP (typically charted in the engine's operating manuals as maximum MP for continuous operation or something like that), but of course that's not necessary the best thing to do.

Edit: actually, I stated rather poorly, or non-rigorously...in principle, you physically can't produce very high power at restricted RPM, it would be more appropriate to speak in terms of torque, but that's for another discussion!

-Esa

[DC3]

Hi Akar,

In this case a Navy training film is cited. In the warbirds of WWII there was a power limitation many times on take off. In these cases not only could the aircraft be overboosted on take off but if kept at take off power for longer than a specified length of time the additional power would harm to the engines. Thus the requirement to reduce power 2 to 5 minutes after take off. On the general aviation fleet this typically is not necessary and the engines can be run at full power for an indefinite length of time, although I don't think most engine manufacturers would recommend this. Because I have followed you for a long time on these forums I am assuming I misunderstood your statement about most naturally aspirated airplanes are free of any power limitations, perhap you are referring to non-turbocharged / supercharged engines.

--Mike

[AKar]

Mike,

Yes indeed - "naturally aspirated", as in non-supercharged. Not too many warbirds with such engines, but as we know, they are common in GA. They can typically be operated at full take-off power without limitations, though it is good to note that in most sensible operations, the altitude gained during climb will soon restrict the actual power produced regardless of the full throttle being used. Of course, if using (IMO, highly dubious!) recommendation to climb at VY or some other relatively low airspeed, some engine temperature issues are invariably encountered - in many instances, actually made worse by power reduction for climb!

[...]but if kept at take off power for longer than a specified length of time the additional power would harm to the engines.

This is actually a bit complicated. In general, exceeding that time limit does no harm. Apparently the limit is in place for two reasons: First, to limit the total amount of time the engine is operated at high power settings in an attempt to limit the total time at high power settings. In that sense, the engine does not care whether it is operated at full power continuously or in several 5 minute intervals. But...

Second, some engines may not be able to cool appropriately at full power. In those cases, especially when discussing the liquid-cooled engines in many warbirds, the hard time limit actually implies something because the cooling system may quickly start to lose its effect if the engine's temperatures slowly creep high enough. Still, properly monitoring the engine's temperatures, and assuming the monitoring system is accurate, one can operate it at high powers, exceeding the time limit, with no ill effects. It is still about the actual engine temperatures and other conditions, not about the time limit per se.

This was actually rather neatly discussed in T-6 flight manual, in an answer to a "common question" that was something like: If I operate the engine at full power for [whatever the time limit was], how long do I need to keep it at lower power until I can apply full power again?

The answer was, in essence, precisely this! Technically speaking, it makes no real difference; if you really need to, you can exceed the time limit, temperatures and pressures allowing, with no direct adverse effects. And if keeping those temperatures and pressures appropriate requires you to reduce power, then the very same parameters tell you how long you need to let it rest.

However, it is good to remember that in this case, this limitation is actually found under the "Limitations", so one must be expecting some exceeding in other parameters if using handbook practices.

-Esa

[AKar]

It is often asked what the consequences would be if the 5-minute limit at Take-off Power were exceeded. Another frequent inquiry is how long a period must be allowed after the specified time limit has elapsed until Take-off Power can again be used. These questions are difficult to answer, since the time limit specified does not mean that engine damage will occur if the limit is exceeded. Instead, the limit means to keep the total operating time at high power to a reasonable minimum in the interest of prolonging engine life.

It is generally accepted that high-power operation of an engine results in increased wear and necessitates more frequent overhaul than low-power operation. However, it is apparent that a certain percentage of operating time must be at full power. The engine manufacturer allows for this in qualification tests in which much of the running is done at Take-off Power to prove ability to withstand the resulting loads. It is established in these runs that the engine will handle sustained high power without damage. Nevertheless, it is still the aim of the manufacturer and to the best interest of the pilot to keep within reasonable limits the amount of high-power time accumulated in the field. The most satisfactory method for accomplishing this is to establish time limits that will keep pilots constantly aware of the desire to hold high-power periods to the shortest period that the flight plan will allow, so that the total accumulated time and resulting wear can be kept to a minimum. How the time at high power is accumulated is of secondary importance; i.e., it is no worse from the standpoint of engine wear to operate at Take-off Power for one hour straight than it is to operate in twelve 5-minute stretches, provided engine temperatures and pressures are within limits. In fact, the former procedure may even be preferable, as it eliminates temperature cycles which also promote engine wear. Thus, if flight conditions occasionally require exceeding time limits, this should not cause concern so long as constant effort is made to keep the over-all time at Take-off Power to the minimum practicable.

Another factor to be remembered in operating engines at high power is that full Take-off Power is to be preferred over take-off rpm with reduced manifold pressure. This procedure results in less engine wear for two reasons. First, the higher resulting brake horsepower decreases the time required to obtain the objective of such high-power operation. At take-off, for example, the use of full power decreases the time required to reach an altitude and airspeed where it is safe to reduce power and shortens the time required to reach the airspeed that will provide more favorable cylinder cooling. Second, high rpm results in high loads on the reciprocating parts because of inertia forces. As these loads are partially off-set by the gas pressure in the cylinder, the higher cylinder pressures resulting from use of full take-off manifold pressure will give lower net loads and less wear. Sustained high rpm is a major cause of engine wear. It requires more "rpm minutes" and "piston-ring miles” to take off with reduced manifold pressure. In addition to the engine wear factor, a take-off at reduced power is comparable to starting with approximately one-third of the runway behind the airplane. Therefore full power should always be used on all take-offs.

Albeit this discussion actually ignores certain aspects of very high-power operation (remember, for instance, that not all parts of the engine actually are cooled by the primary cooling system - some are cooled by the engine oil, and accumulated heat in certain parts can cause oil to overheat and another parts to lose their effective lubrication and further reduced cooling and increased heating - maybe this isn't a concern in T-6G?)... but given that this is an operating manual, what I quoted is an excellent introduction into this operating aspect. Too bad this level of discussion is seldom seen these days in the operating manuals.

-Esa

[DC3]

Still, properly monitoring the engine's temperatures, and assuming the monitoring system is accurate, one can operate it at high powers, exceeding the time limit, with no ill effects. It is still about the actual engine temperatures and other conditions, not about the time limit per se.

Esa,

Being around mechanical things most of my adult life I would have to agree 100% with this statement. If the machinery is operated within limits (and has the proper oil) it will run very well and usually for a long time. (I am assuming there are no structural defects that are pushed over the breaking edge by high temperatures, unusual torque, or other stress inducing actions.)

Keep up the good work, I love reading your answers. Always informative.

--Mike

[francesco.doenz]

Yes, very informative discussion, as often in this A2A community, and comprehensive even for someone not at all around mechanical things in daily life...my knowledge comes only from reading, and consequently short-term memory....as for "the prop lever always ahead of the throttle" from Mackado's Private Pilot Handbook!
Thanks for your inputs!

[AKar]

A great introduction into the wear vs. power is actually given by electric motors. In those the idea remains relatively simple, because unless one stalls a motor by gross overload, or does something equally stupid, their "wear" in service is almost solely a function of thermal stress (ignoring bearings and like, which of course may need servicing!). Also, unlike airplane piston engines, most industrial electric motors have their performance graphed very accurately. However, the interesting part is this: the motors are designed to run at their nominal values in different ways. Some may be designed to continuous usage - in general, these can be operated at their nominal rated power all the time. And just about any of these motors can exceed their nominal values by great margin with no immediate adverse effects. Typically their uses are actually designed to overload the motor: in many instances it would make little sense to invest in such a large motor that can nominally provide the highest transient loads encountered in use if those are only encountered for a very small percentage of total time in service.

One can think this as a two-way door: one can increase the nominal values while decreasing duty cycle and vice versa. A smaller motor can be used for much higher loads, and it is not directly harmful, but the load themselves cannot be continuous in that case. We find examples of every kind in typical airplanes too! An example of a motor designed for continuous use is the one in equipment cooling fan. It is designed to give its rated output continuously. A completely opposite example would be the engine starter: compared to what it could take continuously, it is grossly overloaded - but that is acceptable because it is seldom used and only for short periods. From between would be things like flaps motor, gear motor and autopilot servos. Those are all designed for different levels of intermittent operation: flaps and gear motors provide their rated output while for autopilot servomotors the loading varies. None of them is dimensioned to give their full output continuously. Putting that another way around: compared to their would-be continuous rating, they are overloaded in use, per design. Likely, if we took the flaps motor for instance, and provided some highly efficient cooling scheme for it, we could make it run continuously with no adverse effects. It is the combination of heat capacity, thermal ratings of materials, cooling efficiency and heat production which all together determine the acceptable duty cycle, and it needs to have some margin, for instance, allowing for different ambient temperatures. If that margin is there today, it can of course be used to either decrease the total loading or to push the motor a bit longer.

In simplistic view, this can be applied to airplane piston engines as well. Some engines are "big enough" for the horsepower produced to provide that continuously. Some others may provide the power required with narrower margins. In those cases it may come up as a viable option to restrict the rated output for a limited period of time. Here the thermal issue rises its head also: the manufacturer may know that the engine is not necessarily cooled enough to remain within healthy temperatures while providing full rated power continuously. Luckily for us, however, we typically can monitor these temperatures, and therefore we can see how much 'margin' there still is available for us to use if needed. However, of course, we can't see into every corner of the engine, and therefore when 'overloading' an engine for an extended periods of time, I'd avoid getting too close to manufacturer's redline limitations. Lycoming for instance allows for (but recommends against!) CHTs of 500 °F - that's way too high, and should never be reached except as a last measure in an emergency. I don't even want to know what is happening to the oil which is sprayed to the bottom of the piston to cool it down in those conditions!

But compared to electric motors, the issue is further complicated by the fact the airplane piston engines meet high inertial forces and varying combustion chamber pressures. The latter actually show up in CHT, but not instantly. This is the reason why it is generally suggested to reduce MP before RPM. The engine is designed to provide reasonably well-timed ignition and combustion event in takeoff power operation. If you brutally reduced the RPM, what actually happens is that the combustion event (typically started by the ignition spark at somewhere in between 25° to 20° before top-dead center, depending on the engine), which is still happening at about the same pace as before, reaches its peak pressure sooner, and therefore closer to the piston's top-dead position. This increases the pressures and temperatures dramatically. Modern automobiles, when detecting such conditions, retard the ignition timing somewhat, but their lookup tables are not typically designed for such abuse either (nor could they save everything even if they were!). However, careful reductions in RPM can be done in many engines even at full sea-level MP with no immediate adverse effects, albeit it is pushing the pressures up some.

On the other hand, there also were the inertial stresses from parts reciprocating at high speeds. Some engines may produce their rated power at such a high RPM that the wear the engine accumulates at that speed would be much enough to possibly reduce the engine's life in service. As mentioned, sustained high RPM is one major cause for mechanical wear, and in many instances they are partly offset by the combustion pressures. Anyways, in such cases we may see some engines where it is the RPM that is time-limited. An example of such an engine is the TCM IO-520-D, used in Cessna A185 airplanes. In such a case, we can be almost certain that the time limit is in place to reduce the overall, accumulated wear - and in principle, the rule of a single, continuous full-power run of one hour being equivalent to twelve shorter, 5-minute periods, holds true. It also "breaks the rule" of reducing MP before RPM: the maximum continuous power is defined as full throttle, but reduced RPM.

I strongly believe it is beneficial to have some basic understanding of what can be behind these limitations, and to know how to read the engine specs. While one may be operating well within limitations, if one is aware what are the issues the manufacturers attempt to avoid with such limitations in the first place, I think one can make a better use of his engine, being able to push it where there are vast, unused margins while at the same time avoiding unnecessary wear caused by just-in-limits operation at conditions which the engine is limited against.

These are, of course, engine-specific issues: if one flies a high-boost GA (or a warbird even), and reduces the RPM a good amount while keeping the redline MP....oh my! Add some leaning (many POHs of turbocharged GA airplanes actually recommend a leaning to some fixed GPH-figure - absolutely horrible thing to do!!!), and the havoc is ready.

On the other hand, if flying a Cessna A185, using full throttle from the takeoff, and reducing the RPM from full 2850 down to 2700 without touching the throttle is actually a proper way to operate the airplane. Just keep the nose down to add some airspeed for cooling and rich-enough mixture to limit the combustion speed, and she's happy as ever. Here the reduction in MP, if one chooses to do that for some reason, actually leans the mixture a bit, reduces your airspeed and your climb rate - all doing more harm than good!

Two engines - two partly opposite methods!

-Esa

[DHenriques_]

I was surprised to see in a United States navy training film (at 2min 35 sec viewing time)

https://www.youtube.com/watch?v=3UZmEha ... e=youtu.be

that just after take-off the pilot retards the throttle BEFORE the prop? Is'nt it against the rules? And this is a teaching film!?
Many thanks for your inputs!

I've been reading this thread with some interest and now will offer the following comment.
Although it is possible to operate an aircraft engine in over boost and get away with it for short periods and there are engines where RPM is recommended reduced while leaving MP set after takeoff, the general rule requiring the reduction of MP before RPM stands as the proper way to view any recommended power reduction after takeoff.
The so called "rule" is general in nature and simply "covers the field and avoids trouble" for aircraft engines GENERALLY.
Therefore the habit pattern taught for MP before Prop for reduction and Prop before MP for power increase is a good practice and where power changes are indicated by the manufacturer while in operation stands as correct procedure.
One can write volumes on why this engine or that engine will perform well while you treat it this way or that way and it is quite true that operation over square is viable and used regularly, but to keep aircraft engines in good shape generally there are "good practices" that cover these general situations and the way to reduce and increase power in these engines is correct the way it is taught and considered correct by the industry.
Some "rules" are better left alone and the rule for MP vs RPM is fine as it stands.
Now the TIME LIMIT for operation at specific MP vs RPM is an entirely new subject and should be discussed as such. Only the procedure for how power and RPM are CHANGED is what I am discussing here.
Dudley Henriques

[AKar]

Although it is possible to operate an aircraft engine in over boost and get away with it for short periods

And to add in a further comment to this, an intentional over-boost, or exceeding the quoted maximum MP for a given RPM should, to my best knowledge, totally be avoided. In those instances, one is likely about to exceed the maximum design pressures within the combustion chamber - especially at some mixture settings. To the engine that is equivalent to what pulling an over-g is to the structure.

-Esa

[DHenriques_]

Although it is possible to operate an aircraft engine in over boost and get away with it for short periods

And to add in a further comment to this, an intentional over-boost, or exceeding the quoted maximum MP for a given RPM should, to my best knowledge, totally be avoided. In those instances, one is likely about to exceed the maximum design pressures within the combustion chamber - especially at some mixture settings. To the engine that is equivalent to what pulling an over-g is to the structure.

-Esa

Yes, and if the complete sentence I wrote is read instead of half of it, I believe I made this fairly clear.

"Although it is possible to operate an aircraft engine in over boost and get away with it for short periods and there are engines where RPM is recommended reduced while leaving MP set after takeoff, the general rule requiring the reduction of MP before RPM stands as the proper way to view any recommended power reduction after takeoff."

[AKar]

Deleted due to unfortunate typo.

Sorry about that! See below for the original

-Esa

[DHenriques_]

Yes, I'm sorry about that. A full quote:

Although it is possible to operate an aircraft engine in over boost and get away with it for short periods and there are engines where RPM is recommended reduced while leaving MP set after takeoff, the general rule requiring the reduction of MP before RPM stands as the proper way to view any recommended power reduction after takeoff.

It indeed remains that in case both the RPM and MP are to be reduced, the RPM should go down first. Vice versa to the opposite direction. I've got no data whatsoever to suggest otherwise unless pushing the engine into a one-off.

-Esa

I'm sorry, as I have stated before, this is totally incorrect. In a power reduction after takeoff (if a reduction for both manifold pressure and RPM is indicated by the engine manufacturer) manifold pressure is ALWAYS reduced first........NOT RPM! \
The reverse is true for power increase. RPM before manifold pressure.
Dudley Henriques