Idle power & it's effects

[shumway76]

Regarding piston engines (be it single or multi engine aircraft), it's "idle" speed is normally manually set at around 1000-1200RPM, while it's physical (when throttle pulled back all the way) is less than that, around 700RPM or so.
Is there a reason for it being set at roughly 1000RPM as idle?
I noticed that when throttle is physically pulled back all the way, the engine doesn't sound "good", as though it's on the verge of quitting.

When taxiing - if power is set at "idle" (1000RPM), this sometimes causes the taxi speed to be a bit high. I sometimes leave the power at 1000RPM & apply brakes to slow down, then release the brakes but this is frowned upon by some instructors.
The "logical" way to slow down would be reduce power, but I find that once I reach the safe taxi speed, to maintain it would require throttle lever at physical idle - pulled all the way back, 700RPM, but at this setting as mentioned earlier, the engine doesn't sound "good".
Any thoughts?

[jxk]

1200rpm to keep plugs from fouling and battery charging.
6/700rpm minimum thrust for landing.

[BEagle]

I sometimes leave the power at 1000RPM & apply brakes to slow down, then release the brakes but this is frowned upon by some instructors.

A bad habit - it will cause brake wear and might even overheat the brakes. If you need to slow down, use idle power first. Do not use power against brakes.

If the engine sounds lumpy at 6-700 rpm that's normal. If it stops running, it should be adjusted.

[The500man]

pulled all the way back, 700RPM, but at this setting as mentioned earlier, the engine doesn't sound "good".

Have you considered leaning the mixture? I think most people are taught not to do this on the ground because of the risk of forgetting and destroying the engine in the run-up or when taking-off. The POH may say not to as well. Just a thought.

[mikehallam]

I agree with the question. basically I find I 'need' two idle settings.

One to ensure the engine doesn't run too slow or rough on the ground, but a second lower throttle opening - pull it harder ? - to ensure as little driving power as possible comes from the prop when landing. The windmilling effect will ensure the prop runs quite fast enough.

Of course you could still find, say on roll out, that it'll go rough & too slow, might even stop if you've overdone it, because no wind aid to rotate the prop. That's when - or before really - that you revert to the 'normal idle'.

mike hallam.

[Jan Olieslagers]

An interesting discussion. Are the arguments and answers the same for direct drive as for reduction like the Rotaxen?

[Morris542]

Of course you could still find, say on roll out, that it'll go rough & too slow, might even stop if you've overdone it, because no wind aid to roatate the prop. That's when - or before really - that you revert to the 'normal idle'.




That happened to me yesterday! On roll out, the engine was beginning to run rough and was very slow to respond when I did apply some throttle. It caused me a bit of concern at the time as all the other indications (fuel pressure, t+ps) were normal. Next time I'll be much more aware and apply a tad more throttle sooner to prevent the rough running.

[flybymike]

I was taught by my hoary old CFI to maintain 1200rpm and use brakes against power if that was necessary. He maintained that brake fade would not be an issue at taxi speeds, that resultant brake pad wear would be minimal, and that in any event brake pads are cheaper and less hassle than oiled up plugs, valves and combustion chambers.
Likewise when setting mixture in the cruise he always used to say "Fuel is cheaper than engines."

[IO540]

I don't agree. You can wear out your brake pads pretty fast by taxiing on the brakes (I did so in my early days, to my great suprise).

The other issues are solved by leaning when on the ground, to the point where the engine is almost not wanting to run. That keeps the plugs clean, and there is absolutely no risk of engine damage at such miniscule power settings.

[WILCO.XMG]

very intelligent instructor you had flybymike

[thing]

That happened to me yesterday! On roll out, the engine was beginning to run rough and was very slow to respond when I did apply some throttle. It caused me a bit of concern at the time as all the other indications (fuel pressure, t+ps) were normal. Next time I'll be much more aware and apply a tad more throttle sooner to prevent the rough running.

Exactly the same thing happened to me last week, you weren't in a 152 by any chance?

[Pilot DAR]

Dead idle is useful for allowing the plane to slow down gently. Use brakes if you must, but not if you don't have to - someone has to pay for them. On aircraft not fitted with brakes (generally skiplanes, floatplanes, and seemingly a Tiger Moth I flew a few times!), not only will you want to taxi at dead idle, but it can be helpful to use only one mag, to get the engine turning even more slowly.

Yes, fouling can be a problem at slow engine speeds (particularly on one mag), so a good run up, with a bit of leaning can be important. Assure you have smooth engine operation before takeoff.

Aircraft equipped with composite props will probably require higher idle speeds, as there is less flywheel affect from the lighter prop to carry the engine through compression strokes at the lowest speeds.

Though lumpy, engines should run consistently at dead idle, and be able to smoothly accelerate to high idle with cautious throttle movement. If not, ask that the idle stop be checked. Check this during your run up, as an engine which will run at dead idle when you're stopped on the ground, will not stop while you're rolling out on the runway, and this is a nice thing to know about the plane before you fly it.

When advancing the throttle from idle, smooth and steady, don't jam it. Injected engines handle jamming the throttle better than carb engines, but still avoid. Carb engines generally have accelerator pumps to compensate for jamming the throttle, but if the plane you're flying does not, or it's in poor shape, jamming the throttle will cause it to quit - even in flight. A Cub skiplane I used to fly was like this, you'd burst a bit of throttle on short final, or during a touch and go, and it would quit. You'd have to get out and hand prop it - that's a good way to teach the value of gentle throttle use!

If an engine idles smoothly, but stumbles upon advancing the throttle, it's as likely a technique problem, than an engine condition problem.

[Johnm]

Don't rely on brakes against power, you'll need to close the throttle if the brakes fail so why not close it anyway rather than ride the brakes?

If the engine runs rough with the throttle closed lean the mixture. It's good practice to lean the mixture aggressively for taxi anyway as it keeps things cleaner inside the engine and there's less spare fuel to wash the oil off the bores.

If you lean it so far you have to enrichen to speed up the taxi there's no chance of forgetting to enrich at power checks either. In any event if your memory is that bad use check lists like I do, that way you won't forget carb heat or mag drop checks either.

The "fuel is cheaper than engines" comment is so stupid it would be laughable if it hadn't come from an instructor. In some parts of the world where the density altitude is high, you'll need to lean pretty aggressively to get enough power to get the thing airborne!

[FlyingStone]

Piston engine idle (the real idle, throttle fully retarded) is usually set so low (600-700 RPM when standstill) so that it doesn't produce virtually any thrust. Thrust at such low powers would seem to have almost no affect, but you can find in performance sections of some aircraft that if the idle is set to 1000 RPM, your landing distance and roll increase, usually up to 10%. I prefer fouled spark plugs (if there is even possible to foul the plugs during few seconds that RPM is below 1000 RPM on landing roll) than overruning the runway anyday, because some engineer/pilot thinks that piston engine management requires IQ over 200 and that ground RPM must be set by device rather than pilot. I think that what kind of behaviour (roughness etc.) engine show below 1000 RPM is of little importance. First of all, I hope you aren't going below 1000 RPM unless you need to stop the aircraft on the ground. If the engine runs smooth from 1000 RPM to full power, even at LOP, I'll take it. I once flew an aircraft with an engine who idled perfectly smooth at idle (600 RPM), but the prop was terribly out of balance just at RPM for a normal powered approach, kind of the same rougness if you pull your mixture in cruise from the point that the rougness starts, and then a bit further to lean - not the sound you would like on your every approach :)

Like Pilot DAR said, if you have a short runway, leave the throttle fully retarded until you either stop or reach safe taxi speed. As far as holding brakes against the ground RPM goes, if your aircraft accelerates significantly with 1000 RPM set (which is probably quite unlikely, unless you're taxiing on a downslope taxiway), step on the brakes, slow the aircraft and then release them and use if neccessary again when the aircraft reaches previous speeds. Don't drag the brakes, because it wears them out faster than you think. Some poeple don't like to wait on parking during engine warm-up, so they set 1200-1400 RPM and taxiing they go - and it's mostly people, who taxi at near rotation speeds, rush to holding point where they then have to wait for another few minutes, blocking all the traffic who is ready to depart. What they don't realise is, taxiing at 20+ knots and having the power set at 1200 RPM, you don't do any good for engine warmup, since the airflow will be quite sufficient in cooling the engine - again, it's usually the people who do warmup at full rich mixture.

[Morris542]

That happened to me yesterday! On roll out, the engine was beginning to run rough and was very slow to respond when I did apply some throttle. It caused me a bit of concern at the time as all the other indications (fuel pressure, t+ps) were normal. Next time I'll be much more aware and apply a tad more throttle sooner to prevent the rough running.

Exactly the same thing happened to me last week, you weren't in a 152 by any chance?

Nope. PA28.

[Pace]

This thread is quite amusing :) Please jump into my Citation (also has brakes and pads) close both thrust levers and experience the residual thrust which will actually accelerate the aircraft without braking :E

And no this one does not have reversers

So maybe Flybymikes instructor was not quite so stupid?

Pace

[mikehallam]

A Citation is hardly the kind of 'plane generally thought to be covered by this section of Pprune !

Regular prop planes can certainly float on more than desirable if the engine's 'low' power setting is still giving some prop thrust. That for me is the more important criterion.

BTW. How did you find a way to get your Citation Permitted under the LAA or even BMAA ?

mike hallam.

[flybymike]

So maybe Flybymikes instructor was not quite so stupid?


Well apparently he learned nothing about all this during an RAF flying career and 30 years civilian instructing.

[Torque Tonight]

Well in my RAF career I was consistently taught not to brake against power. That was on SEP, Bulldog, and a few hours taxying JPs around Cosford. Obviously the argument is not so applicable to helicopters. In my current airline role long taxis such Madrid can generate brake cooling issues from normal brake use even when not against power. Pace's steed may well have enough residual thrust to push it along quite nicely but I bet he wouldn't use the brakes without the thrust levers closed!

[Pilot DAR]

close both thrust levers and experience the residual thrust

Try running the engines on only one mag....

[Aerozepplin]

Flybymike, despite your instructor's experience being far greater than my own, in my opinion the phrase "Fuel is cheaper than engines." is not a smart one.

There are pages and pages of good discussion regarding leaning on Pprune, and many more elsewhere (Avweb being excellent). Too much fuel fouls plugs and valve guides, possibly leading to gliding! Likewise running out of fuel leads to gliding! I've developed a dislike for 'cute' phrases like the above because it often over simplifies an important subject.

Anyway, sorry for hijacking, this be about idle and taxi (but you can/should still lean on the ground!), but my childlike fascination with leaning and engine management takes over.

[Pace]

Torque tonight

I was really making a point that brakes are there to be used when required!
When you land on a fairly short strip a pilot will be fairly hard on the brakes.
There is a mindset that taxiing the brakes are some sort of delicate contraption that should be avoided at all costs which is rubbish,
Obviously you don't ride the brakes with high power settings any more than you would drive your car with one foot on the accelerator and one on the brake,
It is not equally good practice to design your engine management to avoid use of brakes,
The brakes on landing will be used far far harder than anything you will do taxiing,

Pace

[Robin400]

700 rpm is the recommended idle setting for a warm small aero engine.
After prolonged running at this rpm, carb icing is a probable cause for stoppage with the throttle closed. The Lycoming 0235 is particularly susceptible to this problem.

[dublinpilot]

700 rpm is the recommended idle setting for a warm small aero engine

Recommended by whom? I was surprised recently when on an idle Sunday afternoon I decided to re-read our aircraft POH (as I hadn't read through it in full for some years) to find that it recommended warming that engine at 1400-1500rpm, and if a hold was encountered while waiting, to use the same rpm setting.

[A and C]

It is not often that I come across such stupid statements, to deal with brakes being used to keep control of the speed when the engine is set to 1200 RPM I only have to think of the badly burnt Extra 300 that I inspected in Florida, overheating brakes had set fire to the spats, the resultant fire had resulted in the aircraft being written off.

The fuel cheaper than engines statement also fails to hold water, assuming that the leaning of the engine is carried out badly and that you write off a set of cylinders at half life it much cheaper to change four cylinders that except a 20 % higher fuel burn. Using a Lycoming O-360 as an example over 1000 hours you can save £12960 in fuel costs by leaning the engine rather than running it full rich, a set of new cylinders in is will cost you IRO £ 5500 and fitting will cost IRO £ 800.
This assumes poor engine. Management I am sure that those of you with engine monitoring equipment could run the cylinders to 2000 hours without damage.

[Pace]

A and C

I am sure nobody including Mikes instructor is recommending more than idle power and using brakes to control speed.
Obviously brakes against power for prolonged periods will lead to overheating and fade.

What I thought we were discussing was residual thrust at idle power setting?
I gave an example of a Citation which I fly which does not have thrust reversers.

On level ground with thrust levers at Idle the aircraft will accelerate with residual thrust which means you will have to stab the brakes every so often to keep the speed where you want it.

It is not a fault with this particular Citation but a known common trait.

The same goes for light GA I see nothing wrong with using the brakes to help control speed in the sense of gentle sporadic pressure as required.

That is very different to having a highish idle setting and keeping your foot planted till they overheat and fade.

Pace

[Pilot DAR]

aircraft POH (as I hadn't read through it in full for some years) to find that it recommended warming that engine at 1400-1500rpm, and if a hold was encountered while waiting, to use the same rpm setting.

My guess would be that this has more to do with keeping the alternator turning fast enough to carry the electrical load, than the characteristics of the engine itself. Though both warming up, and holding would involve the use of brakes, the aircraft would be stopped, so they are not being worn or heated up, so use them as much as you like!

[A and C]

I don't think that most of the people on this forum fly aircraft that will require more than reducing the power to control the taxi speed on level ground, riding the brakes is usually unnecessary on the types of aircraft that most of this forum readers fly.

The high residual thrust that is exhibited by some turbine types is usually due to the minimum engine RPM required to keep the CSD or IDG on line, the correct way to control the taxi speed in this case is to brake the aircraft almost to a stop and then release the brakes and let the speed build until the taxi speed is such that you need to repeat the process, no doubt instructions will differ between aircraft manufacturers but this seems to be the norm for most of the turbine aircraft that I have flown.

[Robin400]

Idle rpm is when the throttle is fully closed.
Thiis setting is determined by the engineer on engine installation.
This figure is around 500 to 700 rpm and is set to a figure which gives the minimum vibration. This setting allows the engine to continue to run at the end of the landing roll, prior to opening the throttle for taxy.

[24Carrot]

Going back to the OP:
I noticed that when throttle is physically pulled back all the way, the engine doesn't sound "good", as though it's on the verge of quitting.

But at least if does not quit while the aircraft is stationary, it is even less likely to do so while airflow is driving the prop blades. At least, that's what I comfort myself with as I listen to it splutter.:hmm:

[john ball]

I find that with my RV6, that even 1000rpm and the aircraft is taxiing too fast, so it is the usual practice of going along then throttling back until slow enough then opening up a bit again and so on. In the Lycoming flyer manual it says that stationary the engine should be run at 1200rpm to keep enough cold air over the cylinders and 1000rpm taxyiing. In my aircraft, I am in the situation where on start up the cylinder head temps get very high before the oil is warm. So I have to be very careful with the gradual warm up of the engine, especially in the winter. This is due to tight cowlings and no cowl flaps like some aircraft for example a Mooney or Beech Bonanza.

[Robin400]

500 to 700 rpm is the recommended idle setting for a warm small aero engine.


A WARM engine is when it has reached normal operating temp.
WARMING is allowing it to reach normal operating temperature, around 1,000 to 1,200 rpm. POH will give the exact figure for specific type.

[flybymike]

Idle rpm is when the throttle is fully closed.
Thiis setting is determined by the engineer on engine installation.
This figure is around 500 to 700 rpm and is set to a figure which gives the minimum vibration. This setting allows the engine to continue to run at the end of the landing roll, prior to opening the throttle for taxy.

OOh! You bad bad boy!:ooh: You must not open the throttle above 700rpm for taxi lest thee burst into flames and perish!

Like Pace I simply cannot understand all the angst generated by this stuff. Pootling along at 3 knots and 1200rpm with the occasional dab on the brakes is not going to kill anyone. In 28 years of multi aircraft ownership I can only recall, a couple of pad renewals anyway, and I cannot understand why road vehicles are not exploding all over the place on a daily basis. Their brakes get far more use at far higher speeds than the average puddle jumper. Get a life folks.

[Robin400]

I have not made any comment regarding the rpm while taxiing and are not going to either.:ok: