Idle power & it's effects
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Idle power & it's effects
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?
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?
I sometimes leave the power at 1000RPM & apply brakes to slow down, then release the brakes but this is frowned upon by some instructors.
If the engine sounds lumpy at 6-700 rpm that's normal. If it stops running, it should be adjusted.
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pulled all the way back, 700RPM, but at this setting as mentioned earlier, the engine doesn't sound "good".
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.
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.
Last edited by mikehallam; 4th Aug 2011 at 23:07.
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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'.
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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."
Likewise when setting mixture in the cruise he always used to say "Fuel is cheaper than engines."
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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.
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.
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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.
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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.
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.
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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!
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!
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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.
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.
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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?
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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 
And no this one does not have reversers
So maybe Flybymikes instructor was not quite so stupid?
Pace
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 And no this one does not have reversers
So maybe Flybymikes instructor was not quite so stupid?
Pace
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.
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.
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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!