Idle check during run-ups

[Genghis the Engineer]

Like everybody else, I am a good boy and do my engine run-up checks, including a check that the engine idles at about the right speed.

If I'm perfectly honest, I've never given enormous thought as to exactly why this was important, until last week.

My job, as most of you probably know, involves testing (and analysing the testing) of small piston-prop aeroplanes. Well, last week we were doing some take-off and landing performance tests of a known type, fitted with an engine we've not seen on that type before. It happened that this aircraft has a slightly disfunctional throttle mechanism, and as a result it was idling at about 30% higher revs than would be normal. This wasn't enough to make the aircraft roll forward with the brakes off, but was about enough to keep it moving once started if you didn't apply the brakes.

No big deal? We didn't think so, until after a day's flight testing I went off and analysed the flight test results.

One thing it did was reduce the stall speed by about 3 knots, wihch is arguably nice to have, but again no big deal.

But, what this bit of residual thrust did was increase the amount of float on landing quite substantially, and also reduce the braking efficiency a bit (since the brakes now had more to work against).

The upshot was an increase in landing distance of roughly 60%: considerably more than any safety factor you'd normally apply to landing distances.


So, somebody has to redesign a throttle lever, I'll have more testing and analysis to do.... and I come away from this with significantly enhanced appreciation of the importance of idle RPM being within limits on my run-ups.

Just thought I'd share that.

G

[Rod1]

The MCR01 is very light and does not have very good breaks. The extreme landing routine used by a number of owners is to switch the engine off after touchdown, stop the aircraft and restart to clear the runway. One owner who took me for a flight insisted it made a 50% reduction in landing roll!

I hope to run some tests of my own later this year……

Rod1

[waldopepper42]

I was taught my my instructor on my very first lesson that that was the reason for the idle check! Then again, I learned to fly at Netherthorpe where any lengthening of the landing run could lead to an introduction to the "arrester hedge" :O

[stiknruda]

Another very good reason for the check is to ensure that with zero airspeed the donkey won't actually stop. Unlikely on final with airspeed and a windmilling prop but not unheard of in aerobatics; spin entries, stall turns, tail-slides, etc!

Stik

[High Wing Drifter]

Me too Waldo :ok: Although I have to say I mentioned this to another instructor (different FTO) during a checkout and he just shook his head and frowned :rolleyes:

[FireFoxDown]

Very interesting Genghis . . . Never had it properly explained to me by any of my instructors! Im quite amazed at the figure of 60%!

You learn something new . . .(you know the rest) :ok:

[Whirlybird]

I never had it explained to me, but I just assumed it was to make sure the engine didn't stop...particularly important in something like the C150 with a Continental engine, as I've had it come close to stopping at that point a couple of times. :eek: But I never thought to notice if the engine was idling too fast! Thanks, Genghis, now I know, I'm not likely to hit the hedge if I ever go to Netherthorpe...well, I might, but I won't have that as an excuse. :)

[Paris Dakar]

Thanks for that Genghis :ok:

I too believed that the check was done purely to ensure that the engine wouldn't potentially stall on final etc. I didn't really consider the problem of an engine developing higher revs.....................but I will now.

PD

[slim_slag]

How did you know what the landing distance would have been if the engine you had never seen before was set to idle at the lower speed? Just looking at the 60% figure, I'm sure you are correct in the distance increases. Did you come in slower because of the lower stall speed? How much would that have helped?

[Genghis the Engineer]

Because we'd previously tested the same aircraft type, at the same weight, on a similar surface, with the same test pilot flying at the same speeds (within a knot anyhow) - but a different engine fitted (which used a different throttle mechanism and thus idled properly).

Lower approach speeds is a possibility, but it means a different manual for each engine type, possibly uncomfortable attitudes, and also an aircraft which might behave differently when being landed at idle, and with the engine dead (e.g. deadsticking after an engine failure). For all those reasons, we didn't.

G

[muffin]

Ah, that would account for why my landing run seemed to go on for ever last week. We had just got the aircraft back in the air after 7 months in bits and I was trying to get the tickover speed set right. Whereas I realised that it would obviously make a difference I did not expect that much.

[shortstripper]

No offense Gengis, but I'm amazed that's the first time you had call to figure that one out :confused:

However, now I'm thinking about throttles ... What do you consider the best way to have a spring return set on a cabled throttle? Set for full on, or full off in the event of a disconnection? I know it's a rare occurance but what if? Both ways have advantages and disadvantages. As an ex glider pilot, I favour off, but a WW1 pilot (not many around I admit) used to a "blip switch" might prefer full on? What do you reckon?

SS

[squawking 7700]

Had a problem similar to this on Tatenhill's Robin, left throttle
had less movement than the right!
Instructor commented that I should be closing the throttle all the way, I said "I am" as we floated down the runway.
When I used the right throttle and flew left handed, all was OK
as idle dropped another 100rpm which made all the difference.

7700

[bar shaker]

I understand that this has been known to manufacturers for many years.

Then again, if the aircraft is fitted with a Jab...

[QNH 1013]

When I was learning to fly, my instructor told me that if I was ever running out of runway and heading for the hedge I should "cut the mags" because it would make a significant difference to the ground roll.

The thrust from a prop at idle rpm is still significant; try walking behind one!

[Ringway Flyer]

Worth bearing in mind that most idle checks are carried out with a cold-ish engine, so if it's a bit fast when cold, it'll certainly make for an interesting arrival when it's warmed up after a sortie....:p

[Genghis the Engineer]

Shortstripper wrote
No offense Gengis, but I'm amazed that's the first time you had call to figure that one out

However, now I'm thinking about throttles ... What do you consider the best way to have a spring return set on a cabled throttle? Set for full on, or full off in the event of a disconnection? I know it's a rare occurance but what if? Both ways have advantages and disadvantages. As an ex glider pilot, I favour off, but a WW1 pilot (not many around I admit) used to a "blip switch" might prefer full on? What do you reckon?

I think that I'd grasped, somewhere amongst my couple of aeronautics degrees and 90 types the importance of closing throttle to land - it was the magnitude of the effect of a slightly high ground idle that took me by surprise, not the fact that it was there. (Also this was a Rotax 912 engine, which has generally pretty low residual thrust due to the stepdown gearbox.)

As to the other question, it's a good debating point, but my money is on a "fail open" throttle. My reasoning is that I can always climb or fly fast with a fully open throttle, then simply kill the engine when safely in glide range of a good landing field. With a "fail closed" throttle you basically have a slightly extended normal glide range and that's it.

G

[Milt]

Optimum idle/minimum RPM with turboprops and jets can also be significant and often more difficult to determine with the types of instrumentation available.

Turboprops running at constant speed often only have a minimum turbine inlet temperature and/or pressure ratio to go by with questionable accuracies at the low end. Then most of any excess thrust roll out problems can often be cancelled out by going into reverse

[Tinstaafl]

I've experienced high idle landing distances before and always check to ensure a reasonable throttle closed idle RPM.

Not just on singles either. Asymmetric idle in twins can be a pain in the arse.

[PH-UKU]

To reduce idle speeds (and hence taxi-speeds) we often cut onto one mag.
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.
.
.
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Oh, should mention that is on floats - as once the donkey starts up ....... we're moving and there's no going back ..... or stopping ......quickly..... :p

That also explains why that Offspring song (No Brakes) always springs to mind at startup. :cool:

[Kingy]

I make a point of setting the idle speed on my aircraft as low as possible as I need to fly into some extremely short strips sometimes. I find that if the engine will just about run during the initial slow running checks, it usually results in a smooth even idle of perhaps 5-600rpm when warm.. just right. Also I have found that with a metal prop, a slightly lower idle can be used.

I say this as a stick and string PFA type, rather as a licensed engineer however..

Kingy

[muffin]

We find that the limiting factor on how low you can idle is the roughness of the engine at tickover. We are using a Rotax 912 and if you let it get below 1100 RPM or so you are afraid the engine is going to jump out of its mountings. While warming up, we usually increase the revs to 1700 or so and hold it on the brakes. No amount of carb balancing will seem to allow an idle of less than 12-1300 RPM with any chance of keeping the screwdriver in the adjusting screw slot.

We are a bit paranoid about engine stability as we found all 4 mounting brackets broken during an inspection last year. It has taken 7 months to get it all repaired and the engine is much smoother at low revs now.

What stable idle speed do others get down to on this engine?

[shortstripper]

Sorry Gengis ... it was the way you wrote that made it look like it was the first time you had experienced it. I know you would have known about it of course.

I see your point about throttle wide open, and I guess that probably is the best as your options are increased. However, many a well set up forced landing has been ruined by an engine stuttering back to life at the wrong moment ... hence "switches off" once committed.

Muffin ....

My mate with a Zenair 701 has a Rotax 912, it often stops as he rounds out due to its inability to tick over smoothly at speeds low enough to produce no thrust. He prefers that to a higher tick over, the reason is as demonstrated by Gengis. And yes they do shake about a bit at low tick over don't they? :uhoh:

SS

[Genghis the Engineer]

As has been said, the startup vibrations from the 912 is pretty horrible, and the 912s even worse. Rotax' solution to this is a "slipper clutch", which seems to make a huge difference to startup and low-RPM vibrations. I've never done one myself, but I believe that it adds about 1½kg and can be retro-fitted, presumably Skydrive can give you chapter and verse.

Incidentally, the analysis I did on this particular aeroplane showed that it was the final approach (float!) that was most extended rather than the ground roll - so if you are really determined turning the switches off at about 100ft would be the way ahead.

Would I have the nerves to do that? Nope, I'd go to an airfield with a longer runway!

G

[bar shaker]

A 912 with properly balanced carbs purrs like a pussy cat on idle.

Alll are a bit rough on start up, the 912S being rougher on start up and slightly rougher on idle, but still not bad when running right. Its the price you pay for a high compression ratio and 20% more power.

Genghis, what aircraft were you testing?

[muffin]

I had a look at the Rotax web site, which specifies an idle speed of 1400 RPM on the 912. There is also an SB that says you must not idle below 1200 due to the risk of damage. I tried the experiment today in a 15kt headwind. I touched down at normal speed where the numbers would be if we had any. The runway has a slight up slope and I did not touch the brakes at all. I used up most of the 450 metre runway despite the up slope. I then repeated the exercise landing at the same speed but cutting the mags on touch down. I rolled to a halt in less than 200 metres.

I expected a reduction but was surprised how much it was. The stationary idle prior to takeoff was 1800 RPM and with power off prior to landing about 2000 RPM presumably due to windmill effect. It was also interesting how long the prop took to stop when I cut the mags due to the same effect.

This is a useful exercise to try for yourself so you get a feel for how short you can land if you really have to.

[Genghis the Engineer]

Genghis, what aircraft were you testing?
Sorry, not really my information to give out - and irrelevant to the context of the thread.

G

[windy1]

Another thing you can do at idle check is to pull back the mixture slightly and watch the revs. If they go up, then the mixture may set a bit rich at idle. Not really critical but just something to mention at next maintenance.

[ChrisVJ]

Jemima has a 912S. It is mounted on a bracket on the root tube by 4 bolts into the bottom (only) of the block. The engine wobbles around a good bit on start up and shut down but actually runs quite smoothly the rest of the time although it does not much like full idle during descent.

A number of the non certified 912S engines were supplied without the clutch for a while. The clutch needs regular adjustment and they hoped to do away with that. The reason given for the clutch was to safeguard the crank in the event of a prop strike. Unfortunately a considerable number of engines without the clutch have developed gearbox vibration at about 3k+ revs after about 70 hours and mostly the owners have been obliged to go with Rotax suggested upgrade of installing the clutch. (Ouch $$$$)

When I first set up the engine I used the factory recommended idle at about 1400 revs but as the weather got colder it occasionally got lumpy. Our group (about twenty of us) were discussing it at the time and we reckoned we might go for 1600 to a max of about 1800 for the Winter. Finally I've set mine for 1600 year round.

Last November I had the engine quit on landing when I pulled it to idle. Twenty seconds later she restarted and idled just fine. I did all the checks and then flew another circuit (close in!) and the same thing happened. According to Rotax the 912S is not subject to icing but I have seen a couple of pictures of a thin film of ice formed on the inside wall of the carb, I reckon it is just enough to close off the air when the butterfly closes but does not affect airflow when the throttle is open. As soon as she quits the throttle body warms up and so she runs.

As it happens it was only a few days before her annual and she now has new plugs, also I have only flown since in weather dry enough to turn running bull into jerky in about twenty minutes. so no re-occurrencies. We'll see. . . . .