Low power operation (Piston) ??

[661 KTAS]

Hi folks,

My company operates a fleet of Lycoming O540 powered aircraft (Piper Lance) for joyflight/sightseeing operations.

Company SOP's require us to fly at a very low power setting all in the name of longer engine life and increased specific air range.

Typical power settings are 22"/2200rpm leaned to about 800c EGT and 11 USGAL/HR at 1000' (about 50-55% power i think)

Is this a flase ecomony ??

As far as im concerned, these engines are designed to run at much higher power settings - do they operate well at such low settings ?

Will 'running in' engines at this low power setting cause cylinders to glaze ? If so, what are the long term effects of using these power settings ?

Any info greatly appriecated !

[411A]

661KTAS
Am not an authority on Lycoming engines (except the IGSO540 series, superb...IMO), but new engines/cylinders should be run very hard for the first 10 to 15 hours (max BMEP) to ensure longer life.

OTOH, if you look back many years to the piston engine airliner days, these engines were routinely cruised at 45-50% of rated power, for longer life.
If your flights are for sightseeing (endurance, 30 minutes, one hour, etc), a lower cruise power setting will result in lower fuel burn.

Conversely, if the idea is to cover as many miles in the allotted time, then higher power settings will be appropriate.
The key to longer engine life is for it to be operated every day, whatever the power.

Hope this has been helpful.

[Crossunder]

Hi!

http://www.avweb.com/news/columns/182544-1.html

Maybe you'll find what you're lookin' for?!

[Miserlou]

I noticed your using the squared rule.

Check the engine handbook. There should be some other settings which may give better fuel consumption or same for higher output.

[javelin]

Running at too low a power setting for extended periods is asking for trouble. These engines were designed years ago when tolerances were less exact and are designed to be run at moderate power setting continuously. I have about 700 hours in PA32 types - nearly all para flying and we never babied the engines. The main thing is avoiding sudden temperature changes we never lost a cylinder in 10 years I was operating them - the Cherokee 6 had 3500 hours on it when I stopped flying it. I would not run an IO540 at less than 23/23 for long periods - lean to peak then richen 50 degrees, you won't get too much trouble at that. Also check the prop notes, you may be in an avoid area at such a low setting.

[bookworm]

Lycoming's Considerations for Low Power Low RPM cruise (http://www.lycoming.textron.com/main.jsp?bodyPage=support/publications/keyReprints/operation/lowPowerLowRPM.html) may be what you're looking for.

[sudden Winds]

Hi,

The way I see it, it´s important to ensure proper cooling and adequate mixture management.
low power settings combined with speeds a bit higher than best range, shouldn´t be a problem, because heat generation is proportional to cooling drag generation

(besides you´re low, air is dense and cooling drag is important, if you were to fly at low power settings at higher altitudes that would be more of a problem, because even tho temp decreases, air density and cooling drag decrease dramatically).

Now, if the airplane is heavily loaded, and you ´re told to maintain constant power settings as opposed to constant speeds, you might overheat your engine, because you´re flying a lot slower and producing a lot of heat.

I fly a baron, and sometimes I use low (45% bhp) power settings but cruise at the fastest speed that setting gives me...ensuring enough engine cooling...and cycle the throttles thru a higher power setting and back once every 20 min or so, to avoid constant power settings for long.

Remember that the same power setting can give you diff speeds in level flight..one corresponding to the 1st regime of flight, and the other one in the back side of the power curve (2nd regime) THAT is the one you want to stay away from.

Mixture management and cowl flap operation are very important too. You don´t want your engine too cold either.

Most engines' mixture controls are supposed to be left alone at the full rich position from 0 to 3000 feet. I normally lean them just a bit closely monitoring EGT and CHT indicators.

The combination you want to avoid is heavy, slow and too lean. If you stay away from that, then TBO should be the one predicted by the manufacturer.

Hope this helps,

SW
www.patagonianskies.com.ar

[Hudson]

Javelin. Suggest it might be worth your while to have a long hard look at the AVWEB site under the title Pelican's Perch written by John Deakin. From what I have read in his articles on leaning mixtures, your policy of leaning 50 rich of peak is a sure fire method of getting detonation under certain RPM/MP combinations.

There is a list of Deakin's articles in a recent (last week?) edition of AVWEB. He debunks many myths on light aircraft engine handling and certainly comes across as a real expert in what he is talking about.

[411A]

Sudden Winds,
You may be interested to know that in the piston airliner days of yesteryear, these aircraft were ALWAYS cruised at a constant power setting, regardless of weight.
Higher the weight, slower the speed, and v/v.
Take the DC6 for example, cruised at 1100 BHP, always...approximately 45% of (wet) rated power, with a fuel consumption of approximately 95 gal/hr (per engine).

As for cooling, that's what the cowl flaps (and where fitted, oil shutters) are for.

And as for 'cycling' power every so often, ah no, not done in these machine either.
The only time power was adjusted (in cruise) was when it was necessary to shift blower speeds...'round about 12,000 feet or so.

About the only time you would want to vary the RPM in cruise would be with a new engine/cylinders, as per the manufacturers instructions.

Flat (opposed) engines have the same combustion events occuring as radial engines of long ago and can successfully be operated lean of peak in cruise PROVIDED the fuel/air mixture has equal flow to each cylinder, an area in which the radial engines excelled.

[Jhieminga]

I've read some of John Deakin's columns, and will definitively read the rest of them as they sure shed some new light on the methods used to operate piston engines these days. I'm currently working on my ATPL and it is quite interesting to see how my FTO keeps on teaching the old wives tales that have crept into aviation ('squared' power settings, leaning ROP). Of course they own these airplanes, which leaves it up to them to decide how they want them flown, and also if everything works out anybody finishing the training syllabus here will quickly move on to jets/turboprops anyway. But it still surprises me sometimes to see how ignorant of new views on these issues people can be.

I do see a few 'buts' though: as 411A already mentioned the fuel/air distribution in a radial is a lot better than what a flat six/four can achieve. Also the engine instruments at our disposal don't always provide enough info to be able to run the engine to its full potential. Anybody wishing to run their engines like John Deakin advocates should start with a good look at the engine itself as you might need to invest a bit before you can exploit its full potential.

[LeadSled]

661kt,

Have a very close look at the POH, paying particular attention to any RPM ranges prohibited by the airframe/engine/prop combination, then use the lowest RPM and highest MP to achieve the performance you want. All these old engines were designed originally for low rpm/high boost, it minimizes the piston speed and maximizes the fuel burn time, and the whole design is crude.

Look at what the engine is certified for, particularly if it has Gamijectors, where you can operate lean of peak ( and also many IO-540’s by original certification), but please don’t ever operate “peak +50 degrees”, you are MAXIMISING the chances of detonation. Deacon is right, but this is not new, it’s just that, in my opinion, TCM and Lycoming are run by the marketing and finance departments, by people who don’t know much about piston engines.

As for how to operate big pistons, Pratt & Whitney and Wright never agreed, Wright always recommended varying the RPM and boost ( for the same SHP) regularly to change the combustion conditions, they always maintained that it would move various deposits more easily , and reduce the probability of plug fouling. But always low RPM, high boost.

I really believe that the temperature control is crucial, the more constant you can keep the temp, the less temp. cycle fatigue. Finally, don’t let the prop drive the engine, the pistons are designed to be pushed down the bore, not pulled, and this goes treble for any geared engine, reduction gears are just that, step down, not step up, reverse loading is death in the afternoon.

Tootle pip!!