fuel saving trick in a 3 engine plane?

[jsoceanlord]

i recently learned that if fuel is tight, and you're light on load, that in my piston 3 engine trislander, you can feather the rear engine and you can travel the greatest distance that way. would that work in a 727 or dc-10?

 

[banned username 2]

i recently learned that if fuel is tight, and you're light on load, that in my piston 3 engine trislander, you can feather the rear engine and you can travel the greatest distance that way. would that work in a 727 or dc-10?

Well in the Falcon 900EX we have the same range on 3 engines as we do on two engines... On two engines it will take you longer to get there and you will be at a lower altitude, thus burning more fuel for a longer time...

An engine loss for us doesn't afffect range...

 

[firstthird]

Don't know about pistons but in a turboprop P-3 we used to shut down an engine all the time to save gas. More for loitering around on station than transitting though. There was even a procedure for shutting down 2 of the 4 engines although this wasn't done all that often. For a turboprop at least, the theory is that by shutting down one engine and running the other 3 at a higher temperature you have 1 less compressor to drive and the 3 remaining engines are operating more efficiently since you are running them at the higher temperature.

That was all about flying around max time though, it gets more complicated when you start talking max range due to ceilings and power available, best speed, etc. Short story, we wouldn't shut one down to save gas for transit unless of an unusual situation, i.e. stuck at 10,000 feet for a pressurization problem.

 

[bigsky]

I am going by memory but I seem to recall seeing somewhere that fuel consumption would increase about 15% if one engine was failed in the DC10/MD11.

 

[RichardFitzwel9]

i recently learned that if fuel is tight, and you're light on load, that in my piston 3 engine trislander, you can feather the rear engine and you can travel the greatest distance that way. would that work in a 727 or dc-10?

Not sure how that'd work? Heard that 727 and -10 engines are kinda hard to feather

 

[Loafman]

Engines don't feather. Some propellors do. I'd like to see a feathered engine. Maybe they get feathered when they suck up a big bird or someone shoots a chicken into them.

(just a pet peeve of mine from hearing too many sim students wanting to feather engines)

 

[CAP10B]

You might save money on gas, but how much is it going to cost you for shock cooling those cylinders on the rear engine?

just something to think about before you start to pull it back

 

[Chunk]

Shock cooling?

An exerpt:


Shock Cooling: Myth or Reality?
Powerplant management guru Kas Thomas of TBO ADVISOR examines the physics and metallurgy of "shock cooling" and concludes that, contrary to the conventional wisdom, it is not a major contributor to cylinder head cracking.

This article first appeared in the March 1996 issue of TBO ADVISOR and appears here by permission. The article is Copyright 1996 by Kas Thomas.

By Kas Thomas (kthomas@avweb.com)


--------------------------------------------------------------------------------

Photo: FredWeick made many contributions to our knowledge of engine cooling.


Not long ago, a writer for a major aviation publication called to ask my opinion(s) on the subject of shock cooling. It turns out the caller had already written his article, but he wanted to run some ideas by me to make sure he wasn't missing something. Since I get a lot of calls on this subject, I had some ready answers for him. Not necessarily correct answers--just ready answers.

I don't think anybody has provably correct answers to questions involving shock cooling of aircraft engines. To my way of thinking, there is no scientific proof that shock cooling plays a significant role in cylinder damage in aviation. "Scientific proof" is perhaps a poor choice of words. What I'm simply trying to say is, the hard evidence is scanty. I know of no fleet studies on this subject. I know of no pilot who can say "I went up and did this and this and this to the engine, and then when I landed I found these cracks that weren't there before."

Still, it's hard to argue with common sense, and common sense says that if you thermal-cycle a piece of cast aluminum (especially while beating on it!) you just might induce it to crack. Pilots can perhaps be forgiven for harboring a strong gut feeling that yanking the throttle back is a good way to bring on cylinder cracking. Certainly, many millions of dollars' worth of spoiler kits and CHT systems have been sold to pilots on this basis over the years.

My own gut tells me that shock cooling--while bound to induce dimensional changes in the engine--is not a great contributor to cylinder cracking. We know it induces dimensional changes, because (for example) valve sticking has been induced in some engines by sudden power reductions. (A Lycoming Flyer article once stated: "Engineering tests have demonstrated that valves will stick when a large amount of very cold air is directed over an engine which has been quickly throttled back after operating at normal running temperatures." See 101 Ways to Extend the Life of Your Engine, page 96.) But it's a big jump to go from that to saying you can make a cylinder head crack just by pulling the throttle back too quickly.

To my knowledge, Bob Hoover has not experienced any problem with cylinder-head cracking on his Shrike, despite his rather odd predisposition to feather both engines while in a redline dive. (Maybe this is what FAA meant by "cognitive defect"? Just kidding.)

Besides which, I think any careful examination of the concept of "cooling" (as it applies to current aircraft engines) will leave one virtually empty-handed, because I think it could be argued that cooling fins on aircraft cylinders are of mainly ornamental value. I suspect that you could hacksaw much of the finnage off, say, a TSIO-520's cylinders and not affect inflight CHT readings by very much. As it happens, this is exactly what Continental did when it created the "lightweight" Crusader engine--the TSIO-520-AE used in the Cessna T303. The cooling fins on this engine are fewer in number, and about half the size of, those on a standard TSIO-520. And yet, CHTs in the T303 are remarkably cool. (One of our readers, in fact, reports a problem in getting CHTs to stay in the green; see this month's "Questions and Answers," page 26.)

Various investigators have done "energy balance sheets" on aircraft engines, and the result is always the same: Only about 12% of the heat energy generated in combustion goes out an "air-cooled" engine's cooling fins. The biggest fraction (around 44%) goes right out the exhaust pipe, of course. Another 8% or so finds its way into the oil--which is quite interesting, because it means the oil plays almost as big a role in cooling your engine as air does. The remaining energy shows up as work at the crankshaft.

Throttle placement doesn't have nearly as direct an effect on CHT as you might think. Back in 1983, there was an SAE paper (No. 830718) by three Texas A&M researchers who tried to correlate OAT (outside air temp), CHT (cylinder head temp), EGT (exhaust gas temp), power settings, air density, and cowl pressure drop in Lycoming TIO-540 engines. Their work was partly based on the NACA Cooling Correlation (NACA Report No. 683, published in 1940), which in turn was based on pioneering work done by Fred Weick in the late 1920s. The Texas A&M group merely extended NACA's approach, verifying their results with inflight measurements taken on a Piper Turbo Aztec and a Rockwell 700. One of their key findings was that the difference between CHT and OAT is proportional to the difference between EGT and CHT, which is (if you dwell on it long enough) intuitive, since the difference between the average exhaust temperature and CHT is what "drives" CHT changes to begin with. (If this isn't intuitive to you, you may want to go back and re-read Fourier's classic Analytic Theory of Heat.) This portion of the group's findings might be summarized by saying that the stored heat of the cylinder head is proportional to the input heat, represented by EGT minus CHT.

But there are two aspects to cylinder cooling. One is the "supply side" aspect (which we have just been taking about--all this business about EGT minus CHT), while the other is the taking-away of heat, or "cooling" aspect. The Texas A&M group accounted for this too. They found that the stored heat is proportional to the input heat--proportional, that is, by a factor y. The factor y, in turn, is made up of engine power raised to a certain exponent, divided by cooling airflow delta-p raised to a certain exponent. The engine-power exponent is fractional; for the Rockwell 700 it turns out to be 0.33. (It varies from plane to plane depending, apparently, on peculiarities of engine installation and operating envelope.) The air-cooling delta-p exponent is also fractional (0.29). In plain English: CHT depends on the cube root of engine power, divided by the cube root (roughly) of the cooling-airflow pressure drop.

contd.

 

[Chunk]

part 2

contd.'

After a few rough scratchpad calculations, you find that cutting an engine's power by half (but leaving airspeed constant, such as in a descent) results in a CHT drop of only 10% or so, or about 80¡ F. (Recall that in calculations of this sort, you want to use a Rankine temperature scale, which begins at absolute zero, or minus-460°F.) Most of the time, a 50% power cut is accompanied by some loss of indicated airspeed, which would tend to offset the CHT drop, making it less than 80° F. The numbers are within reason, evidently. But is this kind of CHT drop capable of trashing a set of cylinders? I doubt it.

Of course, the rate of the drop is plainly an important factor here (not just the magnitude of the drop). In this connection, I am reminded of an experiment once done by John Schwaner (of Sacramento Sky Ranch). It seems Schwaner, curious as to whether he could "crack" a cylinder at will, in a shop environment, one time took a cylinder that was heated to several hundred degrees in an oven (I believe it was an O-320 jug, although here I'm going from memory) and dunked it in a bucket of cold acetone. The abruptly cooled cylinder was later examined, and no abnormalities could be found in it.

And then there's ordinary rain. Every pilot flies through rain at one time or another, and rain should be a very effective coolant (more so than mere air, certainly)--yet no one, as far as I can determine, ascribes cylinder damage to flying through too much rain. In fact, most pilots (I think) consider just the opposite to be true; namely, that flying through rain is good for an engine, because of the extra cooling.

Let us assume that a moderate downpour contains one cubic centimeter (one gram) of water per cubic meter, and let us further assume a cooling airflow of 100 cubic meters per minute for a high-performance engine. (David Thurston's Design for Flying suggests 77 cubic meters per minute as typical for many engines.) We might reasonably expect, therefore, that 100 grams of water might enter the cowling per minute while flying in rain. Considering that water has a heat of vaporization of about 540 cal/g, it's not impossible for 100 g/min of rain influx to give about 54,000 cal/min of cooling, which is about 200 British Thermal Units per minute.

The question is, how does this compare with the heat of combustion? We can do a rough calculation this way: We know that (by ASTM spec) avgas contains a minimum 18,720 BTU per pound or about 112,320 BTU per gallon. If an O-470 burns 13 gal/hr in cruise (or 78 lb/hr, roughly), the engine is capable of producing 24,336 BTU per minute of combustion heat--if combustion is 100% efficient. In the real world of mixture maldistribution, rich mixtures, and incomplete combustion, we can safely say that probably no more than 21,000 BTU/min of heat is actually liberated, of which 12%, or 2,520 BTU/min goes to the outside world via the cylinder cooling fins. If rainwater cooling was 100% efficient (no droplets escaping between cooling fins; all of the water 100% evaporated in contact with fins), we might expect rain to reduce the cylinder fins' burden by about 8% (200 divided by 2,520). If you could somehow translate this into a direct CHT reduction, it might mean a reduction of 64°F (assuming your CHT started out at 800° Rankine). That's a pretty sizable reduction of CHT. In fact, it should qualify as shock cooling.

I think the fact that Navajos and 421s aren't raining engine parts down on unsuspecting civilians while flying through precip (I was going to say while penetrating virga--but decided against it) is pretty good evidence that "sudden cooling" of an air-cooled engine does not contribute in any dramatic way to cylinder-head cracking.

If shock cooling were a definite hazard, your engine should fall apart when you bring the mixture into idle cutoff at the end of a flight. CHTs fall at a rate of 100°F/min or more in the first seconds of shutdown--triple the rate that starts the typical "shock cooling" annunciator blinking. Does anyone complain that repeated shutdowns are causing head cracking? Of course not.

Then why are we worried about pulling the throttle back?


--------------------------------------------------------------------------------

Kas Thomas (kthomas@avweb.com) is among the best-known aviation technical writers and a world-recognized expert on piston aircraft engines. Kas is a frequent speaker at Oshkosh and AOPA Expo, and has written hundreds of articles on technical topcis for Light Plane Maintenance (which he founded), The Aviation Consumer, General Aviation News & Flyer, Private Pilot, Plane & Pilot, and many others. He is author of numerous aviation books and is the editor-in-chief of TBO Advisor magazine. Thomas holds ASMEL, instrument and rotorcraft ratings and is the owner of a Cessna 310. Kas lives in Wilton, Connecticut, with his wife Rita and their two children Justin and Mallory.

 

[BigFlyr]

It's really a simple matter of physics... If the third engine is not required to maintain performance then it is obviously "extra" and burning extra fuel, so it can be shut down. However, I don't know of any aircraft that are equiped with extra engines, therefore if one was shut down, the remaining operating engine/s would have to make-up for the loss of power by increasing their fuel consumption. You don't get something for nothing.

 

[Tri-holer]

I always remember going through training which was not as scientific as one of the previous articles. During this training, we were told the 727 will burn 9,000 pounds per hour in cruise. 3K per engine. If you loose an engine, you will burn 9,000 pounds per hour, 4.5K per engine. And if you loose 2 engines, you will burn (drum roll please) 9,000 pounds per hour, all out of that last remaining engine.

I guess the inverse of that is that if you loose that final engine, you will be saving 9,000 pounds per hour - and that is a significant savings!

 

[BigFlyr]

Triholer,

Exactly what I was getting at... also learned when I flew the ol' Jurassic Classic. It's a matter of fuel burn to get the results. If it takes 9000 lbs per hour and you can only burn 8000 lbs per hour, you're going down!

 

[KigAir]

Re: Re: fuel saving trick in a 3 engine plane?

Well in the Falcon 900EX we have the same range on 3 engines as we do on two engines... On two engines it will take you longer to get there and you will be at a lower altitude, thus burning more fuel for a longer time...

An engine loss for us doesn't afffect range...

Okay, stupid question from a low time pilot. How do you achieve the same range burning more fuel while flying slower??

 

[ClassG]

Yes, but what if you didn't just feather the prop and/or shut down the engine? What if you set the third engine speed at something around 'zero-thrust'. Anyone have any background on what that would do for you?

 

[banned username 2]

Re: Re: Re: fuel saving trick in a 3 engine plane?

Okay, stupid question from a low time pilot. How do you achieve the same range burning more fuel while flying slower??

Sorry what I meant to say was...

"Well in the Falcon 900EX we have the same range on 3 engines as we do on two engines... On two engines it will take you longer to get there and you will be at a lower altitude, thus burning more fuel per remaining engine for a longer period of time...

An engine loss for us doesn't afffect range..."

Example....

at cruise we are burning 650 PPH per engine (1,950 PPH total), on 3 engines at FL410... If we lose an engine we drift down to FL 310 and burn 975 PPH per remaining engine (1,950 PPH total)

 

[Snoopy58]

Well in the Falcon 900EX we have the same range on 3 engines as we do on two engines... On two engines it will take you longer to get there and you will be at a lower altitude, thus burning more fuel for a longer time...
An engine loss for us doesn't afffect range...

To which KigAir asked:

Okay, stupid question from a low time pilot. How do you achieve the same range burning more fuel while flying slower??

That's "burning more fuel" IN THE OPERATING ENGINES (than each one would have been in the "all engines operating" regime). Total fuel burn per mile, in the Falcon's case, doesn't much change.

If 9000pph = 9000pph = 9000pph independent of the number of engines burning that 9000#, then the number of operating engines wouldn't much matter. HOWEVER, some engines have an "overhead" such that you get the same thrust out of 1900pph on 3 that you do out of 1500pph on 4 -- so for the same thrust you save 300 pph shutting down one engine. Gas for free? Not exactly; you aren't operating any of the pumps (hydraulics, oil), generators, or compressors (i.e. producing bleed air) on that engine. On a perfectly efficient airplane, all of that same work (producing the needed bleed air, generating the necessary electrical load, etc) would shift exactly to the other engines, but the reality is that you DO gain some efficiencies by shutting down an engine.

That is why, to answer ClassG's question, setting the engine to zero-thrust instead of shutting it down will NOT be a good deal for you -- all of the fuel needed just to "keep the engine running" will still be burned, but the remaining engines will have the fuel consumption that they would have had if you were just running on (N-1) engines.

Different engines (and airframes) work differently; your milage may vary. But in a P-3 or C-130 (similar airframe, same engines) it IS more fuel efficient to shut down an engine or two (higher temperature in the engine = more energy obtained per pound of fuel) (all other things -- altitude, winds, etc being equal).