If recips were made nowadays

PWR4360-59B said:

And then the costs involved to purchase and then the low TBO etc etc.

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2000 - 4000 hours TBO, I believe.

That, surely, is quite good for a general aviation engine?

Its the cost involved to do the overhaul. Minds well just buy a new plane.

PWR4360-59B said:

And then the costs involved to purchase and then the low TBO etc etc.

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TBO of 2000 hours, same as an equivalent piston engine.
Maintenance costs are less per flight hour.
Reliability is higher.
Weight is lower, so more available payload - more income.

The RR 500 was 'suspended' in 2012 unless it's making a comeback.
The roughly 500hp mark is the dividing line between reciprocating engines and turbines for the last 50-60 years.
The dividing line is bit blurry because light helicopters show a much more improvement with a turbine engine than fixed wing aircraft do.
When you are doing a vertical take-off every pound counts, there is no using another few hundred feet of runway. And nobody wanted to ride in helicopter powered by a reciprocating engine for hours anyway, (and piston powered helicopters were fuel hogs for weight/distance compared to fixed wing aircraft).
Reciprocating engines used in helicopters have shorter service lives than the same basic engine used in a fixed wing aircraft as a general rule of thumb.

Small turbines got a tremendous boost from the light helicopter and the light helicopter would not be what it is without the turbine engine.
The old Allison 250 (granddaddy to the RR500) was very quickly made to give up to 420hp but in some helicopters they were limited to 317hp because of the transmission. This lead to "flat-rating" the engine. A torque sensor was built in that limited the power to 317hp but that meant that you had 317 hp form sea level to whatever altitude on a given day/or location the air would support 317hp. With 103hp "in the bank" you could use that 317hp at thousands of feet above sea level. The early Allison 250 weighed 136lbs. A V0-540 Lycoming of 305hp for take-off weighed around 440lbs.
Allison pretty much sewed up the market for small turbines in the US and much of the world for decades. Great for Alison, not so great for making any large advances.

The small fixed wing planes with piston engines was a much harder market to crack. But the 500-1000/1200hp catagory was like shooting fish in a barrel. Continental and Lycoming had no reciprocating engines in that catagory and the engines from P & W and Wright dated back to the early 1930s.
Continental and Lycoming tried to move up into the 300-400hp catagory and snipe the old 350-450hp radials but success was mixed.
Things like Beech Queen Airs with Lycoming IGSO-480 producing 340 hp (250 kW) with a 1400-hour TBO (and using 48 inches of manifold pressure to do it) meant that operating costs were not as low as expected. The reduction gear and supercharger caused a massive increase in overhaul costs compared to a lower powered I0-480 and unless flown very carefully, many of these engines did not come anywhere close to the 1400hr life.
The P & W Canada PT-6 turbo-prop weighed 1/2 of the 6 cylinder reciprocating engines did and offered around 550hp from the start and rapidly increased.
P & W Canada had more competition than Allison (and the somewhat bigger engine responded better to some improvements, air doesn't scale all that well).

Really big turbo props became a niche market once the turbo-fan showed up in production form.

A lot of engine development was driven by market forces. As the price of fuel fluctuates the interest in "new" engines ebbs and flows and with the millions needed to certify and bring an engine to production (hundreds of millions for a big engine, if not in the billions) large, expensive reciprocating engines are going to stay in the land of fantasy/dreams.

Shortround6 said:

Allison pretty much sewed up the market for small turbines in the US and much of the world for decades. Great for Allison, not so great for making any large advances.

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There were only a small number of competitors to the Allison such as the Turbomeca Arriel. I have not heard much on it recently but I presume it is still around.

As for development the Allison 250 had a turboprop version, upright versions, inverted versions, versions mounted at an angle, versions with the PTO at the front and with the PTO at the rear so the design is certainly never been locked in the one basic configuration like most piston engines. The basic 250 engine originally had an 8 stage compressor (7 axial and 1 centrifugal) but ever since the C-28 they have had more efficient compression from a single stage centrifugal compressor and current engine have a pressure ratio of over 9:1 - far more than any piston supercharger. That is around 120 lbs boost in piston measurements.

The 250C-47 version is now at 700s hp, a far cry from the 250 shp of the early engines like the T63-A5. I would consider that quite a large advance, not as much as the PT-6 (450 to 1,940 shp) admittedly, but certainly not insignificant. I cannot think of any aircraft piston engine that almost tripled its power like the Allison, let alone more than quadrupled it like the PT-6

MiTasol said:

The 250C-47 version is now at 700s hp, a far cry from the 250 shp of the early engines like the T63-A5. I would consider that quite a large advance, not as much as the PT-6 (450 to 1,940 shp) admittedly, but certainly not insignificant. I cannot think of any aircraft piston engine that almost tripled its power like the Allison, let alone more than quadrupled it like the PT-6

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Wright Cyclone. Type tested in 1927 at 500hp (as the R-1750) and finished at 1525hp. Of course you could no more interchange any parts between the two extremes any more than you can interchange parts between a 450/500hp PT-6 and 1900hp PT-6
The PT-6s did use the same fuel though.

Getting back to this on piston engines.

PWR4360-59B said:

Yawn !!! Scaling up you keep the basic cylinder dimensions the same, you just add more, for your motorcycle engine experiment. That is how multicylinder engines came about,

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Ok, take 10 of the motorcycle engines arrange them in 5 banks of two 4 cylinder engine each (or other arrangement?) convert to dual plugs (80 spark plugs per engine, a real maintenance treat) and you have a 1450 reciprocating engine, Turbo charge to maintain power at altitude.
Does anyone really believe such an engine offers any real improvement over even a conventional reciprocating engine let alone a shaft turbine?

I would also take the Duke engine with a grain of salt.

It may very well have some major advantages over some conventional reciprocating engines but when they keep touting the elimination of friction in the valve train one wonders what else they are hyping.
Conventional car piston engines (in a book on tuning for racing) have about 80% of their friction total in the pistons and piston rings rubbing the cylinder walls.
This leaves about 20% to be divided between the main and rod bearings and the valve train.
Allison went from a bit over 100hp being consumed by friction in the early engines to about 200hp being consumed by friction in the late engines, yes in part by the use of stronger valve springs. However they also changed the piston rings to a new design (and higher tension?) to prevent blowby at the high combustion pressures generated by the higher boosts being used.

Now please note the Duke engine uses a rotating cylinder block and a fixed cylinder head with ports. So you have the entire engine block rotating in/on some sort of bearings and the top of the cylinder head sliding by the seals used to isolate the the different areas of operation (intake. combustion and exhaust) which a conventional engine does not have. Now maybe the friction of the cylinder block sliding past these seals is lower than "conventional" valve train (pushrod, single cam, dual cam? how many valves per cylinder?) but is it enough of a difference to make any meaningful change in the fuel consumption?
They are still using piston rings on the pistons.
I also wonder what it costs to build this thing. They talk about the counter rotating engine block helping to reduce vibration but that means the block assembly has to be balanced. Not just the pistons and rods and what not. Nobody cares if a standard reciprocating engine block is a little heavier on one side than the other compared to the next engine block on the production line.

Comparing aircraft piston engine TBOH to gas turbines doesn't tend to end well for the engines with pistons.

One example is the AGT-1500, compared to the diesel in the M60. The mean tine between removals, for major repairs, of the diesel was 25 hours (no typo), while, once the air filter was sorted out, is was over 1,000 for the turbine. Tanks operated fewer than 1,000 hours per year.

Over hauls on turbines far exceed recips. Turbines are far more susceptible to FOD, dirt, sand, small rocks etc. The mentioned initial cost of $263,000 to $323,000 makes a recip look real good even after a few overhuals.
Rolls-Royce M250 Turboshaft Engine | PowerWeb

Cool site, a nice PW jet engine a cool 30 some million, and then you gota pay for the fuel too boot.

PWR4360-59B said:

Over hauls on turbines far exceed recips. Turbines are far more susceptible to FOD, dirt, sand, small rocks etc. The mentioned initial cost of $263,000 to $323,000 makes a recip look real good even after a few overhuals.
Rolls-Royce M250 Turboshaft Engine | PowerWeb

Cool site, a nice PW jet engine a cool 30 some million, and then you gota pay for the fuel too boot.

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Yes, that $30 million engine probably has ten times the TBOH (except modern aircraft engines are maintained on-condition, not overhauled) and produces power equivalent to a few dozen R-3350s.

swampyankee said:

Yes, that $30 million engine probably has ten times the TBOH (except modern aircraft engines are maintained on-condition, not overhauled) and produces power equivalent to a few dozen R-3350s.

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I bet you would be surprised, I don't think military engines have a very high TBO, and especially the afterburner parts. Yeah and suck 1000 gallons in less than 5 minutes at full rating.

Yep, all those after burner parts. Fuel spray bars, flame holders and the adjustable nozzle.

Much, much harder to make and install than hundreds of pistons, cylinders, valves and other parts in a highly stressed piston engine. (sarcasm)

Mig 23 after burner


US Navy 030201-N-9605S-028 Aviation Machinist's Mate 2nd Class Lloyd Keeling, from Exeter, Calif., inspects the flame holder of the afterburner to an F-A-18E

Basing the utility of engine on it's fuel burn per hour (or minute) without considering the amount of work it is doing tends to paint a false picture.

You want to see a really crappy reciprocating aircraft?

Stats for the early versions with P&W R-1340 engines look good, crew 2, up to ten passengers, max range 440 miles (with reserves no less).
However the truth was not quite so good. Max disposable load was 2050lbs (difference between empty weight and max gross) fuel capacity was 190 gallons (1140lbs). leaving 860 lbs for oil and crew + passengers. Or put it another way. with two crew and only 8 passengers at 170lbs each you had 350lbs left for fuel and oil. later Versions got 700hp engines and a higher gross weight.
A lot of good work was done by the Sikorsky S-55 But the numbers given for it in many sources far exceed it's actual capabilities. A handful of them were later given gas turbines and that increase the disposable load by 900lbs. You could actually fill the seats and have enough fuel to fly for several hours instead of shuttling from one side of the airport to other and refueling between "hops".

Just for laughs I looked up the pilots manual for the P-80A (available in the tech section) which is about as bad as it gets for turbine engine with some books claiming 1.23lb/st/hr.

at 30,000ft and cruising at 285mph indicated it got between 1.5 to 2 mpg (while carrying drop tanks) which is all you were going to get out of the P-47 trying to fly high and fast. The P-80 was "cruising" at around 450-475mph. The plane burned about 215 gallons an hour but 470 gallons was good for over 900 miles.
Low altitude could be 1/2 that.
The early jets were very dependent on altitude.

The cost of a little fuel nozzle for an after burner 20 years ago was 20K, I think it took 6

PWR4360-59B said:

I bet you would be surprised, I don't think military engines have a very high TBO, and especially the afterburner parts. Yeah and suck 1000 gallons in less than 5 minutes at full rating.

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Military aircraft are not the only places gas turbines are used, so including commercial engines in comparisons isn't inappropriate.

Gas turbines don't scale down well to light aircraft size, so I don't expect to see Lycoming or Continental's range wiped out by turbines. On the other hand, even the the first generation of postwar turbojets to push pistons aside from military applications: fuel efficiency was, as is, not that important to combat aircraft users.

No jets, military aircraft will be slower, smaller, and less serviceable. Fighters would just about all have a single engine — a less reliable single engine — and be much less capable than even low-end current fighters. International air travel would be much less, not only because the aircraft would be slower but because piston engine aircraft took much longer to turn around.

The military is using lots of recips and I think even talk of Wankel like engines for drones, especially if they want something that has a very long extended flight time. They are looking for economy with some of them, and turbines do not offer economy of fuel use.
Quieter military drones that fly farther? The Pentagon thinks this engine could be the answer


Revolutionary Coaxial Drone with Diesel Piston Engine - Optionally Piloted Helicopter - UAS VISION
A diesel piston engine consumes about 60% less fuel than a turbine. Here ya go. That alone makes it worth trying.

Just flush your toilet and you will see the fuel consumption of a turbine, Oh and just check out the youtube videos of agent jz flowing the jet fuel nozzle to see the fuel consumption.

PWR4360-59B said:

The military is using lots of recips and I think even talk of Wankel like engines for drones, especially if they want something that has a very long extended flight time. They are looking for economy with some of them, and turbines do not offer economy of fuel use.

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Yes, because they're itsy, bitsy engines. I suspect the break point is about 500 shp.

If you want itsy bitsy, then I guess a 787 will have to have a few hundred on each wing.

PWR4360-59B said:

If you want itsy bitsy, then I guess a 787 will have to have a few hundred on each wing.

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Just so; an airplane with the capabilities of a 787, or even 757, isn't possible with piston engines.

Piston engines don't scale up well.

PWR4360-59B said:

The military is using lots of recips and I think even talk of Wankel like engines for drones, especially if they want something that has a very long extended flight time. They are looking for economy with some of them, and turbines do not offer economy of fuel use.
Quieter military drones that fly farther? The Pentagon thinks this engine could be the answer


Revolutionary Coaxial Drone with Diesel Piston Engine - Optionally Piloted Helicopter - UAS VISION
A diesel piston engine consumes about 60% less fuel than a turbine. Here ya go. That alone makes it worth trying.

Just flush your toilet and you will see the fuel consumption of a turbine, Oh and just check out the youtube videos of agent jz flowing the jet fuel nozzle to see the fuel consumption.

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Gee, according to the first link they are using a Wankel. I like the thinking though, exhaust noise too loud? don't put a muffler on it, put in a whole new engine.
and guess what, if you have similar pressure/velocity and volume of gas at the exhaust port/pipe end you will have similar noise levels.

I will grant that small turbines had pretty crappy fuel economy.

I also like the wishful thinking and "gee whiz, aren't we terrific" in the second link.
Co-axial drone helicopter??

Mid 60s. and from Wiki.
" The DSN-1 was powered by a Porsche YO-95-6 72 hp piston engine and carried one Mark 43 homing torpedo. The next developmental version was the DSN-2/QH-50B that was powered by two Porsche YO-95-6 engines and also carried a single Mk 43. Serial production of the DASH began with the third version, the DSN-3/QH-50C, in which a 255 hp (190 kW) Boeing T50-4 turboshaft engine replaced the piston engine and the payload was increased to two Mark 44 torpedoes."

BTW a mk 43 torpedo weighs 265lbs and a MK 44 weighs 432lbs. Later versions of the Dash saw Boeing turboshafts of over 300hp.

and if you really believe you can get a 300hp diesel to give you that kind of power on 32 liters an hour than I am an Ethiopian Prince who needs to get 10 million dollars out of the country in 3 days, just email me your bank account numbers (and credit card numbers) and I will transfer the money and let you keep 1 million dollars for your trouble

32 liters an hour even in an engine that gets 0.25lbs/hp/hr is only good for 230hp. At 0.33lb/hp/hr you have 172.8hp. Now maybe that is all the drone needs to stay in the air without much of a payload, but somebody is shading the truth when they claim 330hp max and an average fuel consumption of 32 liters an hour.

The engine may very well max out out 330hp and it might very well run at a much reduced power rating on 32liters/hour. But that is not the impression they are giving.



You keep harping on the fuel consumption and ignoring the power produced.

Sure jets used a lot of fuel. Now lets rig up a barrel and hose and use it to show the flow of an 8000hp piston engine (or the fuel used by four 2000hp engines, running rich mixture) four R-2800s running at 2000hp us just over 19 gallons a minute. You could fill a 5 gallon jerry can in just under 15 seconds, noce demsotration. Proves nothing.
single engine jet fighters had engines with the equivalent of 8000hp in 1949.

How may 2000hp piston engines are you going to need to equal the power of the engine in F-35?

and the actual fact is that some of the latest turbo props equal the fuel consumption of all but the very best reciprocating engines and guess what, they used power recovery turbines that added weight, cost and maintenance problems.

so far the break point between turbines and reciprocating engines seems to be somewhere in the 300-400hp range.