From manpower to horse power (staying inline or going radi(c)al?)

[Just Schmidt]

Having just in another thread done my part to rehabilitate Greg from Gregs air and auto on youtube, I thought about his claim that jet-engines cost less than piston engines, at least in manpower. hearing that was a surprize to me.

but it got me thinking about the relationship between in line engines and radials, whether the one kind is in general more economic in manpower than the other. Some qualifications are necessary.

For combat aircraft I think that a radial comparable to an in line in capacity should have maybe 20 to 30% more horsepower to be able to compete, given the larger frontal area.

I think turbochargers and coolers in general should be considered part of the aircraft rather than the engine, I'm not sure what the conventions are.

The methods of production will also be an influence, so comparison is perhaps most fair if done between engines from the same country at approximately the same time.

As an example we could take the Ha-40 and Ha-112 II as used in the Ki-61 and Ki-100 respectively. I'm already feeling out of my depths here, but I hope the example illustrates what I am thinking about.

Then i suppose (perhaps wrongly) that the number of cylinders has a lot to say, but maybe it's just as important how they are cooled? Puppet valves versus sleeve valves?

It is manpower expended I'm asking about. Thus the very important metallurgical aspect is not what I'm directly interested in, but maybe it is not meaningful to disregard this aspect? Likewise it is the production side that have my interest, though of course logistics and serviceability is a very important practical factor.

So to all our engine experts, is it all all possible to generalize on this subject? Please take your time to explain to me exactly why it is a stupid question, if you feel this to be the case.

 

[tomo pauk]

Piston engines required cooling system(s) and, if they were good, a good/excellent type of supercharging. They also require propellers. In general they have a lot of moving parts, and lots of things to control (RPM, boost, ignition timing, mixture strength, prop pitch). Compressor and turbine blades will require far less of machining than it will be the case with pistons, crankshafts, cashafts, valves. Jet engines require far less springs.

So I'd say that, as complete powerplants and if we consider mass production, jet engines were cheaper.

 

[pbehn]

There are some natural type limits with these sort of engines. The bore and stroke has an optimum around 6 inches with the bore slightly less than the stroke. You cant make any more than a V12 without the crank twisting. You cant have more than 9 cylinders in each row of a two row radial. Water cooling allows more boost pressure but air cooled engines could use water methanol injection for short periods which is a different way to get water cooling. Frontal area of the engine is only part of the discussion, the radiators on a Griffon Spitfire were huge. Fuel consumption is a factor, the R2800 used approx 2x the fuel that a Merlin needed, so the longest range escort was actually the one that carried the least fuel. In mass production although a water cooled V 12 sounds more simple to make, I think that in practice it was easier to make an 18 cylinder radial. 125,000 R2800 engines means 2,250,000 cylinders, that is the type of volume mass production really likes.

 

[tomo pauk]

As an example we could take the Ha-40 and Ha-112 II as used in the Ki-61 and Ki-100 respectively. I'm already feeling out of my depths here, but I hope the example illustrates what I am thinking about.

We can also take a look on the Ha-140 powered Ki-61 version. On same HP available, it was supposed to be faster than Ki-100 by a good deal. Or the LaGG-3, on considerably less power was about as fast as the La-5. It took an excellent 14 cyl engine or a very good 18 cyl engine to beat a very good V12, speed-vise.

Granted, a lot of things was related to the institutional knowledge in a company, ie. it made a lot of sense for P&W, Wright, Nakajima or Mitsubishi to do the radial engines instead of trying to make a switch to the V12.

 

[Thumpalumpacus]

Piston engines required cooling system(s) and, if they were good, a good/excellent type of supercharging. They also require propellers. In general they have a lot of moving parts, and lots of things to control (RPM, boost, ignition timing, mixture strength, prop pitch). Compressor and turbine blades will require far less of machining than it will be the case with pistons, crankshafts, cashafts, valves. Jet engines require far less springs.

So I'd say that, as complete powerplants and if we consider mass production, jet engines were cheaper.

Does this include the necessary and often expensive alloys required for the critical parts of each respective engine-type? I'm under the impression that the metals used in jet engines were quite a bit more expensive than for ICEs. Is that impression accurate?

 

[tomo pauk]

Does this include the necessary and often expensive alloys required for the critical parts of each respective engine-type? I'm under the impression that the metals used in jet engines were quite a bit more expensive than for ICEs. Is that impression accurate?

Piston engines also used expensive materials, eg. nickel, silver, sometimes (like Germans) replacing nickel with chromium for valve plating and turbine blades. Hollow, air-cooled turbine blades circumvented somewhat the need for expensive materials.

 

[SaparotRob]

Did they have an inkling about hollow turbine blades or did that come out of trial and error?

 

[tomo pauk]

Did they have an inkling about hollow turbine blades or did that come out of trial and error?

Engineers can came out with sleek solutions to the problems sometime
We can note that Wright also made hollow turbine blades for their turbocharger, 1000 or so of these were installed, of all things, on the Curtiss SC-1 floatplane.

 

[SaparotRob]

The things one learns here.

 

[Thumpalumpacus]

Piston engines also used expensive materials, eg. nickel, silver, sometimes (like Germans) replacing nickel with chromium for valve plating and turbine blades. Hollow, air-cooled turbine blades circumvented somewhat the need for expensive materials.

This is why I dig this place: direct question, followed by plain answer. Thank you much for a quick and succinct answer.

Is there any website you know of that lays out unit-cost for the various engine types?

 

[tomo pauk]

Is there any website you know of that lays out unit-cost for the various engine types?

For the US types, I'd take a look at the USAF statistical digest docs: link.

 

[Thumpalumpacus]

For the US types, I'd take a look at the USAF statistical digest docs: link.

Bookmarked. Thank you again.

 

[tomo pauk]

Does this include the necessary and often expensive alloys required for the critical parts of each respective engine-type? I'm under the impression that the metals used in jet engines were quite a bit more expensive than for ICEs. Is that impression accurate?

FWIW, on another forum, a member stated the following (here):

According to figures for Ni usage per engine in Kay's "German Jet Engine and Gas Turbine Development 1930-1945" (2002), the entire production run of Jumo 004 engines, for example (some 6010 engines) used approximately 40 metric tons of nickel. This is not an insignificant amount, but compared to 1944 Ni supplies (10900 tons), consumption (9500 tons), or stocks (7900 tons) (U.S. Strategic Bombing Survey), the needs amount to little more than rounding error.

(nickel being the most critical material for alloys able to withstand high temperatures)

 

[Howard Gibson]

Having just in another thread done my part to rehabilitate Greg from Gregs air and auto on youtube, I thought about his claim that jet-engines cost less than piston engines, at least in manpower. hearing that was a surprize to me.

It does not surprise me. If turbojets and turboprops were cheaper, why are they not used in general aviation?

Turbines, especially ones that run at high temperature, need to be precisely manufactured from heat resistant materials. The blades/nozzles all are separate parts, right? There may not be fewer parts for manufacturing.

Turbojets and turboprops are cheaper to run, since there is only one primary moving part. That matters to an airline. They can go faster. I suspect that they are less fuel efficient than spark ignition engines. Bill Gunston's book on Development of Piston Aero Engines shows a heat diagram of what almost certainly is a Rolls Royce Merlin of some sort. It has around 30% thermal efficiency. Does anybody have efficiency data for a modern jet engine?

 

[Howard Gibson]

but it got me thinking about the relationship between in line engines and radials, whether the one kind is in general more economic in manpower than the other. Some qualifications are necessary.

I did a plant visit when I was in college back in the nineteen seventies. I believe it was General Motors' engine plant in St. Catharines, Ontario. They were building 350ci V8s. They were casting and machining the engine blocks in-house.

They had a machine that picked something like four engine blocks. It flipped one side horizontal, and it ran a huge broach over top, machining one cylinder bank for the heads. Then, it flipped ninety degrees and it did the other side. Four engine blocks machined for heads in a matter of seconds. Another machine bored out either four or eight cylinders at a time, I don't completely recall. There was a lot of tooling that could do nothing other than manufacture parts for a GM 350ci V8, but heck, they were manufacturing 350ci V8s!

I am reading up on Vicker's geodetic construction. Its looks complicated, and officials complained about the cost of it. Vickers built over 11,000 Wellingtons. They built one of them in one day. They must have designed tooling to simplify and automate fabrication of geodetic parts, and that may have made the design economical.

My contribution to the "inline" versus radial debate is to point out that...
1. There no inline engines in WWII fighters. Most of the liquid cooled engines were V12s.
2. The best aircraft engines of WWII were the Rolls Royce Merlin and the Pratt and Whitney R2800 Double Wasp. The quality of design and manufacturing probably is more important than the configuration.

 

[nuuumannn]

I suspect that they are less fuel efficient than spark ignition engines.

Modern gas turbines are among the most fuel efficient engines in use. Taking a beast like the RR Trent, only ten percent of the total thrust produced, at maximum equating to over 100,000lbs, comes from the engine hot section. The other ninety percent is from that big fan at the front. Modern gas turbines require specialist alloys because they require heat to be efficient, and lots of it. The trade-off being that the heat being generated within the combustion chamber and in the turbine section is hotter than the metal these things are made of, this is mitigated by advanced cooling methods. That heat translates directly to energy to turn the turbines, so heat equals power. Gas turbines require constant maintenance, boroscope inspections, igniter changes, component inspections, oil and fuel filter replacement, patch samples for metal contamination etc. These engines operate at very high altitudes though, where they are most efficient.

The primary reason why gas turbines aren't used as much in GA is because there is no need. For the performance of modern piston engined aeroplanes, the complexity of gas turbines is just not necessary for given performance increases. Overhauling and maintaining modern piston engines is a lot more cost effective for the types of operations GA aircraft undertake, especially when Lycoming offer an entirely new engine for lower than the cost of overhaul. While turboprops offer excellent performance and power-to-weight ratio, they are thirsty compared to piston engines at low altitudes and they require as much maintenance as a piston, but the cost of Jet A1 is low. These days, there are lots of single-engined turbines out there earning their keep, Cessna Caravans, Pilatus PC-12s etc powered by the great and reliable PT6 engine, but there's no need to put a gas turbine in the aero club's old Cessna 172 to give it pep, just a waste of money because it's never going to operate within the parameters that the gas turbine is at its most efficient.

 

[Howard Gibson]

Modern gas turbines are among the most fuel efficient engines in use.

Do you have numbers?

 

[nuuumannn]

Do you have numbers?

I could probably look for them, but I'm rather busy at the moment. I used to work on gas turbines and during one of my engineering courses we carried out an exercise with the tutor comparing load factor with distance travelled and fuel consumed between a modern triple spool hi-bypass gas turbine and piston engines in cars and light aircraft. Not exactly an even comparison, but the point was and is obvious; comparing the size and weight of load carried by the big airliners and airlifters and their ranges with their fuel loads means that their efficiencies are obvious.

 

[wuzak]

I could probably look for them, but I'm rather busy at the moment. I used to work on gas turbines and during one of my engineering courses we carried out an exercise with the tutor comparing load factor with distance travelled and fuel consumed between a modern triple spool hi-bypass gas turbine and piston engines in cars and light aircraft. Not exactly an even comparison, but the point was and is obvious; comparing the size and weight of load carried by the big airliners and airlifters and their ranges with their fuel loads means that their efficiencies are obvious.

I believe small gas turbines are similar to regular piston engines in efficiency.

The big gas turbines can be in the low-to-mid 40% efficiency range, IIRC.

 

[nuuumannn]

I believe small gas turbines are similar to regular piston engines in efficiency.

The big gas turbines can be in the low-to-mid 40% efficiency range, IIRC.

The top one's debateable and it depends on what you are trying to do and how high you are trying to do it at. The bottom one, what you're not saying is that that "40 percent efficiency" compares favourably to piston engines.

Let's look at a real world example using a small gas turbine; the PT-6 weighs around 300lbs give or take, yet the most powerful variants can produce power outputs equivalent to 1,900hp, compare that with your average 1,900 hp piston engine. The PW Twin Wasp for example weighs more than three times the PT-6, so using that specific example, make a comparison between the Bazler Turbo 67 and a regular DC-3. The PT-6 powered version is faster in the climb, cruise and maximum speeds, it can carry a heavier load across a greater distance at a higher height than the regular DC-3. Now some of that efficiency comes from modern props etc, but it plainly illustrates that small gas turbines are very efficient, more so than their equivalent in similar power output piston engines. Again though, its about application. Gas turbines are far more efficient at altitude.

PT6A - Pratt & Whitney

www.pwc.ca

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