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Author Topic: Gas Turbine Development Question  (Read 2993 times)

Offline CaseyKnight

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Gas Turbine Development Question
« on: October 30, 2016, 01:46:52 pm »
I'm asking this on behalf of somebody else

1. Turbine Efficiency

In 1923, Edgar Buckingham published a report regarding the efficiency of gas-turbine engines.  According to his calculations, a gas-turbine would become more efficient with speed, but would be 1/4 that of the piston/propeller combination and unsuitable for operations. 

I'm curious how he came up with these figures: I have never really found anything to explain how he ran his experiment.


2. Turbine Weight

Throughout the 1930's and 1940's, there was a pervasive attitude among those at NACA and the aircraft power-plant industry that gas-turbines would suffer from excessive weight.  Why did they believe this?

Offline CJGibson

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Re: Gas Turbine Development Question
« Reply #1 on: October 30, 2016, 02:37:44 pm »
Q2 - in the Museum of Transport in Glasgow there is a steam turbine. It is massive and heavy. If that was your experience of turbines I'm not surprised they reckoned they'd be heavy.

Chris
« Last Edit: October 30, 2016, 04:25:48 pm by CJGibson »

Offline red admiral

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Re: Gas Turbine Development Question
« Reply #2 on: November 02, 2016, 12:49:28 pm »
Propulsive efficiency is related to difference in jet velocity and aircraft velocity. If you're doing 100kts as in the early 1920s a turbojet is a really pants propulsion method. Lots of the work in the 1920s and 30s on gas turbines looked at reducing the jet velocity i.e. turbofans and turboprops. This is what AA Griffith was working on for example to maximise efficiency from a gas turbine. Turbofans and turboprops are heavier and more complicated than single spool turbojets and didn't really work with the technology available in the 20s and 30s.

Whittle was initially interested in turbojets and shaw that they would enable faster higher aircraft; but in the late 20s these faster (400mph+) higher aircraft weren't technically achieveable. For the slower aircraft of the period a turbofan or prop was a better choice. By the time Whittle had his mechanically "simple" turbojet working in the late 30s aircraft design e.g. aerodynamics had almost improved to the point that a turbojet was a good choice for propulsion.

Summary is that technology moved on, both in gas turbines and aircraft design.

Offline charleybarley

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Re: Gas Turbine Development Question
« Reply #3 on: November 08, 2016, 04:03:44 pm »
"how he ran his experiment"  I think it was all based on calculations.
"I'm curious how he came up with these figures"
 Here is his report
http://naca.central.cranfield.ac.uk/reports/1924/naca-report-159.pdf

Offline red admiral

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Re: Gas Turbine Development Question
« Reply #4 on: November 09, 2016, 01:53:40 pm »
"how he ran his experiment"  I think it was all based on calculations.
"I'm curious how he came up with these figures"
 Here is his report
http://naca.central.cranfield.ac.uk/reports/1924/naca-report-159.pdf

Summary is that pure turbojets aren't very efficient below 350mph, which is true today. Jet velocity is much higher than freestream velocity and so propulsive efficiency is poor.

This sums it up; piston/prop efficiency is generally arouhd 0.8 up to c.400mph

https://www.google.co.uk/search?q=propulsive+efficiency&prmd=ivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjHz_jl05zQAhVsBsAKHdSQDQAQ_AUIBygB&biw=800&bih=1280#imgrc=DRsj7kOH7JGA4M%3A 

Offline charleybarley

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Re: Gas Turbine Development Question
« Reply #5 on: November 10, 2016, 11:50:40 am »

1. Turbine Efficiency

In 1923, Edgar Buckingham published a report regarding the efficiency of gas-turbine engines.  According to his calculations, a gas-turbine would become more efficient with speed, but would be 1/4 that of the piston/propeller combination and unsuitable for operations. 

If you have looked at Buckingham's report then read no further. If not, I have noted a couple of things.

He wasn't referring to a gas turbine. His arrangement, what he called an obvious alternative to an airscrew, replaced the propeller with a piston compressor, combustion chamber and propelling nozzle.
From a propulsive efficiency point of view only, both speeds which determine it were very different from subsequent gas turbine applications.
His aircraft speed was 250 mph. Whittle's enthusiasm for jet propulsion was for speeds of 500 mph.
His exhaust velocity was about a mile per second. Turbojet exhausts were less than a third of that.
So from that point of view alone the first jet aircraft were way ahead of his arrangement.

He does mention an advantage over the airscrew of being able to swing the nozzle (todays thrust vectoring).

Offline CaseyKnight

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Re: Gas Turbine Development Question
« Reply #6 on: February 20, 2017, 01:59:47 pm »
I'm posting these replies on behalf of somebody else

red admiral

Quote
Propulsive efficiency is related to difference in jet velocity and aircraft velocity. If you're doing 100kts as in the early 1920s a turbojet is a really pants propulsion method. Lots of the work in the 1920s and 30s on gas turbines looked at reducing the jet velocity i.e. turbofans and turboprops. This is what AA Griffith was working on for example to maximise efficiency from a gas turbine. Turbofans and turboprops are heavier and more complicated than single spool turbojets and didn't really work with the technology available in the 20s and 30s.

Whittle was initially interested in turbojets and shaw that they would enable faster higher aircraft; but in the late 20s these faster (400mph+) higher aircraft weren't technically achieveable. For the slower aircraft of the period a turbofan or prop was a better choice. By the time Whittle had his mechanically "simple" turbojet working in the late 30s aircraft design e.g. aerodynamics had almost improved to the point that a turbojet was a good choice for propulsion.
Whittle anticipated a high velocity from his design, do you have any idea what he was expected?


charleybarley

Quote
If you have looked at Buckingham's report then read no further. If not, I have noted a couple of things.

He wasn't referring to a gas turbine. His arrangement, what he called an obvious alternative to an airscrew, replaced the propeller with a piston compressor, combustion chamber and propelling nozzle.
Why was this report used almost as a cudgel against those supporting gas-turbine if it wasn't even about them?

Quote
From a propulsive efficiency point of view only, both speeds which determine it were very different from subsequent gas turbine applications. His aircraft speed was 250 mph. Whittle's enthusiasm for jet propulsion was for speeds of 500 mph.
By February of 1923, speeds of 232.91 mph were already achieved, by November, 250 mph would be exceeded.  I'm not sure how much Buckingham studied aircraft design and speed records, but that does raise a question.  The fact that Whittle seemed to start his research a few years later explains his interest in higher speeds.  By the end of March 1928, a speed of 300 mph would be exceeded (318.62).

Quote
His exhaust velocity was about a mile per second. Turbojet exhausts were less than a third of that.
During the conceptualization & development of gas-turbines in the UK and US, what did we expect in velocities?

Offline charleybarley

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Re: Gas Turbine Development Question
« Reply #7 on: March 04, 2017, 01:30:07 pm »
Quote
Why was this report used almost as a cudgel against those supporting gas-turbine if it wasn't even about them?
On page 85 B says the arrangement was no good. ie no prospect whatever that will ever be of any practical value. No one who had read the report would champion the idea.

Quote
During the conceptualization & development of gas-turbines in the UK and US, what did we expect in velocities?
In his James Clayton Lecture (available on line p.424) Whittle says 1,720 ft/sec for the design of his first model. This is typical for a turbojet., eg 1953 Olympus engine 1740 ft/sec.

Offline charleybarley

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Re: Gas Turbine Development Question
« Reply #8 on: March 06, 2017, 02:39:06 pm »
20 years after (1943) Buckingham's report  another NACA report No.802, "NACA Investigation of a Jet-Propulsion System Applicable to Flight" by Ellis and Brown, re-evaluates B's report for aircraft speeds of 500mph. It still uses a piston engine to compress the air but uses a blower/fan instead of a piston compressor. In addition it only burns up to 1/2 the air (instead of B's all) so the jet velocities are lower.

The report states that a gas turbine "would hamper jet propulsion development as they are still experimental." This was written one year after the first flight of a US jet, the Bell Airacomet, powered by a gas turbine jet engine and  3 months after a production order had been placed for these gas turbine powered aircraft.

The chairman of NACA's committee on gas turbines and jet propulsion was present for the first flight.
« Last Edit: March 06, 2017, 02:57:23 pm by charleybarley »

Offline LowObservable

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Re: Gas Turbine Development Question
« Reply #9 on: March 11, 2017, 09:04:52 am »
There really hasn't been any successful straight-wing jet that has to haul a load over a distance for a living. The closest thing is the Cessna Nearjet - but efficiency, as opposed to mission performance, has never been that important to executive aircraft.

Offline CaseyKnight

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Re: Gas Turbine Development Question
« Reply #10 on: March 20, 2017, 11:24:57 am »
I'm replying on behalf of someone else

charleybarley

Quote
On page 85 B says the arrangement was no good. ie no prospect whatever that will ever be of any practical value. No one who had read the report would champion the idea.
There were numerous errors, which he made.  Of the following, I can find the following
  • Fuel/Air Ratios: Even in piston engines, the fuel/air ratio does not leave only nitrogen and carbon-dioxide behind; there's actually a significant amount of unused oxygen left over.  I'm not sure how common this knowledge was but the fuel/air ratios were known.
  • Aspirator/Ejector Devices: He explicitly left them out of the hypothetical concept (I'm not sure what an aspirator is though I'd assume forced induction?), and that's a mistake because for the same compression-ratio and fuel-to-air ratio you could get the same thrust with an improved nozzle-system.
  • Compression-Ratios: He assumed that a compression ratios above 15:1 would be of little value and below 7:1 would be useless.  While low-pressure ratios are undesirable, the fact is that 7:1 would have done quite well on an aircraft capable of achieving speeds in excess of 350-550 mph.  Though he didn't contemplate these speeds, he did understand that ram-compression increases thrust.  By the late 1920's, speeds in excess of 350 mph would be achieved, around 400-440 by 1934, and by 1936, NACA was looking for an aircraft of 550 mph.
  • Exhaust-Velocity/Temperature: He projected exhaust velocities of 5280 fps @ 2,500 F.  While he was expecting an aircraft only capable of 250 mph, the fact is this kind of exhaust velocity was frankly excessive for all but supersonic aircraft.  He appeared to realize that exhaust velocity would be best if it was close to aircraft velocity.  Lowering exhaust velocity would increase efficiency for one thing.
The only thing that seems to indicate a turbine would be the comment about a ballistic pendulum.  I'm not exactly sure what was known when in the United States, but what I do know is this
  • Steam-turbines often ran with their turbines in a stalled condition: This had to do with the pitch of the turbine blades and the geometry of the blades, which meant they'd extract the energy to spin a turbine, but leave no surplus left over to continue to push the plane.  If I recall, Alan Griffith realized this back in the late 1920's (I'm not sure if this was known in the United States).
  • Turbo-supercharger proponents did believe that it would be possible to extract useful thrust at altitude as back-pressure would be lowered and compression would remain high in the engine.
  • Proponents of gas-turbines were concerned about the excessive weight of the engine: Indicating that they did seem to understand that thrust could be produced, just that it would be too heavy (it would appear that there were a multitude of reasons excessive weight were expected, of which I may not know all of them, but one might have been that steam-turbines were big and heavy).