Piston exhaust thrust.

[Groggy]

These figures are from RR and show clearly the importance of exhaust thrust for performance at high altitude, high speed for a Spitfire with a Griffon 61(37 litres) (not sure which mark of Spitfire?).
This follows on from the work that Farnborough did circa 1940 when it was found that half the effective thrust for a Hurricane at 30,000ft was from the ejector exhaust.
There is some remarkable data on a two-stroke RR Crecy (26 litre) producing estimates of about 550mph.if fitted in a Spitfire.
There are estimates by Farnborough for exhaust thrust + prop for a projected development of the two-stroke engine that would have given the Mustang an estimated 600mph. That is if an airscrew could have been made to function at those speeds.
A Spitfire had a mock up of the RR Crecy engine fitted at one stage. But fitting it to a Spitfire or Mosquito was then thought to be problematic


Super-chargergear Altitude(ft)Withoutexhaustthrust-Withexhaustthrust

MS 5,000 345 351
13,500FTH 388 400
23,500 373 399
FS
31,500FTH 419 454
40,000 377 424

[Nick Sumner]

That is if an airscrew could have been made to function at those speeds.

How could an airscrew be made to function at those speeds? I have a mental vision of a contra prop with 6 or 7 short blades on each hub!

[Red Admiral]

Propfan.

http://upload.wikimedia.org/wikipedia/commons/thumb/6/61/Antonow_An_70_prop_detail_rvb_jno_MACS_2001_0037.0 3.jpg/237px-Antonow_An_70_prop_detail_rvb_jno_MACS_2001_0037.0 3.jpg

[Groggy]

[quote]Originally posted by Nick Sumner

That is if an airscrew could have been made to function at those speeds.

How could an airscrew be made to function at those speeds? I have a mental vision of a contra prop with 6 or 7 short blades on each hub!

[
/quote]
Hi,
Nick that’s something to think of, but add a few more blades and you have a UDF, this should work with a piston engine almost as well as with a turbine. A UDF was designed by 1944? Farnborough had tested it by 1945?
A question to ponder, would replacing the airscrew with a UDF at the front have improved the top speed for the last marks of MB 5, Spitfire, Mustang and the P47? Say at least 510mph + for the last three?
Or would a contra-rotating hybrid UDF with a truncated airscrew be needed to take off with?
Admiral like the picture - the idea of designing a prop-fan with a slide rule is something! Interesting challenge.

[Red Admiral]

What about a compressor like the Campini "thermojet"?

Add a few more stages to the compressor instead of just 3. Then use the exhaust from your Crecy mixed with the afterburners - if you want to run them all the time.

[Nick Sumner]

Hi Groggy,

Forgive my ignorance, but what is a UDF?

[Wuzak]

UnDucted Fan

[Groggy]

quote:Originally posted by Nick Sumner

Hi Groggy,

Forgive my ignorance, but what is a UDF?


Hi Nick,
Imagine a large compressor with two rows of exposed contra-rotating blades. The blades a little longer than usual maybe 7 or 8 inches long and turning at much higher speed than a normal airscrew. About 45 to 50 inch diameter for a turbine driven unit, a bit larger diameter for a piston powered unit. The unit should convert a greater amount of power in to thrust at high speeds than a conventional airscrew. The boys at Reno should think about it!
The noise level generated would be interesting, anyone know what modern UDFs were like?

[Nick Sumner]

Thanks Groggy and Wuzak - was this an invention that was around in the mid forties?

[Wuzak]

quote:Originally posted by Nick Sumner

Thanks Groggy and Wuzak - was this an invention that was around in the mid forties?


There was a discussion somewhere in here about such a device, which apprently the British bolted up to the rear of their turbojets in late WW2. That may have been ducted, though.

I'll see if I can find it later.

Generally I think that UDFs are a latter 20th century development.

[Groggy]

quote:Originally posted by Nick Sumner

Thanks Groggy and Wuzak - was this an invention that was around in the mid forties?

Hi folks,
Yes it was around then, I did post something sometime ago, but will try and find the documents again,

[Red Admiral]

"Whittle's interest in jet propulsion had begun during his final Cranwell year, in which he sought a better means of propulsion for high-speed aircraft at high altitude. For various reasons, rocket units and gas-turbine driven propellors (a form of Propjet) were rejected but it was not until 1929 at Wittering that "the penny dropped", as Whittle puts it, namely that he could use the turbine output as a propulsive jet."

"By 1936 Whittle's thinking had advanced even further, progressing beyond the simple turbojet so as to envisage all of derivative types known today; the turbojet with reheat, the ducted fan and the bypass engine."

"The Metrovick F3 aft turbofan (my emphasis) of 1943 and two further forms of turbofan, modified by Whittle from his W2/700 turbojet ran as early as 1942. By 1944 Whittle's LR1 turbofan - having a by-pass ratio of 3 with ten axial stages and one centrifugal stage, giving a projected thrust of 27kN - was about to run when the project was cancelled."

"Subsequently, even greater power outputs were to be obtained from some of hte later versions of the last engines listed in Table 1, particulary in case the Centaurus. Even the earlier Merlin proved capable of considerable improvement. Gunston citing Derby's developed engine of 1944 which produced 2640bhp(1969kW) at an unprecenedted BMEP value of 2786kN/m^2."

"In 1935 Meredith at the RAE seems to have been hte first to suggest that the liquid cooled engine's heat energy discharged through the radiator, and therefore lost to the passing stream might be used to produce a thrust which compensated for cooling drag. Essentially the idea was that by adding heat energy at the radiator, the duct's airflow when returned to ambient pressure at the exit should produce significant jet thrust. Meredith's calculations suggest that, by speeds of about 300mph the jet thrust might have increased sufficiently as to equal the cooling drag. A variable area nozzle would be needed at the duct's exit so as to accomodate the speed range of the aircraft."

The Uk's contribution to the development of Aeronautics, parts 3 and 4 by J Ackroyd.

I was sure there was something about the Crecy, but I couldn't find anything....

[Wuzak]

Quite interesting to see that Whittle had begun developing axial flow turbojets before the end of WW2.

It took Rolls Royce a few years after WW2 before they got one into production.

One wonders what would have happened if the Metrovicks F2 had been given a production order and a similar amount of resources was spent on that as was the Whittle jets.

Also interesting to note that A.A.Griffith wrote a paper on turbine engines for aircraft in 1926, producing a design which would later be developed into the Metrovicks F1 (a turboprop, with free shaft turbine). Yet it wasn't until the end of WW2 that engineers began developing turbo compound engines, where the exhaust of the piston engine drove a turbine for additional power.

http://en.wikipedia.org/wiki/Metrovick_F.2

[Groggy]

quote:Originally posted by Red Admiral

"Whittle's interest in jet propulsion had begun during his final Cranwell year, in which he sought a better means of propulsion for high-speed aircraft at high altitude. For various reasons, rocket units and gas-turbine driven propellors (a form of Propjet) were rejected but it was not until 1929 at Wittering that "the penny dropped", as Whittle puts it, namely that he could use the turbine output as a propulsive jet."

"By 1936 Whittle's thinking had advanced even further, progressing beyond the simple turbojet so as to envisage all of derivative types known today; the turbojet with reheat, the ducted fan and the bypass engine."

"The Metrovick F3 aft turbofan (my emphasis) of 1943 and two further forms of turbofan, modified by Whittle from his W2/700 turbojet ran as early as 1942. By 1944 Whittle's LR1 turbofan - having a by-pass ratio of 3 with ten axial stages and one centrifugal stage, giving a projected thrust of 27kN - was about to run when the project was cancelled."

"Subsequently, even greater power outputs were to be obtained from some of hte later versions of the last engines listed in Table 1, particulary in case the Centaurus. Even the earlier Merlin proved capable of considerable improvement. Gunston citing Derby's developed engine of 1944 which produced 2640bhp(1969kW) at an unprecenedted BMEP value of 2786kN/m^2."

"In 1935 Meredith at the RAE seems to have been hte first to suggest that the liquid cooled engine's heat energy discharged through the radiator, and therefore lost to the passing stream might be used to produce a thrust which compensated for cooling drag. Essentially the idea was that by adding heat energy at the radiator, the duct's airflow when returned to ambient pressure at the exit should produce significant jet thrust. Meredith's calculations suggest that, by speeds of about 300mph the jet thrust might have increased sufficiently as to equal the cooling drag. A variable area nozzle would be needed at the duct's exit so as to accomodate the speed range of the aircraft."

The Uk's contribution to the development of Aeronautics, parts 3 and 4 by J Ackroyd.

I was sure there was something about the Crecy, but I couldn't find anything....




Hi Folks,
Some French pre-war aircraft had radiators suspended in pods, were these examples of Merediths principle?

"In 1935 Meredith at the RAE seems to have been hte first to suggest that the liquid cooled engine's heat energy discharged through the radiator, and therefore lost to the passing stream might be used to produce a thrust which compensated for cooling drag.

[Groggy]

Hi Folks

Have just found a referance to 1940 Me110 with the thrust stack to each engine reconfigured to give positive thrust. has anyone got performance details?



These figures are from RR and show clearly the importance of exhaust thrust for performance at high altitude, high speed for a Spitfire with a Griffon 61(37 litres) (not sure which mark of Spitfire?).
This follows on from the work that Farnborough did circa 1940 when it was found that half the effective thrust for a Hurricane at 30,000ft was from the ejector exhaust.
There is some remarkable data on a two-stroke RR Crecy (26 litre) producing estimates of about 550mph.if fitted in a Spitfire.
There are estimates by Farnborough for exhaust thrust + prop for a projected development of the two-stroke engine that would have given the Mustang an estimated 600mph. That is if an airscrew could have been made to function at those speeds.
A Spitfire had a mock up of the RR Crecy engine fitted at one stage. But fitting it to a Spitfire or Mosquito was then thought to be problematic


Super-chargergear Altitude(ft)Withoutexhaustthrust-Withexhaustthrust

MS 5,000 345 351
13,500FTH 388 400
23,500 373 399
FS
31,500FTH 419 454
40,000 377 424

[GregP]

I'm not a big fan of UDFs and don;t think a 7 or 8 inch blade would do much of anything in a UDF. Inside a duct, yes. In a UDF I think the blades would have to be considerably longer to be effective.

I could be wrong, but no UDF has yet made it into production, so it is quite likely that claims are not really what they seem. If they were efficient and good, we'd have an airliner with UDFs making good efficiency now.

Look at what is flying now that is new, and high bypass turbofans seem to be where the efficiency is. Think Boeing 787 ...

All the UDFs I have seen discussed had fans about as big as a large propeller with wide blades; generally about 2/3 the diameter of a normal prop, with many wide blades in contraprop fashion. I recall seeing womething that looked like a DC-9 with two large UDFs at the rear, and they weren't anything near 7 - 8 inch blades. More like 6 - 8 feet.

I have nothing against props, but I'd ride in a turbofan before putting up with the prop noise if I were going on a vacation flight. I think UDFs would have to be as quiet as a jet to be accepted by the public.

The military is a different story. There, performance counts. If a UDF could outperform a jet, they'd opt for the UDF, assuming it could handle battle damage as well as a jet.

I've flown ducted fans in RC and their throttle response is not nearly as good a a prop. They need a LOT of RPM to work well. Things wear out quickly when you run them hard all the time. I'd need to see some efficiencya nd maintenance numbers that compare fvorably with jets and props before going with a UDF.

[Red Admiral]

I could be wrong, but no UDF has yet made it into production, so it is quite likely that claims are not really what they seem. If they were efficient and good, we'd have an airliner with UDFs making good efficiency now.

UDFs definitely give higher propulsive efficiency and are the longer-term aspiration. It's simply that there are other problems involved, such as noise, gearing and mechanical integrity. Even gearing is a pretty big issue - sounds fairly simply until for realise you have to put ~ 80,000shp through a gearbox that's ~2ft cube.

Performance counts in the civil world - if you can drop fuel consumption by 25-40% that's likely worth the increase in noise signature (probably still a decrease over current turbofans though).

With current turbofans you're getting towards 120" fan blades, that's a 10ft prop and pretty similar to a Spitfire for example. But the Merlin in the Spitfire is producing ~ 2000lbf as opposed to 90,000lbf.

[GregP]

Hi Red, mostly I was taling about a practical UDF. The avrocar flew, too, but never got practical or even good at it. Might be different today with modern engines, but who cana fford to chance another failure with it?

A UDF may be temporarily eficient but, if it can't do it day in and day out without breaking apart or requiring expensive maintenance at impractical intervals, it's not a practical solution.

Like I said, until I see one working practically and it shows real improvement over present technology in a measurable manner, I'm not a big fan of them at all. Just my opinion.

[Groggy]

Hi Greg.

The idea was tried out circa 1945 and increased the thrust of the Metrovik jet from about 2,300lb to 4, 300lb but became a lot less efficient above 500mph.
There is a drawing in Flight or Aeroplane circa 1946 with blades more as you suggest.
There is a book about these early designs maybe published by FAST?


I'm not a big fan of UDFs and don;t think a 7 or 8 inch blade would do much of anything in a UDF. Inside a duct, yes. In a UDF I think the blades would have to be considerably longer to be effective.

I could be wrong, but no UDF has yet made it into production, so it is quite likely that claims are not really what they seem. If they were efficient and good, we'd have an airliner with UDFs making good efficiency now.

Look at what is flying now that is new, and high bypass turbofans seem to be where the efficiency is. Think Boeing 787 ...

All the UDFs I have seen discussed had fans about as big as a large propeller with wide blades; generally about 2/3 the diameter of a normal prop, with many wide blades in contraprop fashion. I recall seeing womething that looked like a DC-9 with two large UDFs at the rear, and they weren't anything near 7 - 8 inch blades. More like 6 - 8 feet.

I have nothing against props, but I'd ride in a turbofan before putting up with the prop noise if I were going on a vacation flight. I think UDFs would have to be as quiet as a jet to be accepted by the public.

The military is a different story. There, performance counts. If a UDF could outperform a jet, they'd opt for the UDF, assuming it could handle battle damage as well as a jet.

I've flown ducted fans in RC and their throttle response is not nearly as good a a prop. They need a LOT of RPM to work well. Things wear out quickly when you run them hard all the time. I'd need to see some efficiencya nd maintenance numbers that compare fvorably with jets and props before going with a UDF.