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Meredith Effect


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#1 Flo

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Posted 07 May 2012 - 03:26 PM

Quite famous for 'improving' the performance of Mustangs and Mosquitos.

As I understand it- billy basic and probably not quite right- it's a mechanism for turning the hot air flowing through a radiator into thrust.

Are there any figures available for how effective it was on various aircraft?

Did it have a more pronounced effect at altitude?

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#2 Edgar Brooks

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Posted 07 May 2012 - 09:42 PM

From what I can glean, from the little in Kew, it involves taking air through a narrow(ish) opening, then widening the chamber, which makes the air expand and slow down, then feeding it through the matrix and into a narrower chamber behind, which has the effect of speeding the air back up again, or, as in the Mustang, making it faster than when it went in. The Spitfire used a partial Meredith, but not as efficient as that on the P-51. The same idea was used for the early jet engines, except the air was mixed with fuel, and exploded out of the back end, pushing the airframe forward.

#3 Wuzak

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Posted 07 May 2012 - 10:32 PM

IIRC Joe Smith said that they never developed that part of the Spitfire very well. The Spitfire's radiator ducts had two issues - the exit hole was too big, and it only had a two position flap.

#4 NeoConShooter

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Posted 07 May 2012 - 10:34 PM

Quite famous for 'improving' the performance of Mustangs and Mosquitos.

As I understand it- billy basic and probably not quite right- it's a mechanism for turning the hot air flowing through a radiator into thrust.

Are there any figures available for how effective it was on various aircraft?

Did it have a more pronounced effect at altitude?

Hope this helps?
Chapter 5: High-speed Cowlings, Air Inlets and Outlets, and Internal-Flow Systems

THE RAMJET INVESTIGATION

[161] In 1936, F. W. Meredith pointed out that the waste heat of a piston engine which is transferred to the cooling-air flow in a radiator is not all lost; it produces a small thrust provided the pressure at the exhaust of the radiator tubes is higher than the free static pressure of flight (ref. 192). This phenomenon became known as the "Meredith effect." Its mechanism was something of a mystery to many engineers of that period. A common fallacious notion was that the radial engine, because its fins were hotter than usual radiator temperatures of liquid-cooled engines, would enjoy greater benefits. (This mistaken notion still existed as late as 1949 and is stated by Schlaifer to constitute an "inherent advantage of the radial engine" (ref. 41).) The Meredith effect was so small at 1936 airspeeds that it could conveniently be neglected in performance estimates both by those who did not understand it and by those who doubted that such an effect really existed.

In our engineering analysis of the effects of heat in internal flow systems, the conversion of heat to thrust power was clearly the most [162] intriguing aspect. Thinking in terms of flight speeds of 550 mph, we calculated ideal thermal efficiencies of as much as 10 percent, and by Mach 1.5 the heated duct would have a thermal efficiency comparable to an internal combustion engine. Clearly, the insignificant "Meredith effect" had the potential to become a primary jet-propulsion system. (The term "ramjet" was not then in general use, and we were unaware that there were several discussions of propulsive ducts in the literature starting with Lorin in 1913 and including later treatments by Carter, V Leduc, Roy, and others.)

Excited at these prospects, I arranged a meeting with Langley's leading propulsion analyst at our Power Plant Division, Ben Pinkel. I also talked briefly with D. T. Williams, a young physicist whom Pinkel had recently assigned to analyze propulsive ducts at high subsonic speeds, including the effect of an engine-driver blower typical of the Campini system under study by Jacobs. Neither man showed any real hope for these systems, and Pinkel, reflecting the general attitude of most of the propulsion community at that time, patiently explained "the great weakness of all forms of jet propulsion-excessive fuel consumption compared to piston engines". When Williams' work was published about a year later (ref. 193), its primary conclusion emphasized the same point, showing on overall propulsive efficiency at Mach 0.8 on the order of one-sixth that of a piston-engine driving a propeller. Both men felt that tests of a propulsive duct in the 8-foot high-speed tunnel would be of little value. The duct and heater losses would, they speculated, largely nullify any possibility of net thrust at Mach 0.75.

In fairness to Pinkel and Williams it should be recalled that in 1940 the aircraft industry generally saw no possibility for supersonic aircraft. Mach 0.8 was regarded as a rather optimistic upper limit for the future. The potential of the turbojet for large improvements over the Campini cycle was not recognized either, and it is not mentioned in Williams' paper.

In spite of my disappointing session with Pinkel and Williams I resolved to proceed with the propulsive duct test. At the very least it would establish the Meredith effect as a major design factor at high speeds. Our 8-foot, high-speed tunnel afforded a unique tool for such an experiment. Stack solidly supported the idea. In promoting the project....


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



[163] FIGURE 42.- "Heat model" used in the first NACA investigation of a propulsive-duct (ramjet) system in the 8-Foot High-Speed Tunnel in February and March 1941. Model incorporated a 160-kw heater. Nose B and cusped outlet from ref. 179.
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....we decided not to mention the jet propulsion implications in order to avoid the negative reactions of the propulsion people.

The nacelle model chosen for the tests embodied our universal Nose B shape together with our most effective cusped tail outlet (fig. 42). The all-metal nacelle was supported on a new thin metal wing selected to avoid the local area of flow separation that existed in the wing/body juncture of my inlet-outlet model. (In reviewing my original work at the request of Mr. Miller, A. M. Kuethe, who was employed briefly by NACA during the war, had endorsed my findings generally but had raised questions about possible drag interactions involving the separated flow. These would now be answered. By comparing the inlet results from the new model with the original data, we found no measurable effect of the separated flow.)

How to add heat at a high rate was our primary design problem. Combustion of fuel in the 8-foot tunnel was quite out of the question for many reasons. A search of the electrical heater catalogs with help from G. T. Strailman, Langley's principal electrical engineer, turned up no [164] high-output heater capable of being fitted into our 11-inch diameter duct. Baals and I therefore became high-capacity heater designers and produced a 160-kw, three-phase, 15000 F heat exchanger with 32 square feet of surface area in the form of 1.5-inch-wide Nichrome ribbon woven on reinforced asbestos millboard supports. This heater produced air temperature rises of about 3000 F at high speeds with very small frictional losses. The rates of heat input were larger than those due to piston-engine cooling, but still only a small fraction of the heat of combustion of kerosene.

Testing of the "heat model" started in February 1941, the first NACA wind tunnel investigation of a propulsive duct producing thrust. At a Mach number of about 0.5, the propulsive effect had become equal to the internal drag, and beyond this speed substantial net thrust was developed by the internal flow. At the highest test speed, Mach 0.75, the heated duct developed the respectable thermal efficiency of some 9.5 percent, close to the ideal theoretical value. As expected, the phenomena depended on the ratio of duct pressure to stream pressure, and was independent of heater surface temperatures per se. In all other respects, the careful measurements of these tests confirmed the calculations made by our engineering relations for analysis of this kind of internal flow system (ref.187).

COMMENTARY

In 1941 during the period of our propulsive-duct investigations, Stewart Way, of Westinghouse, made an analysis of the subsonic propulsion possibilities of "open-duct jet propulsion," his name for what was later called the ramjet. He also apparently conducted some tests with an electrically-heated model at about the same time of our high-speed tests in February and March of 1941, although the experimental work was never published (ref. 194), and we knew nothing of Way's work until years later. In the first version of our internal-flow-system report which was issued in September 1942 as a confidential document (ref. 189), the propulsive duct data were included but there was no emphasis in the title or text that the first NACA tests of a potentially important jet-propulsion system had [165] been made. Our "heat-model" tests rather definitely settled once and for all the doubts and arguments about the Meredith effect. Whether they had any impact on ramjet development is questionable. The revelation of the British and German turbojets shortly after our paper was issued had such an enormous impact that all the scattered U.S. activities in jet propulsion were in effect rendered insignificant. Almost overnight the propulsion community reversed its attitudes. By war's end, the ramjet was under vigorous development for missile applications. Both the Langley and the Lewis Laboratories of NACA had organized ramjet projects, concentrating on the prime problems of combustion and burner design which we had not been able to deal with in our 1941 project.

NACA was now being severely criticized for its prior general neglect of jet propulsion and it was clearly desirable to highlight whatever had been done. Accordingly, our report was reorganized to emphasize the tests of the ramjet system, and the words "Ram-jet System" were added to the title. The revised version is included in the 28th Annual Report of the NACA, dated 1943 but actually issued after the war.

#5 Flo

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Posted 07 May 2012 - 10:35 PM

:eek: So it could have been even quicker?

Cheers Wuzak, every day's a school day..

#6 Flo

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Posted 07 May 2012 - 10:43 PM

Thank you, too. Fascinating stuff.

Have you got anything related to production, WW2 aircraft? I'm aware of the (possibly exaggerated) effect on liquid cooled engines, do you know if there was any effect on radials?

(Beyond ref 41, given above.)

Edited by Flo, 07 May 2012 - 10:44 PM.
punctuation


#7 Wuzak

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Posted 08 May 2012 - 12:16 AM

Thank you, too. Fascinating stuff.

Have you got anything related to production, WW2 aircraft? I'm aware of the (possibly exaggerated) effect on liquid cooled engines, do you know if there was any effect on radials?

(Beyond ref 41, given above.)


The NACA cowling used on teh amjority of radials used teh principal to greatly reduce the installed drag of the air-cooled radial.

The cowl flaps were designed to regulate mass flow and thus cooling (and resultant thrust/drag).

#8 NeoConShooter

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Posted 08 May 2012 - 12:38 AM

Thank you, too. Fascinating stuff.

Have you got anything related to production, WW2 aircraft? I'm aware of the (possibly exaggerated) effect on liquid cooled engines, do you know if there was any effect on radials?

(Beyond ref 41, given above.)


Yes, there were several planes with air cooled radial engines that made significant use of the Meredith effect! ( IIRC, their designations were XF-12R? Rainbow and the B-36!) But as much as I know, none during the war.:confused: By the way, the Spit never made use of the Meredith system design radiators and no LC plane was nearly as effective as either of the two ACR planes that I am certain did use it. The Mustang was IIRC, the first plane to use it and it was not nearly as effective there as it was in the later ACR planes. See the size and cross sections of the two radiator systems to see the absence of the Meredith effect requirements in the Spitfire, any. The Rainbow made 200 pounds of thrust from the cooling system of each 3,000 HP engine. I can not remember the exact figures for the B-36, which were not as good as the -12's, but still some significant extension of range.

#9 NeoConShooter

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Posted 08 May 2012 - 12:40 AM

The NACA cowling used on teh amjority of radials used teh principal to greatly reduce the installed drag of the air-cooled radial.

The cowl flaps were designed to regulate mass flow and thus cooling (and resultant thrust/drag).


The NACA cowling had nothing to do with the Meredeth effect. It caused most air to by-pass the engine at speed, thus reducing drag and the exit flaps were to prevent over cooling at speed.

#10 Wuzak

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Posted 08 May 2012 - 01:32 AM

The NACA cowling had nothing to do with the Meredeth effect. It caused most air to by-pass the engine at speed, thus reducing drag and the exit flaps were to prevent over cooling at speed.


It very much did.

The air came in a narrow slot, was expanded to go through the cooling matrix (ie the engine) and then exited through an adjustable nozzle.

#11 NeoConShooter

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Posted 08 May 2012 - 01:57 AM

It very much did.

The air came in a narrow slot, was expanded to go through the cooling matrix (ie the engine) and then exited through an adjustable nozzle.


The flaps were there to stop airflow at high speed. The slower the plane was going, the wider they opened. Exactly the oposite of what would be required if it did use the Meredeth Effect.

#12 Wuzak

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Posted 08 May 2012 - 02:24 AM

The flaps were there to stop airflow at high speed. The slower the plane was going, the wider they opened. Exactly the oposite of what would be required if it did use the Meredeth Effect.


You are incorrect.

The flap position is not dependent on the aircraft's speed.The flaps are used to control teh mass airflow of air through the engine.

At low speed and low power the flaps would probably be closed because there isn't much cooling required.

At WEP at high speed the flaps may need to be open as the engine temperatures, especially the cylinder head temperature, may become too high otherwise.

From wiki:

The cowling enhanced speed through drag reduction and utilising the heat of the engine to generate thrust.



#13 GregP

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Posted 08 May 2012 - 02:37 AM

You don't stop the airflow through a NACA duct or it creates a drag point, exactly the same as if a jet intake were too large and air werr to be both flowing into and back out of the inlet, as on the Bell YP-59A Airacomet.

The flaps might slow the flow through the NACA duct but they certainly won't stop it entirely without adverse drag penalties.

What do you think, Flo and Drgondog and Wuzak?

Guess we posted at the same time, huh Wuzak? Thoufg we said essentially the wame thing, I like Wuzak's wording better than my own.

Edited by GregP, 08 May 2012 - 02:42 AM.


#14 Wuzak

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Posted 08 May 2012 - 02:52 AM

Looks like it Greg.

I was thhinking about the P-51's situation. I'm quite sure that the best thrust from the radiator occurs when the flap is closed.

As I said before, Joe Smith admitted that the exit of the radiator ducts on the Spitfire were too large - not maxmising the potential thrust and causing over-cooling in some situations.

I've wondered if the Spitfire XIV would have been faster had it been given the same style radiators as the Spiteful (which are like the Bf109's).

On radials when the cowl flaps are closed there is still an air path - it is just smaller than when they are open. The flaps also must have soem effect on drag when they are open.

#15 NeoConShooter

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Posted 08 May 2012 - 03:09 AM

You are incorrect.

The flap position is not dependent on the aircraft's speed.Yes it does!The flaps are used to control teh mass airflow of air through the engine.True.

At low speed and low power the flaps would probably be closed because there isn't much cooling required.Major mistake! The flaps will be all the way open because at low speed there is not enough air to cool the engine at idle.

At WEP at high speed the flaps may need to be open as the engine temperatures, especially the cylinder head temperature, may become too high otherwise.Wrong again! At high speed there is so much air that the engine will not reach the right temperature for correct operation. By closing the flaps, more air is spilled out side the cowling.

From wiki:


Again, you know what they say about Wiki?

#16 GregP

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Posted 08 May 2012 - 03:36 AM

The P-51 radiator does not produce thrust; that is a myth. What it does is reduce the cooling drag. In the case of the P-51, it can eliminate most, but not all the cooling drag. So the P-51 flies as though the raduiator is mostly not there. It isn't thrust, it is drag reduction. There cannot BE a positive thrust from the P-51 radiator.

There might be from the Republic Rainbow exhaust system, but that is NOT a cooling system producing thrust, that is exhaust thrust, which is possible and several aircraft have taken advantage of it.

Radiators will never produce thrust, only more or less drag.

Neo is so wrong below that I don't really know where to start. I seriously wonder if he hasd ever operated an air-cooled piston engine in an aircraft. I think he is Gaston or a close cousin. Maybe he is The Pipe. Hello, The Pipe here. Remember him? He was about to start posting in third person when he mysteriously disappeared. Maybe he is back.

At low speed and low power, such as when you are gliding toward landing on final or coming down base, the cooling flaps are usually mostly closed, and your ramblings won't change that. As you increase power and the temperature rises, the cooling flaps are progressively opened to keep the temperatures in the green. If you open the cooling flaps at low power, you usually shock-cool the engine and it gets expensive due to cracked cylinders. Better stay away from aircraft with air-cooled engines Neo. You'll buy a few before you are done, if you fly them the way you post. Best stick with the engine manufacturer's recommendations in the pilot's operating handbook.

Edited by GregP, 08 May 2012 - 03:48 AM.


#17 Wuzak

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Posted 08 May 2012 - 03:47 AM



Again, you know what they say about Wiki?


Yep, it's a better source than your memory!

#18 Edgar Brooks

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Posted 08 May 2012 - 06:49 AM

Again, you know what they say about Wiki?

What people say about Wiki is immaterial; the flap(s) on the radiators opened as the coolant got hotter (with increased engine revolutions and speed,) and closed as things cooled down when the aircraft slowed. On early aircraft, with a single radiator, the flap was opened by the pilot, as he saw the coolant temperature rise, then closed as he slowed.
On the twin-radiator airframes, the flaps opened automatically by a temperature-controlled microswitch; the Mk.IX's temperature setting was 115C, which was what it was supposed to reach (and no higher) in cruise. During take-off, climb, and combat, the temperature was expected to rise above that, and the flaps opened.

#19 NeoConShooter

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Posted 15 May 2012 - 12:59 AM

You don't stop the airflow through a NACA duct or it creates a drag point, exactly the same as if a jet intake were too large and air werr to be both flowing into and back out of the inlet, as on the Bell YP-59A Airacomet.

The flaps might slow the flow through the NACA duct but they certainly won't stop it entirely without adverse drag penalties.

What do you think, Flo and Drgondog and Wuzak?

Guess we posted at the same time, huh Wuzak? Thoufg we said essentially the wame thing, I like Wuzak's wording better than my own.


It works by forcing air to go outside the cowl instead of through it.

#20 NeoConShooter

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Posted 15 May 2012 - 01:12 AM

The P-51 radiator does not produce thrust; that is a myth. What it does is reduce the cooling drag. In the case of the P-51, it can eliminate most, but not all the cooling drag. So the P-51 flies as though the raduiator is mostly not there. It isn't thrust, it is drag reduction. There cannot BE a positive thrust from the P-51 radiator.
True!
There might be from the Republic Rainbow exhaust system, but that is NOT a cooling system producing thrust, that is exhaust thrust, which is possible and several aircraft have taken advantage of it.
No. The Republic Rainbow developed thrust because the engine was cooled by fan air that was drawn in through a small opening, slowed and heated by the cooling fins where it expanded, was then mixed with the exhaust to provide over 200 pounds of thrust through a small opening.
Radiators will never produce thrust, only more or less drag.
Unless the air it uses is forched through it by a fan.
Neo is so wrong below that I don't really know where to start. I seriously wonder if he hasd ever operated an air-cooled piston engine in an aircraft. I think he is Gaston or a close cousin. Maybe he is The Pipe. Hello, The Pipe here. Remember him? He was about to start posting in third person when he mysteriously disappeared. Maybe he is back.

At low speed and low power, such as when you are gliding toward landing on final or coming down base, the cooling flaps are usually mostly closed, and your ramblings won't change that. As you increase power and the temperature rises, the cooling flaps are progressively opened to keep the temperatures in the green. If you open the cooling flaps at low power, you usually shock-cool the engine and it gets expensive due to cracked cylinders. Better stay away from aircraft with air-cooled engines Neo. You'll buy a few before you are done, if you fly them the way you post. Best stick with the engine manufacturer's recommendations in the pilot's operating handbook.

I do and would. But if you watch films of P-47s from WW-II, the flaps are closed at high speed and open on take off. Same-o, same-o for film of Zeros taking off from the carriers before PH!




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