Page 10 of 11 FirstFirst 1 2 3 4 5 6 7 8 9 10 11 LastLast
Results 136 to 150 of 152

Thread: Meredith Effect and the P-51

  1. #136
    Senior Member
    Join Date
    Jun 2009
    Location
    Connecticut
    Posts
    7,867
    Post Thanks / Like
    The " the Merredith effect" was first put out in a paper read in 1935. It was the same paper that advanced the idea of jet thrust from the engine exhaust. The theory was hardly a secret.

    The problem was in turning the theory into reality.
    You needed a radiator core that was of the right size (cross section) and thickness to have the low pressure drop through the radiator and not so large or thin to limit the amount of heat transfered to any given unit of air.
    You also needed air ducts leading to and from the radiator of the right size and the right change of cross section to keep the pressure changes and air flows with in the limits they needed to be order to get any usable thrust from the whole system. Or even a significant reduction in drag.

    too short a duct or too abrupt a change in cross section of the duct could ruin the anticipated results.

    With a real shortage of wind tunnels and such to experiment with before the war and little experience to go on it was going to take a while (or inspired guesses) to get it right.


  2. #137
    Junior Member
    Join Date
    Jun 2011
    Posts
    3
    Post Thanks / Like
    If someone has already made this point, I apologize.

    Five hundred cubic feet of air raised 200 degrees F per second corresponds to 47 horsepower.

    Forty-seven horsepower into the system and 47 horsepower out.

    If the so-called Meredith effect could make 100% use of the temperature rise of the air going through the scoop and cooler, it would at best reduce the drag of the scoop by 47 horsepower.

    In the real world, the Meredith effect, if it exists at all, probably does not work at 100% efficiency; and so we should expect a less than 47 horsepower benefit from the Meredith effect.

    =====

    The designers of the scoop meant for the scoop to capture a large volume of high velocity air with a minimal cost in frontal area.

    Because the scoop originally functioned in the turbulent boundary area, they did not get the flow and volume they expected.

    The designers lowered the scoop in order to get it into clear air, but with the knowledge that they paid a price in increased frontal area in order to do so.

    A radiator that exchanges heat with the air works best with air travelling at a certain velocity, meaning, with air that spends enough time in contact with the radiator.

    The designers directed the high velocity air from the scoop into a larger volume area where the air expanded to fill the space, slowed down and decreased in pressure.

    The original volume of air that came into the system through a small scoop then passed through a radiator with a much large cross-section area, at a slower, more optimal velocity, so that the air spent more time in the radiator and thus absorbed more heat (in fact, 200 degrees F per second).

    As this hot air passed through the radiator, it expanded while still in the radiator; and, since the dimensions of the radiator constrained the air's expansion, the air could only expand in the direction of travel, which results in an increase in velocity.

    The increased velocity air then enters the second or exit chamber, where it has room to expand.

    Even after expanding, the air has more energy than it had when it entered the scoop; and, in fact, a potential of 47 horsepower of extra energy.

    Here I run into trouble with some of Atwood's math.

    In his article, he states that at 1000lbs of propeller thrust, the radiator drag equates to 400lbs of thrust lost to drag, or 40%.

    He further states that the Meredith effect recovers 350lbs of that thrust, or 35%, for a 5% difference.

    However, he then describes this difference not as 5% but as 3%.

    Let's work with the 5% figure.

    The P-51D has a max continuous horsepower rating of 1380 horsepower, and a War Emergency Power rating of 1720 horsepower.

    One of those two horsepower ratings corresponds to Atwoods' use of the the phrase "full power."

    Five percent of 1380 horsepower equals 69 horsepower, or 22 horsepower more than the 47 horsepower potentially available to a perfect system under optimal conditions.

    Five percent of 1720 horsepower equals 86 horsepower, or 39 horsepower more than the 47 horsepower potentially available to a perfect system under optimal conditions.

    =====

    To me, the brilliance of the P-51's radiator system involves the small cross-section scoop pulling in high velocity air and then slowing it down by directing it into a large-volume, high cross-sectional area, low-velocity radiator system.

    Maximum cooling for minimum frontal area.

    This, and not exit thrust, explains the effectiveness of the P-51's radiator system.

    Perhaps the system produced some small amount of exit thrust that compensated in a small way for frontal area, but the real benefit happened before the hot gas exited the system.

    The placement and dimensions of the scoop, the dimensions of the pre-radiator chamber, and the dimensions of the radiator itself made the biggest difference, and not so much what happened after the radiator other than basic streamlining.

    A huge low-pressure, low-velocity radiator fed by a small high-velocity scoop:brilliant designing.

  3. #138
    Senior Member drgondog's Avatar
    Join Date
    Jun 2006
    Location
    Scurry, Texas
    Posts
    6,750
    Post Thanks / Like
    I would agree all your points. IMO Atwood was blowing smoke and Schmeud categorically denied the math Atwood presented.

    The key elements of the design - going from P-51/A36 to the P-51B was a.) improved flow into the radiator, and b.) reduced drag. The original design, founded on the new radiatior cowling design obtained from Curtis, was fine for the P-51 to match the cowl design before the P-51B caused the wing to drop some 3-4".

    The P-51H design was closer in appearance to the earlier Mustangs.
    "The difference between stupidity and genius is that genius has its limits." - Albert Einstein

  4. #139
    Junior Member
    Join Date
    Jun 2011
    Posts
    3
    Post Thanks / Like
    Placing the radiator inside the fuselage instead of tacking it on to a wing or embedding it in a wing allows a smaller inlet to feed a much larger radiator, provides the enclosed volume necessary for the high velocity inlet air to expand and slow down, allows the air to spend more time in contact with the ultra-large radiator, and thus transfers more heat to the air that came into the system through the comparatively small (compared to the size of the radiator) scoop.

  5. #140
    Senior Member drgondog's Avatar
    Join Date
    Jun 2006
    Location
    Scurry, Texas
    Posts
    6,750
    Post Thanks / Like
    understand the distinction - merely commenting about changes within airframe evolution - not between different aircraft altogether.
    "The difference between stupidity and genius is that genius has its limits." - Albert Einstein

  6. #141
    Junior Member
    Join Date
    Jun 2011
    Posts
    3
    Post Thanks / Like
    Quote Originally Posted by drgondog View Post
    understand the distinction - merely commenting about changes within airframe evolution - not between different aircraft altogether.
    I didn't mean my post as a rebuttal or a correction.

    I have struggled to understand and write what I wrote for some time, and now that it has started coming out of me I can't stop.

  7. #142
    Senior Member drgondog's Avatar
    Join Date
    Jun 2006
    Location
    Scurry, Texas
    Posts
    6,750
    Post Thanks / Like
    No offense taken - excellent engineering reduction of the system.
    "The difference between stupidity and genius is that genius has its limits." - Albert Einstein

  8. #143
    Senior Member krieghund's Avatar
    Join Date
    Sep 2006
    Location
    Riyadh
    Posts
    611
    Post Thanks / Like
    This is probably the best way to go to reduce the drag of the radiator.
    Attached Thumbnails Attached Thumbnails -stiletto-84_page_1.jpg   -stiletto-84_page_2.jpg  


  9. #144
    Senior Member
    Join Date
    Jul 2005
    Posts
    219
    Post Thanks / Like
    Five hundred cubic feet of air raised 200 degrees F per second corresponds to 47 horsepower.
    It has been a while but I would be interested in knowing how you found the volume and temperature raise. A few years ago I looked at this. North American claimed that the radiator created 300lb of thrust. I assumed this was at maximum speed (+700km/h) and after efficiency losses. Then I converted thrust into HP.

    300lb of thrust = 136 kgf
    [1 kgf = 2.20 pounds]

    703km/h = ~195m/s

    Power (W) = Thrust velocity
    Power (W) = 136kgf 195m/s
    Power (W) = 260000 watts = 260kW
    260kW= 354HP

    Some of the conversion rates might vary a bit. This method is applied for turbines but I don't know if it holds for propeller engines.

  10. #145
    Senior Member
    Join Date
    Apr 2011
    Location
    High Wycombe, England (home of the Mosquito)
    Posts
    594
    Post Thanks / Like
    Does anyone actually have a copy of this alleged material by Meredith? I say "alleged" because there is nothing substantial in our National Archives, apart from a few single papers, including original sketches, none of which mention radiators. His entire thrust (if you'll pardon the pun) concerned jet propulsion, and nothing else, with typically luke-warm reaction from senior officers in the Air Ministry.
    A year later, together with another scientist, he came up with an idea on how to dispel boundary layer drag, using small holes in the wings' surfaces, through which air would be blown (forerunner of post-war ducts, presumably.) They got as far as taking out an initial patent, but delayed so long that others published similar information, and any lead was lost.

  11. #146
    Senior Member
    Join Date
    Jun 2009
    Location
    Connecticut
    Posts
    7,867
    Post Thanks / Like
    I think "alleged material" is a bit strong.

    See: clark yh | air ministry | scientific research | 1936 | 2349 | Flight Archive

    second report under technical literature.

    apparently you could get a reprint of the 13 page report for 9 pence?

  12. #147
    Senior Member
    Join Date
    Apr 2011
    Location
    High Wycombe, England (home of the Mosquito)
    Posts
    594
    Post Thanks / Like
    One of the definitions of "allege" is to "cite or quote in a discussion," which is why I used it; thanks for the link, which means that I have a lot more digging to do, to find the original.
    Interestingly, the Spitfire was ordered in December 1934, and this report was published several months later, so which came first, or was the shape of the Spitfire's radiator changed as a result of Meredith?
    I've found the original tenders for the Hawker "High-Speed Monoplane" (never called, officially, the Fury Monoplane,) but, so far, not the Supermarine Type 300, so I know what my homework (in Kew) has to be, because both should be in there somewhere.

  13. #148
    Senior Member
    Join Date
    Jun 2009
    Location
    Connecticut
    Posts
    7,867
    Post Thanks / Like
    Quote Originally Posted by Edgar Brooks View Post
    One of the definitions of "allege" is to "cite or quote in a discussion," which is why I used it;
    Well, they do say that the Americans and the British are one people separated by a common language

    Considering some of the multiple configurations some other aircraft went through for radiator design on prototypes after the first flight, the apparent "fact" that Meredith didn't "publish" or finish the report until Aug of 1935 doesn't really concern me that much except for historical accuracy. The Specification was changed in April to go from four guns to eight after all so the design wasn't cast in stone yet. I believe (from other sources?) that this report by Meredith was read at a conference first (??) before being "published"?

  14. #149
    Senior Member
    Join Date
    Apr 2011
    Location
    High Wycombe, England (home of the Mosquito)
    Posts
    594
    Post Thanks / Like
    I believe that I've found it (at least the file in Kew,) and that was entirely due to the reference/title of the report which you supplied, so thanks again for that; the difficulty seems to stem from the file having no mention of Meredith, himself, and there's a possible reason for that, but it will take more digging.
    There are other references to him being investigated for being "involved in left-wing activity" some time up to the end of 1949, and he apparently confessed to being a Soviet agent before the war (he seems to have left the RAE in 1938.) There are 3 files, on that subject, which were closed until 2006, but I'm not sure that they'll be of any interest to aviation enthusiasts.
    Last edited by Edgar Brooks; 12-03-2012 at 08:08 PM.

  15. #150
    Senior Member
    Join Date
    Apr 2011
    Location
    High Wycombe, England (home of the Mosquito)
    Posts
    594
    Post Thanks / Like
    Found it, and, after all that, there are two copies, in separate files. It consists of 20 pages, and I suspect you'll need a degree in maths to get the full gist of it. Due to the size, I won't make any attempt to put it on here, but if anyone would like a copy, let me know. In common with RAE practice, he notes any other source to which he referred, and there's only one:- "Further experiments on honeycomb radiators," by Harris & Caygill, in November 1924.
    Oddly, Meredith seems to have been more concerned with the temperature of the coolant, and its heat transfer to the airflow, than the design of the radiator housing, in fact he states that the radiator becomes more efficient as the aircraft's speed increases, with the optimum "changeover" from drag to thrust being around 300mph, in average U.K. ambient temperature conditions (in another publication, that I have, it states that experience showed it to be nearer 400 than 300.)
    Meredith left the RAE in 1938, and worked at Smiths Industries from then; he remained "of interest" to the authorites until 1958 (when he would have been 63,) but was never imprisoned, arrested, or even charged, possibly because any information he'd passed on in the 1930s was common knowledge, and certainly because his work at Smiths was never of a sensitive nature. (It's possible, of course, that the invasion of Hungary, in 1956, made him think twice [as it did for so many communist sympathisers,] since all surveillance on him stopped in 1958, and no more is heard of him from that date.) The Intelligence files on him were closed to view until 2006, which means of course that he (and possible even his children) has died, so there's no chance of any stigma being attached to any relatives.
    Mention is continually made of his brilliance, and there's a sneaking suspicion that the authorities were prepared to allow him a lot of leeway, for fear of losing him, if he decamped to Russia. As well as his jet propulsion and radiator papers, he also did innovative work on instrument capsules (papers which I haven't yet looked at,) so to say that he never invented anything is way wide of the mark.

Page 10 of 11 FirstFirst 1 2 3 4 5 6 7 8 9 10 11 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •