Talk:Meredith effect

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No such thing[edit]

This article should be checked for deletion. A radiator can not produce thrust as long as the air is not forced to expand one way. Unlike for a jet-engine or a ram-jet there is no mentioning of compressing the air prior to heating. Only the exhaust gases of an internal combustion engine themselves maybe used as jets. --Moritzgedig (talk) 12:09, 24 January 2014 (UTC)

There is such a topic as "The Meredith effect", but whether it really works or not is controversial. Regardless of whether the effect exists, or is a fallacy, it may be worth retaining the article just to gather all the information on it together either way. Not to say this is a hoax, but this is similar to the way Wikipedia documents notable hoaxes as well. For instance none of the computer virus hoaxes listed in Virus hoax are "real", but they are notable enough to have an article to explain that they are not real. - Ahunt (talk) 13:09, 24 January 2014 (UTC)
By "controversy" I think we mean "people who don't understand it". It's not that complicated.
If you do recommend to delete this, will you be going after the ramjet and SR-71 articles next? Andy Dingley (talk) 13:18, 24 January 2014 (UTC)
I just read the refs cited and I don't think there is a case for deleting this, as it meets WP:GNG. It needs expanding, though to explain the subject better!- Ahunt (talk) 13:20, 24 January 2014 (UTC)
The effect is definitely real, and was indeed a significant feature in the Spitfire and Hurricane designs as the article currently states. ISTR its aircraft application was developed at Farnborough during the early 1930's and it added around 5-10 kts to the Spitfire's measured airspeed. FYI a ramjet only compresses the air when moving at supersonic speeds (compressible flow): it still works at subsonic speeds (incompressible flow) but much less efficiently. At say 600 knots the only difference between a Spitfire's radiator and a Bloodhound's ramjet is that the Bloodhound has to burn fuel to heat the air while the Spit recycles waste heat from the engine coolant. Doubting whether it works is no more than acknowledging that you do not understand how. HTH — Cheers, Steelpillow (Talk) 13:45, 24 January 2014 (UTC)
I think the refs cited establishe that the effect exists, although it is a small effect. I have added some text to the article to explain how it works. It seems fine now, although more could always be added, of course. - Ahunt (talk) 13:48, 24 January 2014 (UTC)

@ Andy Dingley, Steelpillow: I also do not understand how "free energy power plants" work. Maybe you could explain like you explained this. Oh wait you didn't explain it. What surface does the thrust work on? How come the air does not go around the duct? Will the thrust exceed the drag? Was it thrust or just reduced drag? No one of us has to understand it for it to be real, but people just thinking they understand it does not make it real either. --Moritzgedig (talk) 10:12, 25 January 2014 (UTC)

There are two ways to look at this: truth or WP:Verifiability.
Wikipedia relies on proof by authority: we believe something because WP:RS tell us so. Logically this is considered a fallacy, but it works better for collaborative editing projects because all editors assume that they themselves know "the truth". If we appeal to "reliable" sources, that's better than the collective madness of crowds we'd otherwise see. If you look out for such sources, either from thermodynamics texts or from aircraft history, you'll find this effect widely discussed and described for the Spitfire and the Mustang. As a general rule, things in mid-century British aerospace with a well-known name attached to them will have originally had papers by that author presented to one of the chartered engineering societies and reported in Flight, so searching the archives might turn up a useful primary source for them.
Alternatively you could try to understand it. It's a duct, and heat energy is added. As a result, thrust is generated. Note that you have to carry this out in a "duct", which has a more complex meaning in fluid dynamics than is obvious. Ducts are generally shaped internally, so that their pressure and velocity vary in a deliberate manner, to achieve some useful purpose. Yes, the effect here is asymmetrical but "obviously" adding a simple radiator to the outside of an aircraft would be symmetrical and so wouldn't produce a nett thrust in one direction. However this effect isn't in a symmetrical situation: it's inside a shaped duct and there's considerable airflow (so much airflow it's not going to be disturbed by the heat added). Only in this duct does the radiator produce useful thrust: the deliberate Meredith effect is the process and result of designing the internals of such a duct, so that the energy goes mostly to adding forwards thrust, rather than disrupting the airflow and accelerating it forwards in an unhelpful manner.
Formula 1 cars (some years ago) had a problem with the anti-Meredith effect. They added so much heat to a small, relatively slower, airstream (from cramming the powerful radiator of a massive engine into the smallest possible draggy airflow) that they could instead stall the airflow through the radiator duct, as the extra heat added wasn't creating thrust, it was raising the pressure inside the duct too much and disrupting the flow.
Andy Dingley (talk) 11:03, 25 January 2014 (UTC)
@Moritzgedig: I can answer your questions in part, however I do not have references to hand so please take my explanations as my own understanding and not definitive (the reason I gave none before). The duct referred to by Andy Dingley typically has an expanding exhaust section, often including a nozzle or, on the Spitfire, an adjustable flap. Thrust is gained from the pressure of heated air against this expanding section. This may or may not overcome the drag of the installation. I seem to recall that the early Spitfire installations merely reduced overall drag and that only in the later marks was a net thrust created, but I need to confirm that. Some air may or may not go around the duct, depending on the intake geometry and other conditions, but the key point is the same as for a ramjet - enough air is "rammed" into the intake by the speed of travel for the thing to work. The expanding exhaust is directed backwards by the expanding nozzle (in direct reaction against the thrust produced) and this also helps to "draw" air from the inlet through the heat exchanger and maintain flow. — Cheers, Steelpillow (Talk) 11:40, 25 January 2014 (UTC)
Just for the record - I was wrong about the exhaust duct, it is convergent not divergent and does not operate on the pronciple I describe above. — Cheers, Steelpillow (Talk) 10:13, 30 April 2014 (UTC)

@User:Steelpillow "a ramjet [...] still works at subsonic speeds [...] but much less efficiently."
Ok, the air is compressible at the speeds these planes were going. At M=0.3 or 230mph compressibility can no longer be fully ignored as I did. At M=0.5 the effect becomes noteworthy. The article now mentions compression. --Moritzgedig (talk) 18:27, 25 January 2014 (UTC)

I have removed the discussion of compressibility from the lead, because I think it did not strike a sensible balance for the lead. I have found a brief description of the ramjet in Kermode, "Mechanics of flight", Eighth edition (1972) and it is broad enough to include the Meredith principle: "It relies on the forward speed, or ram effect, to collect and compress the air which then flows over some source of heat – the Germans originally tried a coal-burning brazier! – from which it gains energy and so flows out of the duct at a higher speed than that at which it entered." Interestingly, Kermode feels it necessary to emphasise the fact that it works in bold type and to print a full-page photograph of a missile employing them. Evidently scepticism is commonplace. Anyway, note that if we exchange the German's brazier in their ramjet for a Glycol heat-exchanger then we arrive at the Meredith device. I should like to see a section on the "Principle of operation". My problem is that I have nothing to use as a source, and I fear that if we post our personal speculations that will go against WP:VERIFIABILITY and/or WP:OR (original research). Until a reliable reference can be found, it is probably better to say as little as possible. — Cheers, Steelpillow (Talk) 19:39, 25 January 2014 (UTC)
What I wrote was not unsourced, the source is already given. It neither is an explanation nor speculative. Compression is needed in any such Brayton cycle device (assuming that it is and that it is a known TD process). "it did not strike a sensible balance for the lead" That might be the case, but the lead already touches on the subject of needed speed. --Moritzgedig (talk) 09:36, 26 January 2014 (UTC)
Fair enough. I have taken a deep breath and created a section for the technical analysis. Feel free to make corrections (quite likely needed) and/or further contributions. — Cheers, Steelpillow (Talk) 13:55, 26 January 2014 (UTC)

A 1941 Flight article with an explanation, and quote from Meredith here: [1] and an earlier 1937 article based on a lecture by Roy Fedden here: [2] and another article mentioning 'Mr Meredith of the RAE' here; [3]— Preceding unsigned comment added by 80.7.147.13 (talk) 20:41, 1 March 2014 (UTC)

BTW, F. W. Meredith later worked on automatic pilots and after he left the RAE, took up a position at Smiths Instruments. — Preceding unsigned comment added by 80.7.147.13 (talk) 19:59, 8 March 2014 (UTC)

1935 or 1936 ?[edit]

I find August 1935 http://naca.central.cranfield.ac.uk/reports/arc/rm/1683.pdf as the year of publication not 1936 as the article states. Vandalism? --Moritzgedig (talk) 17:47, 25 January 2014 (UTC)

Look closely at the cover sheet. The Director of Scientific Research appears to have passed it on to the Aeronautical Research Committee in August 1935, but the committee did not publish it until the date shown on the second line from the bottom, just above the price: 1936. — Cheers, Steelpillow (Talk) 19:09, 25 January 2014 (UTC)

Principles of Operation wrong[edit]

A sub-sonic Meredith effect ducted radiator differs from a supersonic ramjet. A supersonic ramjet uses a constriction and resulting shockwave to compress and slow the air before further heating it, but constrictions only compress supersonic flows. A subsonic flow is instead expanded and accelerated by a constriction. So a Meredith effect duct first diverges, causing the flow to compress and slow before it meets the heat source, and then the exit nozzle is purely convergent in contrast to a ram jet's convergent-divergent nozzle. These facts were key to my understanding of the effect, but were only implicit in Meredith's original paper. They are confirmed by diagrams of the P51's radiator duct, e.g. http://i101.photobucket.com/albums/m56/babraham227/51.gif And here is a site that seems to have it straight: http://contrails.free.fr/engine_aerodyn_radia_en.php But the wikipedia article seems to be describing the principles of operation of a supersonic ramjet. In particular, while the forwards thrust is developed on the divergent section of the duct, unlike in a supersonic ramjet that section is not the exit but the inlet (called a diffuser). A Meredith ducted radiator is a subsonic ramjet, but as subsonic ram jets are quite different to supersonic ones, and probably more people are familiar with the supersonic version, it probably doesn't help to say so. thomas 86.159.241.198 (talk) 20:23, 28 April 2014 (UTC)

I agree that the current version has it wrong about the exit duct but then, I don't think the web page you link to is correct about everything either. The idea that the forward section of the outer profile can create thrust is not widely accepted: close examination of the pressure profile over such a body invariably reveals a greater drag force on the leading edge region (where the airflow turns sharply down) which overpowers any small thrust from lower pressure region between it and the point of maximum thickness. What we need are reliable sources. Still, if anybody can meanwhile kill the divergent exhaust story, that would be good. — Cheers, Steelpillow (Talk) 21:41, 28 April 2014 (UTC)
I have now changed the discussion of flow in the rear section. I believe that the accelerated airflow causes a reduced pressure on the duct walls, but I am not sure if that is the correct/only explanation for the forward thrust, so I have left it all a bit vague. — Cheers, Steelpillow (Talk) 09:59, 30 April 2014 (UTC)
As i see it, if the Meredith effect produces positive thrust, that must be explicable in terms of Newton's laws, as a heat engine, and in terms of the forces from the air flow acting on the duct and radiator.
Newton's laws say that, to produce forwards thrust, the system must throw air backwards; there must be more momentum in the wake than up-stream, so i can see why you're identifying the converging exit nozzle where the air expands and accelerates backwards as the place where the thrust is produced. That also fits with the heat engine view: to convert heat into work, the air must be following a pressurise -> heat -> expand cycle, with the expand phase doing more work than was done to pressurise the air.
But if you look at the pressure of the airflow acting on the duct, it's not so clear. Throughout the duct the pressure is above ambient, returning back to ambient only at the end of the exit nozzle. It's above zero everywhere. This implies a rear-wards force on the inside of the converging rear section of the duct and a forwards force only in the diverging front section.
So i think it's the high pressure air in the duct ahead of the radiator that communicates the forwards thrust to the plane, but the existence of that zone of high pressure depends on the convergent exit nozzle returning the flow to ambient pressure. With a cold radiator the forwards force on the front part of the duct is balanced by the rearwards force on the exit nozzle.
Heating the air causes it to expand, and to maintain continuity of flow it must accelerate rearwards, implying an additional pressure drop across the radiator. Unlike the skin-friction component, this additional pressure drop mostly doesn't act on the radiator because the radiator is mostly empty space. But it does reduce the rear-wards forces on the exit nozzle. (It's only if you're looking at the difference that radiator heat makes that you'd see a forwards force on the exit nozzle.)--Moth78 (talk) 13:14, 22 May 2014 (UTC) (thomas)

Huh?[edit]

"Many engineers did not understand the operating principles of the effect. A common mistake was the idea that the air-cooled radial engine would benefit most, because its fins ran hotter than the radiator of a liquid-cooled engine, with the mistake persisting even as late as 1949."

So, WHY is this wrong? The article doesn't say. The description appears to state that all you need to get thrust is to heat the air into a convergent/divergent nozzle. So why can't this be done on a radial?

Maury Markowitz (talk) 14:51, 26 May 2014 (UTC)

The source does not explain why. I believe it is because the important thing is not the temperature but the amount of heat transferred, and that is the same for any engine with a given power output and efficiency. But since that is not sourced, it cannot be said. — Cheers, Steelpillow (Talk) 16:31, 26 May 2014 (UTC)
Just to clarify, it can indeed be done on a radial, but the engine does not benefit MORE than a liquid-cooled type. — Cheers, Steelpillow (Talk) 16:35, 26 May 2014 (UTC)