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View Full Version : P-51 Radiator = Thrust



dangerlaef
07-09-2008, 08:37 PM
Hi folks,
I assume I lot of you know about the P-51s radiator actually creating a jet thrust effect,
known as the "Meredith Effect".

In game, it seems everyone recommends closing the radiator for top speed.

So am I right in thinking the P-51s radiator thrust is not modelled?

dangerlaef
07-09-2008, 08:37 PM
Hi folks,
I assume I lot of you know about the P-51s radiator actually creating a jet thrust effect,
known as the "Meredith Effect".

In game, it seems everyone recommends closing the radiator for top speed.

So am I right in thinking the P-51s radiator thrust is not modelled?

VW-IceFire
07-09-2008, 08:59 PM
Oi...this one could get long. We had multipage arguments about this a few years back. Until it finally got worked out.

Long story short...the Meredith Effect decreases the drag of the radiator but it does not cancel it out. Its still a cleaner airframe with the radiator port closed just like any other airplane. Its just that when open the drag is slightly less because of forward thinking design. There's a reason that North American managed to get Spitfire XIV levels of top speed and overall performance without the added 500 horsepower.

GH_Klingstroem
07-09-2008, 09:02 PM
However the top speed decreases as much for the p51 as for any other plane in game when the radiator is open! A loss of ca 25km/h

M_Gunz
07-09-2008, 11:55 PM
That radiator gave some thrust but less than the drag. The outgoing air did push, so 'thrust'.
But it was only effective so much at high speeds and automatic operation. The rear gate did
constrict the flow to make it work, at least from what the more technical descriptions said.

drag - thrust = less net drag than otherwise.

Blutarski2004
07-10-2008, 04:56 AM
There was a lengthy article in Flight Journal [IIRC] on the influence of the Meredith Effect radiator upon the Mustang's performance. It was pretty dramatic compared to conventional radiator design. Whereas a conventional radiator might produce a net 400 lbs of drag, the drag cost of the Meredith design was something like 100 lbs.

I'm guessing that was one of the important factors in the Mustang's very high cruising speed.

KIMURA
07-10-2008, 05:49 AM
the difference between expected(calculated)drag and effective drag means thrust. The exhausting warm air fro mthe coller is slower than a/c acuatlly flies, so to speak of thrust is a bit questionable. http://forums.ubi.com/images/smilies/16x16_smiley-indifferent.gif

Aaron_GT
07-10-2008, 07:04 AM
The ultimate thrust from the P-51 was a P-51A the RAE modified by putting a series of small ramjet tubes in the rear of the radiator enclosure. Such a system was originally suggested as a ventral pack for the Spitfire I to boost the speed in 1940, but development time meant that it was not possible in that timeframe.

Kurfurst__
07-10-2008, 08:21 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Blutarski2004:
There was a lengthy article in Flight Journal [IIRC] on the influence of the Meredith Effect radiator upon the Mustang's performance. It was pretty dramatic compared to conventional radiator design. Whereas a conventional radiator might produce a net 400 lbs of drag, the drag cost of the Meredith design was something like 100 lbs.

I'm guessing that was one of the important factors in the Mustang's very high cruising speed. </div></BLOCKQUOTE>

What constitutes is a 'conventional radiator design'...? WW1 radiators or?

I do not think there was anything particularly unique in the Mustang radiator internal layout compared to other mainstay fighters of the era. They all generate a certain amount of thrust.

Meredith Effect Radiator sounds like to me as Elliptical Planform Wing. Or Hamilton Standard Propeller. Or Supercharged Engine. All with capitals suggesting that it something special about it, unique only to this and that aircraft, a sort of built-in-magic-wamd when this was not the case. The same technologies were used with varying degree of success/expertise.

Blutarski2004
07-10-2008, 08:35 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by KIMURA:
the difference between expected(calculated)drag and effective drag means thrust. The exhausting warm air fro mthe coller is slower than a/c acuatlly flies, so to speak of thrust is a bit questionable. http://forums.ubi.com/images/smilies/16x16_smiley-indifferent.gif </div></BLOCKQUOTE>


..... True in conventional radiator design, but the Flight Journal article insists that effective thrust WAS produced - not enough to cancel radiator drag altogether, but sufficient to substantially reduce its net effect.

Blutarski2004
07-10-2008, 08:44 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kurfurst__:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Blutarski2004:
There was a lengthy article in Flight Journal [IIRC] on the influence of the Meredith Effect radiator upon the Mustang's performance. It was pretty dramatic compared to conventional radiator design. Whereas a conventional radiator might produce a net 400 lbs of drag, the drag cost of the Meredith design was something like 100 lbs.

I'm guessing that was one of the important factors in the Mustang's very high cruising speed. </div></BLOCKQUOTE>

What constitutes is a 'conventional radiator design'...? WW1 radiators or?

I do not think there was anything particularly unique in the Mustang radiator internal layout compared to other mainstay fighters of the era. They all generate a certain amount of thrust.

Meredith Effect Radiator sounds like to me as Elliptical Planform Wing. Or Hamilton Standard Propeller. Or Supercharged Engine. All with capitals suggesting that it something special about it, unique only to this and that aircraft, a sort of built-in-magic-wamd when this was not the case. The same technologies were used with varying degree of success/expertise. </div></BLOCKQUOTE>


..... There is nothing about a radiator system based upon the Meredith Effect principle that makes it unique to the Mustang. I never said there was.

The same can be said for an elliptical planform wing; it reflects a basic aerodynamic principle used in many aircraft designs. It was not unique to the Spitfire. Likewise for leading edge slats - a basic piece of useful aviation technology not unique to the Bf109.

What's your point?????

Bremspropeller
07-10-2008, 08:49 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">..... True in conventional radiator design, but the Flight Journal article insists that effective thrust WAS produced - not enough to cancel radiator drag altogether, but sufficient to substantially reduce its net effect. </div></BLOCKQUOTE>


Yeah, b/c the air was heated, thus expanded and produced sort of thrust.

Yet the device's drag was still grater than it's thrust. They'd only minimize drag, not produce effective thrust on top of the prop's thrust.

Blutarski2004
07-10-2008, 09:08 AM
If anyone is interested in an engineer's analysis of the influence of the Meredith Effect radiator design on the Mustang's performance, I recommend the following article which appeared in the Jun 1999 issue of Flight Journal: "An Engineer's Perspective on the Mustang" by Atwood J Leland, an engineer who worked on the design of the P51:

http://findarticles.com/p/articles/mi_qa3897/is_199906/...829?tag=artBody;col1 (http://findarticles.com/p/articles/mi_qa3897/is_199906/ai_n8870829?tag=artBody;col1)

excerpts -

"I therefore offered Mr. Thomas sketches and other descriptions of a Mustang design that had the main radiator in the rear of the fuselage. The alternatives were wing radiators such as those used on the Spitfire and the Bf 109, and under-engine radiators such the P-40's; both positions limited radiator size and the length and size of the ducting that could be used to handle and control the cooling air.

&lt;snip&gt;

... the Mustang's cooling drag was much lower. This was the result of using a ducted radiator with a large area and a slow-speed airflow through it (Pr and P2); closing up the exit and creating a backpressure restored the momentum of the cooling of air (momentum lost in radiator transit). This was possible because of the radiator's cooling capability, which, to be adequate in a full-power climb, was much more than that required at high speed and high dynamic pressure. According to calculations given in a supporting paper, the drag created by momentum loss in passing through the radiator can be reduced from some 400 pounds to close to 30 to 40 pounds because of the offsetting momentum of the jet thrust from the radiator exit (V2)."


The author also provides a good analysis on the true contribution of the Mustang's laminar flow wing design. It did not deliver a reduction in drag, as intended, but an increase in critical mach which endowed the Mustang with its excellent high speed dive performance.

Kurfurst__
07-10-2008, 09:59 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Blutarski2004:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kurfurst__:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Blutarski2004:
There was a lengthy article in Flight Journal [IIRC] on the influence of the Meredith Effect radiator upon the Mustang's performance. It was pretty dramatic compared to conventional radiator design. Whereas a conventional radiator might produce a net 400 lbs of drag, the drag cost of the Meredith design was something like 100 lbs.

I'm guessing that was one of the important factors in the Mustang's very high cruising speed. </div></BLOCKQUOTE>

What constitutes is a 'conventional radiator design'...? WW1 radiators or?

I do not think there was anything particularly unique in the Mustang radiator internal layout compared to other mainstay fighters of the era. They all generate a certain amount of thrust.

Meredith Effect Radiator sounds like to me as Elliptical Planform Wing. Or Hamilton Standard Propeller. Or Supercharged Engine. All with capitals suggesting that it something special about it, unique only to this and that aircraft, a sort of built-in-magic-wamd when this was not the case. The same technologies were used with varying degree of success/expertise. </div></BLOCKQUOTE>


..... There is nothing about a radiator system based upon the Meredith Effect principle that makes it unique to the Mustang. I never said there was. Varying degree of thrust was produced by ducted radiators on WW2 fighters/bombers/etc., but there appears to be general consensus that none of them were capable of producing net thrust.

The same can be said for an elliptical planform wing; it reflects a basic aerodynamic principle used in many aircraft designs. It was not unique to the Spitfire. Likewise for leading edge slats - a basic piece of useful aviation technology not unique to the Bf109.

What's your point????? </div></BLOCKQUOTE>

Its just noting that what you have noted above in this post, ie. that the Junkers ducted radiator (or other aviation technology mentioned) was not particularly unique to single aircraft. Today some tend to over-emphasize the importance of ordinary technical solutions as if were some sort of unique 'special power' possessed by an aircraft type. There are plenty of examples of that in the literature.

M_Gunz
07-10-2008, 10:43 AM
Yeah there's people who like to magnify on different details as if somehow that changes the
performance figures. We've seen a lot of that done on this forum in years gone past.

One detail of the P-51 radiator efficiency article that many seem to miss is that the real
savings only comes about at high speeds. You don't get that lower drag in regular flight.
Determining P-51 drag coefficient at top speed, it won't be the same under 400mph for example.

Blutarski2004
07-10-2008, 11:15 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kurfurst__:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Blutarski2004:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kurfurst__:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Blutarski2004:
There was a lengthy article in Flight Journal [IIRC] on the influence of the Meredith Effect radiator upon the Mustang's performance. It was pretty dramatic compared to conventional radiator design. Whereas a conventional radiator might produce a net 400 lbs of drag, the drag cost of the Meredith design was something like 100 lbs.

I'm guessing that was one of the important factors in the Mustang's very high cruising speed. </div></BLOCKQUOTE>

What constitutes is a 'conventional radiator design'...? WW1 radiators or?

I do not think there was anything particularly unique in the Mustang radiator internal layout compared to other mainstay fighters of the era. They all generate a certain amount of thrust.

Meredith Effect Radiator sounds like to me as Elliptical Planform Wing. Or Hamilton Standard Propeller. Or Supercharged Engine. All with capitals suggesting that it something special about it, unique only to this and that aircraft, a sort of built-in-magic-wamd when this was not the case. The same technologies were used with varying degree of success/expertise. </div></BLOCKQUOTE>


..... There is nothing about a radiator system based upon the Meredith Effect principle that makes it unique to the Mustang. I never said there was. Varying degree of thrust was produced by ducted radiators on WW2 fighters/bombers/etc., but there appears to be general consensus that none of them were capable of producing net thrust.

The same can be said for an elliptical planform wing; it reflects a basic aerodynamic principle used in many aircraft designs. It was not unique to the Spitfire. Likewise for leading edge slats - a basic piece of useful aviation technology not unique to the Bf109.

What's your point????? </div></BLOCKQUOTE>

Its just noting that what you have noted above in this post, ie. that the Junkers ducted radiator (or other aviation technology mentioned) was not particularly unique to single aircraft. Today some tend to over-emphasize the importance of ordinary technical solutions as if were some sort of unique 'special power' possessed by an aircraft type. There are plenty of examples of that in the literature. </div></BLOCKQUOTE>


..... For the sake of good order, let me point out that the quote you attributed to me in the above post differs from what I actually wrote. Perhaps it was a pasting error. My actual statement was as follows -


quote -

..... There is nothing about a radiator system based upon the Meredith Effect principle that makes it unique to the Mustang. I never said there was.

The same can be said for an elliptical planform wing; it reflects a basic aerodynamic principle used in many aircraft designs. It was not unique to the Spitfire. Likewise for leading edge slats - a basic piece of useful aviation technology not unique to the Bf109.

What's your point?????

- unquote

DmdSeeker
07-10-2008, 02:47 PM
While carefull radiator design certainly minimised and to a certain extent offset the drag caused by the cooling system, carefull exhaust design for inline engines made a much larget thrust effect.

As far as I know, no radial engine exhaust was designed in to produce any kind of exhaust thrust.

DKoor
07-10-2008, 03:40 PM
P-51 is the best, it won the war.

KrashanTopolova
07-10-2008, 06:10 PM
Now this is my kind of topic!

I remember someone claiming on here some time ago that a plane could take off from a conveyor belt...

anyway,

the radiator thrust-drag argument is a contradiction in terms (or maybe a contradistinction of berms): there is only useful thrust when all drag is overcome by thrust.
Similarly, there is only added thrust when total thrust becomes positive.

a radiator on an aircraft cannot produce thrust nor can it add to it. It can only ameliorate drag if designed appropriately.

Care needs to be taken over the term 'jet'. Could it be that a jet - stream weather pattern results from an air mass moving between areas of different pressure. While a 'jet thrust' is produced by artificially compressing an air column as it passes resulting in two areas of different pressure (in front and behind).
A 'jet' however is a result of that one and same singular process.

The Mustang radiator did not produce a jet thrust. It was a jet created by the radiator which ameliorated the loss of aircraft momentum caused by airflow passing throught the radiator; as noted in one the posts above.

heywooood
07-10-2008, 06:25 PM
ameliaerharterate????

..she disappeared in flight too...eh

Kettenhunde
07-11-2008, 04:38 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> As far as I know, no radial engine exhaust was designed in to produce any kind of exhaust thrust. </div></BLOCKQUOTE>

Exhaust thrust was a well known phenomenon. Radial Engines used it too.

All the Best,

Crumpp

DmdSeeker
07-11-2008, 04:55 AM
Of course exhaust thrust was known of, you'd only have to stand behind a '20's Le Mans racer to know it http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

However, inlines lend them selves to unrestricted ejector style exhaust stubs, while _as far as I know_ radials seem to use a collector system.

Can you mention any radials where exhaust thrust was exploited?

I seem to remember a figure of 400 lbs thrust being posted for a packard Mustang at full chat, but that seems quite a bit over the top to me.

luftluuver
07-11-2008, 05:23 AM
The A6M5 went to ejector exhaust from collector exhaust of previous models.

Kettenhunde
07-11-2008, 06:18 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Can you mention any radials where exhaust thrust was exploited?
</div></BLOCKQUOTE>

How about the one we are spending several hundred thousand dollars on and own some 28 examples to get three running?

The BMW801 series...

DmdSeeker
07-11-2008, 01:36 PM
The Fw gills produced thrust? Cool, I never knew that!

Any idea if the effect was measured?

I've heard it say (but don't really know) that Merlin exhausts went from being design for ultimate motor efficiency (terrestial racing thinking) to being deliberately designed for thrust at the possible expense of ultimate horse power because the effect was sufficient enough to warrent it.

I wonder what a difference the 60's east german research into two stroke expansion exhausts could have made if it hadn't had to compete with jets http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

M_Gunz
07-11-2008, 01:43 PM
Henri Coanda had a short lived piston engine with augmented exhaust thrust plane.
Then it caught fire from that exhaust during takeoff....
And there was a Caproni, IIRC, that was a piston driven jet that at least did fly.

Aaron_GT
07-11-2008, 03:53 PM
Just a bit more on what M_Gunz said.

In the classic (Whittle) jet the compressor at the front of the engine is driven by a turbine in the exhaust flow. In the Italian engine, though, the compressor was driven by a reciprocating engine. There have been other engines of a compound nature, e.g. the turborocket which is a ramjet with auxiliary compression for low speed driven by a rocket motor exhaust.

The Italian engine had the advantage that you weren't relying on the feedback between the turbine to compressor being exactly right to keep the engine going - the early jets were prone to runaway, flameout, etc. if the feedback got out of kilter. With a compound engine with the compressor driven by a reciprocating engine you could simply change the throttle settings on the reciprocating engine to change the compressor settings to suit the fuel flow and airspeed.

Kettenhunde
07-11-2008, 04:59 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> The Fw gills produced thrust? </div></BLOCKQUOTE>

The cowl flaps are part of the cooling system not the engine exhaust system.

http://www.white1foundation.org/parts/cowlflaps18.jpg

AFAIK they produce no significant thrust.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> I've heard it say (but don't really know) that Merlin exhausts went from being design for ultimate motor efficiency (terrestial racing thinking) to being deliberately designed for thrust at the possible expense of ultimate horse power because the effect was sufficient enough to warrent it.
</div></BLOCKQUOTE>

Exhaust thrust is significant especially at high velocity and high altitudes.

Look at it like this:

Thrust at altitude is a function of density ratio to the power of an experimentally determined exponent. Many references use X=1 and some use X=.8.

This means that a WWII era jet engine producing 1600lbs of thrust at sea level is able to produce some 716lbs of thrust at 25,000ft.

The BMW801D2 produces ~264lbs of exhaust thrust at 25,000ft. This means the engine is producing some 37% of the thrust a WWII jet engine is making at that altitude from the exhaust alone.

Another way to look at it is than in the FW190 at 25,000ft, 25% of the thrust at top level speed comes from the exhaust.

That is rather simple explanation but you get the trend and the idea.

All the best,

Crumpp

KrashanTopolova
07-11-2008, 06:04 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kettenhunde:


Another way to look at it is than in the FW190 at 25,000ft, 25% of the thrust at top level speed comes from the exhaust.

Sounds intuitively true. at the thin air of altitude and the declining efficiency of the prop at highest speed (rpm) exhaust thrust would be the only thrust able to add anything. And at the margin this would be significant. The prop is also an airfoil subject to the effect of thinner air besides having an inherent limitation in efficiency (which the jet engine does not suffer from).

Crumpp </div></BLOCKQUOTE>