P-51 Mustang family aside, any other good implementations of ventral Meredith radiators, or good ways to do it?

[BarnOwlLover]

We all know that the Mustang probably in the end did it best, and it evolved from the NA-73X prototype though to the P-51H and F-82 Twin Mustang. But excepting those, did any other World War II fighters or recon planes (in service, tested or designed) that did this as well or close to as successfully as the Mustang line? And if good examples can't be found, what would've been good practices for, say, converting the Spitfire design to using it, or improving other fighters with ventral radiators?

 

[tomo pauk]

A lot of fighters was manufactured with ventral radiator, semi-burried, that used Merredith's principle. Soviet and Italian fighters, D.520, Ki-61. They were probably close, drag-wise, to the early radiators on the Mustangs, but not as good as the later, with the pronounced boundary-layer splitter?
Better radiator set-up on the Ki-61 was one of improvements over the Ki-60, later being slower despite being with the much smaller wing (Ki-61 might've have a tad more power, though). Ki-61 aso have had the oil cooler next to the coolant radiator. French supposedly improved the radiator on the D.520 with the boundary-layer splitter-diverter some time after the Armistice.

 

[wuzak]

And if good examples can't be found, what would've been good practices for, say, converting the Spitfire design to using it, or improving other fighters with ventral radiators?

The Spitfire's radiator/radiators were designed with Meredith in mind, just not very well.

 

[BarnOwlLover]

True, but as you said, they didn't do the effect as well as hoped, and even Joe Smith wanted to convert Spitfires to using a ventral Meredith radiator (presumably influenced by the Mustang), but the Air Ministry weren't in favor of it due to concerns over production delays, the Spitfires (late Merlin and Griffon models) were adequate, and the RAF were buying/obtaining Mustangs under Lend Lease.

 

[GrauGeist]

The P-51's ventral radiator was designed to separate from the boundary layer. The KI-61 and MC.202 did not have that feature.

The He100 and Me309 with their retractable radiators surely did not have that advantage.

 

[tomo pauk]

The P-51's ventral radiator was designed to separate from the boundary layer. The KI-61 and MC.202 did not have that feature.

Original radiator on XP-51/Mustang I was changed towards A-36 and Mustang II, and changed again for the Merlin Mustang where the intake gained the prominent distance against the fuselage. So perhaps the early radiators were not as perfect as we often think about them?
I'm also not sure how much the oil radiator on the MC.202 messed the airflow towards the entrance of the coolant radiator.

 

[Shortround6]

A lot of fighters was manufactured with ventral radiator, semi-burried, that used Merredith's principle

There is an awful lot of stuff going on with radiator design. In one old text book I have there is a 10 page chapter just as an over view. "Aircraft Power Plants" by Arthur Fraas

Leaving Meredith aside for the moment the in early radiators were hung out in the air with no duct/fairing. People figured out rather quickly that there 3 problems.
"1. Excessive turbulence and eddy losses in the air stream flowing around the radiator.
2. Turbulence and eddy losses in the airstream passing thought the radiator.
3. High skin friction losses in the air stream passing over the heat-transfer surfaces."

Aircraft designers can be working to address all 3 of these problems without doing anything to try for the Meredith effect. Although some things will certainly help reach the Meredith effect.
I would also note that when Meredith presented his paper in 1934 Ethylene Glycol had only been introduced as a coolant for several years and most liquid cooled engines used water.
Ethylene Gylcol permitted cooling fluid temperatures of 250-280 degrees instead of the 212 degrees (max) of water and the difference in heat transfer between the engine and the liquid and the liquid and cooling air allowed for a 70% reduction in radiator size.
Meredith's paper was partially about using ethylene glycol in radiators in ducts.

There was an example in the book using the P-40 (long nose) which goes to item #3. The book claims that the radiators in the P-40 had 147 square feet of cooling surface which was about 30% of the wetted area of the wing (top and bottom) which is a considerable source of drag right there, assuming that the speed of the air flowing through the radiator cores was same speed as the air flowing over the wing. Power losses in the radiator vary with the cube of the speed of the cooling airflow. If you use twice the size of the radiator and can cut the speed of the air flowing through the radiator to 1/2 you will have a drag of 1/8 of the original. You will also need to double the weight of the radiator (and size of the duct at that point) so a compromise maybe/will be reached.


Boundary layer splitters are important to point #2. They help eliminate some of the turbulence and eddies and help make sure the air flowing into the radiator is closer to the same speed and going in the same direction. This is beneficial to the Meredith effect but it is also beneficial to just cooling the engine with the smallest radiator/radiator drag possible.

Point #1 is pretty much self evident.

Meredith also proposed routing the exhaust gasses into the radiator duct (after the actual radiator) to greatly increase the heat of the cooling airflow as it exited the duct for a large increase in thrust. I don't know it this was tried but it never saw service.

You can have a well designed, low drag radiator installation that is not trying to actually use the Meredith effect. It took a while but it turns out the Meredith effect was not that easy to actually get, at least to point of actually getting positive thrust and not just lower drag. You do need a long and rather sizable duct to get it work. Which has weight and drag penalties of it's own.

 

[BarnOwlLover]

That's why I think that in terms of aero on like the P-51/F-82 and the DH Mosquito/Hornet, it wasn't just the Meredith effect that gave the performance, it was a combination of things. Same with the laminar flow wing. It's not a single magic bullet, but it adds up. The perfect laminar flow wing and Meredith effect is probably unachievable, but a net gain is still a net gain and is better than nothing.

Also look at the Hawker Tempest V vs the Tempest II. Even though the Bristol Centaurus radial engine was wider and heavier, it's installation was cleaner (no chin radiator) and, also with out the radiator, the Tempest II was only about 20 lbs heavier than the Tempest V. The cleaned up aero was largely responsible for the increased top speed (as well as any horsepower gains from the Centaurus) of 442 mph vs 435 mph.

Also, even though Joe Smith wanted to modify the Spitfire to use a Mustang-type radiator, there was a report that some simple detail upgrades to the Spitfire IX could lead it to have performance similar to the Merlin Mustangs (P-51B/C/D/K). The Mustang X had a top speed of 433 mph, without a fully optimized engine installation that would come starting with the XP-78/XP-51B, boosting speed to 440 mph. Also, the original Mosquito prototype when it tested Merlin 60 series engines went from 392 mph to 437 mph.

So the big gains there was the increased power, though the aero also helped big time in the end, and also shows perhaps how off in aero the Spitfire was, though a lot of that (from the same report) was due to manufacturing practices/fit and finish.

 

[wingnuts]

The CAC CA-15 used a ventral radiator, but it only made it to the prototype stage as some idiot had invented the jet engine.

 

[tomo pauk]

Some differences between Ki-60 and Ki-61 respective fuselages can be seen here, including the cooling systems' layout. Note the much taller radiator set-up for the Ki-61, with the greater percentage being buried in the fuselage, also the sheet metal directing the airflow. Also note that oil cooler is relocated and it shares the 'box' with coolant radiator.
Ki-60 also had a taller fuselage (making it a better candidate for a seamless installation of a radial engine...).
All of these improvements made possible for the Ki-61 to be faster, despite it's bigger wing.

 

[Shortround6]

Look at the CAC CA-15 in post #9. Granted it is trying to cool a two stage Griffon engine and is trying to make thrust at close to 450mph.

However it uses a considerable amount of length to slow the air down (expand it), get it through the radiator/s and then compress the air down and accelerate up to the speed needed to exit. I just find it hard to believe it could be done in 1/3 to 1/2 the length. Compare that duct to the pair of ducts under a Griffon Spitfire for length.

That is what the Meredith effect calls for. Not just reducing the drag created by the radiator and it's airflow but actually creating positive thrust.
Now somebody/designer can try for positive thrust and miss by a small amount but using short duct has got some real problems.

At 350mph the air is entering the duct at 513ft/sec ( 165m/s) it has to be slowed down, sent through through the radiator, pick up heat, expand and then be compressed down in volume and accelerated back up to a speed higher than the intake speed. A longer duct gives a smoother airflow and more time for it to happen. The air can spend twice time (or more) inside the duct while those changes happen even if the air is flowing at the same or similar speeds.

 

[GrauGeist]

If you look at the proposed KI-62, you'll see that the engineers were trying to address the cooling duct issue with a more streamlined design.

How it would have performed, is anyone's guess, but it does appear to present less drag than the cooling system on the KI-60/KI-61.

 

[Shortround6]

Back in the 1960s/early 70s the US car companies were trying to fit "ram air" to some of their hi-performance cars. Might have been good marketing but the actual benefits tended to vary. Part of the problem was that street cars weren't all that fast, you weren't going to get much "ram air" until you were going well over the legal speed limit.

Another thing is that you had two different effects going on.
Buick Grand sport was one of the worst.

Yes the chrome "vents" in the hood were able to open from the inside.
Chevy had "cowl induction" scoop face the windshield.

Oldsmobile started with 2 huge scoops under the bumper and later (because of marketing) went for two large scoops on the hood.

Note the two scoops under the headlights.
All were GM cars.

You did have two things going on as I mentioned. One was the the underhood air was around 120-140 degrees in normal car as the air intakes for the carb were sucking in the air after it had gone through the radiator so just sucking in cool air helped HP wither or not the car was actually moving. On the Buick it didn't too long before somebody figured out that the those low scoops were actual in a boundary effect area (a lot of turbulence coming off the flat nose of the care and over the hood) and the fact that the air pressure flowing over the hood was actually lower than normal atmospheric pressure. Cold air boost yes, ram air................NO. In fact it may have lowered the air pressure.

Chevy got the cold air boost and what they tried to capture was that after the air ran over the hood it hit the windshield had made a high pressure area at the base of the windshield and Chevy was trying to tap into that high pressure area without using a giant hood scoop. Faster the Chevy went the more effect they might have gotten.

Olds put two huge scoops in a high pressure area on the nose of the car It probably gave the most benefit but it could not be seen from a 100 yds away and was changed by the marketing guys.

Now measuring performance gain was a bit tricky as the popular magazines just measured 1/4 mile standing start times. How much gain was just providing 70-90 degree induction air and how much was due to a fraction of inch boost in intake pressure which changes with the speed of the car?
They were ALL "ram air" systems (or so the advertising men said) but there were some significate differences.

 

[tomo pauk]

If you look at the proposed KI-62, you'll see that the engineers were trying to address the cooling duct issue with a more streamlined design.

Different companies have different views on how some things need to be adressed. Even within the same company, there were attempts on improving the cooling system(s). MTT changed the cooling system on Bf 109 from E to F, and then again from F to G (if we discard the pre-Emil 109s, and forget the oil coolers for a moment). Spitfire was also a subject of changes. P-51 went through 4 different cooling systems.
How much the Ki-62 actually succeeded to decrease the cooling drag vs. for example the Ki-61 is nowhere quantified to the best of my knowleddge. We even don't know if it succeeded in that intention. Macchi tried the similar set-up on MC.202, their fighters still kept the belly radiator anyway.

How it would have performed, is anyone's guess, but it does appear to present less drag than the cooling system on the KI-60/KI-61.

It does?
Ki-60 and Ki-61 have had different cooling systems between the two.

 

[Shortround6]

and then again from F to G

Well, they went for the boundary layer splitter on the F over E, duct the boundary layer up and over the radiator core. Which is more elegant/efficient.
Then with the G they had choice, they needed a bigger core to cool the engine. Keep the boundary layer duct and increase the frontal area of the radiator (make it deeper/lower or wider) or get of the duct and raise the radiator higher into the wing. A bit less efficient using the boundary layer but a trade off vs the larger size duct/frontal area?

 

[tomo pauk]

Well, they went for the boundary layer splitter on the F over E, duct the boundary layer up and over the radiator core. Which is more elegant/efficient.

The radiators on the F were also wider and less tall, meaning that greater percentage of the radiator's frontal area can be buried in the wing. Difference was also in the exit flaps, that were probably less draggy than the single flap on the E.

Then with the G they had choice, they needed a bigger core to cool the engine. Keep the boundary layer duct and increase the frontal area of the radiator (make it deeper/lower or wider) or get of the duct and raise the radiator higher into the wing. A bit less efficient using the boundary layer but a trade off vs the larger size duct/frontal area?

All of that, plus perhaps the boundary layer duct was not as efficient as the designers thought it will be?

 

[Shortround6]

All of that, plus perhaps the boundary layer duct was not as efficient as the designers thought it will be?

Could be.

Ducting the boundary layer through that distance and with a few bends in way might have meant more drag than they thought.
Every bend = drag
every cm of duct length = drag

 

[GrauGeist]

Ki-60 and Ki-61 have had different cooling systems between the two.

The KI-60 had a large and long radiator housing, the KI-61's radiator housing was shortened, but still bulky.

The KI-62's radiator was to have been moved forward of the cockpit, in a streamlined housing with an adjustable exit flap.

 

[tomo pauk]

The KI-62's radiator was to have been moved forward of the cockpit, in a streamlined housing with an adjustable exit flap.

There was an exit flap on the Ki-61, too. Also housed in a streamlined housing.

 

[Shortround6]

See post #11. the flap adjusts.
Just about every liquid cooled engine from the early 30s on had an adjustable flap on the cooling system. Many WW I liquid cooled engines had them too.

Tomo has faster fingers.