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P-51 type radiator

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Russell

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Nov 29, 2015
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Ft Worth, Texas
Bruce wrote:
"Hi Russell - do you have any pics of your radiator set up that you would like to post? Does it extend up into the fuselage?"



Bruce, the designers of WWII were very good so I mostly copied the P-51 type scoop. About 2/3 of the radiator extends into the baggage compartment. Photo 1 is very early in the design, you are looking into the baggage compartment from the pilot seat area, it shows where I decided to place the radiator. Photo 7 gives a good idea of where it ended up. This is one of our YouTube video and shows some of the design and wind tunnel tests for the scoop. https://youtu.be/njir235mVuI
 

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TXFlyGuy

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Russell - Very impressive! Love that wind tunnel! I'll wager that Titan has never done that thorough testing on their radiator intake. The T-51 scoop appears to be very effective, with no reports of engine over heating that I am aware of. In fact, the complaint most often is running a bit cool. Our radiator exhaust door is manually controlled. We also have our engine oil cooler located in the belly intake.
This is a later generation design of the scoop, being an exact 3/4 scale replica of the P-51.

IMG_0099.JPG

IMG_0718.JPG

P1010355.JPG
 

Russell

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Ft Worth, Texas
Myron, the only time that my temps might go higher than normal is during long taxing with a stiff crosswind. The wind blows the prop blast away from the scoop. When I am able to point the plane into the wind the temps come down.

I also found that I have too much cooling. For drag reduction, when we race or as the OAT drops below 80F we install a sleeve on the front of the scoop that reduces the inlet area by about 20%.
 

pfarber

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Couple of questions if I might ask:

Do you think the 'reversal' of the tufts could have been due to the fin count of the radiator fins being to dense? What fin spacing is the radiator using? Did you use a manometer before/after/inside the radiator/tunnel to see actual pressures??

What is the expansion ratio (inlet to radiator face). I think it needs to be quiet low otherwise you get separation from the walls.

I'll assume you are after the Meredith affect, but why did you decide this over a cowl mounted radiator?

Thank you!
 

brucec995

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Oct 17, 2016
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South Eastern NSW, Australia.
Bruce wrote:
"Hi Russell - do you have any pics of your radiator set up that you would like to post? Does it extend up into the fuselage?"



Bruce, the designers of WWII were very good so I mostly copied the P-51 type scoop. About 2/3 of the radiator extends into the baggage compartment. Photo 1 is very early in the design, you are looking into the baggage compartment from the pilot seat area, it shows where I decided to place the radiator. Photo 7 gives a good idea of where it ended up. This is one of our YouTube video and shows some of the design and wind tunnel tests for the scoop. https://youtu.be/njir235mVuI
Great work Russell! - more ideas for my consideration. I am planning a belly intake and have been considering placing the radiator at an angle (not 90 degrees to the air flow) for a lower profile, but have been reading posts on lowering drag as much as possible and a kayes and London style of setup may not be the most efficient in this regard, though apparently cools well enough. I had a question regarding your temps when on the ground but you have answered that already

Thanks.
 

Russell

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Joined
Nov 29, 2015
Messages
43
Location
Ft Worth, Texas
Couple of questions if I might ask:

Do you think the 'reversal' of the tufts could have been due to the fin count of the radiator fins being to dense? What fin spacing is the radiator using? Did you use a manometer before/after/inside the radiator/tunnel to see actual pressures??

What is the expansion ratio (inlet to radiator face). I think it needs to be quiet low otherwise you get separation from the walls.

I'll assume you are after the Meredith affect, but why did you decide this over a cowl mounted radiator?

Thank you!
Pfarber, my radiator was built by Rod Davis in Arizona. Ron builds a lot of the radiators for race cars like those at Indy. He said that aircraft and racing car demands are similar in that they should have very low drag … I went with his recommendation. The radiator is a two row, single pass with 14 fins per inch and no fin angle.

After entering the scoop, air travels about 3.5 feet before meeting the radiator. I found that air entering the scoop acts much like a garden hose pointed at a wall. The water stays in a tight group until impacting the wall then goes into chaos. Only with the addition of guide vanes was I able to correct most of that. No, I did not measure the pressures. The expansion ratio is about 9.6 to 1.

I have never seen cowl mounted radiators that gave both effective and efficient cooling. Take two of the same aircraft with similar power. Let’s say two RV-7s, one has a Lyc the other has an auto engine with cowl radiators. To qualify under my rules the auto RV must cool well in normal operations, be faster, more economical, and out climb the Lyc RV. This doesn’t mean it can’t be done … just that I have never seen it. That is why I prefer a P-51 scoop.
 

Russell

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Nov 29, 2015
Messages
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Location
Ft Worth, Texas
Great work Russell! - more ideas for my consideration. I am planning a belly intake and have been considering placing the radiator at an angle (not 90 degrees to the air flow) for a lower profile, but have been reading posts on lowering drag as much as possible and a kayes and London style of setup may not be the most efficient in this regard, though apparently cools well enough. I had a question regarding your temps when on the ground but you have answered that already

Thanks.
Bruce, my radiator has about a 10 degree tilt. I now wish I had put it in at 90 degrees. The radiator core is 16 inches X 15 inches X 2.75 inches. With the exception of long taxing in a crosswind, I find I have too much radiator.

I must warn you that building a belly scoop is far faaaaar more difficult and time consuming than cowl mounts … you must decide what is important to you.

This is a fun little video that my wife and I put together about a year ago.
She has come up with a great idea for the next video but broke her leg putting up Christmas decorations ... our next video flight has been delayed for several months
 

pictsidhe

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The key to a good radiator installation is the diffuser. My goto book on diffusers is Japikse and Baines 'Diffuser design technology'. Kays and London describes a fairly simple though probably not very efficient way of ducting to a radiator to give fairly even air flow across the core. It's something that you can probably experiment your way to an acceptable solution.
 

TXFlyGuy

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The key to a good radiator installation is the diffuser. My goto book on diffusers is Japikse and Baines 'Diffuser design technology'. Kays and London describes a fairly simple though probably not very efficient way of ducting to a radiator to give fairly even air flow across the core. It's something that you can probably experiment your way to an acceptable solution.
Assuming North American got their design right, what approach did they take?
Wonder how much wind tunnel testing they did...like Russell?
 

pictsidhe

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It took NAA a few iterations to get their diffuser right. The last was made either after flight testing, or after early models. I forget the details. Building an almost scale P-51 installation will probably need some tweaking. NAA stuffed as much expansion into as small a space as possible. Change something, such as core resistance, it can seperate and not work as expected. It is something you can experiment with, but if it ends up bigger than your allotted airframe space, you have a headache... Splitter vanes can really shorten a diffuser, but need to be done right. Note the lack of contemporary copycats of the P-51 doghouse.
 

Riggerrob

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If you need a longer expansion chamber, you can use the entire aft fuselage, like the 1939 vintage Heston-Napier Racer. The lower half of its aft fuselage is devoted to cooling ducts. It’s cooling air exits from a pair of ports beside the rudder hinge.
 

TXFlyGuy

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With the duct work, and the wind tunnel tests that show a bit of "thrust" gained from the hot exhaust on the P-51, how much if any thrust is gained today from the exhaust?
 

wrmiles

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Oct 13, 2019
Messages
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In my experience in oil cooler installations on turboprop aircraft, the problem was always that the avaiable space and location prevented efficient diffuser and ducting design. I once proposed a P-51 type scoop on the bottom of the nacelle, but it was turned down.
 

Russell

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Location
Ft Worth, Texas
Air enters the P-51 scoop under the wing area of the fuselage. This is a naturally high pressure area that helps push the air in. The air exits where the fuselage is narrowing, this should be a lower pressure area and should help pull air out. The above is one of the reasons that I decided to go with a belly scoop.

I have never tested the above statement and am likely to be wrong … air flow seldom does what I assumed it will do.
 
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