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downdraft vs updraft cooling

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any one know the advantages of down draft cooling with the modified upper inlets over the updraft cooling with the naca scoop.

Lynn

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I know very little about free air cooling as it relates with aircraft engines but I will try to explain how I think it works. As a refrigeration mechanic my understanding of the concept in downflow cooling - we call it reverse flow cooling in my trade - is that the cold air coming into the air scoop will first make contact with the warmer section of the engine's cylinders, that is the top horizintal sections ( since heat or hot air rises). Then as the partially warmed up air keeps going down it makes contact with the cooler portion of the cylinders, that is the bottom horizontal portion. In this arrangement maximum heat exchange takes place as the coldest air flow contacts the warmest section of the cylinders first resulting in maximum heat transfer.

 

In upflow cooling, the cold air coming in the air scoop makes contact with the cooler bottom portion of the engine's cylinders picking up heat and then flows to the top portion. Because the air has already picked up some heat before it makes contact with the warmer top portion of the cylinders the heat transfer that takes place at the warmest location is less. Hence downflow cooling is preferable when maximum heat exchange is desired. I hope this helps and if my understanding of the concept as it relates to aircraft engines is wrong, I would hope to hear from anyone who reads this post. Thanks.

 

Bruno Caron

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My understanding of downdraft vs. updraft is as follows.

 

With updraft, high speed air is forced to change direction quite abruptly to route it over the cylinders.

 

With downdraft cooling, the air flow is almost a straight line over the cylinders.

 

With my EZ, you can see the cooling fins of the cylinders by looking straight back through the air intakes.

 

We built a plenum around the cylinders, so the air has no where to go but over the cooling fins, and then out the back.

 

If you want a writeup of this system, email me and I can send you a doc file.

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One of the reported advantages of downdraft cooling, is that the engines were designed for that. With updraft, the exhaust pipes and induction systems are in the way of the cooling air, making the journey to the cooling fins more difficult. With downdraft, the air goes directly to the cooling fins, and exits below.

 

Not having built my plane yet, I have no idea which one is better for a pusher application. Tractor aircraft have easy ram air inlets, while the top surface of our planes is theoretically a low-pressure area. Personally, I'm guessing that either system can be good or bad in our applications, depending on the amount of work done to control the airflow under the cowl.

 

-- Len

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The more I look at downdraft installations, the more I'm dissapointed. . . I'm on the way with mine (pic attached), but wont do the transplant for another year or so.

 

There is a massive amount of data (particularly online) out there on inlets, diffusers, plenums, and outlets... There is equal amounts of data concerning air cooled heat exchangers; oil coolers, cylinders, etc. There are NACA reports that descirbe plenum cooling of an A-65 for example, and literary works by Kuchemann & Weber (Aerodynamics of Propulsion, check out http://www.rotaryeng.net/how-to-cool12.html for a snippet), and Kays &London that take the next step beyond Hoerner. Try a trumpet diffuser instead of a straight or straight-walled divergent duct. Try revising exit geometry to re-accelerate exit flow.

 

Don't give up efficiency becuase you see it on someone else aeroplane.

 

Cheers,

 

Wayne

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any one know the advantages of down draft cooling with the modified upper inlets over the updraft cooling with the naca scoop.

Lynn

With updraft/NACA, you have to create a rather complicated sealed plenum below the engine by using extensive baffling and, as well, add ramps in the bottom cowl to force the air up through the forward cylinders. This baffling is a lot of work to fit and troublesome to seal up. Air does not really like to flow into a NACA, so the opening has to be bigger than for a ram opening. You're also generally pushing heat from around the exhaust pipes up through the cylinder fins.

 

With downdraft, the plenum above the engine can be made much smaller and more easily sealed. Ram openings can be fairly small. Less air is used and it is used more efficiently. There are no real good plans AFAIK for how to do a downdraft installation but lots of pictures out there of how various builders have done it, so a resourceful builder can do what all real homebuilders do: steal, copy and/or borrow.

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Wayne, beautiful work. Looks like it was made for a fighter jet.

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Once I converted to downdraft cooling, I had zero issues on my Longez in terms of cooling.

 

Downdraft cooling is extremely easy to baffle. Don't have to baffle around the alternator, starter, or exhaust pipes.

 

I don't have a lot of pictures, but you can see some on my website with the cowls off.

 

I borrowed a lot of ideas from a lot of places. I used George Shell's basic shape for the upper cowl----and like him, I kept the NACA, walled it off, and piped air to the oil cooler, ram air to the carb, cooling to the mags and alt diodes.

 

Unlike George, I did not wall off the two sides of the engine---I tried to keep the baffling simple.

 

I used the Berkut idea of having a nice flat aft baffle----and then built "shelves" to get around the cylinder shape.

 

Also with downdraft cooling is that all the air goes to the top of the engine before it goes thru the engine. In updraft cooling, the air meanders around the bottom of the engine, the wing roots, out the wing roots, etc before it comes up---and by then, you have lost some air.

 

The other "good" with downdraft cooling is that you really know what the cylinder temps are. The Lycoming engines "health data" in regards to temp are based on downdraft cooling with thermocouples plugged into the boss.

 

So am I going to change my cozy to downdraft? Nope. Working fine as is, The Cozy does not lend itself as well to downdraft because the entire engine sits behind the plane---where the Longez engine sticks out wider than the fuselage. It could be done with an S turn---but I am not going to bother.

 

I have seen a couple people get in a little trouble with their downdraft when they build their scoops too close to the fuselage

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I have completed my downdraft mods to primer stage. Will fly this weekend. Two separate carbon plenums/diffusers fed by NACA ducts. Induction and oil are self contained (inlet through throttled exit). Only cowl air is cylinder exit air.

 

Not sure why the picture didn't show up. . .

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That looks the grouse Wayne. Now I know who's NACA downdraft duct picture has been in my 'admire' file for a while now.

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That pic is so great!! Looks very Very snarky...:D How about a pic showing the front sides?

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Thanks for the kind words on my LEZ. Here are some more shots for those that are interested. I'll try and find a couple of shots from the front.

 

cheers

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Wayne, Spod,

 

Are you two making it to Cowra?

 

All going well, I'll be doing my first post downdraft modification test flight tomorrow. That should give me an indication on whether the system is going to operate satisfactorily, albeit at lower ambient temps. I'd like to get confidence time on the system (and engine/airframe) prior to a trip like Cowra, but it's also a great trip to gather test data. I tend to avoid airshows a bit these days as well, but do like to catch up with other builders/flyers. We'll see...

 

Cheers

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Hi Wayne

Beautiful, sure hopes it cools as good as it looks. Fantastic workmanship.

 

Jack

E-Racer Extreme = getting there.

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Thanks mate. I appreciate that.

 

Will be good to see you back in the air in short order as well. Are you building another beautiful ERacer?

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Thanks mate. I appreciate that.

 

Will be good to see you back in the air in short order as well. Are you building another beautiful ERacer?

great looking job on the down draft. when I first flew my IO-360 the down draft worked very good from first flight but the oil cooling was not good until adding the second 9 row cooler. good luck on the first flight .

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Are you two making it to Cowra?

Don't know yet. Woooorrrrrrkkkkkk:sad:

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Don't know yet. Woooorrrrrrkkkkkk:sad:

Ya, me too. I feel myself coming down with something.

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That's OK, as long as you don't feel yourself in public...

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great looking job on the down draft. when I first flew my IO-360 the down draft worked very good from first flight but the oil cooling was not good until adding the second 9 row cooler. good luck on the first flight .

Cheers Lynn. Thanks for the motivation to get this mod done.

 

I decided to feed my induction and oil cooler in a self contained system. I only have the one SW 10599R 9 row. I may need a second unit like yourself. I went with a oblique diffuser to the cooler and mounted it longitudinally. I found this in Aerodynamics of Propulsion by Kuchemann and Weber. The oil outlet is throttled with a butterfly valve on a push-pull line back to the throttle quadrant.

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I never get tired of looking at Wayne's plane

... and I never get tired of looking at your workmanship. Superb.

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