Hangar Flying => Hangar Flying => Topic started by: vonjet on March 17, 2012, 08:21:13 PM
Title: Downdraft vs updraft cooling
Post by: vonjet on March 17, 2012, 08:21:13 PM
Which is more effecient any why? Speed gains from either?
Title: Re: Downdraft vs updraft cooling
Post by: easyrider on March 19, 2012, 02:23:41 PM
Heat rises I am a great believer in BKB (Burt Knows Best) The NACA inlet on my LongEZ works fine, why change? Easyrider
Title: Re: Downdraft vs updraft cooling
Post by: LongEZDaveA on March 19, 2012, 04:36:33 PM
The updraft works fine and I don't think I'll ever change mine, BUT I don't like that the air has to go by the hot headers first to get to the cylinders and the CHT probes are on the cool side of the cylinder. If I had it to do again, I'd probably go down draft.
Title: Re: Downdraft vs updraft cooling
Post by: Bruce Hughes on March 19, 2012, 07:03:37 PM
If I knew what I know now, I would have gone with downdraft. I think the front of the upper cowling adds drag, air flow through the oil cooler could be arranged better, and the back of the lower cowling around the alternator and starter would give less drag. Don't know; I am NOT an engineer.
Bruce Hughes :)
Title: Re: Downdraft vs updraft cooling
Post by: Bill James on March 19, 2012, 10:51:59 PM
That was my reasoning Dave, cooling air having to go up through the exhausts with my initial updraft armpit installation. Early on, working with the plans-baffling and listening to others talk about how much they had done with it still not working, it became overwhelming to think about what it would take to adequately seal the cowl area. As I thought through the plenums and DD flow, the details all fell in to place. It may seem that I went to a lot of extra work to go to DD with fully enclosed cylinders in upper and lower plenums, but to me it was a smaller job than sealing the entire cowl, especially with some other things i wanted to try. And DD was the early combination that worked for my plane. That’s what worked earliest and best. Lots of folks have great cooling with the stock system. And some don’t. After testing the DD/plenums cooling for a couple of years, flying wide open at 2000 ft on 100 degree days, and climbing hard to 10K feet, and I couldn’t overheat it, I realized that it worked too well during normal flying, so I closed down the inlet size several times. Even after the downsizing i cant overheat it. I am happy for anyone that has good cooling, however they do it. The primary cooling work in my installation is the draw of the augmented cooling; using exhaust air to pull air through the cylinders. While the plenums fit well, I have observed that in my installation the seal of the plenums doesn’t have to be perfect because all of the air is being sucked out with the exhaust. The inlets work well, but it doesn’t really matter if some air is pulled into the plenum from inside the cowl. That reminds me of several other observations... Some random thoughts, getting randomer :) Bill
Title: Re: Downdraft vs updraft cooling
Post by: Bob Holliston on March 20, 2012, 06:55:56 PM
I like updraft cooling because (among other things) that cooling air hits the cylinder right where it needs it the most - the area around the exhaust port.Cylinders seldom crack anywhere else. Also, I've never actually seen a fast downdraft cooled EZ and I raced R.A.C.E. races for seven years. That one updraft opening does it all: Cools the cylinders, cools the oil AND provides ram air to the intake. It cools best when climbing and least when decending. When Klaus (who has the distinction of owning the fastest Variez AND the fastest LongEZ on this planet switches to downdraft so will I.
Title: Re: Downdraft vs updraft cooling
Post by: timdoell on March 20, 2012, 08:17:21 PM
Don't forget the ability to prevent shock cylinder cooling by slightly lowering the speed brake to spoil air over the NACALA on rapid decent
Title: Re: Downdraft vs updraft cooling
Post by: easyrider on March 22, 2012, 10:37:18 AM
I have found that cyl.head temps increase in climb with my updraught NACA setup I ramp climb,1k" at at a time. This keeps the temps reasonable Once in level flight the cht's are all within 20F Easy rider
Title: Re: Downdraft vs updraft cooling
Post by: dorr on April 20, 2012, 02:53:16 PM
When we had guys convert to O-320 engines we had several detailed comparisons available from them. Same with 4 pipe exhaust and ram air boxes. We then just about all outperformed Rutan's original design by 30 mph with those additions. But with bigger engines there was a need for good cooling. We moved the oil cooler to the back under the starter and angled toward the intake and got better oil cooling, but it was baffling that was key to cht - good tight baffles. Nobody should need to stair step at all in climb. Berkut went to arm-pit cooling, which, with interior baffles to direct the air at the cylinders has proven to be fool proof. But with the Injected O-360 though a large oil cooler is key.
What I haven't yet seen from any Downdraft guy is the proof - in converting to Downdraft is it faster with just that change - that's key information. Our updraft O-320s were doing 210 to 230 mph flat out at 7000'. And a few years back we had a lot of photos of very bad downdraft intake designs - lots of drag added out front of the top cowl. I might be able to find the photos of good and bad intake designs if you are doing downdraft cooling and hit me off-line.
Beagle canardfinder at att dot net
Title: Re: Downdraft vs updraft cooling
Post by: easyrider on April 20, 2012, 06:35:22 PM
I posted that I stair step climb and it works. My front end cylinders get hotter in climb(3&4) When I level out they drop quickly and stay at about 360-380F in cruise What do you think I should do to keep temps down in climb? Easyrider
Title: Re: Downdraft vs updraft cooling
Post by: LongEZDaveA on April 20, 2012, 08:07:40 PM
What do you think I should do to keep temps down in climb? Easyrider
Do you have a diffusor? When you run your hand along the NACA as it enters the cowling, you should feel a smooth surface that expands into the cowling. There should be little or no abrupt changes, only smooth expanding curves entering the cowl.
You have to seal as much of the leaks in you baffling as possible. You want as few paths for the air as possible that don't go through cooling fins or oil cooler or into the engine induction system.
Your cylinder baffles need to wrap around the cylinders, such that the air has to go into a small area at the bottom of the cylinders and follow the cooling fins around the cylinders to a slightly larger area at the top. The best way I know how to do this is with the silicone/bid baffles. They are wrapped around the cylinders, touching the ends of the cooling fins, forcing the air to travel from the bottom of the cylinders to the top before being released. If there are any gaps between your baffles that wrap around the cylinders and the ends of the cooling fins, the air will go through the gaps and not cool the fins.
I have the Herb Sanders, Sport Flight NACA inlet that is about 3" x 15". It is larger than I need for my O-235 and if I would improve my #4 cooling, I could go to a smaller inlet and pick up a little more speed. Some day....
One thing that I don't hear discussed much is the affect of drag reduction on engine cooling. Anything that you do to decrease drag, increases engine cooling. If the engine puts out X amount of HP, and you get Y airspeed, you will get Z CHTs. If you decrease the drag, such that you get more than Y airspeed at the same X HP, that provides more cooling air with the same amount of heating from the engine. Put well designed gear leg fairings on, change from football pants to pressure recovery (Klaus/CSA), seal your canopy/airframe leaks, etc. It all adds up to more cooling airspeed for the same engine power/BTUs, which results in lower CHTs.
With this approach, I'm able to climb continuously as high as I care to at 125 MPH indicated, with no more than 375 F CHT on my hottest cylinder (#4). When I race at WOT and low altitude, I am able to keep the cylinders no higher than 375 F also. My cylinder temps are terribly unbalanced and are nowhere near your 20 degree spread.
Title: Re: Downdraft vs updraft cooling
Post by: Bill James on April 21, 2012, 02:31:09 PM
Which is more effecient any why? Speed gains from either?
So. How do you like your answers? :) I like 'em. I remember two experiences. "There I was," standing on the ramp filled with 80+ canards, looking at a very crude airplane. He and his wife were preparing to depart. During the weekend most folks gave it a wide berth, pretending not to have noticed it. The previous evening, the builder/pilot had received the CSA 2000 hour patch. He said that he had landed in 48 states. He had started out building with two partners. They were still building.
"There I was," standing on the ramp at Truckee, having flown 1400 miles nonstop from Fort Worth. Another pilot was showing me his well-used Eze. For years I had envied him from afar as one of the really active guys. They had all started 15 years before me and I was glad to finally meet him because of seeing his name in the CPs. His plane didn’t have wheel pants. He looked at my plane and then his, and said that he had never made a flight longer than 300 miles, or leaned the engine...
They were having all the fun, with pretty bacsic airplanes.
Title: Re: Downdraft vs updraft cooling
Post by: GlennBob on April 22, 2012, 11:21:51 PM
Bill,
There is a lot to be said for that . . . .and . . .(correct me if I'm wrong) but I believe you've said it. My plane would have been in the air probably about 3 or 4 years ago . . .if I could have painted it with the body . ." good enough ". But I can't bring myself to settle for second best because I know everyone will be looking it over and judging it's appearance. Just think of the miles and years I could have spent in flying. It's a tough battle.
I remember seeing one bird years ago at OSH that had a basketball sized delamination on the top strake. The owner had written with a marker : " Must fly, . .not fix " ! !
Hope I get mine in the air soon ! !
Glennbob
Title: Re: Downdraft vs updraft cooling
Post by: Bill James on April 23, 2012, 09:57:36 AM
Yes, the planes are inspiring in many different ways. We start with an excellent design. The examples I mention were impressive and inspiring to me because they were simpler than normal and had long term durability due to the attention that HAD been devoted to maintaining them safe for flight so well for so long. Bill