More about downdraft cooling

[Charles Davis]

Currently, my plane still has the large NACA belly scoop used for updraft cooling.

I plan to convert to downdraft cooling with a plenum arrangement.

Now, what to do about feeding the carb and oil cooler?

I'd prefer to use the Hal Hunt Ram air box.  

A few questions:
1. Does everyone agree that the oil cooler should be mounted aft of the carb?  Or, has anyone mounted it close to the firewall?
2. Since the engine floats, I assume the Ram Air box is not connected to the firewall, but that the inlet "floats" inside the underbelly Naca inlet.
3. Does anyone have pics or can you share with me how you've configured your carb and oil cooler when using downdraft cooling?

One idea I had was to reduce the size of the belly Naca inlet and use it just to feed the carb and oil cooler.

Thanks for any guidance or pics if you have them.

[Waiter]

CD;

I'm working on cowling mounting this week, so Hopefully, This problem will be solved by next week this time!

When I made my cowl, I included two small "armpit" scoops into the lower cowl, one for combustion air, the other for oil cooler.

HOWEVER,

I did see a couple on the Internet that did what your thinking; close off the old NACA scoop and build in a smaller scoop for combustion and oil cooler.

I chose the armpits for a couple reasons;

1) My sump tank now occupies most of the area that used to be the NACA scoop. I cut the NACA scoop out and now have a straight belly. I installed a 5/8 sheet of plywood in this area just in case I have a belly landing. it protects the passengers behind and also the sump tank.

2) The lower cowl does a downward transition in order to clear the exhaust pipes. Airflow must compress and move down about three inches (drag) to go around this transition

Having the small intakes gets rid of the transition. The drag is still there, but its now cooling drag.

My plan right now is to have a SCAT hose for the combustion air. The SCAT will need to be disconnected to remove the lower cowl.

I plan on forming a duct that the bottom of the oil cooler will mount to (cool air in from the intake), The Upper cowl will have a mating duct that will fit over the top of the oil cooler, the top duct will route the hot air out the back of the cowl.

Waiter

[Charles Davis]

[My only concern about scat tube feeding the carb is if the tube collapsed or if it came off at the joints.]

I'm no expert, but imagine this:
1. You make the belly intake so its area is equal to the area of the Hal Hunt intake plus the area of the oil cooler.
2. Place the oil cooler aft of the carb like a lot of the updraft installs.
3. Build fences in the lower cowl to channel the air around the the carb box  and through the oil cooler.

I like the Hal Hunt intake because it's a proven design, and I'd rather than buy than build when I can.... I know that is contrarian to the opinion of many excellent builders.

I wanted to mount the oil cooler on the firewall (to shift the weight forward), but that firewall is so small on an EZ and the cooler takes up a lot of space.

[Drew]

I did downdraft cooling and for simplicity, sealed off the lower NACA scoop.  I put 3 SCAT flanged connectors on the sealed off naca wall and built 1 rectangular flanged aluminum connector.  The biggest SCAT connector (3in?) went to an oil cooler box that I built that mounted behind the carb.  The 2 smaller connectors went to go cool other stuff.  I built a fiberglass extension on the hal hunt ram box to fit over the rectangular flange slip joint.  Don't have any other pictures that is not already posted on my web page.

[Charles Davis]

Drew,

Interesting.  What did you cool off with the extra hoses?  (Mag, Alternator?)

What is the size of the opening for the carb inlet?

Thanks for the info.

[Charles Davis]

Drew,

Did the downdraft cooling meet your expectations?

[Drew]

I based the downdraft cooling based on what George Shell did with his Longez (4 inch diameter round scoops and a sealed off Naca).  Unlike George, I really opened up the exit area.  If I were a speed freak (and willing to put in more construction time), I would have cut off the Naca scoop----and build a more appropriate size naca scoop for the oil cooler----and ram air to the carb (similar to the lower scoop on the berkut).

Since I kept the big naca scoop, I tried to fill up as much of the scoop as possible with flanges for scat---otherwise you just have a miniature speedbrake out there.  At the same time, you will want to avoid unnecessarily pressurizing the lower cowl.  For my oil cooler, no problem---it was pointed out the back end anyway.  My additional cooling went to the mags and the electronics on the back of the alternator.

On my Cozy, I am considering adding a small naca scoop on the cowl to feed separate air into the oil cooler.  Currently, the oil cooler is stealing cooling air from the engine.

The carb inlet was just a straight run extension from the hal hunt box.  Carved a block of foam to extend from the hal hunt box all the way to the firewall.  Glassed over the box and foam extension---then removed the foam.  Then made a rectangular aluminum slip joint to mount the the firewall.  The metal slip joint fit inside the hal hunt extension.  Had no issues with any kind of cracking.

[GlennBob]

Drew,

Other things to consider cooling :  if you have a vacuum pump, . .their life is extended dramatically if cooled.  Fuel pumps,  manual and electric.  Back of the alt is good (as you mentioned).

Where do you get your smaller naca scoops ?  Do you order from Spruce ?  or just make 'em yourself ?

Glennbob

[Charles Davis]

Just a quick word of appreciation for all the comments here.

I can see there is many ways to do this and they all work well.

This thread has really been very educational and has helped me solidify my ideas.

Thanks, guys.

[mwilli7119]

Charles,

Thanks for your pictures. they are very good and you sure did a good job.  I am going to do something similar. You should post those somewhere so people can see them.

Mike Williams

[Charles Davis]

Great idea.  I should have thought of that earlier.  I have the webspace to host the files now.

Here they are:
http://www.thesolacegroup.com/Downloads/DowndraftCooling.pdf

[Drew]

Glennbob,
Glad you mentioned the vac pump---I was cooling that too.

For my Longez, the original large naca scoop was used for oil cooling, carb ram air, and accessory cooling.  The downdraft scoops were not naca scoops.

On my Cozy, I am considering using a small naca scoop for oil cooling.  If I go this route, I would buy the scoop.

Charles,
Your pix look great!  I'll be very interested in how it cools.  I built ram scoops (non-naca) because I was afraid that there may be some separation off the top of the strake---and that there may not be enough air actually going in them.  Nacas on the bottom or sides of the plane are much easier to feed.  Not many people were building downdraft cooling when I did mine---so I made sure that the scoops were well into the airstream.  Terry S (naca) has had trouble with his---but Bill James has not.  George Shell and myself had no issues either.  

And I am sure that you have found that the greatest thing about downdraft cooling is not having to baffle around all the crap on the back of the engine (starter, alternator, oil cooler, etc).  Cool!!   Sorry, couldn't help that one.  

[GlennBob]

Charles.

I remember seeing these pics before.   I too would like to know how well this system performs.  Bill James said somthing about  " heat treating " the plenums to avoid distortion.  Did you do that ?  How is that done ?

Drew,

Your pics look like you just cut a hole in the forward side of you upper cowl.  You said you " built " the inlets.  How much more was involved ?

I am seriously considering downdraft cooling.  After looking at how small my underside naca scoop is and then all the junk in the way before the air ever gets to the cylinders, . .it only makes a lot more sense to see the fins right inside the upper openings ! !

 :wink: Glennbob

[Charles Davis]

Can't say how it works because I'm not flying yet, and probably won't be for another year, as I'm building a house and that seems to be taking all my time and money.

The inlets were made by taking a naca template and scaling it up on a copier.

I have not heat treated the plenums.  

I read something about heat treating (curing) composites before.  Build a heat chamber; an box made from plywood and wrap with insulation.  Feed it with an electric or gas heater, and use a fan to circulate the air.  Monitor closely with a temp gauge.

Bring the temp up slowly, and keep it there for several hours. then bring it back down.

The idea is to raise the temp above what the part will encounter in service.

I do not know what those temps are, or how high you can raise the kevlar temp before damaging it.

[Drew]

George Shells pix are better than the ones I have posted.  My upper cowl pretty much looked like his.

http://www.geocities.com/~kleb/homebuilt/shell/

Similar to Charles, I cut a rectangular hole in the front part of the cowl.  I then nailed a couple of blocks of foam together to get the approximate size of the inlet.  I cut a for and aft template out of thin aluminum.  Both templates were the same.  The template's purposes was to get me the circular shape of the inlet----and hot wire the inside of the inlet.  At this point you have a shoebox looking inlet with a big hole in the middle.  The next thing to do is to put the inlets into the cowl holes and align them to the proper angles.  Then you need to "glue in" a bunch of transition foam pieces-----very similar to how you see people making their own cowls from scratch.  Once everything is pretty much in place, you can knock down the edges of the inlets with foam sanding blocks----pretty easy to get to the shape that you want.  The harder part is getting the transition to look nice.  Spend all of your time making it look good in foam---fixing in fiberglass is much harder.  When it is all done and glassed in, you can dremel out the old portion of the cowl (some of the glass and all the foam)that is not longer needed.  The only foam that was left in the inlet was the leading edge shape of the inlet---all other foam was removed.

One difference my upper cowl had from George's was I had an oil door in the left inlet.  I used the old hinge mechanism from the portion of cowl that I cut out.