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Velocity Top-of-Cowl NACA Cooling

Wayne Hicks

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I'd like to hear from velocity builders who've gone the downdraft cooling route with the top-of-cowl NACA scoop.


How well does the downdraft cooling work?

Do you get adequate flow during climb?

Do you get equal cooling on all cylinders?

What type and size of engine?



Wayne Hicks

Wayne Hicks

Cozy IV Plans #678


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  • 4 months later...


I didn't notice your post until just yesterday.

I don't have downdraft yet, but it is going to be the next significant thing on my Velocity. I am totally fed up with the armpit scoops that came with it. They are inadequate (I have to run rich ALL the time - as I lean out, the CHTs go through the roof - bad for plugs, range temperament, etc.) and require the hideous baffling common to all Rutan updraft setups. I saw guys putting in turtleback NACA ducts on Velocitys at the factory and it looked easy enough. I ordered the plans and they seem easy enough to do. I would hazard a guess that the (free?) Velocity NACA plans would work perfectly on a Cozy.


What I aim to is:

1. Cut the holes in the firewall

2. Cut the ducts in the turtleback

3. Bend the ramp down and glass it in.

4. Build a "nipple" for each duct out the aft side of the firewall 1" or so.

5. Make an Al baffle box around the top of the engine just clear of the cylinders and case seam

6. Shrink wrap the baffle box, sculpt drywall mud ala' John Slade's cowl to form a plenum

7. Lay up a couple of plies of BID over the mud

8. Sculpt a couple of foam duct cores from the plenum to the "nipples" in (4). Skin the ducts and glass them to the plenum

9. Remove the mud & shrink wrap. Baffle the jugs with BID/RTV. Install some blind nuts on the flanges of the baffle box and you're home. The plenums come off with about 6 screws each for access.


My analysis so far is:

Up side:

All of the usual downdraft advantages (cooling air not preheated by exhaust, CHT probes on hot side of heads, etc.

The plenums come off in a heartbeat for access to top of engine.

There is only barely enough baffling in the engine compartment to notice

There is NO baffling in the way of access to ANYTHING

Minimal baffling => minimum (squared) baffle leaks => Much more effective cooling

Better plenum => better flow => more even cooling

Smaller plenum with more direct routing => easier to "trim" airflow for even cooling


Down Side:

Retrofits are always awkward. Better to do it when making the turtleback. I didn't have that opportunity

Cutting ducts in turtleback tears up the upholstery in the back seat which will probably require replacing the entire headliner

Will have to re-do cowl (remove armpit scoops).


The plus' easily outweigh the minus'. All I need is an opportunity to do it.


What are you planning to do? How you planning to do it? Or you just sniffing around weighing the factors?


I think it's a great idea. I've neard NOBODY on the Velocity forum do anything but rave about it.


Hope this helps some. I've THOUGHT a lot, DONE damn little so far .... Jim S.

...Destiny's Plaything...

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  • 1 year later...

Velocity sells a pair of boxes to go over the cylinders for downdraft cooling. I bought them for my AeroCanard. I was planning to put the NACA scoops up on the turtle back but I think that is a low pressure area and I wonder if some external scoops on the upper cowl wouldn't be better. BTW the baffle boxes have a lot of square corners, look like they could be improved upon for airflow optimization.

Long Ez, AeroCanard

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  • 1 month later...

I have been flying my Velocity (173 Elite RG) for 1.5 years and have 150 hours on it. I have the downdraft cooling and have been very pleased with it.


CHT with extended ground idling generally stays under 300 degrees.


My CHT's with a 4000 ft climb from take off stay under 360 degrees if I use 110 knots cruise climb.


I have had a couple of hot take offs due to waiting on other traffic, 90 degrees outside, where CHT's have approached 380 after takeoff. I just lower the nose and temps come right down.


The rear cylinders will run about 10 degrees warmer at low speeds, and about 10 degrees cooler at higher speeds. Temperature spreads across the 4 cylinders (IO360 - 200HP) stay within 20 degrees or usually less.


I made cylinder wraps to force the cooling air to go all the way around he cylinders - the opening at the bottom of the wraps are only 1.5" or so. This helps a bunch.


Bottom line, the Velocity downdraft NACA system WORKS GREAT!!!

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As you can see by my previous post I am building an AeroCanard and want the down draft cooling. Can you tell me the particulars? I have the phlenum that Velocity sells that fits over an IO-360. Did you use this? If not how did you make your phlenum? How big are your Naca scoops?

Sounds like you have the kind of results that I would like.

Long Ez, AeroCanard

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Thanks for giving Velo info.....I have felt like the red headed step child on the two canard forums for a while ;) Since I am still building, I am unable to give real flying data.


All the best,



Christopher Barber

Velocity SE/FG w/yoke. Zoom, zoom, zoom.



Live with Passion...

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I did use the Velocity engine plenum. See photos at http://home.sprintmail.com/~romott/page45.html


The NACA inlets on top of the fuselage are approximately 3 x 9" each.


I believe there is a high pressure area on top of the fuselage caused by air going over the windshield trying to reattach to the fuselage, similar to what happens to a NASCAR race car - the top of the car would appear to blank out the spoiler attached to the rear of the car. However, with the air coming back down after being lifted by the windshield shape, there is plenty of downforce (pressure) at the spoiler, and like wise for the NACA inlet scoop on a Velocity. Different shaped fuselages may have a different result.

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Oh, and by the way, a picture of the cylinder wraps is at http://home.sprintmail.com/~romott/page55.html .


Also, be sure you close up the opening at the left front cylinder. The Velocity supplied plenum leaves this area open for some reason - or perhaps the newer plenums fixed this problem.


Several keys to successful NACA cooling:


1. Tightly fitting plenum

2. cylinder wraps under bottom of plenum

3. my cowl is within 1" of the front edge of the prop (helps pull hot air out of the cowl).

4. Shape of the Velocity fuselage (?) - see my previous post.

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Thanks for the info and pictures. It looks like your phlenum is much better than the one I bought. I spent $200 and I don't think I can use it. Yours is much better. Mine has all square corners and I don't think the airflow would be worth a hoot. They must have found this out and sold AeroCad the old one and I bought it.

Long Ez, AeroCanard

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  • 19 years later...

I have just restarted working on my Velocity 173 RG Unidoor . I know that this an old topic but I am at the stage where I need to consider using the top mounted NACA scoops for engine cooling and induction air. As I am living in Australia I have not had any opportunities to look at other builders installations in detail and would appreciate pictures and details of same. I am planning on installing an IO 360 Lycoming engine and a DUC Cobra 3 blade ground adjustable propeller and 8 inch Saber extension. One of the details I am most interested in is the size of the outlets of the NACA ducts, as I will need to cut these into my bulkhead shortly. Another is details of the induction air system used including any filtration system and alternate air device.

Thanks in advance, Jim

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Found one site with a Google Images search for "velocity aircraft intakes cooling" but using variations on that search and digging into some of the photos will likely discover others.  Canardzone doesn't talk much about Velocitys


You might find some good info here and the old Reflector archives and photos referenced in that site.


Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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I just noticed that this thread was started by Wayne Hicks in 2002--22 years ago!--and wondered about him.  AFAIK, he did not finish his project or sell it and it might still be stored in a hangar at Chesapeake Regional in Virginia.  Wayne was the go-to guy for builders needing examples of good workmanship in the early 2000s and I see that his stuff is still posted.  I do not find any likely pilots by that name in the FAA registry.


Lots of builder links here but I am afraid many of them are dead


Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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