REFLECTOR:Belly NACA

William C. Cox reflector@tvbf.org
Sat, 19 Jul 2003 19:24:13 -0700


Chris asks:

> As y'all may know, I plan to use a Mazda Rotary on my SE fixed gear.
> With the engine mount I already have, there is ample room to position
> a full size radiator under the engine mount angled facing forward. 
Yep.  This is where my radiators are as well.

> Unlike the dual NACA's that need to be split on top of the fuselage 
> and go to each bank of cylinder heads for a Lyc/Frank/Cont, I should
> only need one air source to the radiator set up to provide adequate 
> cooling.  I have seen this done with a NACA scoop installed on the 
> bottom of a Cozy but the under the engine set up on another Velo I 
> saw had a "big ol' scoop hanging under the fuselage> (which is not my 
> preference).
At Scott Lower's (formerly of Hangar 18) suggestion, we did almost
exactly this.  We put a pair of NACAs on the belly, and I've got a
dual oil-cooler rig to catch the air from them to cool my Subaru
2.2 liter four-banger.
 
> Do y'all see a problem with me doing this on my Velocity? I would
> appreciate the groups input as to problems I may encounter or
> reasons this may be a bone-head idea. My thoughts as to issues I need
> to address include:

> 1) Do NACA's work ok on the bottom of the plane?  Yep, start with the
> basics. I have read contradictory views on NACA's use.  Some seem to
> love them on their Velocity's, some don't.
I believe they'll work better on the bottom of the plane than on the
top - more high-pressure air there.  The only liability that I see is
the risk of little stones banging up the radiators.  Actually my
radiators are two oil coolers from Earl's in Long Beach, CA.  There's
about a three-inch space between them, which I have filled with a
fiberglass piece to catch combustion air and feed it through a 3-inch
hose to the throttle body.  So the scoop under the left wing goes away.

> 2) I will need to cut out about 1.5 inches high, by 9-ish inches wide,
> hole from the bottom of the "gear bulkhead" to fit the NACA scoop. I would
> reinforce the missing material with some Triax.  Should this
> reinforcement be enough? Thoughts?
Should work fine.  I'd be inclined to use two layers of Triax over the
bulkheads fore and aft sides near the scoop cut-outs, but I've not done
any calculations of the necessary reinforcement - that's just a guess.
We cut out the firewall as well, so the scoops outlet is into the engine
compartment, and we can mount the radiators to the back of the firewall.

> 3) I will need to raise "sump tank" about 1.5 - 2 inches since the
> NACA scoop will be where the sump sets.  It seems easy enough to elevate
> the sump and rest it on the top of the NACA scoop once it is installed
> and create a solid pedestal for the sump to rest upon.  Will the added
> height effect the gravity flow from the main tanks?  The pictures I
> have seen seem to indicate this is a non issue as the main tank lines
> seem more than high enough to accommodate a slightly elevated sump
> tank....but photo's can be deceiving.
Even sitting on top of the scoops, the sump top is below the bottom of
my (Velocity Classic) strakes.

> 4)  Of course, before I decided on using the rotary, I already
> installed my two top mounted NACA scoops on my SE.  Does anyone have a
> use for them....such a supplemental/primary cooling of something such
> as accessories (alternator, potential turbocharger etc) or would I be
> better off closing them back up. Also, if they were closed up, would
> the airspeed be potentially increased. Also, I can't imagine a good
> way to duct the flow from the top NACA's around the pipes and engine
> to underneath the engine and mount to the bottom mounted
> radiator....thus necessitating this entire question.  The radiators
> can be set up in different ways, it is just under the engine mount
> seems clearly the best.
I'd ditch them as sources of unneeded drag, once you have the belly
scoops.  I plan to add a little NACA on the cabin top, for rear seat
cooling, but you surely don't need two big scoops for that.

> 5)  Finally, I know I could just "build" the scoop into the bottom of
> the fuselage, but I am inclined to just buy a pre-molded scoop from
> Velocity, Inc.  I know technically, there is a right NACA scoop and a
> left NACA scoop, but on the two that came with my kit, I can see VERY
> little difference.  Anyway, the standard scoops are about 9 inches
> wide.....how wide are the scoops on the XL and would the larger XL
> scoop prove a better choice.
I, too, would buy the factory's scoops.  I'd use the same ones that
you have on top as the belly scoops, but buy a new pair to save work
and time.

> Thanks for your input, considered opinions and time as to what I may
> be missing and/or other thoughts.

I need to get some pictures, and I'll have them in about three weeks,
if y'all are interested.