Looks like a plenty big heat exchanger is possible under the engine.
The air scoop is as low as it is to insure maximum effect from
the prop slip stream. At take off and climb it should add about
ten percent to the airspeed and about 40% more dynamic pressure.
It should also cool well while taxiing.
Cowling length here is about 44 inches long.
Perhaps Will James can build you RV10 guys a custom cowl.

Paul Lamar

    BTW a bit of brain fade there on my part. 10% improvement in air
    speed flowing
    into the scoop is only a 20% improvement in dynamic pressure
    and not 40%.

    Paul Lamar

Where V1 = equivalent airspeed at heat exchanger face and V0 = airframe
speed, on our underslung system, we were able to get an "equivalent"
airspeed (V1) at the heat exchanger face of 72 mph at standstill,
reaching V1/V0 of 1.0 at 72 mph, decreasing to .91 V0 at 100 mph and
finally to .69 V0 at cruise.  As you know, when one gets an "equivalent"
airspeed of 72 mph at zero mph airframe speed the V1/V0 is infinity
since V0 is zero!!  Hope this data point gives some guidance to others.

For our GM evap core heat exchangers, we used from .62-.84 (62%-84%) of
the available [dynamic] pressure at the heat exchanger face depending upon
airspeed.  That is higher than what would be a theoretical "best" of
approx. 0.4 (40%) which would minimize the drag per unit of cooling air
mass, but Hey!!, the price was right...$3.00/each!! with $35 worth of
welding inlet 90's.

As I recall, one should get either as close as possible to the prop with
the heat exchanger inlet or....get at least three (3) propeller chords
(3 x 5 = 15 approx.) back from the prop in order to avoid TOTALLY
stirred-up air at the inlet.  Obviously we were fine at about 3 feet!!

This is further evidence of what one can learn with a super-duper water
manometer test!!

Doug in CO

Your my main man on this subject Doug. I quoted one of your earlier messages
in my How To Cool Your Wankel article on our web site.  Thanks for all the help.

Paul Lamar

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