Hi Paul,
I have been building a Zenith CH801.  This is a four passenger STOL aircraft.  It is not a fast airplane and cruises at about 105 knots.  Its strengths are its take off and landing characteristics.  In fact, its high degree of attack in take off could present cooling concerns.  It stalls at about 35 knots and climbs out at about 70 knots.

I have purchased your book How to Cool Your Wankel.  I have had a radiator made which is one inch thick by 40 inches by 20 inches.  It has been folded in a "U" shape as pictured on page 25 of your book.  The radiator has been mounted horizontally under my rotary engine as is shown on page 43 of your book.  The automotive Mazda oil cooler, which is 5 inches by about 22 inches, will be mounted on the side of my cowl as is pictured on page 44 of your book.

I have included a picture of my plane as I begin fiberglass construction of the front of the cowl.  I have created a duct opening at the front of the radiator for air intake of 5 inches by 20 inches.
As I viewed airplanes at AirVenture in Oshkosh this past week, I saw no airplane with such a large duct opening.  I was wondering if there is a formula for creating the size of the duct opening for both the radiator and the oil cooler?  It is for that reason I am writing you.

Hopefully, you will be able to help me.

Sincerely,

Ken Johnson

I assume this is a p-port and you have 250 HP. If you wish to cool 250 HP
while climbing out at 70 kts you will need a lot of air going through
the cooling system. The air flow is proportional to the speed squared.
If you climbed out at 100 kts you would have twice the air flow and the
opening, in theory, would be half as large.

You are on the cutting edge here. I know of no
other airplane with 250 HP that climbs out at 70 KTS other than perhaps
a Helio Courier. What makes this extraordinary  airplane possible is the extraordinary power to weight ratio of the Mazda rotary P-port.

Twenty percent of the core surface area is about right for 70 Kts.
You can always reduce that opening later but it is better to over cool
on the first flight rather than under cool. Under cooling on the first
flight can cause a panic and perhaps a crash. Don't worry about it until
after you fly it for awhile in the hot summer. Then you can reduce the size
of the opening and see if that adds to the top speed.

What gear box is that? What is the ratio?  Take some side view pictures
and send them. It looks rather blunt from the front view but a side view
might give a different impression.

Paul Lamar