Al,
I did mean "intake"!
On both their planes, the Mustang II and the RV-4 the intake area is biiger than the exit!!
As mentioned, if you need I can email the scans of the mag! You also might want to read up on: http://www.cafefoundation.org/aprs/RV-4.pdf
Thomas J.

    ----- Original Message -----
    *From:* Al Gietzen <mailto:ALVentures@cox.net>
    *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
    *Sent:* Wednesday, April 06, 2005 11:29 PM
    *Subject:* [FlyRotary] Re: rule of thumb and RV-3 sizes-

        ..cut

        Note: the exhaust area requirement is greater than the inlet
        combination of oil and coolant due to the now considerable
        hotter air temperature. Rule of thumb: Coolant air inlet
        opening for 200 HP coolant cooled engine ~60 sq. in., oil
        inlet opening ~30 sq. in. equals a total opening of 90 sq. in.
        A good place to start with exhaust opening is 1.4 times the
        inlets or 126 sq. in. Close the exit area down with cowl flaps
        to as little as 80% of the inlet combination at cruise conditions!

        ...cut

        I do understand that an air-cooled installation is a different
        animal, but by the end of the day ( or at the cowl exit...for
        that matter) everything is air cooled. The interesting thing
        is that both run a intake/exit ratio of 78%, the intake (I
        think you meant ‘exhaust) being bigger - just about confirming
        the above statement!

        Schmidtbauer mentiones the " rule of thumb" - exit about 150%
        of inlet.

        I think for our purposes, the 1.5 ratio is more applicable
        than 1.28. It can be lower IF you have well shaped entrance
        and exit ducting. Typically we focus on entry ducting, and
        then have rapid expansion at the exit from the core – sudden
        expansion pressure loss; losing all momentum and then, usually
        a rather sudden acceleration out a fairly small opening.
        Definitely less than optimum.

        I don't think there is a great difference on the amout of
        cooling necessary, as the efficiency of both engines are
        fairly close. So some heat goes out the exhaust and the rest
        has to be cooled. For sure you need different ducting, but the
        amount of heat energy should be about the same and you want to
        get rid of it with the least drag, either way.

        You’re right. The biggest difference is the higher rejection
        temp, and larger delta T of the air on the air-cooled engines
        allows for a lower air flow rate.

        Al