----- Original Message -----

From: Al Gietzen

To: Rotary motors in aircraft

Sent: Wednesday, April 06, 2005 11:29 PM

Subject: [FlyRotary] Re: rule of thumb and RV-3 sizes-

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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!

 

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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