Al, do you recall the CFMs of your fan?  And/or dimensions?

 

Bill B 

Bill; your point is valid, but I’ll take the liberty to add and clarify.

 

You will need to run between 75% and 100%  in your plane most of the time! The rotary is well up to the task, but you need to concern yourself with a totally different operation pattern. Radiator wise the rule of thumb is a minimum of 2ci of radiator per HP for piston engines, Rotaries like closer to 3 cubic inches per HP.

I actually find that with the extra power of the 20B, cruising at about 50% is just fine.  Unless you’re in a hurry, since speed goes up by the cube root of power, an extra 25% + dosesn’t get you that much.  I can fly 200 mph at 9.2 gph, or 210 mph at about 11.0 gph.  I tend to think it is not worth $7 of fuel to gain 3 minutes each hour of flight.  The numbers for a Lancair or Velocity will be different, but you get my drift.

 

But your right, of course; this is quite a different application than racing.  The limiting factor on cooling is takeoff and climb. If you get that done, then you’ll start wondering how to reduce the unnecessary cooling drag when cruising.

 Be really careful about fans in your plane. At the higher air speeds we see a fan can become a real restriction. Also not many fans, (only the REALLY expensive ones), are designed to be freewheeling in a 200 MPH airstream!

I use an auxiliary fan behind the in-cowl radiator (which is handling most of the coolant heat load) in my Velocity (rear engine, of course). It’s a 4-blade SPAL fan that covers about 65% of the radiator.  With the fan on I can sit and wait for takeoff for long periods of time without concern.  It gets turned off when I get clearance.  When I installed the fan, I did not notice any reduction in the cooling ability in the air, so I conclude that it does not restrict the airflow. The scoop inlet is about ¼ the rad area, so the air exiting the rad is 50 mph or less. I haven’t concerned myself about the fan spinning in flight, and it has operated for about 60 hours of flying and doesn’t show noticeable signs of wear.  Although I should perhaps rig the relay to short the fan terminals when turned off.

The fan has a secondary function which I think is also important.  I turn it on as I’m about to shutdown, and leave it running a few minutes after shutdown. It definitely reduces the in-cowl temps while things (especially the exhaust system) is cooling down.  Good for things like the coils and alternator.

Radiator wise the rule of thumb is a minimum of 2ci of radiator per HP for piston engines, Rotaries like closer to 3 cubic inches per HP.

Unless I’m overlooking something; except for a bit lower BSFC (maybe 10 – 15%) there is no reason to believe that a rotary takes any more cooling capacity per hp than a piston engine. The split between oil and coolant heat loads is quite different. The rotary puts close to 1/3 of the waste heat into the oil, whereas a piston engine may be more like 10% or less. So although the combined heat rejection requirement is a bit higher, the coolant side requirement (radiator) of the rotary is actually less; the oil cooler quite a bit more.

As my installation has turned out, the airflow through the oil cooler is less than anticipated, and the effectiveness of the in-cowl rad greater.  I typically see oil temps 20 to 40F higher than coolant – so I’m about to face some re-plumbing to put in an oil/water heat exchanger.

Al