From: "Charlie England" <flyrotary@lancaironline.net>
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Subject: Re: [FlyRotary] Re: rebuild
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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Date: Thu, 23 Jun 2016 11:52:44 -0500
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On 6/23/2016 9:52 AM, Andrew Martin wrote:
> Thankyou for your replies, makes it a lot easier to make sense of 
> whats going on, looks like I  do  need much more inlet area, 
> maybe just too much optimised for cruise, not much good if I just cant 
> get there in the first place though.
>
> Good to hear your system is operating ok Bobby. An early photo of your 
> setup on flyrotary is what I based mine on. Albeit a lower power 
> system being NA Renesis. Don't suppose you also have air temp delta's 
> to go along with the airflow data areas on your radiators. Would be 
> interesting to know the actual btu's for comparison.
>
> I'm now thinking my best bet to try next, is to use current duct 
> solely for the radiator and feed the oil with a new duct somehow, then!!
> Andrew
>
>
> -- 
> Regards
> Andrew Martin
> Martin Ag

Andrew,

If it will help you get more comfortable with a large inlet, here are a 
few quick & dirty points about cooling drag.

Some research papers have indicated that as long as the 'lip' around the 
inlet is shaped correctly (basically a large radius lip), you can make 
the inlet significantly oversized without affecting drag by a noticeable 
amount. The way you do it is to control the *exit* size (cowl flap). 
With the exit flap open, you get large flow, more cooling, and a bit of 
drag during high power operation (climbs). Closing down the exit flap 
'throttles' the flow, letting the excess air volume divert around the 
inlet rather than flowing through the cooling path. Doing this can 
simplify duct design and/or improve performance, because 'pressure 
recovery' happens in front of the inlet, instead of within the duct, 
where trying to turn the air at high speed or expand it is difficult to 
do without causing turbulence inside the duct, which increases drag and 
impedes cooling air flow.

I'm sure that others here are more technically qualified to describe it, 
but as I said, this is the Q&D version.

On the subject of separate ducts: IIRC, some have found that even with 
separate inlet ducts, if the coolers dump into the same space the more 
free-flowing cooler can still rob flow from the other. Not always a 
problem, but can be.

Charlie