From: "Charlie England ceengland7@gmail.com" <flyrotary@lancaironline.net>
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Subject: Re: [FlyRotary] Re: Inlet radius ratio
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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Date: Tue, 7 Jan 2020 19:05:27 -0600
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He's talking about the radius of the 'lip' around the cooling inlet(s). 
Since I'm most familiar with the RVx family, I'll use those as examples. 
If you look at a 'stock' RV cowl, the leading edge of the cowl has a 
very fat radius; probably close to an inch. If you look at one of the 
after-market 'Sam James' cowls (the ones with circular inlets), the 
leading edge lip around the inlets is much sharper; probably 1/2" radius 
or less.

The inner/outer thing is about (for both of the above) having a 
non-constant radius. That's true for almost all inlets; think about 
something that very roughly resembles a 'French curve'. The radius at 
the inside of the inlet is fairly sharp, but gets bigger as it moves out 
and back. A visual analogy would be the leading edge of a flat bottom 
airfoil. If you stretch/rotate the wing cross section into a circle, 
with the bottom toward the inside, you'd get a similar shape.

The fat/thin radius issue:

IIRC, research has shown that *if* the flow through the system is 
exactly right for what's needed, and there's no 'spillage' around the 
lip of the inlet, and slowing of the air with the attendant increase in 
pressure happens *inside the duct* ('internal diffusion'), then a sharp 
lip has less drag. But if the system is designed to have the air build 
up and slow down in front of the inlet ('external diffusion') then a fat 
radius has less drag. Slightly higher drag than a perfect internal 
diffusion duct, but in the real world, typically lower drag than the 
more common imperfectly executed internal diffusion duct. It's supposed 
to also have the advantage of allowing an inlet sized to flow plenty of 
air in climb profile (high power/low speed), with an exit flap used to 
reduce flow through the system at cruise, where the will by necessity be 
significant spillage around the inlet.

Or, as Lynn likes to say, I could be wrong...

Charlie

On 1/7/2020 4:11 PM, Marc Wiese cardmarc@charter.net wrote:
> Can someone sketch this out, I'm having difficulty following where these curves are supposed to be?
> Marc
>
> -----Original Message-----
> From: Rotary motors in aircraft
> Sent: Tuesday, January 07, 2020 10:57 AM
> To: Rotary motors in aircraft <flyrotary@lancaironline.net>
> Subject: [FlyRotary] Inlet radius ratio
>
> Tracy reported a marked increase in cooling when changing the inner/outer radius of the duct inlet. The outer radius of the lip should apparently be greater than the inner radius of the lip. He got the optimum ratio from an NACA paper, but don't remember which.
>
> I think the goal is maximum pressure recovery at low speeds (100 mph) and high angle of attack and of course minimum drag at low and high speeds (output cowl flap to lessen flow at high speeds making the air flow easily around the inlet and cowl at higher speeds).
>
> I've been trying to locate that paper but keep finding papers on turbine inlets and supersonic inlets.
>
> Does anyone know what that NACA paper may be? Or even just the optimum radius ratio?
>
> Finn