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Date: Wed, 07 Jan 2009 20:00:20 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Spinner size
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Al Gietzen wrote:
>
> I have the James rotary cowl--front “face” has a 13.75” diameter. I 
> also have the Vans 13” spinner that is already cut to fit my CATTO 
> prop. This leaves about a 3/8 lip of cowl showing all the way around 
> the spinner. Take a look at the picture.
>
> I know any amount of lip will cause drag, but is this enough to 
> warrant a new spinner? UHS makes a 14” one that would probably do the 
> trick.
>
> Chris
>
> My opinion is that is more a matter of appearance than function. The 
> air flow at that point is very turbulent, and any resultant drag would 
> be less than measurable.
>
> BTW; on the post about the fuel pressure sensor I thought you were the 
> other Chris with the Velocity.
>
> Al G
>

I can't even spell aerodynamicist (& neither can my spell checker), but 
everything I've read says that it's better to have the cowl face bigger 
than the spinner bigger. My impression is that the air will speed up 
slightly but not be greatly disturbed as it continues around the cowl, 
but if the spinner is bigger it will cause really bad turbulence as the 
air tries to turn the corner at the edge of the spinner to fill that gap.

Will James assured me that the stock Van's spinner fits his cowl, so I'm 
going with his assurance.

FWIW,

Charlie