Also for what its worth, testing of the Lo-mcfadyean-sti XS cowls has just
been completed. These are comprised XS cowls with the underslung portacabin
removed. The radiator sits in a new duct under the cowls. The duct is much
slimmer and curvier, set further back from prop blade interference and has
about half of the inlet area of the XS. The oil cooler is removed from
behind the water radiator and is now smaller and placed behind the port
nostril.
In terms of cooling, it all works rather well.
In terms of aesthetics, it looks loads better (IMHO).
In terms of overall drag, I have no idea whether they are any better or
not. A throttle -closed VNE dive with the former BMW cowls (not too
dissimilar to the Std. XS) lost about 4000fpm; now its about 3000 fpm
(although to confuse matters, prop pitch is now coarser).
For good measure and to reduce wetted area(!) the cowls were also reduced
in length by about 2" (because the Classic engine mount has been used). The
secret to the cooling success is a new outlet duct at the base of the
firewall through the area that the monowheel likes to occupy (or is
occupied by the nose gear stuff on the Tri). The inlet duct is also
"properly" dimensioned and shaped so that it isn't running in a stalled or
'vena-contracta' condition. You too can make this mod., but you may first
have to change to fixed conventional gear to create the opportunity for
the additional exit duct.
Duncan McFadyean
PS does anyone have a NACA duct splash mold I could borrow?
On Saturday, May 18, 2002 9:40 PM, Fred Fillinger
[SMTP:fillinger@ameritech.net] wrote:
>> FWIW, on the aerodynamics aspect, the substance in Hoerner's "Fluid
> Dynamic Drag" re cowlings is this. Once you have the problem of a
> propeller, aerodynamics aren't much more than the least wetted area,
> limited by "practical considerations" of what's inside. The shape is
> a transition to the fuselage with curves of decreasing radii. Thus,
> whether an alternator "bump" is worse than a larger cowling is a
> matter of experimentation, or just aesthetics at a possible price.
>
> Biggest issue is cooling drag, meaning smallest, most efficient inlets
> to still do the job. Whether a NACA scoop is better for the
> intercooler, or larger central duct and split the flow, or totally new
> arrangement would involve expert opinion. But I suspect it still
> reduces to trial and error even for Lopresti. It's curious also that
> Lopresti obtains the biggest gains on the faster aircraft, which I
> guess is the "velocity squared" phenomenon: only 5 MPH on the Arrow
> and no cowl kit for the slower but bigger market PA-28 fixed-gear
> versions. In comparison, the XS cowl appears to have the type of
> cleanups the aftermarket guys do.
>
> For landing lights, I have small halogens, 35W to 75W with narrow-beam
> reflectors that are tiny and only $4 each, to play with. Besides room
> to fit, having 2 small lamps means less heat each to deal with the
> heat issues, and more focused light pattern to maybe make up for the
> lower candlepower compared to the short-life aircraft lamp.
>
> Regards,
> Fred F.
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