Hi All,
This subject is really interesting and I imagine some of the
refinements/improvements
that have been found to increase cooling for 914's could be used on 912
powered aircraft without cooling problems to improve cooling efficiency and
thereby
reduce drag. I would very much like to see some diagrams, photos with
explanatory
notes when the experts reach some conclusions!
John
There are some historical reasons why the standard XS cowl arrangement
performs
badly on the ground. When the XS was developed from the Classic,
the factory moved the engine up to give them better prop clearance. This
had the unintended consequence of forcing them to use a smaller diameter
spinner, which exposed the propeller cuffs and roots. These cuffs thrash
around during ground running producing no thrust and a large amount
of turbulent air.
The designers threw every known orifice at the new cowl; two Lo-Presti's
(popular at the time), two NACA vents (because all real aeroplanes have
to have NACA vents), two louvres to complete the set and a shoe-box
underneath
to keep the radiator out of the cold (I think someone added that
for a bet).
The problem when ground running is that all this swirling airmass is
largely
passing across the mouths of these vents - not into them. The Lo-Presti's
are positioned adjacent to the blade roots and the radiator, positioned
way back in its duct, is sitting in stagnant air. It's not until the
aircraft reaches flying speed that the apparent wind comes more from
the front that air begins to enter the vents/ducts and the cooling
improves.
This is one reason why Frans Veldman's front mounted radiator arrangement
is so much more effective. High twist blades such as the Woodcomp
have been shown to give better ground-running temperatures because
they produce a much better airflow at the blade roots than many of the
flatter blade options.
I started with the prop/spinner and worked back. I lowered the engine to
the original Classic position - but maintained the XS forward position.
This allowed me to design a much larger spinner to completely shroud the
blade cuffs and most of the blade root.
--------
Europa mono Nr 192
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