From: Sky2high@aol.com
Message-ID: <400fd463.361d8cf5@aol.com>
Date: Fri, 9 Oct 1998 00:11:33 EDT
To: lancair.list@olsusa.com
Subject: Bugs on my cowling, where is my spinner?
Mime-Version: 1.0

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Some of us must find the answer to Zen-like questions.  It isn't called
"experimental" for nothing.

One day, after several summertime Illinois (high pest count) flights, I
noticed a spray of bugs, moon-sliver shaped (upside-down smile sometimes
called a frown) on the cowl just behing the spinner.  1/2 to 3/4 of an inch at
the top, disappearing at the horizontal split of the cowlings halves.  Of
course, this starts at the outside edge of the vertical cowl face (I hope you
have this pictured in your mind).  So... some facts and questions:

1. My propeller expert says that there is no way the air could make a right
angle turn in the 1/4 to 3/8 inch space between the spinner and the cowl and
deposit bugs in that pattern.  He said a strange pressure gradient may cause a
backflow on the upper cowling.  Anybody tufted the upper cowl behind the prop
lately?

2. My cowl is not sloppy.  I put more fasteners closer together than Lancair
recommended and a layer of glass on the inside edge of the cowl for strength
and smoothness in th efastener area.  Once it is on, it doesn't move more than
a 1/16 to 1/8 of an inch, up or down, measured at the front.

3. Yes, my engine/spinner exhibited a small sag of 1/8 inch on the ground.  I
would have thought that the engine would pull somewhat upward or at least
neutral in the air, as opposed to the several hundred pounds pulling downward
on the ground.

4. As part of an engine cooling experiment, I have a vertical aluminum plate
screwed to the upper cowl  (clearing the prop shaft) to reduce wasted non-
cooling drag air from entering the lower plenum, while also increasing the
pressure differential between the upper and lower engine cooling compartments
(this worked).  

The bug "frown" size was reduced to about 3/8 inch.  Was this because of
reduced airflow or because it stiffened the front upper cowling so the low
pressure could not deform it (bow it upward) directly into the airflow behind
the spinner?  Or, is the entire cowling, subject to the low pressure above and
the high pressure below, deformed upward enough to catch these bugs and create
drag (ugh!)?

Finally,does anybody have any data or experience to indicate that the spinner
should actually be positioned above the cowling (1/4 inch or more) to ensure
that, in flight, the air flows smoothly over the spinner and the cowling
behind it?  

Remember, speed in the air beats beauty on the ground.

Scott Krueger
N92EX