The Need For Speed...
20P originally had the standard windshield up to the wing,
standard doors and a full Lexan rear enclosure. It worked OK,
but in terms of drag, the MKIII is a badminton birdie on a grand
scale. Picture pushing a large badminton cone through the air,
and visualize how the air would swirl around the back side, and
that's kinda' how it is. Also, the radiator mounted at the rear
of the cage was originally inefficient because of the disturbed
airflow caused by the windshield and doors. One of the first performance
modifications I made to 20P was a shortened windshield. With the
shortened windshield and doors, it cooled the engine very well.
The picture on the left shows it in that configuration, and I
flew it that way from 1996 to 2001.
In spring 2002, I changed the shape of the windshield,
added an air scoop along the top of the windshield, enclosed the
rear upper fuselage in an effort to be able to have an efficient
radiator and an enclosed cockpit for winter flying. It did not
work very well, so there are no pictures. Some things are better
forgotten... During the spring of '03 I changed it to the
configuration on the upper right, which was a great improvement.
The upper rear cabin top was made of 1/16" Lexan, which could
be opened along the side for summer ventilation, or closed in
the winter. However the shape was not optimum, and the radiator
cooling was marginal on hot days. During the spring of '08 I changed
it to the way it looks in the two pictures below.
20P is now a true parasol wing configuration, there
is a 5" gap between the bottom of the wing and the top of
the windshield/turtledeck. Flight characteristics are very good,
the fuselage is more streamlined and cuts the air more efficiently.
Airspeeds fully enclosed are 50mph at 4500 rpm, 60mph at 5000
rpm, 70mph at 5400 rpm, 80mph at 5800 rpm, and around 86 mph at
6450 rpm, and that's flat out. The rate of climb is improved with
the wing completely exposed, solo with full fuel on a summer day
typically gives a climb rate of around 800-900'/minute@6250 rpm.
In the winter, it gets even better, a climb rate of 1000-1200'/min
@ 6250 rpm. These airspeed and climb numbers are an improvement
over what the airplane would do new and in stock configuration,
which shows just how adaptable to tweaking the MKIII really is.
The biggest hurdle the MKIII has to overcome in
order to be efficient at cutting through the air is it's width,
normally made even worse by the standard doors. Replacing the
standard doors with flat doors reduces the width by a good margin.
Something very important which is easily overlooked is the improvement
of airflow into the prop. Stock, the top half of the prop is typically
in clean air, and the bottom half of the prop is partially blanked,
or in trashy air, caused primarily by the windshield which goes
to the wing leading edge, and to a lesser extent, the bulging
doors. This turbulence causes the top half of the prop to be more
efficient than the bottom half, and requires more nose up trim
to counteract the stronger push, or thrust vector above the airplane.
By getting more clean air into the lower half of the prop, you
increase total thrust, & you balance out the thrust vector
much better, which reduces the amount of up elevator trim needed,
which reduces drag.
The original doors were very convenient in that
they could open and lay across the windshield when getting in
or out, the current doors are not as convenient. That is as far
as they go, and there is a prop that holds them open. However,
they are not too bad, and since you only spend a minute getting
in or out, and a lot longer than that flying, it is not a bad
trade-off.
Something that I was not real happy with was the location
of the muffler. Being hung out in the air alongside the engine
surely wasn't helping air flow, so I moved it. I replaced the
stock curlicue expansion pipe with the Quicksilver based 90 degree
pipe, and hung the muffler behind the radiator, behind and below
the engine, where it lies in air flowing between the wing and
the cabin top. It is supported by two 1.5" x 1/8" aluminum
angles that run under the stock motor mount plate, and stick out
toward the rear. This also has the advantage of keeping the muffler
in the same plane of vibration as the engine, instead of cantilevered
out to the side, now it shakes a whole lot less. This muffler
position will only fit if you have the Ivo prop hub extension
or similar. Sorry, I don't know if the wings will still fold or
not, because I don't fold mine, I take 'em off. The fuel pump
is mounted on one of the muffler mounting angles, a convenient
place. All fuel line is 1/4" aluminum fuel line tubing, joined
with standard automobile rubber fuel hose.
The muffler is hung with rubber muffler hangers
from Advance Auto Parts. ( Why not? ) I got tired of the Rotax
exhaust springs breaking and fabricated some compression bolts
to hold the various pieces of the exhaust in place. It uses 5/16"
bolts compressing springs that are trapped by the bolts. This
has proven to be a very reliable arrangement. The bolts gradually
wear where they go through the brackets and can be replaced during
annual condition inspection. The fairing in front of the engine
is non-structural and is made from .024 2024T3 which attaches
to the fuel tank via piano hinges epoxied to the top of the fuel
tank. It supports and streamlines the oil injection tank, which
came off a Murray lawn mower, holds 1.5 gallons, and was free.
There is a tubular sight gauge running down the back side of the
oil tank so you can see how much oil is in it. The fairing also
houses two Hobbs meters, one for the airframe, and one for the
engine. That way I don't have to do any math... The fairing is
for eliminating the turbulence formed by the black nitrogen launching
canister for the parachute, and redirects the airflow around the
oil tank and engine. Don't know how much it improves the total
streamlining equation, but it can't hurt, and it makes a great
place to put the Hobbs and the oil tank.
Those nice plastic fairings that Kolb uses to streamline
the lift struts also work great on the landing gear. You may not
think that would make much difference, but the first time I flew
after installing them, the airplane wanted to fly nose high, and
required noticable retrimming. That means some drag was eliminated.
