The Need For Speed...
One of the first performance modifications I made to 20P
was a shortened windshield. The aircraft originally had the standard
windshield up to the wing, standard full length 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 open back side, and that's how the
standard cockpit 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. With the shortened windshield
and doors, it cooled the engine very well. This is how it was
from 1996 to 2001. Flew great. Then in 2001 I froze my tush coming
back from the Kolb Fly-In that September, and decided to enclose
it and heat it. I don't do cold as good as I usta'..
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 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
below which works very well.
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. This was done to improve the wing characteristics
and improve airflow to the prop. I was somewhat concerned about
cockpit headroom, but it turned out not to be a problem. (I stand
5'10" and fit OK) The rear of the fuselage is enclosed with
removable Lexan panels which can be quickly removed or replaced
as the day warms up or cools off. It also accepts the original
half doors for hot summer flying, so it is now a MKIII Convertible.
(Targa? Spyder?)
Flight characteristics are identical in all enclosure
configurations, and there is minimal trim change with airspeed
change. With all the Lexan in place, the fuselage is more streamlined
and cuts the air more efficiently. New airspeeds fully enclosed
are 50mph at 4500 rpm, 60mph at 5000 rpm, 70mph at 5400 rpm, 80mph
at 5800 rpm, and around 88 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@6200 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. These new doors
reduce 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 picture on the right shows the vortex generators
along the rear edge of the doors. I suspect they are like feeding
crackers to a dead man, they don't help much, but they don't hurt
anything either. Years ago, I tuft tested the side of the MKIII
fuselage in flight, and the area aft of the doors is a mess of
turbulence. That is part of the reason why TNK came out with the
MKIII Xtra, which has modified fuselage sides to improve the aerodynamics
of this high-drag area. There is no total cure for a standard
MKIII, but the combination of narrower doors, vortex generators
along the sides (maybe?) and lower windshield, combined with the
tapered turtle deck, help a whole lot.
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 that is already turbulent. 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 muffler hangers from Advance Auto
Parts. ( Why not? ) I got tired of the stock springs breaking
and fabricated some compression bolts to hold the exhaust in place.
It uses 5/16" bolts compressing springs that are trapped
by the bolts. Will it be more reliable in the long run? I'll keep
you posted. One things for sure, if one of them breaks, that IvoProp
is in BIG trouble. 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.
