Van's Air Force
Western Canada Wing
|
Van's Air Force
Western Canada Wing |
John is the EAA Technical Counselor
for Chapter 277, and has inspected quite a few RV's of various flavors.
“It is a good design, one of a handful that I actually recommend to people
who are looking for an airplane to build,” he says.
Corky is a regular contributor to the
newsgroup, who always has interesting insights into airplane building and
aerodynamics.
However, I can make a few general statements. Cooling drag, for a piston engine airplane, is one of the largest contributors of drag at cruising speeds. That is because you must have enough cooling to keep things from overheating on a hot day with an extended slow speed climb. That is your critical condition. Very few airplanes actually have enough cooling to climb for any length of time at the maximum climb rate speed on a hot day without overheating somewhere. You will often find a “cruise climb” recommended for that reason. The higher airspeed in a “cruise climb” allows slightly better cooling.
Clearly, the mods that do the most to increase speed when airplanes are “modded” for speed have to do with reducing cooling drag. The P-51 is a case in point, as are all of the airplanes that have been worked over by LoPresti. The first thing he does is clean up the cooling system. That is probably where the largest gains can be made for minimum effort.
The next largest contributor to drag is probably that caused by gaps and intersections. Good fairing and gap seals are the next largest contributor and the next best return for effort. Notice the plethora of speed mods that do things like seal gaps and fair protrusions. This includes sealing around wheel wells and around doors and windows.
The old Lindbergh trick was to go up and
fly in the rain. Look around you at the airplane. Every place
you see water piling up tells you where you need a fairing! It even
gives you a good idea of the size and shape of the fairing. Some
are quite counter-intuitive.
A simple rule that will help clean up
an airplane design is this. Every time air has to turn a corner or
change direction it causes drag. The sharper the corner, or the greater
the change in direction, the larger the drag increment. That is why
the most drag comes from the back side of something pushed through the
air. You have to move air to fill the gap where you just went by.
You can see the result of drag with a boat by the wake. Airplanes make a similar wake and the magnitude of the wake is a direct measure of the energy spent making the wake. That wake-making energy is what we call “drag.” Look at the wake behind a canoe and compare that to the wake behind a standard V-bottom powerboat. The drag and the wake of a boat increases alarmingly at a certain speed based on the waterline length. This is similar to the effect of Reynolds Number in aerodynamics. Drag can be quite low at speeds low enough to remain relatively laminar. As speed increases and Reynolds Number increases, laminar flow becomes much harder to maintain.
If you place a burning cigarette in an ashtray in a still room, you can see a smooth tight stream of smoke rising above it. Suddenly, a few inches above the burning cigarette, the tight smooth column of smoke will break into swirls and eddies and increase markedly in size. That is the laminar/turbulent transition point. Anything will trip the flow mode. There is no way you can force the stream to turn and stay laminar. The turbulent flow has a lot more wake, hence drag
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