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Drag Percentages

This is a reproduction of two posts to the rec.aviation.homebuilt newsgroup on the internet.  The posts, by John Johnson and Corky Scott present some interesting ideas about the drag on a light airplane, and were inspired by a question about the percentages of drag from various sources on an “average” homebuilt airplane..

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.

Charles K. (Corky) Scott

There are lots of places to counter drag.  Take a look at the “Mister Mulligan” next time you see it at a big fly in for hints on where to pay attention to streamlining and other details.
With an air-cooled flat engine, “efficient” and “airflow” really shouldn’t be spoken together.  The intake openings are in an ok space, but often the openings are simply flat cutouts against the nose.  You’ll notice that the most efficient openings are round holes with a protruding lip around them and often they are smaller in size than what you are used to.
The air must go past the cylinders, which are very draggy.  Then the air, which has just become heated and therefore wants to rise, is forced down and out the bottom of the cowl, usually.  Often the air must go by numerous objects within the engine compartment, each of which makes their contribution to the drag of the flow.  Then the air is going downward and must transition to horizontal flow to match that of the air passing the fuselage.  A torturous route for air to follow.
Some aerodynamicists attempt to alleviate the flow problem by exiting the air in front of the windshield, which is good in that at least the heated air wants to go up.  But it’s bad in that close to the windshield it is a high pressure zone.  Air exhausted into a high pressure zone will flow very poorly, if at all.
That’s why, in theory anyway, it’s possible to set up a more efficient flow-through system using a radiator for a liquid cooled engine than to design something for an air cooled flat four.  Unfortunately, it’s still not easy because unless you use a radiator specifically designed for your space considerations, you will face compromises in fitting it.  Lots of guys lay the radiator down flat and bend flow through it.  This works, but obviously isn’t as good as flow straight through an upright radiator so the air doesn’t have to bend much.
Then again when you’re dealing with an airplane that only cruises at 130 max and has a big high-lift wing, would having the perfect cooling system make any difference?

John Johnson

Unfortunately, I can’t give you a list of numbers, like 30% cooling, 28% wing, 37% fuselage, 10% gear, etc.  Any numbers I gave would only apply to one specific design in one specific condition.  I know, you did specify the “cruise” condition, which is probably the most reasonable speed to evaluate an airplane for drag.

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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