We all know that air has mass and that it will take a force (energy) to change the direction of any mass in motion (Newton). Aerodynamic drag reduction is just a way of changing the direction of localized air flows without disrupting the main air flow around the aircraft. This change of direction must be done as smoothly as possible.
If you think of the aircraft as moving through a thick fluid you can imagine when this fluid must give way to the airframe and might interfere with other flows. This virtualisation will help where to look for improvements on your particular aircraft, and in addition consulting with other builders of the same type is also advisable.
This page will look at the parts and sections of an aircraft which can changed or modified by the builder to reduce drag (streamlining the air flow) and to improve the performance of the aircraft.
When looking at an aircraft one can see a number of items sticking out into the wind, so to speak. These items are usually already streamlined to a large extent by the aircraft/ part manufacturer. But there is always room for improvement, and this is where the aircraft kit builder shines.
One good example is the landing gear. Some manufacturers have them streamlined so well (Lancair, DynAero and Pipistrel) that the difference between retracting or a good fairing / streamlining job is really minimal. The extra weight of creating a retractable gear outweighs the small loss of performance by properly streamlining the landing gear.
Fairing a landing gear leg concentrates on four major points: the intersection between the fuselage and the gear leg, the leg itself, the intersection between the gear leg and wheel and the wheel pant. These four places need all to be streamlined if it is to have any effect at all.
This is a form of interference drag (like the intersections mentioned above) where proper fairing separates and guides the interfering air flows and this reduces drag. You will see this fairing at the wing root where it is attached to the fuselage. It can also be applied where the horizontal stabilizer or vertical fin enters the tail section in the fuselage.
Cooling inlets are needed to let air enter the cowling to cool the engine. But these are sometimes too large creating a lot of pressure in the cowling. As air exits under cowling it has been warmed up. Warm air expands so these exits must be even larger than the inlets. It is best to create a low pressure differential between the inlet/ exit so that warm air is sucked out of the cowling at the underside.
If you need to design your own cowling, take a look around at similar engine/ aircraft combinations to see whats possible with round air inlets.
Metal aircraft usually have no choice but to put the radio antennas outside. If you must do that use the streamlined types or if possible the thin metal rod types. Keep in mind that these thin metal rods have a bit worse SWR (standing wave ratio, or effectiveness) that the thicker ones on the ends of the frequency band (near 108 and 137 MHz). This will usually not a problem with the higher powered COM sets with greater than 10 watts of RF output.
If at all possible, lowering your canopy reduces the frontal area and drag. But the headroom might become to small for the larger pilots out there. And while you're at it: creating a turtle deck behind the canopy keeps the air flow from becoming turbulent and this reduces drag too.
Using a smaller tail wheel and streamlining this can reduce drag. One needs to ask how much trouble is involved and how much is there to be gained as you will have somewhat less controllability on the ground during landing roll out and taxi.
The gaps between aileron or flaps and the wing can be sealed with a special tape. Keep in mind that with fowler flaps you do NOT want that gap to be sealed off (when that flap is down) as that will disrupt the air flow over the flap and increase stall speed!
Other gaps around the aircraft, tail, stabilizers and even not so perfectly fitted fairings can be sealed to reduce turbulence. Again use special tape for that (3M) as the suns UV rays hardens the tape and possibly the glue might damage the paint job. This tape will usually need annual replacement.
Sometimes wing gap seals can produce an anoying tone or sound when flaps are extended. Pipistrel Alpha Trainer was known for this audible aerodynamic effect. Removing the top seal solved that.
You as the builder can experiment with sleeker windshields, improved engine cowls, vortex generator kits and more. There are a number of manufacturers busy in the aircraft speed market with STC'ed improvements which can be used by homebuilders too.
This combination creates the power and thrust you need, keeping them in tip top shape maintenance wise will result in a higher performance. Engine intake and exhaust can be optimized so that air gets in and out of the engine with the least restriction which will result in a couple more horsepower or kilowatt for you.
Also installing an electronic engine ignition will increase efficiency and lowers fuel burn or produce higher power, whichever you choose. Read more about that here.
Kent Paser managed to squiggle 57 knots more out of his Mustang II top speed, I can only imagine what can be done with your aircraft with some time, effort and creativity.