/Library

/Building Kit Aircraft

/Aerodynamic CleanupEvery aircraft builder is interested in improving the speed of his airplane, or at least reducing its fuel consumption so that he/she gets the highest mileage out of every gallon or liter of fuel in the tanks or even the lowest hourly rate.

Basically the form of the aircraft defines how much drag it generates and, in un-accelerated flight, how much thrust the engine propeller combination will need to produce to keep up the required speed. The math is simple: this costs fuel.

We as the builder can try to keep the weight as low as possible (more lift results in more induced drag). But by paying attention while we construct the airframe we can also reduce parasite drag by streamlining parts as much as possible.

Thus by reducing the amount of drag (parasite) and keeping the empty weight to a minimum we consequently reduce fuel consumption and we can increase the payload and/or the range of our aircraft. We therefore have increased the efficiency of the aircraft / engine combination and the cost for the flight is reduced.

This section will explore what can be done to reduce weight and parasite drag to improve the performance of our aircraft.

When an aircraft in is un-accelerated level flight, the thrust is equal to the drag of the aircraft, remember Newton's first law. And we also know that Thrust (Drag) = Horsepower × Propeller Efficiency. Both variables thrust and drag are expressed in either pounds or kilograms.

This formula tells us that if you increase the thrust or decrease the drag, or even do both, the aircraft will go faster. But there is a mathematical relationship between power, drag and speed. The speed increases with the cube root of the power increase or in formula form: Speed Increase = Power Increase^{1/3}.

Look at the next example to clarify this: lets assume your aircraft has an IAS of 100 kts with a 150 bhp Lycoming engine and you are curious to know how fast it will cruise with a 180 bhp Lycoming. The increase in power is 1,2 (results from 180 / 150). The cube root of that increase (1,2^{1/3}) equals 1,063. Thus the increase in speed is 100 × 1,063 = 106,3 kts. In reality you will not go that much faster by increasing engine power alone.

Drag also increases with the square of the speed. So if your aircraft total drag was 200 lbs at 100 kts then increasing the speed to 106,3 kts the total drag would end up to be (106,3 / 100)^{2} × 200 lbs = 226 lbs. That totals to 13 % more drag from a 6,3 % increase in speed, let alone the increase in fuel flow...

We can conclude that drag reduction is the single best way to get additional speed from a certain design, and the faster the airplane gets, the more important each little improvement becomes.

In flight, any airplane produces drag. This total drag is a combination, or the sum of induced drag (as a result of lift produced by the wing) and parasite drag. Note that: when an aircraft increases speed, induced drag becomes less (due to a lower angle of attack) and parasite drag increases.

This is drag as a result of generating lift by the aircraft. We as a pilot have some influence on induced drag by extending or retracting flaps and or slats. As an aircraft kit builder there is not much we can do about this (apart from redesigning large parts of the airplane, which some have done). With higher airspeeds, in level flight, this form of drag reduces (lower AoA).

Parasite drag is the sum of profile (the shape of the fuselage) and interference drag (this occurs where the wings, antennas, tail and undercarriage are attached on the fuselage). Thus every part of the aircraft that sticks out in the wind generates this form of drag and if we can keep this to a minimum, we have an advantage. In contrary to induced drag, this form of drag increases with speed.

By regularly washing and waxing the aircraft and keeping the surface as clean and smooth as possible (yes, the propeller too) we reduce drag in the boundary layer caused by dirt and bugs. And minor side effect is that your aircraft will look great when visiting other flying clubs and airports.

By properly aligning the wings, flaps, tail feathers and all control surfaces in such a way that the aircraft flies straight and level without any input from you and no with control surface deflection (zero trim), drag is also reduced enormously. Imagine an aircraft that flies left turns hands off, in this situation you need constant aileron input to the right causing drag and performance will suffer.

Do spend some time accurately rigging your aircraft, checking wing geometry and proper aileron and flap alignment, it will pay off in an easier to fly (hands off) aircraft with the lowest drag and fuel consumption as possible.

This is where we as the kit builder can do our best to reduce these drag effects and enjoy somewhat higher cruise speed and/or lower fuel consumption and reduced flying cost. But first we need to explain about these forms of drag, so that the builder can apply this knowledge to his or her airplane and really get results from their building efforts.

Our next pages will go into parasite and induced drag and we will see how we can clean up aerodynamic drag to enjoy the benefits of our efforts.