Myth 5 - Smooth Composite Airplanes Provide Laminar Flow And Low Drag

Don't bet on it!

If you are talking about a low speed sailplane, that glass smooth surface really pays off. If you talking about a high speed jet, it is not that important.

That is because total drag is an accumulation of several components, skin friction being only one item. As speed (Reynolds Number) increases, other factors become more significant, skin friction less of an issue. Other contributors include:

Drag due to lift As air passes over the wing creating lift, drag is created. Higher aspect ratio wings (wing span divided by wing chord) tend to reduce drag. Winglets can reduce tip vorticies which will also reduce drag.

Profile drag As the aircraft passes through the air, the molecules of air have to physically move out of the way to let the aircraft pass. It takes energy to move these is called profile drag.

Skin friction drag Drag due to air passing over the skin. It can be minimized by minimizing wetted area (read exterior surface) and is influenced by the roughness of the skin. Laminar flow is ideal but in a higher speed conditions, only small portions of the aircraft see laminar flow no matter how slick the surface.

Interference drag Intersections of surfaces like the wing to fuselage, horizontal tail to vertical tail, wing to tip tank etc. Fairings or aerodynamic blending can reduce drag on intersections. Burt Rutan designed the Pond Racer to break the world speed record for piston driven propeller aircraft. It had 32 intersections. Tough to break records carrying that kind of burden.

Trim drag Any time an airfoil is not streamlined it has additional drag. When a trim tab deflects it moves the control surface in the opposite direction. The added drag is trim drag. Most jet aircraft have variable incidence stabilizers which during cruise conditions keep the elevator streamlined.

Cooling drag On a single engine piston airplane, cooling drag can account for as much as 15% of total drag just to get the required airflow over the engine.

Neglecting any of the drag components can dramatically boost the drag count. A perfect design can only minimize drag. The cost to optimize and lower drag through flight test can be huge...the results often almost nil.