September 29, 2017, ©. Leeham Co: Airbus flew its “Blade” laminar flow research aircraft for the first time this week. It’s a project in the European Clean Sky research program.
The “Blade” aircraft is a modified Airbus A340-300, where the outer wings have been replaced with special laminar flow wing sections. We will spend a couple of Corners to understand why this research is done and why it’s important.
The drag force of an aircraft makes it necessary to have engines on the aircraft, generating thrust to offset the drag. The higher the drag, the higher the fuel consumption.
Aircraft drag has two principle components for subsonic airliners. The drag due to size, called parasitic drag, and drag due to weight, called induced drag.
Of the two, the parasitic drag is the dominant part in most flight situations. Most of the parasitic drag is due to the air’s friction against the aircraft’s skin. The rougher and larger the aircraft skin, the higher the skin friction drag.
The parasitic drag for an Airbus A320 with 150 passengers on a typical two-hour flight is 5.000lbf at mid-weight (then flying at Flight Level 350, 35,000ft). The induced drag is then 2,500lbf.
This means the two engines need to counter 7,500lbf of drag. If we could reduce the drag we could reduce the engine thrust and thereby the aircraft’s fuel consumption.
The airflow near the aircraft skin is turbulent on more than 95% of the aircraft surface at these conditions. Turbulent flow creates more drag than if the surface could be made so the flow stayed laminar.
The difference in drag between laminar and turbulent flow is large. The parasitic drag could be halved if the flow could be kept laminar over most of the aircraft surface. This is difficult to achieve, but if the flow could be kept laminar on more than today’s initial 5% of the surface, a lot would be gained.
It’s difficult to create aircraft surfaces that keep the airflow laminar over a larger proportion than 5%-10% of the aircraft surface. When the air hits, for example, a wing’s leading edge, the flow starts as laminar, but then quickly (within inches) turns turbulent.
To keep the flow laminar over a longer distance, one needs a smooth surface and a special curvature of the wing profile. This is what is researched with the Blade aircraft. It has special “laminar flow” outer wing sections, Figure 2.
How such surfaces are done and why they are difficult to introduce on production aircraft, we will cover in the next Corner.