Tackling Frictional forces and smoothing flow. Part 1

Saric's team at Texas A&M University's Flight Research Laboratory use a Cessna O-2 Skymaster to carry a 30-deg-swept-wing airfoil section perpendicularly under its left wing to flight-test laminar flow techniques for transonic wings.

For Aerospace Maufacturers, particularly those in the business of building transports, the Holy Grail of their continuing quest is to reduce the dragcreating effects of friction on the skins of their aircraft.

Friction accounts for half the total drag on a modern jet transport; the rest comes from pressure induced and wave drag. An aircraft’s need to generate lift in order to stay in the air creates these latter three forces, so not a great deal can be done to minimize their effects. Wing fences or winglets can shave only a few percentage points off an aircraft’s induced drag budget, and only if the plane remains cruising long enough. However, if skin friction can be reduced appreciably, an aircraft will achieve a proportional saving in the amount of fuel it burns, conferring benefits on range and operating cost. This is the reason for the industry’s fascination with laminar flow control. If greater amounts of boundary-layer air can be made to flow over an aircraft’s wing, fuselage, and empennage without becoming turbulent, the plane will burn proportionally less fuel. About half the total skin friction experienced by an aircraft is on its fuselage and empennage, and about half is on its wings, says William Saric, a professor at Texas A&M University’s Dept. of Aerospace Engineering and the director of its Flight Research Laboratory. Recently, companies have experimented successfully with riblets raised rib-like protuberances applied along fuselages and empennages in areas where aircraft have turbulent boundary-layer air to reduce skin friction (and fuel burn) by 2-5%.

Fuel savings from laminar flow

According to Saric, achieving wing laminar flow would complement the use of riblets elsewhere on an aircraft and would yield additional fuel savings. He estimates that wing laminar flow control potentially offers fuelburn savings of 10% to 12%—roughly equal to the savings a new generation of turbofan engines offers compared with the preceding generation.

The calculation is simple: Skin friction accounts for half the drag on an aircraft, and the wings account for half of that skin friction—hence they represent a quarter of the total friction drag. But not all skin friction on the wings can be nullified: Saric notes there is a limit to the degree of wing laminar flow that can be achieved. Laminar flow breaks down as a result of disturbances within boundarylayer air. As these disturbances grow and become more unstable, they create turbulence. The boundary layer can remain laminar as the flow accelerates to its minimum pressure at about 60% of chord. However, the air must decelerate efficiently to atmospheric pressure by the time it reaches the wing’s trailing edge; this ensures that boundary-layer disturbances create turbulence in the pressure recovery region over the control surfaces.

Since laminar flow is only possible over about 60% of the wing, total wing friction can potentially be reduced by only 60% at most—or about one-eighth of total skin friction on the aircraft. Saric says laminar flow over the wing’s upper surface would produce about 60% of the wing friction reduction benefit, and laminar flow over the lower surface about 40%.

Tried and tested techniques

Various approaches have sought to achieve laminar flow control, and some have seen fair success. Creating a 2D airfoil (a very thin airfoil with a sharp leading edge), and ensuring the wing leading edge and surface are highly polished, is the best known way to achieve natural laminar flow. Saric says it is also much easier to achieve it on a wing with no sweep angle, or only a small one, than on a transonic or supersonic swept wing.

Another option is to use weak suction at the surface. Boeing used this technique successfully in the 1990s in an experiment with a 757. This approach combined natural laminar flow control—using an accelerating pressure gradient in the swept-wing airfoil— with tiny holes in the leading edge of the wing. Suction applied through those holes helped control leading-edge airflow contamination and crossflow instabilities.

But in a swept wing that carries fuel and features high-lift leading-edge devices, installing the ducting needed to produce leading-edge suction presents engineering problems that may not be solved easily (or cheaply) in a production aircraft. Saric’s team at Texas A&M is pursuing a different approach that, while still at the technology demonstration stage, might eventually offer a simpler way to produce laminar-flow control benefits.

More to follow.