The angle of incidence is measured by the angle at which the wing is attached to the fuselage.

)

David Anderson

Fermi National Accelerator Laboratory

Batavia IL 60510

dfa@fnal.gov

&

Scott Eberhardt

University of Washington

Seattle WA 91895-2400

scott@aa.washington.edu

As discussed by Gail Craig (Stop Abusing Bernoulli! How Airplanes Really Fly

The wing as a pump

Air has viscosity

Why should a fluid follow a curved surface? The answer is viscosity: the resistance to flow which also gives the air a kind of "stickiness." Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface. The relative velocity between the surface and the nearest air molecules is exactly zero. (That is why one cannot hose the dust off of a car and why there is dust on the backside of the fans in a wind tunnel.) Just above the surface the fluid has some small velocity. The farther one goes from the surface the faster the fluid is moving until the external velocity is reached (note that this occurs in less than an inch). Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the "boundary layer".

The wing as air "scoop"

At cruise, the power requirement is dominated by parasitic power. Since this goes as the speed cubed an increase in engine size gives one a faster rate of climb but does little to improve the cruise speed of the plane.

Wing efficiency

Power and wing loading

Wing vortices

Ground effect

Another common phenomenon that is misunderstood is that of ground effect. That is the increased efficiency of a wing when flying within a wing length of the ground. A low-wing airplane will experience a reduction in drag by 50% just before it touches down. There is a great deal of confusion about ground effect. Many pilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.

Conclusions

· The amount of air diverted by the wing is proportional to the speed of the wing and the air density.

· The vertical velocity of the diverted air is proportional to the speed of the wing and the angle of attack.

Now let us look at some situations from the physical point of view and from the perspective of the popular explanation.

Axis of Rotation

The imaginary line which extends crosswise, wing tip to wing tip, is the lateral axis. Motion about the lateral axis is pitch and is produced by movement of the elevators at the rear of the horizontal tail assembly.

DETAILS OF MODERN AIRSHIPS - 1927

roll and pitch.

© 2000

But such is the nature of progress - the testing of ideas. It took the lives of airmen to drive the investigations which led to today's understanding of these matters and which allow our complacent and sometimes arrogant review of history.

Forces Acting on an Airplane

Laminar Flow Airfoil

The P-51 Mustang is the first aircraft every intentionally designed to use laminar flow airfoils. However, wartime NACA research data I have shows that Mustangs were not manufactured with a sufficient degree of surface quality to maintain much laminar flow on the wing. The RAE found that the P-63, despite being designed with laminar airfoils, also was not manufactured with sufficient surface quality to have much laminar flow.

The B-24 bomber's "Davis" airfoil was also a laminar flow airfoil, which predates the Mustang's. However, the designers of the B-24 only knew that their airfoil had very low drag in the wind tunnel. They did not know that it was a laminar flow airfoil.

The boundary layer concept is credited to the great German aerodynamicist, Ludwig Prandtl. Prandtl hyposthesized and proved the existence of the boundary layer long before the Mustang was a gleam in anyone's eye.

A favorable pressure gradient is required to maintain laminar flow. Laminar flow airfoils are designed to have long favorable pressure gradients. All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30 and 75% of chord.

Profile drag

Surface friction drag

Surface friction drag can amount to more than 30% of the total drag under cruise conditions.

At low AOA the high pressures near the leading edge produce a component of force in the rearward (i.e. drag) direction, while the low pressures ahead of the maximum thickness point tend to suck the wing section forward, giving a thrust effect. The low pressures aft of the maximum thickness point tend to suck the wing rearwards, since they act on rearward-facing surfaces. Without the influence of the boundary layer, the normal pressure forces due to the above drag and thrust components would exactly cancel.

The Swept Wing

Over the course of the past half a century, jet-powered flight has vastly changed the way we all live. However, the basic principle of jet propulsion is neither new nor complicated.

. To this basic engine, other components may be added, including:

to recover and direct the gas energy and possibly divert the thrust for vertical takeoff and landing as well as changing direction of aircraft flight.

, a long "tailpipe" behind the turbine into which additional fuel is sprayed and burned to provide additional thrust.

, which blocks the gas rushing toward the rear of the engine, thus forcing the gases forward to provide additional braking of aircraft.

in front of the compressor to increase thrust and reduce fuel consumption.

.

A turbojet engine.

A turboprop, or turboshaft engine.

A turboprop engine uses thrust to turn a propeller. As in a turbojet, hot gases flowing through the engine rotate a turbine wheel that drives the compressor. The gases then pass through another turbine, called a power turbine. This power turbine is coupled to the shaft, which drives the propeller through gear connections.

A high bypass turbofan engine.

A low bypass turbofan engine.

For supersonic flight, a low bypass fan is utilized, and an augmentor is added for additional thrust.