Fighter Wing: A Guided Tour of an Air Force Combat Wing

THRUST

This is the force that causes an aircraft to move through the air. It is provided by an aircraft's engines, and has the same effect on the aircraft whether it is pulled through the air with a propeller or pushed with a jet engine. Thrust is conventionally measured in pounds or newtons. The more thrust an aircraft's engines can generate, the faster the aircraft will travel, and the more lift the wings will provide. Similarly, when you step on your car's accelerator, the engine produces more power, the wheels spin faster, and the car moves along the road at a higher speed. This action also causes the air to move past the car at a higher speed.

^{An illustration of the four primary forces on a powered aircraft: thrust, drag, lift, and weight.}

^{Jack Ryan Enterprises, Ltd., by Laura Alpher}

In the world of combat aircraft design, the engine's raw propulsion power is expressed as its thrust-to- weight ratio. This ratio compares the amount of thrust that the engines can produce to the weight of the aircraft. The higher the ratio, the more powerful the aircraft. For most combat aircraft, this ratio is around 0.7 to 0.9. However, really high-performance models, like the F-15 and -16, have thrust-to-weight ratios greater than 1.0 and can accelerate while going straight up!

LIFT

Lift is the force that pushes an object up due to the unbalanced movement of air past it. In an aircraft, the unbalance comes from the different curvature of the upper and lower surfaces of the wings (the upper surface has more curve than the lower), and the movement of air is provided as a consequence of the engine's thrust. When the moving air comes in contact with the leading edge of the wing, the air separates. Part of the flow passes over the top of the wing, and the remainder below. Given the shape of an aircraft's wing, the air stream on top has to travel a greater distance than the stream below. If both air streams are to arrive at the trailing edge at the same time, then the air stream above the wing must have a higher speed.

In aerodynamics, there is a simple, but neat, relationship between the speed of a gas and its pressure: The faster a gas travels, the lower its pressure and vice versa. This principle is called Bernoulli's Law, in honor of the 18th-century Italian scientist who first investigated it experimentally. So if the air stream above the wing is moving faster than the air stream below the wing, air pressure above the wing will be lower than below the wing. This difference causes the air below to push upward and 'lift' the wing up. As the speed of an aircraft increases, the pressure difference grows and produces more lift. This wing's angle, called the angle of attack (AOA) of the aircraft, can have a significant effect on lift.

Initially, lift increases as AOA increases, but only up to a certain point. Beyond this point, the AOA is too large and the air flow over the wing stops. Without the air flow, there is no pressure difference and the wing no longer produces lift. When this situation occurs, the wing (and the aircraft) is said to have stalled. Now, a high AOA isn't the only thing that will cause an aircraft to stall. If an aircraft's speed gets too low, the air no longer moves fast enough over the wings to generate adequate lift, and again the aircraft will stall — and any pilot will tell you that stalls can be really bad for your health.

DRAG

Drag is the force that wants to slow the aircraft down. In essence, drag is friction; it resists the movement of the aircraft. This is a tough concept to grasp, because we can't see air. But while air may be invisible, it still has weight and inertia. We've all taken a walk on a windy day and felt the air pushing against us. That is drag. As an aircraft moves through the air, it pushes the air out of its way, and the air pushes back. At supersonic speeds, this air resistance can be very significant, as a huge amount of air is rapidly pushed out of the way and the friction generated can rapidly heat the aircraft's body to temperatures over 500deg F/260deg C.

There are two types of drag, parasitic and induced. Parasitic drag is wind resistance associated with the various bumps, lumps, and other structures on an aircraft. Anything that makes the aircraft's surface rough or uneven, like bombs, rivet heads, drop tanks, radio antennae, paint, and control surfaces (rudder, canards), increases the aircraft's wind resistance. Induced drag is more difficult to understand because it is directly linked to lift. In other words, if lift is being generated by the wings, so too is induced drag. Since drag is unavoidable, the best that can be done is to minimize it and understand the limits it places on the aircraft's performance. And the limits are significant. Drag degrades the aircraft's ability to accelerate and maneuver and increases fuel consumption, which affects combat range/radius. Therefore, a good understanding of drag is needed not only by aircraft designers, but by aviators as well.

WEIGHT

Weight is the result of gravitational attraction of the Earth, which pulls the mass of the aircraft toward the Earth's center. As such it is in direct opposition to lift. Of all the forces involved with flying, gravity is the most persistent. To some extent, we can control the other three. But gravity is beyond our control. In the end, it always wins (unless you're riding a spacecraft fast enough to escape the Earth's gravity entirely — about 25,000mph [40,000 kph]!). Thrust, lift, and drag are all accounted for in the design process of the aircraft. But when thrust or lift become insufficient to maintain the aircraft aloft, gravity will bring the plane down.

ENGINES

Once you understand the physics of flight, and you can build a sufficiently lightweight power plant, getting an aircraft into the air is a relatively simple matter. But operating high-performance aircraft in the hostile environment faced by today's military aircraft is quite another thing. These machines are anything but simple.

With complexity comes problems. The heart of a good aircraft is a good engine — the thing that makes it go! More fighter programs have been plagued by engine troubles than by any other source of grief. So, what's the big deal in making a good jet engine, you might ask? Well, try and imagine building a 3,000-to-4,000 lb./1,363.6-to-1,818 kg. machine that produces over seven times its own weight in thrust and is made with tolerances tighter than the finest Swiss watch. It has to operate reliably for years, even when pilots under the stress of combat or the spur of competition push it beyond its design limits.

To give you a better picture of how exact these engines are made, look at a human hair. While it may look pretty thin to you, it would barely fit between many of the moving parts in a jet engine. That's what I mean by tight tolerances! Now, let's spin some of those parts at thousands of revolutions per minute and expose a few of them to temperatures so high that most metal alloys would melt instantly. One can now begin to appreciate the mechanical and thermal stresses that a jet engine must be designed to handle every time it runs. Should even one of the rapidly rotating compressor or turbine wheels fail under these stresses and come into contact with the stationary casing, the resulting fragments would shred the aircraft just as effectively as missile or cannon fire.

Since a combat aircraft's performance is so closely tied to its propulsion plant, the limits of engine