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 TFX | by Digital Image Design

TFX: by Digital Image Design

HUD (Heads Up Display) from TXX, 1993

APPENDIX SIX
THE SIMULATION
LIFT

As seen below, airflow over a wing generates lift because after the flow splits in two at the front of the wing, the shape of the wing causes the air over the top to travel faster in order to meet up with the air under the wing, as the curvature gives it a greater distance to travel. Even if the wing is symmetrical, angling it to the airflow means that the air does not split at the front of the wing, but on the underside, producing the same effect as an asymmetric ('cambered') wing.

In general, the major variation of CL occurs when angle of attack a is changed. As seen below, at a certain angle for a given plane, CL is zero. As a increases, so does CL, up to a point when CL is highest. After this, the wing is at too high an angle for the air to flow smoothly from the leading edge to the trailing edge of the wing. It begins to 'separate' or stall, producing a wake of low-pressure air behind the wing. As pressure behind the wing is low, this pulls back on the wing, acting as drag, without producing any extra lift.

Separation can occur unevenly on the wing, leading to unwanted rolling or yawing moments. So, increasing angle of attack beyond the stall angle is not only pointless, but dangerous.
Now return to the lift equation:

LIFT L = 1/2rV2 S CL

Hold r and S constant for a minute, and assume that, for level
flight, L must equal weight W. We are left with V and CL.
Bunching the rest together:

V2 CL = CONSTANT
or:
CL = CONSTANT/V2

So, if we change one of these two variables, the other must also have a new value in order for the equation still to work. Decreasing V, we must increase CL. However, we have already learned that CL can only go so high before the wing stalls. So, V can only go so low, before you are flying along at the stall angle, and when you try to decrease speed further and increase a to get more lift, you will find that you don't get any more lift, and you will stall. This speed is called the stalling speed and is the slowest speed at which the plane can fly level.

The maximum lift depends very much on the shape of the wing. Many airplanes use special methods to change the shape of the wings during flight. Two examples are shown below:

The Tornado has a variable sweep (swing wing) design. At low speeds, the wings have a low sweep angle (they are almost at right angles to the airflow). This gives high lift, but produces more drag than the opposite setting, used at high speeds, whereby the wings are swept back sharply, producing lower lift and drag coefficients (as speed is high enough to use lower lift coefficients). The YF-22 uses flaps to increase lift. These hinged devices at the leading and trailing edges increase the effective camber (curvature) of the wings, producing greater maximum lift coefficients, again at the expense of drag.

Both of these are examples of the Mission Adaptive Wing (MAW), the concept of in-flight adjustments to wing shape making airplanes more versatile. In TFX, both the F-22 and the Eurofighter have a leading and trailing edge flap MAW configuration. Both types of flap increase the stalling angle and maximum CL, but trailing edge flaps also increase the lift at all angles of attack (see below).

The flight control system automatically takes care of flap adjustments, selecting the best settings at all speeds and altitudes. You can see the positions of the MAW by selecting the control display on one of the cockpit MFDs.

The F-117A has no high lift devices of any kind, and is thus forced to take off and land at high speeds, and often fly at very high angles of attack. Luckily, the sharp sweep of the F-117A's wings, and the shape of the plane's body give higher lift than other planes would have without flaps.

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From the manual