Aerodynamic center

The neutral point (NP ), or aerodynamic center of a profile, the fixed point with a constant torque in the moderate angle of attack. It lies approximately to the chord in 25 % of the tread depth.

The torque or its Momentbeiwert Cm always refers to the fixed neutral point on the wing profile. The Momentbeiwert CM25 refers exactly to the t/4-Punkt the tendon.

Stability computations, the concept to the constant torque, and the fixed position NP to the wing is easier to handle, than the pressure point (DP ) and the angle -dependent position of the profile chord. The Momentbeiwert is approximately proportional to the profile curvature and its reserve ( Cm = f * xf * -6).

Definition

Neutral point

Is the lift coefficient and is the Momentbeiwert

Momentbeiwert

Sometimes the Momentbeiwert CM25 a profile is shown in a diagram called " curve" to the angle or the lift coefficient. However, since the change of this curve in the normal angle of attack is low in profile descriptions often only the mean value of the torque at the neutral point Cm used.

The distance DP - NP is in relative proportion to the tread depth (t = 1). The Momentbeiwert often a sign. Minus means that the leading edge is pushed down.

The pressure point hike over the angle of attack is about:

Location of the neutral point

For a profile in incompressible, inviscid flow, a constant neutral point arises from the skeletal theory at 25 % of the tread depth (t / 4), which is independent of the profile shape and buckle. In the real flow around profiles, this only shifts slightly due to influences of Reynolds number and Mach number. It remains almost constant at changed angle of attack. The generated by R. Eppler laminar it is about 26%, for profiles for slow aircraft further ahead (approx. 24%). In the supersonic region it is for the flat plate at 50 % tread depth.

In addition to the neutral point of profiles can be divided:

• The neutral point of the wing (mean aerodynamic center, MAC).
• The neutral point of the entire aircraft (wings, fuselage, tail ). It can be roughly calculated from the neutral points of the individual components and their geometric position.

The neutral point of the aircraft lies with the conventional configurations behind the neutral point of the wing. Below the neutral point of the airplane will be described.

Longitudinal stability

Stability along the stability of the aircraft about its transverse axis. The corresponding movement is called a " nod ". A stable behavior must occur to a restoring moment on the aircraft by a small disturbance in the angle of attack that attempts to reduce the interference. At this restoring torque contributes to one at a low center of gravity and on the other, the lever arm to the horizontal stabilizer. Importantly, the distance aircraft center of gravity ( SGP) to aircraft neutral point (NP ).

• SWP before NP. The additional buoyancy generated with respect to the SGP a top-heavy (back -handed ) moment ⇒ stable behavior
• SWP in NP. The additional buoyancy produces no torque, the disorder remains indifferent behavior ⇒ ( = neutral stability)
• SWP behind NP. The additional buoyancy generated with respect to the SGP a tail -heavy moment ⇒ unstable behavior

In practice, the position of the neutral point exclusively from the wing and tail layout and the arrangement of these two surfaces depends on each other. With an increase in the angle of attack to the buoyancy increased by. Since the pitching moment about the neutral point but must remain constant, the additional buoyancy can only attack in the neutral point itself. The focus should therefore lie in the neutral point.

Due to the demand for stable flight shows that the anticipatory wing must generate a greater lift coefficient than the following. The use of positively curved wings with Negative Profile moment further increases this difference. This observation applies to both conventional aircraft and configurations for ducks and for swept and unswept flying wing. The front wings ( or portion of the wing ), always has the greater angle of incidence as the rear.

A measure of the steering stability, is the restoring torque per pitch angle. Another dimensionless stability measure is calculated on the mean aerodynamic chord distance of the center of gravity in front of the neutral point.

Trim speed

The trim speed of an aircraft is usually set so that the best glide ratio is achieved. This is done by horizontal positioning of the center of gravity ( SP) or adjusting the horizontal stabilizer (see Einstellwinkeldifferenz ).

Control the speed of a moving aircraft is, to change these trim or this setting. This is achieved by mechanically moving the center of gravity, or by aerodynamic shifting of the pressure point. The pressure point may with the tailplane or - to move the rear flaps on the wing - slightly. For motor-driven aircraft, the location and the strength of the drive plays an essential role in the stability and control. However, all aircraft have to fly unpowered also stable with appropriately adapted trim, the above is generally true in aviation.