Pressure point (fluid mechanics)

The pressure point (DP) of a body in the flow is where all effective flow forces can be summarized. There is no torque in relation to the pressure point ; this distinguishes it from the neutral point .

With lift profiles , the pressure point is behind the neutral point. The distance between these two points is: ${\displaystyle x}$

${\displaystyle x={\frac {C_{\mathrm {m} }}{C_{\mathrm {a} }}}\cdot t}$

in which

• ${\displaystyle t}$ the chord
• ${\displaystyle C_{\mathrm {m} }}$ the moment coefficient in relation to the neutral point and
• ${\displaystyle C_{\mathrm {a} }}$is the lift coefficient .

The pressure point of the wing profile is at the intersection of the air force resultant and the profile chord . Its position on the chord depends on the curvature of the profile and the angle of attack of the wing . With curved profiles, the pressure point is usually one third of the surface depth ( ). ${\displaystyle x\approx 0{,}33\cdot t}$

Pressure point hike

Pressure point position on the profile chord

On curved wing profiles, the pressure point changes its position in the direction of flow depending on the angle of attack: it moves forward on the profile chord when the angle of attack is increased. With a DP position behind the quarter point of the tread depth, every increase in the angle of attack causes an upward torque change ( nose up).

The profile curvature and its backward position are the geometric measure for the pressure point migration . The torque around the neutral point is the dynamic measure for the pressure point migration.

With symmetrical wing profiles, the pressure point is at the point of the neutral point, regardless of the angle of attack, i.e. at 1/4 of the wing depth. A similar behavior can be achieved with a suitably corrugated skeleton line of an S-shaped profile .

stability

The air force vector at the pressure point develops a torque at the fixed neutral point on the profile. This torque around the transverse axis ( longitudinal stability ) is compensated and stabilized in stationary gliding ( trim speed ) with the paraglider by the low position of the center of gravity . The glider achieves the same purpose by positioning the pressure point with the horizontal stabilizer .

control

To control the speed ( nodding movement around the transverse axis), the center of gravity is shifted horizontally on the hang glider . With the paraglider the position of the pressure point is changed horizontally with the wing trailing edge or the accelerator and with the glider with the horizontal stabilizer.

Stability and control

For stability and control, setting (trimming) and shifting (controlling) the pressure point or the center of gravity means the same thing. The “pressure point center of gravity” geometry (distance, direction) is decisive.

See also

• Stagnation point

literature

• Götsch, Ernst - Luftfahrzeugtechnik , Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8

swell

• nasa.gov: Center of Pressure
• Thuros Werkstatt: Aerodynamic Basics , accessed on June 7, 2018
• raketenbauen.de: Calculate the flight stability of a model rocket