Polar curve (aerodynamics)

A polar chart (in short: Polar ) is in fluid mechanics is a graphical representation of the forces acting on a body is flocked forces for different angle of attack. Shown are not the forces themselves, as they are very dependent inter alia on the flow velocity, but dimensionless coefficients. This representation has been developed by Otto Lilienthalplatz to assess the aerodynamic properties of the blades. The polar diagram is used to date for the characterization of profiles and aircraft.

  • 2.1 -resolved polar for the above example

Lilienthal Polar

The actual polar diagram, the so-called Lilienthalplatz polar, is a plot of coefficient of lift on the ordinate ( vertical axis) against the resistance coefficient of the abscissa ( horizontal axis). In addition to this resistance, the polar moment of polar, in which the torque coefficient is plotted on the angle of attack exists. The distance between the coordinate origin and a point on this curve is referred to as Polstrahl. When the resistance of Polar increase in Polstrahls is the glide ratio E for each point.

Drag polars allow a conclusion on the aerodynamic quality of a body. For example, in wing sections in glider area of ​​operation, high-speed flight or good slow flight characteristics, seen from the curve.

Specific points on the Lilienthal Polar exemplified by a profile

The figure at right shows a calculated with XFOIL Lilienthal polar of the profile NACA 2412 It is possible to find a number of specific points marked.:

  • " 1 " minimum buoyancy. The profile here has the smallest ( most negative ) on lift coefficient. This point corresponds to the minimum flight speed in horizontal inverted flight.
  • " 2 " zero lift. The profile generates no lift. This point corresponds to the parabolic flight. The drag coefficient at this point is referred to.
  • " 3 " smallest total air force. The Polar here has the smallest distance to the origin. is only for profiles that have above a pronounced Laminardelle, significantly different from. An airplane reached here in the almost vertical dive the largest aerodynamically possible speed.
  • " 4" Minimal resistance. For symmetric profiles it is usually (but not necessarily! ) At.
  • " 5" Best glide, the contact point of the steepest Polstrahls. The glide is minimal here, an aircraft reaches here, glide the greatest distance at a given altitude loss (). This point is linked to the best glide speed (V *). For jet airplanes, this is also the best climb speed and minimum thrust. For propeller airplanes it is the speed of least resistance, but not the speed for minimum power, this is 0.76 × V *.
  • " 6 " Best climbing, lowest sinking. The so-called climbing speed is maximum. Here's an aircraft in the gliding has the lowest rate of descent. The minimum sink speed for propeller and jet aircraft is 0.76 × V *.

Note: The points " 5" and "6" are found quite analogously for the negative part of the polar curve.

  • "7" maximum lift. The profile reaches its greatest impetus, becomes maximum. This is the lowest airspeed in level flight.

The points shown are found not only on profile polars but also on polars for total aircraft.

Resolved Polar

In polars the resolution of the force coefficients are plotted on the ordinate above the angle of attack on the abscissa. Widely used is the dissolved polar plot of lift coefficient for angle of attack. It is characterized by an almost linear curve at low angles of attack, with symmetrical airfoils through the origin. The course tends at high angles of attack, running through the apex and then falls in the so called the stall, again. The course at this vertex, the maximum attainable lift coefficient characterizes the demolition behavior of an airfoil or aircraft.

Resolved polars illustrate the influence of variables such as the Reynolds number or shape parameters such as buoyancy aids and surface texture on individual coefficients of a body being streamed. In ground-based vehicles such as the crosswind influence on the driving stability is crucial.

Resolved polar for the above example

In the dissolved polar of the NACA 2412, the course of lift, drag and moment coefficient is plotted in each case on the angle of attack. For the uniqueness of the Momentenbeiwertes its reference point has yet to be specified. If this is missing, then refers to the moment usually (and also in this example) to a point at 25 % tread depth ( " t / 4"). It can still read some other important variables:

  • : Gives the angle of attack for zero lift itself.
  • : The lift at zero angle of attack.
  • : The buoyant rise.
  • The size of the linear part of the lift polar. Here enters A., to no detachment.
  • : The angle of attack at maximum lift.
  • : The moment coefficient at zero lift.
  • The (virtually) horizontal course of the torque curve in the linear part of the example shows that the neutral point ( very close to ) the moment reference (in this case T / 4).
Aerodynamics
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