The Katzmayr effect

"Never heard of" will probably be your first response. Me neither, until Prof. Loek Boermans drew my attention to this effect during a discussion about a flightsimulator in development at the Technical Universty Delft.

As early as 1922 Prof. Katzmayr has considered this effect, but anno 2010 nobody in sailplane flying seems to have heard of it. The principle is quite simple:

- When flying in a steady situation and hit by a positive air gust, than the angle of attack will increase by some 2 to 3 degrees. Because of this and the positive inclination of the liftcurve, the liftcoëfficient will increase by some 50%. Therefore the liftvector itself will increase by this same amount and will also rotate forward. The result of this is that the sailplane is lifted up and accelarated in the forward direction. An additional effect is that the sailplane pitches shortly forward because of the effect of the gust on the horizontal stabiliser. Every sailplane pilot will recognise these effects of course.

- When flying in a steady situation and hit by a negative air gust, than the angle of attack will decrease and so will the liftcoëfficënt and the lift itself. The liftvector rotates backward. The result is that the sailplane is pushed downward and decellerated. Also, the horizontal stabiliser will pitch the sailplane shortly backward. These phenomena are a little more difficult to recognize by sailplane pilots, but they surely occur of course.

The changes in the lift as a result of equal positive and negative gusts are identical but of opposite sign. On the average, effects in height cancel out. However the rotated liftvectors L+ and L- are different in magnitude, as indicated in an algebraic and graphical way in figure 1. L+ is larger than L- since Cl(alfa0+alfag) is larger than Cl(alfa0-alfag) and therefore the thrust vector dT+ is larger than the thrustvector dT-. So, in a situation of randum gusts the sailplane will on the average experience a positive thrust vector and will accellerate. The pilot will automatically transfer this average accelleration into height by pulling slightly on the stick or by selecting a little more positive flap setting. Dr. Wolfram Gorisch derived a relation which shows a typical climb rate of 0,4m/s in bumpy conditions for a modern sailplane; an interesting result of course when gliding straight ahead through turbulent air between thermals or to your final destination. When having a wing profile with a plateau in the Cl-Alfa curve, be sure to fly fast enough with a small angle of attack to allow positive gusts to have their full effect in generating L+.

Please do not hesitate to ask me more.

Figure 1: The Katzmayr effect described in an algebraïc and graphical way

ir. K.P. Termaat