Polar diagram
 

I found it in an old PPL German book. Could someone explain me please the diagram and eventually translate it? Thanks



These diagrams do still exist albeit under a fresh name, "Drag Polar Curves".

senkrechter Sturz => vertical fall
Rückenflug => inverted flight
Rückentrudeln (not sure, here) => inverted stall
Schnellflug => fast cruise
Reiseflug => cruise
bester Gleitwinkel => optimal rate of descent (or "angle of descent")
maximale Reichweite => maximum range
bestes Steigen => best climb
geringstes Sinken => least descent
Anstellwinkel wird kleiner => decreasing angle of attack
Anstellwinkel wird größer => increasing angle of attack
Abkippen => stall
Klappe => flaps

hope this helps

Coefficient of drag on the horizontal axis, coefficient of lift (also AOA, as they move together) on the vertical. So unlike most graphs that show whatever you're controlling on the horizontal axis and the result on the vertical, this one shows what you're controlling on the vertical. Each curve shows a certain flap configuration.

The highest point on each curve is the highest lift coefficient, aka stall. You can see the higher the flap setting, the higher the max lift coefficient, meaning the lower the stall speed.

At any point along a curve, the ratio of lift to drag coefficients is represented by the slope between that point and the origin. (The graphical result of dividing top by bottom. Bigger ratio means steeper slope.) So if you're looking for the highest L/D ratio, (meaning the most efficient flight regime) for a particular flap config curve, it'll be shown by finding the spot on that curve that has the steepest slope to the origin, which in essence means where it bulges out the most to the top left. Aka, the tangent point. Here, that max slope is shown for the clean wing ("Polarentengente") and the point of the clean wing curve that touches it is "bester Gleitwinkel." Any lower on the curve, both lift and drag coefficients decrease, but in a ratio such that drag dominates. Any higher, they both increase, but again drag dominates. So that point, your best L/D, is your best glide.

For higher flap settings, you can see their tangent slopes to the origin are shallower, meaning their best lift/drag ratios are less, manning the best glide angle you can achieve is worse. Also those worse best glide angles occur at higher lift coefficients, meaning lower speeds.

Rückentrudeln => inverted spin

Would that be inverted stall, rather than spin? I don't know German, but I know aerodynamics enough to know that the airfoil would not know if it were stalled, or spinning, just it wasn't creating lift anymore...

No, I was wrong, ArrowFly is right. "Trudeln" does translate to "spin". And "Rücken" to "back", in the anatomical sense.