Wing Twist Angles

Prior to 1972 Standard Cirri were manufactured with a washout twist of -0.75 degrees. Early that year, however, the twist was increased to -1.5 degrees. This change occurred somewhere around serial number 175. According to Klaus Holighaus, this was to tame the hot landing characteristics of the early model. However, George Moffat, who owned both models, observed that it did more than that. He wrote in his book, Winning on the Wind, that it "distinctly enhanced performance." Apparently it made the new ship climb "much better" and it gave it a "much faster rate of roll" ("slightly over three seconds" compared to "a touch under four"). Indicated stall speed was reduced from 36 kts to 33 kts. George went on to say "early impressions that I was climbing well soon became convictions that I could go up through anything." Flying these ships at Vrsac, he and Ben Green "grew used to outclimbing all comers." In addition glide performance was improved at speeds up to 85 kts. Finally, as Klaus had intended, the new ship had a slower safe approach speed with "much shorter and safe landings."

In a letter to Torben Kristensen (#641), J. Laude, of Schempp-Hirth, indicated that the twist is localized to the "outer end" of the wings. Evidently this refers to the whole aileron panel where the airfoil is Wortmann FX 66-17 A II-182. (The inner wing panel is a linear transition zone from the root rib, where the airfoil is Wortmann FX S 02-196, to the aileron panel.) J. Laude, further observed that this "improved the flying characteristics at lower speeds, with more stability in curves, specially during thermaling. A measurable negative influence in performance at higher speeds could not be observed."

In January, 2004, Matthieu Scherrer, a French aeronautics student who had been following this web site, shared with the group his 3D calculations on the Standard Cirrus wing that give theoretical insight into the effect of the added wing twist. His results are presented below. In these graphs, Cl is the local coefficient of lift. Local refers to the value at particular span stations, as opposed to the effective Cl of the entire wing, for example. Span positions are normalized and designated as y/b, where y is the actual span coordinate, presumably from the centerline of the aircraft, and b is half the span of the full wing. Negative values indicate the left wing and positive values the right wing.

Local Cl distribution vs AOA, -0.75 degrees
Cl of original wing (-0.75 deg twist) vs angle of attack

Local Cl distribution vs AOA, -1.5 degrees
Cl of modified wing (-1.5 deg twist) vs angle of attack

Local Cl distribution: 0.15, 0.70, 1.35
Comparison at high, cruising, and thermaling speeds

Cl=0.15, high speed
Comparison at high speed

Cl=0.70, transition speed
Comparison at cruising speed

Cl=1.35, thermalling speed
Comparison at thermalling speed