Drag Polar Invariance with Flexibility

MOST modern transport aircraft wings exhibit high aspect ratios (typically nine), and consequently undergo large deformations between ground and cruise, and beyond. The most noticeable effect is bending.Alarge bendingmodificationwill strongly contribute to the twist law alteration, and twist is amajor aerodynamic parameter.Wind-tunnelmodels undergo smaller deformations than flying aircraft, but these are often by no means negligible. Their aerodynamic effects can be quantified in pressurized wind tunnels by changing dynamic pressure. They can also be modeled by aeroelastic computational simulations. A puzzling thing is that, often in wind-tunnel tests, the effect is obvious on CL(α), Cm(α), and CL(Cm) curves but almost indiscernible on CL(CD) drag polars. The same phenomenon can happen with numerical simulations comparing a rigid-wing polar and a flexible-wing polar.It is the purpose of this Note to explain under which conditions this phenomenon may take place.