Does less dihedral improve the performance of a wing?

From : Don Stackhouse

In level flight less dihedral is usually (but not always) better. In turning flight this is often not the case.

We noticed back in the early days of developing the first Monarchs that reduced dihedral usually resulted in higher (i.e.:worse) sink rates in tight thermal turns. Investigating further, we also noticed that flatland, low altitude soaring birds such as Turkey Vultures and Prairie Falcons tended to use lots of dihedral, while slope soarers and higher altitude thermal soarers like the Black Vulture and the California Condor tended to keep their wings flat (BTW, yes, my younger brother just happens to be a professional ornithologist).

Stability is often cited as the explanation for the birds behavior, but this theory doesn't stand up to scrutiny. The slope soarers face as much turbulence as the low altitude thermal soarers, yet they don't use high amounts of dihedral. Besides, why would a flying creature that already has the ultimate form of "active fly-by-wire stability augmentation", massively variable geometry, and relaxed stability need to add more dihedral to increase stability, especially if it costs performance to do so? It just doesn't make sense. Added to that we had the corraborating evidence from our own experiments that there was a PERFORMANCE benefit to dihedral in certain flight modes.

We later figured it out mathematically. I'll spare you the grizzly details, but in a small radius turn (typical in working low altitude, small diameter thermals) the flow field around the wing is highly non-uniform, and a good dihedral arrangement can take advantage of this to help certain portions of the wing do a better job. This phenomenon shows up when the radius of the turn is small in comparison to the wingspan. It's particularly effective at lower Reynolds numbers. The California Condor doesn't take this approach because it typically works higher altitude lift where the thermals are larger in diameter and its circles aren't so tight. We've also observed Turkey vultures increasing their dihedral when making tight circles down low, then reducing their dihedral when they widen out their turns higher in the thermal.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net