On Monday, May 18, 2015 at 8:00:29 PM UTC-7, wrote:
On Monday, May 18, 2015 at 3:56:47 PM UTC-4, Jonathan St. Cloud wrote:
Can an aerodynamicists explain the reasoning behind the bent up wing tips of the Schempp-Hirth and Jonkers gliders. I thought Schempp first started this on the Nimbus4 purely to keep the outboard tips from getting scraped, but now the tips are bent on shorter wing birds. Someone fairly knowledgeable once told me the the bent tips actually hurt performance in the run but help in climb. I just wanted to get a bt more educated not the reasoning. I noticed the Quintus has bent tips but the same wing on the Antares 23 were straight.

Here is a theory: When you are banked at 40° when thermalling the inside
wing tip is closer to the core than the outside wingtip. With the strong dihedral of the wingtip this puts the bank angle of the wing tip closest to the center of the thermal at say ~28° instead of the ~40° so the lift vector is closer to perpendicular to the lift = better climb rate.

What i don't understand is that when flying straight/level with an effectively high dihedral the lift vector of both wings are pointing perpendicular to the wings, so the vectors are not pointing up, but rather inward. They balance each other out of course but seems like if they were perpendicular to gravity they would be most efficient. Isn't dihedral costly on performance, but helps on handling?

Curious if anyone else makes any sense of this. I'm more asking than telling.
Chris
Not an aerodynamic expert!

Where to begin.

In your example the inner wing is going slower and produces slightly less lift (everything else being equal) and the outer tip is going slightly faster and produces more lift, so by your logic the extra dihedral overall would go towards producing more inward lift than upward lift.

Of course none of that really explains use of polyhedral versus dihedral. Both are used to create spiral mode stability. Simple V dihedral is easier to manufacture, whereas polyhedral will produce similar spiral stability with more of the wing carrying a lower dihedral angle overall because of the longer moment arm out at the wingtip. In either case, the dihedral effect occurs through the coupling of roll and sideslip.

It's really not first and foremost a performance thing. The angles involved are pretty small in the first place so the cosine for the dihedral angle will be close to 1 and the sine will be close to 0, meaning the performance effect will be tiny. You mention something like 12 degrees of dihedral in your example, but that much would produce a pronounced dutch roll and would be a pretty unpleasant to fly airplane. More typical is a couple of degrees..

Also keep in mind how angle of attack is produced by pitching the aircraft. The whole idea of producing lift parallel to the span requires either a fixed angle of incidence relative to the direction of the travel (such as for winglets) or lift that is created as a function of sideslip to create spiral stability (as is the case for dihedral and polyhedral). Wikipedia has a decent explanation. http://en.wikipedia.org/wiki/Dihedral_(aeronautics).

9B