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Higher pressure air on the underside of a wing generally rolls outboard around the wingtip and over the top of the wing reducing lift and therefore increasing induced drag.

My intuition would tell me a wing with a dihedral would increase this effect as the higher pressure airflow on the bottom of the wing would flow outboard easier than anhedral. Is the opposite true with anhedral? Would the airflow flow inboard?

Are there any other factors that could contribute to efficiency?

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The lateral flow effect you are talking about affects both top and bottom so would roughly cancel out. Pressure gradient induced lateral flow is outbound on bottom but inbound on top. The wing is at a fairly low angle of attack in cruise flight; even near stall AoA is only about 18-20 degrees and retains fairly normal flow up to that point.

With very large dihedral or anhedral the isolated airfoil section would produce lift just as efficiently as a flat wing but the overall aircraft is less efficient because it wastes the lift produced. At 45 degrees from level the wing would need to produce lift equal to 1.4 times the aircraft weight. Same as being in a 45 degree bank but with no turn as the wing vector forces are fighting each other. At low angles the wasted lift is not significant compared to the effect on handling and stability. At 5degrees dihedral or anhedral the lift is 1.0038 times the gross weight, or 0.38% less efficient.

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