Date: 18 May 98 16:02:58 From: email@example.com (Mark Drela) Organization: Massachvsetts Institvte of Technology References: 1 Followups: 1
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In article <airliners.1998.772@ohare.Chicago.COM>, kls@ohare.Chicago.COM (Karl Swartz) writes: > I can see how "positive" dihedral (like the 777) might help cancel out > unwanted roll. This might also explain why the 777 has greater dihedral > than the 747, since an engine failure on a 777 would produce greater yaw > which in turn would trigger a roll. (The same sort of yaw-induced roll > which is one of the theories for the USAir 427 crash.) > > I can't see what "negative" dihedral (like the An-124) would accomplish, > or why it would even be desireable. The main effect of dihedral is to give a roll moment in response to a yaw angle (Cn_beta stability derivative). On most aircraft, having Cn_beta close to zero (i.e. little or no "dihedral effect") is desirable, since this makes the aircraft more controllable on crosswind landings and takeoffs. Another reason for having a near-zero Cn_beta is that a sudden yaw angle from an engine failure should NOT produce a roll moment. This would only complicate life for the scrambling pilot. On the other hand, having zero Cn_beta makes the airplane spirally unstable, but this is easily countered by the pilot or by any rudimentary wing-leveling autopilot. Free-flight model aircraft must have spiral stability, and hence always have large amounts of dihedral. On a low-winger, the flow around the fuselage at a yaw angle acts on the wing which then generates a roll moment as though the wing had negative dihedral (bad). Hence, positive dihedral is added to compensate and return Cn_beta close to zero. On a high-winger, the effect is opposite, and negative dihedral is added to compensate. Mark Drela First Law of Aviation: MIT Aero & Astro "Takeoff is optional, landing is compulsory"