Jon S. Berndt wrote: > Snap roll: > > This is indeed the recipe for a snap roll: starting from a speed slightly > above the stall, apply a sudden yaw with the rudder, apply opposite aileron, > and pull back on the yoke. SNAP! --- One wing stalls and the plane rolls > over. > > [I liked the clever use of the word, "recipe" with the phrase "snap *roll*"] > > This would be hard to model using lookup tables, but it might be possible > using JSBSim functions and a table or tables, together. Could be fun. I need > to think about this one. The first idea that comes to mind is that if the > aircraft speed minus the yaw rate times some characteristic lateral length > (span/2?) falls below the stall speed, then a rolling moment would be > generated - maybe a yawing moment, too. > > Jon > > > > _______________________________________________ > Flightgear-devel mailing list > Flightgear-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/flightgear-devel >
My understanding is that a snap roll is initiated by yaw-roll coupling. The lower wing is put into the turbulent flow behind the fuselage by the hard yaw. This imparts a strong roll moment, and the result is that the AOA of the upper wing goes down, while the AOA of the lower wing goes up into the stall region. At that point the partial loss of lift on the down wing becomes almost complete, while the upper wing only loses a small amount of lift. If it were done at a low AOA you would only get roll damping as the low wing would go into a high AOA high lift regime, while the upper wing would go into a low AOA low lift regime. You need to be close enough to stall that the lower wing goes past the high lift regime and into the stall regime. I may be wrong about that. If the roll were initiated on the back side of the lift curve, the upper wing would actually gain lift in the roll, and the lower one would lose it as it goes into stall. I'm not sure which is right, but I'm pretty sure that to get a roll going fast enough to get only one wing into a stall you have to have the yaw-roll coupling. Otherwise roll damping would limit you to a mere barrel roll. So for JSBSim, you would need to add another dimension to your lookup tables that indicates the loss of lift as an airfoil goes through the turbulent wake of other elements like the fuselage. Not a bad idea really, but it's a lot of data and probably pretty hard to find. You would also need separate R/L wing elements. Josh _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
