Originally Posted by
SansAnhedral
The comment made in that post about yaw is interesting.
I think a historical knock on tiltrotor maneuverability was probably based on yaw rate, which is primarily affected by rotor flapping limits. The V-22 has around 10 degrees of flapping cooked into the hub and I do not believe meets level 1 HQ yaw rate (probably exacerbated in crosswinds). The V-280 on the other hand showed some very impressive ADS-33 level 1 yaw maneuvers for its gross weight, thanks wholly to the massive increase to 15 degrees of flapping. When ground taxi turning in their demo footage, the flapping can look pretty wild!
Inherent to tiltrotors, the control power from flapping is applied at the wingtips, creating a solidly effective yaw moment arm.
Now with the compound coaxial pusher prop design, you have the large and heavy static pusher prop assembly, gearbox/bearings, massive stabilizer, and tailwheel assembly - a huge amount of dead weight at the end of a very long moment arm from the mast. The only way to achieve yaw control is by differential torque between the rotors, which have been designed to be as stiff and lightweight as possible. This is a massively different situation from a conventional helicopter where the yaw moment is applied by the tail rotor at that same location perpendicular to the longitudinal axis.
I wonder, at the same medium to large gross weights, is a high flapping tiltrotor going to have substantially better yaw control than a rigid rotor coaxial design with a pusher assembly? I would say almost certainly. It's a real shame that Defiant hasn't even attempted (that we know of) high yaw rate maneuvers that could be compared to the myriad V-280 demos. Perhaps they have and the results were disappointing.
A coaxial design will be limited in yaw when operating at maximum lift. Reducing collective on one of the rotors to change the reaction torque, reduces lift. If Sikorsky could have demonstrated a high yaw rate, they would have.