Actually, this may apply to all big twins(and even smaller ones as well according to Boeing). Has anybody noticed during a strong crosswind takeoff, the need to use rudder input in one direction too maintain centerline during the initial takeoff roll transitioning to the requirement for opposite rudder input at higher speeds(classic weathervaning rudder input) to maintain the centerline?
I recently read an article about the 777x in Aviation Week which stated.....
"Another area of test focus will be whether the 777-9 has the potential for increased susceptibility to a phenomenon known as Downwind Turning Effect on Takeoff. First encountered during initial tests of the baseline 777, the effect occurs on the ground in strong crosswinds and manifests itself as a thrust-induced yawing moment in the windward direction. The effect, which increases with thrust, occurs only at the start if the takeoff run at speeds up to 70 knots and appears counterintuitive to pilots as it works against the aircraft’s natural aerodynamic weathercock yawing moment."
A few weeks later I went to a Boeing symposium and they discussed what they termed the Anti-Weathervaning Effect.
This is the effect according to Boeing.....
"High crosswinds at low airspeeds(less than 80 knots):
- Air is flowing across nacelle inlet
- A force is imparted on the inlet as air is drawn into the nacelle(from ‘across’ to ‘in’)
- As the nacelles are ahead of the main gear the force generates a yawing moment away from the wind.
- At these speeds, aerodynamic force on the vertical tail is small so the resulting yawing moment into the wind is small
- At low airspeeds, the yawing moment generated by the inlet flow outweighs the yawing moment of the vertical tail so the airplane turns away from the wind
At higher airspeeds:
- Airflow is more aligned with the inlet
- Force on the vertical tail increases
- Vertical tail yawing moment outweighs engine yawing moment and airplane weathervanes"
