787 flaperon behaviour on takeoff roll

[tcasblue]

Based on other responses and combined......

This is a design feature to protect the flaperon, and particularly its actuators, from vibration damage(fatigue) caused by engine thrust impingement loads. When the flaps are extended(such as during taxi), hydraulic pressure to the actuators cause the ailerons droop to the takeoff position in order to assist in providing lift.

When the engines are powered up at the start of the roll, the flaperon actuators are bypassed, removing the hydraulic pressure that has been holding them in position. Once the hydraulic pressure is bypassed, the flaperon can then free float in the airstream which reduces the vibration load on the actuators(extending their life).

With little forward airspeed, the flaperons droop downward from the takeoff position due to gravity(sort of like some airbus ailerons droop downward when depowered). As the airspeed increases, the increasing airflow causes the flaperons to rise(freefloat) to the horizontal position. Above a set speed(around 80 knots), the bypass is removed so the flaperon is hydraulically actuated again and is commanded downward to its drooped takeoff flap position.

So basically, what you are seeing is the result of the flaperons being hydraulically depowered during the first part of the takeoff roll and the subsequent affect on its free floating position at low and increasing speed until such point as they become powered again.









reduce the fatigue on those silly actuators when the thrust right in front of that surface is rather large. The flaperons will float as the lift on the wing increases.

With takeoff thrust, until reaching 80kts, the flaperons are in BYPASS mode so they droop all the way down due to gravity, then upon reaching 80kts, power to actuator is restored and they resume the preset drooped position.
With takeoff thrust, until reaching 80kts, the flaperons are in BYPASS mode so they droop all the way down due to gravity, then upon reaching 80kts, power to actuator is restored and they resume the preset drooped position.