KL-666 wrote:From a 787 pilot i recently heard that the plane automatically extends flaps in cruise. Not much, somewhere between 1.5 and 3 degrees as a form of trim. The increase in camber should move the center of lift a bit aft, and thereby unload the horizontal stabilizer, resulting in a more smooth positioning of the airfoils in the airflow. Ultimately resulting in less fuel burn.
Kind regards, Vincent
Yeh but the center of lift of a plane in level flight is exactly at the CoG..... otherwise it wouldn't be in level flight.
The center of pressure of the main wing is aft of the CoG thus rotating the nose down, the horizontal stabilisers see an angle of attack roughly equivalent to the negative incidence of the inner wing section... ie if the AoA is zero and the inner wing incidence is at 2 degs then the h-stab see's -2 (level flight - things change when not in this state).. BTW this is why the h-stabs are not always -ve incidence as the wings effect the free stream air that the h-stabs see.
So changing the camber of the inner main wing results in the free stream air flowing off the back of this part of the wing being at a more -ve angle resulting in the h-stab being more effective and thus less elevator (back stick) is required. What also has to be considered is that the overall wing due to twist is possibly flying at a -ve AoA, still generating lift, but also a thrust vector as opposed to a drag vector, and you want to keep this.
There's a lot happening
p.s.
Last evening I modeled the effect of the main wing on AoA of the h-stab and added it to the Beagle Pup, tonight I'll factor in the effects flaps have.
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchell
