Thans you everyone for your replies!
Very interesting points here.
I see what Mad scientist is saying, but I have a few questions:
Do the ground spoilers completely spoil the airflow? I thought there was a re-attachment point aft of the spoiler where some flow over the trailing edge still exists. Do you think the flaps in the approach setting might promote this re-attachment (by increasing chord and sfc area) and increase lift?
What about the lift produced by the lower half of the wing?
Is it significant at those speeds and so close to the ground? How do the flap settings affect it?
Haw Eye also has an interesting point. When the flaps retract they would be passing through all those settings that add more lift than drag (such as T/O flap settings).
For this reason I find it hard to explain why on the 727
you would want to retract the flaps only till 25degrees. Maybe only for a FOD-protection reason. And evidently even at 25 degrees, it was determined that they were creating more aerodynamic drag than the increase in rolling-friction-drag if they had been fully retracted.
Plus, correct me if i'm wrong, but increasing the weight-on-wheels only increases the MAXIMUM amount of braking action that is achievable (the point that can only be ridden by the anti-skid). For normal braking, it should not affect the performance significantly.
(I think its safe to say that the small
percentage increase in rolling friction attributed to a weight increase is negligeble when compared to amount of energy dissipated by the brakes, or the amount of aerodynamic drag produced by the aircraft while shooting down the runway at over 100kts.)
Tankengine,
it was actually a typo, I meant to write "airliners".
Though I'm not sure I agree when you say "the airlines have no say in the matter". The cu$tomer is always first....
I think the main questions we have to answer now are: How much added lift do the flaps provide (compared to a clean wing) at those speeds and following ground spoiler deployment?
How much does this reduction in weight-on-wheels affect the deceleration rate and how does it compare to the amount of aerodynamic drag produced by the flaps?
Thanks again for your contributions, keep `em coming
palgia