Why does optimum flap configuration change?

[WhySoTough]

Just curious.
When calculating your v speeds and choosing the option of "optimum configuration", you sometimes get flaps 1, sometimes flap 2 and sometimes 3.
What does this depend on?
Also, if we choose the option of flap 1 when optimum was flap 2, what exactly changes?

And finally, more flaps will require less ground distance on takeoff but will give you a less climb gradient later, is that correct?

[Cough]

Correct...

If you have a short runway, with no climbout obstacles then flap 3 is easy. Long runway with a few far out obstacles then flap 1. When it gets complicated is when you have a medium length runway, a few obstacles and a quartering tailwind....

If optimum was 2 and you choose 1 then V speeds probably a few kt higher with maybe a bit less flex.

[WhySoTough]

Cough,

Thanks a lot for the response.
V speeds will go a little higher, because just like in general all speeds will go lower with more flaps(stall speed etc), and hence higher with less flaps.
Why will flex go lower?
I know these sound as though they're stupid questions to you, but I'd appreciate the help.
Thanks again.

[SloppyJoe]

You have to accelerate to a higher speed with a lower flap setting on the same runway so will require more thrust if the runway is limiting, a lower flex. If the runway was not limiting in either case then flex would not change.

[WhySoTough]

Okay understood. Thanks sloppy.
Now just to clarify one thing to remove the doubt from my head.
The more the flex temp, the more you're basically tricking the aircraft to believe the temperature outside is higher, hence it can provide less thrust with a higher outside air temp / lower density.
Is this correct?
Thanks a ton :O

[framer]

WST,
Although you can probably remember the answers given to you here, it won´t help with your overall understanding much. If I were you I would draw a side view of a runway on a white board and go through each limiting factor and draw it on the board.
EG, draw the ASDA, the s/e climb out dry screen height, s/e climb out wet, the four climb sectors and their %, the net take-off area. Spend an hour or two putting different obstacles into the drawing and seeing what it will effect, draw take off rolls and flight paths for different flap settings etc.
The whole takeoff performance subject makes a lot of sense and is not difficult if you view it completely and from a bit of a distance rather than getting in too close.
If you just want answers for an exam then that will be a bit too much like hard work. If you want to be a proficient airline pilot it might help. Also, keep in mind that answers on a web site may or may not be correct and need verifying.Hope that helps.