Chicken6...a bit hard to explain how lift/drag ratio alters when flap is lowered but take it from me it does...alot.
You may notice a reduction in ROD when 10 flap is lowered , but I guarantee that your airspeed reduce aswell...and that is why we lower flap, so that we can land at a lower airspeed.
What will be altered is your glide angle, which steepens when you lower flap, alter your glide speed from defined best glide and your glide angle will steepen.
Basically we always try to fly at our best lift to drag ratio.
A PA28 for example has an optimum l/d ratio of approx 10:1..we can prove this thus.
ROD in a glide at best glide 700fpm approx
Best Glide speed = 73kts
Using PFM maths (73 * 6080)/ 60=7397.3
7397.3/700 = 10 LD ratio = 10.....
L/D ratio S+L is thus
MGW=2400lbs
Engine HP(max)=160 * 2.2 (lbs of thrust or something)=352
lbs of thrust at cruise pwr is 75% of max
so = 264lbs, in the cruise s+l @ 100kts
Drag = thrust therefore drag = 264lbs
@mgw S+L lift must = weight so lift equals 2400 lbs...Drag is 264lbs 2400/264 approx 9
So L/D in S+L flight is 9:1
So I hope you can see from this that if you start sticking flaps and stuff out then your L/D ratio is screwed and you will not achieve optimum perf.
Cheers