On Wednesday, May 4, 2016 at 1:01:08 PM UTC-5, son_of_flubber wrote:
On Wednesday, May 4, 2016 at 12:58:11 PM UTC-4, wrote:

I teach that dive brakes/spoilers control the rate of descent(at constant air speed) and control rate of deceleration in and after the round out. The change in drag resulting from the lift to arrest the rate of descent is pretty much meaningless compared to the power of the air brakes.
UH

Let's say the glide ratio of a steep descent is 1:7 and IAS is 70 knots when entering the flare. So the vertical component of the airspeed before flare is 10 knots. The horizontal component of the airspeed before the flare is 60 knots. (Are these assumptions correct?) After the flare the vertical component of the airspeed goes to +/- zero. What is the horizontal airspeed at the end of the flare? 60 knots?

Your airspeed vector during descent is the hypotenuse of an upside down right triangle. The long axis is the 7 horizontal component and the short axis is the 1 vertical component. The square of the hypotenuse is equal to the sum of the squares of the other two sides of that triangle.

My algebra tells me that at IAS 70 knots at the beginning of the flare, for a 7:1 descent rate, your vertical descent rate is around 9.9899 knots and your horizontal speed is around 69.2965 knots, not 60 knots.

If you are targeting 60 knots at the end of the flare, you need to burn off about 9 knots of excess horizontal velocity.