mikej_45 wrote:

Does anyone know why a plane needs 70% of its take off speed by the time it reaches the halfway point of its total take off distance???

There is a simple rule of thumb that explains acceleration in a prop airplane like this:

"10 times the square root of the percentage of liftoff distance required is equal to the percentage of lift off speed that should be attained in that distance. "

Since an airplane will normally stop in less distance than it will take to accelerate (Not true for all surfaces) it makes somewhat sense to check this speed at the haflway point - gives you enough distance to stop again.

Using the rule, "10 times the square root of the percentage of liftoff distance," we use 50 percent for the liftoff distance and the square root of 50 is 7.07. Ten times 7.07 equates to 70.7 percent of the liftoff speed should be obtained at the halfway point to guarantee takeoff in the remaining half of the runway. If you have the speed, continue. If you do not have the speed abort.

Of course this method should not be used with complex aircraft