Originally Posted by
dmhpilot
140 knots at 90,000 lbs (making numbers up, feel free to insert real ones but convert to SI units) =
105,813,216 joules.
145 knots at 90,000 lbs = 113,562,811 joules
A 5 knot increase in speed in this example is a 7.3% increase in kinetic energy to dissipate. The speeds are the most important thing. Even assuming a mass of 1 kg, a 3.6% increase in speed increases energy 7.3%.
Can you run the numbers again using an additional 2 second delay to thrust reverser deployment over a set stopping distance? That's more to the the point I was trying to make.
Thrust reverser effectiveness increases at higher speeds. Deployment of TR in the 717 depends on nose-wheel touchdown, which takes longer at Flaps 25 vs Flaps 40. I think that the delay of TR deployment may have equal or greater impact on stopping distance than the higher approach speed.