I made a kinetic energy graph for our J-3, at max GW. As speed increases, KE increases at an increasing rate.

Jumping only 15 MPH from 35 to 50, our kinetic energy more than DOUBLES. Add another 15 MPH to 65, it TRIPLES.

If I can have my Cub touch down at 38 instead of 50, my braking/rollout distance is basically cut in half.

There are several online KE calculators, the numbers for your aircraft may be different, but the curve will look the same.



This is why "a Lear jet 25G will float about 100 extra feet down the runway for each knot over its proper landing speed." — Richard Collins

Thanks, FlyingSignPainter for your excellent charts and graphs. I have too little physics education to fully appreciate this one, but experience has demonstrated that kinetic energy in low ground effect is really good stuff on takeoff and something to be avoided by deceleration before low ground effect on landing. Keep them coming.

Clicks with the old rule of thumb that doubling your landing speed will quadruple your landing distance.

hotrod180 wrote:Clicks with the old rule of thumb that doubling your landing speed will quadruple your landing distance.

Sometimes. It's not a linear Y=X*4 graph. The ratio changes as you speed up, it could be more or less than a quadrupling, plus parasite drag also increases with speed and has an effect.

Surplus Kinetic Energy (AKA "Zoom Reserve") has to go somewhere in order to land and stop. It can be dissipated instantly (by crashing), or it can be dissipated over time, and also at different phases of landing- you can bleed off speed before final, on final, progressively dissipate KE on short final while approaching touchdown, while floating in ground effect before touchdown (eats up a lot of distance due to reduction in induced drag), with your brakes after touchdown, or some combination of them

"progressive dissipate KE on short final while approaching touchdown." FlyingSignPainter is talking about the apparent brisk walk rate of closure approach, but smart enough to use physics terms so people don't throw rocks. Just use elevator to make the numbers continue to come to you at what appears to be a brisk walk. Manage the resultant sink with increased power.