Re: Pulling a Stop to Runway Overruns
To achieve the claimed 20% improvement in braking effect would require increasing the mainwheel reaction force by the same amount, and having brakes capable to dealing with the increased torque. Assuming the latter, lets look at the 20% increase in download.
Assuming that the plane has just touched down, the speed will be somewhere close to stall speed. Also, let's assume that without the suggested elevator technique, there is zero aerodynamic lift or downforce, so that the mainwheel reaction is essentially the aircraft weight. (ignoring the nosewheel reaction component of maybe 5% for now)
Now, if I want to increase the mainwheel reaction by 20%, that means, taking moments about the nose gear and assuming the tail is as far behing the mains as the nose in front, I need a download of approx 10% of aircraft weight at the tail to generate that 20% download. If the tail is about 25% of the wing area and similarly efficient, that means I have to get to about 40% of the tail stall CL, which means I'm going to need substantial amounts of elevator to get there.
Now, the plane continues to slow until we get to about 70%Vs. I still need to generate 10% of the aircraft weight at the tail, but that now is something like 80-90% of the tail stall capability. Clearly, at much lower speeds I'm not going to be able to generate enough download to get my 20% number, and eventually it won't much matter where the elevator is.
What does that lot mean?
It means that to achieve the kinds of braking efficiency improvements talked about requires SIGNIFICANT elevator movements because it requires significant redistribution of the wheel reactions.
It also means, if you look at the impact on nosegear reaction, that in order to achieve 20% more mainwheel download, I need to offload the nosegear by 10% - maybe more depending on geometry. Since a typical nosegear load is of this magnitude this means I am definitely risking raising the nose with this technique.