I'm doing a project: Study of aircraft braking and pre-sizing of associated subsystems. We tried to find criteria for the reverser in the braking phase. I read FAR 25, but I did not find it. Is there a criterion such as how many percent of the braking force must be applied by the reverser in certification documents?
As @John's answer alluded to, the rules for actual landing distance (25.125) and the accelerate-stop distance on dry runway (25.109) effectively prevent a manufacturer from taking credit for thrust reverser (T/R) when demonstrating/calculating for these distances. However, 25.109 does allow a manufacturer to take credit for T/R when dealing with a wet runway. This means that having a (good) T/R gives an operator more flexibility when dealing with adverse weather conditions, and this translates to more money saved (vs sitting on the tarmac).
Therefore, just like any other performance numbers, the performance of T/R is subject to market objectives. There is no regulatory requirement on how powerful the T/R has to be at max reverse.
That being said, this NASA memo has some helpful information on why airlines want thrust reverser at all. The following two graphs are cited from this source, which outline the percentage of work done by the respective braking systems for a typical jetliner on dry and wet runways. You can perhaps back-calculate the thrust of T/R from your design point:
Other factors to consider for design:
- Foreign object debris ingestion from T/R deployment, especially at lower speed
- Compressor stall from hot reversed gas, especially at low speed
- Inadvertent deployment of thrust reverser in-flight. For most airplanes, this would be catastrophic.
- Controllability in single T/R deployment on ground
- For rear mounted engines, aerodynamic interference with rudder effectiveness
Thrust reversers are not accounted for in performance criteria for accelerate/stop and landing distance values, so there aren't really any cert requirements for what you would think of as minimum reverser braking performance. Cert requirements mostly focus on controlability during landing/reject with asymmetric reverse thrust, and the safety related aspects of (preventing) reverser function in flight, that sort of thing.
In terms of stopping performance, their use is a performance "bonus" and the pilot has to option to leave them stowed, unstow them but leave them at idle, or use any reverse thrust amount between idle reverse and max reverse desired during braking on any landing or reject.