Understanding the friction behavior of brake lining materials is fundamental to the ability to predict brake system performance. Of particular interest to the aviation community, where carbon/carbon composite heatsinks are commonly used, is the aircraft response at deceleration onset. There are two performance measures defining brake system performance at braking onset: deceleration onset rate and system response time. The latter is strictly a function of the brake system hydraulics and is not affected by brake lining friction. The former performance measure is a function of both system hydraulics and brake lining friction. Previously to the work herein, carbon heatsink friction was thought to be unpredictable at braking onset. That being the case, a predictive capability for deceleration onset rate was not previously undertaken. This meant that assessment of this performance measure waited until aircraft taxi tests were performed. This practice resulted in a system being completely designed, fabricated and partially qualified before its deceleration onset rate performance was assessed. Significant cost and schedule impacts have occurred trying to produce acceptable deceleration onset rates at that stage of the development cycle. A predictive capability for deceleration onset performance is desirable.
Full Scale Development (FSD) testing of the Harrier II aircraft during the early 1980’s, revealed that the FSD brakes had an excessive deceleration onset rate. In pilot terms, they were deemed ‘grabby’. The development of an acceptable brake for Harrier II included several configuration changes and took almost three years to define an acceptable production configuration. Whereas the raw material used to fabricate the heatsink in the production brake is no longer commercially available, a new carbon heatsink must be developed. The risk of reintroducing a ‘grabby’ brake problem and the finite inventory of the current heatsink justified a risk reducing analysis effort.
The analysis effort consisted of developing an analytical tool and validating it with both dynamometer and taxi tests. This effort showed that the unpredictability of the friction coefficient at deceleration onset was due to theoretical errors in the manner in which friction is calculated. An improved method of calculating friction lead to a relationship between friction and pressure with an integration constant that is thought to be a function of temperature. The new analytical tool, which includes the new friction model, satisfactorily predicts the brake system deceleration onset rate performance. An acceptable prototype brake system configuration was produced during the initial taxi tests. Boeing is now under contract to qualify the new brake configuration and introduce it into the USMC Harrier II fleet.