Mission-critical applications in wireless environments depend on a cross layer approach for Quality of Service. Dealing efficiently with variable link quality and dynamic topologies involves QoS integration at the link, network and transport layers interfaces. In this paper we present a new contention-based Medium Access Control protocol for wireless ad-hoc networks of unmanned air vehicles. A receiver-initiated access control protocol with sender scheduling provides the mechanisms to implement effective MAC-layer bandwidth allocation with support for ad-hoc scenarios. A variable RTS lead time procedure integrated with binary exponential backoff improves fairness and provides low access latency for critical data frames. A major protocol design factor is efficient operation under heavy load. Performance measurements from simulations indicate consistent performance and promising results.