The helicopter made a hard landing during an aborted takeoff following a medical evacuation demonstration at Nantucket. After the presentation, the aircraft was changed from a 2 litter configuration. The pilot noted that the cyclic retention cap was on the cyclic stick mounting point but did not know who put it there. The rack was then put in place and held there by one of the retaining clips and the stretcher mounted in position. Controls were checked for freedom of movement and the flight took off. In the initial climb, cyclic was moved forward with no problem but later in the climb the nose continued up and forward cyclic was attempted but would not go forward of the position it was in. The pilot lowered collective and descended hitting the ground tail low. The aircraft bounced and collective was held down until the aircraft twisted to a stop. Investigation revealed that control interference by a cyclic stick retention knob, inadvertently installed, prevented forward cyclic movement because of contact with the left litter rack.