pbr Prince Harry has joined teenagers training for a charity trip to Africa, and stopped to sign the plaster cast of a girl with a broken arm.br ppbr The young Royal joined the cadets for the final hour of their 25km walk through the Black Mountains in the Brecon Beacons, mid Wales, as they prepared for their trip to Lesotho, southern Africa, to volunteer with his charity Sentebale.br ppbr Prince Harry led the way as he chatted with cadets on the walk back to their base, where he was presented with a commemorative plaque and met the rest of the 60 youngsters who will go to Africa in August.br ppbr Charlotte Wilkinson-Burnett, 17, from the Air Training Corps, who broke her wrist playing hockey, asked the Prince to sign her white plaster cast, which had already been written on by her friends. Afterwards the teenager, from Barrow-upon-Soar in Leicestershire, showed off where he had written: "Get well soon. Harry."br ppbr Addressing the group, the Prince, who was in the army branch of the cadets at Eton, said: "It is so nice to see so many of you turn out this afternoon. You will really enjoy it, it will be a hell of an adventure."br ppbr During their three-week trip the cadets, who raised much of the money for the trip themselves, will trek through the Drakensburg Mountains, take tours of military sites and work on Sentebale development projects, helping build and paint schools.