W. E. Johns used the name Ranger for many types of aircraft in his books. In this case, it refers to a bush-plane which Worrals and Frecks chose (on the recommendation of Eddie Clark) for their mission to the Canadian far north in Worrals in the Wastelands. According to the text, it is a twin-engined, six seater float-plane specially designed for the transportation of forest fire-fighters. According to Eddie Clark, the Ranger was already obsolete by the time Worrals used it but it had been designed by a man who knew the country. It was robustly constructed and reliable even if it did not have spectacular performance or good looks. It had a squat nacelle and bulky fuselage which gave plenty of space for stores besides carrying two crew and four passengers.
This Ranger is definitely not the Ranger which Biggles used in Biggles Gets His Men, nor the Ranger light aircraft which Frecks and Worrals used in Worrals Takes a Hand.
Some ideal candidates like the DHC Beaver are single-engined. The Twin Otter appeared too late for the story.