The thermal performance of an air-heated propeller, installed on a test airplane, was evaluated by observations of the ice-prevention propertiesof the propeller during flight in natural-icing conditions and by the collection of thermal data on the propeller during flight in clear air and in clouds at temperatures above freezing. The test propeller was equipped with hollow steel blades of a standard design which were altered to permit heated air to enter the blade cavities at the propeller hub and to leave the cavities at the blade tips. No provisions were made to control the distribution of air flow inside the blades.