Ron, You asked:
>Can someone who is knowledgeable in this area explain why a geared >motor is (or can be) more efficient than a direct drive. If you >consider the extra weight of the drive, say 30-35 grams for a good >planetary gear box, such as cosmotec or graupner(?), and the extra >weight of a larger prop, say 5-10 grams, and factor in the extra >drag from the gearbox, is a geared motor really better than a direct >drive? I am also a novice in this area so I picked up Martin Simon's book, Model Aircraft Aerodynamics, and read the section on propellers. The starting point is the fact that a motor has a limited range of rpm and torque over which it runs efficiently and puts out a high level of power. For a direct-drive propeller, the pitch of the propeller has to be matched with the target rpm so as to pull the plane forward at the target speed of the airplane. (The pitch is the distance that the propeller slices forward in one rotation.) For a propeller with a pitch that is too high, the torque increases, the motor slows down and the propeller flails away at the air with poor efficiency. So the first step is to obtain the proper match between the motor, the propeller pitch and the speed of the plane. With the pitch optimized, there is a second consideration. Larger-diameter propellers are more efficient as described in the book. The total thrust is given by the area traversed by the propeller (a disk with a diameter equal to the length of the propeller) multiplied by the pressure differential between the front and back of the propeller. The same thrust can be obtained with either a large-diameter propeller with a low pressure difference or a small-diameter propeller with a large pressure difference. However, the efficiency of the propeller is directly related to the pressure difference. The smaller pressure difference (larger diameter) gives higher efficiency. High-rpm motors are not well matched to large-diameter propellers. Therefore, the strategy is to allow the motor to run at a good rpm while the larger propeller rotates at a lower rpm. This means using a gear box. Dick -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Dr. Richard C. Williamson Phone: 781-981-7857 Room C-317 FAX: 781-981-0122 Lincoln Laboratory Email: [EMAIL PROTECTED] Massachusetts Institute of Technology 244 Wood Street Lexington, MA 02420-9108 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
