I have an old balsa plane of my father’s from the early days of RC electrics. It has a motor on it called a promax 400 7.2 volt - can someone help me determine an appropriate updated motor?
That's and old brushed motor. Not very powerful (about 60-80watts). 1 inch diameter and 1.5 inches long puts its approx C-pack size but a lot lighter. It might be easier to simply spec a new motor for the plane regardless of the original brushed unit. What are the specs for the plane (wingspan, estimated all up weight with motor and battery)?
The size of the old motor & size of the prop are great ideas. I would also look at the all up flying weight of the plane and the flying style you prefer.
• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.