4 1/2inches. I have a Saito72 in mine also. Stuff the battery pack over the wing and all the way to the trailing edge. Put the reciever right in front of it. Don't use a spinner. I use the tank that came with the plane. With the 72 you'll fly 15 to 20 minutes. I set my timer for 10 minutes and normally land with 40 to 50% fuel remaining.