The Concorde RG-25 battery in my 2000 7GCAA is getting weak. I’m thinking of replacing it with an Odyssey SBS J16 using the installation kit from ACA. I’d save about 7 lbs off the aircraft’s empty weight thus increasing its useful load, which is always nice. But I’m not sure if it will be an equally good savings in flight. Because the current battery location is behind the back seat, it provides a lot of aft balance for the aircraft. Getting rid of 7 lbs from this location would put the single pilot CG very close to the forward limit. Also by my calculations, the horizontal stabilizer/elevator will still need to generate approximately 6 lbs of aerodynamic downward force to keep the airplane longitudinally balanced in level, moderate speed flight. I came up with this figure as follows:
The wing’s leading edge is station zero (reference datum) for weight and balance.
The wing is 60 inches wide with a cambered airfoil, so I’m taking a ballpark location that the center of pressure (lift) would be approximately one third of its width (cord) from the leading edge, so at station 20.
The battery is at station 86 (inches). So its distance from the center of lift is 86” – 20” = 66”. Thus the lost moment when putting in a lighter battery is 66” x - 7 lbs = - 602 in lbs
The approximate center of downward aerodynamic force of the horizontal stabilizer/elevator is at station 111 (inches). Thus the distance of downward aerodynamic force from the center of lift is 111” – 20 “ = 91”.
So the amount of downward force in pounds that the horizontal stabilizer/elevator needs to generate to keep the fuselage level is – 602 lbs/in ÷ 91” = 6.6 lbs
Thus in flight, the load on the wing will only be lessened by about one pound, from 7 lbs to 6.6 lb. So have I really gained much? Also to create the additional downward force, the tail will now be creating more drag so now my cruise speed will be slightly less, probably imperceptibly less, but still less.
Also the Odyssey battery has less rated capacity than the Concorde, 12.3 verses 22 @ C1 (1 hour rate in ampere hours). So once again, have I really gained much by going to a smaller battery with less capacity?
Any independent analysis or opinions would be helpful!
The wing’s leading edge is station zero (reference datum) for weight and balance.
The wing is 60 inches wide with a cambered airfoil, so I’m taking a ballpark location that the center of pressure (lift) would be approximately one third of its width (cord) from the leading edge, so at station 20.
The battery is at station 86 (inches). So its distance from the center of lift is 86” – 20” = 66”. Thus the lost moment when putting in a lighter battery is 66” x - 7 lbs = - 602 in lbs
The approximate center of downward aerodynamic force of the horizontal stabilizer/elevator is at station 111 (inches). Thus the distance of downward aerodynamic force from the center of lift is 111” – 20 “ = 91”.
So the amount of downward force in pounds that the horizontal stabilizer/elevator needs to generate to keep the fuselage level is – 602 lbs/in ÷ 91” = 6.6 lbs
Thus in flight, the load on the wing will only be lessened by about one pound, from 7 lbs to 6.6 lb. So have I really gained much? Also to create the additional downward force, the tail will now be creating more drag so now my cruise speed will be slightly less, probably imperceptibly less, but still less.
Also the Odyssey battery has less rated capacity than the Concorde, 12.3 verses 22 @ C1 (1 hour rate in ampere hours). So once again, have I really gained much by going to a smaller battery with less capacity?
Any independent analysis or opinions would be helpful!
