Post by jd on May 29, 2013 3:50:15 GMT 1
I'd like to apologize in advance for the long post, but I wanted to share an experience I've had with my Super Cub hoping it can help someone else not do what I did, or think I did. I don't think I’ve seen this discussed here before, and I have to wonder why. Maybe I'm making too much of something that amounts to nothing, anyway, here goes....
Some time ago, I performed a “flatten wing and add ailerons” mod to my Super Cub. Before the aileron mod, I had reinforced the battery box and gone to a brushless motor, placing the ESC on the underside of the fuse behind the cowl.
In flattening the wing, I simply sawed the wing in three pieces, removing the center “flat” section and carefully gluing the two remaining pieces back together, adding a 10 inch long carbon fiber tube at the thickest point on the wing for added strength. The wing was nice and straight and felt plenty rigid. I then cut the ailerons, servo pockets and so on. I also swapped the stock radio for a Spektrum AR 6210.
I have never had much luck balancing the Cub on my fingers, or on a homemade pencil type device either. None of my previous mods had seemed to have much impact on center of gravity, and I didn't expect this one too either, as all of the work had taken place very near the center of gravity. I expected that I could overcome whatever small changes had been made with a few clicks of trim. That was NOT the case!
The first flight with the reconfigured wing was...ummm.. very exiting to say the least. She flew extremely tail heavy, nearly uncontrollable. I somehow managed to get her back on the ground in one piece and took her home.
I removed the ESC from under the fuse and paced it in the cowl, inside the motor mount, took her back to the field and tried again. This time she still felt tail heavy, but with about half of the available down trim, was flyable. I've flown the plane that way for over a year now. She has some funky quirks such as, she tracks “funny” in tight turns, refuses to do an outside loop, and tends to nose over on landings in grass just before she stops. I attributed these to my poor piloting/building skills and thought little more of it.
If you are still with me, I'm almost to the point of this post. Last week I bought a proper COG machine to carefully balance another model. Just for grins, I set the machine to the proper 2.25 inches COG and placed the cub on the machine to see just how tail heavy she was, and to maybe balance her with some lead to the nose. To my near disbelief I found that she in in fact not tail heavy, but instead, considerably nose heavy. Her center of gravity is currently 1 7/8 inches from the leading edge instead of 2 ¼ inches as it should be. Now I'm baffled. How can a nose heavy plane require so much down elevator to fly straight and level? The wing cradle looks good, in fact the entire airplane is in pretty good shape.
After some head scratching, I remembered that I had bought a new stock wing before the aileron mod and never used it. Looking closely at the bottom of the stock wing, I think I may have solved my mystery. The stock wing is flat where it meets the fuse, with a “bend” where the dihedral starts. The “bend” is barely noticeable at the leading edge of the wing, becoming more noticeable towards the trailing edge. It seems that this would in effect give the working portions of the wing a slight downward tilt in relation to the flat center section of the wing and the wing cradle. To check my theory, with the wing inverted I measured about 7 ¼ inches out from the “bend” and made two marks square with the leading edge, the first 1 ½ inches from the LE, the other 6 inches from the LE. Placing a straight edge on the center flat section of the wing and measuring from the bottom of the straight edge to the marks on the wing, the forward mark measures ½ inch, the rear, ¾ of an inch.
So....it seems to me that when I removed the center section of wing and placed the outer sections in the wing cradle, I unknowingly changed the wing's angle of incidence, creating more lift. I have to wonder if that also moved the center of lift forward, or if it was just the extra lift that I interpreted to be a tail heavy condition.
Either way, it seems my next job is to get the COG back where it should be, then try to determine the proper amount of shim required under the rear of the wing to restore the proper angle of incidence.
If I’ve missed something here, or stated something incorrectly, please correct me, and if anyone has any tips for resetting the wing angle, I’d sure appreciate it.
Thanks!
Some time ago, I performed a “flatten wing and add ailerons” mod to my Super Cub. Before the aileron mod, I had reinforced the battery box and gone to a brushless motor, placing the ESC on the underside of the fuse behind the cowl.
In flattening the wing, I simply sawed the wing in three pieces, removing the center “flat” section and carefully gluing the two remaining pieces back together, adding a 10 inch long carbon fiber tube at the thickest point on the wing for added strength. The wing was nice and straight and felt plenty rigid. I then cut the ailerons, servo pockets and so on. I also swapped the stock radio for a Spektrum AR 6210.
I have never had much luck balancing the Cub on my fingers, or on a homemade pencil type device either. None of my previous mods had seemed to have much impact on center of gravity, and I didn't expect this one too either, as all of the work had taken place very near the center of gravity. I expected that I could overcome whatever small changes had been made with a few clicks of trim. That was NOT the case!
The first flight with the reconfigured wing was...ummm.. very exiting to say the least. She flew extremely tail heavy, nearly uncontrollable. I somehow managed to get her back on the ground in one piece and took her home.
I removed the ESC from under the fuse and paced it in the cowl, inside the motor mount, took her back to the field and tried again. This time she still felt tail heavy, but with about half of the available down trim, was flyable. I've flown the plane that way for over a year now. She has some funky quirks such as, she tracks “funny” in tight turns, refuses to do an outside loop, and tends to nose over on landings in grass just before she stops. I attributed these to my poor piloting/building skills and thought little more of it.
If you are still with me, I'm almost to the point of this post. Last week I bought a proper COG machine to carefully balance another model. Just for grins, I set the machine to the proper 2.25 inches COG and placed the cub on the machine to see just how tail heavy she was, and to maybe balance her with some lead to the nose. To my near disbelief I found that she in in fact not tail heavy, but instead, considerably nose heavy. Her center of gravity is currently 1 7/8 inches from the leading edge instead of 2 ¼ inches as it should be. Now I'm baffled. How can a nose heavy plane require so much down elevator to fly straight and level? The wing cradle looks good, in fact the entire airplane is in pretty good shape.
After some head scratching, I remembered that I had bought a new stock wing before the aileron mod and never used it. Looking closely at the bottom of the stock wing, I think I may have solved my mystery. The stock wing is flat where it meets the fuse, with a “bend” where the dihedral starts. The “bend” is barely noticeable at the leading edge of the wing, becoming more noticeable towards the trailing edge. It seems that this would in effect give the working portions of the wing a slight downward tilt in relation to the flat center section of the wing and the wing cradle. To check my theory, with the wing inverted I measured about 7 ¼ inches out from the “bend” and made two marks square with the leading edge, the first 1 ½ inches from the LE, the other 6 inches from the LE. Placing a straight edge on the center flat section of the wing and measuring from the bottom of the straight edge to the marks on the wing, the forward mark measures ½ inch, the rear, ¾ of an inch.
So....it seems to me that when I removed the center section of wing and placed the outer sections in the wing cradle, I unknowingly changed the wing's angle of incidence, creating more lift. I have to wonder if that also moved the center of lift forward, or if it was just the extra lift that I interpreted to be a tail heavy condition.
Either way, it seems my next job is to get the COG back where it should be, then try to determine the proper amount of shim required under the rear of the wing to restore the proper angle of incidence.
If I’ve missed something here, or stated something incorrectly, please correct me, and if anyone has any tips for resetting the wing angle, I’d sure appreciate it.
Thanks!
No flying today, nice day or not!