Citabria and Decathlon Fuel Tanks

[Thumper]

Read in more than a few places online about damage to/leaky fuel tanks particularly from snap rolls in these aircraft. Surprised to hear that because its an approved maneuver in the manual with an entry speed. Apparently its frequent enough that some aerobatic "schools" and rentals prohibit them completely. I'm interested in seeing others inputs on the matter?

 

[Bartman]

Thumper,

As I understand it, the problem was worst in pre-95 ACA airplanes and it happened when pilots snapped the plane with nearly full fuel tanks causing internal baffle welds to crack. The baffles are welded to the outer shell of the tank via tabs on the baffles that protrude through slots in the shell. the tabs are welded around the edge of the slots. ACA increased the number of tabs/slots around the perimeter of the tanks and seems to have reduced the frequency that we hear about this problem. Bellanca 8KCAB's don't seem to have this problem although snap rolls with full fuel tanks are probably bad all around.

 

[Bob Turner]

I will not do snap rolls in my own airplane. I think they load up the empennage. I have seen several failures of 2003 Decathlon tanks, due to aggressive aero.

 

[Thumper]

Thumper,

As I understand it, the problem was worst in pre-95 ACA airplanes and it happened when pilots snapped the plane with nearly full fuel tanks causing internal baffle welds to crack. The baffles are welded to the outer shell of the tank via tabs on the baffles that protrude through slots in the shell. the tabs are welded around the edge of the slots. ACA increased the number of tabs/slots around the perimeter of the tanks and seems to have reduced the frequency that we hear about this problem. Bellanca 8KCAB's don't seem to have this problem although snap rolls with full fuel tanks are probably bad all around.

Thanks for the response, that being said I have a Bellanca built '74 7KCAB do you think the occasional snap would be fine in that case? I know its hard to say but I'd just like to practice a few here and there rather then doing them all day long like say an aerobatic schoolhouse would. Going with less than full for aerobatics in general seems wise in which case would you say its best to go with the minimum required or just say keep it at half tanks?

 

[Thumper]

I will not do snap rolls in my own airplane. I think they load up the empennage. I have seen several failures of 2003 Decathlon tanks, due to aggressive aero.

Aero definitely seems to up the wear and tear on these planes in ways they wouldn't normally experience it from the research i've done thus far. What would you consider to be aggresive? As my aerobatic career is in its very very infant stages I would just assume anything negative which is pretty much off the table anyways for my Citabria, tailslides which I know are a bad idea in both Decathlons and Citabrias, and then "whippy" maneuvers such as a snap roll.

 

[Bob Turner]

Why not start with aileron rolls, slow rolls, and inverted flight? Those are gentle on the airframe and take a great deal more skill than a snap roll.

 

[Thumper]

Why not start with aileron rolls, slow rolls, and inverted flight? Those are gentle on the airframe and take a great deal more skill than a snap roll.

Certainly will be starting with those and refining my skills especially with the slow roll as my aerobatic training was in a Great Lakes not a citabria or decathlon. Snaps were fun in that plane hence wanting to try them in my Citabria, was curious though when I heard about how they are off limits by many since my instructor never really said anything about what could go wrong with those other than making sure not to inadvertently tail slide in a hammerhead hence the post.

 

[Bob Turner]

Snap maneuvers put loads on airframes beyond simple G pulling. A loop is little more stress than a 60 degree banked turn. A snaproll induces serious stress in tail and wing surfaces. Do snap rolls in somebody else’s aircraft.

Opinion.

Maneuvering speed is that speed below which a sudden full application of controls will not cause structural problems. So if you enter a snap roll at, say, 70 mph, you are a third of the way between stall speed and some speed that will cause you problems. Why beat your aircraft up like that for a maneuver that pretty much requires zero skill?

 

[Bartman]

Thanks for the response, that being said I have a Bellanca built '74 7KCAB do you think the occasional snap would be fine in that case? I know its hard to say but I'd just like to practice a few here and there rather then doing them all day long like say an aerobatic schoolhouse would. Going with less than full for aerobatics in general seems wise in which case would you say its best to go with the minimum required or just say keep it at half tanks?

Wish I could help but I'm just starting to learn myself. Make sure you have no less than what you need for the flight plus reserves (have to say that, right?). If you're out there and you've got full tanks, probably not such a great idea to do snaps regardless of the model year Decathlon being flown.