Hi all

I am building my first kit, a SIG Kadet LT-40 Afte building the left wing and taking it off the builind board I noticed that the ribs are not 100% flush with the trailing edge on the bottom , they all stick out about 1/16th. Obviously they are then slightly sunken in on top. Would that cause any problems sanding them flush between the trailing edge and the bottom back spar? Would it affect flight charactistics? I am trying my best to get things 110% right, but that made me wonder how "inaccurate" a trainer can be built and still fly properly.

Marcel

I don't think you have to worry at all about changing the airfoil by sanding the ribs level on the bottom The larger issue is whether the attachment joints with the TE have been weakened by reducing the effective contact area. Seems to me that if you sand something off the bottom you'll need to add a bit of balsa to the top rear of the ribs and sand to shape and this could be a delicate operation.

But then I don't know anything about this kit. Is the TE notched to receive the ribs? If so, this is probably not a problem.

I think the largest danger for first-time builders is not using the right glues in the right places. The most likely disasters will be bits of the model coming off in flight! Second would be not spending time to making sure the wing is properly aligned with the elevator.

Thanks for the tips.

The trailing edge notched and about 1/2 inch thick so surface area would not be a problem I think, and those ribs are glued tight .

The instruction manual of the LT-40 is brilliant. Tells you exactly what glue to use and where. Medium CA is called for most of the building work (Even joining when joining the wings together, but using epoxy seems like a better idea) and then after the construction of the wing it is advised to reglue all the joints with another coat of eith thin or tick ca ensuring that a small but visible fillet of glue is seen.

Hi Marcel,

oooopss!

Easily done on your first model. If the top section is to be covered in tissue/fabric, then you are best to glue some thin strips along the top of the ribs, from the top spar to the trailing edge, using aliphatic or PVA glue or something that will sand easily; then cut these back again until they are flush with the trailing edge (using as long as a sanding block as possible, allowing several ribs to be filed at the same time). Make sure you don't scallop the t/e between ribs.

If the top is to be sheeted, then the recess may be of value allowing a flush finish to the trailing edge.

Either way, the bottom just needs sanding, taking care to match the correct curvature if any.

For future reference, when pinning the timbers to the building board, always pack the trailing edge up to the correct height, remembering to extend the packing to support the ribs as well, but I guess you know that now!

Wings are critical, so any warp could be disasterous, particularly when reducing speed to land, as a phenomenum called 'tip stalling' could well occur, this is when the angle of attack to the airflow of the wing tip is higher than that of the rest of the wing, and the flow of air over the tip is interupted, allowing the tip to drop, or stall, casusing the plane to roll, usually into the ground!

Keep the whole wing as straight as possible, and if anything, allow the wing tip to point at a slightly lower angle of attack to the airflow than the rest of the wing which will help eliminate the tip stall.

Not knowing the model, I can't say much more, other than standard advice; keep behind of C of G as light as possible, otherwise you will end up puttting weight in the front just to balance the plane, remember for best flight, build it light.

Mark