If the instructions are followed the slingshot will work and the plane will fly smoothly.
What you need:
A4 paper (any size actually, but a4 is best).
Why not have a look at my other instructables:
http://www.instructables.com/id/Knex-Mini-Transformer-Jet/ Knex Transformer Jet
http://www.instructables.com/id/Knex-mini-transformer-tank/ Knex Transformer Tank
http://www.instructables.com/id/Knex-mech-warriorrobot/ Knex Mech Warrior/Robot
http://www.instructables.com/id/Knex-mini-motor-bikefinger-bike/ Knex Mini Motor Bike/ Finger Bike
Hope you like it.
Step 1: All step
1. Fold the paper in half, horizontaly
2. fold in the top corners, to create a point at the top
3.Fold the side in of the plane in again, ths makes it thinner and more like a jet
4. Fold together the paper, this is what it looks like now
5. Fold down one side of the plane to create a wing, fold it down so the wing is the depth of the plane
6. Exactly the same as step 5, but opposite side
7. This is what it should look like now
8. Now with your scissors cut out the square, as shown in the pic.
9. cut out the tail, with the scissors