Source: 2008 Q2 Beartracks, Glenn Patterson
We ran a trial with a piece of wing skin to learn the process for building the wings. We are home schooled the same as every builder. Keeping the ribs straight and the noses from flexing was a little frustrating. The skin is laid over the ribs and then it is impossible to see what the nose rib is doing. It may be partly the way we assembled the wing skins. It was important to our build to use the ratchet straps and boards to keep the skins flat & tight across all the ribs as we clecoed the skin on. The wing skins fit great and are dead straight with the clecoes in.
HOLD THE NOSE RIBS STRAIGHT. A board was laid through the top lightening hole and anchored back to the wing support stand. Each rib was set with a square. A small block is pushed up against the rib on each side and held with a C-clamp to keep it square. We worked in from both ends so there was never a mile of board inside the wing. Reach up inside and remove the clamps when it is time to move the ribs to play peek-a-boo with the predrilled wing skin holes. The blocking has kept the ribs up straight for as long as needed so they can come out. THE GREEN PAINTERS TAPE was used everywhere to draw the centerlines. It provides good contrast and allows thin lines to be drawn on with a pen or a superfine marker.
STIFFEN THE NOSE RIB AT THE LEADING EDGE. We want to be able to use the ratchet straps to their maximum advantage to install the wing skins. The rib tips are at risk of bending when trying to move them around under tight skin. The tips dig into the skin and bind. The solution was to temporarily reinforce the leading edge to protect the ribs from buckling. The real prime purpose of the reinforcements was to help in forming the exact leading edge profile as it was designed. Reinforced rib tips helped to be able to move the rib noses and allowed good pressure with the ratchet straps to bring the wing skin tight to leading edge on the ribs.
MDF wood tips in the photos were added to prevent the nose rib top from bending and to give more support for forming the leading edge. The MDF pieces are held with one screw so they should self align with the rib and the skin. Tighten the screws to snug but the tip should rotate a little bit on the screw. Make sure that the MDF tips are a perfect match & fit up to the leading edge on the aluminum rib tips. Install a plywood backing washer to avoid buckling in the other direction & to hold it all together similar to the photo. All the nose ribs that lined up with a middle ribs were reinforced as that is where the ratchet straps go. A full size wood template of the wing was used in the project & that is the pattern used to form the MDF nose pieces. The nose tips were machine to shape with a flushcut bearing guide bit in a router table. A long piece of MDF x 9 inches wide was the material of choice. Place the full size template on the MDF board and drill through the jig pin hole. Countersink the hole on the underside of the MDF then put a snug fitting screw through to secure the template on top of the MDF. C-clamp the board to the template further back so the two pieces stay together during the machining. The C clamp should not interfere with running the wood on the router table. Set the guide bearing up to run against the template and high enough to cut the MDF. This will cut the MDF to the exact template shape. Cut the nose shapes with the router & trim with a jigsaw.
This photo illustrates the final fit of the wing skin to the leading edge. The photo with the reflections of the aluminum & shadows makes a gap appear much more than it measures. A .012 feeler gauge would not fit in anywhere. That is a good fit. Three pieces of 3/8” wood door stop x 4 ft long were put fairly close together and used to work the skin over the leading edge with the ratchet straps. Be careful not to put too much tension on the straps as it could buckle the wing. Three straps can create a lot of pressure. An option is to put some clamps and boards on to temporarily reinforce the main ribs. You have invested a lot of your life to this point so use good sense here. Slacken the straps when you are not working as the strap tension can be affected by temperature & humidity. Finally check back to the first photo. There is a 50 lb. fishing line running the full length of the wing through the nose rib lightening holes.
The line has 10 lbs of iron hanging on it for tension. Both ends of the wing are level and the string in this case was set at 41mm to the spar web at both ends. The wing frame was adjusted and supported so that measurement was the same for the length of the wing. Place a skin and check the string measurements as it can change as skins are added plus tons of clecoes. Keep checking the string measurements at every step. Check the vertical spar offset measurements for movement too. Our inspector told us of a fellow that built a perfect wing except it had a ever so slight curve down the leading edge. Let’s not go there.
– Bearhawk 727 – Bob, Jim & Glenn