How To 3D Print an Aeroplane - Day 4 - Wings and Aerofoils
Time to start modelling one of the wings. A really cool thing about building stuff that has a line of symmetry is you only need to do it once. We are going to model the left wing, when it is finished, with all the bits we need like hinges and stuff, we will simply mirror it onto the other side.
A word on aerofoils…. You can overthink this. It is a model plane, it’s not going supersonic, its not carrying 300 passengers for 10 hours. Anyone who has ever built a foam board plane and seen it fly knows this. You can use a flat piece of board as a wing and it will fly. Now, in full size aircraft the aerofoil is of critical importance, it has an effect on the efficiency, top speed, stall speed, pretty much everything. Aerofoils can be categorised using a set of numbers called the NACA number. You can learn more about them here.
If you want to, its pretty easy to find the NACA aerofoil used on the 337 and use it in your model. I have used an estimation and I think it will fly just fine. I created sketch on the longitudinal plane and used the spline tool to outline the aerofoil - here you could place a canvas of the NACA aerofoil and trace if you want it to be exact.
To create the wing, the inboard part is just extruded out to where the tail boom is, as this part of the wing is straight. Don’t worry about the dihedral yet as it is easier to work on the wing when it is flat, later when we are finished, we can add the dihedral. Dihedral gives us lateral stability, it causes a wing that has been disturbed upwards to produce less lift and thus to move back down.
For the outboard section, create an offset plane from the end of our inboard wing to just before the wingtip on the sketch. Create a sketch on this plane and project the outline of the wing onto this new sketch. This new sketch can then be scaled down to give us the wings taper.
Washout. Washout is a reduced angle of incidence at the tip compared to the root. This means the root reaches the stalling angle of attack first and thus the stall spreads out from the root to the tip - avoiding a tip stall and impending death spin. Adding washout into the wing is a good idea. Cessna and Piper have washout designed into to all their wings to make them safer for GA pilots. On our model I rotate the wing tip sketch by just a couple of degrees to have a lower angle of incidence.
Now you can complete the loft from the inboard wing to the wing tip sketch.
We have a wing shape the matches the canvas. I talked about how the exact shape of the aerofoil is probably not that important. What is more important for how the model will fly is the wing loading.
The wing loading doesn’t scale linearly with size, in other words, it's a good idea to make the wing proportionally bigger in our model than on the real thing. You can probably scale the wing up about 15% without loosing any scale looks.
For the winglet I have gone with the style seen on the Flying Bulls 337.