RAY’S RANT #7
The next subject we have to consider, are the wings of the project. Those interested in flying looked to the birds, and tried to imitate them with the first try at airplanes. I was the Wright Brothers who did the technical work to bring about the first flight of an airplane. This is a big subject, and includes the theory of flight, which I will tackle at a later time. For those that are new to the hobby, and are interested in building, we need to look at some features that are vital to having a good flying model. Thankfully, when we build from a kit, or plans, we know that the resulting model should fly, providing a plane was built from the plans and flown. The advent of ARF’s means we have to look at all the aspects of the model. We usually advise beginners to buy an ARF trainer, so they can get started learning to fly right away. Right now, I think the Sig LT40 trainer is one of the best. It has a larger wing than many trainers, as well as a longer fuse which helps with stability, and slower flight, which makes learning easier.
Wings come in many shapes, and one of terms we use to describe them is Aspect Ratio. This tells us the relation between the wingspan and width (chord) of the wing. The wing of a glider would be called a high aspect ratio wing, because it is long and thin, and designed for high lift and slow speed. On the other end of the scale, we would have a low aspect ratio wing, and these would be found on racing planes, and those designed for speed.
Our models are usually in the middle of these two extremes, because in a trainer we want a wing that will give good lift, a slower speed, so we can learn to react to the input needed to fly the plane. So a good trainer needs to have a relatively high aspect ratio, a flat bottom airfoil (Clark Y) and a constant chord wing. As we gain experience flying, we would then go to a model with a different airfoil, a semi- symmetrical airfoil. This would give us more speed and aerobatic capability than the trainer wing, and require a higher level of flying experience. We would still retain the constant chord with the wing. The next level we would go to is called a double taper wing, where the wing narrows toward the wing tip in chord and thickness. This type of wing would be used on high level aerobatic monoplanes that we see being used in the air shows today.
In picking a particular airplane to build, we would need to look at the wing layout, its airfoil, and aspect ratio. Suppose we wanted to build a model of the Stinson SR-9, and a quick look at the plan form of the wing are extremely narrow wingtips. This should raise a red flag for you, as it raises the problem of wing tip stall. Other planes in this category are the DC3, the DH Mosquito, and many others with a similar wing shape. In the early days, those who built SR9 models, would raise both ailerons to try and defeat the wing tip stall problem. This helped somewhat, but was not the answer.
When coming in to land, the wing needs to stall at the root of the wing and progress outward. In a double tapered wing, this is hard to accomplish. The new technology we use now, we twist the tip with a couple of degrees of washout. This makes the end of the wing fly at a lesser angle of attack than the root part, so the tip remains flying while the root stalls, allowing the plane to settle in a wing level mode. Sometimes we have to thicken the wingtip’s airfoil so that it will help combat the wing tip stall.
The wing construction on most models is built up from using strong wing spars, and ribs. When we do what was a fabric covered wing, we usually sheet the leading edge back to the main spar, top and bottom. This makes a strong D spar setup. If we are doing a metal covered wing, we sheet the whole wing, which makes a strong wing. In a one piece wing, we have to put in plywood doublers, to stop the wing from folding upwards so the wing tips meet. The load on wings is quite high, and so the centre section needs to be strengthened with strong spar doublers. Some of us have learned this the hard way, when wings have failed in flight. Also, in low winged planes with the wheels in the wing, the doublers help to strengthen the wing for those firm arrivals.
I have built some wings with Styrofoam cores, and sheeting them with 1/16th balsa. The two halves are glued together, with a plywood doubler, then a 4” fiberglass strip is epoxyied around the joint. This makes a very nice wing that is strong, but is heavier than the built up wing. On a pattern ship designed to do aerobatics, the heavier weight is not a problem usually, as the fuselage is lighter than a scale models.
A well designed wing will make a nice flying model.
Also, a straight wing, without warps will make a nice flying model as well.
Most kits today would have tabs on the wing ribs, so that the wing can be assembled on a flat surface, and keep it straight.
There are also a number of wing jigs around homemade or commercially made that will help in assembling the wing so there are no warps.
I use a piece of ¼” plate glass to build wings on, it is absolutely flat with no twists.
On top of that I use a ceiling tile so that I can use pins in the assembly process.
Whatever you use to build the on, it has to be flat with no twists or warps.
Be sure to follow the instructions when building if they are available.
On one model I had the instruction to assemble the wing upside down. Well, I have always built the wing with the top side up, so I went ahead and did it my way. In the end I had to build a new wing, as the washout was in the wing, and so it came out as washin, and didn’t fly very well that way.
So it pays to follow the instruction, and be careful of any changes you make.
In the next blog I want to share our experience with the C-46 project, and changes we had to make. Every time we construct a model, we learn things and lessons, so we won’t make mistakes the next time.