RE: Pay It Forward Build Thread!! Sig LT-40 build
Wow, once again time as gotten away from me again. It's amazing how quickly things can get behind. I've been pretty busy at work, and around the building room I've been working on getting some projects finished up and moved out the door. Unfortunately, documenting this build got put on the back burner. It wasn't intentional, but it did happen. So, with that said, let me work on getting caught up. We are actually getting close to finishing up this project. What's left is balancing the plane, and then covering the plane.
With the plane completely assembled “in the bones” it's time to balance it. It's important to note that when a plane is balanced we need to make sure that the CG is in the correct location (this is balance from front to back), and just as important it to laterally balance the plane (side to side). Many people fail to make sure the lateral balance it correct and wind up with a plane that doesn't fly as well as it should . With a lateral balance that is not correct you will have a plane that will always dip a wing in flight. This can be corrected with aileron trim, but a heavy wing can also cause some weird things to happen when performing maneuvers. Spending a little bit of time to balance laterally will be rewarded with a better flying plane, so I encourage everybody to make sure it's correct.
Ok, let's jump in and start to check the lateral balance. The easiest way (for me at least) to check this is by using heavy (25 lbs test) fishing line. Very simply, we're going to use the fishing line to pick up the plane and see where it balances. Tie one end of the fishing line around the tail section of the plane (picture #1) and the other end around the shaft of the motor behind the spinner (pictures #2 and #3). As you can see in picture #4 set the plane level on your work surface with the fishing line attached ready to check the balance. Now pick up the plane using the fishing line. If you have a hard time doing it yourself get somebody to help you lift it. The trick is to lift the plane so that you aren't touching the plane at all. Looking at pictures #5 and #6 you can see that the plane is dropping the right wing. Before making any adjustments you may want to check your balance several times to make sure you getting a true reading of the balance of the plane. Since we know that the right wing drops we will have to add some weight to the left wing. This next step is why prefer to balance before covering the plane, because if we had the plane covered we would have to cut the covering to add weight to our wingtip. This way we can put it in place before covering. You can use anything you want to adding weight, but most hobby shops sell lead weight that has a self adhesive backing on it. Another great source for weights is your local tire change shop. They will usually save the old weights used to balance tires in a big can. If you go in and ask for them they will usually give them to you at no charge. The weights that I used here actually are sold for adding weight to pine car derby cars. Like I said, you can use just about anything. As you'll see in just a minute how you attach the weight becomes very important when you are adding weight. But first, find out how much weight you are going to need to get the plane to balance (pictures #7 and #8). Very simply, start placing weight on the tip of the left wing and check the balance. Keep doing this until the plane will now balance when you lift it up (pictures #9 and #10). As I said above, how you attach your weights becomes very important at this point. Why? Because epoxy adds weight. If you are using self-adhesive weights there won't be any added weight at this point, but if you use epoxy to put the weights in place you need to take into account the weight of the epoxy. Because of this you will actually have to put in a little bit less weight because the epoxy will make up the difference. Once your weight is in place recheck your balance to see if you got correct. Trust me, it may sound a bit difficult but you'll get the hang of in pretty quickly. Pictures #11 and #12 show the weights epoxied in place at the wingtip. Once final note here, you want to put your weights as far out on the wing tip as possible. If you try to put your weights in closer to the fuselage you actually have to add more weight to get it to balance than you will if it's placed at the wing tips.
Pictures
1. Attach the fishing line around the tail section
2. Attach the fishing line around the crankshaft of the engine
3. Attach the fishing line around the crankshaft of the engine
4. Set the plane level on your work surface.
5. Lift the plane using the fishing line
6. Observe the wing tips as you lift the plane
7. Use weights on the wing tips to adjust the lateral balance
8. Use weights on the wing tips to adjust the lateral balance
9. As you add weight to the wing tips recheck your balance until the plane hangs level
10. As you add weight to the wing tips recheck your balance until the plane hangs level
11. Epoxy the weights into place in the wingtips
12. Epoxy the weights into place in the wingtips
Until next time
Ken