Kadet LT-25 with Kadet LT-40 Wing
 

Builders,

Thinking about using a extra wing I have from a LT-40 ARF, on a LT-25 Kit I am building. Has anyone tried this before?
Planning on adjusting wing saddle forward as required to keep TE of wing where it is shown on the LT-25 Plans, so that Tail Moment is unchanged. Balance would be where called for on the LT-40 wing Power will be a Saito FA40a. Any thoughts?? Thanks.

Randy

I have not tried that, and I wouldn't. Keeping the tail arm the same is not the issue; you have a different wing. If the tail arm of the LT-25 is shorter than that of the LT-40, and if the stab area is less, the CG will need to be farther forward to have the same effect. Also, the point where the engine is mounted will be relatively close to the balance point compared to an LT-40 (aggravated by the smaller tail volume), and the Saito 40a is a light engine, so my guess is you would end up having to add nose weight to get it to balance. I would just build the LT-25 wing and use the Saito 40a. It's a very nice combination. Jim

As noted the stock LT-25 is a nice airplane in its own right. My question would be "What do you want to accomplish?" If you can afford the airplanes, "I just wanted to see what would happen" is a valid answer. The process of getting it to fly well will be educational. As long as the C of G is correct relative to the wing, a .40 class engine will fly a .40 class wing. A way to answer the question of "How will it fly?" is to look around for an airplane with similar proportions. The points made above about tail moment and tail volume are relevant. I suspect that the resulting plane would fly kind of like a Slow Poke with undersized control surfaces. Unfortunately, I don't have the source right at hand but there are a sort of set of golden proportions for model airplanes that generally fly well. These proportions vary somewhat with size, speed and task that the aircraft is designed for and designers manipulate one proportion or another to promote various traits in the aircraft. There is, as they say, more than one way to skin a cat, but there is a reason why most airplanes tend to conform to the same general shape and proportion. Experimentation is a part of our hobby and is a way to gain a greater understanding of aerodynamics. On the whole, if you are trying to improve a trainer, I have to agree with Buzzard Bait. If you are trying to thrash together a wing with a partial kit to get one flying airplane, then yeah, it will probably fly. Good luck and have fun.

The location of the CG as a proportion of the wing chord does not depend on just the wing, it also depends on the length of the tail arm and the area of the stab. It is proportional to...

stab area/wing area X tail arm/wing chord

So if the tail arm and stab area are reduced compared to the LT-40, the CG position as a proportion of the wing chord will be smaller, i.e., farther forward.

It sounds to me as though you want a longer wingspan. IS that it? If so, why not cut a few more wing ribs and add them to the existing build. Not that big of a deal.A couple more bays on each side shouldn't cause any problems.

On my Kadet Sr I extended my wing by 1 bay on each side and it just added to the floating effect the wing had.
The LT-40 balances on the main spar. If you put the balance of the wing in the same position as the LT-25 the throws will probably will need to be reduced because of the shorter moment between the wing and the stabilizer. The tail control surfaces will become more sensitive. I would suggest that you balance it a little bit nose heavy, perhaps mounting the servos last to set your balance point. By that I mean after the plane is completely built including covering, fuel tank, engine, batteries, because the covering on the fuse behind the wing will affect the balance. Have it ready to fly except the servo position. Make a servo tray that will mount on 1/4 inch squares attached to the sides of the fuselage. Adjust the balance by moving the tray for best balance before the first flight. This way the plane shouldn't need any additional excess weight to balance it. After that use the batteries to refine the balance to your way of flying if necessary. Trainers give you the best point for the model to fly best. Other planes give a range to adjust the balance so it can be tailored to your flying style. Keep in mind , there is a saying, a tail heavy plane flies once, a nose heavy plane lives to fly another day. Good Luck with your project :)