Hi guys.
I scratch built a small dlg and it flew fine until i decided to finally throw it by the wing tip. When it was released it basically traded ends, the tail continued to rotate the the right. It came back down to the ground continuing to rotate about all three axis.

I'm guessing that the issue is the vertical tail/ rudder area is not large enough. Is this the correct analysis?

Thanks for any feed back

Spamman127

That's possibly the problem. I'd have to see a picture of the plane and maybe a video of the launch to be able to give good feedback.

Brandon

Here's a picture of it. My suspicions arise from the fact that it flies great when not launched discus style.

Good lookin' plane! I like the checkerboard pattern! I suspect that you have two problems. The first is that your vertical is entirely above the tailboom. When you discus launch it, the vertical has to provide the lateral force to stop the yaw inertia and return it to straight flight. With all of its area above the boom, that force creates another moment about the roll axis, causing the plane to roll (and from the sounds of things, pretty hard). This will also twist the tailboom pretty hard, and could cause it to break. That's why all commercial DLGs have almost equal fin area above and below the tailboom. The second problem is simply its size, like you said. I would estimate that it ought to be 1.5 to 2 times the size it currently is. If you just added about the same size fin below the boom, it would probably launch MUCH better.

One other thing to keep in mind is that there is a balance between fin size and amount of dihedral in relation to stability. Increasing the size of your vertical will affect this if you don't increase the dihedral as well. This is why DLGs have such a large amount of dihedral. It doesn't look like yours has any, but that could just be that the photo doesn't show it very well. I would try flying it after adding more fin area below the boom and see if it has a hard time tracking in a straight line. If you find it wants to wander slowly into a spiral dive, you may want to consider adding dihedral.

Probably more information than you wanted to hear, but there ya go. BTW, what is the wing span?

Brandon

Its a 36 inch span with no dihedral. There is a small amount of fin on the bottom but it is probably about a third of the size of that on the top. I'll try adding some to the bottom and see if that helps. This is why the TLAR method is not the best method for design, eh ?

It was a good TLAR for a javelin glider!

Brandon covered it. Just as a side note, I tried to throw someone else's homemade DLG very similar to this once. It would glide fine with an overhand toss, but would just tumble when discus launched. I think it had the same issues. Tail too small, not enough sub-rudder, not enough dihedral, and it was also way too heavy (like 17oz).

Try punching your dimensions into SailplaneCalc and see what the results are. It helped me build a 32" DLG that flies very well.

I don't know if you already did this, but the rudder pushrod is normally run on the side of the tailboom. This way, flexing of the boom is counter acted by rudder response.

Originally Posted by like.2.fly

Brandon covered it. Just as a side note, I tried to throw someone else's homemade DLG very similar to this once. It would glide fine with an overhand toss, but would just tumble when discus launched. I think it had the same issues. Tail too small, not enough sub-rudder, not enough dihedral, and it was also way too heavy (like 17oz).

I remember that thing! I recall it being over 20 oz. Either way, I got a kick out of that. He used a golf shaft for a tail boom. That thing by itself must have been like 60-80 grams. Small difference from our 17g booms... Gotta love Larry

Finally got a chance to try my modifications today. It flies much better now, well, because it actually flies off the launch. Nothing spectacular, but it was fun until the girls soccer team kicked us off of the field.

Thanks for the help,
Spamman127

Good to hear! Have fun with it.

Glad we could help out