Elastic Launched Glider C

[bjt4888]

Fanjiatian,
Here is a methodology that my students have used toadd washout to the Littl Sweep wing tips:
1. Usually, the glide circle produced by the plan specs is between 30' and 35', a little too big for a single basketball court sized gym. The students add a very small amount of additional right rudder in by bending the TE of the rudder about 1/16" and breathing on it and holding about 10 seconds (it is necessary to sand the rudder to about .025" from it's original thickness of 1/32" in order to make it more bendable). After this shot amount of time bending the TE, of course, it does not stay 1/16" deflected after you stop actively bending it. This adds an almost imperceptible amount of additional right rudder. Maybe 1/64" or 1/100". A test flight will confirm if it is enough or if the procedure needs to be repeated. A good result for this step is to reduce the glide circle so much that it starts to spin in a little (maybe a circle of almost 20').
2. After the above step, bend in the wingtip washout to about 1/32". It is not necessary to be super precise, 1/16" washout is ok too. Deliberately bend in the washout a little less on the right wing tip (ex. 1/32" on the left tip and 1/16" on the right). This will cause the glide circle to open up and flatten out again. The circle should open to about 25' and the glide should be very smooth, and, best of all, the glider will recover better at transition and when flying in bouncy air giving better average flight times.

The beneficial results of these trimming steps are easiest to observe on very light gliders and, as you have observed, it takes careful testing and documentation of test variables to produce the best result.

Bjt4888

[bjt4888]

Fanjiatian,
Sorry, I meant 1/32" washout on the right wing tip and 1/16" on the left. Less washout for the inboard wing panel relative to the glide circle direction.

Bjt4888

[fanjiatian]

Ah I see! So less washout on the right wing if you're doing a rightwards turn? The amount of washout is different for both sides?
Do you add the washout before you glue the wingtips to the wing or during testing? How do you make sure the washout stays?

[bjt4888]

Correct, less washout for the right wingtip for a glider that circles to the right. Bend in the washout during test flying. It is not necessary to washout during construction. Be sure to understand my previous post; only the last 1" of the wingtip TE (1" along the span) is washed out. The chordwise distance that is bent up is only about 1/4". And, yes, bent in washout tends to unbend and reduce between sessions. As long as some washout is still present and as long as it appears to still be slightly greater on the left wingtip, I usually suggest to the students that they fly it the way it is and not try to bend the washout back to where it was unless they are unhappy with how it is flying.

Bjt4888

[Smithy0013]

I've built twenty second gliders in a relatively small gym and from my experience, washout just simply is not needed. So long as you taper the wings that alone should move you from an airfoil to approaching basically a flat plate at the tip. I have yet to have problems with tip stall because i didnt put a twist in my wing and personally i think youre risking more than you have to gain. Correct me if I'm wrong and there is some other advantage to washout other than stalling.

[bjt4888]

Smithy0013,
Congratulations on your success. 20 second flights in a low ceiling gym is quite good. The purpose of tip washout is to reduce lift induced drag. As approximately 50% of drag for these small gliders is induced drag, (other types of drag are "skin" drag and profile drag) reducing it will improve the lift drag ratio and reduce the sink rate. Tapered wings, swept wings, wing tip plates and progressively varying the airfoil over the span are other methods of minimizing induced drag. If you want to study this concept you can read about it on the NASA Glenn Research Center Guide to Aerodynamics.
It is true that tip washout may only affect duration a small amount and it is tricky to bend into thin wings accurately and would be one of the last refinements to employ after minimizing weight and selecting an optimal design. I think that I can safely state that all AMA national championship and FAI world championship indoor model gliders use tip washout.

Bjt4888

[sciencegreek]

I've built twenty second gliders in a relatively small gym and from my experience, washout just simply is not needed. So long as you taper the wings that alone should move you from an airfoil to approaching basically a flat plate at the tip. I have yet to have problems with tip stall because i didnt put a twist in my wing and personally i think youre risking more than you have to gain. Correct me if I'm wrong and there is some other advantage to washout other than stalling.

Just out of curiosity...how low of a ceiling are we talking about here?
From the competitors I have seen in Long Island there has not been a plane that has broken one second per foot.

[Smithy0013]

So I looked into it more and it looks like the the lower AOA provided by washout reduces the tip vortex created because theres less pressure differential between the top and bottom of the wing right? Sounds like it would be interesting to put it in a small home made wind tunnel with some smoke and look at the before and after vortex produced. Also while at it, has anyone done a before washout and after washout time test? If it adds 1 or 2 seconds I would say great and absolutely use it. But if its coming down to tenths of a second i would probably leave that stuff to the national champs rather than try to deal with the bending and the way it may bend back in storage and just feels very messy. Maybe add it for states. I think on my next glider ill try adding washout and see times before and after. Unfortunately I dont know when ill get to this because were currently on a shortage of matierials (and time for that matter!). And no sorry i dont know the height of our ceiling but i would guess around 20 to 25 feet. Just a guess. Its northwestern lehigh in pennsylvania Im not sure if anyone here is familiar with that gym. We hosted an invitational a little bit ago

[Smithy0013]

Ok so has anyone solved the canard instability problem? Im trying to do it on paper but its just untable no matter how i move stuff around. Or is longitudinal instability just something you have to deal with

[JonB]

Ok so has anyone solved the canard instability problem? Im trying to do it on paper but its just untable no matter how i move stuff around. Or is longitudinal instability just something you have to deal with

We can get stable flights IF we get a good transition. The transition is REALLY delicate. It will transition for a while, then (with no changes to the glider) will stop (nose dive). I believe they are just extremely fragile- any minor change to the glider seems to impact the transition more than anything else.