Elastic Launched Glider C

[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.

[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

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.

well yeah that fragility is the thing. The way Canards are is that any change in pitch is compounded rather than damped as they are in a traditional aircraft. I don't know if it should rightly even be called a horozontal "stabalizer" because of that. So on the conventional glider if it transtioned to an angle less than its comfortable glide angle it would just pitch up and you're good. On these if your transition is down at all, that gets compounded and you either nose dive or do an outside loop. Frustration

[JonB]

Agreed. For those coming to Nationals, be prepared for frustration (I hope records are broken, but realistically it will be less than ideal flight conditions). The humidity fluctuations can create havoc on really thin wing profiles and the AC will not be turned off (just accept it and move on...). The canards are so fragile that even the humidity change from day to day will change how it flies. Each glider takes on a "personality" and issues will be compounded in these conditions.

[jander14indoor]

I must be missing something here. You can certainly make a canard statically and dynamically stable.

See following references on canard stability:
http://en.wikipedia.org/wiki/Canard_(aeronautics)
http://www.desktop.aero/appliedaero/con ... ility.html

hmm, is the issue be as described in this reference: http://www.desktop.aero/appliedaero/con ... nards.html
"One possible explanation for this lies in the fact that aircraft performance is much more sensitive to canard size than to tail size. The proper canard size depends strongly on the most critical design condition (e.g. climb unimportant, high speed cruise critical vs. a long endurance design with high speeds unnecessary.) Here we consider simple methods of estimating key performance parameters for a given design. For a particular project, several canard sizes may be tried to achieve the best compromise."

And gliders have several very different critical design conditions so no one canard works well?

Regards,

Jeff Anderson
Livonia, MI

[Smithy0013]

I must be missing something here. You can certainly make a canard statically and dynamically stable.

See following references on canard stability:
http://en.wikipedia.org/wiki/Canard_(aeronautics)
http://www.desktop.aero/appliedaero/con ... ility.html

hmm, is the issue be as described in this reference: http://www.desktop.aero/appliedaero/con ... nards.html
"One possible explanation for this lies in the fact that aircraft performance is much more sensitive to canard size than to tail size. The proper canard size depends strongly on the most critical design condition (e.g. climb unimportant, high speed cruise critical vs. a long endurance design with high speeds unnecessary.) Here we consider simple methods of estimating key performance parameters for a given design. For a particular project, several canard sizes may be tried to achieve the best compromise."

And gliders have several very different critical design conditions so no one canard works well?

Regards,

Jeff Anderson
Livonia, MI

Well yeah they can be designed to have acceptable handling characteristics for a pilot. But since both surfaces are producing positive lift then CG has to be somewhere in between the two. So if a disturbance in the air causes the canard to tilt downward, both surfaces are going to have a decreased angle with respect to the airflow. This causes the plane to pitch down more. This further pitch down creates an even further decreased angle with respect to airflow. These compound until you're diving. That's what I'm talking about with inherent instability. There's a reason you get such a substancial bonus and it's not just cause you cant buy a kit

[bjt4888]

Smithy0013,
Jeff Anderson has given you a very big hint in his post. Test different size forplanes. Also, see the canard design calculator I posted a link to several pages back in this forum. With a conservative static stability margin entered into this calculator (like 15%), and all other area and dimension data entered, the calculator will give you a good suggested center of gravity. A variety of foreplane moment arm distances should also be tested. Playing with the calculator should give you some guidance as to what length forplane moment arm distance best compliments different forplane surface areas.
This problem is solvable. I went through three generations before hitting on design parameters that produce good static and dynamically stable flight. Transition is another story, but flights average between 17.5 and 19.5 seconds for a launch height of about 24 ft. Keep testing and good luck.

Bjt4888
Brian Turnbull
Academy of Model Aeronautics member since 1972 (off and on)