Elastic Launched Glider C

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Re: Elastic Launched Glider C

Post by chalker7 » November 28th, 2012, 6:02 pm

314chocolate wrote:Just a thought, is there any reason to scale up a model? If you truely keep everything the same, wouldn't the fight time be the same, but you would just have a bigger glider? It might be an obvious answer, but even if it is, could someone please point it out to me?

Thanks for the info about the carbon fiber!
Why do you think the flight time would be the same? I think there is a more nuanced answer about the utility of scaling and modeling in engineering buried in your question, but first I'd like to hear a bit about your logic.
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Re: Elastic Launched Glider C

Post by retired1 » November 28th, 2012, 7:03 pm

illusionist wrote:What are approximate fuselage dimensions that have worked well? I'm currently using 3/8 x 1/8 as listed in the simple simon plans, but it's extremely heavy. If I use a thinner piece, I would need to create an external hook for the rubber. What are the best ways to go about doing so? Would something like what was used in wright stuff planes or helicopters be enough for the forces of launch?
If the stick is too heavy, try a lighter weight balsa. Some hobby shop in your area should have some lighter material.
Then you could sand off some of the sharp corners in areas that the wing and tail do not locate on. You could thin both the height and thickness aft of the wing. (Underside for the tail) You could try 3/32" thick material. Some delicate planes actually use 1/16". Probably not a good idea for a first glider.

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Re: Elastic Launched Glider C

Post by jander14indoor » November 29th, 2012, 7:54 am

illusionist wrote:What are approximate fuselage dimensions that have worked well? I'm currently using 3/8 x 1/8 as listed in the simple simon plans, but it's extremely heavy. If I use a thinner piece, I would need to create an external hook for the rubber. What are the best ways to go about doing so? Would something like what was used in wright stuff planes or helicopters be enough for the forces of launch?
The Simple Simon is a robust design with many opportunities to save weight. Recommended as a good starting point because of that robustness and ease of build, but as you suspect, there are many places to improve it. The fuselage being one.

After I built my first one, I noticed I needed a lot of ballast for balance, as well as the oversize stick. Some things I've tried to increase strength AND reduce weight. I've used as small as 1/4 by 3/32 for the basic stick. Have to watch getting too thin or too light as the tip tends to break off in the inevitible crashes while trimming. The front third is a place to consider carbon fiber reinforcement to allow lighter wood overall and keep strength. You might also want to lengthen the nose in front of the wing so less clay is needed reducing overall weight. As to the hook, you need a blunt nose to meet SO requirements anyway, as well as clay ballast, so here's what i did. I slabbed a couple of short piece (say 1/2 inch or so) of 3/32 sheet wide enough to take the nose out to the required size to the bottom of the motor stick. Back edge slanted up and forward to make a hook. Round the front profile to meet the bluntness requirement. Put a small amount of CA glue on the inside of the hook to prevent the rubber from splitting off the lower piece.

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Re: Elastic Launched Glider C

Post by illusionist » November 29th, 2012, 12:14 pm

jander14indoor wrote:
illusionist wrote:What are approximate fuselage dimensions that have worked well? I'm currently using 3/8 x 1/8 as listed in the simple simon plans, but it's extremely heavy. If I use a thinner piece, I would need to create an external hook for the rubber. What are the best ways to go about doing so? Would something like what was used in wright stuff planes or helicopters be enough for the forces of launch?
The Simple Simon is a robust design with many opportunities to save weight. Recommended as a good starting point because of that robustness and ease of build, but as you suspect, there are many places to improve it. The fuselage being one.

After I built my first one, I noticed I needed a lot of ballast for balance, as well as the oversize stick. Some things I've tried to increase strength AND reduce weight. I've used as small as 1/4 by 3/32 for the basic stick. Have to watch getting too thin or too light as the tip tends to break off in the inevitible crashes while trimming. The front third is a place to consider carbon fiber reinforcement to allow lighter wood overall and keep strength. You might also want to lengthen the nose in front of the wing so less clay is needed reducing overall weight. As to the hook, you need a blunt nose to meet SO requirements anyway, as well as clay ballast, so here's what i did. I slabbed a couple of short piece (say 1/2 inch or so) of 3/32 sheet wide enough to take the nose out to the required size to the bottom of the motor stick. Back edge slanted up and forward to make a hook. Round the front profile to meet the bluntness requirement. Put a small amount of CA glue on the inside of the hook to prevent the rubber from splitting off the lower piece.

Jeff Anderson
Livonia, MI
Alright, I'll try those two things. Thanks, Mr. Anderson.

What should be a target weight? I know that some of the lightest are < 3g, but what's reasonable? 4-5 without ballast?

Another question about wood thickness: 1/32 wood is what I would like to use for the wings, however it bends quite a bit. I believe that the flexing during launch will cause the front surfaces of the main wing to flare up, causing the glider to loop over. Has this effect been seen, or should I not worry about it?
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Re: Elastic Launched Glider C

Post by jander14indoor » November 29th, 2012, 12:50 pm

1/32 wings can work in a glider this size, correct wood selection is important, and this is where selective stiffening by carbon fibers can be useful. But good wood selection shold be enough to get started. Pick C-grain and check the sheet beforehand for stiffness across the grain.

The dihedral joints themselves can add quite a bit of stiffness along the chord.

Target weight, as LIGHT as you can get it and the glider survive the launch. You should be targetting 4-5 gm WITH ballast and work to get down to 2-3 grams. You will be amazed at the difference. Weight is ALWAYS the enemy in flight.

Possible hint, possible red herring. Definitely advanced topic. The openness in the rules about definition of the catapult MAY allow SO gliders to be significantly lighter than AMA gliders ultimately. Competition will sort this one out.
- AMA catapults must be short, and held by one person. This limits the stretch length on the rubber and that puts a lower limit on the acceleration (force) to achieve the speed needed to reach any given ceiling.
- SO catapults HAVE NO SUCH LIMIT. You should be able to devise a system which has a longer stretch, alowing a longer space to accelerate, allowing LOWER acceleration (force) during launch so a lighter glider MAY be possible. Of course you have to reach the same velocity (possibly more, things are never simple) to get to the same height so that lower limit on strength is the same.
- How this all plays out as a system is why I suspect there is advantage to be gained here, but warn this is ONLY speculation on my part. Something I hope to explore, but since I'm not an SO competitor, its kind of low on the list of way to much stuff to do. Left to you students to explore...

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Re: Elastic Launched Glider C

Post by hunterboy+ » November 30th, 2012, 2:23 pm

Is there a good place to get balsa wood :?: . I don't have any hobby shops were I live.

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Re: Elastic Launched Glider C

Post by retired1 » November 30th, 2012, 2:38 pm

On line balsa is frequently of poor quality. There are a couple of exceptions and shipping will eat you alive.

I would go with a kit. Stan Buddenbohm has one called the SO lilt'l sweep. 2 gliders for $30 or $40 dollars. Kirk has a how to build it at this site. Very light.
Freedom flight models has a 4 glider kit for a bit over $50 including shipping. Fuselage is delicate. Wing requires sanding to shape. Good instructions.

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Re: Elastic Launched Glider C

Post by 314chocolate » December 1st, 2012, 8:24 am

chalker7 wrote:
314chocolate wrote:Just a thought, is there any reason to scale up a model? If you truely keep everything the same, wouldn't the fight time be the same, but you would just have a bigger glider? It might be an obvious answer, but even if it is, could someone please point it out to me?

Thanks for the info about the carbon fiber!
Why do you think the flight time would be the same? I think there is a more nuanced answer about the utility of scaling and modeling in engineering buried in your question, but first I'd like to hear a bit about your logic.
Even though you are increasing the area of the wing, and therefore you would think it would increase the glide time, you would also be increasing the weight of the glider. If you are truely staying to the same ratios, wouldn't the effect of the increased wing area be negated by the increased weight? But if you don't scale up the wood when you do the model, that would throw the calculations off.
Like I said, not sure if its true or not.

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Re: Elastic Launched Glider C

Post by BlueRed07 » December 1st, 2012, 1:58 pm

Is it ok to make the hook out of a thin piece of metal as opposed to wood? I am not sure if metal is allowed, however, due to the part in the rules stating that the functional components (including the fuselage) must be made out of wood, foam, etc. I have tried both and it seems like the metal is far less likely to break on a bad launch.

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Re: Elastic Launched Glider C

Post by theging » December 2nd, 2012, 10:11 am

jander14indoor wrote:1/32 wings can work in a glider this size, correct wood selection is important, and this is where selective stiffening by carbon fibers can be useful. But good wood selection shold be enough to get started. Pick C-grain and check the sheet beforehand for stiffness across the grain.

The dihedral joints themselves can add quite a bit of stiffness along the chord.

Target weight, as LIGHT as you can get it and the glider survive the launch. You should be targetting 4-5 gm WITH ballast and work to get down to 2-3 grams. You will be amazed at the difference. Weight is ALWAYS the enemy in flight.

Possible hint, possible red herring. Definitely advanced topic. The openness in the rules about definition of the catapult MAY allow SO gliders to be significantly lighter than AMA gliders ultimately. Competition will sort this one out.
- AMA catapults must be short, and held by one person. This limits the stretch length on the rubber and that puts a lower limit on the acceleration (force) to achieve the speed needed to reach any given ceiling.
- SO catapults HAVE NO SUCH LIMIT. You should be able to devise a system which has a longer stretch, alowing a longer space to accelerate, allowing LOWER acceleration (force) during launch so a lighter glider MAY be possible. Of course you have to reach the same velocity (possibly more, things are never simple) to get to the same height so that lower limit on strength is the same.
- How this all plays out as a system is why I suspect there is advantage to be gained here, but warn this is ONLY speculation on my part. Something I hope to explore, but since I'm not an SO competitor, its kind of low on the list of way to much stuff to do. Left to you students to explore...

Jeff Anderson
Livonia, MI
I just had one that weighed about 3.5 grams that held up just fine for the first few launches and actually got some good times (I got one launch flirting with the upper teens range) but unfortunately somehow it hit my launch handle and the wing shattered into about a dozen pieces. I guess the moral of my story is you can probably get it under three grams as long as you're careful (unlike me) and don't hit tables or anything. Thanks for the launch idea, Mr. Anderson. I may be trying that in the future with really thin competition rubber.

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