Hello, everyone,

I am undertaking a competition for which I must build a glider out of wood and plastic film, the objective of which is to be lauched in a gymnasium, then to stay aloft the longest. There can be no onboard power source, and no radio control, just free flight. The thing will be launched by an elastic band, and is to have a WS of no more than a foot, weigh no more than 10g.
I have a decent intuition of how regular sized airframes behave, mostly from messing around with my buddies' gliders. Unfortunately, I usually build airframes to optimize lift to drag, time aloft usually being an issue of battery life. The wingspan limit rather limits the range of shapes viable for consideration. Of course, scaling the plane down so much will also reduce the Reynolds number of the airflow, making the air thicker and stickier.
I'd really appreciate it if someone here can lend some advice, especially regarding the flight characteristics of very small planes. Also, any tips for minimizing weight during construction would be very nice. Thanks!
Thanks!

My first suggestion is to google for "catapult glider plans". Download some, study them then build one. Even if you end up designing your own, building one from plans will give you the experience to see how it's done.

If possible find a plan with an article or build description attached. I know some of the plans are just simple outlines and assumes you know how things are built.

With regards to RE number, if you look at small competition free flight gliders you'll notice that unlike bigger gliders they have fairly low aspect ratio with quite thick chords. The reason for this is to make the wings fly at a higher RE. So that's one solution. At these sizes the loss of efficiency due to low aspect ratio is significantly less than the gain in efficiency due to longer chords.

As with RC, to build light you should build the tail light. This means opting for simpler structure, using thinner wood etc.

I wouldn't bother with a plastic film as the traditional solid balsa wing is still the world record holder. The current world record for hand launched (not catapult!) is very close to 2 minutes.

Finally read build logs for "serious" competition gliders. Either AMA classes or FAI classes. I don't think the FAI has a class for catapult launched gliders but there's always the F1N indoor hand launched class.

IMHO, this is the priority list:

1. Light weight
2. Light weight
3. Light weight, again...
4. A minimal amount of glue, thinned out....to support the first 3 points...
5. Lo drag. A thin, but nevertheless a "proper" airfoil....not a "flat plate"...

Then....check the weight...

Compare the weights of various plastic films....you may find considerable variations...

Something like what you see in this thread;

http://www.rcgroups.com/forums/showthread.php?t=1797199

But built as a glider.

Forget launching it with any sort of rubber band. Just trim it then hold it as high as you can over your head and push it out at the normal gliding speed. Assuming you can reach up to around 7 feet you should be able to achieve about 15 seconds.

The other option is to build a rubber or hand launch glider. But then you'll need to learn to trim it well so it launches upward and transitions with next to no altitude loss. That's a tricky thing if you don't have time to do the trimming before the compeition. In fact it's pretty much impossible.

With a 12 inch span limit you want LOTS of wing area to keep the loading down. And you will want to pick very light wood for the wings and tail. 1/16 x4 for the wings shaped to a crescent shaped leading edge and straight trailing edge and curved over a 1/8 camber rib at the root and mid span points will do the job. The leading and trailing edges should be sanded well back to keep the weight low.

Tail surfaces can be made from 1/32 and sanded down to 1/64 and still be more than strong enough for a typical school gym flight pattern. If by some odd chance your gym is more around the 40 foot mark for the cieling the wings can then be done from light 3/32 instead of 1/16.

Trimming is a whole other issue. We'll get to that once you iron down your building plan.

There's simply no need to use any sort of plastic film on this model. It'll just end up complicating things and making the final model too heavy.

For some other references google or bing or whatever for "low ceiling hand launch glider" and see what comes up.

Dare Designs used to market something called a Zing Wing, a flying wing catapult glider. Made for outdoors they would certainly be too heavy for your purposes. Perhaps you can design your own, much lighter with a smaller rubber band catapult. Nothing but wing, no fuselage and tail to haul around.

To avoid reinventing the wheel- here are some kits to get you started...

http://www.stevensaero.com/Free-Flight-p-1-c-30.html

http://www.a2zcorp.us/store/category.asp?Cguid={E9A6AC0D-A7FC-4160-954B-BFAEBFA01739}&Category=ModelKitsByBrand:Sting%20Ae ro

The other guy to look for is Stan Buddenbohm, but his stuff tends to be larger.

Does someone want to try to explain why air molecules act differently in a model size versus full size? I'm sorry, but I don't buy this "air is stickier" argument- I think in these sizes it's probably got a lot more to do with skin drag versus frontal area...Molecules are really pretty small- relative to the size of a model wing versus full scale wing- the number of interactions with a surface has got to be pretty similar.

Sam

I'm not an engineer and hopefully somebody with an education in aerodynamics will answer your question better.
As I understand it, the air molecules are not acting differently at all. It's just that our wings are so much smaller than a real airplane and the Reynolds Number is a ratio of the inertial forces and the air viscosity, so I guess it equates to big differences in performance.

Hi Eric

I'm a chemist- so that statement has always bugged me. I'm pretty sure that if we substituted helium for air under standard temperature and pressure- nothing would really change. Note that helium is less "sticky" on a molecular level than air. I think the answer is in the Reynold's number which shows that flow in a longer object has less drag than a smaller object if I've got it right. Don't think this has anything to do with stickiness of air molecules- it's just the percentage of the surface interactions versus the overall mass of the flow.

Sam

Interesting....I know they use helium (rather than regular atmosphere) in hypersonic wind tunnels where the prototype models are far smaller in size than the actual aircraft they represent. This is probably something to do with the velocity of the stream and surface heating. But it's still just magic to me really.....

If you're flying in helium under stp, you will have to go a WHOLE lot faster. Two or three times as fast, due to the lower density. However, it sounds like appa609 won't have to cope with that!

Suggest making the glider JUST heavy enough to almost get to the ceiling. So it's important to know the ceiling height and to do a bit of practice. Ideally, you put all the energy you have in the rubber into a glider which is light enough that it gets to just under the ceiling and levels out into a nice glide. At 12 inch span, you'll want the weight to be considerably under 10g unless you're in a blimp hangar or something. Probably, if your building skills are up to it, and you have some time to practice, a traditional indoor flapped clg will be best. If your skills aren't up to it, you could make an unflapped one. This kind of glider, with a 12 inch span, is known as a "standard" catapult glider to the indoor guys.
If you go to this URL, you can download "Winning Indoor Designs", which has a couple of standard catapult gliders in it:
http://indoornewsandviews.wordpress.com/downloads/
You will almost certainly find more in the various INAV issues available there for download. References to "category 1" or "category 2" sites refers to ceiling height.

Trimming these little beasties is a whole other thing. There's probably a good article someplace, but I don't remember where.

What size is the gym (including how high)?

Are you required to use plastic film?

Hi Linc

Drat- you're right- forgot about the density issue...

Sam

http://www.rcgroups.com/forums/showthread.php?t=1072383

Here is some reading on indoor hand launched gliders. Catapult launch design would be similar, I think.

Thank you all for all the responses! I will look through all in detail. That matchbox airframe is certainly amazing for its light weight.
According to the official rules, t must be launched by elastic potential energy from ground level, unfortunately, as I have some paper planes that will do 10 s or better with a good toss.
The height of the venue is unannounced, but the venue of the regional competition looked to be about 10 m. I feel like the best option in this respect is to have detachable masses to adjust AUW on the spot.
Thanks for the info on RE, I am no aerodynamicist. It seems intuitive that wider wings are necessary at small scales for lift, and the flying wing sounds like a good idea, though stability would be a particular concern.
The nose of the craft is required to be blunt, measuring at least 3/8" in diameter. It is not gigantic, but an inconvenience.
That is a great collection of glider designs. I will have to try a few.
Where do I go for really light balsa? My school's material is rather thick and (I suspect) denser than hobby grade stuff. Thanks!


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Preliminary design for wing
1/16" balsa wing: trapezoidal 4.5" root to 3.5" tip, 1/8" camber at 1.5" from LE, 5 degrees dihedral .