Elastic Launched Glider C

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jander14indoor
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Re: Elastic Launched Glider C

Post by jander14indoor » October 8th, 2012, 8:05 pm

Warning, this is a new year, and some of you may not be familiar with my style. You will find that sometimes I (and others) talk about VERY advanced concepts for these flying events that I don't recommend for teams just starting. Other times you'll see advice for teams just getting started to help them be successful fast. I (I won't speak for others, but I suspect they ascribe to the following) do that for a couple of reasons. I talk at high level to show you where you can go if you work. To set a high goal. But you don't get to that goal without starting lower. And, these things seem to have a steep learning curve at first. And, I hate timing the race to the floor. Its boring, disheartening and VERY hard to learn much useful until you can make devices that FLY. REALLY fly. And you all can.

I try to be clear what level I'm talking about so students don't (sorry, tortured cliche coming) try to fly before they can walk.

The rest of this conversation (other than the advice to always build light which is especially important for beginners) is on the advanced to very advanced end.
retired1 wrote:I find the plan for the feather shooter a bit unusual. For the wing, it calls for 1/8" 5 # A grain balsa, yet the listed high point is 0.038 or just a bit over 1/32. Any idea why he is starting that thick and sanding so much off??? Also has a 0.020 x 0.020 basswood leading edge that appears to be sanded to near nothing.
Back to the topic. That's a VERY sophisticated design with characteristics tailored to max duration on low ceilings, not a starter design. Adapted to SO rules it is more of a third or fourth iteration design due to advanced building skills required to be executed correctly. Trimming will also take more skill.

The wing is carved to that curved shape out of the 1/8 inch starting section. The underside is carved out to give the under camber and the top sanded down to the aerodynamic shape. If you took that curve you see in profile I suspect you will find the thing barely fits in the 1/8 inch height when the leading and trailing edge touch the bottom of the plank. You can't get a very stable shape like that by forming the balsa with heat or steam. Oh, and the very thin leading an trailing edge is important to reduce drag.

You could start with thinner balsa and form the wing with heat. You'd have to glue in the full chord rib to keep it stable, and I suspect that's one of the thingss Chalker7 sent you to see in that design. But that design only has that rib for the first 1.7 inch of the wing.

This design doesn't have a full length rib for a reason. Though it is difficult to see in that copy, there is a slit along the center line of the wing for the last 1.3 inches of the wing span. PART of the reason it is sanded so thin along the trailing edge is so the wing actually flattens on launch reducing drag for the launch to the ceiling. It then recurls to increase lift (and drag and pitch down) as the speed drops and it starts to glide.

Of course the thin wing is also to keep weight down. Note how it is ballasted UP to 2.0 gm.

And this is an example of why I've mentioned you will have different gliders for max performance depending on the ceiling height. That things great in low ceilings, but the wing recurls too fast and the drag will never let it reach a really tall ceiling.

Almost forgot about the leading edge. Dent resistance. Balsa that thin just will NOT hold up, basswood does better. I wonder if a carbon fiber might not do almost as well. Hmmm...

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Re: Elastic Launched Glider C

Post by iwonder » October 8th, 2012, 8:23 pm

Wow you weren't kidding about jumping straight to a very complex design :D I really like the ingenuity of having the trailing edge flatten out at launch though, rather cool...

Two questions though, what's the benifit of the under camber? I understand that the standard teaching about different leangths of travel on different side of a wing isn't entirely correct, but wouldn't an under camber make the top and bottom almost the same length and thus negate any effect of the pressure differential? If so, what's holding the glider up(angle of attack is the only other thing I can think of...)? Sorry if that's misinformed or confusing...

Now the simpler question... Is there a good starter kit for this event? There were a few helicopter kits that really helped my team, but as this is a new event and I don't follow the competitions outside of scioly, does any have recommendations for a kit to get the general idea of the event? It doesn't have to fit the rules of scioly, but a beginner level kit is perfered(for the record, I'm not building it :D )

Thanks.

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Re: Elastic Launched Glider C

Post by jander14indoor » October 9th, 2012, 12:03 pm

iwonder wrote:<SNIP because I don't have time at the moment to answer the harder question, but I will unless someone beats me to it (I hope!!)>

Now the simpler question... Is there a good starter kit for this event? There were a few helicopter kits that really helped my team, but as this is a new event and I don't follow the competitions outside of scioly, does any have recommendations for a kit to get the general idea of the event? It doesn't have to fit the rules of scioly, but a beginner level kit is perfered(for the record, I'm not building it :D )

Thanks.
I searched google on "catapult glider kits". Here's some kits that approxiamately meet the rules I hope the links work:

Beginnner kit, listed is out of stock at this source, but you should be able to find it other places. It is hand launched, add a hook to make it catapult.
http://www.a2zcorp.us/store/ProductDeta ... delKits:FF Glider

Smaller, but very easy to build and you get the basic idea.
http://www.a2zcorp.us/store/ProductDeta ... delKits:FF Glider

Not really indoor if I remember correctly, but about right size and again a place to start.
http://www.a2zcorp.us/store/ProductDeta ... delKits:FF Glider

Looks like it might have been built to SO rules, at least an early version. At little more difficult, but still fairly basic.
http://www.a2zcorp.us/store/ProductDeta ... delKits:FF Glider

Forget the launcher, but the glider looks pretty basic.
http://www.pitsco.com/store/detail.aspx?ID=3296&bhcp=1

No Kits, but some nifty materiels and a folding wind glider called the Zing Wing. Not a kit, so not legal, BUT... interesting concept.
http://www.modelresearchlabs.com/pricelist.htm

Another beginner kit source
http://www.acsupplyco.com/aerospace/midwestair/502.htm

REAL beginner design
http://www.scienceguy.org/Articles/Hamm ... lider.aspx

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Re: Elastic Launched Glider C

Post by chalker7 » October 9th, 2012, 1:25 pm

The A2Z links Jeff mentions above appear to be broken. This might work better https://www.a2zcorp.us/store/category.a ... F%20Glider It's the full category, none of which are specifically designed for SO but all of which are probably informative and will help get you on your way.

And yes, I apologize for not qualifying my previous link with a disclaimer that it was a very advanced set of plans (it held the national record in Catapult Launch Glider once upon a time.) I simply wanted a set of plans that illustrated the use of a pylon/rib to induce camber (curvature) into the wing. That being said, Jeff did a great job of outlining many of the subtle features on the glider and how you might be able to use those on your designs.

*Edit - The link system seems to not like what we're trying to do above. Just copy and paste the complete links and you should be fine.
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Re: Elastic Launched Glider C

Post by retired1 » October 9th, 2012, 3:36 pm

Well, I feel stupid, I have a larger plan of the feather shooter as well as some comments on it, so I had been aware of the under camber.

For beginners, I would suggest that you buy a very cheap glider to get used to the basics before you spend the money on an expensive kit.

Unfortunately, TailSpinAviation appears to have gone out of business, but their site is still somewhat active and they have several CLG's that are cheap (but you can not buy them there). You might be able to take the names and google it to see where they are sold by others.

Some are foam wings and some are all foam.

I have a kit from RetroRC called the sky jack that is a nice kit, but far too heavy. It will take a bunch of sanding.

The kits from A2Z will require modification to be legal.

I copied several plans from AMA gliders a while back. That site should be mandatory reading for all team members before spending any money. It may not be the most organized for SO purposes, but it is great information for free.

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Re: Elastic Launched Glider C

Post by jander14indoor » October 9th, 2012, 4:30 pm

iwonder wrote:<SNIP>
Two questions though, what's the benifit of the under camber? I understand that the standard teaching about different leangths of travel on different side of a wing isn't entirely correct, but wouldn't an under camber make the top and bottom almost the same length and thus negate any effect of the pressure differential? If so, what's holding the glider up(angle of attack is the only other thing I can think of...)? Sorry if that's misinformed or confusing...

<SNIP question already answered>
Thanks.
I think I have some time to try to answer this one. Real aero engineers feel free to step in and correct as needed. I switched to Metallurgy as a Junior in college. More inspiring prof. Of course I came back to it in my masters where turbine engine design was a minor. For lift and airfoils same stuff, less of the stability stuff that drove me nuts at the time. Anyway...

Warning, high level stuff coming.

I’ll start with a summary. Gliders tend to spend most of their time at a single operating condition. Gliding near stall. Turns out a highly undercambered wing has the best lift-drag ratio in this condition so it gets used when you can get away with it in the other conditions (launch).

Unfortunately that standard teaching is worse than not entirely correct. It's mostly flat wrong. If you want to know the right mathematical way to describe things, look up circulation theory, Richard von Mises, Ludwig Prandtl and Theodore von Karman. They really established the rigorous theory and math of flow around a wing. Bernoulli’s theories were important to that work, but not as commonly understood.

The following is an attempt to describe that non-mathematically.

Let’s start at a high level of detail where you don't have to worry about the airfoil and how it works, but look at the wing as a system. That way we can look at inputs and outputs only for a moment. The wing becomes a black box. We'll assume straight and level flight. Hold the wing stationary and consider the air to be flowing past the wing.

Start with an empty box.
- Air goes in and out with the same velocity, let’s make it horizontal for now. Note, VELOCITY, not speed. This is vector stuff, direction is important.
- Since there is no change in the air flow in and out, there is no force on the system

Now, we want to get some lift out of this system.
- Insert wing. Details unimportant at the moment, but to get lift (force upwards on the wing) you have to push DOWN on the air.
- This changes the air’s velocity. Now the air goes in horizontal, and comes out at the same mass flow rate, but with a DOWNWARDS pointing velocity vector.
- To maintain the mass flow balance (what goes in has to come out, conservation of matter and all), you have to slow the air in the horizontal direction too so you get drag (force forwards on the air, backwards on the wing, vectors).
o This is even IF you could create a theoretically dragless wing shape. Which you can't.
o But you can imagine it. Which is why aeronautical types tend to break drag up into induced and form (sure of the first, not sure of the second term, might be parasitic, hmm… might be three pieces, one from things like surface roughness, hmm… Oh well, later), one from the basic creation of lift (induced) and one from the form.

OK, that’s the basic lift model.
- It shows why speed increases lift (more mass flow, F=mA after all)
- Angle of attack increases lift (more A or change of wind velocity vector) and wing area (more mass intersected).
- It also shows why more lift means more drag. Irregardless of airfoil shape.

Now, let’s look a little closer and look at that airfoil. Because to fill out that lift equation, you need a constant which IS influenced by the shape of the wing.
- Its ONLY purpose is to turn that airflow with as little FORM drag as possible adding to the basic induced drag from lift.
- But how does it work?
o Turns out, its not that stuff about Bernoulli and speed differences. The Bernoulli stuff works great on air flowing through a tube of varying cross section, but not so much in free stream. Otherwise a flat plate wing wouldn’t work, and they perfectly good wings (in some conditions).
o Coanda had a MUCH better model. A fluid flow tends to follow nearby surfaces.
o Assuming non separated flow (more on that in a moment, but it’s almost always bad stuff) the amount of air deflection tends to be the average of the top and bottom of the TRAILING edge of an airfoil. That flow following the surfaces
o That’s why a flat plate wing works. For small angles of attack the flow comes off the trialing edge at the angle of attack and lift vs form drag is just fine.
- But you are going to say, “Mr Anderson, what about the REST of the wing. Lots of different shapes in the real world, can’t be totally unimportant.” Of course it isn’t. That’s where we get back to form drag and flow separation.
o Lets go back to the flat plate.
 It is fine at low angles of attack, air stays attached, is deflected with little turbulence. But only creates a little change in flow.
 Unfortunately, it goes to garbage at high because the flow separates from the top surface,
 This messes up Coanda flow something fierce, the air flow tumbles in random directions.
 Causing LOTS of drag and lift no longer follows angle of attack directly.
 If it gets TOO bad, lift goes away entirely and you’ve stalled.
o So, what do we do.
 I’ll start with a simple curve shape. Think of a metal venetion blind slat. Kind of like that undercambered wing, but thinner.
 Set that slat so the leading edge is parallel to the incoming flow.
 For reasonable conditions, the flow will nicely follow the top and bottom surfaces with little drag. But lots of flow turning, lots of lift. You still get the induced drag of course.
 For that specific condition, it turns out you get about the lowest form drag.
 For minor variations, it works OK, but then, like the flat plate, drops of sharply for too large variations of the conditions.
 In fact, the blades in a turbine where the operating conditions tend to be fairly constant are shaped mostly on that model with just enough thickening to meet structural requirements.
- Again, you’ll say “Mr Anderson, that’s not what modern wings are like. They tend to have flat or convex bottoms. Why?”
o Well, that gets back to operating conditions. Remember I mentioned the above shapes operate under only limited conditions,
o Airplane wings on the other hand have to work under a much larger range of conditions and the shapes you are familiar with give up ideal lift to drag at one condition to gain OK lift to drag under a wide range of conditions.
o And let’s not even talk about speeds near sound. Air starts doing really weird stuff there and that affects wing shape again.
o And finally, I’ll point out slats and flaps. Modern jets when landing extend slats and flaps. This does two things. One it increases area. Second, if you looked at the wing in cross section you would see that undercamber airfoil. They need that to get enough lift at the slower speeds!

Note, I’ve left out MOST of the math. It becomes important in describing the details of forces and pressures and velocities and modeling airflow and performance of a specific airfoil under specific and varying conditions. Here, you need to look at something called circulatory flow and Richard von Mises, Ludwig Prandtl and Theodore von Karman. They really established the math of flow around a wing. Bernoulli was only a part and not as commonly understood.

So, why do low ceiling gliders tend to have undercamber wings? Well, they spend most of their time at a single, slow speed (gliding) and need to have the best lift/drag properties at that speed.

Why does a low ceiling glider have flaps that straighten? Because that nice slow speed undercamber is too dragging at launch velocity, you’d never get to the ceiling.

Why might a high ceiling glider have LESS undercamber, or none? Two reasons. More time climbing, so you need an airfoil to compromise on launch vs glide conditions. Structural, much larger launch forces, that thin wing just won’t hold up.

Hmm, way too much, I've spentmore time that I really have already. Probably some errors or oversimplifications, hopefully not to egregious, experts feel free to correct.

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Re: Elastic Launched Glider C

Post by retired1 » October 10th, 2012, 2:48 pm

https://www.a2zcorp.us/store/ will get you to A2Z.

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Re: Elastic Launched Glider C

Post by illusionist » October 10th, 2012, 5:27 pm

A cool video showing a Science Olympiad ELG launch: http://www.youtube.com/watch?v=Mp5YCny0 ... re=related
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Re: Elastic Launched Glider C

Post by himlynx » October 26th, 2012, 10:58 am

Ref Two questions though, what's the benifit of the under camber? I understand that the standard teaching about different leangths of travel on different side of a wing isn't entirely correct, but wouldn't an under camber make the top and bottom almost the same length and thus negate any effect of the pressure differential? If so, what's holding the glider up(angle of attack is the only other thing I can think of...)? Sorry if that's misinformed or confusing...


Have a look at http://en.wikipedia.org/wiki/Airfoil It explains airfoils, starting with the basics and then goes on to advanced stuff. The first part is good enough for Sc Oly.

Let's first look at a symmetric airfoil. A vertical fin requires a symmetric airfoil because it has to work equally well in both directions. Symmetric, or near symmetric, airfoils are also used, for eg, in competitive aerobatic airplanes, because they spend much time inverted. A symmetric airfoil produces zero lift at zero angle of attack (AOA). It produces lift only at a positive AOA.

An airfoil with positive camber does produce lift even at zero AOA. Other things being equal (AOA, wing area, air density, aspect ratio, sweep back) it produces more lift than the symmetric airfoil. Therefore, it is said to have a higher co-efficient of lift.

An airfoil with a flat undersurface produces no pressure difference below the wing at zero AOA. However, a symmetric, or near symmetric, airfoil actually has lower than ambient pressure at the under surface. Therefore, it is actually pulling the airplane down. Conversely, the under camber produces high pressure under the wing ie it helps to push the airplane up. Bernoulli works all the time. The under camber makes the air slow down, thus raising its pressure. Since both upper and lower surfaces are pushing upwards, such a wing has a high co-efficient of lift.

Lift coefficient increases with AOA till we reach the stall, when it drops. Contrary to popular notion, there is some lift even after stall.

The total lift is proportional to lift co-efficient x air density x wing area x (airspeed squared). For an airplane to remain in level flight, or a gentle glide, lift must balance the weight. Lift = Weight or L = W. This is achieved by a suitable combination of lift co-efficient, air density, wing area and (airspeed squared).

For Sc Oly, air density does not vary and can be ignored. Under cambered wings have a high co-efficient of lift and hence can fly at a lower speed.

Under cambered wings are mainly used in gliders and slow speed airplanes. Their drawback is that they also have a high co-efficient of drag and, hence are unsuitable for high speed airplanes.

For a glider, maximum distance covered (called range) is achieved at maximum lift to drag ratio ( L/D ). In fact the glide angle (to the horizontal) is the inverse tangent of D/L.

However, in Sc Oly we are looking for maximum time in the air. This is achieved at an AOA higher than the above but lower than stall. You can control AOA by trimming the horizontal stabilizer, or the wing, or the center of gravity, or a combination thereof.

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Re: Elastic Launched Glider C

Post by Eel13 » November 4th, 2012, 5:52 pm

We have broken a couple of gliders because the rubber band didn't fall off of the hook. Can anyone share a tip on hook shape?

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