Feb 14, 2013, 09:32 PM
made of foam
appa609's Avatar
Thanks, that article was very helpful. Is sounds like it's saying that I have to try to reduce angular moment by moving the CG back so that it turns round quicker. Noting that, I have moved the wing back to the 1/3 position, and reduced the size of the rear stabs.
I'm actually rather worried that the forces of launch will break something: the fuse is only a 1/8 by 1/8 of balsa.

Anyways, I have a second model. I've slimmed down the wings, added a LE and TE, and moved it backwards. The fuse is 10.5 inches now, opposed to 9 before. Fuse is reduced to single stick 1/8 x 1/8. the horizontal and vertical stabs are each about 60% the prior area to reduce weight and pitch stability.
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Feb 14, 2013, 09:39 PM
made of foam
appa609's Avatar
It turns out I can use foams too, which are less dense than balsa. Is it worth the effort to ship some 1mm depron?
Feb 14, 2013, 10:28 PM
Registered User
slebetman's Avatar
That's starting to look good.

I've made several small gliders with 1mm foam (hotwired from a 6mm sheet) and they flew quite well. I've even made some catapult launched gliders with roughly 6 inch span. So it may be worth trying. But for larger wings I'm not sure if the foam will be able to withstand the catapult launch without reinforcement of some sort. That's one of the advantages of balsa - it's quite stiff for it's weight.
Feb 15, 2013, 12:25 AM
B for Bruce
BMatthews's Avatar
Originally Posted by appa609 View Post
It turns out I can use foams too, which are less dense than balsa. Is it worth the effort to ship some 1mm depron?

NO! ! ! ! Balsa give you fibers along the grain which act as spars to withstand the loads during the launch. 1mm Depron will fold under the launch stress and likely would even be marginal for adequite stiffness during regular gliding.

Your other post about the turning ability missed the mark. The reasoning for the balance point being located very near the overall planform neutral point is to avoid excessive pitch stability. It basically prevents the model looping around and hitting you backside of your forehead....

The more or less ideal transition takes place when the model climbs in a spiral path through about 1/3 to 1/2 turn towards the ceiling, runs low on speed while nose up and at around a 30 to 40 degree bank angle then sideslips and drops the nose at the same time. The side slip acts on the wing's dihedral to restore the model to a level attitude at the same time as the nose is dropping back to level. And if it all works just right the speed that the model falls to during the climb happens to match the speed needed for a nice minimum sink glide and the model simply glides away from the transition. In reality a slight nose down and recovery occurs a little more than half the time. But it's not all that rare to see the model simply fall into a proper glide perfectly. And when that happens at about one foot below the max ceiling height you darn well hope that someone has a stop watch running on the flight as it's generally a good one.

You and your competitors in this competition are all capable of making a nice gliding model. The WINNER is going to be the one that has it trimmed spot on for the climb and transition. Trimming and practice to know just exactly how far back to pull the model against the rubber so it transitions just below the ceiling without actually hitting it is the route to success.

Back in my teens and early 20's I was flying indoor models a lot. My best ever glider for the low ceiling gym managed a 35.4 second flight under a 28 foot ceiling after something like 200 attempts. And I'm not exagerating. The gym had 8 feet of exposed warren truss ceiling support structure and I had to thread the glider through 3 trusses, past 2 lights and a cross bar before it was down below the structure. The leading edge of the poor model by the time of this local record flight looked like the hounds of hell had used it for a chew toy.

The real key to the success and the obviously low sink rate was a primo piece of 3/32 contest wood that I used for the wing. The wood was strong but amazingly light which provided the 23 to 24 inch glider with a lower than usual overall weight. That and the contest grade 1/32 tail wood that I sanded down to .016 at the center and around .010 around the edges and the fin which was sanded down until I could just about read thru it.

The 1/8 fuselage wood was medium density balsa which I tested for fibrous content by cutting a bit and breaking and evaluating the jaggedness of the break for long strand wood fibers. These days the folks "cheat" a little by using decent balsa and glueing on a few strands of carbon fiber along the sides to provide the sort of strength that I had to find from testing some 20 to 30 sheets of wood.

I'm not suggesting that you need to get THAT serious. But it might provide you with an inside view of what truly serious indoor model flying is about.

Getting back to your model. I'd suggest that you consider the realities of forming and shaping wood for a 12 inch glider. Your airfoil design is pretty much wishful thinking if you have not shaped balsa or other materials by hand and have the skills needed for this sort of accuracy. Realistically for the 12 inch span rule you're going to slightly relieve the first 1/2 to 3/4 inch of 1/16 sheet at the leading edge and maybe the last 1 inch at the trailing edge to thin the edges down to around .020 inch. That'll at least lower the weight of the wing. Although frankly there's a lot to be said for using 1/32 or 1/20 and simply sand down the trailing edge a little and curve the sheet over a 1/8 inch camber rib for a 3 inch root chord and call it good.

I gave up on 1/32 sheet for the wings on gliders for that 28 foot gym because I kept tearing the wood at the root of the wing along the leading edge. But I was hand launching and could well have been dragging the model belly first which caused the tear. Also my designs that used 1/32 were about 16 inch span with a 3.5 to 4 inch root chord. Your 12 inch max span requirement makes using good quality 1/32 sheet for the wings VERY doable. Especially given the low ceiling of the flying site to be used.

Now to the latest design. The wing is still WAY too big for the size of the tail and tail moment arm. You simply will not be able to get the balance point all that far back. You mis-interpreted our earlier comments. Even your previous design had tail surfaces that were too small for the size of wing. The tail areas are based on the wing area and tail moment arm length.

There is a great online CG locating tool at http://adamone.rchomepage.com/cg_calc.htm . Run your design through this calculator. If your neutral point isn't back at around 40% of the wing root chord then you made the wing too big. In the box which asks for % stability use "2". You'll likely move it back from there but it's a good starting point.

You mention that there is a 12 inch max span. What about overall length? It's quite permissable with this type of model to make the overall length the same or more than the span. In fact it's generally very effective at obtaining a good design which flies nicely and has a good climb to glide transition. If you've looked up plans for handlaunch and catapult gliders you'll have seen this already in the designs you found.
Last edited by BMatthews; Feb 15, 2013 at 12:45 AM.
Feb 15, 2013, 10:20 PM
made of foam
appa609's Avatar
length is unrestricted. I kept it short to try to save weight on the fuse. Now I've elongated to 12 inches. Wing has been made sleeker, at 2.5" root chord, with 1/32", though I am not sure I'll find this thickness at my LHS. H-stab almost doubled in area, and wing has been moved forwards 1.375 inches. According to the calculator, NP is now 1.00 inch behind the root LE, or 40.0% root chord. I assume that 2% static margin means the CG is 2% further forwards than the NP with respect to the RLE. Latest revision:
Feb 16, 2013, 12:00 AM
B for Bruce
BMatthews's Avatar
Yes, that's what the 2% means.

Oddly enough now your vertical is likely larger than it should be. And I don't know if it's perspective but the nose seems rather long and the tail rather short. A LITTLE nose weight is OK. And a longer tail moment will allow you to move the balance point back even further. A short tail moment arm tends to make for a poor transition. So trading a little added nose weight for a longer tail isn't a bad trade.

Again, look to the plans available online to get a feel for what the top view of the glider should look like. When you're learning like this plagarism isn't a bad thing. And the models that show up in the winner's circles don't get there by accident. The lowly small all sheet balsa glider has had a LONG development program that has covered the better part of a 100 year span of time.

1/32 is a stock size in any balsa bin. The trick is finding decent sheet stock which is light and does not have lots of built in stress that makes it look like a potatoe chip. If you don't start with flat wood you can't build a good wing. If you can't find any at the one shop check another. Also Michael's craft stores generally have a balsa bin. But the one time I checked it out I'm not sure if the wood in the bin would be able to actually float on water. It was THAT heavy.

One message I hope you're getting from all this discussion is that you should not wait to the actual day of the trials. You should be buying wood and building and flying well ahead of time. At 12 inch span the models are only going to cost about a buck a model even with the horrendous price of balsa these days. By all means build one of the designs you've done up. But try some other options. In fact here's one to consider just for fun.
Feb 16, 2013, 12:11 AM
B for Bruce
BMatthews's Avatar
That's what I get for writing a reply then stepping away to make dinner and come back and hit "submit" thinking I was done.

I forgot to add the drawing and some instructions. The drawing below is actually for an outdoor fun model plan I did up a while back. To adapt it to your indoor sintuation the wing would be done from 1/32 with ribs at the center and mid span shaped as shown in the side view of the carved thicker sheet wing. And instead of only the fin being 1/32 both tail surfaces would be 1/32 sanded down a little thinner. The fuselage should be a little less deep and made from 1/8 medium sheet instead of 3/16 very hard.

The idea behind the rounded tip shapes is partly for looks and partly to give the wing a slight sweep back effect for tracking in the climb and partly to avoid the sharp tips of a trapezoid wing such as your design from becoming splintered by hitting things.
Feb 16, 2013, 10:55 AM
Registered User
GliderJim's Avatar
I like the looks of this one...
Feb 16, 2013, 01:59 PM
B for Bruce
BMatthews's Avatar
Exactly. Notice the proportions of the wing to stabilizer areas and how many chord lengths long the tail is between the wing's trailing edge and the stabilizer.

The key to minimizing the nose weight is to make everything aft of the balance point lighter. As already mentioned this means sanding down the 1/32 sheet and tapering the fuselage.

This can lead to broken tail booms. But every time I broke a tail boom I reacted by making the stabilizer and fin even thinner rather than making the fuselage deeper and thicker. The reason the fuselage breaks is due to the momentum of the mass of the tail surfaces. Lighter surfaces, less momentum.

To get the most out of all this realistically you will need to build and flight trim a good three to four models before you learn what you need to make a successful bid at winning this competition. Less typing and more cutting and sanding should be your goal over the next week or two. The good news is that even as a beginner it should not take more than an evening to make a glider.

Hints on sanding the sheet for the tails thinner-
  • Use fresh sandpaper of 220 grit for the basic thinning down then switch to 400 to polish the wood to a nice surface.
  • Cut out the stabilizer by itself. Cut out a piece of sheet large enough for three or four fins as a separate piece.
  • Sand the stab thinner by holding it down at one tip and pushing the sanding block away from your finger towards the other tip. Two or three strokes then reverse the tips. This will taper the tips to be thinner than the center which is what you want. Repeast the two or three strokes and reversing end to end until the tips are down to about .010 and the center is about .015 to .018 thick.
  • Do the same to the fin wood but try to thin it evenly so the whole sheet is about .010 to .012 thick in the end
  • A set of digital or dial calipers comes in handy for measuring your progress.
  • When thinned out nicely "polish" the wood with a few strokes of some 400 grit wetordry sandpaper.
  • The key to avoiding curling the wood is to use light pressure on the sanding block. If you use old worn paper and press hard the skin stress will make the wood curl. Fresh, sharp snandpaper cuts with light pressure and doesn't compress the surface of the balsa and produce that sort of stress in the wood.
Feb 16, 2013, 10:22 PM
made of foam
appa609's Avatar
Awesome. I am picking up some wood tomorrow at the Lhs, and I will start building. Probably go from a plan, first. The long nose is probably from sketchup's perspective distortion. The wing is 1/3 the fuse back, as suggested.
As far as adhesive, I only know that I want very little of it. Ca or watered down wood glue?
Feb 17, 2013, 12:54 AM
B for Bruce
BMatthews's Avatar
CA is fine. But to keep the amount you use to a minimum I'd suggest good fitting joints and using only a little of the thin stuff.
Feb 17, 2013, 07:11 AM
Balsa Flies Better!
Man- do I not like CA in this application. Problem- it's brittle and if the joint lets go, its often very hard to get a good bond again- you've got to sand down to fresh wood. Also- catapult launches are going to put a lot of stress on the airframe.

I'd use either Titebond II- and not watered down- water weakens the bond although you can probably get away with adding water to Titebond I- its water soluble-

Or- Ambroid. Get use to double gluing. Ambroid (or Sigment) is light, easy to sand, flexible, easy to repair. It's also much more adjustable than anything else- flood the joint with acetone and you can get it to release which can allow you to reposition a stab for stab tilt.

Feb 17, 2013, 11:04 AM
B for Bruce
BMatthews's Avatar
Actually you're right. But it does make for faster building.

The Titebond would work nicely but you'll want to squeeze out a little puddle of three or four drops of glue then use something like a straightened paper clip to transfer the glue to the wood so you use a very fine bead of it. And only just enough that you don't get much squeezeout. You'll need to make up some small scrap balsa "clothepins" using small rubber bands or some other tricks to clamp or otherwise support the parts until the glue dries.

The Ambroid or Sigment is also a great choice. Sigment is quite thick though which makes it hard to use in small amounts. I'd suggest putting some in a sealable container and thinning it with a few drops of acetone or lacquer thinner so it becomes a little more liquid like. About the consistency of one of the thinner pancake syrups is about right. The stuff dries FAST so you'll need to work quite quickly to get it from the container to the joint and then push the parts together so it soaks into the wood decently well before it gets too thick.

Another option for Ambroid or Sigment is to thin it until it's like water. Then apply it with a small bottle that has a bit of the very thin teflon tubing attached for control. A fellow on another internet group does it this way. The joints are positioned dry so they fit tightly. Then he wicks on just enough of this water like mix to soak into the joint but not flood the surface of the parts. Being so thin it dries in 3 to 5 minutes. He then applies a second coat in the same manner again using control to prevent it flooding out too far from the joint. 3 to 5 minutes later it's dry enough for careful handling. By working on a variety of stages at one time the waiting time isn't an issue.
Feb 17, 2013, 12:18 PM
FF lift master...sometimes
CA is absolutely fine for CLGs, even full-power launches with outdoor CLGs. I use medium viscosity for the wing/fuse joint.

Fully on-topic, I note that Stan B. has a kit for a glider appropriate for S.O. This would be a great option. The kit includes materials for 2 gliders, and Stan's stuff is the very best.


Feb 17, 2013, 01:42 PM
Registered User
I haven't followed this closely, but is there a plan of Stan Stoy's Coot available? That airplane set a number of low ceiling indoor handlaunch records.

One thing about the long nose. It is good for reducing needed nose weight, but it contributes to the barbell effect. Moment of inertia is proportional to the square of the distance of the weight from the CG. May not be a problem.

Best advice is to get building and practicing. Being able to make the airplane fly as you want is a big step forward to success in competition.

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