AR-40 Combat Glider

plan view

We have a good sized group of RC combat glider pilots who fly in Pacifica California during the excellent summer winds that we generally get between June and September. Here is a nice video of a large group of Colorado pilots battling it out on a good day.

Most pilots use commercial glider kits such as the Zagi, Combat Wings, and the Bee. These are all very good planes but the sport has progressed such that you have to make a lot of modifications to make them combat-ready. This caused me to dream about what the ultimate combat glider should be. I obsessed over this project for a year and the result appears to be excellent. These are the plans that I created with all of the features and flying characteristics that I want in a combat glider. I will try to include enough information for you to build one yourself. My criteria are:
Having learned from painful experience with my own radical designs, I now understand why the general plans for flying-wing combat gliders all look largely the same. With the design of the AR-40 I chose to stay closer to the proven designs but modified to accommodate my preferred materials and design constraints.

The first main choice I needed to make was to select the exact aspect ratio that I wanted. The aspect ratio in its simplest form is wing span divided by width but a better definition is twice the wing span divided by the sum of the root chord and tip chord, including the elevon width. Using this formula I measured all of the gliders that my fellow pilots and I have been flying and entered them into a spreadsheet to calculate all their aspect ratios. I was then able to sort the list of gliders by aspect ratio and select the aspect ratio I wanted based on what I had seen of each glider's performance. The name of this glider is "AR-40" simply because I chose the aspect ratio of 4.0 as being my ideal.

Here are the specs. Note: Optional cut-out for base plate shown in red.

Here is the Visio drawing file in case you want to edit it.

The main unusual feature of this design is the single large, unbreakable carbon fiber spar that runs from tip to tip. This allows the glider to retain as much energy as possible through high-G turns and still be flexible enough to absorb the force of hard collisions and landings. In order to keep the entire length of the spar within the foam core I needed to reduce the amount of sweep. This makes it a bit less stable in pitch. This much is not a problem for experienced pilots but is probably not ideal for beginners. Another potential disadvantage of this choice is that the spar and the glue to hold it are placed well behind the center of gravity. That means that substantial weight will need to be added forward of the CG to compensate and that guarantees that in order to fly well, the resulting plane will be quite heavy for its size. Again, that is not a big problem for experienced pilots but increases the speed and danger to people and the plane. I bought the rod from Goodwinds.com here. It is product #020099, with a .281" outer diameter, .186 inner diameter.

I had the wing cut with a computer-controlled cutting machine by the helpful folks at FlyingFoam.com. If you send them the above specifications and diagram, they can cut and send you the cores for about the same price as a commercial glider kit. I also recommend that experienced pilots experiment with your own ideas using mine as a starting point. If you don't do anything crazy you should be OK and you may even improve on the design.

Note that from here on, I'll give a complete describing of how I built it out, but if you have a preferred materials and methods of building out a combat glider, then you can stop here. The most important things are the core cutting parameters and the carbon spar. All the rest are minor details.

The other main bit of strengthening that I use is the base plate shown in red above. I use 1/8" hobby plywood glued directly to the foam. The folks at Flying Foam can cut the spar hole as well as the base plate cut-out, which is a huge help. The cut-out gives you a nice flat surface on which to attach the base plate. In addition to stiffening the typically weak central region, it provides a nice foundation on which to attach all of your components. I bolted the servos and battery directly to the plate though you can use servo tape or other means. It also allows you to easily cut out and transfer all the components into future planes once your first one needs a body transplant.

This image shows the cut-out. Just reattach the portions that you don't remove.



This shows the main components attached:


And this shows how the final assembly will look:


My servos are very strong and fast. They're also $20 each at Hobby King which is expensive compared to other components but cheap for what you get. You can easily get away with less if you want to save money. I originally used a lipo battery and UBEC unit above but had lots of trouble with them (no fires as some worried) and do not recommend them. I later replaced it with a 5-cell NiMh. Hobby King also has really cheap Spektrum receivers.

The last bit of foam cutting is to create the trailing edge. That could have been done by Flying Foam but I wanted to decide later how and where I want it to be cut. You may want to have them do that but this operation was very quick and easy. Here you see that I have taped down a metal ruler and am cutting on a 45 degree angle. Doing that on top and bottom gives a nice result. The trailing edge ends up rather fat but that's how the airfoil was designed and I didn't want to risk modifying it. See the diagram to see what I mean.
Cutting the trailing edge

The next order of business is to paint the foam. I always use fluorescent spray paint so that I can more easily spot it in a crowded fur ball. Contrasting top and bottom designs help to see which side is up after a collision.

blocks
Be sure to paint and save the cut-out foam blocks. You will want to slice off the topmost painted parts and glue them back in right before you add the skin.

The secret is to use the very least paint possible so that the skin adhesive will attach to the foam and not just the paint. Even knowing that I still used too much paint. You should probably not paint the foam. Instead, add paint or decals on top of the skin just to be safe. I'll probably have to iron the skin back down after each weekend of combat which is unfortunate.

Next up, install the base plate:
gluing prep

I attached the base plate with Super 77 adhesive spray. Here I have masked off the areas that I do not want to glue in preparation for spraying. That stuff tends to get all over the place so take your time and be careful with this step. After spraying, wait a couple of minutes for it to get tacky and then press it into place with a lot of pressure.

Next, install the push rods. I highly recommend Golden Rod or other flexible push rods as they let you place your control horns close to the center of the elevons, plus the bend keeps your servos from stripping during collisions. Using the circular servo horns guarantees that the servo arms won't break either.You may have wondered why I position the servo horns towards the leading edge and closest to the center line. This allows the push rods to be bent the least and for the entire linkage to happen in a small space that is part of the rectangular servo cut-outs.

Important: Attach the push rods to the bottom-most part of the servo horn! I.E. closest to the base plate. This is to allow the push rods to be buried in the foam for as much of its travel as possible. Here you see where they exit on top and understand why they connect to the servos near the bottom of the glider.

push rods 

The skin that I prefer is document lamination plastic. It's the stuff used on driver's licenses. This stuff is extremely strong and light and comes with heat-activated adhesive. You use it just like you would use Monocoat or Ultracoat though you need more heat and pressure. The thinner rolls will even heat shrink a little which makes it ring like a bell when you tap it. Mitch who introduced me to this stuff posted information on our mailing list here which references an even longer discussion on the RC Groups forum.

There are two types of plastic "DI" and "CP". The CP type stretches a bit and is my preference. They also come in a number of different thicknesses of plastic and adhesive. These are specified as a fraction with the first number being the thickness of the plastic in mils (thousandths of an inch) and the second being the thickness of adhesive. The two numbers must add to 5 or 10. Most builders use 3/2 CP which is excellent. For the wing above I used 4/6 CP which is one of the main reasons that it is heavy. I recommend that you get a roll of 3/2 CP. You can get it from LaminatorWarehouse.com. though I notice that they don't use the fraction specification anymore so you should call in your order and make sure you know what you're getting. You'll want an 18 inch width.This stuff comes in 200 foot rolls which is enough for 10 or 15 gliders, so try to go in on a roll with some other pilots if you can. The folks at Laminator Warehouse are very nice. They normally sell rolls in pairs but will make exceptions for us especially if they already have some broken sets.

Finally, construct and install the elevens and winglets. Every builder has their own preferred materials and design for these parts. My winglets are made from kitty litter containers though large detergent bottles also work well. Cloroplast is another popular winglet material as it is very light and cheap but also is easily broken and torn off in battle.
Winglets
The popsicle sticks distribute the force across a wide area. When installed as you see above they almost never break.The disadvantage of my winglet construction is that it is relatively heavy. It doesn't weigh very much but since they are at the very back of the plane, that weight must be compensated with ballast in the nose meaning that you pay twice for all weight placed behind the CG, and probably triple for weight at the tips. Therefore any weight that you can save behind the CG will allow you to build a lighter plane.

Note that I have not yet cut off the tips of the control horns in the photo above. You can leave them like that while you are testing which hole you want to attach the push rods to, but be sure to cut them off once you've decided, otherwise they are easily broken in collisions and will draw complaints from other pilots if they cut into their wings.

The red thing at the front of the battery cage is about 1.7 ounces of lead. That turned out to not be enough in the end and should have been closer to 3 ounces for the heavy planes that I build. You want to put all such ballast as far forward as possible but no closer than about 1.5 inches from the leading edge for your safety and those around you. The point of adding ballast is to put the center of gravity where you want it using the least dead weight. In the diagram above the CG is meant to be 7.81 inches behind the nose. That means that if you balance the finished plane on a point or horizontal edge, it will lie on that line. Even small changes of CG can make a big difference in how a plane flies. If it is too far back it won't fly at all. If it is too far forward it will be very sluggish. 7.81 inches is my preference though beginning pilots will probably be happier with 7.5 or less. Here is a lovely CG calculator on the web that you can use to find out where it should be for the responsiveness you prefer. Adjust the "CG Position" buttons to suit your flying skills and preferences. I must emphasize the importance of choosing a proper CG and carefully adjusting your build to put it where you want it. That may take a bit of trial and error due to the weight and placement of all your components and materials but it is crucial to get the CG right.

Here is what the CG calculator looks like:
CG calculator

See the "responsiveness" control in red above.

Here is a close-up of the base plate on the underside of the finished glider with the CG positions marked. The horizontal line is where I decided that I liked it.
base plate with CG

Once getting the CG right I found that roll control was much more sluggish than pitch. That's true in general but seemed a bit excessive in this case. I bumped aileron throw to 125% of my transmitter's default, and all was perfect for my tastes. You will certainly want to experiment to find your ideal settings.

The Result

The AR-40 turned out much better than I had hoped! When well-trimmed it has an unusual tendency to continue on a straight line when you let go of the stick. I was used to planes that would quickly recover on their own and turn lazy circles. At first this made me nervous because a lost of radio contact could leave it flying out to sea or other poor locations, but now I like the behavior. The plane goes where you point it, so be careful where you point it!

And how does it stand up to combat? Well look what happened when it met head-on with another glider. That glider was an old hand-me-down that had been covered in Monocoat or similar skin and it nearly cut it in two. Notice the damage to my glider on the right. Believe it or not, Dano's plane was fixed in less than 5 minutes using 3 pieces of fiber tape and thrown back out into the mix where it did just fine. Of course he'll spend some time later gluing all the foam back together, and should probably reskin with Docu-lam, though he's also interested in building an AR-40.



If you build a plane from these plans, please Let me know. Likewise if you have any questions or corrections.

Well, that's about it. Here's a snap of our happy group mixing it up in Pacifica, California. Happy hunting!
Glider combat