AR-40 Combat Glider
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.
- 48 inch wing span which is the most common size and which I feel
is not too large that it lumbers, and not too small making it too
- Bullet proof materials and construction to stand up to constant punishment.
- A relatively low aspect ratio for the excellent self-recovery and nimble aerobatics.
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: 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
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
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. Here
is the lipo battery and UBEC unit
needed to step it down to 5 volts. I also added a 12 volt switch
between the battery and UBEC because if the switch is downstream of the
UBEC, it will drain the battery when turned off even if it not connected to the
receiver. 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.
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.
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 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
have to iron the skin back down after each weekend of combat which is
Next up, install the base plate:
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
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.
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
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:
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
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.
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!