Bungee Launching RC Sailplanes

Bungee Launching RC Sailplanes ("High Start")

What it does:

A bungee setup allows you to fly an RC sailplane from a flat surface, such as a schoolyard or playfield. It is especially useful when learning to fly as you will get a lot of practice flying your aircraft in different orientations and in landing. As you develop skill, you can even bring your sailplane back to your hand, catching it just at the stall.

A bungee will also enable you to fly from completely flat locations, for a launch will place your glider several hundred feet off of the ground. Under the right conditions you may then be able to catch a thermal (a rising torus of warm air) and climb to higher altitudes.

Aircraft requirements:

You will need a tow hook on the bottom of your aircraft. It is on a line that angles forward, but mostly downward, from the center of gravity. (The inverted aircraft shown here balances on the wing spar.)

What it is:

A bungee setup for launching your glider consists of a length of elastic material, about ten times that length in string, and a drag parachute, with a ring at the top of the parachute that fits in the tow hook on your glider. The elastic end is secured to a stake in the ground (or other stationary object).

How it works:

The glider is attached to the drag parachute and walked back, stretching the elastic to about three and a half times its relaxed length. The model is released at a 45 degree upward angle, and considerable (but not full) up elevator is added. The pull on the tow hook is in effect, artificial gravity, which allows the aircraft to glide downward relative to the airstream while accellerating and climbing at a steep angle. Without the extra up elevator all you get is a very fast shallow climb with little gain in altitude. (Too much elevator can cause a premature release of the parachute, where you may have to quickly complete a loop to get the glider into stable flight.) As the elastic relaxes, you gradually relax the up elevator. The parachute then inflates and drags the tow ring off of the hook.

How to get it:

You can purchase a kit from your hobby shop or mail order house or you can build one from scratch following the instructions below.

Building a bungee:

A good elastic for our purposes is latex surgical tubing. 25 feet should be enough for most uses.

Sizing the elastic:

The diameter of the tubing is determined by the model size:

For light models up to 20 ounces (560 gm), wingspans to 60 inches (1.5 meter), or light 6 foot (1.8 meter) wingspan models, use 1/4 inch outside diameter (O.D.) tubing.
For 6 foot, 2 meter, and light 100 inch models 22-36 ounces, use 5/16 inch O.D. tubing.
For heavy U.S. standard class (100 inch), use 1/2 inch tubing.

Care of elastic:

This elastic will last longer if you store it in a cool place, keep it out of sunlight when possible, and dust it with talcum powder to keep it from sticking to itself.

You will also need a parachute. When there is tension in the line the parachute will be colapsed. As the tension decreases, the parachute inflates and performs two functions. First, it drags the ring off of the glider and second, it reduces the likelyhood of the line ending up in a tangled pile.

Materials:

A bungee set-up (in this case for a light two meter sailplane such as a Pussycat or Gentle Lady), consists of the following:

25 feet of 5/16" outside diameter latex surgical tubing. This amber colored material can be obtained from a medical supply house. This is the stretchy rubber stuff - not the clear plastic and not the synthetic black rubber.
250 feet of polyester or nylon cordage. Something a little stronger than kite string such as nylon cordage for tying boxes is suitable. Kite string is fine for light duty use (1/4" tubing).
Three one inch (25mm) rings. Key rings are suitable for light duty use.
One eight inch spike. A second spike can be used as an axle for the reel instead of a dowel.
A large washer with an inside diameter to fit the shank of the spike. This is to keep the ring from slipping off of the end of the spike.
A small, simple parachute. You can sew one up out of polyester or nylon cloth. This should be about 12" (30 cm) diameter. If you divide up the sheet into eight triangles and use six of the triangles for gores that will be fine. A simple conical parachute is all that is needed. When you sew each seam you must include a shroud line, so the shroud lines are continuous from bottom ring to top ring.
A 3 inch (7.5cm) length of heavy cardboard tube. This will form the hub of the reel. The kind of tube used as a core for a large roll of carpet is especially suitable. Do not use the lightweight kind used for wrapping paper. If you start from a square cut end, you can mark a guide line for a square cut by wrapping a piece of paper around the tube and marking it with a pencil. After cutting, finish the cut end against some coarse sandpaper on a flat surface.
A small round headed wood screw (U.S. #10 or #12, 3/8" long). This will be used to hook the parachute ring when you reel up the bungee.
An eight inch (200 cm) length of dowel at least 1/2 (12.5 mm) inch in diameter. You will use this as an axle for the reel.
A one inch (25mm) length of the same dowel. You will use this to form a crank handle for the reel.
Two disks of plywood or hardboard You can use regular or thin plywood(such as packing box wood) or wood hardboard composition material (such as Masonite). The size of the disk used will depend upon the size of the cardboard tube. The disks must be thick enough to support the crank handle. The diameter of the disks should be about 4" larger than the diameter of the tube.
Epoxy glue
Three rubber bands.

Building the reel:

Using a protracter, mark the center of the disks, a guide circle so that you can accurately locate the hub, and the outer diameter of the disks.

Cut out the disks.

Drill the center of each disk to form a bearing for the axle, which must rotate freely. Drill one of the disks near the edge (not too close) to form a tight fitting hole for the crank pin.

Trial fit the pieces before gluing, making sure that the disks are parallel and that you can see the guide lines for centering the hub. Epoxy the hub to each disk, being careful to align the hub. Do one disk at a time to ensure proper alignment. When you set the second disk, insert the axle and make sure that it is perpendicular to the face.

Glue in the crank pin using the epoxy. Make sure that the pin does not protrude into the inside of the reel. After the epoxy starts to set, but before it is hard, build a fillet on the outside. A little fiber material (of any kind) will allow a greater build up of material here.

Insert the screw in the middle of the hub. Leave enough protruding to accept the key ring. Soak the area around the screw with thin CA or remove the screw and put a little bit of expoxy in the hole and reinsert the screw.

Asembling the bungee:

Attach each end of the elastic tubing to a key ring. The best way is to form a short loop in the elastic. Thread this loop through the ring and back over the top of the ring. Secure the free end of the tubing to the main part with a rubber band. Tie the string to one of the rings (a bowline is the best knot). Tie the other end of the string to the lower parachute ring. Put the axle in the reel, hook the parachute top ring over the screw on the reel hub and wind up all the material. This will leave the elastic end free, ready to be staked into the ground.

You will also need a hammer to drive the spike

You can secure the washer to the spike with a rubber band wrapped around the spike. Use a backpack or other convenient bag to hold the loaded reel, spike, washer, axle dowel and hammer.

Deploying the bungee:

The launch must be made into the wind. Your stake will be set at the upwind side of the launch area. Put the washer on the spike, the spike through the ring at the free end of the elastic tubing, and drive the spike at an angle (top away from where the glider will be). With everything laid out, pace off the relaxed length of the elastic.

Prelaunch check:

Always check that you can operate your controls and that your transmitter antenna is up before you release the glider!.

Stretching the elastic

After the slack is out of the system, pace off three and a half times the distance of the relaxed elastic length.

Launch

Perform your prelaunch check and release the glider at a steep angle. Pull addtional up elevator to keep the speed down and the climb rate up. Ease off the up as the tension is released. You may be able to gain a little more altitude at the top if tension is still in the line by sharply pulling up to release the tow hook. This will give a little additional zoom off of the top but be prepared to push down to avoid a stall.

For another page on bungee launching, with a more technical information, see http://www.charlesriverrc.org/articles/design/dickwilliamson_histartphysics.htm

If you have comments or questions, e-mail me (Attn: LGB)

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