Piggyback Carrier

This is a simple and relatively simple-to-build project. It is great for launching gliders, testing odd flying projects and assisting while teaching beginners. What: I wanted to launch strange things in the air a couple of years ago, and none of the carriers I had seen were flexible or "evolved" enough. I made this one and found that it worked better than I could hope. Build one and you won't be disappointed. It's advantages compared to other schemes are: Single, reliable release point. Other devices use separate rubber bands on each wing that have to release simultaneously, or else. Also, I don't much like rubber bands for this kind of thing. Can carry a very wide variety of airplanes. Because the wing supports are easily adjustable in height and pitch, practically any plane can be lifted. Modifications required to the carrier plane are minor and will not detract from either function or appearance when removed. Only modification to the carried plane is installation of a release clip with two #2 screws. Some simple way to protect the clip on landing is recommended but optional.
The mechanism is built around the Vortac Bomb Release, and therefore the size/weight of planes that can be carried is limited by the capacity of the plastic release clip. I have not, and do not recommend that planes heavier than 25 ozs be carried aloft using this setup. This allows all hand-launch gliders, Speed 400-powered electrics and, of course, The Gentle Lady. If you can machine aluminum release clips, heavier 2M and other larger planes could be easily be carried, the release is certainly strong enough.
Who: The instructions here are not very detailed for two reasons: the many and varied planes useable as carriers make it impossible to cover them all adequately, and the modifications required to the dihedral joint of the carrier, if poorly planned or executed could easily fail. These are not difficult mods, but you should have built several kits already before you attempt this project. Some experience modifying or scratch-building will also come in handy. Why: If you belong to a club with a training program, this will enable a simple and inexpensive plane like a Gentle Lady to be easily carried aloft. It will let a beginner get decent stick time with minimum investment on his/her part. A Gentle Lady, 2-3 channel radio, glue, Monokote etc. runs about $150, a world of difference from the "entry price of a .40-powered ARF with starter, fieldbox and other accessories. Being also slower and easier to both fly and fix it should help more beginners get into and stick around this hobby. If you are among the afore mentioned beginners, print this out and bring it to the field or local club meeting. I would be very surprised if you don't find at least one person willing to add this to one of their planes and if you don't go out and find a different club. If you are the type that likes to try odd things that may or may not quite fly and are not yet, or never will be, powered, this will enable you to get them to altitude quickly and painlessly. If you're into gliders, or want to try them and there is no glider field with a winch around, this will let you get far above hi-start altitudes, and, provided you already have a carrier plane handy, is cheaper and easier to set up than a winch. Also, this will not stress your wings the way a winch can; build your glider "stock" and go have fun! Even though limited to fairly light planes it will give you a taste for soaring to see if you want to keep going. I suspect that larger planes like the Telemaster could carry 2 planes aloft one on each wing panel for vicious EPP glider combat even if there is no slope site nearby. You've heard it before: Your imagination is the limit! Where: As you will see from the pictures I built this for a Sig LT-40. The plane has very low wing loading, will not stall with power on, is large, stable, and easy to work on. The pictures you see are on the model stripped for winter-time mods. I have no inflight pictures yet. When the weather improves I'll post some. The LT-40 has two sets of spars which make it very easy to mount the blocks for the wing supports. If you build this for a plane with a single set of spars, you must add false spars that bridge at least 3 ribs to mount the blocks to. While the force imparted downward to the wing is not significant, do not expect a single unsupported rib to carry this. High-wing trainer types like Kadets and Telemasters are natural candidates, but there is no reason why a stable low-wing plane cannot accommodate this as well. The most critical dimension is that the two wings be separated by a height at least 50% of the chord length of the larger one. There is no reason that the pylon cannot be attached to the top of the fuse. The supports attach to the wing in the normal manner, except check to make sure the bottom of the 4-40 rods have enough ground clearance. It may not be possible to use this with a standard-gear airplane. The pylon is built such that the CGs of the two planes are stacked above each other. This ensures no trim changes upon release and ensures that any plane can be lifted by attaching it at its own CG point. With standard gear, at the start of the ground roll, the weight of the lifter displaces the CG of the two-pack backwards, and may prevent rotation to take-off attitude. Excess power or an extra-long tail skid can help this, but it would likely be easier to just switch to tricycle gear and avoid these complications. At all times use care and discretion.
How: Pylon: As mentioned, the height of the pylon should be greater than 50% of the chord length. Make a copy of a center rib and use this to draw out the shape of the pylon. Since we will most likely be working on a flat or semi-flat bottom airfoil, draw a line perpendicular with the bottom of the wing through the CG. At the height you've chosen draw a line parallel with the bottom of the wing. The bomb release will be installed centered over this intersection. Extend lines for the leading and trailing edges of the pylon to the top surface of the wing. Draw in hardwood pieces at the top, LE and TE of the pylon. Draw in reinforcements to mount whatever servo you've chosen (any size will do), and also for the bellcrank pivot, wheel or whatever you will use to activate the release. What I did was to fill in the perimeter of the first three pieces with balsa, cut out room for and install the hardwood attach points, then cover both sides with 1/64" plywood. The hole was put in for lightness and to reduce vertical area. It's probably not necessary, but I've since found that the hole makes a nice handle when wiping down the wing or carrying it about.
Saddle: Find three points on the center of the wing that can be reinforced from underneath to carry the required weight and stress. I attached pieces of plywood to the bottom surface of the top skin, and then attached threaded wing blocks to these. Use slow-setting epoxy to give you enough time to position everything just right.
The shape of the saddle is largely determined by the placement of the attachment holes. Once you've finished these, mark out a rough size and shape for the saddle. In order for this conform to the compound curve at the wing joint, I used 3 layers of 1/64" ply with relief cuts around the perimeter to allow them to bend to the required shape. In order to maintain adequate strength make sure the cuts from one layer don't overlap another layer's cuts. It may be helpful to work out this step with a few pieces cut from a file folder, it's about the same stiffness and thickness. Once you're happy with this step, cover the top of the wing with plastic stretch wrap, spread slow CA or epoxy over the three layers of ply, sandwich them together in place on the wing and press down to have them assume the required shape. It's easier to leave the pieces oversize and cut them to their final shape at a later point. When dry, locate and drill the holes for the attach points. Screw the piece down to check the fit, and draw its final outline. At this point you can also attach the pylon to the saddle. Use epoxy or slow CA for the initial joint, making sure it's properly aligned, then make a fillet all the way around from epoxy and microbaloons. Paint the piece, install the bomb release, servo and release actuator and you're done. You could actuate the release with the servo arm directly, but this will make adjusting the servo tricky. My setup releases with the servo at the end of its travel. This allows the use of either a proportional or a retract gear channel, and adjusting travel with the clevis.
Wing supports: Make these from a single piece of hardwood drilled through the center, or else as follows: take a piece of 1/4" or 3/16" hardwood. Chamfer one edge and then cut into 16 oversized pieces. Glue them together in sets of four, with the chamfers facing each other. Use slow CA and align them carefully. Enlarge the resulting hole until it clears a 4-40 screw. If you have a xerox copy of a rib from the plan it will make the job easier. Figure out what the spacing of the rods will be and drill and tap two 4-40 holes in a 3/16" x 1/2" piece of hardwood. Screw in the 4-40 rods, then slip a block over each and glue the blocks to the ribs/spars. Repeat on the other panel. It's worthwhile at this point to make sure that the spacing of the blocks and supports are interchangeable so you don't have to worry about which belongs to which side. Sheet around them to give the covering someplace to stick. Glue a dowel across the top of each hardwood block and you're done. The wing supports are held in place and adjusted with threaded aluminum spacers available from electronics supply houses, machine shop suppliers etc. Adjust the position of the support with the top pieces and then lock the thing in place by tightening the bottom one. Finger pressure has proven sufficient to me. One final note: When you taxi out you you will inevitably cause others to stop and stare. Make sure that the plane you are lifting is turned on, otherwise it will take you a long time to live it down.

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