120% FT 3D Swappable with Dowloadable Plans

December 11, 2013|clough42

Here are my plans for a 120% size FT3D. I think it hits the sweet spot of size, weight and power, and it flies easily on common 3s-2200 batteries.

This plane is 20% larger than the original, giving it 44% more wing area, enabling it to loft a heavier motor and battery while still maintaining a very light wing loading. Unlike other scaled up FT3D models, this one keeps the height and width of the fuselage the same, which maintains the foam geometry and allows it to use a standard swappable power pod.

I have lots of 3s2200 batteries, so this plane is sized and equipped to use these batteries. I recommend a 3530-size 1100KV motor with an 11x7 prop. This combo pulls a peak of about 30A and generates about 1750g of thrust. The plane weighs only 850g, with the battery, giving it a thrust-to-weight ratio over 2:1. Since most flying (even hovering) requires 50% throttle or less, flight times are very respectable. Depending on how you fly, you will get six to ten minutes on a battery.

This 120% version goes together just like the original FT 3D. You can reference the original scratch build videos for instructions. The spar is different, so see the detailed instructions below.

The thing you will notice first when building this plane is that the fuselage is longer than a standard 30 inch sheet of foam. Just join another ten inches or so of foam to the end of a sheet to make a sheet large enough. Make a clean, straight cut on each piece of foam so they fit together neatly. Put a piece of Extreme packing tape on one side, then open up the joint and apply a bead of hot glue. Close the joint, hold the foam flat on your building board and squeegee the excess glue to seal the side without the tape. This is the same technique used to glue the tail on the Bloody Wonder. There is no need for tape on the glue side of the foam. Just cut the pieces so the tape will be on the inside of the fuselage and you will be all set. Because the joint wraps around the fuselage, the geometry makes it very strong.

This plane needs a carbon fiber spar. I built the first version with a balsa spar, but it snapped in flight when I loaded up the wing, so something stronger was needed. This spar is a carbon fiber arrow shaft, purchased at Wal-Mart for $4. To make a pocket in the wing, glue down a regular spar strip to each side of the wing, just like the original FT3D, and then add two narrow strips to each wing half, leaving a pocket for the spar. When the wing folds over, it leaves a hollow foam spar with a channel in it just the right size for the arrow shaft.

Be sure to trim the foam portions of the spar short so they fit into the hole in the fuselage, but do not extend into the interior of the fuselage. This allows the power pod to sit lower, giving the plane a zero degree thrust angle. The original FT3D has significant down-thrust built in—apparently due to interference with the foam spar. If you want down-thrust in this model, you can easily move the skewer holding the back of the power pod up to get whatever angle you desire. When the pod is all the way down against the spar, as marked in the drawings, the motor angle is at zero degrees.

I recommend 1.8mm carbon fiber rods for all of the servo linkages. These fit easily into the standard HobbyKing linkage stoppers. They are light, easily adjustable, and they completely eliminate pushrod flexing.

Reinforce the landing gear attachment points with carbon fiber strips. Wood will also work, but will crack more easily. Glue 1.5x5mm carbon fiber strips to the inside and outside of the fuselage bottom and attach the landing gear with zip ties that punch through the foam and go around both the inside and outside carbon strips. This is very durable and easily keeps the landing gear from tearing through the foam on hard landings.

To keep the paper hinges from tearing or separating, reinforce all of the hinge joints with Extreme packing tape. Just place a 1 inch wide strip across the joint on the paper side. Then, fold the control surface back on itself and wrap a second piece of tape around the open foam edges. Make sure it adheres well to the foam. When you fold the surface back down, it will be virtually indestructible. Do this at each end of each hinge, and you won’t have any trouble with tearing.

You can easily add a tail wheel for taxiing. A standard .40 size tail wheel is about the right size. Just glue a piece of popsicle stick to the bottom of the tail and screw the wheel bracket on. I also use a little hot glue to make it stronger. Bend the rudder rod back and capture it between a couple of small sticks or carbon strips embedded in the bottom edge of the rudder. Do not glue it to the rudder. Since the hinge point of the rudder is offset slightly from the steering axis, it needs to be able to move a bit where it contacts the rudder.

Use the longest servo arm you can get on the elevator. I cut the short side off the asymmetrical horn that came with the servo, because it was the longest. This plane likes lots of elevator throw for harriers. Also, make sure you have the elevator connected to the upper servo. The shaft of the top servo is aligned with the hinge of the elevator to optimize the geometry and give the largest, most consistent throw possible.

A couple of bonus tips: You can cut a hole in the bottom of the power pod and make an intake duct from a plastic spoon to get airflow over the ESC. Dental floss picks make great servo horns. Just clip off the floss and the handles and drill some holes for the push rods.