Swappable Extra 300L with Plans

Having worked my way through the FT Bloody Wonder and the FT 3D builds, I thought I would try my hand at designing for build number three. This article describes the result. I used the FT 3D as a base and made a number of changes:

• Double-tapered wings with shaped ailerons
• Two-part fuselage that tapers from front to back
• Elevator and horizontal stabilizer mounted forward a la the full-scale plane

More on those in just a bit. First, some video.

Proof of flight

No, I am not a very good 3D pilot. Flight characteristics are similar to the FT 3D, at least to my limited perception.

Wings

I made three main changes to the FT 3D wing.

The first change was to use the Armin Wing style of construction. Instead of folding a series of creases along the leading edge, I put a single crease at the very front, removed the paper from the inside of the wing, and used gentle pressure to coax the foamboard into a smooth curve.

I actually developed this technique earlier, while building my FT 3D.

The second change was to add shaped ailerons. With a smooth curve at the front of the wing, the flat ailerons looked a bit funny. Putting the hinge line across the top surface of the wing would have required a gigantic bevel on the bottom, so I used internal hinges. The hinges I found online were all meant for use in wood, so I made my own more flexible hinges out of gift cards.

The flat part of the hinges mounts to a foam spar inside the aileron.

The "pin" part of each hinge goes into a former that extends up to the main spar. These formers also help maintain the airfoil's shape.

The aileron is made from a single piece of foam board, with the foam removed from the center section.

I fold the two halves over, and the paper forms a semicircle. The foam that is removed from the center becomes the aileron spar.

The shape of the aileron allows it to rotate inside the wing with minimal disruption of airflow.

The third change was to make the wing double-tapered.

I had thought that a double-tapered wing would be more difficult, but it was actually a bit easier to put together. The thickest part of the wing is at the same location throughout the entire airfoil, so it's simpler to make a consistent airfoil profile from root to tip.

Fuselage

The fuselage consists of two parts. The inner part looks a lot like the FT 3D's box fuselage. I added a curved turtle deck at the back, created by removing paper from one side of the foam board and slowly bending the sheet.

A second, outer section of the fuselage gives the front half of the plane a taper and also adds some reinforcement to the wing joints and to the parts of the plane that come in contact with the ground during a belly landing.

The construction technique for the outer fuselage is based on the Experimental Airlines fuselage tube. Instead of removing strips of paper at evenly-spaced intervals, I changed the pattern of paper removal to create a tube that transitions between four different cross-sections.

The air scoops on either side of the propeller are functional; small holes on the top of the inner fuselage channel air to the speed controller inside the power pod.

Tail

The tail is very similar to that of the FT 3D. I moved the horizontal stabilizer forward to match the appearance of the full-scale plane. This movement required splitting the horizontal stabilizer and elevator into two parts.

A plastic gift card (hotel key card, actually) reinforces the portion of the elevator that goes through the fuselage.

Canopy

The fuselage has a space to place a bubble canopy with an access hatch underneath. I don't yet have the skills and tools to make clear plastic canopies, so I built one out of some foam formers covered with poster board.

Plans

PDF Plans: Extra 300L Plans 2014-04-08

The first three pages are 20x30 inches. I've had little luck with tiled plans, so I scaled these pages down to letter size and used the red measurements to transfer the plans to the foam board.

The last three pages are meant to be printed at 100% on letter-size paper.

Wingspan: 40 inches. There was extra space on the foamboard, so I scaled up the plane by 110%. Later, I discovered just how much bigger a 110% model is...

All-up weight: About 800 grams. Roughly the same wing loading as the original.

Build log: See this forum thread. If you have questions about the build, please post them in the thread so that everyone can see the answers!

Building Materials

• 3 sheets of Dollar Tree foam board, plus a few scraps
• A small amount of poster board (available at Dollar Tree next to the foam board)
• A few inches of thin steel wire (hardware section in Dollar Tree)
• Paper surgical tape (first aid section in Dollar Tree)
• Main spar: one 36 by 3/8 inch square basswood dowel (98 cents at Home Depot [link]; make sure you get a straight one!) 
• Horizontal stabilizer spar: About 15cm of 2 to 3mm carbon fiber (now available at HobbyKing's US warehouse; shipping is cheap if you add it to a larger order). If CF is hard to come by, a 3mm hardwood dowel should work.
• 2 plastic gift cards
• 2 control horns (I like the Flite Test laser cut control horns)
• 2 control rods and clevises (I'm using DuBro mini nylon quick links, about 75 cents each for clevis and threaded control rod)
• 2 paper clips or 3 inches of 0.75mm music wire (for aileron control rods)
• White Gorilla Glue

Electronics

• 400-watt, 480-size motor (I'm using an NTM 28-36 1000KV). Note that the motor and speed controller together should weigh about 120 grams, or the plane will not balance out.
• 40-amp speed controller for 4S, 50 amps if running on 3S (I'm using a 40A BlueSeries on 4S)
• The biggest propeller that your motor can swing without overheating (I'm running 10x5e and 11x5.5e APC props)
• 1800mAh 4S or 2200mAh 3S battery (I'm using a Zippy Compact 1800mAh 4S)
• Four 12-gram servos (I'm using the Turnigy 50090M, a "9-gram" servo that actually weighs 12 grams)
• 5+ channel receiver
• Programmable radio