Double Swappable

by webbhm | February 18, 2013 | (13) Posted in Projects

Background

This is my addition to the swappable scratch-built series.  It uses the standard power pod,and the tail is similar to the FT Flyer, but with different dimensions.  It is a double swappable, because not only is the power pod swappable, but you can swap the main wing - use either the plain wing, or build one with airelons.

This plane could also be called "Scrappy" as it was not a deliberate design. I had built the Nutball and FT Flyer swappables, and was working on a plane similar to Ed's "Axion". Due to trail and error, I ended up with some spare parts and realized I had the makings of another plane. I didn't think the swappable power pod would have sufficient thrust, but with nothing to loose I gave it a try; and was quite happily surprised with the results. Going back and looking at the numbers (see the bottom of this post), I should not have been surprised.

Construction

To start, build the three main components (see related articles for details of each):

  • The power pod
  • A 30 inch square tube fuselage (2.5 inch inside dimensions)
  • A 30 inch armin wing - 5 inch cord, no airelons.

Fusalage

The tube for the fuselage is modified by cutting off a bottom triangular section, starting from the front top edge, to a point 6 inches back on the bottom.  The top of the fuselage is the side with the seam and reinforcement.  Do this for both ends.   The wing sits 10 inches back from the front edge.  Drive two bamboo skewers through the sides, up against the top at 7 inches and 13 inches from the front.  These are used to attach the rubber bands that hold the wing in place.

Glue a 2.5 x 2.5 inch piece of foamboard inside the front top edge of the fuseladge.  Then like on the other swappables, insert two skewers through the top holes of the power pod firewall.  

The power pod in held in place at the front by these skewers, and rubber bands stretched from the landing gear skewers on the bottom of the power pod, to the front wing skewers on the fuselage.

Tail

The horizontal tail is a rectangle 15 inches wide and 7 inches long.  Draw a rectangle 1.5 inches from the back edge for the elevator, then another 1.5 inches for the rectangular are of the tail.  Connect the corners (3 inches from back edge) to the centter of the front for where to cut to make the triangular front leading edge.
The vertical tail starts as a rectangle 6 inches wide and 7 inches tall. Mark off a rectangle 1.5 inches from the back edge for the rudder, and another 1.5 inches in (total of 3 inches in) on the top where the slope of the leading edge begins.  Connect this point the the front bottom corner.  Cut a triangle off of the bottom of the rudder to give the elevator clearance.
Prepare the elevator and rudder as with the other swappables (follow the FT Flyer plans).  Glue the horozontal tail to the top of the fusaladge, letting the elevator hang off the back with room to move freely.  Glue the vertical tail to the horozontal tail.  I mounted the servos 10 inches from the back edge, though this location was fairly arbitrary.  Due to the long fusaladge, I installed servo wire extensions.
Hook up and adjust the servos, attach the power pod, and strap on the wing and you are ready to fly.  I initially did not expect the small motor to give much performance to this larger airplane, but the airfoil makes so much difference that it flies great.

Yes, you are seeing correctly, the wheels don't match, one was lost in recovery from an arboreal landing. Not having a spare, I used the idea from the new swappable kits to make a replacement foamboard wheel.

Wing(s)

This is a double swappable, because not only can you swap the power pod with the other airplane designs, but depending upon your interests and desire to experiment, you can swap out different wing designs.

The first design (which I am using) adds a bit of dihedral to the basic armin wing to stabalize flying (I did this because I am using the plane for aerial photography).  I sliced the tip of the wing off at at 30 degree angle, and glued on a 5x6 rectangle of foam board (and rounded the corners).  It is hard to make a clean angled cut, but hot glue does not require precision for firmly attaching a workable winglet.
For other wing designs, consider the armin wing with airelons, or install two servos on the wing and try eleverons.  Feel free to experiment with different wing lengths and chord widths.

Accessories

I need to do some more testing, but this plane may meet my needs for some conservation work I am experimenting with - a stable platform for photography (hence why this design has winglets, and not airelons). I made a small removable 'sled' that attaches under the wing, and it easily flys with a keychain video camera.

The numbers look good for carrying a Android phone which can give both HD video as well as time lapse photography. Even with this small motor, another 4 oz should keep the numbers in a comfortable trainer range; if it doesn't work, I can always swap out another power pod with a bigger motor.

Here is the plane by the numbers:

Full out weight: (with 1000mw 3 cell LiPo battery): 16 oz
Wing area: 1 sq ft
Wing load: 16 oz/sq ft (nicely fits a light trainer definition ~15 oz/sq ft)
Stall speed: 14.8 mph (= 3.7 * sqrt(weight/area)) This is a rough calculation, but good for a start
Turnigy Park 300 Brushless outrunner 1380kv = 77 watt (good for up to a 1 lb trainer: 60-75 watt/lb)
1380kv * 11.5V = 15870 rpm
15870 rpm * 8x4 prop = 63480 inches/minute = ~60 mph max speed (theoretical)
Even if the real max speed is half of this, it gives a nice cushion between stall speed and full throttle.

 

Update 3/17/2013

I was having trouble with the original wing.  Even trimmind down the winglets did not change the flight characteristics.  It seemed to 'tip-stall'; turning right and nosing over.  I am still not sure what the problem was, but since this is a double swappable, I swapped out the wing.  It was easier to try something new than be frustrated trying to fix the problem.

My current wing roughly follows the dihedral idea of the Swappable Trainer.  I made two 20 inch Armin airfoils.  The bottom side was 5 inches, and the top was cut to 6.5 inches (cut the form board 11.5 inches wide).  This left some excess overhang, but rather than cutting it off or turning it into airelons, I left it at a 6.25 inch cord.  I trimmed the ends a bit to join at an angle and form a dihedral, and made a small bottom brace similar to the one in the Swappable Trainer that gives a flat mounting surface.  Several flights with this wing have all had smooth stability and control.

- Find this article & others like it at www.FliteTest.com -

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COMMENTS

Adib Vahedi on February 21, 2013
Looms great you have done a good job building it! You should put a video of you flying it!

Keep up the great work!!!!!!!!!!!!!!!!!!! ;)
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cloud9photos on February 21, 2013
NICE!!!!
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WizzyWiggs on February 21, 2013
Wow, that's a great looking plane, especially taking the fact it was built from scraps. Can't wait to see the specs about the plans you made, wouldn't mind trying to build it. What model of the motor did you use?
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webbhm on February 23, 2013
Turnigy Park 300 Brushless outrunner 1380kv. I am not aware that there are different models of this. This is all the std Hobby King parts listed in the swappable bujild.
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ronnie.burchfield. on February 27, 2013
great looking plane man
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Milwaukee Dan on November 30, 2014
I'm new to the rc world and I've been thinking about designing a trainer of my own. My hand sketches look very similar to your design and I have a question regarding your motor attachment on your power pot. Looking at the last photograph before detail drawings I see the that the motor appears not to be mounted square to the axis of the plane (this may be an optical illusion) . Did you have to adjust the thrust angle and why? Overall it looks very good and I'd like to see a video of its flight characteristics. Good article.
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webbhm

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Double Swappable