Frank's Cobra Z build- by Pilot

[Frank v B]

Sorry for the delay in reporting.  Had technical difficulties*.


Fuse: built and ready to sand.  Made out of 3/32" sides and 1/16 top and bottom.  There is a hatch in the top front to plug in batteries.  Note: the nose was lengthened about 3/4" to accommodate the motor and the CG.  The original Cobra was designed for the Norvel .061 with the tank mount.

Please note that I used bright (uncoloured) pins this time to avoid Andy and Bruce from reading too much into this build.  Braille for my detractors.

Wing done and rough sanded.  A whopping 24" span or, ........ as Michael calls it....... "a stabilizer". Servo installation is next.

Frank

* the camera was somewhere in my car and took 4 days to find.  Andy enjoyed the silence.

[Frank v B]

- Sanding completed
- added a plywood triangle to spread the load for the wing bolt.  Note the grain is chord-wise.
- built the hatch.  Will add magnets later.
- second thought on the strength.  Added a strand of Kevlar at the firewall (360 degrees around the fuse) and the wing hold-down at the fuselage. Attached it with 20 minute epoxy* and wrapped it in wax paper so it will dry flat. Added two rubber bands to press it flat.  Note the colours for Andy's sake.  The original design had a fuselage doubler which I omitted.

Frank

* Bruce- ran out of 5 minute epoxy.

[Frank v B]

Installed the two aileron servos in the wing (HXT 500's) and the elevator servo in the fuse (Hitec HS 55).  Glued them in place with epoxy.  That's commitment!

The Kevlar is barely visible at the front and back end of the hatch (top of fuse).  Stabilizer and fin done and sanded.

Frank

[Frank v B]

Wing measured and aligned, leading edge dowels drilled, wing hold down screw drilled and tapped, fin and elevator test fitted in place.

Next: motor and ailerons.... then covering.

Frank

[piker]

Very cute.  A pocket Rocket!

Are you going to dress it up with a cowl?

[Frank v B]

Piker,

Thank you. 
Making a cowl will take longer than building the entire plane.   If it gets too cute it will do a "Hoffer"* on its maiden flight.  Remember, Ugly Planes last longer.  Will attach the motor and see how much room there is.  The jury is still out.


Frank


* A "Hoffer" is aviation's version of a face plant.

[Frank v B]

Piker,

re: "Are you going to dress it up with a cowl?"

I tried but it looks odd.

ps: what kind of cowl befits a classy version of an Ugly Stick type plane??

Frank

[Frank v B]

Motor decision made.

Two choices- the old Cobra used a 150 watt system (red motor).... or a 330 watt motor (Turnigy 2826/6 2200KV., 34 amp max)

The larger motor weighs an ounce more.  The plane will probably weigh 10oz  That's over 500 watts per lb.  EdEdge will be impressed.  I was starting to opt for the smaller motor and heard EdEdge yelling at me and calling me "Chicken".

I have this vision of punching it the first time on the maiden flight, the motor flying off and seconds later chewing off the tail.  Well, at least it will prove the earth is round.


https://www.youtube.com/watch?v=FgqW_hgpuEI

Frank

[bweaver]

@Frank v B I would recommend replacing the motors you are considering with the biggest one you have.  To h... with the weight.  The largest motor that will fit in any one of those cowls will do.  You can always add a little weight to the back end of the plane, or put the battery back there to balance the plane.  It is safer that way for the battery.     

[Frank v B]

Bruce,

re "..the largest motor that will fit"

option 1 - 1380 watts
option 2 - 6000 watts.

Frank

ps: Welcome back!

[Frank v B]

Covering 101 completed.  Just some accent stripes to go and then final assembly.

Frank

[Frank v B]

Finished!!!

Attached all the pushrods, control horns and re-installed the motor.  The prop is a 5x5 APC.  Will try 3 cells.

Flying this thing is next.

Frank

[Frank v B]

Just stuffed all the electronics in and got the wing wigglers to work.  Here are the stats:

Motor- Turnigy 2826 2200KV outrunner
Prop- 5x5 APC
ESC- 25 amp Electrify
Battery- 3 cell 1300 Venom battery 20 C
Power readings- 19.4 amps, 204 watts
Total weight- 13.4 oz
Watts/lb- 244.  Note: this is a 34 amp continuous motor or 450 watts/lb in this plane but I would have to increase the ESC and C rating for the battery.  My name is not EdEdge!

Ready for Sunday's visit to the field.

Frank

[Frank v B]

Maidened it this morning.  Thanks for the power chuck by Bruce.  Surprising elements were:
- it flew very well and neutral
- surprisingly fast and straight as an arrow.
- quick turning


The only minor problem was it ran out of "up" elevator on landing but I aimed for the tall grass in front of the pilot station.  Will give it 10% more "up" throw.  I do not like dual rates because that starts the old fumbling for switches problem. 

Note: I used a larger battery than normal to make it nose heavy.  Nose heaviness makes fast planes easier to control but contributes to the landing problem.

Frank