Toronto Electric Model Aviation Club Forum

Toronto Electric Model Aviation Club (TEMAC) => Building / Construction => Topic started by: Frank v B on February 03, 2019, 09:52:56 pm

Title: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 03, 2019, 09:52:56 pm
Have started the Cobra Z by Pilot models.  Built it 20 years ago with a Norvel .061, then electrified it with a 150 watt out runner on 3 cells.  This plane is small at 24" span but it had unlimited vertical.  Just bought a snarly 2826 2200KV at the Milton swap meet this morning and uses a 5x5 prop at 300 plus watts.  See if I can make it balance otherwise I will use the original 2805 bell motor.

It has a straight wing and a box fuselage so the build should be fast.

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Andy Hoffer on February 04, 2019, 12:11:21 pm
Hey @Frank v B ,

I love the push-pin morse code.  We should talk about whether it is even possible to save your soul.   ;D

Happy building!

Andy
Title: Re: Frank's Cobra Z build- by Pilot
Post by: bweaver on February 04, 2019, 10:00:42 pm
@Andy Hoffer

Re: I love the push-pin morse code.

Yah, but did you notice he spelt it wrong... or did he intend to?   :-\

What's up @Frank v B ?
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 05, 2019, 09:59:36 pm
More Morse code,

Coloured rubber bands to hold the trailing edge on while the glue was drying.
Black clamps to hold the shear webs while the glue was drying. 

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: bweaver on February 06, 2019, 12:22:41 pm
.... ._  .... ._ .... ._ .... ._

... _. ._ _ _ ._. _

 ;D  and the construction looks good too! What are you using for glue, or is the wood that warped that you need that many clamps to hold it in place?  ::)
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 06, 2019, 06:06:34 pm
Bruce,

The glue I used was carpenters glue. You can see the glue drops on the wax paper in the top right hand corner of the picture.  I use round toothpicks to apply it to the wood.  The clamps were to solve a problem. The glue expands the wood and causes it to curl while it is curing.  Without the clamps on each end of the shear web the ends would curl away from the spar and weakens it.

Of course you use CA so you wouldn't know this. ;)

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: bweaver on February 06, 2019, 09:46:09 pm
I don't like shear webs.  They are always breaking off.  Carpenters Glue...? We used that in Kindergarten.

CA works for me, so yes, I don't need to know this thank you.  Saves on having to buy so many clamps and pins, etc.
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 07, 2019, 06:15:29 pm
Bruce:

re:"I don't like shear webs. They are always breaking off."


I guess you need to use shearless webs. ;)

Yep, carpenter's glue.  I can't use CA in my house or it would be the equivalent of a thousand Cha's being miffed at me. :)  If I use 5 minute epoxy I get lambasted on the Forum.  Welcome to my life. :D

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Andy Hoffer on February 07, 2019, 09:14:44 pm
Bruce:

re:"I don't like shear webs. They are always breaking off."


I guess you need to use shearless webs. ;)

Yep, carpenter's glue.  I can't use CA in my house or it would be the equivalent of a thousand Cha's being miffed at me. :)  If I use 5 minute epoxy I get lambasted on the Forum.  Welcome to my life. :D

OK @Frank v B .  I'm taking the bait.  What is a "Cha" (besides being half of a great Cuban dance)?

Andy
Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 07, 2019, 10:48:20 pm
Andy,

re: "What is a "Cha" (besides being half of a great Cuban dance)?"

Sorry about the lack of info. 

"Cha" is (was) Bruce and Katie's 5 lb Chihuahua watch dog that fiercely protected the household.

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 07, 2019, 10:53:51 pm
Update:

Attached the leading edge sheeting.  Did both sides at the same time to minimize the wing curling.  Attached it with 15 minute epoxy.  Put in the pins and to hold the balsa in place.  Then rubber banded it to a piece of 1x6 poplar to keep it straight when it cured.  First photo

Attached the trailing edge sheeting top and bottom, pinned through all 3 layers of balsa and clamped it to the 1x6 to keep it straight while curing. (Second and third photos.)

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 10, 2019, 09:27:11 pm
Center sheeting completed both sides.

Next step: fuselage.

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Andy Hoffer on February 11, 2019, 10:00:02 am
I'll bet @Frank v B has some interesting stories to go with the bent pins sitting on the sidelines!!  :D

Andy
Title: Re: Frank's Cobra Z build- by Pilot
Post by: piker on February 11, 2019, 10:10:51 am
Very true.  And have we talked before about that archaeological dig at the front of the bench?  There's a lot of history there I think.

I'm also curious about the unusual center section sheeting on the Cobra.

Robert
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 11, 2019, 10:47:38 pm
Piker,

re "I'm also curious about the unusual center section sheeting on the Cobra."

Unusual for two reasons:
- I have not decided which side of the wing is up.  That will be decided when I see if the wing is straight.  It is a symmetrical airfoil.
- there will be two aileron servos buried in the wing.  It needs one side to hold the servo and the other side for the output slot.  Once I decide I will trim the center sheeting properly.

"Don't make a decision until you have to" is my guiding principle.

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 18, 2019, 09:35:23 am
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. :D

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. ;D
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 18, 2019, 12:17:33 pm
- 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. ;)

Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 18, 2019, 04:13:07 pm
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

Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 18, 2019, 09:27:28 pm
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
Title: Re: Frank's Cobra Z build- by Pilot
Post by: piker on February 19, 2019, 10:30:51 am
Very cute.  A pocket Rocket!

Are you going to dress it up with a cowl?
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 19, 2019, 11:35:06 am
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. ;D
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 19, 2019, 08:40:52 pm
Piker,

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

I tried but it looks odd. ;D ;D

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

Frank
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 19, 2019, 08:59:00 pm
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
Title: Re: Frank's Cobra Z build- by Pilot
Post by: bweaver on February 19, 2019, 09:08:23 pm
@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.      ;D
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 19, 2019, 10:11:46 pm
Bruce,

re "..the largest motor that will fit"

option 1 - 1380 watts
option 2 - 6000 watts.

Frank

ps: Welcome back!
Title: Re: Frank's Cobra Z build- by Pilot
Post by: Frank v B on February 21, 2019, 09:45:06 pm
Covering 101 completed.  Just some accent stripes to go and then final assembly.

Frank