34" span Chipmunk- House of Balsa- build.

[msatin]

Good progress Frank

[sihinch]

I'm so glad this kit is getting built and thanks so much for sharing it with us all, Frank.

Have you got any idea yet of finishing scheme?

[Frank v B]

Simon:

re: your "Have you got any idea yet of finishing scheme?"

Yep!

When I started reading Simon Chapman's magazine collection I came across this in an RCM&E magazine.
The main reasons:
- the accents on the fuse are linear , fore and aft.  It removes the clumpiness of the nose whenever it is a single colour. (yellow or grey)
- I have the colours- I still cannot drive and do not want to impose on TEMAC"s Three Amigos* who have volunteered to drive me around the city.  Oh, yes, as well,  I am cheap!!
- it is not the MD** trademarked colour scheme.

Frank

*   Mark, Andy, Graham.
** Michael and David- the yellow with red tips.

[sihinch]

Gorgeous! My favourite scheme actually! Bravo.

[Frank v B]

Picking the power system. (Experienced modellers, please stop yawning)

step 1- Gather important background info:
- declared weight of model- 21-28 oz. Note it was designed for a Cox TD.049, a very lightweight but insanely powerful IC motor.  Have to pick the top end of the weight.
- space in the nose.  The nose ring decides what size motor can fit.
- prop size max- .049's used 6x4 props.

step 2- inrunner vs outrunner? an inrunner has a fixed outside case with the shaft spinning more like an old brushed motor. The outside case can touch the fuselage.  An outrunner has the outside bell spinning.  It needs space/clearance and cannot touch the fuse.  As well, the wires need space to lead back or the spinning bell will sever the wire(s).  Don't ask me how I know! 

Inrunners- generally-much higher KV* and need smaller props (kv's run from 2500-4500)
Outrunner- lower KV (800-1200), swinging a larger prop- 8-11" for a small 450 size motor.

Step 3- How many watts do we want/need?  A watt is VOLTS (11.1v) x the AMPS the motor draws.
Guideline:
Cub-type: 80 watts per pound
P-51- 100 plus watts per pound
aerobatic/3D planes- 150+ watts per pound
Pylon racer: 200+ watts per pound
If your name is Ben Feist or EdEdge: they start to feel at home at a kilowatt (1000 watts) per pound.

guidelines:
"always overpower an airplane.  You can always throttle back...if you have to!"
"There is nothing more difficult than trying to fly a marginally powered airplane".

Decision Time
power needed for this approx. 2 lb airplane- 200 watts minimum. (2 pounds x 100 watts per pound)
motor type- inrunner because of the prop size and minimum nose clearance (interior)

My pick
An E-Flite Series 6 inrunner 2700 kv.  This is a 400 watt max 36 amp max (burst) motor on 3 cells.  It is a 28mm diameter motor same as the old Speed 400 size. I have 4 of these and are amazing in small fast planes.  One is in my Nooner pylon racer.  Known results at different APC prop sizes:
- 4.1" x 4.1" - 18 amps (200 watts)
- 5" x 5"       - 25 amps (278 watts)
- 6" x 4"       - 28 amps (310 watts)

Will pick the 6x4" prop so it has bite for take-off.  The smaller the prop the more difficult the acceleration for take-off.  Again, please don't ask me how I know.

I will team it up with a 35-40 amp ESC.
The photo shows the motor with two parts to the nose ring.

Decision made.

Frank

ps: An indication of how far this hobby has come.  This motor is the same size as the old Speed 400 brushed motor at 100 watts Max.  This motor has 4 times the power for the same size and weight. EFLITE has a 32 size inrunner motor of the same size for their jets that produces over 1600 watts (over 2 horsepower) or 16 times the original speed 400 motor.  I believe in technology... and flip phones!

*a KV is the number of revolutions per volt of input power in a no-load situation.

[GordPayne]

Thank you for such a clear and well-presented note Frank. I'll keep it as a starting point reference. I had read that inrunners are not for airplanes so the one I have is gathering dust. I'll definitely look to it for future projects.

[msatin]

Thanks for this Frank - Great breakdown!
I will weigh my Fiasco tomorrow, but the kit indicated a weight of about 30oz (about 1.9lbs)
Based on this I selected a 2826 1,100KV outrunner rated at 186W on 3S & a 30A esc.
I was planning on using a 10x5 prop.
I suspect that I'm probably underpowered a little

[Frank v B]

Thanks Gord and Mark.

Mark-  Your Fiasco will probably be fine for your kind of flying.  3D requires enough power to prop-hang.  You and I don't do that kind of flying.  I suspect it would/could be a handful at 120 watts/pound

It is only a rough guideline.
Now you know why you bought a wattmeter.

Frank

[GuyOReilly]

Great explanation on power requirements.
You must have a lot of time on your hands to write such epistle. 
Keep up the good work.
Guy

[Frank v B]

Ailerons.

This model uses the old torque rods with a single servo in the center of the wing. See reason below*. Problem- the rod breaks through the balsa aileron, either top or bottom (see photo 59).  Here is how I fix it.

Step 1- cut a piece of tubing, either brass or aluminum, that just fits the diameter of the torque rod. Photo 60
Step 2- enlarge the hole in the aileron so the tube fits.
Step 3- place a drop of oil on the end of the torque rod for 2 reasons: i) for easy fitting and ii) so glue will not stick to it. Photo 61
Step 4- epoxy in place, lay a piece of fibreglass cloth over the edge so it covers the top and bottom, add a piece of waxed paper, add a piece of wood on both sides to squeeze the glue and add a clamp.  The pin is necessary so the wood will not slide off the angled aileron when the clamps are added.  Photo 63, 69.
Step 5- slip the aileron in place to make sure the tube is angled the right way before the glue cures. (photo 67) Remove and add clamps around.

Notice that I do not cut the extra cloth at the trailing edge until after it cures.  reasons i) maximum size and strengthening, ii) easier to trim once cured.  One pass of a #11 blade.  iii) you cannot fold cloth around a sharp bend.  It will radius and create two voids between the wood and the cloth, one on top, one on the bottom.

Guaranteed it will not pull apart.   I do this on elevators as well when there is a "U" shaped joiner wire.

Frank


*Back Story:   The reason torque rods were used was that a mini servo in the '90's was $28.00.  A "Y" harness cost $15.00.  Individual aileron servos in the wing would have cost $71.00 versus a single servo in the middle at $28...but that was in '90's dollars.  Today's equivalent of $140 versus $56 dollars.  The price of a fully flyable second hand foamy airplanes today.  Now a servo is $6.00 and a Y harness is $4.00..in 2023 dollars.  A non-issue.

[Frank v B]

Fibreglassing the wing center section.

Step 1- sand the area
Step 2- cut the fibreglass cloth.  I used 1/2 ounce cloth- bought at a swap meet.  See photo 71- note- the narrow glass cloth came with the kit.  The wider one is the one I am using.  reason- It is better to have the cloth be wider than the fuse so the fuse does not crush the thin balsa wing sheeting.  I know the pilot who will be flying this plane.
Step 3- apply 5 minute epoxy to the wing skin wider than the cloth.  I use folded foam as a brush (see photo 72).  This way there are no sharp edges to pull the glass.  Do not brush the epoxy, dab it otherwise the cloth will stretch.  Lay the glass cloth over the glue. 
Note, like covering- do the bottom first so the top overlaps at the bottom where it cannot be seen.  Do not worry about missed spots.  Can be filled in tomorrow.
Step 4- apply 2 pieces of waxed paper.  Use your fingers to squeeze the epoxy through the cloth.  Let it dry overnight. See photo 73, 74
Step 5- repeat for the top glass cloth (tomorrow). Photo 75 shows the top.  Note the bottom glass overlaps the top about 3/4" at the leading edge and trailing edge.

Frank

[Frank v B]

Finishing the fuse.


- added the cheek blocks, top front deck and whole cabin structure. This will allow me to futz with the motor installation tomorrow.
- note the wing drying in the background.

Letting it dry/cure overnight.

F.

[Frank v B]

details

- shaped the bottom balsa nose block.  Cut it into 3 pieces with a razor saw so there would be a hatch- Photo 97
- installed the rudder and elevator servos.  Photo 98
- made the plywood motor mount.  Made it large, held the motor in place and scribed the nose shape onto the back of the motor mount-  Cut it to shape. Photo 99
- with a drill, dug out the thick plywood IC motor firewall until I could fit in a Dremel Sanding drum and open the hole enough so the motor could slip through in case of replacement, crash, etc.  Photo 00.  Oh, yes- the sanding drum started to char the plywood and the smoke alarm went off in my shop.  My wife yelled down "are you trying burn us out of our house again".  "Sorry Dear."
- Glued the nose motor mount in place and am letting it dry overnight. (no photo, stay tuned)

Frank

[Frank v B]

The motor is in!

photo 04- The motor mount cured overnight.
photo 06- Oops! the nose ring supplied in the kit blocked the air holes.  Queue the Dremel sanding disk to open the air holes
photo 07- the motor can now breathe again.
photo 08- nose is finished.

[Frank v B]

Hatches How to locate magnetic catches without needing to be accurate.

These rare earth magnets are available at Lee Valley Tools and Princess Auto.
Design considerations.  I always put the registration pin or the tongue closest to the prop.  The magnetic catches are downwind (rear of hatch) so that the wind cannot catch it and open up.  I can prove to you that magnets closures on the front can blow open.

Step 1- drill a random hole on one side of the hatch end.  I used the fixed portion of the hatch.  Make the hole larger than the magnet, fill it with 5 minute epoxy (photo 52.  Note it is crooked.  Does not matter), lay a small piece of waxed paper over top and place any flat metal object across the magnet (photo 53).  Used a screw driver bit.  It automatically pulls the magnet flush with the surface.  It cannot sink into the epoxy as it cures.
Let it cure.

Step 2- Locate the magnet on the hatch.  I dabbed red paint on the fixed magnet (photo 56) and closed the hatch.  It transferred the paint onto the wood of the hatch (photo 57).  Take a drill bit about 50% larger than the magnet and drill a hole.  Fill it with epoxy, place the magnet in place (photo 58- note magnet is crooked, no problem), insert a piece of waxed paper close the hatch until the glue dries (photo 59).  Hatch complete!  Note: watch the polarity of the magnets.  Don't ask me how I know.  :-)

This method is fool proof, no building accuracy required and works perfectly every time.  The waxed paper is to avoid oozing glue from permanently gluing the hatch shut.

Frank