The BTC-2 build

Next, the wing is flipped over, the upper spar glued in place, and the upper ribs glued in.  The bellcrank is temporarly put in place so I could mark where the leadout lines are to go, and leadout guides (old servo grommet thimbles) glued in place.  Then, the ribs are cut with a razor saw for control lead channels, the bellcrank installed, and the wing sheeting glued in place.









This wing method not only ensures a warp-free wing, but also an incredibly strong wing.  This edge photo shows that rather than a C cell or D cell leading edge with wing ribs and trailing edge sheet, my wing ends up with four separate triangular boxes.  


The torsional strength of this wing is amazing!  I can’t twist the wing. It’d break before it twists.  And there is no span-wise flex at all.  Having the spars glued to the center wing sheet, and then glued to the upper sheeting triangulates perfectly.  This wing really IS built to crash.  And it’s still pretty light.  Using extra light contest 1/16” balsa sheet for the center sheeting, and 1/32” sheeting for the top and bottom, the total wing weight isn’t much more than a slab wing of hard 1/8” sheet balsa.

Now to finish off the wingtips.

The Warpless Mark

That's RADICAL man!!! Are you putting gear on it.. or are you gonna' try a "wing-stooge"? Looks like about a 12-13" wingspan.. 36-40 squares?

Well, I completed closing out the wingtips.  The seams may not match, but after sanding, the wing profile matches top to bottom.  Again, if I'd just stuck to flat wingtips instead of getting fancy, this would have been so much faster.  I'm my own worst enemy when it comes to building.  Here's how it looks with all the sheeting complete:





So, looking at the pictures, does anything stand out as strange?

The Stand Out Mark

roddie wrote:That's RADICAL man!!! Are you putting gear on it.. or are you gonna' try a "wing-stooge"? Looks like about a 12-13" wingspan.. 36-40 squares?

No gear.  It will need a stooge or someone to pit. The wingspan is 12.5" and 46-47 square inches.

The Stooge Mark

Nothing strange, but are you able to install the pushrod with it all closed up?

Cribbs74 wrote:Nothing strange, but are you able to install the pushrod with it all closed up?

Ding... I bet you get a prize for that.

Cribbs74 wrote:Nothing strange, but are you able to install the pushrod with it all closed up?

Bingo!  As I was applying the sheet to the bottom and leading edges, I kept saying, “Don’t forget the pushrod slot! Don’t forget! Don’t forget!”  But, when I got finished sheeting the wing and looked at it, I remembered.  No one listens to what I say. Not even me….  

So, I marked the wing centerline and cut the pushrod slot.  After making sure the pushrod had free motion I glued the wing to the fuse, then the stab and elevator and fin.  Just needs the firewall glued in place and paint.




Not knowing how the sheeting would affect the balance, I made the nose ¾” longer than I figured, so I’d be able to cut it down to the proper length at finish time.  To make balancing easier, I marked the fuselage at the balance point, and then drilled a 1/16” hole at that point.  I cut a toothpick to size, and glued it in place so that no matter what changes I end up making, I always have the sides of the toothpick protruding for me to hold between my fingers to balance the plane.  It’s getting late tonight, so I’ll start cutting the nose back tomorrow.  Here’s a picture of the engine taped in place for an initial balance check.  Way nose heavy.  That’s good, 'cause it’d bite if it was tail heavy and I had to add length to the nose…




The Picky Mark

Great idea, build, and documentation.

The only thing I would do different would be to remove a lot of the original slab wing balsa between the rib bays for a just as sturdy but lighter wing

Well done, I wish I had the time to copy this plane

When you sand it all down try to make the leading edge as rounded and blunt as possible. Also sand down the angle of the spar to make the top and bottom more rounded vs sharp diamond airfoil. This will let the wing flow through the air better, be less "twitchy", and prevent wing stalls in sharp maneuvers

Diamond wings were popular way back on combat airplanes but those were always over powered. Easy and Fast to build but had very nasty habits for flying if put in the air with all the sharp angles

Hey Mark,

Looks similar to the li'l Gem! More robust though.

Have you considered skipping the paint and just sanding it and spraying TF clear on it? Since it's made to crash it would make repairs easier.

Ron

fredvon4 wrote:The only thing I would do different would be to remove a lot of the original slab wing balsa between the rib bays for a just as sturdy but lighter wing

When you sand it all down try to make the leading edge as rounded and blunt as possible. Also sand down the angle of the spar to make the top and bottom more rounded vs sharp diamond airfoil. This will let the wing flow through the air better, be less "twitchy", and prevent wing stalls in sharp maneuvers

Fred, My inital idea for the build was to build a square frame from 1/16", with a 1 1/2" strip from the leading edge back to the spar location, and then a 3/4" trailing edge piece and wing tips, leaving the center open and making a sort-of E-cell leading edge.  But, for the 2-3 grams saved (the whole 1/16 x 3 x 36" sheet only weighed 9 grams), I figured it'd be quicker and easier to just do the plank.

As for the pointy angles, the top and bottom aren't that sharp.  I might sand a little more bluntness into the l.e., but not too much.  It's not too sharp as it is.

The Blunt Mark

Cribbs74 wrote:Hey Mark,

Looks similar to the li'l Gem! More robust though.

Have you considered skipping the paint and just sanding it and spraying TF clear on it? Since it's made to crash it would make repairs easier.

Ron

Ron, the L'il Gem I built last year was the inspiration for this. I thought about just clear, but I have a plan in mind for the finish.

The Inspired Mark

Just some food for thought Mark... The basis for this design was to be a quick-building/crash resistant model.. Right? I realize that the wing is already built and you've calculated for balance... but you might want to consider on a future similar diamond airfoil design; using iron-on covering, instead of the sheeting. (at least on the outer rib-bays) I think you could save a lot of weight.. (not to mention balsa) It's "quick".. and you already have a lot of strength in that sheet wing with top and bottom spars, to begin with.

Some modelers hate iron-on coverings.. and I would certainly understand if that was the case, but it's more durable and quicker to apply than a silk-span/doped covering, and easy to patch too.

I'm only trying to help keep you on track with your mission to not spend too much time on a model that's meant to go through some thrashing. Cutting, fitting and sanding all of that wing sheeting must have taken a lot of time.

roddie wrote:Just some food for thought Mark... The basis for this design was to be a quick-building/crash resistant model.. Right? I realize that the wing is already built and you've calculated for balance... but you might want to consider on a future similar diamond airfoil design; using iron-on covering, instead of the sheeting. (at least on the outer rib-bays) I think you could save a lot of weight.. (not to mention balsa) It's "quick".. and you already have a lot of strength in that sheet wing with top and bottom spars, to begin with.

Some modelers hate iron-on coverings.. and I would certainly understand if that was the case, but it's more durable and quicker to apply than a silk-span/doped covering, and easy to patch too.

I'm only trying to help keep you on track with your mission to not spend too much time on a model that's meant to go through some thrashing. Cutting, fitting and sanding all of that wing sheeting must have taken a lot of time.

Yeah, I spent more time than  I wanted.  If I'd gone with square tips, I'd have shaved off about a fifth to a quarter of the building time.  As to using Monokote and eliminating the rear half sheeting, I was going to say, “Yeah, I know it would have been lighter with the panels cut out. But I thought about it and paint is lighter than Monokote, so the trade-off between painted wood and Monokoted open space is negligible”.  But, I could be, and often am, wrong.  So I decided to do the math.  Since I’ve read for years that painted wood is lighter than Monokote, I allowed for 2 grams difference in weight for painting.  Why two grams?  Three seemed too much, and one seemed too little.  Besides, it’s just a guesstimate, anyway.

The center slab of balsa came from a 9 gram sheet of balsa, so a 2 ½” by 11” panel, or 27 in², that I could have cut out, but didn’t, comes to 2.29 grams.  The upper and lower sections that would have been cut out are 54 in², from a 11 gram sheet of 1/32 balsa, which comes to 5.5 grams.  So the saved weight would have been 7.79 grams.  Call it 8 grams.  Then subtract the 2 grams added for Monokote, and the difference is back down to 6 grams, or just shy of a quarter of an ounce.  I'll take the hit.

Monokote actually is my covering of choice for the last 30 years.  But, using Monokote on the Little Gem I built last year, I decided paint was easier for these small models.

The Shy Mark

Well, it’s officially an airplane!  A build doesn’t officially become an airplane until you either 1) cut yourself and bleed on it, thereby offering a blood sacrifice to the balsa gods, or b) glue yourself to it, signifying your true and everlasting attachment to the plane.


You can see above that I glued the firewall onto the plane.   You can also see that I decided to make it more crash resistant by gluing on 1/32” ply doublers.  With the firewall attached, it’s on to paint.

The Everlasting Mark

As I said earlier, I thought about a clear LustreKote finish, but since I wanted to use a helicopter engine, and experiment with the transparent yellow helicopter tank, I decided to go with transparent yellow for a finish.  It didn’t come out EXACTLY as it looked in my head, but it’s okay.  I just gave it a couple of light coats to seal the wood and fuelproof the airframe. All these photos are much darker than the finish actually is. I'll try and take a couple of pictures when its light outside so you can see the actual shade of yellow the plane is.





Yes, I know, the needle is down and the fuel pickup is up.  I’m just doing this because I wanted to see how it would fly with a stock helicopter engine setup, and the only way the stock tank fuel pickup would work is to mount it this way.  I bent up a temporary piano wire guard for the needle.  After I fly it that way its first day out, just to say that I did it, I’ll rotate the backplate to put the needle up, make a smaller guard/skid to protect the fuel hose on the bottom, and swap the yellow two blade prop for a 3 blade prop.  I’ll use this tank for all further flights:


I got the tank in a parts lot that I bought on eBay, so I don’t know exactly how big it is, but it looks like four or five cc’s.  Should be more than enough for a good flight on a PeeWee.

The Transparent Mark

Well, while weighing other things, I also weighed the BTC-2 this evening. As with the BTC-1, it’s heavier than I could have wished.  But, at 3.2 ounces ready to fly, I think a PeeWee can carry it around on 25’ leads.


The Heavy Mark

Since I had the son this week, I had him help me with this one, too.  As with the Min-Win Trainer, the throws were set to minimum for the first flights to feel it out. I'll increase it a little on this one also and see what I get.  I tried it first on a 3-bladed prop.  It revved up higher and got a little more speed, but not much oomph.  I only did one loop, and it lost a good bit of energy by the time it came around. No chance of a second consecutive loop.  I'll have to play with it to see how I can get more power out of the little PeeWee.  As before, not much more than going around in circles on the first flights.  These are more to just show that it does really fly.



And I swapped out the prop for a 2-bladed one to see if I could trade speed for torque.  It was a little slower, but gained no traction in the air.  I couldn't even get enough pull for a loop.  More experimentation on the way.



The Oomphless Mark