Speed,Combat,Scale,Racing > Scale Models

Ercoupe Build (slow)

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wwwarbird:

 That's a pretty good sized model, a nice size for scale though. A whole different ballgame, but over the years I've considered doing an Ercoupe stunt model many times. This one will be an interesting build to watch.

Tim Wescott:
All ribs cut.  I had originally planned on a 2" spacing, but I got to thinking and I decided that 2.5" would be OK (I'm not trying to match the original).  I have the main rib in a CAD program, so I just scaled it down to fit between the LE and TE for the two tip ribs.

20 main ribs
2 mid-tip ribs
2 end-tip ribs

I need to cut holes for lines in the ribs next, then start bending wood for the wingtips.

Fred Cronenwett:
That is a great size, big enough you can install a servo and other hardware and not be worried about wing loading. Small models can not tolerate adding the scale details and heavy items such as extra servos.

Another thing that happens with the larger models is that the wing loading per square foot can go up and still fly great. My 96" span B-29 flies with a 39 ounce per square foot wing loading, that same wing loading on a smaller model it would not get the off the ground. the 14 lb overall weight for the B-29 is a pig by stunt standards but flies just like the real B-29 so it's a good model for CL scale.

Math is on your side, if you double the size of the model from 30" to 60" you have doubled the size of the model but the wing area is 400% the area of the smaller model.

30" model would have 142.6 sq inches
60" model would have 570.4 sq inches

You can easily fly the larger model even if the model comes in at 5.5 lbs, but with the smaller 30" model you would be lucky to get airborne if the model came in at 1.25 lbs (or 20 ounces). I tend to round to the nearest 1/4 lb with the scale models instead of quoting model weight to the nearest ounce.

Fred

Tim Wescott:
Small progress made on airplane - large progress made on shop.

Sunday or Monday I went into the back shop to find some wood for a tip form, and found that the last of the florescent fixtures had died.  I am sick and tired of florescents, so I've spent my evenings this week installing LED shoplights.  The good news is that it's brighter back there than it has been for years, and I still have the other half of the fixtures to install.  The better news is that it was all overhead work, and my right shoulder hasn't been bugging me for it -- so maybe in 2019 I can get back to flying stunt!

After taking a break from building to fix up my back shop where all the carpentry stuff lives, I ended up making the tip form out of cardboard, in the front shop.  I've got six 22x3/8x1/8 sticks soaking in ammonia water now; tomorrow I'll see if I can bend them.  If I can't hand bend them, I'll heat bend them.  I was going to say "I'll have to figure out how", but as I typed that a solution popped into my head.  Now that I can SEE my welding bench Igrab some pipe and make up a 4" or 6" diameter form that I can hold in my vise and heat up with a propane torch while I'm bending wood over it -- that should be both effective and gonzo, and if I'm careful, perhaps even let me finish the job before I burn down my house.

One of these days I'm going to have to try steam bending balsa.  I've had really, really good success bending wet balsa over a soldering iron for small rubber-powered airplanes.  So balsa seems to be good for heat-bending.  On the other hand, I've seen web sites that say you just cannot steam-bend kiln-dried wood.  I'm not sure if that means that other heat-bending techniques work, or that when you're working with thin sections of wood you can effectively undo the kiln drying by overnight soaking in water.

Will Hinton:
Tim, I heat bend kiln dried wood all the time, and hardwood at that when I'm building acoustic guitars.  I've bent maple, rosewood, koa, cherry, mahogany and the like.  I do soak them over night before doing it, but I can't imagine having problems heat bending balsa, even really hard balsa if it's plenty wet.
I feed the torch into one end of an aluminum tube with a baffle an inch from the opposite end to retain as much heat as possible.  I use a four inch and a two inch tubing, depending on the radius of the bend I need.
Just go slow and watch the fingers!  Good luck with it.

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