PC 21 Scratch Build

[Joker 53150]

If you think you're going to cut more sticks in the future you may want to look into a "balsa stripper". It does a great job giving you very uniform sticks from sheets. I bought one, and eventually 3D printed a version I like a lot better.

 

[Jackson T]

If you think you're going to cut more sticks in the future you may want to look into a "balsa stripper". It does a great job giving you very uniform sticks from sheets. I bought one, and eventually 3D printed a version I like a lot better.

I did use a balsa stripper. I guess the stanley knife in the photo might be misleading . It was probably you that recommended I get a balsa stripper on my 4m glider build. It's is very handy indeed! I saw your 3D printed version, it looks pretty good!

 

[Jackson T]

I've made a start!

It's summer here in Australia, and it's hot like you wouldn't believe! It was 37 degrees C (99 F) in my work space at 10:10 this morning, and it's 42 degrees (108 F) when I was just out there working now!

 

[Grifflyer]

I've made a start! View attachment 151419
It's summer here in Australia, and it's hot like you wouldn't believe! It was 37 degrees C (99 F) in my work space at 10:10 this morning, and it's 42 degrees (108 F) when I was just out there working now!

Cool!! Will this be a square fuse or rounded?

 

[Jackson T]

Cool!! Will this be a square fuse or rounded?

It'll be just like the FT planes, a square fuselage with a rounded turtledeck on top. You should make one out of foamboard, I recon you'd do a great job!

 

[Jackson T]

I glued in the vertical and diagonal supports in this arvo, then made the other fuselage side. I hadn't actually done much structural design in cad on this project, just the fuselage outline and the wing. All the vertical support locations where made up as I went along . The gap in the centre-ish spot is where the wing will go, so I'm leaving it open for now.

 

[Jackson T]

This is the wing plan so far. The root chord is 175mm and the tip is 100mm. The aileron's are 40mm at the root and 25mm at the tip. The spar is so wide because it'll be made of 1/8th thick balsa, so it has to be wide to make up for it's skinniness. I may change the spar design before I build it, it just depends on what thickness balsa I've got on hand.

 

[Jackson T]

Made some fuselage formers yesterday.

I know they look a bit skimpy, but I'm kinda just making things up as I go, and I can always reinforce it later. I'm trying to use up all my 1/8th scrap balsa on this bird, it's kinda my "scrap plane" before I build my next huge motor glider .

 

[TooJung2Die]

I know they look a bit skimpy

Not really. The diagonals give the box tremendous stiffness. You could've even left out the additional side strips. The more I build the more I believe in the mantra, "Build to fly, not to crash". In most cases lighter airplanes fly better than heavy and less weight means less inertia in a crash. Planting it full throttle nose first will wreck anything.

 

[Jackson T]

"Build to fly, not to crash"

I like that saying! Good to know it's not too weak. I know it's strong enough for the flight loads, but it does feel like I might break it with my hands one day.

 

[Hai-Lee]

I like that saying! Good to know it's not too weak. I know it's strong enough for the flight loads, but it does feel like I might break it with my hands one day.

On a frame build such as the one you are doing I often stiffen the area forward of the wing TE with 1 mm balsa sheeting and sand it to paper thin at the wing TE. It adds significant strength to where the torsional stresses from the motor torque, propeller drag, and the wing control surfaces are greatest. It also adds a little more crash resistance in the case of a rough landing.

Have fun!

 

[Jackson T]

On a frame build such as the one you are doing I often stiffen the area forward of the wing TE with 1 mm balsa sheeting and sand it to paper thin at the wing TE. It adds significant strength to where the torsional stresses from the motor torque, propeller drag, and the wing control surfaces are greatest. It also adds a little more crash resistance in the case of a rough landing.

Have fun!

That is a good idea, but I don't think I'll use any sheet on this one. I know it's not ideal, but I'm trying to build this one out of scraps. if it still feels flimsy when I'm done putting in some more members, I guess I can add some sheet then. Great idea though, and thanks for the tip!

 

[Jackson T]

This is what I'm up to.

This is the hatch. For my first go at curved sheeting I think it turned out alright. In hindsight, I probably could have sanded the 1/16th balsa down a bit first to make it easier to bend.

 

[Grifflyer]

This is what I'm up to. View attachment 155027
This is the hatch. For my first go at curved sheeting I think it turned out alright. In hindsight, I probably could have sanded the 1/16th balsa down a bit first to make it easier to bend.
View attachment 155028

That hatch looks great!
How you get the balsa to curve without breaking it?

 

[Jackson T]

That hatch looks great!
How you get the balsa to curve without breaking it?

The 1/16th I was using can handle the radius of curvature I wanted, I just made sure I bent it slowly, nothing special.

 

[FPVAirCombat]

In the olden days, builders would wet the balsa first to increase its resiliency to cracking and then bend it to shape. And as the balsa dries, it would take on the new shape (stay bent).

 

[Joker 53150]

In the olden days, builders would wet the balsa first to increase its resiliency to cracking and then bend it to shape. And as the balsa dries, it would take on the new shape (stay bent).

We still do that, and we often use Windex with ammonia as it appears to help the balsa bend without breaking better than water alone.

 

[Jackson T]

I finally got some more work done on the PC 21. The 1/8th former on the diagonal is what the clear canopy is going to glue onto at the front.

 

[speedbirdted]

Pretty. If you are having issues bending balsa sheeting into place, soaking it in warm water can really loosen it up and allow it to bend at crazy angles, and the best part is if you do it correctly it holds its shape almost perfectly after it dries. You can also do it with acetone, which works even better, but you have to be careful not to fumigate yourself by doing it in an enclosed space.

Is the wing going to run through the fuselage? Honestly in this case I would be more inclined to have a two-piece wing and have a carbon spar affixed in one of them, while you have a spar tube living inside the fuselage. The wing would then be held in with nylon bolts running through tabs extending from the inside edge of the wings (these will not be taking any bending load, only tensile load) and then the nuts retaining said bolts would be attached to a piece of ply or some other hardpoint inside the fuselage.

 

[Jackson T]

Pretty. If you are having issues bending balsa sheeting into place, soaking it in warm water can really loosen it up and allow it to bend at crazy angles, and the best part is if you do it correctly it holds its shape almost perfectly after it dries. You can also do it with acetone, which works even better, but you have to be careful not to fumigate yourself by doing it in an enclosed space.

Is the wing going to run through the fuselage? Honestly in this case I would be more inclined to have a two-piece wing and have a carbon spar affixed in one of them, while you have a spar tube living inside the fuselage. The wing would then be held in with nylon bolts running through tabs extending from the inside edge of the wings (these will not be taking any bending load, only tensile load) and then the nuts retaining said bolts would be attached to a piece of ply or some other hardpoint inside the fuselage.

Thanks for the tip! I haven't finished planning the wing/fuselage junction yet, but it will be all balsa, as this build is kind of a "leftover balsa" build following my 4m glider project.