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4 Metre Glider Scratch Build

What should I build next?


  • Total voters
    7
  • Poll closed .

Jackson T

Well-known member
#1
Hey guys, I like to create design challenges for myself when I build. On my last build, the Balsa Baby Baron, the challenge was to make a extremely portable plane, so it would fit easily into a backpack without being prone to damage. This time, it's size. I like motor gliders, so I decided to make my next project a HUGE motor glider. Because the wing is so big, I decided to make it a 3 piece wing with aluminium and balsa wing joiners for transport. I've just finished designing it, so now it's time to start building!

Specs:
Wingspan: 400cm (157.5 in.)
Length: ~170cm (67 in.)
Wing Area: 93dm2 (1441.5 sq. in.)
Aerofoil: SD7037
Estimated Flying Weight: 1500-2000 grams (53-71 ounces)

I've already cut out some ribs and the forward fuselage pieces, which will have longerons running off them down to the tail. The balsa I buy from my local hardware store is only 915mm long (36 inches), but my longerons have to be a bit longer. I'm going to have to join two longeron pieces together somehow. Any ideas on how should I do the splice joint?

I'll get some build pics up soon!
 
Last edited:

Joker 53150

Mmmmmmm, balsa.
Mentor
#2
I assume the longerons you're using on the fuselage are simple 1/8" x 1/8" (or similar square stock sizes)? When I've run into this I avoid a simple butt-joint as it's very weak and instead use a scarf joint which gives you more surface area to glue. I also tried to make sure the joint was also supported by a fuselage former.

I found that the smaller the sticks I'm trying to scarf join, the harder it is to make a long and even bevel. I often use my bevel cutter and cut a bunch of them one after another to make sure they all have the same angle.
 

rockyboy

Skill Collector
Mentor
#3
Wingspan: 400cm (157.5 in.)
Aaaack!! :eek:

That's a big one alright!

I assume the longerons you're using on the fuselage are simple 1/8" x 1/8" (or similar square stock sizes)? When I've run into this I avoid a simple butt-joint as it's very weak and instead use a scarf joint which gives you more surface area to glue. I also tried to make sure the joint was also supported by a fuselage former.

I found that the smaller the sticks I'm trying to scarf join, the harder it is to make a long and even bevel. I often use my bevel cutter and cut a bunch of them one after another to make sure they all have the same angle.
+1 to what he said :D
 

Jackson T

Well-known member
#4
I assume the longerons you're using on the fuselage are simple 1/8" x 1/8" (or similar square stock sizes)? When I've run into this I avoid a simple butt-joint as it's very weak and instead use a scarf joint which gives you more surface area to glue. I also tried to make sure the joint was also supported by a fuselage former.

I found that the smaller the sticks I'm trying to scarf join, the harder it is to make a long and even bevel. I often use my bevel cutter and cut a bunch of them one after another to make sure they all have the same angle.
The longerons are 10mm x 3mm (oriented vertically). Is this what you mean by scarf joint? IMG20190314090453.jpg
 

rockyboy

Skill Collector
Mentor
#5
Yep! That's a good scarf joint. If you can get them to be over the ribs that would be a good thing too - just don't have them all in a row across the wing at the same place - spread out the forces :D
 

Jackson T

Well-known member
#7
Yep! That's a good scarf joint. If you can get them to be over the ribs that would be a good thing too - just don't have them all in a row across the wing at the same place - spread out the forces :D
Yeah, I'll make sure they don't line up. By the way, they are actually the rear fuselage's main longerons.
 

Jackson T

Well-known member
#10
I just remembered, my pushrods aren't long enough to go from the wing saddle (where I normally put them) all the way to the tail! Should I try to joint two pushrods with a solder weld or something, or should I mount them half way back inside the rear fuselage? If I put the servos in the rear fuselage, I would make a little access hatch for adjustment should I need it. The least tail heavy moment is found with the servo as far forward as possible even though you add pushrod weight with the longer pushrod, so I want to keep the servos as far forward as I can.
Any suggestions?
 

rockyboy

Skill Collector
Mentor
#11
Lots of ways to extend the control rods and still keep the weight forward. The trick is keeping the rods from flexing. I've seen lightweight wood sticks with little metal rod ends used for this in old time gliders, and I've even done this myself with BBQ skewers that I split in half lengthwise. Tie on a little bit of metal control rod at the ends by wrapping it like crazy with sewing thread and then hit it will a little CA glue to lock it in place.

I've also extended control rods using shrinkwrap by overlapping the rods by a couple inches, leave a little bit of the ends of the overlap stick out and heat the shrinkwrap tight, and then CA glue the ends so you can get both rods glued to the shrinkwrap at each end.
 

Jackson T

Well-known member
#12
Lots of ways to extend the control rods and still keep the weight forward. The trick is keeping the rods from flexing. I've seen lightweight wood sticks with little metal rod ends used for this in old time gliders, and I've even done this myself with BBQ skewers that I split in half lengthwise. Tie on a little bit of metal control rod at the ends by wrapping it like crazy with sewing thread and then hit it will a little CA glue to lock it in place.

I've also extended control rods using shrinkwrap by overlapping the rods by a couple inches, leave a little bit of the ends of the overlap stick out and heat the shrinkwrap tight, and then CA glue the ends so you can get both rods glued to the shrinkwrap at each end.
What about overlapping the pushrods and than wrapping it with sewing thread and CA'ing it?
 

Jackson T

Well-known member
#15
Sound like it would, but I haven't tried it that way. I'd say give it a couple inches of overlap and see how it goes! :D
I think I'll do that. OR, I could just mount the servos back a bit in the fuselage so I don't have to worry about joining pushrods instead.
What would you recommend?
 

Hai-Lee

Old and Bold RC PILOT
#16
I think I'll do that. OR, I could just mount the servos back a bit in the fuselage so I don't have to worry about joining pushrods instead.
What would you recommend?
Wrapping the pushrod overlap and using CA or epoxy will work though CA can be a little brittle so a single crash might cause the join to fail and the rods to slide in the joint. I normally put "Teeth marks" in the pushrod ends, (the ones to be joined), so that the glue has something to really bite into!

As for adjusting the servos if rear mounted I always recommend that the linkage stoppers be mounted external to the fuselage for adjustment purposes. It makes burying servos for balance reasons much more user friendly!

Nice work so far!

Have fun!
 

Jackson T

Well-known member
#18
if you run a very thin wire pushrod in a sheath you can keep the weight down and not worry about flex if the sheath is supported.
I'll be using around 1mm piano wire and I will definitely have guides of some sort every 40cm or so. I'm thinking of using some pieces of drinking straws for the guides. What do you think?
possibly use a pull-pull setup using thru control horns and non-stretching fishing line. fyi- never used this myself.
I'm trying to keep things (relatively) simple on this build for the most part, but I am quite curious on the exact layout and linkages of a pull/pull system. Do you know of any build threads with pull/pull systems?
 

rockyboy

Skill Collector
Mentor
#19
possibly use a pull-pull setup using thru control horns and non-stretching fishing line. fyi- never used this myself.
A brand of fishing line called "Spider Wire" works great for this.

I think I'll do that. OR, I could just mount the servos back a bit in the fuselage so I don't have to worry about joining pushrods instead.
What would you recommend?
I recommend putting the servos where the help you balance the plane without any extra dead weight added. Putting most of the structure together and setting electronics in place to see if you can get the balance point roughly in place is probably a good idea.

I'll be using around 1mm piano wire and I will definitely have guides of some sort every 40cm or so. I'm thinking of using some pieces of drinking straws for the guides. What do you think?

I'm trying to keep things (relatively) simple on this build for the most part, but I am quite curious on the exact layout and linkages of a pull/pull system. Do you know of any build threads with pull/pull systems?
Look for @willsonman build threads - he's done this on a couple of his builds and talked about some of the setup considerations, but I don't remember which specific planes off the top of my head. I'll drop a link back here if I run across the one I'm thinking of.
 

Jackson T

Well-known member
#20
A brand of fishing line called "Spider Wire" works great for this.



I recommend putting the servos where the help you balance the plane without any extra dead weight added. Putting most of the structure together and setting electronics in place to see if you can get the balance point roughly in place is probably a good idea.



Look for @willsonman build threads - he's done this on a couple of his builds and talked about some of the setup considerations, but I don't remember which specific planes off the top of my head. I'll drop a link back here if I run across the one I'm thinking of.
Thanks Rockyboy!