The Sprat - A new year's build

TooJung2Die

Master member
Taken back out, looking good. The jig might seem like a lot of work, but it's totally worth it. Fuselage is arrow straight.
You're right about that. Nice fuselage jig. Working around the jig while putting in the cross pieces may be tedious but it assures the fuselage will be built straight. The fuselage came out really nice!
I have a feeling the battery may have to be placed closer to the nose to get the CG right without adding ballast. You'll know when you do a bare bones assembly and check the CG before covering. You can always make a battery hatch in the nose.
 

speedbirdted

Legendary member
Under rubber power I would guess it was just fine. After all it doesn't say anywhere on the plan to add nose or tail ballast. Therefore just adding weight on the CG (the brick and battery are actually a little bit forward of CG) shouldn't affect it and I'm pretty sure under rubber power, more of the rubber is behind the CG than in front of it so getting rid of it will also push your CG forward a bit. I also removed the mounting hook for the rubber in the tail, having the same effect. The extra weight of a motor will probably cancel out the extra weight of control linkages going to the tail, provided I can keep them light.

Then again, I'm not so sure. Hell, I'm not even sure the designer even bothered to build his own airplane; if he did so he would discover how badly the parts fit together! Which is twofold; it means my CG guesstimation may be totally wrong, and I might also be creating the first Sprat ever built. :p
 

AIRFORGE

Elite member
Under rubber power I would guess it was just fine. After all it doesn't say anywhere on the plan to add nose or tail ballast. Therefore just adding weight on the CG (the brick and battery are actually a little bit forward of CG) shouldn't affect it and I'm pretty sure under rubber power, more of the rubber is behind the CG than in front of it so getting rid of it will also push your CG forward a bit. I also removed the mounting hook for the rubber in the tail, having the same effect. The extra weight of a motor will probably cancel out the extra weight of control linkages going to the tail, provided I can keep them light.

Then again, I'm not so sure. Hell, I'm not even sure the designer even bothered to build his own airplane; if he did so he would discover how badly the parts fit together! Which is twofold; it means my CG guesstimation may be totally wrong, and I might also be creating the first Sprat ever built. :p
LOL! Sprat maiden build! :D(y)
 

speedbirdted

Legendary member
Built the horizontal stabilizer. It uses laminations, something I've never tried before on this scale. I did not want to cut up my plan to make lamination patterns so instead I cropped out the portions of the plan I would need to make them and cut the patterns from foamboard. Laminations are also used on the wingtips and the tip of the vertical stabilizer.

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Four laminations of 1/32x3/32 as called for. The plan underestimates how thick this will be; in retrospect three layers would have been about the thickness specified. I glued the laminations together with Super Phatic so it would be easier to sand later.

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Both sides done. I cut them a bit oversize as the curvature varies a little and I could rotate it around a bit to fit the plans best.

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From here on building the horizontal stabilizer was fairly straightforward. I gave the trailing edge a symmetrical taper before installing it, and to install it with no camber I shimmed it up with pieces of scrap 1/32.

The only change here is the installation of an elevator (singular.) Surprisingly many old-timer type designs have only one elevator on one side. I have flown a few such designs and unless you really start horsing around with the right stick you won't feel it most of the time. I have a feeling on this airplane the elevator won't get a huge amount of use anyway. It'll be treated more or less as a big trim tab. I figure I can keep it lighter by not adding the extra wood of a second elevator at the tail and also getting rid of whatever I would need to go between them. Likely here it would be a short length of music wire because of how the tail is designed.

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Building the elevator. I used the section of trailing edge that I cut away originally as the elevator's trailing edge.

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I then cut away a little bit from the front stick to install a little control horn. I made a few of them because it is very hard to drill a hole though it small enough to not introduce excessive play while also not just breaking it.

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Fishing line hinges installed. Not glued yet: I think I will wait until I cover at least one side with tissue to do that. Or perhaps I will glue in the line on one half to prevent the lines falling out and getting lost all the time.

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Finished. I have a bit to sand off the fuselage so it will sit level. It will not be glued on until I cover everything, carrying over practice from bigger planes.

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Vertical and wing are up next. Maybe I'll also 3d print a little pilot....
 

TooJung2Die

Master member
Great build. The laminated tips look so nice. They're light and strong. I agree with you about the one side elevator. It works remarkably well on this type of airplane. I used a one sided elevator on the Contest Commercial. As you said, you don't won't be using the elevator much. Use the motor to climb and glide it down; repeat. The only time the elevator comes into play is when you're bored of riding that thermal and it's time to go home. 😁
 

speedbirdted

Legendary member
Vertical and rudder up next. First order of business was making another lamination jig, much the same practice as making the horizontal tips. This time though I would have to cut the lamination in half to add the rudder hinge line.

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Here we encounter further examples of the designer's inattentiveness. What is specified to be the rudder's 1/4x1/16 trailing edge strip is drawn as closer to a 1/8 strip 🤨 Having it be 1/16 thick makes no sense either as all the other pieces of the vertical are 3/32 square. I just made it out of some of the spare horizontal trailing edge that I'd already sanded down, which is sitting next to it.

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Building up the rudder next to the vretical. I used the same fishing line hinges and made a little control horn out of 1/32 basswood again. It's hard to make them without screwing it up 😬

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Tail group finished.

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I'm getting decently comfortable with using tissue after making a few test squares now. This one I stuck down with UHU stick and shrank with eze dope sprayed on. I find the shrink is much nicer with eze dope than alcohol + water. However if you spray on eze dope like I did be careful to immediately flush whatever spraying equipment you used with water or it will become plugged!

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TooJung2Die

Master member
Looking good! I had high hopes for Eze Dope. I can't use traditional dopes in the house. I watched all their instructional videos and tried to duplicate the results. It didn't live up to my expectations as a dope replacement in every regard. It works well as a tissue adhesive but not so well as a tissue finish. I don't like the way it wets and sags the paper on every coat. On thin parts like the elevator the tissue can sag enough to touch the other side and get stuck to it. I did try mixing a little bit in water as a shrinking spray. I don't think it worked any better than plain water and it clogs the sprayer as you found out.
 
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speedbirdted

Legendary member
Eze dope seems to work fine for me. I encountered the sagging problems when I tried to brush it on but spraying it on with a perfume bottle caused it to sag much less. I have a can of Brodak clear butyrate somewhere, maybe I'll try that too. Though butyrate tends to overshrink over time and warp stuff, or so I've heard. Maybe I ought to buy some nitrate.
 

TooJung2Die

Master member
Over at RCGroups 'Scale Kit/Scratch Built' a lot of stick and tissue builders are using clear aerosols instead of dope now. I agree, it works great. I have a bottle of clear Aero Gloss dope that may never get opened. I think dope is going the same way Ambroid glue went. That's the same forum that taught me about document laminating film covering.
 

speedbirdted

Legendary member
Built the wing (finally) Started the process by cutting out 12 1/32 wing rib blanks, sticking them together with Super 77, and sticking the template atop of them.

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Scroll saw made quick work of it. Notches were made via sanding and razor saws. You can make two cuts to define the outer edges of the spar notches, and then use an x-acto or a pin to clear out the space in between. SInce it's with the grain, the wood comes out very easily.

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Wingtips required laminations which I'd actually made yesterday while I was making the vertical lamination. The notched trailing edge was made with two razor saw blades in one knife. I found it makes a 1/32 notch and the formers fit in nice and snug. Wing was built in three sections; two wing panels and a center section. This gets complicated later...

This might also be the first time I've ever pulled the ribs apart after gluing without breaking any of them. :)

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The design of the dihedral brace stumped me for a bit. I don't get why there are notches in it. I also don't get why there's any need for the doubled ribs. A single 1/16 rib on each side would be more than strong enough.

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Building the center section. I cut the middle rib down to fit the trailing edge later. I stuck it in a jig to make sure I had the same dihedral on both wingtips before clamping the spars to the brace then gluing.

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Getting real close to airplane noises time now. I still need to sand the LE/TE and wingtips down but for now I ran out of time.

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My estimate for airframe weight was 15 grams. Considering the weight of the wood I used (some is not what I would call light) I'm surprised it's less than expected. Meeting the 50 gram target will be smooth sailing.

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Tomorrow I will build the gear and wheels and hopefully get the finishing touches on like the pushrod routing and electronic placement figured out, and see if I need to move the battery or not. The CG feels tail heavy with no electronics or gear but fortunately everything I need to add from here forward except the pushrod wires and tailskid goes in front of the CG. Then it's tissue time.
 
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Matthewdupreez

Legendary member
Built the wing (finally) Started the process by cutting out 12 1/32 wing rib blanks, sticking them together with Super 77, and sticking the template atop of them.

View attachment 188953

Scroll saw made quick work of it. Notches were made via sanding and razor saws. You can make two cuts to define the outer edges of the spar notches, and then use an x-acto or a pin to clear out the space in between. SInce it's with the grain, the wood comes out very easily.

View attachment 188954

Wingtips required laminations which I'd actually made yesterday while I was making the vertical lamination. The notched trailing edge was made with two razor saw blades in one knife. I found it makes a 1/32 notch and the formers fit in nice and snug. Wing was built in three sections; two wing panels and a center section. This gets complicated later...

This might also be the first time I've ever pulled the ribs apart after gluing without breaking any of them. :)

View attachment 188955

The design of the dihedral brace stumped me for a bit. I don't get why there are notches in it. I also don't get why there's any need for the doubled ribs. A single 1/16 rib on each side would be more than strong enough.

View attachment 188957

Building the center section. I cut the middle rib down to fit the trailing edge later. I stuck it in a jig to make sure I had the same dihedral on both wingtips before clamping the spars to the brace then gluing.

View attachment 188956

Getting real close to airplane noises time now. I still need to sand the LE/TE and wingtips down but for now I ran out of time.

View attachment 188959

My estimate for airframe weight was 15 grams. Considering the weight of the wood I used (some is not what I would call light) I'm surprised it's less than expected. Meeting the 50 gram target will be smooth sailing.

View attachment 188958

Tomorrow I will build the gear and wheels and hopefully get the finishing touches on like the pushrod routing and electronic placement figured out, and see if I need to move the battery or not. The CG feels tail heavy with no electronics or gear but fortunately everything I need to add from here forward except the pushrod wires and tailskid goes in front of the CG. Then it's tissue time.
15 grammmmss!!!!!!!!!! man why can't i afford balsa:cry::cry:
 

speedbirdted

Legendary member
I've worked on the wing some more. Sanded the LE and TE to shape as well as smoothed it a bit in preparation for tissue. Speaking of that....

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Since these bits were otherwise done I decided to cover them to see if I actually knew what I was doing or not. Tissue ended up going down flawlessly. I thought it would be more hassle around compound curves but I cut a generous amount of relief cuts and there were no problems. The elevator fits perfectly and runs with no binding. Fishing line really is a great material for hinges on micro models like this.

Only complaint is the white was a little more transparent than I wanted, but I like the translucent look. It'll look fine on the fuselage.
 

TooJung2Die

Master member
Perfect tissue job. The white looks excellent in the photo. I like seeing the balsa wood through the covering. You said you're using glue stick to tack it down? I think glue stick is the biggest product advancement since I last covered with tissue many decades ago. It makes positioning and smoothing out the tissue so easy. If you screw up somehow it's just as easy to peel off and do over. You can get the tissue so tight it almost doesn't need to be shrunk.
 

speedbirdted

Legendary member
@speedbirdted what servos are you using? Won't normal 5g or 9g ones be way too heavy?
Yeah, a pair of 9g servos would be nearly half the AUW :p I'm using one of those all-in-one bricks which uses a couple of linear servos. It weighs about 3g total.

After attempting to use the tape method to hold the v/h stabs together to glue them and have it fail horribly I got lazy and made this jig to hold them together so I could do something else while the glue dried. If it looks stupid but it works it isn't stupid...

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I also got the wing covered. This was significantly less pretty than the tail covering but I guess it's still passable for the first full wing I've ever covered in tissue. The wingtips put up a bit of fuss. I sprayed it with water to try and get it to stretch a little but it didn't seem to really stretch at all. It didn't become weak, it just wouldn't stretch. Maybe I wasn't getting it wet enough. A light misting is all I really gave it.

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You know what I did immediately after finishing the wing?

I dropped it. :mad:

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So I guess I now get to test my ability to patch holes in tissue as well. I'm thinking I will cut all the tissue from the bay the tear is in and then glue down the edges before adding new tissue on top of it. Or I could just put tissue immediately over the tear, like how you'd apply a second layer of tissue. Help is appreciated...
 

TooJung2Die

Master member
I think that is about as good as you can expect to achieve if you cover the top and the tip of the wing with one piece. I doubt anyone could do it better. 🙂👍 Me, I cover the tips separately. As for the hole, at this point in the build I would strip and recover the whole top if I had enough tissue. More practice. 🙃 If there isn't enough to do the top over then I'd cut out and cover the section between the ribs where the hole is. Paint thinned Eze Dope on top of all the ribs before you cut it out. It'll soak through the tissue to the ribs and stick the tissue to the balsa. If you do that to the top of all the ribs then the patch seams will look like the rest of the wing.

I prefer to replace a section rather than placing a patch over a hole. It makes the repair much less noticeable. There was a hole in the side of this fuselage. Can you tell which section was cut out and patched? Hint, the grain of the tissue goes in a different direction.

Fuselage patch.jpg
 
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