Semroc Scissor Wing Transport Kit Build Thread

[TheAviator]

There are some miniscule parts like the pivot support that are a challenge to assemble if you have Fat Finger Syndrome like me. Tab and slot joints were too tight for me to get them to fit, so to avoid breaking anything, cautious, conservative sanding with something like an emery board or small diamond file will enable the parts to fit together snug and square. Remove just a little more than the char and you should be good.

Just a couple little tips for laser cut tab-and-slot stuff.

1. The tolerances that you can hold during laser cutting are pretty good. Often it's the thickness of the wood that throws of the fit, so check that first. If so, just sand the side of the part gently until it fits.
2. It can help to chamfer the end of the tab to help it slot it. You only need a quick swipe with an emery board to get where you want to be.

Looking good!

 

[Ez2cDave]

I built the original Estes kit with an airfoiled wing and full "Contest finish". Mine is very light and glides great. I would not fly it in a contest because it could thermal away. I usually use an Astron Invader for a second contest flight.

I find it hard to visualize an Estes Scissor Wing Transport "thermalling away".

However, THIS would, definitely.

https://www.nar.org/wp-content/uploads/2018/12/Spanish-S4A-Design-2018-1.pdf

Dave F.

 

[Ez2cDave]

Actually that's a pretty decent glide for a sport model up to the point where the video cuts off. I've seen much worse roll and stability problems with flying wings even when they've got dihedral. Probably not a good idea to fly a SWT in turbulent or gusty wind conditions though.

Right before the video ended, it looked to me like it was "mushing".

https://forums.ubisoft.com/showthread.php/378730-mushing-vs-stalling

https://aviation.stackexchange.com/questions/26932/what-is-a-mushing-glide

Dave F.

 

[Alan15578]

Congrats on the Invader. I built one as a kid and never could get it to launch right. It would put in a squirrely all-over-the-sky ascent before doing some loops and then sorta pancaking down. I just called it my "acrobat" glider, and hilariously the other kids thought it was supposed to fly like that and loved it.

I have seen one Invader that did a power loop on launch, with the low end of loop at head height above the judges table. All of mine boost straight under thrust, they get just slightly "squirrely" on coast, and transition into a flat circling glide after the motor ejects with a small streamer.

 

[Alan15578]

I find it hard to visualize an Estes Scissor Wing Transport "thermalling away".

However, THIS would, definitely.

https://www.nar.org/wp-content/uploads/2018/12/Spanish-S4A-Design-2018-1.pdf

Dave F.

My old Estes SWT weighs 41.5 grams with the airfoiled wing and good finish. With the internal pod it weighs 54.7 grams. I don't know what an actually finished Semroc SWT weighs. I never trust catalog weights. I think the airfoiled wing is a significant performance booster, even when left flat bottomed. It is kind of like the old Flat Cat vs. Round Hound issue. For a sport model you really want the lower performance, but few craftsmen can resist the urge to do it up right.

I suspect the Semroc version is heavier, but I like what I have seen of it on TRF, except for the $46 kit price. One mod that I might try is to make the wing deployment angle variable. The concept for the model is based on the oblique wing having lower drag in the transonic regime, and NASA actually tested two oblique wing aircraft, mainly to asses the flying qualities and autopilot development. Modelers might develop some apprecialtion for the flight control problem if they could set the wing at different angles.

I've seen that and similar FAI designs. It is a bit too flip-flop-flex-crash for my tastes. Once you develop a certain level of skill, the design matters less, and the launching and return matters most. Eventually you start flying to avoid thermals. Saddly, in sport flying you usually wait for a lull in the wind to launch, but this is often the way to catch a thermal.

 

[Ez2cDave]

My old Estes SWT weighs 41.5 grams with the airfoiled wing and good finish. With the internal pod it weighs 54.7 grams. I don't know what an actually finished Semroc SWT weighs. I never trust catalog weights. I think the airfoiled wing is a significant performance booster, even when left flat bottomed. It is kind of like the old Flat Cat vs. Round Hound issue. For a sport model you really want the lower performance, but few craftsmen can resist the urge to do it up right.

Can you post up some pics of your SWT ?

Thanks !

Dave F.

 

[Rktman]

My old Estes SWT weighs 41.5 grams with the airfoiled wing and good finish. With the internal pod it weighs 54.7 grams. I don't know what an actually finished Semroc SWT weighs. I never trust catalog weights. I think the airfoiled wing is a significant performance booster, even when left flat bottomed. It is kind of like the old Flat Cat vs. Round Hound issue. For a sport model you really want the lower performance, but few craftsmen can resist the urge to do it up right.

I suspect the Semroc version is heavier, but I like what I have seen of it on TRF, except for the $46 kit price. One mod that I might try is to make the wing deployment angle variable. The concept for the model is based on the oblique wing having lower drag in the transonic regime, and NASA actually tested two oblique wing aircraft, mainly to asses the flying qualities and autopilot development. Modelers might develop some apprecialtion for the flight control problem if they could set the wing at different angles.

I've seen that and similar FAI designs. It is a bit too flip-flop-flex-crash for my tastes. Once you develop a certain level of skill, the design matters less, and the launching and return matters most. Eventually you start flying to avoid thermals. Saddly, in sport flying you usually wait for a lull in the wind to launch, but this is often the way to catch a thermal.

So true about waiting for a lull in the wind to launch. I've lost many a glider to thermals that way, so I always try to fly as early in the morning as I can. I've just come to accept the risk of loss as part of the enjoyment of flying.

 

[Rktman]

This is the dowel and its standoff. If you’re familiar with the SWT, you know the dowel is the thing that holds the wing locked in launch position. It’s glued onto the motor tube in one of the earliest steps. See anything wrong? I didn’t — until just now.




When I slipped the motor tube into the main body, I noticed that the dowel was warped slightly, just enough so that it won’t slip through the guide tube in the upper nacelle.



A WORD OF ADVICE: DON’T ATTACH THE DOWEL AND MOUNT TO THE MOTOR TUBE UNTIL AFTER YOU’VE ATTACHED THE UPPER NACELLE AND ITS SUPPORT AND THE GUIDE TUBE THAT RUNS THROUGH IT.
That way you can assure it’ll be properly aligned with the guide tube and notch in the wingtip. Also, roll your dowel across a flat surface beforehand to make sure it’s not warped.

At the moment I’m not quite sure how I’ll solve the problem. Replacing that dowel and mount is unlikely, as it’s on there good with epoxy and a TBII fillet. I tried wetting the dowel on the inside of the curve hoping that it would swell and straighten it enough. It didn’t. That trick seems to work on balsa but not on a piece of hardwood.

The only other solution I can think of is to notch the dowel in a couple of spots and then once straight, “freeze” it in place with thin CA, then later go back and fill the notches with epoxy and sand everything smooth.
If anyone has any suggestions, I’ll gladly consider them.

 

[TheAviator]

I suspect the Semroc version is heavier, but I like what I have seen of it on TRF, except for the $46 kit price. One mod that I might try is to make the wing deployment angle variable. The concept for the model is based on the oblique wing having lower drag in the transonic regime, and NASA actually tested two oblique wing aircraft, mainly to asses the flying qualities and autopilot development. Modelers might develop some apprecialtion for the flight control problem if they could set the wing at different angles.

Unfortunately, one of the control problems they found with the oblique wing was that it would roll pretty hard towards the swept back wing. While you could certainly correct for that with radio control, it would not be conducive to a free-flight model.

 

[TheAviator]

This is the dowel and its standoff. If you’re familiar with the SWT, you know the dowel is the thing that holds the wing locked in launch position. It’s glued onto the motor tube in one of the earliest steps. See anything wrong? I didn’t — until just now.

View attachment 505039


When I slipped the motor tube into the main body, I noticed that the dowel was warped slightly, just enough so that it won’t slip through the guide tube in the upper nacelle.

View attachment 505040

A WORD OF ADVICE: DON’T ATTACH THE DOWEL AND MOUNT TO THE MOTOR TUBE UNTIL AFTER YOU’VE ATTACHED THE UPPER NACELLE AND ITS SUPPORT AND THE GUIDE TUBE THAT RUNS THROUGH IT.
That way you can assure it’ll be properly aligned with the guide tube and notch in the wingtip. Also, roll your dowel across a flat surface beforehand to make sure it’s not warped.

At the moment I’m not quite sure how I’ll solve the problem. Replacing that dowel and mount is unlikely, as it’s on there good with epoxy and a CA fillet. I tried wetting the dowel on the inside of the curve hoping that it would swell and straighten it enough. It didn’t. That trick seems to work on balsa but not on a piece of hardwood.

The only other solution I can think of is to notch the dowel in a couple of spots and then once straight, “freeze” it in place with thin CA, then later go back and fill the notches with epoxy and sand everything smooth.
If anyone has any suggestions, I’ll gladly consider them.

I would put some sort of shim to over-bend the dowel in the other direction while it just sits. I've found that the birch dowels will creep with time, so that might let it creep enough to work with your setup.

 

[Ez2cDave]

At the moment I’m not quite sure how I’ll solve the problem. Replacing that dowel and mount is unlikely, as it’s on there good with epoxy and a CA fillet. I tried wetting the dowel on the inside of the curve hoping that it would swell and straighten it enough. It didn’t. That trick seems to work on balsa but not on a piece of hardwood.

The only other solution I can think of is to notch the dowel in a couple of spots and then once straight, “freeze” it in place with thin CA, then later go back and fill the notches with epoxy and sand everything smooth.
If anyone has any suggestions, I’ll gladly consider them.

Eric,

This is going to be "surgery" . . .

Dremel out the wood dowel and replace it with a carbon fiber rod / tube.

No more warpage !

Dave F.

 

[Rktman]

Eric,

This is going to be "surgery" . . .

Dremel out the wood dowel and replace it with a carbon fiber rod / tube.

No more warpage !

Dave F.

What kind of bit do you suggest? The dowel sits on top a balsa spacer totally enclosed by 1/16" balsa "support plates" on either side. Concerned about about what kind of bit to use that won't damage the balsa and yet can route out all of the hardwood dowel?

 

[Ez2cDave]

What kind of bit do you suggest? The dowel sits on top a balsa spacer totally enclosed by 1/16" balsa "support plates" on either side. Concerned about about what kind of bit to use that won't damage the balsa and yet can route out all of the hardwood dowel?

Hi, Eric !

I would use a small "burr" ( round ball ) bit . . . Work slowly & carefully.

There are two types, one with "cutting blades" and the other a "diamond bit" style.

https://www.eternaltools.com/blog/7-facts-about-tungsten-carbide-burs-and-how-to-use-them

https://www.aliexpress.com/item/32858068732.html

https://www.amazon.com/Diamond-Grinding-Spherical-Dremel-Rotary/dp/B077JPDDNV

Cylindrical burrs are another option . . .

https://www.ebay.com/itm/183969887617

Dave F.

 

[Rktman]

While I decide how to fix the warped dowel problem, I’m going to move forward on assembling the other components. Because some of the parts are diminutive and hard to handle when you’ve got 10 thumbs, I’ve found them much easier to assemble while they’re still in their carrier sheets. This is the upper nacelle support and the plastic guide tube (launch lug) that runs through it.




The wing pivot assembly is glued to the main BT. This part used to be preassembled plastic and would oftentimes break from what I understand. Hopefully this redesigned version will be less brittle and have more “give” during hard landings.






The wing pivot plate is glued to the bottom of the wing next. The small dowel is glued into the hole provided to ensure the plate is oriented correctly and centered. The small kidney bean-shaped pieces attached earlier to the wing pivot assembly (see post #30) will ride in these slots, which also limit the rotation of the wing to 90° (oriented back for launch, then perpendicular for glide).




Although it’s a clever way to replace traditional music wire for the elastic anchor point, this balsa tabs like this have a tendency to break off sometimes, so I soaked it with thin CA to strengthen it.




The wing hold down tube (a short piece of launch lug) is installed in the slot provided in the wingtip. This is what the dowel will pass through to hold the wing folded back in launch position.

 

[Rktman]

Some more interesting speed bumps have materialized. First, the wing pivot post supplied is too tall.



Per Randy, the female end of the pivot post just needs to be sanded down. Easy fix since it’s aluminum.

The second more vexing and daunting problem that appeared was that the wing, when mounted, was pitched down about 2°. That totally didn’t make sense. After spending far too much time trying to figure out how that could possibly be and how to correct the problem, the culprit turned out to be the simplest cause: the pivot post was canted 2° from vertical.



That should’ve been impossible, since the flange on the female side of the pivot post should have been absolutely flat, and I simply dropped the post through the hole in the (again, flat) wooden wing pivot assembly and supported everything over the top of an open bottle. Gravity alone should have kept it perfectly vertical.




Hard to tell but if you squint hard, you’ll see that the post isn’t absolutely vertical.




Fortune favors the bold—but I guess in this case, it favors the desperate.
Since I figured it was a lost cause and I’d have to reorder the main body tube and pivot pieces, I had nothing to lose and tried bending the post vertical with pliers. It didn’t work of course, but out of sheer luck it snapped free of the glue enough that I was able to get it very close to vertical (looks to be maybe about a half-degree off). Sanding the wing trailing edge per the instructions should help some, and I’ll likely have to sand the elevator flap-stop to allow a little more angle.
I’m still enjoying the build, but fingers crossed that no more surprises surface.

 

[Rktman]

Re: that too tall pivot post — since I was sanding it down by eye and didn’t want to inadvertently take off too much, I removed as much as I dared and then made a washer out of 1/64” ply to deal with any slop. It has the added benefit of keeping the balsa wing from being crushed when the screw cap is tightened.

 

[Ez2cDave]

Re: that too tall pivot post — since I was sanding it down by eye and didn’t want to inadvertently take off too much, I removed as much as I dared and then made a washer out of 1/64” ply to deal with any slop. It has the added benefit of keeping the balsa wing from being crushed when the screw cap is tightened.

Adapt & overcome . . . Nice fix !

Dave F.

 

[Dan Snow]

I'm wondering if the 2° tilt, if you mean the leading edge is lower than the trailing edge, means you have in effect induced washout on the entire wing, reducing the angle of attack and slowing the onset of a stall? Not an aeronautical engineer so I maybe way out to lunch, hopefully an expert out there than correct me.

I very much appreciate you build thread as I am currently in a DIY build making my own version of this. Question: from what I can see on this and other sources, is there only one elastic powering the wing movement? The one shown on the left wing?

 

[dvdsnyd]

I'm wondering if the 2° tilt, if you mean the leading edge is lower than the trailing edge, means you have in effect induced washout on the entire wing, reducing the angle of attack and slowing the onset of a stall? Not an aeronautical engineer so I maybe way out to lunch, hopefully an expert out there than correct me.

I very much appreciate you build thread as I am currently in a DIY build making my own version of this. Question: from what I can see on this and other sources, is there only one elastic powering the wing movement? The one shown on the left wing?

The 2 deg tilt is not a feature of the rocket. Wing should be parallel with the main body tube.

Take a look at the instructions, available on the erockets site. I think that will help clear up how the mechanism is built and used.

https://www.keepandshare.com/doc10/34420/scissor-wing-transport-thumb-pdf-1-0-meg

 

[dvdsnyd]

Mistakes were made...

 

[Rktman]

The 2 deg tilt is not a feature of the rocket. Wing should be parallel with the main body tube.

Take a look at the instructions, available on the erockets site. I think that will help clear up how the mechanism is built and used.

https://www.keepandshare.com/doc10/34420/scissor-wing-transport-thumb-pdf-1-0-meg

I'm wondering if the 2° tilt, if you mean the leading edge is lower than the trailing edge, means you have in effect induced washout on the entire wing, reducing the angle of attack and slowing the onset of a stall? Not an aeronautical engineer so I maybe way out to lunch, hopefully an expert out there than correct me.

I very much appreciate you build thread as I am currently in a DIY build making my own version of this. Question: from what I can see on this and other sources, is there only one elastic powering the wing movement? The one shown on the left wing?

Be very careful to ensure that the pivot post is installed absolutely vertical or you'll end up with the same problem I did. I'm still not sure how it could've been canted, since the post flange and the surface it sits on are both assumed to be flat.

Just my opinion, but if I was scratch building, I'd substitute a small bolt and locknut from McMasters in place of the binding screw post used in the kit. Much simpler, adjustable if it somehow goes in crooked, and no slotted screw cap that tends to back out/loosen when the wing swivels. This is what the competition S4-A style scissor-flop gliders use; very light, minimal, and reliable. Of course, if you're a purist the appearance might bother you, but I'd rather have a simpler, more easily adjusted and locked-down pivot post.

 

[Rktman]

The build has slowed down lately while fabricating small fixes. I decided on another approach to remedy that down-tilt of the wing. I shimmed the front of the Wing Pivot with 1/64” ply, sanding a taper into the ply’s aft end so that it acts as a wedge to tilt the wing’s leading edge up just enough that it’s now level.




Also doing a lot of balsa sealing. I long ago discovered that self-adhesive hinges don't adhere well to bare balsa and tend to let go easily, probably because of the porous surfaces. So the tape hinges for the elevator will have to wait until after the stab gets sealed and/or painted. Moderate shaping of the wing's trailing edge was done (see note below) and then some sealer will be applied before a light coat of white lacquer or enamel (once the temps and humidity permit).

Note on sanding the wing trailing edge: best done BEFORE installing the Wing Pivot Plate. The instructions would have you do it after, but with the plate installed, the wing won’t sit flat on a surface, making it difficult to sand a consistent airfoil without risking snapping or cracking the balsa wing.

 

[Rktman]

Question: from what I can see on this and other sources, is there only one elastic powering the wing movement? The one shown on the left wing?

Yes, only a single elastic band, a 1/2" diameter dental band in the case of the kit. I guess the anchor points aren't very far apart so it doesn't require anything much longer or with more tension, like in the case of slide wing gliders for example.

Edit: to be precise, 9/16" O.D.

 

[Ez2cDave]

Eric,

I'm surprised at the wing incidence issue. From your build photo's, it looks like everything was "square" when assembled. I, too, am puzzled at how the misalignment happened.

Dave F.

 

[Rktman]

If you’ve been patient enough to follow along this far, I hope I’ve been able to help you by making you aware of some of the things to pay attention to, like the tube marking guide or pivot tube.

Anyway, still working on sealing balsa surfaces. After reinforcing wing, stab, and rudder edges with CA, I used a homemade sealer I came across here on TRF that works real well: Minwax Polycrylic and talc. Cleans up with water and is very low-fume, unlike regular lacquer-based sanding sealer. Hardens and protects the balsa well too. A thinned layer of easily sanded Carpenter’s Wood Filler goes over that before the final coat of sanding sealer. I’m not trying to achieve a glass-smooth finish on the wings or stab, just trying to minimize the grain to allow the tape hinge to have a reasonably smooth, hard surface to adhere to.

 

[Rktman]

Got a ways to go on the nose cone. Lots of fuzzies and roughness. I’ve never tried hardening it with CA, though I know it’s routinely done by a lot of builders. It could really use the protection during trimming tosses, so I won’t be doing any final smoothing and painting till after that’s done.




Just waiting for decent painting weather to finish things up. Rudder and forward wing hold-down will go on today, and elevator will be installed once everything is painted.




The motor pod unit that is ejected is supplied with a 1.75” x 48” crepe streamer, but to minimize any damage to the fins and pod, I elected to substitute a 9” mylar ‘chute, anchoring it at the unit’s CG.

 

[Ez2cDave]

Eric,

Coming along nicely !

How did you resolve the Wing Retention Dowel issue, from post #38 ?

Dave

 

[Rktman]

Eric,

Coming along nicely !

How did you resolve the Wing Retention Dowel issue, from post #38 ?

Dave

Forgot to mention. I ended up carefully slicing the standoff from under the dowel backward in a straight line, then glued the new dowel on top where the old one used to be. I still need to apply an epoxy fillet and do some cosmetic touch-ups, but it's good to go.

 

[Ez2cDave]

Forgot to mention. I ended up carefully slicing the standoff from under the dowel backward in a straight line, then glued the new dowel on top where the old one used to be. I still need to apply an epoxy fillet and do some cosmetic touch-ups, but it's good to go.

Eric,

Good deal . . . Great fix !

Dave F.

 

[tab28682]

Excellent work and great solutions to the minor glitches.