Building a Centuri X-21 clone

[surdumil]

I've started working on building a clone of the Centuri X-21, whose plans are available here at Ye Olde Rocket Shoppe at http://www.oldrocketplans.com/centu...1/cenKBG-21.pdf

One of the things I've noticed is that the wing section can be cut out as one piece instead of as three (Main Wing, Insert, and Runner pieces) by using 4"-wide balsa. This simplifies assembly and, I think, strengthens the rear edge of the wing with a better grain orientation.

A full-size single-piece Main Wing pattern is attached as a .tif file. The pattern includes a 1" square as a dimension reference.

From the original wing pattern, there is a reference mark showing where to mount the under-wing runner. The mark is about 1/16" wide, so I'm assuming that the original kit's balsa sheets were 1/16" thick by 3" wide.

The root edge of the Main Wing, is 5" long. From the assembly diagram, the main body tube appears to be a BT-20 that is 5-1/4" to 5-3/16" long. However, the instructions specify that the motor block is mounted one full engine length in the main body tube. I prefer being able to grip the back end of the motor in case I have to pull it out of there, so I cut the main body to 5" long, mounted the motor block with 3/16" of the motor sticking out, and mounted the wings 3/16" back from the front of the main body.

The wing mounting jig appears to have originally been die-cut from cardboard, apparently about 1/16" thick. I put it together with 1/16" balsa.

I plan to cut the elevon hinges out of a mailing envelope made out of Tyvek. I've had great success with this stuff so far with other glider projects.

The clip for the elevon tension cord is a mystery. The exposed end of it appears in the Bottom View of the assembly drawing, but its shape is not clearly visible. The length of the inserted part of the clip is not mentioned or drawing anywhere. Any suggestions?

The launch lug looks to be about 1-3/4" long from the assembly drawing.

The control stop wires and aluminum tubes are a bit of a mystery as well. I'm thinking that some suitably pliable wire will do the job.

From the length of the root edge of the Forward Wing, the payload section is 2" of BT-20-diameter payload section. I thought the local hobby shop had a few (recent) Estes X-Ray kits around, which include a BT-20-diameter payload section, though I don't know if the tube is 2". I can't find plans for these kits anywhere on the Internet, so I can't find out. If I can't find an X-Ray kit to bash, the payload section will be a standard, boring BT-20 tube.

[surdumil]

Here's a picture of the wing-mounting jig... pretty nifty structure!

[CPMcGraw]

The root edge of the Main Wing, is 5" long. From the assembly diagram, the main body tube appears to be a BT-20 that is 5-1/4" to 5-3/8" long...

From the length of the root edge of the Forward Wing, the payload section is 2" of BT-20-diameter payload section.

Since this was a Centuri design, that should be ST-7 body tubing. Carl at SEMROC has ST-7 in both the spiral paper and the clear tubing. You want to look at CPT-722, which is 2.25" long, for the payload body. Carl also has the ST-760, which is a 6" length of ST-7. If the original was shorter, just trim it down...

[Ltvscout]

Since this was a Centuri design, that should be ST-7 body tubing. Carl at SEMROC has ST-7 in both the spiral paper and the clear tubing. You want to look at CPT-722, which is 2.25" long, for the payload body. Carl also has the ST-760, which is a 6" length of ST-7. If the original was shorter, just trim it down...
You can buy just about every part you need for the X-21, except the hinge material, from this link:

http://www.semroc.com/Store/scripts/ClassicParts.asp?ID=489

[Green Dragon]

"I plan to cut the elevon hinges out of a mailing envelope made out of Tyvek. I've had great success with this stuff so far with other glider projects."

that's a good idea, never heard that before, but the Tyvek is tough, have used for kite covering before, but taped on .

which brings me to a question - what do you glue the tyvek on with ? epoxy ?

~ AL

thinks the X-21 is 'neat enough' just might have to add to the buikld pile myself :-)

[surdumil]

Hi Al,

I've been using yellow glue and white glue for bonding Tyvek to balsa. This has been successful so far.

Here's a picture of the main wings mounted on the body, still sitting on the mounting jig.

The elevons are mounted, but the hinge material is not shown. This is because the mounting jig is designed to be used with the glider's belly side up, as mentioned in the "GLUE WINGS TO FUSELAGE TUBE" step of the X-21 assembly instructions.

[CPMcGraw]

You can buy just about every part you need for the X-21, except the hinge material, from this link:

http://www.semroc.com/Store/scripts/ClassicParts.asp?ID=489

Somehow, I figured Carl had this on his site... :D

Thank for the reminder, Scott...

[surdumil]

Here are a couple of pictures of the X-21 on its mounting jig, with main wings, elevons, bottom root stress support, under wing runner, and bottom stabilizers installed.

The X-21 is starting to look real interesting. The runners and stress supports add some interesting detail, but the stabilizers add some really impressive wing area. From the side, it looks quite x-plane'ish.

[surdumil]

Here are some pictures of the completed main body. Missing items include the elevon tension cord, elevon release line, and release clip.

This model has a serious amount of balsa surface area. Smoothing all that balsa is going to be a long haul.

Next, the nose/payload section needs parts collection and assembly. The main body requires very few difficult-to-find components. The release clip and the balsa bulkhead are about all there is. However, the nose/payload section wants a long balsa plug, a long balsa nose cone, and a 2" 18mm payload section. The balsa stuff can be easily turned. The payload section will need a lucky kit bash, or a substitution with an opaque tube.

[surdumil]

Happy Thanksgiving everyone!

Anyhow, I've manage to bring the pieces together to build the payload section.

I bought an Extes X-Ray kit, which has a 2.75" BT-20-diameter payload section. It cut it down to 2" for the X-21.

The balsa connector was very simple to create with a toy lathe. The nose cone took quite a bit more time, being such a long beast.

Attached are images of the lathe, the nose cone in various stages of completion, payload section parts, the assembled payload, and the structurally complete X-21 model.

[surdumil]

I finished off the final details and the X-21 is complete (except for glide trimming). This includes some tidbits that aren't included in the assembly drawings/instructions or in the Semroc parts set. I hope Craig at Semroc will note the elevon release clip and elevon control stop notes below, or will suggest better alternatives.

For the elevon tension cord I used about 4 1/2 inches of elastic thread. I cut slits in the elevons instead of poking holes. I thought this would ease thread replacement.

I used black sewing thread for the neutral elevon release. I poked holes for this and used a sewing needle to insert the thread into the holes, securing the thread in place with square knots bolstered by surgeons knots.

I was a little stumped for a while regarding the release clip, but found a solution that seems to work well. The X-21 instructions give no details to work from, so here's my suggestion. Get an empty soda can. You know that piece of metal that gets pushed and folded into the can when you pull the tab to open the can? That's the piece of metal that keeps the drinking hole sealed until the can is opened. Get that piece out of the can. Cut a 1/8-inch wide, 3/4-inch long strip from this piece. Cut the corners off of one end of the strip, giving you a pointed end. Bend about 1/16" to 3/32" of the other end of the strip over, but not completely over. Slip the center of the eleveon release thread under this end of the strip and crimp the end over the thread. Voila, a crimped elevon release clip.

I formed control stops out of a straightened-out metal paper clip, adding zig-zags that lay flat against the wing to stop that part of the control stop from twisting when adjusting elevon height.

I gave the rocket a swing test, prepped with a B6-2 motor, and found that the model is stable without needing extra nose weight. I suspect the custom-turned nose is a bit more massive than the original nose.

All that needs doing is to do some glide testing and adjust the control stops for a hand-tossed flat glide. Then, wahoo!!!! Launch time!

[CPMcGraw]

I hope Craig at Semroc will note the elevon release clip and elevon control stop notes below, or will suggest better alternatives...

If only Craig DID work at SEMROC! :D :D :D

You know something I don't???

[surdumil]

Doh!

Sorry Craig. I meant Carl. (I think?)

[surdumil]

Today, the wind dropped down to around zero km/h. What a perfect launch day! I prepped the X-21 with an A8-3. Attached is an image of the X-21 on the launch pad, to give you an idea of the size of it... pretty bad picture though. I need a better digital camera.

The X-21's boost was very straight and very quick. Despite all that balsa, she's a very light bird. Transition to glide was smooth and graceful. The glide was quite flat, with a moderate right spiral. I'd like to open the turn up a bit next time with some adjustments to the elevon stop wires. The landing was gentle and flat.

I was going to send it up again with a B4-2, but as I was tensioning the elevon hold-down thread, the thread snapped. I think the clip nicked the thread or cut through it. I had no thread on hand, so had to put the X-21 away for the day.

A SAROS launch is scheduled for tomorrow. If the wind is acceptable, I can try another flight then.

Pretty cruddy image, eh? I need a better digital camera!

[Ltvscout]

The X-21's boost was very straight and very quick. Despite all that balsa, she's a very light bird. Transition to glide was smooth and graceful. The glide was quite flat, with a moderate right spiral. I'd like to open the turn up a bit next time with some adjustments to the elevon stop wires. The landing was gentle and flat.
Thanks for the flight report!

Now, let's put a grasshopper in the payload section for the next flight to see how she glides then. ;)