Vladimir's ELF DLG Build Log: Assembling and Trimming a V-Tail Discus Launch Glider
Santa was exceptionally good to my family this year. A nice new ELF Discus Launch Glider appeared under the tree. I had never flown a glider before, but they looked like a lot of fun. My first impression on opening the box was amazement on how light everything is.
If you aren't familiar with this glider, it is a 39-inch mosquito class. Fully outfitted, it should weigh just shy of 100 grams—which is pretty good for its pod and boom construction! It is launched into the air with a discus-like spin and it can reach altitudes of about 60 feet without too much effort. The REALLY good ones have been reported at over 100 feet.
It is a two-channel plane, only needing controls for the elevator and rudder. There is no motor (obviously) or ailerons (thankfully!). The simplicity of design and control makes this a great choice for beginners.
Here's a V-Tail in Motion
Which is your favorite? V-Tail or X-Tail?
V-Tail or X-Tail
The first choice you need to make when ordering is the tail configuration. I chose the V-Tail thinking that it would be safer when landing. The X-Tail has part of the rudder that hangs below the carbon fiber tube of the body. Owners of the X-Tail say it isn't a problem for them, but I thought I would be extra-safe, just in case.
The drawback with the V-Tail is that the servos that control the elevator and the rudder must be mixed in order have the glider move where you want it to. Check out the video to the right to see what I mean. With the X-Tail, one servo is dedicated to controlling the rudder and one servo controls the elevator. I'll discuss the configuration of the V-Tail and the radio later. With a quality transmitter like the Spektrum DX6i, this shouldn't be a problem though.
ELF DLG in Flight
Opening the Package
First of all, I was shocked at how light the package was. I don't know why, because the whole point of the glider is to be light, right? The wing was carefully packaged in bubble wrap and I was very impressed at how securely the Aerolite covering adhered to the wing. I have had mixed results with the product when I tried to install it on planes myself so I can appreciate the skill it takes to make this so crisp!
The two-part pod is a translucent yellow and easily dry-fit into the wing. The two push-rods were tucked nicely inside the boom. A small bag of parts including some screws and the teflon tubing to keep the pushrods aligned was also included.
Glue the Pod to the Boom
The first step is to dry-fit the pod to the boom. I tested the fit of all of the electronics first just to make sure there were no major issues. The front of the fixed pod section should be aligned near the spot on the boom where it starts to slope. Set the wings on the mounting pegs and then make sure the removable nose pod fits fine. You don't want to mount the fixed pod section so far back that nose pod won't fit.
Once you are satisfied with the location on the assembly, take it apart and sand the boom with 150 grit sandpaper where the fixed pod section will attach. Put the fixed pod section back on the boom and glue at the three attachment points using medium CA glue. Again, don't use a kicker! Make sure that the two predrilled holes for the pushrods remain clear of the glue.
Installing the Control HornsClick thumbnail to view full-size
Installing the Control Horns
The first step to properly assembling the V-Tail is to install the control horns.
Measure precisely per the instructions (13.5 - 14 mm from the outside edge of the bevel) and cut a slit through the control surface. The instructions call for an 8 - 9 mm slit, but I cut mine about 6 or 7 mm. Then, from the opposite side, I inserted the control horn. The slit was too small, but as I gently pulled it through, it lengthened the opening to create a perfect fit. The edge of the control horn should be flush with the outside of the tail foam and secure it with foam-safe CA glue.
Note: Make sure the control horn is fully seated. The horn should go all the way through to the other side of the tail as it will need the entire thickness of the foam to get the best adhesion. One of the most common failures of this piece is due to it not being installed with the entire depth of the tail.
Assembling the V-Tail
I didn't have a protractor handy, so I downloaded a picture of one from the internet and used that as a template. Temporarily tape the two tail pieces together on the back side. This is used to ensure properly alignment on the next step. Bend the tail pieces to exactly 106 degrees and tape it to the protractor. Then glue them together, but do not use a CA accelerator. I put a piece of plastic under the joint to catch any glue that dripped down - it saves cleaning up a mess later.
Now, use a protractor and glue the two tail pieces at 106 degrees. I did not have a protractor, so I downloaded a picture of one from the internet and printed it. I used that as the gauge. Do not use accelerant during this step. Instead, use some low-tack tape like painters tape to keep the tail assembly at the correct angle until the glue sets up on its own. Just to be safe, I kept it taped overnight.
A Cool Trick to Align the Tail With the Wings
Temporarily bolt the main wing in place. Turn the glide upside down and gently rest the wings on a flat surface like the kitchen counter. The V-Tail will automatically align with the wings as shown in the picture to the right. Cool trick, right?
A thin line of CA glue on the boom will tack the tail in place and secure it well enough to turn the glider right side up. Once the CA has set up, turn the glider right-side-up and run the CA glue along the entire boom where it touches the tail. We don't want THAT falling off mid-flight!
Note: The instructions do not call for it, but I went ahead and sanded to portion of the boom that abuts to the tail surface. I then glued to the sanded surface. I can't imagine that there would be a glue failure on an unsanded surface, but I figured it was better to be safe than sorry.
Aligning the Teflon TubesClick thumbnail to view full-size
Install Teflon Tubing for Pushrods
The next step is to install teflon tubing guides. These tubes guide the carbon pushrods and will route along the upper side of the boom. Cut the tubing into ten 3/8" pieces. Slip five on each pushrod and lay the pushrod from the control horn on the tail into the rear of the fuselage. Spread the five pieces fairly evenly along the boom and then tape the carbon rods to the boom. This will secure them in place and serve as clamps for the teflon tubes.
Now its time to glue. I was a little bit nervous about applying the CA. I decided to dip a pin in CA glue and then touch it to the joint where the teflon touches the boom. It was a lot easier to control the amount of glue using the sewing pin and you really don't need much sticking power. After the glue has set up, flip the boom over and repeat the process on the other side. Once the glue has cured, remove the tape.
Install the Pushrod Tips
- Extend the pushrods beyond the tail to give yourself some working room. Take two of the pushrod tips and bend them to 90 degrees using a pair of pliers. (Note: One end of the pushrod tip is tubular, and one end is a thin piece of metal. Don't bend the tubular section.)
- Slip the opening of the pushrod tip over the end of the pushrod until it is well-seated.
- Crimp the pliable metal over the carbon pushrod. Be careful! You don't want to accidentally bend or snap the carbon pushrods.
- Lastly, add a drop of thin CA glue to the connection. The crimping alone should be enough, but it is better to be safe than sorry!
- When the glue is dry, insert the pushrod tips into the control horn.
An Alternative Technique
As an alternative to adding a drop of CA glue, add an inch or so of heat shrink tubing on to the pushrod before installing and crimping the tip. Afterwards, slide the heat shrink over the joint and shrink it per the manufacturer's instructions.
Servos and Battery
The distributors recommend using the following servos: Dymond D47, Ripmax SD100 or the LSX110. I chose the Dymond D47 because that is what they had in stock. I can't vouch that one model performs better than the next.
I did cut off the mounting horns from the servo body. These were just extra weight and got in the way in the tight confine of the pod. A quick dremel tool cut will take care of it. There is no need to worry about being too neat. (Plus, I will do just about anything to get the opportunity to use a dremel tool!)
The Shread Smart Lipo 240 is the battery to beat. This has an onboard charging circuit, voltage regulator and gives warning beeps when it is running low. It is pretty slick and can be recharged straight from another Lipo. It is a lot more expensive than a regular battery, but I like the features. You certainly could go with a less expensive battery.
Locating the Servos
Lay out the assembly before trying to glue it in place. The battery goes in the very front and the servos behind. I made an ink mark on the boom where the battery would be installed, making sure to leave enough room for the speaker in front.
The servos are installed immediately behind the battery. Make sure you know which servo controls the rudder and which one controls the elevator (or elevator and aileron if you choose to use elevons). The pushrods will be cut to different lengths, so now is the time to plan it out.
Once you are satisfied, gently sand the bottom of the servo and the top of the boom. Then use a bit of CA glue to secure the servos in place.
Shread LIPO Instructions
If you have never used a Shread LIPO, find instructions on how to use and recharge it by doing a quick search online.
Install the Battery
The battery is installed in the very front of the fuselage. Simply add a drop or two of CA glue to keep it in place. Again, route the wires to the receiver box. You will need to remove the plastic connector and insert the wires under the bridge before reinstalling the connector.
I used a toothpick dipped in thin CA glue to hold the battery wires in place along the boom, just to keep them out of the way and neat.
Install Pushrod Tips to Servos
Attach Pushrod Tips to the Servos
Using pliers, make a Z-Bend into the pushrod tips that will connect the servo lever arm to the pushrod. Slip the Z-bend into the lever arm. Make certain that the servo is centered by temporarily connecting the receiver and battery, binding the equipment and testing to make sure the servos are centered. You will need to trim the pushrods to length.
Tip: Use the good practice of measuring twice and cutting once. Cut the pushrod too short, and you are guaranteed a trip back to the hobby shop for new pushrod tips and carbon pushrods. Not fun!
As you read above, I secured the servos in place with CA glue BEFORE I cut the pushrods to length. This caused a bit of a problem as the Z-bend tips had to be inserted into the lever arm before the pushrods were set in place.
To solve this dilemma, I unscrewed the lever arm from the servo, installed the tip onto the pushrod and screwed the lever back in again. I am not sure that was any easier than installing the pushrod tips to the servo and then glueing the servo in place—but it worked! Another option is to remove the pushrod tips from the control horns on the tail and sliding the pushrods back far enough to insert them into the Z-bend tips. I did not choose that option as the pushrod tips fit extremely tightly into the control horn and I did not want to try to remove them.
Make sure everything is at the correct length and the control surfaces are in line with rest of the tail. Once satisfied, crimp the tips and put a small drop of CA glue over the joint. Just in case you need to replace the servo at some point, you can unscrew the lever arm from the servo. That keeps the pushrod, pushrod tip and lever arm together. Simply replace the servo with an identical one and everything should fit right back together without having to reassemble the pushrod mechanism.
Now Is a Good Opportunity to Try It Out
With the servos attached and the pushrod tips secured in place, go ahead and power up the receiver. Sure, neither the receiver nor the battery are permanently affixed, but now is a good time to make sure the servo/control horn mechanism is working fine.
There is a bit of extra space now to work out any kinks. Install the remaining equipment and it will be tighter.
Route the Wires Neatly
You really don't need to do this step, but I like to have a nice, clean interior. I took the lead wires from the servos and gently pulled back the three plastic tabs and removed the red, white and black wires from the plastic connector. The three wires were fed under the plastic bridge to the area where the receiver will be located.
Once the wires are through and are free from obstructions (like the Z-bent pushrod tips) reinstall the plastic connector. Make sure the three wires are installed in the exact same location from where they were removed. I took a picture with my camera phone, just to be sure.
Once the wires are securely seated in the connector, press the plastic tabs back into place. On my servos, I heard a little *snap* when they were secured in place. To make sure the wires stayed out the way, I added a small drop of hot glue to keep the neat, clean and orderly. Look at the pictures to the right to see the process.
Both Hyperflight and Kennedy Composites list receivers that work with this model. I was particularly concerned with issues some folks reported that some of the receivers were having problems when used with the carbon fiber boom.
The AR6255, however, is designed to be used with a carbon airframe. It is a 6 channel DSMX receiver, but you'll only use two of the six channels for the stock setup. The receiver is compatible with Spektrum's Telemetry and Flight Log modules.
Installing the Receiver
Fitting the electronics inside the pod can be a challenge. I used the AR6255 receiver and it was a bit too wide for the pod. I could have removed the plastic covering of the receiver and shrink-wrap it, but I decided to gently bend the pod outward to squeeze the receiver in place. It was a very—VERY tight fit, but it did finally set in place.
I have seen two different setups with the antenna. One routed the antenna out the back of the pod which leaves two small "whiskers" unobstructed. The other was to route them underneath the receiver and forward towards the servos inside the pod. Because the pod is made of plastic and not carbon fiber, there should not be any degradation in the performance of reception, but you can bet I will range testing it before hand. I am also tempted use the Spektrum Flight Log to make sure that the receiver was operating up to par.
Balancing and CG
The Center of Gravity should be 3" behind the leading edge of the wing. You may need to add ballast to balance it appropriately. This setting should be tweaked after you fly it for a while. You may find that the slightest adjustment to the CG will drastically change the flying profile of the glider.
Install the Throwing Peg
The instructions call for sanding the throwing peg to fit comfortable with your fingers and then supergluing the peg in place. Some fliers have found that CA glue is not enough to hold the throwing peg in place. Instead, they used an epoxy with micro balloons. I opted to use this approach.
Another tip is to drill two or three small holes in the section of the throwing peg that inserts into the wing. This allows the epoxy to seep into the hole and bind to the other side of the peg. It offers a bit more strength, but is probably overkill.
Note: You don't have to install both throwing pegs. They are so light, it really won't affect the in-flight stability at all if you install the second one or not.
Review of the Building of the ELF
Here is a great review from Aviation Modeler International that discusses the building of this HLG.
Control Surface Movement
V-Tail Diff (up)
V-Tail Diff (down)
Programming the Spektrum DX6i
V-Tail vs. Elevon: There are two options for your DX6i to control this style of tail. I chose to use the Elevon setting for this project.
The Differential function gives you the opportunity to fine-tune the movement of the throws. The recommended amount is 20% (80% down and 100% up) - but play with it on your V-Tail glider to see what is most comfortable for you.
Which Transmitter Should You Use?
Deflection of the Tail Control SurfacesClick thumbnail to view full-size
We are almost done! There are two thing you may do to the removable front fuselage.
- If you used the Shread LIPO battery, you may want to use your utility knife (or a drill bit) to cut a hole where the on/off plug is located. This way, you never have to worry about removing the canopy as you will have access to the plug from the outside. It also minimizes the scratches on the inside of the canopy.
- Tape the fuselage in place using clear scotch tape. You don't want it falling off!
A Finished Project
Simply screw the wing back on and test all of the control surfaces to make sure they are working properly. Take the glider out to the flying field and gently toss it overhand to get a feel for the reaction and controls. When you are comfortable, try an easy discus launch.
This is a fun and easy build for a first time DLG and the two-channel model will make it easy for the novice to control. Intermediate and expert fliers will enjoy the graceful gliding as well!