Mugi, 153% scale with Rev B Material, Tweaked In, Flying Well

This article was originally published in April 2015 without maiden info with incremental improvements over the next few months.    


Rich Caine and I have been having some fun with the regular Mugi Evo for some time now.  So much fun, in fact, that we decided to make bigger ones.  Rich’s original one went in a few weeks ago due to a radio issue but mine is going strong although I have had a couple of close calls.  

The original Mugi Evo is described quite well online, and in an earlier article under Tim’s Tips.  These guys have done a great job in making the design robust and the construction simple for this awesome flier. 

In general, the mugi is built in the flat on a piece of coroplast and then folded up to form the wing.  The airfoil is unsupported but the coroplast is very strong and well up to the task.  I added a spar on both of mine because I wanted to go fast. 

The original Mugi Evo drawing is in mm because it originates in the UK, about 31” wingspan and made from 2mm coro.   This upscaled version is made from a 24x48” sheet and 4mm coro.  My upscaled drawing is at the end of the article.  If you build one, be sure to get the flutes going the right way to exploit the strength of the coro.  Overall Mugi specs below for both sizes.

Mugi comparison


1.5x scale

Bigger motor




































Mugi Overall Specs

A little bit on adhesives.  The online guys want to sell you some special Mugi adhesive but I prefer to use what I can get locally.  Rich used contact cement on his; with this adhesive you get one shot to get things to align properly.  I wanted a bit more work time, so went with Foam Tack on the original one.  Since the price of Foam Tack has gone a little crazy recently at $16/bottle, I used Plumbers Goop on this one at $5 a tube from Home Depot or Walmart.  It is thick like shoe goo, and sticks well to the coro.  Pic below.

used goop on this one.jpg

I used this on the Mugi this time.  Nice!


Seems all the hobby shops have their own private label version of this adhesive, Victor’s version is shown below but aero micro had the same stuff with their name on it.  Might be good for some things but I didn’t think it had the strength I wanted for Mugi. 

this glue wasn't very good.JPG

Not up to Mugi standards for me

I got the coro from TAP Plastics in Pleasant Hill who stocks it.  I had to buy a full 4’x8’ sheet, but they cut it like I wanted, and it was only $25.  I got material for 2 scaled up Mugis.  It takes one piece for the mugi wing itself and another for all the pieces, so $12 for coro. 


Below is a pic a few steps in, before the spars are glued down.  I layed it out per the scaled plans, drew on the lines made the cutouts including the M around the pusher motor, and scored the fold.  The M reliefs around the motor are to help reduce prop noise.  I used mounting squares under the servos then cable tied them, so that they would not twist under load.  All except the mounting squares is per mugi website guidance. 

pre spar.JPG

Mugi pre Spar Glue


For spars I got a 6mm carbon rod which was full length.  I thought the rod had a little too much give, and I had a shorter piece of 7mm tube on the bench, so I used it to strengthen the center section where most of the load is.  Using both spars together I was happy with the stiffness.  Mostly I used what I had easily available. 


So the spars will dry flat, I weighted them down to cure.  My wife had some old granite tile samples she was going to throw away.  Since they were reasonably heavy, I repurposed them as building weights.  I also had some pieces of half inch steel, painted white, that I have used in the past to weight down the trailing edge of a wing while glue dries. 

 I used a fan to push the adhesive vapors out the window. Goop is made from petroleum distillates so it has a strong smell, but that’s why it’s good. 


with loose spars.jpg

Spars weighted down

Below, post spar gluing with pushrods installed.

ready to fold.jpg

Post spar gluing with pushrods installed


The site recommends carbon tube for pushrods which I’ve used in the past for weight reduction.  I didn’t like the ends I could get locally, so I just used 2-56 threaded rod with a Z bend at the servo side.  Since the servos are not accessible after you fold up the wing, I didn’t want any possibility they could come loose.  I did use ball clevises on the aileron side.  For servos I used Hitec HS225MG servos.  I like the metal gear.     

I setup the radio and servos at this point so the arms are centered properly and they go the correct direction. 


Next is the folding operation, the most critical operation of the build.  This part can be a little challenging as you need a second set of hands.  I recruited my wife for about 5 minutes to get it folded, then used a pile of clamps and let it dry for a day to let the adhesive cure thoroughly. 

Since it is a busy operation to do the fold as well as measure the max camber line, a trick I used is to cut and glue in some spacers at the max camber line, shown in pics below.  This way you don’t need an extra hand to use a ruler, just fold up and over and the camber height is right.  The can be removed later if desired.  So far, I’ve left mine in since they didn’t get in the way of anything. 

spacers 2.JPG

Spacer location on each side near servo


spacers for wing fold.JPG

Spacer Closeup, 1.5” tall at max camber

Once it’s folded, I clamped the trailing edge to let the glue dry for a day, pic below.


Folded and clamped

Next, more doublers go on on the nose and the top and bottom down the center of the wing.  Then the vertical fins and ailerons are installed. 

To check CG, people typically put a mark under the wing, then struggle to get their finger on the marks.  I figured what I need is an indicator I can feel with my fingers, so I get these little adhesive backed rubber feet to use.  Been doing this for some many years now and it’s really convenient.  You can see them in the pic below along with the cable ties holding down the servos.  These rubber feet are available from Home Depot or Lowes or other places.

The CG scales directly and is shown on the plans.  

CG markers.JPG

CG markers you can feel

Motor.  I planned for about 700w and chose this HK motor from the US warehouse at $22 and 2630kv.  We’ll see how this works out.  

I really don’t think such a high kv is needed, maybe 2200 would be enough, but this is what they had. 

Since I had to add 1.5lb to the nose to make CG, I ordered a backup motor, 1100w and a bigger ESC.  It comes from international HK warehouse so will take a while to get.

Below is Mugi completed and ready for paint for better visibility. 

mugi pre paint.JPG

Mugi Pre Paint, No Motor

I wanted some contrasting color to help visibility in the air so I added some white strips on the top.  At least for me, red and white are a good combination.  Pic below. 

mugi painted.JPG

Big Mugi, Ready to Go.


In checking the CG, I was a bit surprised to see it took so much nose weight to hit CG, 1.5lb.  Wow!  There is a lot of material and the motor all behind the CG and not nearly as much near the nose due to the extreme sweep angle.  I had some steel cut which would fit well on both sides of the battery and as far forward as possible.  I figured varying thicknesses (0.25”, 0.375” and 0.5”) of 6” x 1”.  The 0.375” thick ones did the trick and I glued them in permanently.

Pleasanton Steel is in Livermore on Brisa, close to my home and the field and will cut pretty much anything you want.  I kept to standard width and thickness and had them cut to length for the desired weight.  I made a spreadsheet to help me to figure out the dimensions I needed to get close to the desired weight.  If anyone wants a copy of the spreadsheet, just let me know. 

Launching.  I put a hook on the front so I can use a bungee due to the weight.  I used music wire of a diameter that would fit inside one of the flutes on the coro and originally taped it on, but later dug out a channel in the lower doubler.  Pics below

hook pieces.JPG

Hook Pieces

hook assembled.JPG

Hook Assembled with tape on plate


hook mounted.JPG

Hook Mounted, tape on method

new hook, new bungee.jpg

Realigned Hook placement, channel in doubler


I also put a handle on the bottom just under the CG for hand launch which also acts as a skid plate on landing.  I think at 5lb it might be tough from hand launch, but the skid is a good thing. 


RevA materials

Well, the maiden for this one took some time.  I initially used a 700w motor and propped it for 900w with a 5.5x4.5 apc e as a quick loop around the field.  My 3/8” bungee pulled it up, but I had too much reflex and couldn’t trim it out.  So I took it home and remade the pushrods;   this time I bought the carbon tube with titanium ends.  I had to cut into the wing to reconnect the servo arm which didn’t make me happy, but I did it anyway.

The old motor was under sized, so I upscaled to the following 1100w 2200kv 36mm motor from hobby king.  

The mugi way to attach the motor is with cable ties, but the old motor it would rotate at high throttle so I needed to come up with a new method.  I had initial trouble finding a motor mount for the 36mm motor, ended up on a model boat site, and bought the one pictured below.  It had a lot of holes in the mount and I was able to get the needed thrust angle.  I had to rotate the shaft end block 180 degrees to get the needed angle.  If you look closely, you can see water ports that the boat guys use for cooling which are pointed down.  I then cable tied it through the coro. 


mugi 36mm motor mount.JPG

Boat Mount for 36mm Motor


I used a 100A HK speed control and 4S4000 batts that I had from a 3D ship.  1100w on 4s is about roughly 75A.  On an 8x4e prop it was pulling 100A, so I propped down to e at 81A and called it good.  To hit CG, I needed to add an additional half a pound to the nose for a full 2lb of nose weight and 5.6lb AUW.


The First Maiden

 At long last, I finally got to the maiden day.  After a few prop adapter adjustments and bungee adjustments the new bungee pulled it up at max 3x bungee extension.  Maybe “pulled it up” is a bit of an exaggeration since, when released, the bungee pulled the wing off the ramp fine.  But the wing had inadequate speed off the ramp and sagged to the ground,  the bungee pulled it along the ground for a while, then the wing came airborne, so I throttled up and started flying.  I was a bit surprised it would come off the ground like that, but it was flying so I went with it.  Clearly, not like I wanted and I needed to work on the launch mechanism a bit. 

Since my old 3/8” bungee wouldn’t pull it up anymore, so I chatted with my bungee guy and bought a bigger half inch bungee from the following website.  This guy is very good and makes bungee launch equipment for the UAV industry and military applications. 

A bit of flutter.  After a couple of low speed passes, I started to throttle up a bit and noted some control flutter noise and we could actually see it.  Throttled back and landed.  The linkage was tight and packing tape hinge had little play, so decided need to do some aero dynamic balance of the elevons to get the CG in front of the center of pressure (CP).  Conveniently, the coro has open flutes along the edge so I found a piece of 1/8” music wire and slipped into the outside front flute of both ailerons which seemed to take care of it. 

The launch deficiency seemed clear, inadequate lift coming off the ramp.  So, after some discussion with the bungee guy and some colleagues, I made two changes;   a).  longer ramp (double length, same angle)  and b). moved the hook mounting back to get higher angle of attack off the ramp. 

Since the ramp was pretty much falling apart after several years of use, I decided to just rebuild it from scratch and retain the bungee release mechanism which worked well.  This time I went with ¾” pipe and kept the same angle, which yielded twice the vertical distance at the end (24”) to allow for some trajectory sag.  I moved the hook back a couple of inches to 5” from the nose.  Following are some pics.

mugi hook 5.jpg

New Hook Position, 5” from nose

big mugi full maiden.jpg

Ready to Launch, New Ramp


The Real Maiden

You can see from the ramp proportions compared to earlier pic how it is upsized.   The combo of ramp extension and hook movement did the trick and the bungee pulled it up nicely in a little headwind.  Getting to full throttle, I did see and hear a bit more flutter although not as bad as last time, so I cut some more music wire and slid it into the inner side of each aileron up to the control horn.  Hopefully that will cure it.

There is a vid of the initial half inch bungee maiden on youtube.  Here is a link.

The APC 7x4e prop tachs at 23,000 rpm, which is a bit over the APC recommendation at 20,700.  I could prop down a bit more, the next standard size is 6.3x4 with max rpm of 23,000, but with less loading I’d expect the rpm to increase. Not good.    More on this later. 


The hatch.  I did some improvising regarding the hatch. Early on I was trying to use tape as had worked on the smaller one, but it just wasn’t effective with the 4mm coro.  Even using the wide tape, the hatch was always open at landing.  Since the 1/8” music wire fits in the flutes, I made a couple of Ls and cut out flutes as shown below.  Now I have a nice dual slide latches that holds the hatches closed well. 

mugi hatch.jpg

Mugi Hatch

A few more changes

Flutter and hinging.  While I think the weights in the leading edge of the control surfaces helped the flutter some, my control surface hinging leaves a bit to be desired.  The tape seems to have continued to stretch on subsequent flights.  So, I installed a more robust hinging method using Robart Super Hinge Points on the elevons.  These big points fit well into the flutes on the 4mm coro.  I also added storage tape top and bottom, although now it serves more for sealing the surface gaps rather than mechanically attaching the elevons to the wing. 

I was a little surprised at the need for enhanced hinging as tape only worked well for all my Ritewings.  I think this worked because all the Ritewings are symmetrical airfoil and there is no reflex needed.  I think the asymmetrical camber on the mugi needs a bit more substantial hinging.  Pics below. 

mugi hinging.jpg

Hinge pic for scale in 4mm coro

rehing left.jpg



The launching has always been a problem, below is a pic of the bungee journey I’ve taken, 3/8” dia on the left, half inch in the middle and 5/8”(mega) on the right.  And they actually have one that is bigger than the mega!  The 3/8” is 15 ft long with a 30ft nylon leader.  The others are 24’ long with similar leader. 


 3/8”, Half Inch and 5/8” (mega) Bungee

The half inch bungee, at 3x (max) extension, would pull it up if there was a bit of headwind, but the wing would still sag a bit leaving the ramp.  In calm wind, the wing would sag and hit the ground.  No fun when you want to fly.  I wanted a launching method with some authority even in calm wind.  So, I bought the mega at 5/8” diameter mega bungee. 

First attempt with new bungee at 3x extension, pulled the taped on hook plate right off the mugi.  So, I dug out a channel for the music wire hook in the belly plate of the mugi which is glued in lengthwise.  Then put coro over the top and taped as shown below.  This arrangement held well, and pulled the mugi up with authority.  I later removed this tape and glued in the hook, replaced the top plates, and redid the tape. 

The recommendation is 3x extension is the max safe stretch.  I’m currently at 2x stretch on the mega for a reliable launch, see vid below.  Really it’s pretty much like a rocket!  Vid below.

I have a strong launch now with mega bungee, 5/8” diameter.  The mega pulls it up very nicely, even with no headwind. 

For the mega, I had the manufacturer break it into two 12’ pieces so I can use half the length as well.  I’ll be trying the half length shortly to see if it is enough. 

You will note on the vid that the ramp deflects pretty severely at launch time.  So I added some bracing on the sides to help out, new pic below. 

Move hook.  This should give a bit more angle of attack on launch, I’m at 6” from nose now. 

Battery hold down.   On the second flight, I noted that the batteries had come loose in flight.  I suspect that the rather abrupt launch force pulled the Velcro only battery attachment away.  Later I dug out a couple holes to put a Velcro strap around the batts to the bottom of the belly pan and over the glued in hook.  Now they are solid.  I haven’t had any trouble with the addition of the strap to hold the batts into the bottom Velcro.    

Prop change type.  The last change is to move to a non e prop, sport prop.  I had to drill out the hole on the 7x4e which made me a little nervous because I’m over absolute max RPMs.  While the apc sport prop still needs drilled out, the remaining wall is more substantial.  Below is a pic. 


                                                                             7x4e on the top, 7x4 sport on the bottom


The drill out needed for the prop adapter is 0.320”, resulting in a wall thickness of 0.085” on the 7x4e.  Ouch, just too thin a wall!  As mentioned earlier, the max RPM on e props is 145,000/prop diameter, or 20.7k RPM for the 7x4e and I measured 23k, just not safe.    

On the 7x4 sport, the post drill out wall thickness is 0.180”.  Max RPM on the 7x4 sport is 190,000/prop diameter or 27.1k RPM.  I measured 24k RPM so all is good here. 


All tweaked in, finally

Well, I finally got it tweaked in with the items described above and a few more things listed below. 

7x4 sport prop worked well.  76A, 24,000 RPM

Cut elevon roughly in half, so half the area and no more flutter.  5 degree reflex is pretty much perfect. 


5 degrees of reflex seems almost perfect


This was more of an interesting ride than expected, and it took longer that I had anticipated, but all was good fun.  For this one, it was the journey.  I did learn a few things in sorting out all the issues and that kept the process interesting and even more gratifying to get it in the air. 

If you are interested in Mugi, let me know, I’m happy to guide you.  A Mugi of any size is a ton of fun!  It is very satisfying for the builder as it is quick and flying is also fun if you like the speed.

Happy Flying

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