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Mar 06, 2011, 06:11 AM
15 again
Build Log

Scratch-built 120% RC Hangar Rat

The whole story started with a Christmas gift, a micro toy helicopter: the first model I held in my hands in 17 years. I was amazed at how small RC parts have become since I last had my hands on an X-Acto knife. It was not long before the urge to build won the upper hand and I had to succumb, and start looking for the right plane.

I wanted something small and easy to construct, but sturdy enough to fly outdoors in a city park on a calm day. I soon came across the Hangar Rat, a rubber job from 1979, and it was love at first sight. The Hangar Rat's story itself is a wonderful read; here are some links:

How it all started:
A beautiful RC conversion: (in Greek, but hey, we have Google Translate)
Greg Covey's RC conversion:
John Morrison's King Rat: (a 200% scale-up with impressive craftsmanship)
A 150% build: (Too bad the poster seems to have disappeared from RC Groups, I'd love to know more)

So the plan was clear: build my own Rat, using a Walkera transmitter (the brand of my heli) and recycled RC heli parts. The Rat is now nearly complete, but I'll post several messages about the process below. I hope to inspire you and maybe help with your own project. Enjoy!
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Mar 06, 2011, 06:36 AM
15 again

Nailing it down

OK, so I'm building a small plane from scratch. How large should it be? What will it weigh? Will it fly? Lots of tortoring questions, so I started gathering my facts to nail down the design.

I thought the heli will give me an idea of what can be made to fly for 10 minutes or so on a 1-cell 500mAh LiPo. The Walkera CB-100 weighs at 66g, but I'm building a plane that needs less thrust than its own weight, so I can go higher than that.

Starting from the other direction, I tried to gather some info about the original Hangar Rat. Here's what I came up with:

Wing chord: 10 cm
Wing span: 50 cm (projected span less b/c of generous dihedral)
Wing area: 5 sq dm
Weight: 15 g
Wing loading: 3 g/sq dm

The weight was the hardest part to figure out, as it's unfortunately not indicated on the original plans I went with the info that I found on Greg Covey's page.

Now the original is a *very* slow indoor flier, I sure want to go for faster than that, which means higher wind loading. But just how high? Here I started digging into info about slow park fliers, and the info I found on the King Rat page was also great help. I soon had my target: 9-11 g/sq dm.

But what will my plane weigh after all? For the construction I used the original's 15 g, but considering I'll need something bigger and sturdier, and probably also bulkier for lack of skill, I generously assumed 40 g. Looking at the scarce information there is about Walkera parts, I figured the electronics, avec motor, prop and battery, will add up to about 35 g. That makes 75 g total.

For a wing loading at around 10 g/sq dm, a 120% scal of the original "looked about right," giving me these parameters; it's still a slow flier, and if I manage to build lighter, it will be slower still:

Wing chord: 12 cm
Wing span: 60 cm
Wing area: 7.2 sq dm
Weight: 75 g
Wing loading: 10.4 g/sq dm
Mar 06, 2011, 07:03 AM
15 again

The Plan

No, I don't mean the BSG movie. The Cylons sure had much more of a plan than I did when I started. I went with Starick's drawing, which not only has great detail, but is also beautiful. I hope attaching it here is OK; it can be freely downloaded from several sites on the internet.

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Description: Hangar Rat - Max Starick Variant 2, c.1980
Aeromodeller, Vol 63 No. 757 20 Nov -17 Dec. 1998
Original image:

Here's confession time. I did not print the image; I had no idea if I could print it in perfect 1:1 proportion, as the image has no scale in it. I simply held a piece of paper against my monitor and drew the lines manually, then measured the wing chord, which I knew to be 10 cm, to know how much I needed to scale things up. What was supposed to be 10 cm in the plan came out at 5.4 cm on my monitor; from there it was simple arithmetics, and I went on to draw out my own plans using a ruler.

Only later did I realize that my monitor's aspect ratio may not be true either, so there's a fair chance I'm building a model that's a 120% scale only in the horizontal direction, and something different vertically.

The trickiest part for sure was the airfoil, which I drew by hand based on a few calculated points. I doubt it has much to do with the original, but then again, with this type of ultra small and slow-flying wing, it hardly matters a lot. More on that later.
Mar 06, 2011, 07:19 AM
15 again

Tail assembly

Building time at last! I laid out my drawing on the building board and covered it up with a quasi-transparent foil from my grocery's veggie section.

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Description: Vertical stabilizer and rudder drawing, covered by veggie bagName: sDSC01148.jpg
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Description: Finished tail assembly parts, plus a bit of wing

The outer edges are made of 2x3 mm hardwood (pine), which is probably quite an overkill here. It was the inexperienced builder's insecurity that made me go for this extra robustness, but balsa rods of the same dimensions, or even 2x2 mm, would probably have been enough, and saved me several grams.

The cross-beams are 2 mm cutouts from a 0.8 mm balsa sheet. I doubt they add any strength to these tiny structures, but they look great.

Lesson learned: Lighter is enough. Go for balsa next time. Also much easier to work with.
Last edited by glu; Mar 06, 2011 at 07:24 AM.
Mar 06, 2011, 07:47 AM
15 again


It gave me a bit of a headache to figure out how to hinge the control surfaces to the horizontal & vertical stabilizers. The factory-made plastic hinges out there are clearly too big for this small structure. Scotch tape is what foam builders seem to use all the time, and a great plus is that they also seal the joint, but I thought they would look really silly in a kind-of-vintage, Jap tissue covered job. The Greek Hangar Rat page came to the rescue: hand-made hinges from thin electric wire!

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Description: Hinges from thin electric wireName: sDSC01159.jpg
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Description: Drilling holes for the hinges

Each hinge consists of two parts: a spiral, plus a single wire inserted inside and bent 90 degrees on both ends. I created the spiral by winding the electric wire around a thin steel wire, which I'll be using at the end of the push rods later.

Drilling the tiny holes horizontally into the 2x3 wood takes a bit of skill and lots of messing around to find the right position for the holes on the opposing parts. I made the mistake of drilling some of the holes at an angle, and not perdendicular to the edge. I don't know what I was thinking really, but it was a bad idea because it's impossible to push the wire in later, and I had to drill extra perpendicular holes.

I set aside the four structures and the five hinges (two for the rudder, three for the elevator) to assemble after covering. This was yet another bad idea, fitting the covered pieces together and glueing in the wires was a mess.

In order to give the rudder and the elevator clearance to move I sanded the 2x3 mm wood symetrically, adding a 45 degree angled surface on both sides. I was smart enough to do this before covering, but not before building the structure itself. Again, sanding balsa would have been easier than sanding hardwood.

Lesson learned: Sand the rudder and elevator edges to an angle before glueing the frame together.

Lesson learned: Join stabilizer with control surface together using the hinges before covering. Drill perpendicular holes, and allow some 2 mm of the wire to stick out on the inside. You can then bend the wire on the inside of the structure and glue to the wood; much simpler than glueing into the holes after covering.
Mar 06, 2011, 08:12 AM
15 again


The free-flying, rubber-powered Hangar Rat has the control surfaces covered on only one side, and the building instructions invariably say not to dope the Jap tissue ("Rats don't need tight skin"). I was wondering about the reason for this, and came to the conclusion that it's mainly in order to save weight. The extra tissue to cover both sides and the weight of the dope probably makes a perceivable difference on a 15 g model, but I doubt the extra gram matters with my 75 g target, so I deviated here. All parts of the tail assembly are covered on both sides, and I did dope the tissue.

I also think doping the tissue on the main wing makes a difference for a faster flyer, probably preserving the pressure difference between the two surfaces better, though this is only speculation.

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Description: Jap tissue glued to a single edgeName: sDSC01177.jpg
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Description: Fully covered, doped tail assembly parts with hinges installed

The dope I used is quite thick and dries very quickly, which left me barely enough time to glue the tissue to a single edge at a time. I diluted the dope to double volume with nitro thinner for tightening. I wonder if I should try glueing the tissue to the wood with white glue the next time, as that dries slower and would perhaps allow me to pull the tissue tighter with fewer wrinkles.

I cut the tissue with an X-Acto knife along a ruler and tore a stripe off to bare the wood where the two parts will be glued together, and where the horizontal stabilizer will be glued to the stick. In some places this left me with a stripe of only 0.5 mm where the tissue is glued to the wood, but it seems to hold pretty well.
Last edited by glu; Mar 06, 2011 at 08:25 AM.
Mar 06, 2011, 08:37 AM
Registered User
Hi..that's a nice projet ...i realy like that King Rat Project did you find out if the kit or plans are sold or found anywere..
thanks, i will be looking at you projet
Mar 06, 2011, 09:03 AM
15 again
Originally Posted by Lutin View Post
Hi..that's a nice projet ...i realy like that King Rat Project did you find out if the kit or plans are sold or found anywere..
thanks, i will be looking at you projet
Hi Lutin, thanks for reading! I didn't find a whole lot of kits for the Hangar Rat, I guess because the plans are out there for free so the maker wouldn't have IP on the kit, and also coz its such a simple plane to build. Here are the ones Google did find for me: (search for "hanger rat")

I posted the link to Starick's plan, that's perfect to go by to build your own. Good luck!
Mar 06, 2011, 10:30 AM
15 again


My main concern with the wing was that the original design might be too flimsy for my purposes. I'm building a much heavier thing here than the tiny rubber variant, so I expect bigger forces trying to destroy the plane mid-flight.

The dimensions are scaled up 120% and a bit more from the original. Instead of 25 cm, both wing halves are 30 cm long. The ribs are placed 5 cm from each other, and I got the 30 cm length by adding an extra wing bay. The wing chord is 12.5 cm: the ribs themselves have a 12 cm chord, plus the LE and TE.

I went for hardwood LE and TE, which I sanded to fit with the ribs and create at least the semblance of a decent airfoil. The LE is a 3x3 mm rod, the TE is 2x3 mm. Sanding a hardwood rod to have the desired cross-section is not fun... The wing is open bottom, but in the name of robustness I added a 12 cm rod between the TE and LE at both ends and under the rib in the middle.

To get the ribs I cut out a full-depth template that I hardened with some white glue, and I then cut the ribs using the template. I had to cut around both the upper and the lower edge because the LE end of each rib is 3 mm high while the TE is 2 mm; this meant that the "sliding" method did not work for me. I used a 0.8 mm balsa sheet.

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Description: Glueing the full-depth root rib to the LE and TE with the dihedral jigName: sDSC01144.jpg
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Description: Glueing in the ribs, with the wing tacked down to the building board with Blu-TackName: sDSC01171.jpg
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Description: Joining the two fully built and covered wing halves

I first glued the full-depth root rib to the LE and TE using the dihedral jig, and then the 2x3 mm hardwood rod at the far end. At this point I tacked down the wing to the building board and glued in the internal ribs, adding a hardwood rod in the middle, and a 2x0.8 mm balsa rod under the other ribs. I also added a tiny piece of balsa vertically where the airfoil is thickest, you can see this in the third picture.

Covering was less challenging than I had thought, but the dope did dry very quickly and I had to be fast after applying it to the wood to glue the tissue to the structure. At the end I applied the thinned dope to the whole surface, holding the LE and TE flat against the building board with hefty books to avoid warping.

I found that the trickiest part is making sure the root rib is joined to the LE and TE at the right angle. With my wing this didn't work out very well. If the wing is "flat", i.e., suspended so that both tips are at the same height, the root ribs are not precisely vertical. This turned out to be a problem when I attached the cabane struts later.

Lesson learned: Again, I built too heavy. OTOH the 0.8 mm balsa for the ribs is too weak. Better use 1.5 mm balsa for the ribs, maybe even make all ribs full depth, and use balsa for the LE and TE. Also simpler to sand into profile.

Lesson learned: I used Blu-Tack to fix the wing while glueing because I only had a hard building board that doesn't take pins. This doesn't work very well, though; there's clearly a reason why everyone else is using pins
Apr 20, 2011, 03:57 PM
15 again


It's been quite a while... I have long finished the build in the meantime and the Rat has even been maidened - so here goes.

Most of the electronics are recycled from a broken Walkera 4#3 heli I bought off the previous owner online. These used to be brushed main and brushed tail motor, but the one I bought is an upgraded variant with brushless main motor.

Receiver: RX2406C
Servos (elevator & rudder): 2 x HM43Z24
Battery: Walkera 500 mAh 1S LiPo

The rest came from HobbyKing.

Motor: AP05 3000kV 5.4g
ESC: Turnigy DP 3A 1S
Prop: GWS-DD-5030

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Description: Electronics and prop

Whether the ESC would work with the Walkera receiver was a bit of a gamble, but it turned out just right. The idea is to connect the ESC to the main motor's throttle output, and the two servos to the aileron and elevator channels. "Aileron" on the Rat being, of course, the rudder.

The Walkera receiver has a built-in gyro that could be a bit of a problem, but it only affects the tail motor output, which I'm not hooking up with anything at all.

The 5.4g motor allegedly has a thrust of 110g at 7V, but in my 1S setup it feels to be much smaller than that. Whether that's up to motor itself, the ESC or the Walkera LiPo is anybody's guess.
Apr 20, 2011, 04:40 PM
15 again

Final assembly / balancing act

The wing and parts of the tail assembly all built up, the motor mounted on a front board, and all the electronics in place - everything was now ready to put the pieces together.

The balancing act, of course, is about getting the CG right. The original plans put it at about 60% of the chord from the LE, which seemed way, way aft to me. I was wondering what may have been the reason for this; here are the ideas I came up with. The rubber-driven Hangar Rat is probably intended to have a lifting tailplane, so the plane is meant to fly in an upwards pointing attitude. It may also be that the highly cambered wing with an uncovered bottom has a stronger pitching moment than closed-bottom airfoils, but this is pure speculation.

In any case, I decided to put the CG rather at 40% of the chord from the LE. Expecting to mount the receiver, battery, and probably also the servos under the wing, I placed the two struts much farther apart to give myself more room to play around.

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Description: The struts being glued to the stick with epoxy Name: sDSC01220.jpg
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Description: ESC, battery and receiver mounted on the stick Name: sDSC01190.jpg
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Description: Tail skid and the elevator's horn Name: sDSC01221.jpg
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Description: Servos with control rod joiners Name: DSC01223.jpg
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Description: Control rod in the rudder hook

Placing those struts so far apart turned out to be a mistake. As I started experimenting with the right location for the wing, motor, servos, battery, and receiver, it soon turned out that the plane was rather tail-heavy. The only way I could fit it together was by placing the servos behind the rear strut, and all the rest just ahead of the front one. With the two struts closer together I'd have had a lot more options to arrange things.

What with the servos placed so far from the receiver, I had to extend their wire by cutting it in the middle and soldering in an extra piece. I could not, of course, get the ultra-thin wire, so this added an extra 1 or 2 grams to the total weight - an extra consequence, ultimately, of the heavy tail assembly.

Apart from this, the rest was relatively easy. Once I figured out all the locations, I went on to epoxy the structs to the stick and mount the servos and electronics with pieces of double-sided adhesive band. The wires are held in place around the stick by Scotch Tape. The third image shows the tail skid, made up of the same bent steel wire as the landing gear. (Steel wire is hard to bend!)

The front plate holding the motor is glued to the front of the stick, supported on both sides by two tiny pieces of spruce.

The control rods are carbon fiber. At the servos, I attached the rod to the connecting piece that comes with the servo by winding thread around the two and fixing it with CA. At the back, a thin bent steel wire goes into the horn. I attached the steel wire to the rod with a tiny piece of Scotch Tape, and after adjusting for neutral trim, I fixed this end too with a drop of CA.

Lesson learned: Build lighter, build lighter! The reason the stick ended up being so tail-heavy is because of the heavy tail assembly, and this effect is magnified because it's at the end of a long arm, far from the CG. Also, the stick behind the wing is an overkill here - the thickness that was a must on the rubber model is not really needed. I would surely go for a CF stick for this part now.
Last edited by glu; Apr 20, 2011 at 04:52 PM.
May 14, 2011, 07:40 AM
What goes up must come down..
I am working a semi similar model of the Hangar rat except that mine is using thermofoam wings that I had in the closet for years.. the way that I draw the uprights is instead of like yours a single ones I double these so prior to final the location of the wing to the stick it could shift fwd and aft until proper CG is acheived.
May 14, 2011, 09:05 AM
15 again
Thanks for the reply rodair, and good luck with the build! Post a build log or a few pictures if you can.

Leaving a clearance that allows you to move to wing forwards and aft until you get the CG right is a good idea; I chose not to do that to keep the distance between the wing and the tail fixed. Out of my plane's 75g of weight the battery takes up 15g, so moving the battery leaves a lot of room to play around with the CG once the wing is glued in place.

What's the section of your foam wing? The rat being a very slow flyer a curved plate is what appears to be advised to get docile characteristics at such very low Re (30k-60k). I'm curious how a different wing will behave.

Some of the things I'd do differently now are:
  • Add a U bend to the wire at the end of the control rods to allow for some manual trimming
  • Fix the motor with screws and put some foam-like material in between the motor mount and the fireboard so I can adjust the right and down thrust's angle. (Where do you get such tiny screws?)
  • Find a way to mount the wing that allows me to experiment with its angle of incidence too

I've flown, crashed, and repaired the plane quite a bit since my last post, I should be writing an update about that soon. After the initial learning curve it's a great little plane to fly, and it's sure teaching me a whole lot. Flying it is mostly limited by the windy weather we have around here this spring.
May 14, 2011, 12:42 PM
What goes up must come down..

AFT FWD picture added..

Here are some pictures here although I don't want to "hack" your thread here.
If you look at the pictures you'll see that one the parts are moved AFT and then FWD they simply slide on the Fuselage stick.
That gives me about ~1 inch play + incidence adjustment if I do only a rotation point at the back and use a good pin in the front for test flights?
The servos' will remains fixed, the fuselage AFT section was left longer here until some more build are made to the model (TAIL + STAB + linkages...).
These wings are having a lot of undercamber ( as one can see by the Center Rib hold down and they do have some washout ~ 3 deg.
These were made by the defunct TakeOff 2000 Inc. it as called: THERMOFORM BIG WING set, I gather a few and they were colecting dust in a closet until re-discover these when I moved recently as well as some radio gears that came from diff. ARF models and my idea here was to make a simple "test bed" before I start with a more complex CAD drawing this will depend on how she will fly I guess it will stay "as is" simple fast building yet hopefully FUN! etc..
Hope these help.

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