Back to Balsa Roots - Seeking out less covered designs - The Duranita


CrossThread Industries
While electric foamies from FT got me back into the hobby/sport/obsession of model building/flying in early 2014 - and how it did - I am now running parallel foam and balsa programs.

I thought I would write up a build thread on this balsa model in case there is any information that is helpful to someone just thinking about trying their first balsa plane - whether electric or fuel, which this one will be.

The genesis of this build started as a response to a build-off challenge on another site. I chose to find a plane that isn't covered that much or at all. The build-off challenge basis was to build a plane appearing in a US publication up to and including 1965. My sense of nostalgia has been raised from having a ball on the site that SteveWMD runs, reviewing all the old plans that he is posting up there - the earliest plan I've found going back to 1911, a Wright Flyer only 8 years after it first flew. Imagine that - a plan that is 104 years old.

In the world of smallies - 1/2A and A engines up to about .10 ci displacement, a few prolific designers are found on the outerzone (OZ) site. Ken Willard's designs emerged as a bunch of fun-to-build-&-fly planes that take modern miniaturized RC very well.

I actually got started unofficially on this build well back some 7-8 months ago, starting to build up tools for a bench that will get me good results and speed my progress. A mini table saw and a scroll saw from Dremel have really helped out, cutting out patterns and parts. The table saw let's me buy sheets of balsa locally and rip them to sizes I need. The big benefit is I can buy the hardness of balsa I need, make sure it is a straight piece, and cut many straight pieces from it - rather than sorting through balsa for many straight pieces. Also, I had to get a CA glue, CA kicker, wood glue, and a metal building surface. I went to Harbor Freight and got a bunch of magnets for the sheet metal building surface - I had never built this way before but reading up on it, I thought it sounded fast and easy. No sticking pins through and aside balsa pieces into a soft building surface like cork or pine. It turned out to be a good investment, it is really fast to build this way, and very adaptable as you can move magnets around fast to accommodate whatever you are building.

As I started building, I started computing when the last time I built a balsa plane was - and it was at least 24 years ago. I have that unpowered sailplane sitting in the corner of this room, right now - used to get a fare amount of use, took a few repairs well, and is ready to fly.

If you want to learn a something about balsa, I was pointed to a really interesting place on the Sig site, that you may be interested in, too, that has tech information about the different cuts of balsa you can buy - different cuts are good for different things. A really good article on balsa:

You have to get used to identifying the different cuts, and gauging its thickness and strength. I needed hard balsa strips as spars for my wing, but soft balsa for wingtips. Soft is generally lighter and easier to shape, hard is heavier and stronger.

The plane I chose to build, and now is partly done, is the Ken Willard Duranita from 1953, printed in Model Airplane News (MAN). It is a funky biplane, with a flat (no dihedral) smallish lower wing, and a gull upper wing. I thought that was unusual. When I did some googling about the plane, I found virtually no coverage of this plane, whatsoever. That really got me interested - hopefully, no coverage does not equate to not fliable(!). I wish we still had Mr. Willard with us so I could find out more about this design from him. He is in the AMA Hall of Fame.

The desig was originally designed for a .049 diesel engine, as a Free Flight model (FF). There was a lot of skill in designing, building, and then trimming an airplane like this back in the day to get about a minute of flight from it. More than that and it might fly away on a thermal or gust of wind. That seems unbelievably common in the accounts I read about FF flying, which is still active today - just not in populated areas. FF is still big in the UK - great videos on youtube.

Anyway, here is a scanned photo of the plane from the original article:

Screen Shote Page 14.jpg

There will have to be mods to take it from FF to RC as it does not have a true rudder, elevator, or ailerons. I am scaling up the design to 115% and so will have to strengthen the spars, leading edge, and trailing edge.
In the next entry I will post a screen shot of the plans, and what I had to do to get a copy of them in order to build on the bench.

My build criteria for taking plans of OZ is to build planes not covered at all and possibly don't even have a photo of it on the OZ site - plans only. As mentioned, this fits the bill.

The Duranita is an interesting crossover plane from the time with FF was still going gung-ho to the advent of better small engines, especially the ubiquitous Cox engines. So some of the lines have an old-timer look I want to preserve. And, RC gear was gradually becoming more available.

That's it for now, let me know if you have any questions along the way.



Some guy in the desert
Now that's a unique looking design. Those lower wings almost look like an afterthought :D Looking forward to this build for sure!


Senior Member
After I read your post I was thinking i've seen that somewhere before. Turns out it was mountain models.


It's not a biplane but in the description they say the mm kit was based off of a plane called the Spook. A little more googling uncovered the outerzone plans for the Spook. Apparently this plan is from 1940 so maybe that was the inspiration for the Duranita.

Looking forward to your build!


Free Flight Indoorist
Well, I consider myself well versed in old-time and nostalgia era models, and that's one I hadn't seen before. Nice job!
By the way, Free Flight is alive and well across the globe. I'm here in middle Georgia and am part of an active club. We fly on a sod farm west of Atlanta. It's only a 2-3 minute field and I'm hoping they'll work with me to access one of the bigger farms around here. I've sent a couple models down the Chattahoochee river and that's not how I like for planes to end their lives. Anyway, Free Flight is not as much voodoo as it at first seems, and it's not hard to get 2-3 minutes from a good rubber model. An old gassie like a Ramrod can do 3-4 minutes off a 10 second engine run (trust me, you don't want to go much longer than that or you won't even be able to see it!).

Biff45452, the Spook is actually an unrelated pre-war design, and a real classic at that. Josh Bixler was telling me last week that he built several of them as a kid. It's got its own worldwide following, especially in the UK and Oz. I've gathered that the 72" is the definitive version, but I might be wrong about that.


Biplane Guy
Nice looking bipe. I really like these unconventional designs that don't seem to surface often, it really adds to the flying community when weird or experimental designs are tested modified and perfected. Ken Willard is definitely a great designer, I've built his Drake II with DTF and really like that thing, so I hope you have similar thoughts when you are done with this.


Senior Member
Biff45452, the Spook is actually an unrelated pre-war design, and a real classic at that. Josh Bixler was telling me last week that he built several of them as a kid. It's got its own worldwide following, especially in the UK and Oz. I've gathered that the 72" is the definitive version, but I might be wrong about that.

The Spook was designed in 1938 and became a kit in 1940 and the Duranita was designed 13 years later. All I was suggesting is that maybe the Duranita took some inspiration from the Spook. When you look at them side-by-side they definitely seem related.


CrossThread Industries
Thanks for the posts, guys. Great references to the genesis of the Duranita- I'm learning something here and I wasn't even trying.
Very cool.

I finished the lower wing today, outside of detail sanding, prepping and then covering. However I didn't get a photo yet.

Here's a few shots of the start of lower wing build two weeks ago - I will catch up to current situation within a couple of posts...

Used a Dremel Table Saw to make offset cuts in the (enlarged) main spar so the ribs and spar have more of a "bite" - stronger wing.

Ribs cut out and need slots for main spar which is part of original plans, and forward spar, which I added in for strength. The original was very light for its size, used a .049 diesel which I believe would have even less thrust than a Tee Dee .049 for a 43" biplane.

Aligning a stack of ribs on spars, etc.

Ribs, spars, LE and TE glued. First gluing I have done for 24 years, I figger.

Loving building on the steel surface, with magnets. first time using this. Went to a local sheet metal shop and they cut it (6ft x 2ft) for under $50. Magnets cost all of maybe $20 for a bunch of different kinds. Magnets are `fairly cheap at Harbor Freight, and you can use coupons at Hobby Lobby and Michaels. HF has its own coupons, too.

More to come - Cheers - Peter

Sat 10 Jan 2015 - lower wing construction img1-40p.jpg Sat 10 Jan 2015 - lower wing construction img 2-40p.jpg Sat 10 Jan 2015 - lower wing construction img3-40p.jpg Sat 10 Jan 2015 - lower wing construction img4-40p.jpg


CrossThread Industries
Here you go - some detail shots of fabbing the wingtips. This kind of plan left a few things to the imagination but most of it you can easily interpolate. The Dremel moto saw was used to cut 1/8" ply templates to then cut the 1/16" ribs. I took an extra rib and cut the spar notches in it, and used it to just cut the notches in the rest of the ribs. Then gang them all up, sand the top airfoil surface so they are uniform, and I used a medium file to make the notches uniform. Very little sanding/filing is needed.

The wingtips are angled up from the bottom of the wing to meet the main spar at the outboard end. Two special ribs have to be cut, and there are eight contact points to align. Think of a 3 legged stool - that's straightforward - it won't rock because one leg is shorter than the other. But on a 4 legged table, one leg has to be propped up if any or all legs are a different length. think of having 8 legs, that is the equivalent of getting this wingtip to touch all eight points straight and even. Not that hard when I made accurate buts with the moto saw. That helped keep everything building straight, even though it required a little freehand cutting.

The moto saw is like a jig saw - or maybe is a jig saw. I like it because I put it out of the way, and then clamp it to the bench just when I need it. Doesn't take much space and smaller than a jig saw. I didn't get anywhere near perfect rib templates - I had to do a lot of sanding to bring them into uniformity. I made two of them - I now can cut a foam core wing - at least for the bottom wing which is simple and straight, no dihedral.

Cheers, Peter

dremel saws screen shot.jpg

screen shot tue 13 jan 2015 wing build.jpg


CrossThread Industries
The Crux of the Problem, Literally

The Crux of the Duranita is getting the wing spars to be durable in the upper gull wing where they intersect. As I progress with the build I am getting a better picture of how light the original design was for the FF version - almost wispy. I had been thinking from the start about strengthening the upper wing spar at the two sets of joints. The original design calls out for the separate spar sections to be butt-joined at an angle, with 1/16" balsa sheet buttresses glued over the joints. I do not believe that type of joint would last very long on a scaled-up, RC version of the original FF design.

Over the past weekend I set out to get a method of joining the spar segments that was strong, true, and light.
I could have cut the shape out of 1/4" basswood, but it would be wasteful of material, and possibly heavier.

My goal was to be able to cut accurate scarf angles on the two spar pieces to join.
Here's how I did it.

First, I used a 1/4" thick sheet of basswood to set up drawing the necessary angles on 1/4" balsa cut to the spar dimensions - from memory I think it is 7/16" in cross section (x 1/4 of course).

As precisely as humanly possible, I drew the 15 degree lines called out in the design, on the basswood.
Then I butted the spar next to that basswood and carefully extended the lines on the spar.

Making the cuts.
It took me 3 preliminary attempts to get accurate enough to come out with a finished product that would work.
Each of the two pieces had to have 1/2 their thickness removed, at a final 15 degree angle, so the two pieces would mate up.

The points of critical accuracy were
1. get the 15 degree angle line drawn accurately on the spar segment, to start with.
2. cut that line very accurately - but only halfway thru the thickness of the spar.
3. remove exactly half the thickness of the spar in the joint area - so you end up with two inner joint surfaces that are exactly planar with the outside of the spar. If not flat and true to each other, the joint will not be true and the finishes spar crooked.

So, cutting the angle and then removing exactly half the spar thickness were two separate things.
Each had to be done accurately.

I found if I was really careful with an x-acto knife cutting the 15 degree edge, i could get close. But I would have to go back and rework that edge surface another time to get precise. That is, take a thin slice of material off to get it to exactly 15 degrees.
The other cut is done at 90 to the angle cut.
It means slicing/cutting through the longer dimension of the spar, and then at a varying depth to meet the 15 degree angle cut. The first couple tries got me close on this part of the fab, but not close enough, by hand, to be happy with it.
I took this part to the Dremel table saw which did a perfect job of making that ungainly slice. I had to finish off with an x-x-acto right at the point the 15 degree cut had been made - you can't get in with the table saw blade to finish that straight edge. And, finally, I had to make a sanding stick by spray-gluing a strip of sandpaper to the edge of a 1/4" piece of scrap balsa. With this I could really dial in the angle.

The finished center section spar was light and very very strong for what it has to do.

It took me too separate 4-hour sessions to get the hang of this, but it is a technique to be used over and over.

After all that development , the pressure is on to cut the same scarfs for joining the outer wing sections. If I really learned how to do this, things will go well. If not, then I may be starting all over on another pass at the center section - ha.

Pictures should be better at explaining - here you go. Hopefully they make it more obvious the cuts on the different spar segment.

This is a technique that can be used on any spar joint to form dihedral, polyhedral, etc.


Note - one photo I should add is what I used the Dremel table saw for - will add that later.

Butting the 1/4" thick spar segment next to the basswood sheet where the spar break angle had been carefully drawn.

using 1-4 inch basswood base to extend angles onto balsa spars - 15 degr.jpg

Some examples of the preliminary attempts to get the process down - spar segments and some of the tools.

A few prelim attempts to get a feel for technique to get a finished spar.jpg

Another view of prototype pieces, for clarity.

getting joint area for scarf correct - 15 degree angle from wing root.jpg

Sanding stick - 180 grit sandpaper strip spray-glued to the edge of a 1/4" scrap piece.
I found this was absolutely necessary to get a final angle on the edge section, which sets the final angle of the joint.
Remember, you have to get both mating pieces absolutely accurate. It can be done, not hard once you get into it.

sanding stick 2.jpg

Laminating 1/32" balsa over the joints as a buttress.

gorilla wood glue the scarfed joint and buttresses.jpg

Top view of finished center spar section, showing some of the prototype pieces.

finished scarf joint with prototype pieces shown for clarity.jpg

Close up at an oblique angle to show the final 1/32" buttruss laminated on each side over joints, for added strength.

angle showing 1-32 inch buttress over joints to strengthen - highlight.jpg

Lined up perfectly on the plans - essentially finished until the outer wing panels have to be attached.

finished lined up on plans.jpg


CrossThread Industries
Another similar design to the Duranita

Found this on - similar to the other ones you guys found. A nice 1948 Rubber model - on my short list - if a list 100 models long can be considered short. Very nostalgic.

ladybird bipe ff rubber 30WS.jpg

Joker 53150

Mmmmmmm, balsa.
I like your method of handling the gull-wing. The layered areas should give you plenty of strength, and look better than the plans called for as well.


CrossThread Industries
Getting there - wings taking some time...

Well, it has been a little time since my last post but I have been moving ahead on the wing set for the Duranita.

Below is an (unfortunately lo-qual) photo after tonight's build session, both wings. I get in about 1.5 to 2 hours on average each night. With bad weather in NY Saturdays have been good for building but with the days getting longer the call to go fly gets stronger each day.

This plane is entered in the RCGroups AMPLAN buildoff and I have a full build log on RCG (id=PGregory).
Cheers - Poughkeepsie Pete

clamped together.jpg


CrossThread Industries
(Copied from my RCGroup build log).

Getting off the wings for a while, I started the tailfeathers.

At first I thought I would just try building as a solid balsa sheet for the vert stab and rudder. Last night I butted two pieces of 1/8 balsa together. However, I didn't get a good feeling about that as there was a slight inherent cup to this when I got them off the bench and looked at them.

Going for a built-up approach, tonight I started by ripping the sticks I needed from the 1/8 balsa sheet that I had joined last night. The piece was totally reusable. I took a deep breath and decided to use scarfing again where the pieces met for added strength. That added some complexity. I could have just butted them together but the scarfing was just a little bit of extra work. It turned out to go pretty well. the trick was getting the Dremel table saw blade pretty much exactly 1/16 inch, half of the 1/8 balsa, to route out the end material where each stick would mate.

Here is the computer drawing of the expanded rudder I drew, with a couple of assembly shots along the way this eve.

Reshaping the vertical stabilizer for more rudder area and authority.

old vs new vert stab and rudder.jpg

This photo shows scarfed ends of sticks to create very positive attachment, more rigid frame.


Overhead shot of first sticks going into place - keep things square and straight from the beginning.


All the pieces joined using Gorilla wood glue. Not sure if this is better or worse than Titebond. So far, it is sort of rubbery to sand, so I might not use any more. I have to try with Titebond, first, though.

I will shape the pieces a whole lot more before they are finished, some of the parts are very roughed in. Both inside and outside edges will be shaped. Right now looks kind of Frankensteinian.


Joker 53150

Mmmmmmm, balsa.
Personally, I love using Titebond II wood glue. Dry time is a little slow but I'm not rushing. I haven't tried Gorilla Glue for comparison.


CrossThread Industries
Personally, I love using Titebond II wood glue. Dry time is a little slow but I'm not rushing. I haven't tried Gorilla Glue for comparison.

Hey Joker,

I am finding Gorilla Glue (this is the wood glue, not the regular foaming GG) stays rubbery and I am getting bumps at joints when sanding because of that. Nothing catostrophic. It is really really dry right now in NY where I am, relative humidity in low 20's I think in my house. So, drying not a problem.
I thought I bought TB II but it seems to have gone AWOL. I want to compare and see the difference.
Although it takes some time to dry, I like the fact that the balsa does not become another material like when I use CA.

Cheers - Peter


CrossThread Industries
Tailfeathers day...

Cleaned up the vert stab and rudder, from its ugly roughed out state.

Got about 86% done with the horizontical stab and elevator - this turned out to be a bit bigger job than I anticipated, but it went well and it was an exercise in dual tapering of a flying surface.

The flipper is tapered from root to tip, and its thicknesses diminishes to the tip, too.

So this model has turned out to have some complexities I didn't see at the beginning.

Here's the cleaned up surfaces from today - quite a marathon day.


Shaped vert stab and rudder


Getting horz stab and elevator under way. Just dive in. Notice the "V" cut out for the elevator with the original plan underneath.


I used heavy timber to frame things up - things come out more square and straight that way later on. Leaves A LOT of trimming and sanding, though.




Just about done - maybe 86% or so. Ribs in, 1/8" basswood spar layed in.

Flipper seems to be coming out OK...

Last edited:

Joker 53150

Mmmmmmm, balsa.
Nice work, and I really like the before/after on the rudder. Showing the rough assembly before all the clean up work really shows well how working with balsa doesn't need to be perfect. Start rough and sneak up on a finished shape with sandpaper!


CrossThread Industries
Nice work, and I really like the before/after on the rudder. Showing the rough assembly before all the clean up work really shows well how working with balsa doesn't need to be perfect. Start rough and sneak up on a finished shape with sandpaper!

Ha, exactly. It is really gratifying. Get's lighter too!


CrossThread Industries
Short form of my day.

Had to fix a bunch of ribs I accidentally caught with the sanding block.

Had to figure out how I was going to power this plane. That could effect cutting the fuse sides, if I needed to make mods.

And, got cutting the fuse sides today. Trying to keep the plane true to the original look, so didn't want to extend the nose for the large engine. I will work in a different bulkhead system to mount the engine. What I DON'T want to happen is design my way into a box and have a tight engine compartment that makes mounting and working on the engine a pain. You need some access room, especially in a fun, sport plane.

Have to fix a few gotchas that occurred from my overzealous sanding with a sanding block.


Here's the 87% complete flipper.


So nice to turn the corner today and start working on the fuselage - pulling out the plans for such...


Figuring out how to mount an engine that is 2x as big and 2x in weight than the original. Have to add bulkheads and beam mounts for this engine.

firewall and rear beam secure bulkhead.jpg

Bonding two lengths of 1/16 sheeting, edgewise.


Finished with the cuts using a fresh X-Acto blade - cuts like butter.


Finished cutting and sanding to create two identical sides. I CA'ed the two sides together so they stayed registered to each other.



CrossThread Industries
More fuselage

16 days until build-off ends - moving good progress...

Cutting three ply formers/bulkheads up front.
Three Ply Bulkheads.jpg

Cutting square holes for the square-profile engine mount rail.
Drilled pilot, drilled with large bit.
Using Dremel jig saw to cut square holes.
Cutting holes for rails.jpg

Square holes cut.

Holes cut for rails.jpg

Aligning the two bulkheads that bear the engine mounts (oak).

Registering the load bearing bulkheads.jpg

Hot glued the 3 bulkheads together in order to cut the recess for 1/4" sq longerons.

Three bulkheads with longeron recess cut.jpg

Cutting the rails on the band saw.

Cutting Oak Engine Mount Rails.jpg


A test look at the ASP 09 engine perched on the rails.

Engine on the rails.jpg

Fixturing up the stick frame, ahve to cut another of each and build one on top of the other to get perfect mirror.

Fixtured up.jpg