3D 1/72 Cassutt 111M



A couple of years ago I ordered two 1/72 Formula 1 pylon racers from Civilized Models (via Wayback Machine), run by Gary McRorey (although my PayPal invoice said 'Curtis Embrey'). Like so many others, I got burned: no kits and I lost my money. The Formula 1 racers remained on my mind: I love tiny aircraft, pylon racers often have great color schemes, and technically it's also an interesting subject. I had great drawings of the Cassutt 111M, but I had never designed a complete aircraft shape in 3D CAD. The Cassutt looked like an excellent test case. One problem however is that nearly each Cassutt is different - welcome to the world of homebuilt aircraft.



The Cassutt F1 racer

In 1954, Tom Cassutt first flew his own racer for the 'Formula One' pylon racing class. It had race number 111 and registration N20N. Following a common naming convention, the original aircraft was called 'Cassutt Special'. Tom later built a more extreme version, with thinner wings and lower weight, first flown in 1959. It had race number 11 and registration N111U. This aircraft was called the 'Cassutt Special II'.

In 1960, Tom Cassutt started selling plans for his designs. The plans aircraft was called 'Cassutt 111M', '111' from the race number and 'M' for modified since the plans are of a developed version of N20N. Confusingly, the '111M' designator is often written as 'IIIM' which would read as '3M' if you see 'III' as Roman numbers. The plans included one extra sheet for the extreme version, called 'Cassutt 11M' (or 'IIM') in the same style as the 'Cassutt 111M', but leading to some confusion which came first.

An early sixties 'Sport Aviation' issue had an article by Don Berliner, with a three view drawing by Bob Pauley, that alerted the world that the drawings were available. The first plans built Cassutt flew around 1964. More than 2000 plan sets were sold, and some 800 Cassutts have been built and flown (here's a production list). Probably less than 30 of these were the extreme 11M version. 80% of all the Formula One racers are Cassutt based.

The plans ownership and support changed hands several times, to 'Southern Aeronautical Corporation' (Miami Lakes FL), then 'National Aeronautics and Manufacturing Co' (Independance MO, later Arvada CO) and 'CassuTT Aircraft' (Valley City UT).

Starting in the seventies, racing Cassutts were often fitted with other wings, like the 'Wilson wing' (17.5 or 18 ft span, straight leading edge, trailing edge is swept forward, 1971), 'Stockbarger wing' (almost 20 ft span, trapezium planform) or 'Owl wing' (late seventies?). Another development is the use of longer and longer spacers between the engine and propeller, growing from 4" to 12", made possible by lighter composite propellers.

The Cassutt, fitted with a 75 sqft wing and VW engine, was also used in 'Formula V' air racing. The Lasher Renegade I (by Charles Lasher) Formula V racer was largely based on the Cassutt 111M. The 'Formula V' class appears to be dormant since ~2000.




2024: my test year for 3D printing

I had planned to make 2024 a test year for 3D printing, with questions like whether it's time-effective, is it fun to do the 3D CAD, and are the printing results satisfactory? The Cassutt is the most complete project in the first half of the year. Another goal was the get a much better grip on those nasty supports that are required for resin printing. They often ruin pieces because their removal resulted in too much damage. That goal was mostly achieved by learning to work with PrusaSlicer. Another tool that I learned to use is UV Tools, that can be used to check for 'islands', 'resin traps' and other possible printing problems.




3D CAD

I found great scale drawings in the May 1970 issue of 'Aero Modeller', drawn by A.A.P. Lloyd. They were later republished in 'Aircraft Archive: Famous racing and aerobatic planes' from 1989. The drawings show the Airmark (UK) built version, more specifically G-AXDZ and G-AXEA. I picked the latter when there were differences between the two, like the engine cowlings. My 3D model will represent one specific aircraft, but it's fairly easy to modify the 3D CAD model to other versions with detail changes.

I'm using 20 year old 3D CAD software, and drawing fuselage cross-sections is very laborious. Each of the dimensions was measured in the Lloyd drawing.

It was an experiment to see whether the software wanted to 'loft' a continuous part through the six cross-sections - it often complains that it's too difficult. But luckily it managed this time.
Shown here is the resulting loft of the rear fuselage. The lofting process results in an imaginary stringer on the top fuselage, that doesn't show up in the STL export.
Work continued towards the front. The lower fuselage was extended forward, and a sheet metal section under the nose followed. Next was the upper nose, a bit of a guess shapewise. The sequence may look awkward, but I was doing the easiest parts first :-) I'll also need to digitally 'carve out' a cockpit later.

As you can see here, you need to break up the shape in pieces that can be designed relatively easily. That works a lot better than drawing the aircraft from nose to tail as multiple cross sections and lofting through them.
More progress, with basic wings (ailerons are too big in chord though - I misinterpreted the drawing) and extra parts for the nose. I tried modeling the front part of the cowling, and failed miserably.
The lower cowling was next. I made four cross sections and lofted the shape. The first cross section was identical to the propeller spinner, the last identical to the sheet metal on the belly. The guesstimated intermediate cross sections were then endlessly adjusted to create a relatively smooth shape.

I also added the fin and rudder. Both need more work.
The air inlets of the cowling were probably the most difficult parts so far. The inlets are angled back, and again I had to make guesstimated intermediate cross sections for the loft. I think I hit the limits of the software here, I'm expecting some classic modeling work is required after printing (i.e. filler and sanding).

For the record: I now spent three evenings of 3-4 hours on the model, say 10 hours total so far.



Half time review

The ailerons were redone, and have a smaller chord now. I found that they are 8" average chord, and I made the root 9" and the tip 7". The mass balance still needs to be added.

Horizontal tail surfaces were added, but I could not make them look like fabric over tube. Maybe I can add that after printing. I had to make them a bit too thick to make them printable.
Compared to photos, it looks like I got the incidence angle wrong, too small. Usually the trailing edge is on the on the top longeron of the rear fuselage frame.
So far nothing to comment on the front view, except that it shows that he Cassutt was designed for minimal frontal area. It's almost surprising that a pilot fits inside :-)
The elevators and rudder are all mounted with a 0.5" gap, like in the 1:1 plans, used for the hinges.
I think the shape is fairly correct, it looks like a Cassutt to my eye. Still have to design the 'flat wrap' canopy though.



Work continues

Another evening was spent hollowing out the fuselage parts. I had to decide on a wall thickness, and I guessed that 0.75 mm would be stable enough for 3D printing - we will see!

A subtle change was the shape of the rear top fuselage. Cassutts often have different cross-sections right behind the cockpit, to suit the builder/pilot. G-AXEA had a small flat section at the top, and a smaller radius than usual. I changed the CAD model accordingly.
I also added small sections to the cockpit's opening sides. Maybe I'll add the main spar running through the cockpit too.
Another improvement is the addition of tip caps and a mass balance for the aileron. I could only draw a basic shape of the mass balance, the software did not allow me to round off the edges. But I can sand the printed part.

I haven't made 'panel lines' yet by rounding off the edges of (in this case) the wing and/or tip cap. The real plane probably didn't have gaps there, but it's tempting to add a little detail.



A first print

The design is not quite finished, but I thought it would be wise to make a first print at this stage. I made quite a few assumptions, take for example the guesstimated 0.75 mm wall thickness. Will that work?

First task is to disassemble the parts assembly to arrive at the individual parts that you want to print. I decided to break up my model very conventionally, as seen in the right fuselage half here.
I had recently learned how to prepare my own parts for 3D printing, in terms of placing the part in space for the best surface quality, and where supports would be added. The latter can be done automatically, but it can lead to inconvenient attachment points, right on top of delicate but important details. The software of course doesn't know what's important to me, therefore it's very desirable to modify the supports manually.

I also learned the basics of UV Tools, that checks for 'islands' and 'resin containers'. Each part was checked, and only very minor islands were found. I left those in.
Another view of the main parts of the Cassutt: fuselage halves, wings, horizontal tail plane, control surfaces and spinner.

I estimate that I spent 20 hours so far on this project, half a work week.
Club member Wim Hoogendoorn printed the parts for me. The fuselage halves came out great, and the wall thickness looks fine for a model of this size.
The remaining parts: wings, horizontal tail, control surfaces and spinner. One wing wasn't supported correctly, and the lower corner was deformed. I had made a beginner's mistake with the trailing edge of the ailerons: I made them perfectly sharp, and printers can't do that. The aileron gap was far too small, and it's barely visible. The control surfaces are really small!
The right fuselage half didn't print right the first two times. It was solved by adding more supports under the corner that's printed first.
Here's the assembled fuselage, fitting nicely and looking great! I'm very happy with this preliminary result. The faceting on the lower fuselage sides needs to be made more visible.

A funny realisation was that the print striations are more audible than visible! Even with an Optivisor-like glasses I can't really see them - but sand the surfaces and you'll hear them :-)
The wings are 'blu-tacked' on here. The wings need a bit more work, as explained above. But the Cassutt is nicely taking shape.
I applied two coats of Tamiya Surface Primer, and sanded in between. The control surfaces are still being processed.



Another round of CAD work

A long evening was spent on an approximation of the 'flat wrap' canopy shape. It was by far the trickiest part to design. I started with a simple object that matched the three surfaces that the canopy connects too: the rear fuselage, the cockpit sills and the slant-cut half-round nose. In that sense it's sort of a keystone, and therefore a difficult part. I made the simplest possible loft to create a part to check whether the shape would fit in the assembly. Which it did, but not shown here.
The next step was adding two intermediate cross-sections, to create the 'flat wrap' transparancy shape and the bulbous top section. The 'flat wrap' canopy consists of a half-cone and flat sides. The end result looks deceptively simple - it required quite an effort.

This part will serve as a vacforming master. Ideally I can 'remove' the thickness of the vacforming material from the master, using some kind of STL tool.
I added some 'panel lines' to the model. I rounded off the relevant parts with an 0.1 mm radius. I learned that's barely visible on the printed parts, but anything larger looked horribly overscale.

Not visible here are some other refinements: increasing the stringer height on lower fuselage to make it more visible (it wasn't on the first print), adding a main spar running through the cockpit, hollowing out the nose parts to improve the print quality, adding slots for the landing gear legs, and holes for the four exhaust tubes.

Still on the to-do list are: the wheel spats, separating and improving the ailerons, some details under the nose, maybe adding propeller blades, and again maybe an instrument panel.

My time estimate at this point is 35 hours.



New fuselage prints

Here's the printed version of the canopy. In this (first) print of the fuselage halves, I had accidentally deleted the cockpit side parts, hence the gap.
I quickly vacformed two canopies from 0.5 mm PET-G, to establish the wall thickness, to subtract from the canopy master. It was interesting to see that the fine printing steps transferred perfectly to the vacformed parts. It's not important though, since these canopies are just for test purposes.

On second thought, I might do a negative mould for the canopy. I have a few more canopy projects in the queue, and one more wouldn't make much difference.
Club member Wim printed a new set of fuselage sides. The right-hand side was perfect, the left-hand side not, which is puzzling.

Later I got a good reminder of how brittle and vulnerable the 3D printing material is. While doing measurements on on the right fuselage half, it snapped in two in the cockpit area. This reinforces my idea that it's better to have a conventionally cast-resin model.
The left-hand side had problems on its bottom side: there were ridges leading towards each support, and the edge pulled back in scallops. I have no idea what went wrong here. The main landing gear spats and the propeller are last major parts missing.



The last round of CAD work

Another round of CAD work. Visible in the screen shot is that the rear ends of the cowling 'cheeks' were given a cut-out for the wing's leading edge. The wing's roots were reshaped to make them fit against the fuselage sides. The ailerons were redone: I had made the first ones infinitely sharp at their trailing edge, and that cannot be printed. I changed that to 0.3 mm thickness, rounded off. All panel lines were doubled in depth and width, since the previous ones were almost invisible. All holes were made as short as possible, since they acted as resin containers. The slots for the landing gear legs were made a bit wider.

This added another 6 hours to the project, with the total around 40 hours now.

I found an easy way out for the wheel spats: the LS Pitts S2 has spats that are of the right size and shape.



The definitive parts

Club member Wim printed a new partial set of parts, this time with a gray resin. The wings and ailerons are now separate parts. I will reuse some parts of the first print.
I assembled the fuselage, and added the wings (minus ailerons) and horizontal tail. It was coated with Tamiya Surface Primer twice; especially the wing needed that since the showed a faint waviness, requiring sanding. I think it looks sweet!
Details on the lower side include the offset exhausts, and the slots for the landing gear legs. The landing gear legs should connect horizontally, but I'll mount them vertically in the slots.
The first and second kits together. In the second model, you can see the wing spar running through the cockpit.
According to the drawing, the landing gear legs were 6.2 mm long, tapering from 2.0 mm at the fuselage to 1.0 mm at the wheel spat. I had some aircraft grade 0.2 mm strip, that was bent and sanded accordinly. I think this will be strong enough for the tiny model. The spatted wheels are from the LS 1/72 Pitts S2.
A first fit of the landing gear legs in their slots.
The legs were attached to the fuselage with CA glue, and the spatted wheels were similarly glued to the legs.
I had to make a decision what to do with the carburetor inlet and the engine cooling air outlet - the drawings were not very clear about them. I consulted photos and decided on a 1.2 x 0.6 mm strip (2.5 mm long) for the inlet, and a 1.0 x 0.2 mm strip (3.0 mm long) for the outlet. I dug a tiny partial recess for both. A bit of Apoxy Sculpt and / or Mr Surfacer 500 will be required to smoothen things out.
The tail wheel was 1.0 mm diameter. I searched my spares box, and the smallest wheel I found was 4 mm.. Only then I realised how small the wheel actually was! I then tried making a 1.0 mm disc with my Waldron set, but the results were rough. In the end I switched to regular 1.0 mm plastic rod - much easier. I cut a tiny slot in the piece of rod, big enough for a 5.5 mm long piece of 0.2 aluminium of 0.5 mm width, with a kink in the middle. That tiny part was CA-glued to the lower fuselage. It's pretty sturdy!
Then all control surfaces were added. The rudder is straight, the elevators hang down a litle, and I overdid it with the aileron deflections, wanting to show them off. I broke one aileron in the middle after gluing (how?) but was able to glue it back seamlessly. Two rounds of MRP-84 grey primer were used to blend the control surfaces in. The elevators are far too thick, but I'm still learning where the lower limits are with 3D printing.
Another small detail were the aileron hinges, in reality simple steel straps around the aileron torque tube, screwed to the wing skins. I used some random photo-etch pieces, 0.9 x 0.4 mm small, sixteen pieces. Another tiny detail is the 0.3 mm pitot tube, again Albion micro-tubing.
I had postponed detailing the cockpit, but I could no longer avoid it. The model was very delicate to handle by now, so I built a crude handling jig from very old PVC foam. That worked really well, but foamboard would have been nicer.

I started the work on the cockpit by gluing the top longerons inside the cockpit. I used 0.3 mm Albion micro-tubing.
Next I glued a 'pylon' behind the pilot, resting on the longerons. Adding the 'cross', consisting of three pieces, between the top longerons was ten times more difficult.

I robbed a seat from the LS Pitts S2 kit, and after some sanding it fitted nicely. It's not installed here.
I added diagonals belong the top longerons, and more tubing at the upper edge of the cockpit. A seat from the LS Pitts S2 was modified to fit the Cassutt.
I designed an instrument based on the Lloyd drawing, but adapted to the space available in the model's cockpit. I painted one of the 3D prints coal black (Revell 9).
Work continued on the canopy. So far I only had a true-to-size canopy master, that resulted in oversized vacuum-formed canopies. Therefore I made a new version of the canopy master. It's 0.4 mm smaller in vertical direction, and 0.3 mm laterally. These reductions were measured on a canopy vacformed over the first master, using 0.5 mm PET-G sheet.
I designed a custom mask for the 'flat wrap' canopy. This involved a lot of calculations to define the cone-shaped part in the center, see the first paper mask on the left. I then added the sides, but had to fine-tune that in several steps, until I had the one covering the canopy master. The wrinkle-free tape demonstrates this is a true 'flat wrap' canopy. Club member Jan de Wit cut the masks on his Silhouette cutter.
The true-to-size canopy master is seen rear left, an undersized canopy master rear center. More 3D prints of the undersize canopy are seen on the right. In the front you can see a primed and sanded undersized canopy master on its dedicated support, and a piece of 0.5 mm PET-G vacformed over it.
Cutting out a vacformed canopy is never fun, but at least I had the master to put in it, to see where to cut. The fit just needs a tiny bit of sanding to be perfect. The clarity is very nice, so the cockpit detail is still visible.
The last fuselage detail were metal hatches on either side of the fin. I used 0.13 mm plastic card, but it still looks massively thick in the photo.
The 'scimitar' prop blades were made from those of the LS Pitts S2 (again). They were too thin to drill a hole in, instead I cut a slot for the 0.3 mm spring steel wire, CA'd in place. Similar holes were drilled in the 3D printed hub, followed by lots of wire bending to get the blades in the right position. CA glue fixed the wires in the hub and filled any remaining gaps.



Painting

The photos on the ABPic site showed a color alternating between red and orange, depending on film used, lighting, etcetera. I decided to mix MRP-194 Signal Red and MRP-232 International Orange in a 2:1 ratio. I liked the resulting color. The cockpit was masked with a combination of Tamiya tape amd Blu-Tack.



Decals

In parallel with the 3D CAD work, I designed the markings of G-AXDZ and G-AXEA and as shown in the Lloyd drawing, using an old version of CorelDraw. I added a few markings seen in photos, that were added later.



Leasons learned

Some preliminary comments on this undertaking:




Links




Cremona / Maxwell diagram

In a 89-page forum thread on the Homebuilt aircraft forum, I found a partial view of the original plans, showing the Cremona or Maxwell diagram for a fuselage side, for a single loadcase. I'm not 100% sure what loadcase, but it looks like balanced 1G flight, with half the mass and aero loads acting on one fuselage frame side. But there are some forces noted in the diagram that I don't understand. I assume the idea is to multiply the resulting tube loads by limit load G and ultimate load G, and then check each tube for yield / break stress and buckling.

I had never drawn such a diagram before, but I have several old aircraft engineering books that explained the process. I made the first one with pencil and paper, but found out one needs to draw all angles very accurately for the diagram to work. I switched to CorelDraw, overlaying the original diagram with my calculations in red, and got a much better result. I only did the front part of the fuselage, to see whether I could reproduce a part of Cassutt's graph. The agreement is not bad I think, considering the drawing wasn't reproduced (scanned or photogaphed) very accurately.

I do wonder how this analysis translates to the full fuselage truss, where the sides are canted. Also, it's still a simplified analysis assuming pinned joints, whereas the real fuselage has welded joints of course. But that is / was standard practice in aircraft engineering, using a suitable (reduced) factor for the end conditions.





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