S2B OR NOT S2B - CML Distribution

test pilot

] 60’ span Pitts S2B ARTF

- part 1

]

Reviewed by Jason Robinson

S2B OR NOT S2B A 120 SIZE PITTS - YUM, YUM...

W

hen Curtis Pitts introduced his little biplane to the competition aerobatic scene, it was clear that a revolution was about to take place. Legendary, exhilarating, responsive and perfectly balanced are just a few of the words

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used to describe one of the most highly regarded aerobats of all time. For some time I have been looking for an ARTF model of the Pitts to suit the biggest engine that I have, a 120 four stroke. The Great Planes Pitts is too large, the Jamara Pitts too small. I had all but given up,

but recently while flicking through a recent American modelling mag, I saw an advert for the Cermark Pitts S2B, which with its S2B cowl, looked a bit different - and it was designed around a 120 4-stroke! It was confirmed to be fate that I should have one, for when at the NEC show

Model Flyer

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(Model Expo) a couple of weeks later, I saw the very same Cermark Pitts on the CML Distribution stand. They had just become UK distributors for the Cermark range, so I made an order then and there for one of the first batches, due in a matter of weeks. The Cermark version of the

control surfaces will be high, so decent servos are required on ailerons (4), elevator (1) and rudder (1) - the instructions recommend servos with a minimum torque capability of 42oz.in. on the ailerons and 75oz.in on each tail surface. A standard servo is fine for the throttle, however! With this size of servo, I would recommend fitting a 6v Rx battery of at least 1000mAh, a heavyduty switch, fit a visible battery level indicator - and fit a throttle failsafe! OK, let’s go...

ASSEMBLY

legendary Pitts is a big, beautiful ARF - designed and developed by America’s Dave Patrick. It’s claimed to be capable of huge knife edge loops, tumbles, etc - anything you can handle, in fact. The kit features single servo controls on rudder and elevator, mounted at the rear for a solid and precise control. The interplane struts are very quick and positive to fit with neat ‘thumblock’ connectors that make assembly at the field fast and accurate. A look into that cavernous fuselage reveals a laser cut, top quality balsa and plywood structure, with veneered

foam top decks, painted fibreglass cowl and wheel pants, complete good quality hardware including wheels, fuel tank - even a custom Pitts shaped, Spin-Right metal spinner! The standard of finish and covering is first class in a choice of two AVIAT Team colour scheme, either blue on white, or traditional red and white. A sheet of vinyl decals registration, Pitts logo and a variety of ‘stars’ completes the kit, together with a nine page instruction booklet. As this is a highly aerobatic model aircraft, the loads on the

Now if you’ve got the skill and ability to fly the Pitts, you’ve probably built a few aircraft getting there, so you shouldn’t have any difficulties putting it together. I say that because the instructions aren’t the best I’ve seen - just about adequate, I’d say was an accurate assessment - they would certainly fox a beginner, but then again, a beginner wouldn’t be looking to fly the Pitts, would he, or she... Having said that, the steps are logical, well illustrated and the standard of English is very good! One of the very first steps is to fit the ailerons (each designed to be operated by an individual servo) to the wing panels. Here, I must say, I don’t understand the design philosophy employed here. The ailerons are round nosed and the wing panel cut outs, are semi shrouded, so the fit should be nice and aerodynamic. The hinges are large diameter, metal-pinned, plug in type and the holes are accurately produced in the aileron leading edge and in the wing TE sub spar - but then they go and spoil it! To get the benefit of the round nose and shrouding, the hinge pin should be at the centre of the nose radius, however, the indication is that the hinge pin should end up at the aileron leading edge! This means that the top edge of the aileron moves upwards when the aileron is deflected up, fouls the shrouding and limits the amount of throw. The perfect answer would be to inset the hinge into the aileron nose and slot the aileron leading edge to allow full movement of the aileron - I’ve drawn a sketch to show what I mean. As it happens, I checked the throw with the hinge pin at the aileron leading edge and it just allowed the high rate throw advised in the instructions (7/8” up and down) before fouling the shroud, so I left it as is. A photo shows the aileron fully up, showing how the aileron top surface rises above the top of the wing. It is interesting to

note that in this position, the aileron trailing edge is out of step with the wing panel TE, indicating that the aileron should be positioned further forward i.e. hinge pins inset. For aesthetics alone, I would have carried out the mod, but as this is a review... You need some six extension leads to fit out the model, plus a couple of Y leads for the ailerons. The instructions say to fit a Y lead on the Rx - one leg for the top wing and the other for the lower wing, so needing three Y leads. I opted to be able to use the computer aileron differential function, if I needed it, so linked the left hand servos with a Y lead in channel 1 and the right hand servos with a second Y lead into channel 7 (Futaba FF9 Tx). The level of ARTF is such that the model can be literally taken out of the box and the wings and struts fitted, bolts tightened - in about two minutes - the design of the interplane strut attachment is very novel and one of those “why haven’t I thought of that, before?” solutions - very neat and very quick. This biggest job in the whole assembly is fitting and gluing the tail surfaces in place, making sure that they are perfectly aligned. Hard points in the tailplane and fin have to be drilled for the bracing wire fittings (all supplied). The bracing wires being cut from nylon covered fishing trace and crimped around brass plate fittings bolted in place. Again, the high loads during flight mean that this bracing is absolutely necessary - they don’t need to be ‘guitar string’ tight - just ensure there is no slack. The tail hinges are of the ‘furry Mylar’ type - perfectly adequate, as long as they are well-glued in, with plenty of thin cyano ‘wicked’ in to the joint. The rudder and elevator servos are mounted at the back, on opposite sides of the fuselage exactly opposite! Using the larger torque servos (bigger cases) as advised, I found that the bottoms cases touched each other before the mounting lugs touched the fuselage sides - the only solution for a totally firm mount is to fit 1/8” play packing plates under each servo (remove the film from the fuselage sides in way of the packers before gluing them in position - a minor niggle, but something perhaps that the pre-production models should have shown up. The undercarriage is a substantial, pre-bent, aluminium plate with lightning holes in the belly plate - the captive nuts are already drilled and fitted - simply bolt on! Model Flyer

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

test pilot

] 60’ span Pitts S2B ARTF

- part 1

]

Reviewed by Jason Robinson

CONSTRUCTION CERMARK PITTS S2B

The nifty strut attachment method makes assembly very quick...

The bottom of the strut has a locating peg to prevent the strut twisting...

Just locate the strut in the hole and finger tighten the knurled nut - easy!

The ailerons are round nosed in section...

As designed

Preferable

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But the hinges aren’t recessed into the aileron to give true aerodynamic movement - note the inboard hinge has to be trimmed...

...to clear the aileron servo, when mounted in the wing aperture.

The effect of the hinge line in front of a round nose is to lift the top surface of the aileron above the wing surface...

... and the recessed sub spar has narrow shroud strips top and bottom...

...which restricts the amount of aileron throw (aileron shown in full ‘up’ position).

The top wing is fixed at the centre with the mounting plate sandwiched between two cabane struts and secured by two M3 bolts and Nylok nuts.

The tank is planned to be retained by Velcro straps, but I preferred a more rigid location.

The sturdy undercarriage bolts to the fuselage bottom - blind nuts are factory fitted.

The large cowl need minimal trimming to fit a 120 4-stroke...

...Which is almost totally enclosed within it.

The crimped fishing trace tail bracing is necessary to prevent tail stress overload, whilst...

...high torque servos are needed on all control surfaces to prevent flutter - the Blue Bird BMS621 servo has a 90oz torque rating.

The spat attachment method is extremely efficient, again indicating the high level of thought that has been put into the design. Wooden plates have been glued to the inside of the spats, ready to be drilled to match the two holes in the bottom of each the UC legs (either side of the axle stub hole), when the aircraft is placed on its wheels to ensure the spat lower edge aligns with the ground. Drill through the holes and the spats, remove, fit the

captive nuts and bolt them in place - a nice and strong installation.

and I found that it crushed when the mount bolts were tightened, so I threw it away and made up a tapered birch ply plate. The danger of using too soft a packer is that it will continue to crush with vibration, resulting with the engine mount becoming loose. Another unusual feature of the Cermark Pitts is that the cowl is predrilled for the mounting bolts (small caphead bolts). The holes in the cowl are drilled big enough to locate over

short lengths of fuel tubing that have been pre-fitted over the bolts - when you tighten the bolts, the fuel tube diameter expands to firmly grip the holes in the cowl - clever! This does mean, however, that you have to measure the position of your chosen engine on the mounting arms very carefully, to ensure a nice 1/16” gap between the rear of the spinner and the face of the cowl. The instructions show a GMS 1.20 2-stroke being fitted inverted.

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ENGINE INSTALLATION

Most ARTF kits are designed with a firewall with built-in thrustlines - not so the Pitts S2B. A square of tapered wood is supplied to fit under the engine mount to provide the 2 degrees of right thrust and zero downthrust (the tail is set at +1.5 degrees). I say wood - the tapered packer certainly isn’t ply

Model Flyer

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A very pretty vbersion of the aerobatic classic bipe - just cries out for a pilot under that enormous canopy, though!

The Cermark version of the legendary Pitts is a big & beautiful ARF - designed and developed by America’s Dave Patrick ... and can do anything you can handle... I opted for the Magnum 120 FS to be mounted sidewinder, so that the needle was a sensible height to the fuel tank - this did mean cutting a hole in the cowl to clear the rocker cover - and give access to the glow plug, but on the plus side, it allowed the standard manifold to exit at the bottom of the cowl and the needle valve to be accessed from the top of the cowl. Now the fuselage structure is very lightweight meaning that large areas of the nose is unsupported film - not a problem, just needs careful handling, but also the tapered fuselage bottom at the rear of the cowl - the bit that is exposed to the hot exhaust gases! I thought it necessary to fit a silicon ‘dog leg’ exhaust deflector tube to prevent the film surface getting the full heat treatment! Regarding the location of the fuel tank, the instructions recommend using the supplied Velcro straps to secure it in place. I must admit, I couldn’t work out how to do this and found that with my ‘sausage’ fingers it was extremely difficult

to manipulate the tape inside the tank bay - so I opted to locate the tank using 3.4” x 1/4” balsa strip glued to the ply structure to support the rear of the tank, with vertical pieces either side to prevent the tank moving from side to side, with a piece of the strip at the front to support the neck of the tank. With the engine, cowl, prop and spinner fitted, the wings were put on and the position of the battery determined to get the CG sopt on (1/4” in front of the rear cabane strut on the bottom of the top wing) - guess what, the Cermark designers had got it perfect! With the battery in line with the internal Rx/throttle servo tray, the model balanced as advised, so it was a simple matter to fit the battery in the fuse, on the tray, using rubber bands wrapped around the ply lugs designed for that very purpose! However, just assume this will be the case for all engine installations - as the recommended range is pretty big, there is bound to be some variation in optimum battery position placement.

BUILD SUMMARY

This is the first example I’ve seen of the Cermark brand and I have to say the build standard is excellent - ARTFs have come a long way in the last few years and Cermark are certainly at the top end of the pile. The materials used look to be first class, as are the choice of hardware supplied. A few innovative design features show that the designers have really thought about the ease of assembly without compromising

strength or weight. The standard of covering is first class and the whole model was, overall, very pleasant to put together. With a price tag of just under £250, you expect quality and, from this example, with Cermark, you get it... q

NEXT MONTH

In part 2, we take a close look at setting up the Pitts S2B and see how she flies - believe me, it’ll be worth the wait!

SPECIFICATIONS PITTS S2B Manufacturer: UK Distributor; Wing Span: Wing Area: Weight: Motor range:

Radio: Recommended servos: RRP: Part No:

Cermark CML Distribution 60” 1020sq.in. 9lb 12oz. .90-1.08 (2-Stroke) 1.20-1.40 (4-Stroke) 23cc (Petrol) 4 channel (7 servos) CML Blue Bird BMS621 (90oz. torque) £249.99 CER-A103B (Blue) CER-A103R (Red) Model Flyer

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