Super Chipmunk R90S

Super Chipmunk R90S

 

  • This review was first published in 2006, the kit is still available.

When asked if I fancied penning a review of the revised World Models ARTF Super Chipmunk I was, to begin with, not that excited at the prospect. All I really knew about the Chipmunk was that it had been an RAF training aircraft, and as a result I expected it to be very slow, docile and generally a little boring.

However, a quick search on Google demonstrated just how wrong I’d been. What a pedigree! I immediately found lots of references to aerobatic championships from the late ‘60s and ‘70s. Indeed Art Scholl, the owner of this particular variant (N1114V) was one of the best aerobatic pilots of his era, and this was his chosen weapon. Art himself met with an unfortunate end whilst filming a flying sequence for the movie ‘Top Gun’ – neither he nor his Pitts Special were found after he failed to recover from a spin somewhere over the Pacific Ocean. With this entire heritage to hand I viewed the Chipmunk in a completely different light and accepted the kind offer most gratefully!

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WORKSHOP BOUND
Upon opening the box I was presented with a fairly typical, fully covered ARTF. The distributor (Steve Webb Models) also provided two replacement rolls of Toughlon pre-printed covering. Available as a ‘crash kit’ (costing around £5 per roll) one is for the fuselage, the other for the wings – a neat idea!

With an engine and all the other gear to hand, I started to build her straight away. Taking the advice of another clubmate, ex UK IMAC champion Colin Fisher, I decided to opt for a Super Tigre .90 two-stroke – more than enough power. Colin used to fly the Goldberg Chipmunk when he first started in competition and so knows what he’s talking about. Although my model was recommended for .60 two-stroke or .90 four-stroke motors, Colin informed me that he’d flown his Goldberg variant with a .90 two-stroke in various IMAC contests very successfully. This advice, together with a nod from the mag’, decided it. I realised I would have to ‘shoehorn’ the motor in somehow, which I’ll expand on later.
Whilst the instruction book is very clear, I felt that it required the builder to have some previous experience. Many of the more detailed steps are omitted and you have to think around some of the problems.

WING
The wing assembly is very straightforward, except that the hardwood joiner was a bit on the small side and didn’t fit too well! To overcome this I capped it using 3mm hard balsa to force a reasonable fit.

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With plenty of 1-hour epoxy mixed and with the dihedral already set up on the bench, the left wing met the right wing and hopefully they’ll never be parted. Waiting for the epoxy to cure, with the wing taking up most of my bench, I had little else to do other than read the instructions, play about with the retracts and gather the servos for the next step.
The wing uses four servos: one per aileron, one for the retracts and one for the flaps. A ‘proper’ retract servo is needed as the mounting depth is too shallow to accept a high torque, standard-size job, and the required travel probably wouldn’t be sufficient either. The aileron servos sit behind liteply covers which, incidentally, fitted the wing recesses perfectly.

I built a World Models Frontier .40 trainer for my son some time ago and was very impressed with the little rod connectors that were supplied, so it was good to see them here, too. These little gizmos are a very neat and tidy way of connecting piano wire to a servo horn whilst leaving it adjustable. In fact, all the hardware supplied with the Chipmunk is of good quality and gave me no cause to complain.

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The control surfaces all come pre-attached with some very flash metal hinges. No amount of (careful but firm) tugging could loosen them so I decided that further ‘cocktail stick’ type pinning wasn’t necessary.

With the wing looking good to go, I retrieved the family shoehorn from the bottom kitchen drawer and turned my attention to the fuselage and engine installation.

FUSELAGE, MOTOR & FEATHERS
The fuselage is well constructed, with a veritable ‘concert hall’ amount of space for the radio gear; very pleasing to the eye it was truly a shame to have to perform the major surgery necessary for the installation of that big engine. The problem here was the position of the silencer because, with the engine mounted as suggested, the silencer would stick out the side of the model. Ugly, ugly, ugly! Eventually I decided to mount the engine at 120°, therefore aligning the silencer with the centreline of the fuselage on the underside. The only way to do this was to cut away the tank bay floor and then rebuild it with a channel for the silencer. The bare wood, uncovered as a result of the modification, was given several coats of Aerokote fuel proofer to make sure it lasted as long as the rest of the model. With this having dried overnight I then installed the Super Tigre using the kit-supplied engine mount and securing it with double locknuts on all the bolts. A silicon elbow fitted to the end of the silencer would direct the muck away from the fuselage, save on the cleaning a little and knock a bit of the noise off.
I usually perform the first couple of flights on aerobatic models without the cowl, thus permitting some adjustment of the thrust lines without slotting the cowl fixing holes, but with a rapidly approaching deadline to meet I chose to bite the bullet and fit it. Truth is, there’s plenty of side-thrust already installed on the bulkhead, so I assumed that someone has put it there for a good reason!

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The fibreglass cowl is a thing of beauty, with the excellent paintwork being a perfect match for the covering. The supplied ‘blank’ cowl to help judge the cut-outs now seems to be the norm for ARTFs, and is a great help. Screwing cowls to models has always been a bugbear of mine, but I think I’ve perfected my technique. After drilling suitably large holes in the fuselage I screw in a short piece of (M8) nylon bolt smeared with 5-minute epoxy until it’s flush with the sides. This stays in place and provides the perfect reinforcement for the self-tapping screws – a very strong, self-locking fastening system that’s never let me down.

The next task was to fit the supplied tank into its pre-cut bearers and sort the plumbing. The tank is held in place with a 1/4” balsa bar, which I screwed in place rather than use glue, as the instructions suggest – makes it easy to remove should problems arise. I then glued in the servo tray using foaming polyurethane wood glue to fill any gaps.

No real problems with the tail feathers, the tailplane being stuck in after a few dry fits and alignment checks. The fin required several adjustments to get the fit and alignment correct but was likewise glued into place when square. This just left the radio and ancillaries.

WHAT’S LEFT
After completing the rudder linkage I couldn’t help wondering if I’d missed something… and I had! The tail wheel. This turns directly with the rudder and takes quite a bit of fitting, so take your time and get it right.

Servos of choice were standard JR 591s, Futaba 3003s and a Hitec HS 75BB (retract). After looking into various ways of multi-pin connecting the wing, the desire to fly took precedence and I opted to connect them directly into the Rx. I set all the movements as suggested in the instructions and mixed the flaps to down elevator in two stages on my JR 9X Tx. The Rx aerial was routed from the top of the fuselage to the moving part of the rudder and secured with an elastic band.

I spent an unusually long period of time playing with the retracts once again, showing them to various members of the Royle household and failing to impress any of them in the slightest (except for my 2-year old Boxer dog, who barked and tried to eat them). Oh well, perhaps it’s only me who’s impressed with mechanical simplicity! The retract speed was just right, not too slow as to warrant dropping the wheels two circuits before landing, and not so fast that they punch their way through the top of the wing to make inverted landings possible!

With the C of G checked and the model ready to go, the British winter weather lived up to expectations and did its worst for a week. With this there was clearly nothing to do but put the model on a shoebox in the conservatory and play with the wheels again.

LIFT OFF
The weather abated, and with the ST 90 whipping the APC 14 x 8” and aluminium spinner (not the plastic one that comes with the kit) at a fair old rate, I leaned her out in the pits and carried the Chipmunk to the runway. After gunning the engine to around half throttle she nosed over in a very undignified manner on the wintered (and long) grass strip. Not even full up elevator could stop the inevitable from happening. It was with a little embarrassment that I returned to the pits for a brew before enlisting the help of one of our beginners to hold the tail and provide her with a gentle push. “All part of the training,” I told him!

The engine was run up to full throttle, and with a nod of my head the brave helper released. The ‘Chippy’ (we nickname everything at our club) had only travelled about 10’ (3m) before leaping into the air, and I soon realised that the power to weight ratio was just what I’d hoped for – there was loads of grunt available. At just about half throttle I only had to give her a dab or two of down elevator and a touch of left aileron and she was flying a dream.

Basic circuits were the remit for this first flight, but I did try a couple of ‘slow and low’ passes, more for the small crowds and the camera’s benefit than anything else. The slow speed handling was superb, and I put the wheels up for effect. It seemed to me that the Chipmunk flew much smoother with the wheels away, and I was surprised at the difference. With the wheels back down for a ‘dirty’ pass up the strip I added the first stage of flap; she nosed up a little, but then, I was going a little fast.

With a lower speed everything was well, so I added the second stage of flap. She was now flying at a ludicrously slow speed, which surprised me as the flaps aren’t really that big. Mind you, she began to show some disapproval at the tight turns being asked of her, although nothing too nasty manifested itself. 

After a few approaches and overshoots she louched down nicely, but nosed over again just before coming to a rest.
I explored the Chipmunk’s capabilities a bit more on the second flight; rolling with just aileron she loses no noticeable height, loops can be as big as you like and stall turns can be performed as high as you want. The power is awesome, and much to my approval!

Back in the workshop at the end of the day, I rechecked everything to make sure nothing had come loose and moved the C of G back a little (lead added at the tail) as I’d noticed that she was pulling out of the dive slightly.

ROUND TWO
I waited around two weeks to get a decent flying day again, and this brought a new problem. Just at the beginning of a take-off run the wheels decided to retract! Upon investigation I found that the retract servo had fried and stopped driving one way, with the result that the wheels would go up but never come down. So, I disconnected the retract servo and locked the toggle over with a plastic cable tie. The servo positioning meant that only another slim servo would do the job, so borrowing one at the field was out of the question.
Determined to fly her, and with the help of my friendly novice, she was soon airborne again. Actually, I managed to release some up elevator on this particular take-off run and, with that, she made a reasonable length along the strip before getting airborne. After a few circuits Andy Ellison appeared and promptly tried to rip the wings off in an effort to show me that I needn’t be scared. Not sure Art Scholl’s cockpit Voodoo doll thought that, though! With flicks, spins and rolls at every opportunity he obviously liked it but made a few observations about the unbalanced controls, i.e. the up and down elevator felt different.

Andy urged me to be a little more adventurous, so with the transmitter wrested from his grasp I tried inverted flight, and I saw what he meant. The Chipmunk was a little heavy on down elevator compared to the crisp response of up elevator, easily sorted out using the ATVs and differential dual rate settings on my JR 9X. This time the landing was almost perfect, save for the eventual nose-over just as she came to rest. We really must get the grass cut!

WRAPPING UP
If you’re looking for an aerobatic ‘scalie’ that’s a change from the all too-common Extras and CAPs then you won’t go far wrong with this model. The flying characteristics would suit someone practising for a ‘B’ certificate (like me) or a more experienced modeller looking for an addition to his stable. The retracts are interesting, and to be fair I’ll try her again when our strip is its usual bowling green texture to see if the nosing-over problem is any better. The more I moved the C of G back the ‘flickier’ she became, but that may suit a more experienced flyer.

The World Models Super Chipmunk R90S is a revised, cheaper version of the older model (the 90S); the specification is virtually the same, and with a price tag of £149.99 (2006) it’s a lot of plane for your money. The shoehorned ST 90 two-stroke had ample power, though I would probably have opted to fit a four-stroke if I’d had one. Yup, despite my early reservations, the Chipmunk experience has been a suitably happy one!

DATAFILE
Name: Super Chipmunk R 90S
Aircraft type: ARTF semi-scale aerobat
Manufactured by: World Models
UK distributor: Steve Webb Models
Tel. 01928 735225 / 735252 www.stevewebb.co.uk
RRP: £179.99 (April 2011)
Wingspan: 64'' (1620mm)
Fuselage length: 53'' (1340mm)
Wing area: 4.7sq. ft. (0.44sq. m)
All-up weight: 7 lb (3.2kg)
Wing loading: 24oz / sq. ft. (7.3kg / sq. m)
Rec’d engine: .60 two-stroke, .90 four-stroke
Radio req’d: 6-ch, 7 servos (optional inboard flaps)

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