I recently retired my Saturday afternoon hack, a Black Horse Twister. Reflecting on the ease of use of the Twister’s stable mate, an electric SebArt Sukhoi 30e, I decided to go electric for the Twister’s replacement and, after much cogitating, settled on the very compact and well-built 1200mm span wooden version of the WOT4. I had read on these pages and elsewhere of serious C of G issues due to the tail heavy nature of the WOT4e. I also read that the recommended C of G position of 70mm back from the l/e was apparently miscalculated or misprinted in the instructions and that the C of G position should be 90mm back from the l/e. My calculations indicated a position at around 87mm so I was less worried about the C of G issue, especially as I was planning to use a 3200mAh 3s battery weighing at least 50g more than the recommended 2200 to 2500mAh pack, with this additional weight forward of the C of G position.
WOT’S IN THE BOX
The first impression on opening the box was very positive. The wing halves aligned perfectly when assembled on the joining brace and the fin and tailplane sat straight when fitted into their slots. The cowl is beautifully finished and sits well on its mounting lugs. So, to the instructions for the build; the instructions are written in proper English, and provide the obligatory introduction from Mr Foss with a brief outline of his historic model making, complete with a photograph outside the factory circa 1990. They are reasonable in terms of content and build sequence; however, I felt that they could have been a little more comprehensive. There are several good tips to help avoid building errors but overall the instructions are very concise (especially compared to the handbook that I worked through during the recent construction of a Great Planes ARTF).
The hardware was of generally good quality except for the very poor clevises – both metal and nylon. When assembling the closed loop rudder system it became apparent that the metal clevis threads were very shallow and wouldn’t tighten on the brass eyelets. As the thread was M2 it was an easy fix after a rummage through my spares box for replacement M2 clevises. There were also two sets of clevises and eyelets provided for the closed loop system and in the instructions only one set are used at the tail end. The nylon clevises for the ailerons and elevator are pretty flimsy; however, the threaded metal rods provided are not M2 so are not so easy to replace with standard metal M2 clevises. Fortunately, I had a couple of spare Great Planes nylon clevises that did the trick for the elevator (two needed here as the elevator pushrod splits before exiting the fuselage to drive the two separate elevator halves).
As for the ailerons, I trimmed the smart, matt black pushrods to length and fitted the supplied nylon clevises. I then slid the short lengths of plastic tubing that were provided to lock the clevises onto the aileron horns into position. The clevises literally collapsed in the middle. They looked ridiculous and were not convincingly secure for my liking. Each time I looked at the underside of the wing my eyes were drawn to them. It just wouldn’t do to leave them, so, after completing the build l revisited the aileron linkages. I further shortened the linkage rods, filed the ends of them to remove the paint and roughen the surface of the metal and soldered them into M2 connecting rods. No problem then fitting yet another couple of proper M2 clevises from the spares dept. Definitely worth the extra effort!
… AND WOT’S NOT IN THE BOX
There is no recommendation in the instructions for servo size or torque rating. In fact, servos were not even mentioned under the heading 'Items Recommended to Complete'. This aircraft is of a size that will, on flying days, temp me to assemble it at home and load it into the car in one piece when heading off to the local club field, so there is the risk that control surfaces will be bumped.
I decided metal geared servos were not too much of a luxury. I measured the aileron servo mounting holes and then reverted to Google to find suitable servos, settling on four Towerpro MG16R metal geared digital units. These have a torque rating of 2.7 kg/cm at 4.8V – roughly the same torque of the old 'standard' servo that we once would have fitted to a plane of this size without a second thought. They are available from the internet shops for less than a tenner each and from initial inspection and testing appear to be very good value for money.
I had a DYS 3548 1100kv outrunner motor and a 50A ESC that looked about right when compared to the Ripmax recommended set up. I have read that some folks have struggled to find a range of motors that fit the predrilled mounting holes with captive nuts fitted ex-factory. I had no problem here in that the aluminium cross mount for the DYS motor aligned with the holes and only required very minor filing so that the supplied M3 screws would align with the captive nuts without binding on the aluminium mount. The motor had previously been mounted inside a front bulkhead and the biggest challenge was moving the motor shaft through the casing to facilitate a rear mount, as the shaft was an incredibly tight fit in the case.
ParkAero sent the servos to me overnight and I had battery packs so when all the parts were available to weigh I came up with a projected ready to fly weight of around 1,800g. The WOT4 is very solid indeed.
FINISHING TOUCHES
A long stint in the garage one night saw work nearing completion and I was far enough down the line to start checking the C of G. Keeping in mind that I had an average weight motor (156g) and could just squeeze the ESC well forward inside the cowl on the side of the motor mount (where it would be in the airflow that exits at the side of the cowl) and the extra weight in the oversized battery pack, I was ever hopeful that the aircraft would balance on a point somewhere between 85mm to 90mm back from the l/e. Pigs might fly! Despite my best endeavours the model balanced at 100mm back from the l/e and it took 70g of lead mounted well forward, strapped to the side of the motor mount box to bring the C of G to the 90mm mark. Final weight with battery is 1,875g. I hate carrying dead weight in an aircraft but there is no option with this one!
Now, this is where it gets interesting. Whilst my SebArt Sukhoi is intended for a different purpose and flying style, I do find that it can be a great sports aircraft when flown on low rates and I could not help making the comparison. The Sukhoi has 20% more wing area and, with a heavier than standard carbon fibre undercarriage, a chunky 600kv motor and a 4000 4S Li-Po, weighs just under 1,800g. With the suggested 11×5.5 prop on the WOT4, a bench test showed a current draw of 32 amps at 10.5 volts (336 watts). Hmmmm, my Sukhoi takes off with 700 watts on tap and 100g less weight.
I just hoped that, considering its weight, the model wouldn't be a bit light in its loafers from a performance perspective. I'd make sure to take alternative props to the field to flight test. As for the wing loading? This came out at 62.5g/dm2 or 20.5 oz / sq. ft. in the old money. Prior to flying I reassured myself with the positive comments from other owners that have been posted on various forums confirming this aircraft flies well heavy. Time would tell.
FLYING
With C of G at the 90mm mark and control throws set as per the instructions, a fully charged 3s 3200mAh battery was loaded into the fuselage and all control surface received a final check. The Wot4 was then despatched into the cold, damp 10-12 mph December wind blowing more or less straight up the runway. A short roll and the aircraft was airborne. Aileron and rudder trim were spot on, however; a surprising amount of up elevator was required to maintain level flight (I will have to check wing and tail plane incidence as there is no other obvious reason for so much up trim). I had not set rates and found the 18mm throw on the ailerons very lively, even with 50% exponential. The 11×5.5 prop was all or nothing, requiring full power for most of the first flight – lower power settings resulted in the model flying too slow. The first flight was seven minutes and the Li-Po was still showing over 11 volts after landing.
For the second flight I fitted a 10×7 prop and found the aircraft transformed. Adding rates (60% on low setting) meant that the take-off into the blustery wind was far smoother on 'low' than on the first flight with no rates. The take-off roll was probably only about 2 metres straight into wind and the power delivery was much improved, enabling a wider speed range, especially in the windy conditions. I planned another seven minute flight and was closely monitoring battery performance as the timer sounded on my Tx. Motor speed still sounded healthy so I stayed close to the runway for a couple of truncated circuits, landing at walking pace about 10 seconds short of eight minutes. A check of the second battery after landing showed voltage to be only marginally lower than the first. This wasn’t surprising as I was able to throttle back more with the 10×7 prop whilst maintaining sufficient forward motion to not wallow about or fall out of the sky.
The WOT4e lived up to its reputation for enjoying a wide speed range, happily penetrating into the strong winds at high power settings, or flying slowly enough into the wind to have a negative ground speed as it disappeared backwards downwind. With the recommended control surface settings, tight loops, snap rolls and stall turns are all easily performed. The stall was a non-event. The aircraft slowed to the point that it was just gently descending vertically in a high alpha position, wings remaining level. It was a jolly cold day with wind chill dropping the temperature by a good few degrees so I didn’t try knife-edge or extend inverted flight, not trusting my frozen fingers on the sticks.
After returning home and recharging a battery I checked current draw and rpm with the 10×7 prop. The results were 41 amps at 10.2 volts producing 418 watts and 8,750 rpm. This is the better setup as, with the ability to use lower power settings in flight, endurance is not impacted by the higher current draw at full power and overall the performance is much, much better.
CONCLUSIONS
The assembly is very straightforward and the quality of the design and precision of the wood cutting is evident from the excellent fit of airframe components. The aeroplane has a good feel to it and comes without any dry joints or flimsy balsa panels. The film finish is excellent with only very minor attention required on some of the trim. Naturally, there is a weight penalty for such robust construction, but for a sports hack I would prefer to err on the side of industrial strength in favour of a fragile floater.
The supplied clevises are not up to the standard of the kit in general. As these are relatively inexpensive components, this is spoiling the ship for a ha’penny of tar. The instructions need reviewing: in addition to my comments above relating to C of G position, closed loop parts not matching the described method of connecting the system and the lack of servo guidance, the instructions completely omit the fitting of the wing bolt brace for the top of the wing. A list of included parts and a more comprehensive list of required parts would not go astray.
In closing, a great ARTF kit of the classic club sportster. A few minor niggles but good value for money and great fun at the flying field.
FLYING UPDATE – 2 MONTHS LATER
In between the freezing winter days, squeezing in a bit of slope soaring on Sunday mornings and the usual demands of a busy family life here in idyllic Jersey, I have managed to aim my WOT4 skywards 16 times now. Along this journey of discovery I have tried a variety of options focused on fine tuning prop selection, control throws and C of G position. If you are considering venturing forth with a 'woodie' WOT4 ARTF electric my experiences may be helpful to you.
PROPER PROP
On the second outing I managed to damage the 10×7 prop with an untidy return to earth. A club mate helped out with a loaner to get me airborne again and this situation provided me with an opportunity to try some alternative props. I tested a couple of different pitches and diameters and ultimately settled on an 11×7 prop as this provided a further flight performance improvement without breaking the bank from a current perspective.
Static tests indicate that the current draw on full power ranges from 41A to 46A depending on the charge state of the battery. This aircraft will happily laze about on half power and I am typically only applying full power for a few seconds on take-off or through manoeuvres, so maximum current draw sits happily within the 50A constant/60A boost capacity of the ESC without overheating it.
I can also still enjoy a safe 7 minutes of air time without threatening the long term health of my battery packs through regular deep discharging. The batteries come out of the fuselage after a flight pleasantly warm (great on cold days as a gentle hand warmer!) and typically will show a retained charged of around 20-25% depending on how windy a day it is.
TAMING THE SHREW
Control throws as recommended in the instructions seemed OK initially, when 60% low rates were applied. However, it became apparent that full throws on aileron and elevator produced a very nervous aircraft that wasn’t easy to fly smoothly and the degree of control sensitivity on low rates was still a little unpleasant.
Just like the Bank of England in the not too distant past, I applied progressive rate reductions to bring the machinery under control. The intention was to maintain reasonable aerobatic performance on low rates and to have very spritely performance on high rates. I have settled on the following control surface throws, which are the same in each direction. I have provided the throws recommended in the instructions for comparison:
Aileron
10mm low rates
14mm high rates
18mm instructions
Elevator
8mm low rates
12mm high rates
20mm instructions
Rudder
20mm low rates
50mm high rates
40mm instructions
I am not a 3D pilot but do enjoy tame aerobatics and I find that reasonably fast rolls are possible on the low rate aileron setting above. The low rate elevator throw is good for loops and bunts and the higher setting enables very tight radius loops. For most flights I leave the ailerons and elevator on low rates and this is more than sufficient for a range of manoeuvres such as the BMFA 'B' certificate routine. My rudder settings vary significantly from low to high as I have gone for quite a tame throw on low rate to facilitate easier control of tracking down the runway on take-off. I switch to high rate as soon as airborne and leave it on high for the entire flight and, with 50mm throw each way, stall turns are a breeze.
CENTRE OF GRAVITY
This has been debated in the forums at length and, it would appear that those of us with WOT4 woodies may have been unnecessarily increasing our stress levels. It is apparent that Mr Foss did his homework well, producing an airframe that will tolerate a decent C of G range and, should ballast be required for trimming, a capability to carry a little extra weight with no adverse effects on overall flight performance.
Nevertheless, I was keen to remove the 70g ballast I placed in the nose if this was at all possible. I have gone with heavier than recommended 3200mAh 3S batteries weighing 253g, positioned forward of the C of G, and did not want the extra dead weight of ballast for trim if it could be avoided. I started my C of G shifting experiments by adding a small counter weight to the tail plane to see if a trim change rearward was suicidal. The effective ballast change was equivalent to reducing the nose ballast to about 35g, or half. Handling was assessed over two flights in and there was no discernible difference.
I then removed my temporary counter balance and the 70g lump in the nose, ensuring that the ESC was sitting as far forward as possible under the cowl to ease my conscience. One of those rare sunny winter afternoons with little wind demanded a test flight, so off to the field. A brace of batteries was discharged with the more rearward C of G and still no significant change in control sensitivity. To sanity check my set-up I had a very experienced club member who owns a foam WOT4 fly my woodie and he gave the overall performance a thumbs up. I have also flown in very windy conditions with the new setup and there were no surprises.
HARD FACTS
Accurately measuring the C of G position back from the LE is challenging given the wing thickness and my steam-driven balancer, however, after much measuring from the LE and double checking by measuring from the TE, I believe I can state with a degree of accuracy that the C of G is now at 96mm back from the LE, or 38.4% of MAC with the aircraft sitting level on the balancer. The final C of G position came out a little further forward than I expected, probably due to a combination of more accurate measurement, a very slight increase in prop weight due to the move from 10in to 11in diameter and ensuring that the ESC was as far forward as possible. I have now marked this C of G position with a couple of small lengths of trim tape on the underside of the wing as the preferred position.
One other cute discovery, which you may have already made for yourself, is an inbuilt battery hatch tidy to stop the hatch cover from blowing away on windy days when “refuelling”. The spacing of the magnets on the battery hatch cover aligns perfectly with the undercarriage mounting screws. Take the cover off, turn it through 180 degrees and sit the magnets on the screw heads whilst changing batteries.
Good flying and please do provide feedback on your own WOT4e woodie experiences.
DATAFILE
Wingspan – 47" (1.2m)
Fuselage length – 41" (1050mm)
RRP – £94.99
Distributed by: Ripmax
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Thanks to Ripmax and Terry Walters for the pictures.
