Wot 4 XL Mk 2 - Build

[Paul Adams]

Have just got my hands on a Wot 4 XL and thought I would keep you all informed on the build / flying experience. First of all I am an experienced builder / flyer but due to work / other duties, do not get enough time to build from a plan, so could not resist the idea of an ARTF model.

Why did I choose the Wot 4 XL - I have a 26cc petrol engine which I have been trying to find a home for, for some time, its power output is similar to a 120 4 stroke, so this kit ticked the box. I have also over the years built and flown many of Chris Foss's models (middle phase, Phase 5 and Acrowot) and found them all to be good models and all have flown well, but enough about me, back to the Wot 4 XL.

On opening the box the parts are well presented and protected well. Examining the model I have found no damage, just a little slack covering in one place on the fuselage, but that will iron out. The construction of the fuselage is mainly ply, the glue joints look good and it has a large must be 1/4 ply undercarriage plate, which should take some punishment. The undercarriage I believe is anodised aluminium, the cowling is of good quality and light weight and all the pushrods are of good quality with metal fittings and spring retainers. The kit comes complete with very comprehensive instructions with plenty of pictures. So first impressions - very good.

A bit more detail revealed that the instructions are probably from a slightly earlier version, as it refers to fitting the wheels to the undercarriage (already fitted) and the use of fuel tubing over the pushrod clevises (spring clevises fitted), More importantly though is that there appears to be an error in the instructions with regards to the engine mounting. The instructions state a distance from the prop driver to the bulkhead of 155mm, however the cowling is only 137 mm long and the instruction state that it should overlap the fuselage by 9-10 mm. So effectively meaning that the front of the cowl will finish some 127- 128 mm in front of the bulkhead, some 27mm behind the spinner / prop. I therefore believe the distance should be nearer 125mm.

 

Well that's as far as I have got tonight, if people are interested I will continue with the build and flying experience.

[David Ashby - Moderator]

Please do Paul, watching with interest.

 

 

 

 

 

 

 

[Paul Adams]

Slight correction in the above posting, the distance should be nearer 130mm so give 2-3 mm clearance rather than 2-3 mm interference fit.. Hopefully start the build tonight (wife permitting).

[Paul Adams]

 

What's in the box

Progress made tonight:-Wings - Servo's fitted and wings joined. The servo's needed 200mm extensions to give total length of 500mm. The wings had cotton fitted to pull the servo leads through, A very thoughtful addition as without them it would be near impossible to thread the servo leads through. Joining the wings went well but would not recommend trying to do it with a quick setting epoxy, as it does take time to align properly.Fuselage - Openings for tailplane and fin opened up. The instructions say to fit these next, but decided that it would be best to delay this stage until the engine is fitted to prevent damaging them. One thing I did spot though is that the tailplane covering on one side was only sealed at the edges of the tailplane and was not attached to the tailplane in the centre section, so a quick iron and all is good. Glad I spotted before I cut through it to glue it to the fuselageSo far so good, impressed with the build quality. Another update tomorrow. 

Edited By David Ashby - RCME Administrator on 08/03/2011 05:02:21

[Paul Adams]

Build progressing with the engine being attached. I had to stand off the engine from the bulkhead by 20 mm to get the correct length from bulkhead to prop spinner of 130 mm (not 155mm as stated). this was easily done by using a ply pad.

 

Once the engine was mounted, I went back to fitting out the tail plane. I dry fitted the tail plane and decided prior to gluing into place I would see how hard it was to fit the elevator push rod (this is the order of assembly as per the manual). I Found it was impossible. I therefore decided to fit the push rod prior to fitting the tail plane as you could at least have some access to try and align the push rod ends with the slits in the side of the fuselage. Even doing it this way I still had problems, but by straighting the push rod I was finally able to fit it. I then fitted the tail plane without any further problems.

 

Next came the fin, and this is where I discovered what I consider a major design flaw. The instructions say to fit the fin such that it makes contact with the tail plane. On dry fitting the fin it was found that it was short, so using a piece of paper I measured the correct depth and found that it was approximately 1 cm short.

 

I did consider just gluing the fin in as it was, but looking at the construction at the top of the fuselage  (5mm ply) I decided against as if the rudder was knocked (probably loading / unloading the car) the torsional forces applied to the fin would basically rip the rear of the fuselage apart, not a good idea. So a method of extending the bottom of the fin was required.

 

 

 

 

 

 

Edited By Paul Adams on 12/02/2011 09:11:46

[Paul Adams]

Fin Fix

 

Initially I looked round for a piece of balsa to replace the existing fin, but all my stock of balsa is imperial and the fin is made out of 7.5mm hard balsa. So finished up adding a piece of 1/4" balsa to the bottom. To get maximum strength I used a saw tooth joint, so hopefully spreading the load and used five minute epoxy. See below:-

 

I Once this was set I adjusted the length such that the fin sat correctly. A bit of 5 min epoxy and the fin was fixed good and strong. Looking at the design and the way the fin was cut I reckon that the Wot 4 XL Mk 1 probably had the tail plane about 1 cm higher. In the mark 2 version the tail plane was moved but the fin dimensions were not changed.

 

This sort of error should have been picked up in test build of the Mk 2 model as I do not expect to have to bodge the build to correct design errors.

 

[Paul Adams]

I installed the servos tonight and fitted the rudder pull pull wires. The wire tend to untwist when cut so I used a piece of masking tape to keep the wire together whilst fitting. Plenty of spare so not a problem, just something to look out for.

 

One mod I did though was to turn the rudder servo round such that the servo arm is near the back. This will allow me to fit a larger horn to give big movements which wont interfere with the elevator servo.

 

Did a test balance and without the cowling and batteries the C of G comes out a 100mm behind the leading edge of the wing, This is with a 26cc petrol engine up front, the spark ignition system in the place the fuel tank would normally sit and the fuel tank on the C of G (petrol engines work fine with remote fuel tanks and having the fuel tank on the C o f G means no difference in trim required during the flight.

[Paul Adams]

Finally got it all ready to go:-

Control throws set - low rates as per the book, high rates – maximum throws.

Balance point checked and is mid range according to the book,

Weight 10 pounds 2 ounces (4.54 Kg). (A little heavy but with that large wing area should not be a problem.

So now it’s all together time to wait for good weather which occurred last weekend, light winds straight down the runway. So of I went. The Wot 4 XL was assembled, all control throws rechecked for direction and throw. Range check carried out with motor running – All OK. A couple of compulsory photo’s and it’s time for that walk to the strip.

 

[Paul Adams]

On to the strip. Throttle opened up about a quarter at which time the model was accelerating down the strip, slowly increasing the power to about half way the tail plane lifted and the model left the ground and gained height. Had to add a couple of clicks of up trim and one of right aileron, but nothing drastic. Flew around for a couple of circuits to get the feel of the model, all felt very safe although the Aileron were a little twitchy near centre (a bit of expo required), but nothing uncontrollable. Throttled back to test the stall, the model slowed and stalled, but nothing nasty and immediately control surface authority was obtained. A couple more circuits and I closed the throttle down to tick over, the model slowed but required no change in trim. Applied full power and the model took off live a scolded cat, but still level flight. I pulled up into a vertical climb and the model just kept going straight up, so the engine thrust line is about right.

Inverted flight, just a hint of down elevator required, so C of G is in the right ballpark. I will fly it like this for a couple of weeks to get used to it before I start experimenting. Did a moderately fast roll to the left and another to the right, very little elevator required through the inverted stage, Tried a slow roll which produced a nice flat roll (using rudder and elevator to correct). Stall turn, no problem, Knife edge flight required a little down and opposite aileron to keep straight (so C of G could go back a little and a little mixing), but flew a complete circuit knife edge without too much problem. Flick roll, no problem, Took it up high into a spin, center the sticks and comes out within ½ revolution, nose down very nice. Inverted spin, a bit hesitant going in but once in, no problem. Not bad for a first flight, now for the compulsory bit, the landing. Called landing, throttled back and in she came, nice and slow, very stable, perfect three point landing. This model would make anybodies flying look great.

So a quick refuel (only used about 1/3^rd of a tank (5 fluid ounces)), put a bit of expo (30%) into the aileron and elevator and back to the strip. This time line her up, apply the power, and after she left the ground, pull up until she is vertical and power up to the clouds, rolling as you go. Into an inverted spin to bring her back to nearer the ground and do a low flypast at head height inverted. Pull her up into a stall turn, and do a flypast the other direction inverted. Into a half loop and do a long slow roll. At ten minutes the timer went off, but as someone was about to take off, and I knew I had lots of fuel, I kept flying. I was having so much fun I flew for another ten minutes, at which time the engine went dead stick (must have been out of fuel) Straight into the back leg of a landing circuit and touchdown mid strip, what a flight. No vices beautiful flyer, great fun. The only problem is that my jaw hurts from all that grinning. Would I recommend it to anyone else? Defiantly yes.

The next flight I let someone I had helped learn to fly last year have a go, and had to prise his hands off the transmitter to let me have it back.

Had a good check over of the model when I got home to make sure all is well. A couple of loose bolts / nuts caused by the vibration of the large petrol engine, but nothing a bit of lock tight won’t solve. The only other problem I found was that one of the wheels on the main undercarriage was out of alignment, which surprised me as all the landings were good. Had a closer look and the stud axles are mild steel screws, which are defiantly not up to the job, so will be replacing with proper stud axles before I fly it again.

So overall, a couple of problems with the build, the fin needed extending, and the wheel axles, but nothing major. But the flying defiantly makes up for it. I can see it being my favorite model for some time. The 26cc petrol engine is on the large size and will take the model vertically out of sight, but at low throttle the model is an absolute pussycat and the slow speed handling excellent. Even in the very light wind it would nearly hover. Chris Foss has done it again.

Edited By Paul Adams on 07/03/2011 22:01:22

[David Ashby - Moderator]

Excellent flying report Paul, well done

[Peter Wood]

Fantastic report. I am flying a standard Wot 4 (gently), Now in need of repair. I wondered if to to get another one, but not sure if a sc65 4 stroke would manage. It is quoted as 6.7 square feet. What is the area of the standard Wot 4.

 

Peter

[Steve Houghton 1]

I enjoyed reading that report Paul. I too fly a standard Wotty 4 and love it, they are just just great fun. If I ever get a larger car then I may get myself an XL.

[Paul Adams]

So after about thirty flights under its belt, hows it hanging together. Very good, still enjoying flying it. Its slow speed handling is excellent making landings simple. I have flown the standard size Wot 4 and thought they were good, but this is better. A couple of our club experts had a fly and one is looking at getting one himself, so it must be good.

 

Only problems I have had with it is the spinner which after about ten flights fell off in flight. Its never been a good spinner and had defiantly been manufactured down to a price. So a nice new aluminium spinner was fitted.The only other observation I have is I should have replaced the fuel tank with a smaller one. At 15oz and with a petrol engine fitted it will last for well over half an hour.

[Phil Reeves]

I see you are using a CRRC26, what prop are you turning? Because of this thread and that I have a spare CRRC 26 V2 I will now have to get one.

[Paul Adams]

I have tried a couple of props a 16 x 8 and a 18 x 6, both have advantages and disadvantages, so it will depend on your type of flying which suits you best.

 

16 x 8

Advantages - provides plenty of speed and vertical pulling power.

Disadvantages - Provides too much pulling power on tick combined with the ability of the model to fly at very slow speeds makes landings a challenge. To counteract this I have used flaperons allowing shorter landing.

 

18 x 6

Advantages - reduced speed and allows maximum use of the Wot 4's slow speed capability and making landings easy without the use of flaps.

Disadvantage - Engine unloads when in a slight dive.

 

I am waiting for a 17 X 8 prop to arrive to try that. I expect it will perform very similar to the 16 x 8 but should reduce the engine revs and therefore the noise.

 

 

[BB]

I use 17x8 Turnigy from HK. Great slow speed handling & as much vertical as your eyesight can handle. Final approaches are at just above tickover then cut for a 3 pointer everytime.

 

BB

[Derek Brown 1]

Built a Wot 4 XL using a Saito FG20 and just had maiden flight.

Found the same problems with the fin and made a central groove lengthwise in the bottom of the fin and then glued in a ply extension reduced in thickness to form a tenon joint and epoxied in place

Used the Saito metal mount which gave a 5mm engine c/l offset and used 5mm Viton rubber as anti-vibration pads between engine and mount and achieve correct engine c/l. --works well.

Model balanced perfectly with CDI unit and batteries in bay forward of servo mounting plate, secured on a ply mounting plate , sitting on rails glued to fuselage sides, allowing longitudinal adjustment of the plate to achieve correct CG .

As an ab-initio model pilot looking forward to using this as a trainer and easy to prepare model

 

Del

 

 

[Tim Reeves]

Very interesting read, I have been pondering over one of these for a while and have now made up my mind to get one. Have the same engine waiting to be housed.

What servos have you used?

 

Tim

[Paul Adams]

MG966R servos for everything (metal geared to take the petrol engine vibrations) except the throttle servo where a miniature servo was used. Also used a petrol kill switch from Just engines.

 

Paul

[Anthony Pattison 1]

Hi

I'm considering using an OS FT160 twin on WOT4 XL. Heavy lump but similar power to 120 four stroke I guess.

What do we think?

Tony

[Paul Adams]

The CRRC engine weight in at 1150 grams plus ignition system and 4AA battery pack, so the FT160 is slightly lighter so it should not be a problem. Power will be about the same, so shouldd be a good match.

[Paul Thompson 1]

I've just received my XL kit today, though not yet started I've noticed that there's no side thrust built in, did you find it needed any side/down thrust?

You might be interested to know that the faults you mentioned have been rectified namely the fin now extends down to the tail plane, and the undercarriage is supplied with proper stub axles instead of screws.

[Paul Adams]

Have just checked my engine mount and I have not added any down or side thrust, so I assume it must be built in or not much required.

Its good to hear that they have corrected the problems.

Mine had a slight mishap on landing a couple of weeks ago. We have trees at one end of our strip and as such have to do a dog leg approach. I had been instructing all morning, and then had a chance to fly my own model. Unfortunately being used to landing the standard size model all morning (including a standard size Wot 4) I misjudged the distance on the dog led approach with the Wot 4 XL, so instead of being in front of the trees it was behind them. It made quite a bang as it went through the trees. What surprised me most though was the lack of damage. The wing and its hold down plate at the rear had come apart from the fuselage and a few of the joints on the fuselage had come apart, a wing tip was slightly damaged and it had a ding in the wing covering, but that was it, not the carrier bag job I originally thought it would be . Its defiantly a tough model and will be flying again very soon.

[Tim Reeves]

I had my first flight with my XL, and I too had a BIG grin from ear to ear.

My mistake was not to keep an eye on the time on my tx, so ran out of fuel, this was not a problem as it glided well. The problem was when I landed, one of the wheels went into a hole caused by a pesky rabbit and it ripped the under carrage out. Once the aircraft came to a rest after completing a couple of somersaults, I thought there was going to be a lot more damage. There was a small hole in the covering, a sticker soon sorted out that one! On closer inspection of the ply, which the under carrage is mounted on, there did not seem to be much glue in that department, so now it has been repaired and glue applied,

I cannot wait until the next flight.

[Turbycat]

It might be worth fibreglassing the ply undercarriage mount into the bottom of the fuselage from the inside. I managed to rip out the undercarriage twice before i did this. I think it has been mentioned here before too They are pretty tough for an ARTF. I have found that the covering on mine is starting to come off. Mind you it has had a lot of abuse over the past year. I still take it with me every time I go flying. Glad you're enjoying it

Andy