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Chris Foss Wot Knot - Build Thread


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The History of the Winter Build

A club, of which I am a member, hosted a BMFA Electric Fly-in, which was well attended, with modellers bringing an eclectic selection of electric models.  One of these modellers was Mark Agate, who brought along a Mini Blitz, built from the free plan in RCM&E magazine from November 1995.  The model was originally designed around a Cox 0.49 glow engine.  Mark has converted his, very successfully, to electric power.  When chatting to Mark he said that this model was the subject of his club’s winter build.  My club, to my knowledge, had not previously organised a winter build competition; I approached the committee, they were onboard, and the rest is history, so to speak.

Back in 2022 we built the Mini Blitz, in 2023 we built the Sticky, also from a free plan in RCM&E magazine from January 2018.

The idea behind the winter build is to encourage modellers to get back into building, or in many cases, have a go at balsa bashing for the first time.  Success is measured by the number of members who have a go.

 

The 2024 Winter Build Model

I am particularly proud of the subject that I have chosen.  Most modellers have owned a Chris Foss Design’s model at some stage of their modelling life, I know I have had a few.  You may have learnt to fly on an Uno-Wot or a Wot-Trainer, then progressed to a Wot 4 (high-wing), an Acro-Wot (low-wing) or even the Wots-Wot (biplane).  Most modellers I have spoken to were not aware that Chris also designed a mid-wing power model.

I thought I knew of most of the Chris Foss Designs models, but I had never heard of the Wot Knot (mid-wing).  This was never kitted; it was only available as a plan.  Chris designed the Wot Knot in about the mid-seventies, before the first Wot 4, which came along in the late seventies.  The first Wot 4 was a plan only, then Chris started producing kits of the Wot 4 Mk2 around 1980.

 

The Winter Build Competition

The first task is to elect a Winter Build Coordinator; they need to be someone with reasonable computer skills.  The main competition is around the building, covering and finishing of the model.  The committee can organise other, flying competitions, suited to the model, throughout the year.

The entrees need to build and cover their models and email a picture, or two, of the model to the coordinator by a cut-off date (e.g. the end of April).  If my experience is anything to go by, these will arrive imbedded in an email, attached to an email, via WhatsApp or I have been asked to take the pictures myself.

Once all the entries are in, the images need to be collated and numbered.  The coordinator will nominate someone, who is not entering the competition, they do not have to be in the club; their job is to judge their favourite three models, from the numbered pictures, for the first, second and third places in the competition.

 

Model Details

The Wot Knot is a Chris Foss Designs fully aerobatic model:

·       Motor: .19 to .40 cubic inch engines (6.5cc) or an electric setup

·       Wingspan: 46”

·       Spinner: 2”

·       Main Wheels: 2 ¼”

·       Tail Wheel: 1”

 

Getting a Plan / Kit

Plans are available from Chris Foss Designs directly: https://www.chrisfoss.co.uk/.  I sent Chris a cheque payable to “Chris Foss Designs” for £20 and received by plan about a week later.

Chris Foss Designs
Unit 1 Freehold Street
Shoreham by Sea
West Sussex
BN43 5TQ

Chris has confirmed that it a few members club together to have multiple plans send to a single address, then he can reduce the postage costs.

The plan shows a built-up wing, which will definitely be the lightest option.  If you do not fancy tackling a built-up wing, or prefer using foam cores, which will be make a tougher more durable model, then I have organised them to be available as on option.

The model needs a clear canopy; these can be vacuum formed over a mould; these are also available as an option.

 

Where to Source Parts

Future Crafts: https://www.futurecrafts.uk/

·       Laser cut wing ribs: £14.00

·       Laser cut ply formers: £15.00

·       Laser cut fuselage sides: £16.00

·       Laser cut tail, elevator, fin and rudder: £13.00

·       All of the above: £55.00

Vortex Vac Forms: http://www.vortex-vacforms.co.uk/

·       Canopy (CN60 - North American P51 Mustang): £12.00

Cloud Models: https://www.cloudmodels.co.uk/

·       Foam veneered wing panels only: £42.00 plus £7.00 p&p.

·       Wing kit comprising foam veneered panels plus wood leading edge, trailing edge, tips, ailerons and wing joining tape: £53.00 plus £7.00 p&p.

 

How to Power the Model

Electric Setup: 4-Max The Fixed Wing Electric Flight Specialists: https://www.4-max.co.uk/

3S setup should provide around 3.9lbs of static thrust with an estimated top speed of around 57mph

·        Motor: PO-3547-960 £35.99

·        ESC: 4M-HESC50AV2 £45.95

·        Programming Box: 4MHV2-PROGBOX £11.99

·        Propeller: JXF 11x7 £9.68

·        LiPo Battery: PPL-60C3S-2200 £23.50

4S setup should provide around 4.8lbs of static thrust with an estimated top speed of around 62mph.

·        Motor: PO-3547-800 £35.99

·        ESC: 4M-HESC50AV2 £45.95

·        Programming Box: 4MHV2-PROGBOX £11.99

·        Propeller: JXF 11x7 £9.68

·        LiPo Battery; PPL-60C4S-2200 £33.50

Glow Motor:

OS Engines MAX 35AX w/E-3080 Silencer (£184.99) http://www.ripmax.com/Item.aspx?ItemID=L-OS13100

O.S. MAX 25FX II with 892 Silencer (£199.99) https://www.ripmax.com/Item.aspx?ItemID=L-OS12662

SC SC32A-S 2-Stroke Glow R/C Aero (£86.99) https://www.jperkins.com/products/SCEA032524

SC SC36A-S 2-Stroke Glow R/C Aero (£99.99) https://www.jperkins.com/products/SCEA036590

 

Controlling the Model

The model will need either one or two aileron servos (see the section on change of plan).

The model will also need a servo to control the elevator and a servo to control the rudder.  If you are using a glow engine, then you will need another servo to control the throttle.  If you are going to use electric power, then ESC will replace the throttle servo.

I personally think the model would be well suited to mini servo like the Hitec HS-225.  These are smaller than standard but still have four mounting screws.

 

Change of Plan:

The plan suggests using a system involving two bell-cranks in the wing to operate the ailerons from a single central servo.  This will work but could also be replaced by two servos instead of the bell-cranks.  This would reduce the possibility of any play in the aileron linkages.

The plan shows the undercarriage being held on by two ⅛” aluminium plates; these could be replaced by plastic saddle clamps.

With the glow motor installed upright it is not possible to get the centreline of the carburettor lined up with the centreline of the fuel tank.  It is possible if the slow motor is rotated ninety degrees.

 

Good luck and happy building…

 

Note: all prices were correct at the time of writing the article.

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6 hours ago, Antony R said:

Morning, just tried to order a wot knot kit but PayPal gave me a big red warning that future crafts (even though their address is uk) do not post to my uk address? 

 

I will ask my mate Glenn (of Future Crafts to investigate).  This is all very genuine, I am one of the Weston Park commentators, if that makes you feel any better.

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  • 2 weeks later...
  • 4 weeks later...

Glueing in the leading edge spruce strip, while trying to ensure the ribs and spruce strip geometry are correct. I’m using super aliphatic on the spruce strip, as taking it out and using normal aliphatic resin was too difficult to maintain proper geometry within glue working time. It was easier to place everything correctly and then wick in glue around the joins. Will use aliphatic resin for the main spars top and bottom.This order of construction is following what little guidance is available from Chris Foss.  

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Edited by Jonny L
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Bottom spar in. Bit of a tight fit and needed a touch of sanding. Waiting now for the 1/16th square strips to sit either side of the leading edge spruce strip before I can dismount the wing and fit the top spar (the wing is initially built upside down). 

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Edited by Jonny L
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Second spar in, now starting on the vertical webbing. I’m going to diverge from the plan slightly and add a second webbing layer on the front of the spar. Extra strength for very little extra weight. 

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I don't think there's much need to add interspar webs to both sides of the mainspars.  I've never seen (or heard of) a mainspar breakage other than at the join but I usually but the webs in front of the spars  to make clamping the L/E sheeting easier - a Peter Miller tip IIRC.

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2 hours ago, Geoff S said:

I don't think there's much need to add interspar webs to both sides of the mainspars.  I've never seen (or heard of) a mainspar breakage other than at the join but I usually but the webs in front of the spars  to make clamping the L/E sheeting easier - a Peter Miller tip IIRC.

Good tip. I’ve done both side of the spar anyway - looking for torsional rigidity rather outright strength to prevent breakage. Feels like an overall benefit with minimal downside. The Chris Foss design has the web on the back of the spar, perhaps this addition with improve the adhesion of the L/E Sheet as well. An unexpected bonus!

Edited by Jonny L
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