BLOWING UP A HUNTER!

BLOWING UP A HUNTER!

Our latest Lockdown Project is from Cliff Whittaker, who fancied a slightly bigger version of Hawker’s famous jet

The December 2019 edition of RCM&E included the plan of a Hawker Hunter, designed by Tony Nijhuis. The plan looked straightforward and I considered having a go. But with a wingspan of just 25 inches, it looked small, fast and twitchy and I decided it wasn’t for me.

Why? Well, I first got going properly with aeromodelling in the 1980s. I joined a club and built and flew sport models with average success for several years. Then I married, children arrived, I changed my job and we relocated to Sussex. I put my aeroplanes in the garage of our new home – where they stayed untouched for over 20 years!

About five years ago I decided to return to the hobby. Unfortunately, I find flying more difficult now than when I was younger. I get disorientated or miss the runway much more often than I used to and so suffer rather frequent crashes of varying severity. I even keep a black bin liner in my flight box to carry the pieces home in. There’s optimism for you! That is why I decided against the Hunter.

But January 2021 came and the magazine with the Hunter on the cover was still on the coffee table tempting me. If only it were bigger; I would be much more comfortable if it had a wingspan of around 4ft. That was the starting point.

PLANNING THE PROJECT

The thrust for the Tony Nijhuis model was provided by an FMS 50mm ducted fan with a 3S LiPo. According to the 4-Max website the Powerfun 70mm fan (for a 4S) only costs £5 more and has twice the thrust. So why not make the Hunter twice as big with twice the thrust? I considered re-drawing the plan, then had a better idea. I took the plan to my local photocopy/printer shop and got it enlarged from double-sided A3 to two single-sided sheets of A2. Feeling smug I ordered the fan and ESC and started to plan the build.

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Cliff’s enlarged plan was for a 40% bigger Hunter, with a wingspan of 34-inches and double the wing area, not the 48-inch span model he had intended!

To avoid mistakes later I decided to amend my enlarged plan straightaway to reduce the jet pipe cut-outs in the rear frames from 100mm to 70mm diameter to match the fan that was on order. I was surprised to find they were already 70mm on the plan. Then the penny dropped. When you go up a paper size it doubles the area, not the linear length – the lengths only increase by a factor of 1.41!

My enlarged plan was for a 40% bigger Hunter, with a wingspan of 34 inches with double the wing area, not the 48-inch wingspan model I had intended. What an idiot!

As we were then into the first full lockdown the printing shop was closed and I couldn’t go back for another go. After some deliberation I decided to go with what I had. I later realised that was a very good decision because it meant that the distance from the fuselage nose frame to the end of the tailpipe was 35 inches – just within the length of a standard sheet of balsa. If I had actually doubled the linear size it would have meant a join in every longeron and every piece of fuselage skin, which would have been a right pain in the proverbial.

Enlarging by photocopying meant that the thickness of every part on the plan had increased by 41% – call it 50%. Effectively 1/8″ became 3/16″, 1/4″ became 3/8″ etc., but I decided that I would use the wood thicknesses shown on the original plan. That should save weight and the fuselage would be stronger anyway due to its increased diameter. If I could build a model with twice the thrust and twice the wing area, but less than twice the weight of the original, I should be winning.

The exception to the ‘original thickness’ rule had to be the wing – to have enough strength and to fit the slot in the fuselage. On the original model the wing was made from 1/4″ sheet balsa, so mine would be 3/8″. I didn’t have enough 3/8″ balsa to make it, but I had plenty of 1/8″. That got me thinking again…

Why not make the wing using three laminations of 1/8″ sheet? I could vary the grain direction for strength and save weight by making the centre lamination an open structure rather than a continuous sheet. If I did that, I could bury a control snake in the wing to drive the ailerons from one central servo. And that would mean I could have the flap and aileron configuration of the full size instead of the full span ailerons shown on the plan!

I also thought about the final colour scheme. I wanted something bright and easy to see. I chose the prototype Hunter WB188. In 1953 it was modified and painted red to break the speed record for jets in the hands of Neville Duke. WB188 differed externally from other Hunters in having a longer nose radome (easy to do) and a continuous wing leading edge i.e., without the ‘dog tooth’, which was how it was drawn on the plan anyway.

CONSTRUCTION

My building experience is all traditional – balsa, ply and sometimes foam cores; that’s how it was in the 80s. However, no-one who has seen my models close up would ever call me a craftsman. My models tend to have raggedy edges and uneven, sometimes wrinkly, surfaces – rather like me. I hadn’t built a ducted fan or mid wing model before, so the Hunter would present new challenges.

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One of the most satisfying moments of the build was when this shape emerged from the roughly planked construction.

Fuselage – I built the fuselage as drawn on the plan, except for making the nose longer and more pointed. It is a long octagonal box formed from plywood frames, connected by longerons and planked with thick balsa skins that are planed and sanded to the final shape. For me, one of the most satisfying moments of the build was when I put down the razor plane and sanding block to see that I had managed to produce a straight fuselage that had the desired torpedo-like shape.

Wings – As I had decided to have ailerons and flaps, I drew a line on the plan at 70% of the local chord of the outboard panels. This defined the break between the main wing and the moving surfaces. From 1/8th balsa sheet I made the lower lamination of the main wing flat on the building board and cut exit holes for the aileron control snakes. Then I glued on the centre lamination: leading edge, rear ‘spar’, some flat ‘ribs’ and pieces to fit the wingtips to.

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The redesigned wing without the top skin.

Aileron torque rods were fitted to the rear spar to drive the flaps and the aileron snake was glued in. Then the 1/8″ top skin was added, along with the wing tips. The leading edges and tips were sanded to the appropriate symmetrical cross-section. I found that the three laminations made this easy because I simply sanded until I reached the centre lamination at each edge. The final rounding was then achieved with just a few extra strokes of the sanding block.

There then followed a prolonged session of trying to get the wing into the slot in the fuselage. A case of ‘in out, in out, cut the wood about’. While doing this I realised that it would be better to cut and drill the wing centre-section for the radio installation away from the fuselage, with repeated trial fitting at each step. All-in-all there was an awful lot of wing ‘Hokey Cokey’ before I finally glued it to the fuselage.

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The redesigned wing with the top skin.

Three sets of flaps and ailerons were cut from 1/8″ sheet and then glued together to make one set 3/8″ thick. They were then sanded to a triangular section, again by sanding down to the centre lamination, leaving them 1/8″ thick at the trailing edge,

Fin & Tailplane – and a crisis of confidence… The fin, tailplane and elevators were simply made from sheet balsa. When I trial fitted the complete tail unit to the fuselage/wing assembly the tailplane looked frighteningly small. This fed my concerns that the model might be too twitchy for me.

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Tail unit with the original elevators removed and enlarged surfaces fitted.

So, what to do? I couldn’t scale up the fixed part of the tailplane without compromising the existing fin and changing the fin shape would ruin the profile of the model. I did a drawing and found that doubling the chord of the elevators would increase the area of the complete horizontal tail by 30% without being glaringly out of scale (at least to my eye). So, I made a new set of larger elevators, with a note to self to halve the suggested maximum elevator movements when setting up!

I also made a fin/tailplane rear ‘bullet’ fairing, as on the full size. This was carved from scraps of thick sheet glued together.

COMPLETION

Installing the radio was straightforward, except for the flaps. I had failed to appreciate that because the rear spar is swept the torque rod lever ends move together when the flaps go down and apart when the flaps come up. This is okay when the control runs are long but with my design the servo was within a few inches of the lever ends and their lateral movement was comparable with the servo travel.

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Radio bay above the wing. The redundant flap levers are visible.

Over the course of many hours, with much unprintable commentary, I repeatedly proved that I couldn’t make a linkage between the flaps and the servo that would reliably give the same flap angle on both sides. I finally did the sensible thing – I glued the flaps in the fully retracted position!

The rest was simple enough. In one of my boxes of ‘useful things’ I found a generic teardrop canopy that I was able to trim to fit. I covered the whole aeroplane in red Oracover. The silver ring around the jet pipe is self-adhesive aluminium foil from Screwfix. On the internet I found stick-on roundels and fin flashes (using stickers of the French flag cut to shape).

I ordered the WB188 lettering from www.vinylletteringonline.co.uk

Their website lets you choose the font, colour and letter height of your text. What you receive is the individually cut vinyl letters sandwiched between two peelable sheets. The top sheet keeps the letters inline and correctly spaced while you smooth the sticky side onto the model. I found it works really well.

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1/12th scale Neville Duke in his office.

The insertion of a 20g weight into a slot in the bottom of the solid nose had the model balanced on the CG position shown on the enlarged plan. The weight came out at 52 oz, which is a little more than twice the weight of the Tony Nijhuis prototype. Not what I had hoped for! It means my Hunter has a 14% higher wing loading, but that should equate to only a 7% increase in stalling speed (1.07 being the square root of 1.14)*. Disappointing, but not a disaster.

*At constant angle of attack, in this case the stalling angle, lift increases with speed squared.

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Model and modeller (with lockdown hair!)

WOT, NO WHEELS?

I put pictures of the completed model on our club website. Norman, our Chairman, recommended using a bungee to launch it and Stuart (another club stalwart) offered me the use of his bungee cord. I found an excellent article on bungee launching by Dave Royds on the RCM&E website: www.modelflying.co.uk/flying-start

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Moving the CG back should reduce the effectiveness of the fin – fingers crossed!

From Screwfix, I got some 40mm domestic waste pipe and push fit connectors, from which I built a ramp that comes apart easily to go in the car. To avoid having pipe connectors at the upwind end of the launch rails, I bent the rails through 90 degrees by melting them with an electric paint stripping gun. Crude but effective.

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Bright red WB188 should be easy to see in the air.

FIRST FLIGHT

Acutely conscious of my inconsistent flying skills, I took the Hunter to the club field with some trepidation. My vague plan was to have a few flights with one of my other aeroplanes first. If I was having ‘a good day’ I would consider trying the Hunter. As it turned out, the aforementioned Stuart came over to look at the Hunter. He offered to ‘give it a go’ for me and I gratefully accepted. After a couple of false starts, due to not getting the bungee right, the Hunter shot off the ramp like a scalded cat and climbed steadily away in a wide left turn. Within half a circuit of the field it was up at about 400ft. Impressive!

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Hunter on the launching ramp.

Four minutes later it was safely back on the ground.

The good points: the Hunter was stable in pitch, it trimmed out straight and level and had enough thrust.

Things to fix: it was pretty lively on the ailerons and whenever there was any bank angle the nose dropped strongly towards the lowered wing. This couldn’t be prevented by the pilot as the model doesn’t have a rudder.

POST FLIGHT CHANGES

The excessive aileron control was easily fixed by reducing the servo travel. Regarding the dropping of the nose in turns, I believe what we have here is a case of spiral instability. See this link to the FAA library:
www.faatest.com/books/FLT/Chapter17/SpiralInstability.htm

Essentially, spiral instability is caused by having a fin that is too effective for the rest of the aeroplane, because it is too large and/or has a moment arm about the CG that is too long. Put simply, whenever Stuart rolled the Hunter it started to sideslip towards the lowered wing, and it rotated like a weathercock to face the new direction of the airflow – and so the nose dropped towards the lowered wing.

How could I reduce the effectiveness of the fin? Not wanting to change the shape the only option was to move the CG back. Tony Nijhuis warns strongly against that in his article, presumably due to pitch stability concerns. But as the tailplane on my Hunter is proportionately 30% bigger than on his model, I should be able to have the CG further back without running into pitch stability problems. On my enlarged plan moving the CG back 28mm would put it at 25% of the wing mean chord, which ought to be okay?

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Cliff’s enlarged Hunter has flown but the ‘flying’ pics in this article were really handheld.

I also applied the method in Gordon Whitehead’s book ‘Radio Controlled Scale Aircraft’. This gave 22mm behind the balance point marked on the plan as a ‘safe’ position. Removing the nose weight and moving the Rx battery moved the CG back 25mm, which splits the difference, so I have gone for that.

The model now awaits its next outing. I really need a few good consecutive days of flying my other models to get me fully ‘current’ on the sticks before flying the Hunter. At the time of writing the Covid restrictions and the British weather have thwarted me in this.

CONCLUSION

I am very glad I did this. I have pushed my personal boundaries, learned a few lessons and got the aeroplane I wanted. And it has flown successfully.

AND FINALLY…

A confession. Everything written above is true, but the flying shots are fake. I got my daughter to photograph the Hunter while I held it above my head. I then used some free software to paint myself out. In this digital age you can’t believe everything you see!

DATAFILE

Scale: Hawker Hunter WB188
Design: Tony Nijhuis plan adapted by Cliff Whittaker
Wingspan: 35″ (889mm)
Fuselage length: 42″ (1067mm)
Wing area: 2.14 sq. ft. (0.2 sq. m.)
All up weight: 52 oz (1.48 kg)
Wing loading: 24 oz/sq. ft. (7.4 kg/sq. m.)
Powertrain: 70mm Powerfun 3400kV EDF with matched ESC
LiPo: 4S 3000mAh
Functions: Aileron (1), elevator (1), throttle (via ESC)

EDITOR’S NOTE

In the February 2022 issue of RCM&E we published a free Pro-Plan for Tony Nijhuis’ own large version of his Hawker Hunter design, which spans 36-inches and suits the same 70mm EDF units that Cliff used for his own enlarged Hunter. So, if you are interested in building your own larger Hunter, this means that all the design work has been done for you.
Specifications and details are shown below:

Name: Hawker Hunter
Model type: EDF Midi jet
Designed by: Tony Nijhuis
Wingspan: 36” (908mm)
Fuselage length: 40” (1016mm)
All-up weight: 48oz (1.36kg)
Wing loading: 22 oz/sq. ft. (6.55 kg/sq.m.)
Wing area: 0.202 sq.m.

A canopy, combined CNC/wood pack and decal set are available from Tony Nijhuis Designs Ltd (TND), as are additional copies of the plan.
Website: www.tonynijhuisdesigns.co.uk
Email: sales@tonynijhuisdesigns.co.uk
Phone: 07563 518159 (9am to 4pm)

Back issues of the printed version of RCM&E February issue are available here:

RCM&E February 2022 Back Issue