The GK-1 streaking along at 220mph on 20 litres per hour.

The GK-1 is an amazing little plane that has been returned to its rightful place in the sky. It is like an affordable personal jet fighter, and is one of the very few homebuilts to successfully use a water-cooled car engine.  

Looking like a racer even while standing still.

THE GK-1 WAS BUILT IN THE LATE-1980S by Garth Kopke (hence the name GK) as a plane that would give him a wonderful thrill to fly – with speed, but at a cost his young pocket could afford. Dr Russell Phillips – the creator of the Whisper range of aircraft, worked with Garth and writes: There’s a saying out there about the ‘talkers’ and the ‘doers’. This article is about a ‘doer’. In this case a young South African who quietly got on with the job of designing and developing a unique aircraft. The chap in question was Garth Kopke – who sadly is not with us anymore as he died in a fire-bombing crash as a result of structural failure.

Garth started building a KR-2 when still at high school and some years later ZS-WJN took to the skies. This was a successful aircraft because he kept it simple and light. Garth had the usual itch for more speed and for aerobatic capabilities, so it was no surprise that sketches of a sleeker, faster machine started to appear. Russell writes, “In 1988 Garth and I each started building our own ‘Wild Things’ as we called them.

Garth Kopke with the GK-1.

These were hot little planes to be powered by 2-strokes and would be lightweight aerobats. Garth was working in Lesotho and when relocating back to Port Elizabeth he transported his fuselage on the roof of his Mazda 323. About 5 km from PE it fell off the roof and trashed itself. He left it next to the road in disgust and came and told me about it. I raced out and picked up the pieces. I still have some of it. He was really annoyed with himself and more than a tad disappointed. So I gave him my Wild Thing fuselage, which ended up becoming the GK-1.

Garth ate, spoke, slept and talked aeroplanes…. all day, every day. He would arrive at your place at 10 pm on a Sunday night and sketch aircraft until 2 am the next morning….all on tea. His death was a great loss to aviation and homebuilding. Moving on from the ‘Wild Thing’, the new aircraft was to be a single seater with sufficient strength for aerobatics and very low drag.  The issue of an engine became a problem as it is difficult to extract more than about 70 hp from the tried and tested VW’s used by so many homebuilders.

Garth had seen the Honda 1600 car engine in my Citabria. The actual engine used had flown some 200 hrs and had been dyno tested to 105 hp. It had been detuned from its car application and simplified with a change from fuel injection to a carburettor. Where possible, redundancy was built into the ignition system with dual electrics feeding the single plug per cylinder. Garth studied the Masters’ thesis I had done on this engine and decided that was the way to go. All the ingredients were now in place, so all he had to do was design and build an aircraft from scratch! Every few months I would receive a call from Garth giving sketchy details of how he was progressing. Due to his nomadic lifestyle as a cropsprayer/ firebombing pilot, these calls came from many different places. The aircraft project also moved around the countryside quite a bit. I finally got to see the aircraft in Plettenburg Bay when it was almost complete. There is yet another saying (this time an aviation saying) “If it looks right it will fly right”. My first impressions of the unpainted machine in a dusty Plett workshop were exactly that – it looked right! 

“The airspeed indicator was almost on the stop.”

The GK-1 is a machine that achieved and surpassed its design goals The Honda engine flew 200 hrs in Russell Phillips – Citabria. 52 March 2021 Garth was not a qualified aeronautical engineer, yet he produced a machine using common sense and gut feel that I believe would be hard to improve upon, even with an army of engineers. (The sizing of the structural items such as spars etc were all verified by the appropriate experts). The little speedster changed hands a few times after Garth was killed. One of the owners was Carlton Blandford – a KR-2 fan, who flew it out of Tedderfield where he kept, not one but two, KR-2s. But Carlton found Tedderfield a bit tight for the GK-1 so he put it on the market. In 2020 it was acquired by ace aerobatic pilot Neville Ferreira, who had recently sold his Slick 540 unlimited aerobatic plane and who had 700-800 hours on these high performance planes. Neville flew it for our camera shoot and is putting a new lease of life into the GK-1. He has ironed out little niggles with the carburettor and harmonic vibration and given it a snappy new paint scheme.

Neville Ferreira flying the GK-1

ON THE GROUND

The overriding impression is one of diminutive size – and sleek lines – built for speed and without the wide high drag cross section of the horizontally opposed air-cooled aircraft engines. The GK-1 looks a lot like Alex Henshaw’s Mew Gull, which he flew from London to Cape Town and back in an incredible four days and 10 hours, a record which stood for 70 years! For those who know Ken Rands KR-1 and KR-2 designs, there is an immediate similarity as the rear fuselage and tail are almost entirely KR-1. The construction methods are also largely KR derived, using wooden structural elements with a foam core covered with fibreglass. This gives a cost effective and simple to build yet clean composite finish. The wings have two substantial single piece laminated wood spars. The one-piece wing fits into a cut-out in the underside of the fuselage.

The radiator is mounted in the rear fuselage and draws air in through an under-belly scoop, similar to a P-51 Mustang. The engine cowl has no air inlets, thereby reducing cooling drag to the absolute minimum. The Honda 1600 spins a 3 blade Warpdrive ground adjustable propellor. Often car engine installations into aircraft throw up a host of unanticipated problems that can take much time and effort to resolve. One of the more common is vibration from mismatched resonance frequencies, particularly considering the complexities of driving a three bladed propellor with a four-cylinder four-stroke engine through 2.2:1 reduction drive unit. ZU-BLJ was not immune to this problem and required significant experimentation – which required test flying, a return to base, adjustment and then more test flying. Possible culprits which were eliminated flight by flight were: ignition timing, carb icing and tension on the reduction drive belt. In the end the vibration was solved simply by rotating the prop through 60 degrees on the crankshaft hub. Fortunately, the large belt V-drive absorbs much of the engine pulses and the backlash from when the prop drives the engine in a low power dive.

It is immediately evident that much care and skill went into the design and refinement of the GK-1. The workmanship on the aircraft is superb, with great attention to detail. The canopy uses a mould of a glider canopy and blends very neatly into the fuselage lines. Big emphasis has been placed on weight reduction and hence the cockpit is spartan and functional.

The ailerons have aerodynamic balance tabs which help reduce stick forces. The wheels are small, so it is a tar runway aircraft. The mainwheels take 400×5 tyres which are used by RV nosewheels. The spats are a tight fit around the wheels and are blended carefully into the spring steel undercarriage legs. Every attempt has been made to reduce drag, and even the tailwheel is tightly faired.

Exhaust even looks like a mini Merlin.

FLYING THE GK-1

Russell Phillips writes, “My first flight in this machine was with the normal trepidation and nervousness one has at flying something new. In this case, it was really new – this was the first GK1 in the world! It’s a single seater so there was no one to show me the ropes. I admit to butterflies in my belly as I started up and taxied out. The nose is very long but fortunately, forward visibility is not too bad as the nose is also narrow. The first impression, as you open the throttle to taxi, is that you are flying a turbine due to the high-frequency whine and smoothness of the engine.

The engine drives the prop through a toothed belt reduction drive which results in a lot less of the low rpm clatter one gets with other prop speed gear reduction systems. Takeoff is an exhilarating affair as the power to weight ratio is spectacular. The aircraft has reasonable taildragger manners with ample An incredible 5:1 ratio between cruise and stall speeds rudder. Lift-off happens at about 70 mph, and initial climb rate is in excess of 1,200 ft/min. Climbing out at around 90 mph she felt solid and responsive. Once at around 2,000 feet above the ground, 6,500 amsl she settled in the cruise at around 185 mph with the engine at an easy 5300 rpm.

At full throttle, she reminds me of a racing car, as the four cylinders reach 6000 rpm. This is something to get used to after flying 6-cylinder Lycomings with max rpm below 2900 rpm. Cruising at 185 mph I took my hands and feet off the controls and she remained stable without the tendency to pitch or bank, even though she has a straight wing with zero dihedral. The most striking recollection I have of the first flight is the control harmony achieved in this design. I have read of the classic ratios between elevator, aileron and rudder forces which designers aspire to. Some aircraft which apparently have come close to these ideal ratios are the Chipmunk and Pitts S2A. I would liken the delightful control harmony of the GK-1 to the Pitts S2A. Roll rate is yet to be accurately measured, but is in the order of 180 degrees per second, making multiple aileron rolls and hesitation rolls a pleasure.

In straight and level flight the aircraft does what it was really designed for – speed. After levelling off, a quick glance at the airspeed indicator yields a pretty impressive speed, but patience is required as this machine seems to take about two minutes to really get going. The radiator is built into the rear fuselage and has an adjustable cowl flap. Once you are established in level flight, the coolant temp is monitored and Garth Kopke with the GK-1. March 2021 55 the cowl flap progressively closed. The first time I tried this I couldn’t believe the reading I was getting on the airspeed indicator, which was almost on the stop. Neville Ferreira shares his experience of making friends with the GK-1: “My very first takeoff went without a glitch after keeping her on the ground until she wanted to get airborne. The engine turns the ‘wrong’ way, being anticlockwise from behind, so left rudder is needed to keep straight.

My next flight was with a GPS to confirm what the air speed indicator was reading – approximately 160 KIAS (184 mph indicated – at 6,000 ft.)  

This is impressive in a machine burning just 17 litres per hour! On a later flight with an RV-14 for company, flying at 7,500 feet routing from Rhino to Kitty Hawk, the GK-1 settled at 194 mph ground speed straight and level. Catching up with the RV-14 that has just taken off from Kittyhawk, I throttled back in the descent, so as not to bust the 230 mph VNE. This produced a 236 mph ground speed. Back at the airfield after 19 minutes of flying, I had used only 5 litres of fuel.

The cockpit has been upgraded and is very neat and has all you need.

I cruise at an easy 4000 rpm which gives me 160 mph air speed at a fuel burn of just 15 litres per hour. Useful load is 350 lbs so a 200 lb pilot leaves just 150 lbs for fuel. For a lightweight pilot a full 50 litre fuel tank gives a more than ample four-hour endurance with reserve. Although never intended for cross country cruising, there is a 15 kg baggage allowance, but minimal space for even an overnight bag and small toothbrush. Normally a plane built for speed has a demanding wing, but the GK-1’s stall speed is a low 45 mph IAS, with very little wing drop. This is probably due to the use of the well-proven RAF 48 wing section, known for its docile stalling manners.

The circuit and landing can be interesting if you leave the going down and slowing down too late. With no flaps, the only way to lose height is by sideslipping. This is also a useful way to see the runway over that long nose. I calculated my own Vref approach speed by adding 30% to the stall speed of 43 mph and settled on 58 mph across the fence. After a bit of trial and error I find the best approach is to fly Downwind at 90 mph, base at 70 and then to go down and slow down by sideslipping to cross the threshold at 58 mph. Flying the correct approach speed is vital.

The trailing edge of the wing is just 40 cm from the ground, creating a cushion of air that you can just float along for the length of the runway if the speed across the threshold is too fast. A full three point landing must be done, as a wheeler may float you off the end of the runway. The aircraft touches down gently on its spring steel undercarriage. With a few practice landings, a 400 m runway is quite feasible, even with the rather ineffective drum brakes fitted to ZU-BLJ. Disc brakes would improve stopping performance and thus shorten the runaway requirement even more. Keeping it straight during the final stages of the landing roll requires a bit of footwork, but nothing that a competent taildragger flyer would have a problem with.

OWNING A GK-1

Neville Ferreira wants to share the tremendous fun and value proposition that is the GK-1. He says that in comparison to modern high price tag aerobatic aircraft, this offers a kick in the pants experience that you just do not associate 230 mph in level flight with an aircraft at this price point. “Where have you seen a 100 hp engine delivering 200-220 mph speeds with less than 20 lph fuel burn?” he asks. “This little aircraft with a 6-metre wingspan requires only the corner of your hangar, but once you have strapped her on, she flies as well as any $440,000 aircraft.” Although the GK-1 is not marketed as aerobatic, Neville says it is 100% able to perform loops, rolls and any basic manoeuvres. But the fun lies in the flight itself. It is a Reno Racer on steroids with a price tag around the $100 000-$120 000 – depending on options – ready to fly. And the engine sounds just like an Italian sports car! Neville says he is the final stages to start production of this amazing little plane. The 2 seat tandem seater with a tubular steel frame will be offered with the 180Hp Viking engine (also Honda derived), while the single-seater option will be powered by 100-130 Hp engine from the same company.

The neat and tightly-cowled Honda engine with the large belt drive on the right.

DESIGN GOALS:

The GK-1 is a machine that surpassed its design goals. Anyone who has considered designing and building an aircraft singlehandedly from scratch will realise what a notable achievement a project like this represents.

KEY DESIGN FEATURES AND GOALS FOR THE GK-1

Cockpit: The cockpit will accommodate any pilot with an average build and up to 6 feet tall with max weight of 125 kg or 275 lbs.

Stall speed less than 45 mph yet a CRUISE speed of 200 mph @ 5000 rpm (Max rpm 6500). This is an incredible 5:1 ratio between cruise and stall speeds.

VNE: 230 mph, yet an approach speed of just 50-55 mph making for much lower energy in the event of a forced landing.

Fuel burn: 4.5GPH / 17 lph @ 185Mph Under the trading name Great Knots (for GK), Neville is taking orders for the aircraft sold in kit form or completely built, with or without firewall forward components – being engine, prop and all accessories. The engine options are the Viking Aircraft water cooled inline 4-cylinder Honda derived engine range, or UL Power, or the Continental 0-200.

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