Flight test: Sherwood Kub
PUBLISHED: 14:39 16 October 2019
This sporty single-seater microlight is available as a factory-built or in kit form, as a trike or taildragger. It comes with folding wings − and 'attitude' | Words: Dave Unwin - Photos: Keith Wilson
As I turn onto final James Milnes's voice crackles in my headset: "Golf Uniform Bravo - Golf Alpha Kilo, don't forget it's a tailwheel today!" The reason for James's timely reminder is that when I'd flown the same aircraft a few weeks previously it had been configured as a trike, but today it's a taildragger!
Like most things in life, sport flying isn't getting any cheaper. Hangarage, insurance, maintenance, and fuel are all becoming more expensive. But what if you had an aeroplane that lived on a road-legal trailer and fitted in your garage?
One you could fly from practically any friendly farmer's field while burning only eight litres of mogas an hour, and that you could do all the maintenance on? I give you the Sherwood Kub.
Built by The Light Aircraft Company at Little Snoring Airfield in Norfolk, the Kub is a high-wing, single-seater which falls into the snappily-titled Single Seat Deregulated category, or SSDR.
As the name implies, an SSDR aircraft may have only a single seat, and in the UK it must stall at less than 35kt and have a MAUW (maximum all up weight) no greater than 300kg (some countries alternatively specify an empty weight of either 115 or 120kg). And 'deregulated'?
Well, in the UK not only can you do all the maintenance yourself, but there isn't even a requirement for a Permit to Fly.
Before going flying, a look around the TLAC facility reveals quite a few Kubs in various stages of construction, and the large lift (which wouldn't look entirely out of place on an aircraft carrier) that conveys completed aircraft from the production line on the first floor down to the ground.
Unlike some of the SSDRs that I've tested over the years, the Kub looks very well made, and also quite robust. TLAC boss Paul Hendry-Smith explains that although SSDR aircraft don't need specific approvals from a national aviation authority or administration, as the Kub is descended from the Reality Aircraft Kid, it is built to British Civil Airworthiness Requirements, uses aircraft-quality materials and is "a proper aeroplane".
Looking at the test machine reveals an interesting anomaly; unlike any other aircraft I've ever flown, 'Golf Oscar Kilo Uniform Bravo' appears to have both a nosewheel and a tailwheel!
TLAC's Chief Pilot James Milne explains that the Kub shares several similar features with its big brother the Scout, including being offered with different engine options, having quick-folding wings and being easily reconfigured with either a nosewheel or tailwheel undercarriage. "The mainwheels are set up for a nosewheel," he explains, "but we thought we'd leave the tailwheel on for a laugh−see if you'd notice!"
Currently offered by TLAC as either a basic kit, 'Fast-Build' kit or as a ready to fly factory-built SSDR microlight, the Kub is of classic 'rag 'n' tube' design. Construction is primarily of TIG-welded aircraft grade 4130 steel tube, powder-coated for corrosion protection.
The fuselage has a triangular cross-section aft of the cockpit (in this respect resembling an Aeronca Champ, rather than a Piper Cub - Ed), while the wings use Avid Flyer/Kitfox-style tubular aluminium spars and plywood ribs. Oratex UL600 covers the fuselage, wings and tail, with composites used for the cowling, which half-covers the Hirth F-23 engine.
The F-23 is an air-cooled horizontally-opposed two-stroke twin, which produces 50hp at 6,150rpm (TLAC says 6,150; Hirth says 6,500…) and turns its three-blade fixed-pitch prop via a wide Polyflex vee-belt drive with a reduction ratio of 2.2:1.
James explains that initially G-OKUB had been fitted with a Hirth F-33 single-cylinder two-stroke of only 33hp, but this was soon replaced with the F-23, which, he grins, "has transformed it!"
Now, I'm not a huge fan of two-stroke aero-engines. In fact, even my lawnmower's engine is a four-stroke, so I regard the F-23 with a slightly jaundiced air. It's quite an interesting, almost contradictory little engine as−despite Hirth having replaced the old-school magnetos with dual Capacitive Digital Ignition (CDI) units−you still must mix the two-stroke oil into the petrol by hand. I believe automatic oil injection is an option, but even my Yamaha RD400 motorcycle had the oil automatically injected as standard in 1978.
The Hirth does have an excellent power to weight ratio though−50hp from an aero-engine that only weighs 35kg (including the electric starter and twin expansion-chamber exhaust) is not to be sniffed at. It's fed from a pair of wing tanks with a combined capacity of 47 litres via a fuselage-mounted, four-litre header tank.
The main undercarriage is of the split-vee type fitted with chubby low-pressure tyres and heavily slotted Shimano cable-actuated disc brakes. Bungees are used for shock absorption. As both 'third wheels' were fitted for my benefit, it made this particular pre-flight unique, at least in my experience.
The large pneumatic nosewheel is carried by a welded steel-tube frame and free-castors, while the small solid tailwheel is suspended from a single leaf spring and steers via springs through the rudder pedals, up to about thirty degrees each way.
A mixture of struts and wires brace the tailplane, which carries separate elevators with a large trim tab set into the trailing edge of the starboard one. The big fin is pleasingly rounded and carries an equally large rudder.
The constant-chord wings are braced by vee-struts and fold aft using a similar system to the Scout's, but what really catch my eye are the large, single-slotted mechanically-actuated flaps. These have four positions, 0°, 10°, 25° and 40°−but are they really necessary?
This thing has a MAUW of only 300kg and with a wing area of 10.5sq m the wing loading is less than 28.57kg/sq m (6lb/sq ft), so why would it need flaps? It's obvious that without some sort of hinged trailing edge it wouldn't be possible to fold the wings, as they'd foul the fuselage. But do these sections really have to be lift-and-drag-producing aerodynamic flaps? Only one way to find out - fly it!
Access to the cockpit is via a split window/door on the starboard side. The door opens forward and is quite small, while the upward-opening window is big. Most Kubs also have an identical window to port.
Unsurprisingly, neither the seat nor pedals adjust, but luckily it fits me quite nicely so, once firmly strapped in with the well-made Willians four-point harness, I study the surprisingly large cockpit's controls and instruments. Quite predictably it's an exercise in minimalism.
The tall stick carries twin bicycle-type brake levers, while levers for the throttle, trim and flaps are on the port sidewall by your left knee, hip and elbow respectively. The panel continues the minimalist motif with a centrally-mounted MGL Stratomaster Xtreme EFIS as the primary instrument for both flight and engine information, with a back-up analogue altimeter and ASI below it, and a slip ball between them.
The ASI is in mph and somewhat optimistic (about a third of the scale is superfluous) and the altimeter not ideal, as it only has a single pointer.
There are only four circuit breakers and four toggle switches (for the master, avionics master and CDI units), plus a large button for the starter−and that's pretty well it for the electrical services, as the Icom transceiver doesn't count as installed equipment.
Now it's time for my mea culpa moment. James had turned on the master and avionics to brief me on the EFIS, and then said "off you go", so I set throttle and choke, shouted "clear prop" and pressed the starter. The propeller whirled most convincingly but the motor didn't even cough. Further attempts were equally unsatisfactory before realisation dawned on James and me simultaneously−maybe turning on the dual ignition systems might help? This was the first time I'd made this fundamental mistake on a test flight.
Lesson learned: if someone else has turned on some of the systems, it's always best to turn everything off and start again from the beginning. Incidentally, the choke isn't great but they're working on it.
It's spring-loaded to the 'off' position, and as there's no parking brake you run out of hands as you also need to press the starter and guard the throttle. Taxying out using the hand-operated differential brakes is quite easy, once I'd remembered that−as with all aircraft fitted with a castoring nosewheel−it's easier if you keep the speed up a bit.
As you may readily appreciate, the pre-takeoff checks continue the simple theme−and as the F-23 is a two-stroke it doesn't need warming up and you can't even check the oil temperature or pressure. Consequently, my generic single engine piston 'flow check' is quickly completed but−as always when flying a two-stroke−the small 'Master Caution' light in my brain flickers once or twice.
"Have I missed something?" So I wait until the cylinder head temperatures rise slightly then run through the pre-takeoff checks again. Finally convinced I really haven't forgotten anything, it's time to fly.
There's no need to taxi around to Runway 25 so I simply set the flaps to 10°, open the throttle, and take off from the taxiway. Ambient conditions are above ISA, with an airfield elevation of 196ft and an OAT of 20°C, while with both tanks full the Kub's still about 44kg below its 300kg MAUW.
There's a slight crosswind from port but the acceleration is so brisk that the Kub is up and away after roughly fifty metres ground roll. The climb rate is equally impressive, the Vy (best rate of climb speed) of 45kt producing over 1,000fpm.
The weather is no good for air-to-airs, so photographer Keith stays on the ground while I head off to the west to explore the general handling, control and stability. Initial impressions are all good. The handling is fine around all three axes, with low break-out forces and little 'stiction'. Predictably, slow flight is slow.
The strut-braced wing uses a relatively high-lift aerofoil and the loading is only 28.6kg/sq m, which may not sound that low. However, in old money it's less than 6lb/sq ft, or barely half that of a C152. Stalls−either power on or off−are very benign.
There is no artificial stall warner but adequate natural pre-stall buffet. Furthermore, as you approach the stall a reasonable amount of back pressure on the stick is required. Recovery is quick and easy−just release the back pressure. Flaps up, it stalls at around 28kt, and although with full flap and some power you can get it down to around 22, it's almost academic as a sensible approach speed is so far above the stall. The trim is quite precise, although it did seem to run out of aft trim at my weight.
Regarding stick-free stability, the Kub is stable around all three axes, being quite positive longitudinally, softly positive directionally, and just barely positive laterally. The roll rate is quite nippy, as you'd imagine, while the visibility in the turn (and most phases of flight) is quite good for a high-wing aircraft.
The cruise performance is also pretty much what you'd expect. A comfortable cruise speed is around 50-55kt, and although you can bump it up to sixty, the engine is buzzing quite frenetically and you'll be burning a lot more fuel. For example, at fifty knots you're only burning around ten litres per hour, so the full 51 litres provide a still-air range (including thirty minutes' reserve fuel) of around 225 nautical miles.
If you pull the power right back you can certainly improve the endurance−it's just that if there's any appreciable headwind at all then you won't actually be going anywhere! However, when flying an aircraft like a Kub, the journey is at least as important as the destination.
For my first landing I opt for Runway 25, which is wider, longer and directly into wind. This goes well. I can see that Keith has positioned himself by the mown grass strip (aka Runway 28) next to the taxiway to shoot some takeoff and landing shots, so I fly several.
This is great fun. The simple pleasure of a well-flown approach never diminishes, while the subtle and seamless transference of weight from wing to wheel and back again has never paled, especially if you've got an open cockpit (or large window open) and the runway is grass. As mentioned earlier the test aircraft had a third wheel at both ends and just for laughs (and with a bit of application) I even managed a three-pointer.
I typically use about 45kt on final. If it's flat calm you could probably safely shave off another five knots but I'd advise against it. The Kub has plenty of drag and not much inertia−the speed soon washes off. Plus, it sideslips superbly. Furthermore, when landing into just a stiff breeze the ground speed at touchdown is very low, possibly less than ten knots. The brakes are only for taxying.
I also examine the takeoff and landing performance with various flap settings and eventually try the ultimate test by inverting all normal procedures−taking off with full flap and then landing with no flap.
Conclusion: you don't need any flap at any time, except when folding the wings! My experiments had convinced me that the weight, cost, and complexity of the flap system's lever, cables, pulleys and bell cranks are unnecessary, and that (and particularly for the fifty-horse version) simple pip-pins could be used to hold the flaps in place when rigged for flight.
A few weeks later Keith and I return, and the weather is great. This time the Kub's lost the 'training wheel' and is configured purely as a taildragger−and looks a lot better for it. In fact, it looks a little bit like a single-seat miniature Aeronca Champ. Slightly chubby and cheeky-looking, it exudes fun.
Taxying out is, if anything (and unusually) easier in the taildragger than the trike. S-turning is unnecessary, as visibility over- and each side of the nose is good and the tailwheel steers through the rudder pedals, whereas the nosewheel castors and requires differential braking.
The best rate of climb is attained at 45kt, but as soon as I'm above 500ft I speed up to 55, as this not only improves the view over the nose but also gets me clear of the airfield and chasing the camera 152 a bit quicker.
The air-to-airs with the C152 carrying Keith and James are not easy, as the Kub has practically no 'overtake', so I have to use a lot of geometric cut-off for the rejoins.
With all the pictures in the can, I briefly repeated some of the items on the flight test card to see what, if any, changes in performance and handling have been produced by removing the nosewheel.
Unsurprisingly, directional stability is stronger (less keel area in front of the centre of pressure) but longitudinal stability weaker (more weight aft of the centre of gravity) alleviating the limited aft trim. The aeroplane also seems slightly faster (less drag) and is definitely better looking!
An absolute hoot
During my two test flights I had a lot of fun with the Kub−it's an absolute hoot! Even the two-stroke engine impressed me (it really does pull well) and the rate of climb is spectacular.
It was great fun to bumble about the Norfolk sky with my elbow out the open window. The handling is crisp, the roll rate nippy and it's just... well, fun−for there's something very special about flying rag 'n' tube taildraggers from grass, something that is difficult to explain and hard to resist.
In his classic book, Airymouse the great Harald Penrose wrote about his flights in 'a diminutive single seater aeroplane of insignificant horsepower'. Is the Kub the 21st century equivalent of Penrose's Curry Wot?
Possibly... I think he would've liked it, especially the tailwheel version with both windows down.
So, would a Kub work for you? Ultimately, the vital question you need to ask yourself is "Do I want go flying for sixty miles, or sixty minutes?"