Supermarine Spitfire Mk 26

The Supermarine Spitfire Mk 26 is a two-seat Australian homebuilt aircraft currently produced in kit form by Supermarine Aircraft for completion by amateur builders.

Design and development

The Spitfire Mk 26 is an 80% scale replica based on the original Supermarine Spitfire design. Despite its smaller size provision is made for a passenger to be seated in tandem behind the pilot. The stressed skin structure consists of 2024 aluminium alloy skins, former s and longeron s with some fibre-glass mouldings for certain parts such as fairings and air scoops. The design features retractable undercarriage with differential braking to the main wheels. The aircraft has been approved by the British Light Aircraft Association .

Powerplants

Early versions of the Spitfire Mk 26 used an eight cylinder 200 hp Jabiru horizontally-opposed engine but as more power was required for the two-seat variant the company now offers a converted General Motors Isuzu V6 engine . The normally aspirated version of this engine produces 226 hp (168 kW) with a supercharged version producing up to 310 hp (231 kW).

Variants

Mk 25

Single-seat version, no longer produced, 75% scale.

Mk 26

Two-seat version. 80% scale.

Mk 26b

Improved Mk 26. Option of dual controls, 90% scale.

Construction

Construction of the Mk 26 requires basic metalworking skills and tools and makes extensive use of pop rivets . Pre-assembled kits are provided but still leave the builder with 1200 manhours plus of work to be completed.

Flying the Mk26

The aircraft was fitted with a composite four blade fully variable pitch propeller without automatic constant speed control. A manual pitch selection switch on a panel on the upper left front console gave an 8 second adjustment range between fully coarse and fully fine. The Mk 26 has also been flown with a two bladed wooden propeller as fitted to the RV6 aircraft.

Some pilot seating and rudder control adjustments must be made prior to strapping in.
The aircraft is fitted with differential toe brakes on the rudder pedals. They are adjustable for length via panels either side of front lower fuselage. The front seat has three fore aft positions selected by removing two bolts on the seat frame. The rear seat has no adjustments with no stick or rudder controls fitted.

Walkround and Prestart Checks:
A positive check of the undercarriage locking levers, with their big black knobs fully forward on the front starboard cockpit wall, is advised before clambering under the aircraft. Combine this with a check of the 2 tell tale locking pins protruding from the wings. More on the gear later.The aircraft has no fabric surfaces, and only a couple of fibreglass fairings on the base of the fin attached by self tapping screws. The aircraft is otherwise riveted aluminium.
The low wing requires a fair degree of bending and crawling around to check control rod connections, access panels, undersurfaces and undercarriage. The main engine panels on this prototype required 20 minutes to remove and replace (production models have redesigned panels) if a visually check of the engine is desired. Otherwise it is a simple task of checking quick release fasteners and 4 intakes for carby and cooling air.
A look up, for oil leaks, in the lower engine sump area, can be achieved if the engine cooling cowl, which hinges down immediately in front of the firewall, is fully opened on the initial cockpit check. This push /pull control is handily placed at the lower front console.

Fuel and oil caps are on the upper surface of the top engine panel a couple of feet in front of the windscreen. A small step-up device is recommended as standing on the leading edge will damage the surface of the wing.
The oil dip stick, under a quick release panel, is an awkward long reach, fine threaded, plastic affair. Heat soak makes this device difficult to remove and check quickly and accurately. Fuel drain is directly under the 115 litre tank on the under belly.

The end of the U/C down lock pins can be seen in the large holes at the top of each oleo leg and the tell tale pins mechanical linkage connection (vital) should be checked. Otherwise, all other connections and micro switches are internal to the wing root and cannot be checked. Each leg and its controls are a completely separate system. Only one half of the wheel is covered by the oleo mounted door on retraction. The tail wheel sits on a strong leaf spring and is linked to the rudder by a horizontal bar. Electric flaps are fully variable to 55 degrees and are very large extending to almost half the wing span. A large vertical blade radio antennae sits behind the canopy on the upper fuselage.

Cockpit General
Entry is over the port inboard trailing edge with flaps up. A small fold down door is easy to open after sliding the lockable bubble canopy fully rearwards. The removable seat cushion is canvas covered foam. I added a small leather document case with a polystyrene filling to increase my height in the cockpit by approximately one inch (pilot 185cms) until my hard helmet was just clear of the canopy.

Entry is fighter style by standing on the seat and lowering oneself using the cockpit arch and surrounds. Care must be taken to prevent stressing or marking plastic transparencies is recommended. The spade shaped, Spitfire style, control column is comfortable and has trim and RT switches fitted.
A four point Sutton type harness system for both seats provides good restraint and is easy to adjust and lock safely. Instruments and switches were generally well sited and easily read and operated, the main fuel cock is low behind the stick but easily checked.

There was little forward vision available over the long nose and forward wing. This can be improved down the left side only if the side door is opened and one leans out of the cockpit.
A master switch centre console gives 12 volt power and is flanked by two mag switches. Start is by pressing both Boost and Start buttons simultaneously. Boost has no function other than decorative, why not! Having sat in the real thing, this cockpit certainly feels and looks remarkably like the real thing, it just smelt new.

Undercarriage locked or unlocked position lights were not fitted to this aircraft. All aircraft are fitted with 2 inch tell-tale pins mechanically linked to the leg oleo. These pins protrude through the top of each wing by the inboard leading edge to indicate gear position.

Only the colour BLACK showing on a shorter pin length indicates locked down. A longer pin showing RED and BLACK indicates unlocked. NB. These pins will only indicate locked (up or down) if both large undercarriage locking levers are fully forward also.

Difficulty was experienced siting these pins with the canopy fully closed and hard helmet fitted. My helmet touched the canopy despite the concave moulded into the canopy curve.

The undercarriage emergency lowering system: to be used if the gear motor fails in the up position, are two black handles 4 inches long situated either side on the floor behind the stick. The handles are easily reached and pulled separately. This direct cable link pulls a pin thereby disconnecting the control arm of the electric motor from the top of the leg. This enables the pilot to unlock the leg with the black knobbed locking lever starboard cockpit and use G forces to lower it before locking it again by pushing the lever forward. This simple system was seen to work on the ground during trials.

Engine Ground Runs
The sliding canopy can be opened under full power. No parking brake is fitted. The aircraft was tied to a small sapling via a rope attached to the tail-wheel spring with chocks in place.
Full choke was applied for start but cancelled immediately, idle was 800rpm at 5-7 lph. There was no tendency to over-temp during prolonged running. 3000rpm was obtained at full power and fully fine on prop setting. No significant mag drops noted. Pick up and throttle response was smooth and instant over the full rpm range. The engine emits a loud powerful bark and a noticeable torque roll left during slam accelerations to high power.

Taxi
The aircraft moved forward at 1000rpm there is no forward vision and a constant weave is required to clear ahead. Turning is easy, the tail-wheel is liked to the rudder and the progressive toe brakes enable tight areas to be negotiated. The aircraft is capable of turning through 180 degrees almost within its own length.

Take-off and Climb
No flaps were used, zero trim was selected on the electric stick trimmer as indicated by the gauge. There are no aileron or rudder trimming controls.

The undercarriage electric motor switches (they are three position UP, OFF, DOWN) can be selected to the UP position pre take-off if desired. Only by selecting unlock (rearward) on the levers on the starboard cockpit wall does a lever attached to the locking pin move a micro switch allowing the motors to start. These switches were not selected on the ground on these first two sorties. However, subsequent flights proved the viability of this technique as it saves time and some actions after take-off during high cockpit workload situations.

Run-up produced an easily controlled port swing right rudder was immediately effective and no brakes were required to hold it straight. The tail lifted quickly and smoothly pitch control was good throughout the ground roll.

The aircraft flew itself off at 60 knots after approximately 100 metres. Gear retract takes 8-10 seconds and a change of hands on the throttle and stick is needed to bring the locking levers rearwards.

Throttle friction should be tight at this juncture as it is recommended that the right hand remain lightly on the levers to feel the travel of the gear. The locking pin is sprung loaded onto a plate during its travels and it can be felt sliding along this plate and eventually into the up lock as the levers move forward under the same spring pressure. A positive check that the levers are fully forward and that the telltale pins are showing RED will confirm gear is fully up. The two u/c switches can now be set to OFF or DOWN or left in the UP position. The motors having been shut off by the locking levers being checked fully forward.

This rather protracted handling sequence is recommended as there is no trim change, sound or mechanical feel imparted by movement either way during undercarriage selections and one can imagine Mr Murphy visiting the scene, especially with two totally independent systems to get involved with. A simple red LED unlocked light is being fitted to other Spitfires.

Climb at 100 knots gave 2000 fpm. 85 knots achieved 2500 fpm. Engine indications were 3050 rpm (fully fine) at 24 map at 51 litres per hour. Climb to 5000ft was achieved in 3 minutes from wheels roll into the overhead with one climbing turn.

The controls were light and responsive with no significant adverse yaw noted during a steep climbing turn. The canopy and windscreen transparencies were clear and undistorted. Full cowl open position was selected shortly after take-off to maintain oil temperature limits.

Cruise
A 135 knot cruise at 27 lph @ 19 map was achieved with 2350 rpm by selecting 50% coarse pitch or 4 seconds on the switch. I would equate this to approximately 75 % cruise.
A 150 knot cruise was achieved @ 2200rpm fully coarse 24 map and 43 lph the throttle was about 95% fully open.
Subsequent sorties with fixed pitch props and a slightly modified four bladed propeller produced cruise speeds in the region of 150 knots with fuel flows between 30/35 lph at 2000ft.

There was a satisfying surge of speed and deepening engine note on selection of fully coarse. Cowl flap fully open kept oil temps within normal limits at all high power settings this reduced speeds by 2-3 knots. Cylinder head and exhaust gas temps were within limits.

Stalls and Manoeuvre
These were carried out at 5000ft. OAT 15 degrees, half fuel capacity.

Clean Stall power off
Control effectiveness was diminished below 55 knots though full control in all axis was easily maintained, a very light airframe buffet was felt at 50 knots. The nose dropped gently at 44 knots no wing drop was noted. Recovery was instantly achieved with power application and a slight forward movement on the stick. An estimated height loss of approximately 30 feet was experienced.

Stall Full Flap 75% power, level 30 degree banked port turn
Control effectiveness was diminished below 50 knots though full control in all axis was easily maintained, a light airframe buffet was experienced at 45 knots, the nose dropped at 40 knots with a slight roll left. Recovery was instant as full power was applied and the nose lowered slightly, the left wing was easily picked up with a small amount of right rudder. Height loss was less than 100 ft.

High G handling
The aircraft has no negative G capability with the present engine configuration, as oil and fuel starvation will result. No oil pressure fluctuations or engine misfiring was experienced throughout the manoeuvring of up to 4.2 G . The controls were flutter free and very pleasant to use, with no stiffness or over lightness in their feed back to a maximum indicated TAS of 205 knots at 8000ft. This was easily achieved in a 1500ft ROD from 10,000ft. The placarded Vne is envisaged as 190 knots at present. However, I feel the all-metal build and 9 G limit imposed on this airframe gives significant scope for a safe increase in this figure.
A significant torque roll at max power setting was experienced when G stalling. This was predictable and controllable over the speed range used (40-180 knots) during this phase of the air test.
The handling is excellent and the pilots seat position, harness capability and all round visibility during spirited manoeuvres gives one a very pleasant confident feeling making it worthy of its name, if my reading of the real Spitfires handling is correct.

Gliding, Circuit and Landing
The aircraft glides at 85 knots with a rate of descent of 650 fpm with the engine at idle. It was trimmed and flown hands off at this speed with the canopy fully open to simulate abandoning. The aircraft was stable.
Flap and undercarriage can be lowered below 100 knots with no significant trim changes experienced even with full flap on finals.

A curved approach was flown at 75 kts. With the canopy open a very clear and continuous view of ones touch down point is available. There is no obtrusive wind draught in the canopy half-open position and normal RT calls can be heard. Full flap lowered the nose appreciably and low power was required to prevent speed build up, there are no airbrakes fitted. A last look speed of 65 kts was used over the threshold as the flare was initiated and forward vision lost. This can be reduced to 60 kts for short field approaches. The aircraft is easy to land on three points with good control in all 3 axis right down to a very low touch down speed of around 40 knots. Flapless landings were flown adding 10 knots to all speeds.
A crosswind component of 7 knots was handled easily using both crabbed and wing down techniques. Side-slipping is easy to achieve with effective and manageable rudder forces up to 75 knots.

The toe brakes were snatch free with no tendencies to pitch forward from the 3 point attitude. 2 point, main wheel only touch downs were carried out but are not recommended as touch down speeds of below 65 knots were difficult to achieve and significantly more ground roll was used than 3 pointers. 3 point landing roll outs averaged about 200 yards using minimum braking.

Rear Seat Occupied
The first rear seat sortie was flown with a 75 kilo 185 cm pax with 85 litres of fuel. The rear seat passenger must enter first after moving the pilots seat fully forward. This new forward position did not inconvenience control of the aircraft. The presence of legs and feet either side of me were not a distraction. My right elbow had to be lifted slightly higher during the backward movement of the undercarriage locking levers to avoid a large knobbly knee. Sorties over 1 hour duration will require some form of lumbar support for the pax as a slightly crouched curved lower back position is unavoidable due to the shape of the seat.

I have flown 75 minutes sitting in the rear and I used a large oblong en route frequency book tucked behind me to achieve a fair degree of comfort. Vision is restricted forward dramatically by the pilot’s shoulders which are a foot or so in front of you. The pilots shoulder harness are also routed close either side of your head.
However, this rather cramped and claustrophobic feeling soon diminishes as one gets airborne and the aircraft levels off in the cruise when good vision out and down is available through the side windows. Map reading is achievable but folding a large map would not be easy to accomplish.

A couple of medium squashy overnight bags can be stowed either side of the pax’s shoulders. There is also a small storage area beneath the pilots seat for tie-downs and a few tools. A 100 litre fuel / baggage pod has been developed. This fits flush under the belly beneath the pilot and is accessed through a built in door in its side. This pod has not been test flown yet.
It would not be possible to instruct from the rear, there are no controls or useful forward vision.

Take-off with 20 flap in still air was still lively and only another 30 metres was covered before lift-off. No noticeable trim change was experienced throughout the 40 minute flight which was a close formation photographic sortie. The aircraft handling was precise and responsive over its full speed range. Steep turns up to 3 G were carried out. Glide, circuit and landing performance were the same as for solo with only a small amount of nose up trim required as fuel diminished.
The Mk 26 is a full two seater which, though cramped compared to many tandem aircraft, is capable of transporting two large males with overnight gear over its full range and capabilities.

Overall Impressions
The aircraft was well presented with all instruments, radio, transponder and GPS working. The automatic prop controls were not fitted. The engine worked flawlessly, it is meticulously engineered, very light and simple. It sounds and looks lovely and is easy to work on.

The aircraft is difficult to look forward and down out of whilst taxiing or in other high nose attitude flight regimes. The undercarriage system works smoothly and efficiently but owners would be well advised to get further electronic confirmation of locked positions.

The aircraft is easy to land in a 3 point attitude with enough peripheral and forward references to achieve consistent success. Correct approach and touchdown speeds are essential to prevent bouncing and or pitching forward especially on rough surfaces or when tempted to wheel it on to the runway.

The controls are well harmonised and the handling very good throughout the speed range flown (40-205 knots) The aircraft is stable with positive stability characteristics in all three axis. However, during manoeuvre the dynamic and static stability mix nicely towards the negative side of the equation giving a spirited fighter feel to the aircraft’s nature which really gets you loosening your straps and looking up sun whilst feeling for the stick top button!

Richard Lea
December 2001

RAF 20YRS (Mil.fast jet) Hunter Jaguar Lightning
RAAF 4YRS (Mirage)
FRADU NAVY 5YRS (Hunters)
Piston Type Tigermoth, Chipmonk. Winjeel & Cessna
Licenced Commercial Fixed wing & Balloons
TOTAL APPROX 8200hrs
From: Supermarine Aircraft Website