Design Overview of the
Nordenfeld F-10J / Fendrake FG Mk. IV

by: G. Wolfe

Introduction

Affectionally Known as the "Flying Guntruck" among its pilots and mechanics, the Nordenfeld Fendrake is remarkable among fighter aircrafts for its versatility and adaptability. it has served, with distinction, as air superiority fighter, long range bomber escort, fighter bomber, photo-recconaissance, counter-insurgency, close air support, night fighter, torpedo bomber and carrier onboard delivery in armies, air forces and navies of 11 countries around the globe.

First conceived in 768, the Fendrake was intended as future replacement for Nordenfeld F-9 / Mallard then just entering active service, it superficially resembled its predecessor in shape and construction but significantly larger, heavier and packed considerably more horsepower thanks to its turbo-compound CO4100T.67 compression-ignition engine - a novel powerplant design for its time. The turbo-compound engine gives lower fuel consumption compared to spark-ignition engine of the same power class and provides consistent performance over wide range of altitude.

Despite its cumbersome size and weight, the Fendrake is deceptively fast. It can reach 700 kph (378 kn, 435 mph) in level flight without effort and capable of breaking sound barrier in a power dive. Manoeuverability still left something to be desired, but its tremendous excess power always secured upper hand in any air combat scenario.

Outside the powerplant choice, the general layout of the Fendrake is conventional. The fuselage is all-metal, semi-monocoque, stressed skin type with conventional empennage consisted of metal-covered horizontal and vertical fins, all of which also semi-monocoque. The low-mounted wings are full cantilever, stressed skin type with full span twin spar construction - triple spar on later ground attack variants. All three conventionally arranged landing gears are fully retractable with hydraulic power, with fully steerable tail wheel linked to rudder control. The cockpit is enclosed within perspex bubble canopy giving the pilot unrestricted 360 degree view, although forward vision is slightly hampered by heavily framed armoured windshield. Last but by no mean the least, to harness the best of CO4100.67T engine power, the Fendrake is equipped with 4.3-metre wide, four bladed constant-speed propellers, the largest ever mounted on a single engined fighter.

Throughout its long service life, the Fendrake has evolved through 8 major variants and countless minor modification to the F-10J / Fendrake FG.IV, the latest of the series to enter active service. It is also the fourth variant to be designed specifically for ground attack / close support duties, with 276 currently in service with the Imperial air force.

Fuselage Group

The Fendrake fuselage is a semi-monocoque structure fabricated largely of 7075 and 2024 aluminum alloy and divided into three main sections - engine mount, mid section and tail section. The width of fuselage is 1.4 metre and length from tip of spinner to tip of tail is 12.5 metre

The mid and tail section are quite similar in construction. In each section, seven longerons, consisting of built-up H- and T-section 2024 extrusion, carry the bending loads. Shear and torsional loads are carried by the skin which is fabricated from 7075-T6 alclad up to 1.8 mm thick and is reinforced by transverse stiffening frames. In locations where the fuselage has cutouts (for example, the cutout for the cockpit or for the tail wheel doors) there are additional longitudinal stiffeners.

Engine cowling consisted of six removable panels held in place by six joint strips, secured with vibration-proof fastener. The side panels give access to cylinder heads, engine bearers and fuel injection systems; top panels give access to engine accesories, oil and coolant tanks and cowl machine guns; while the bottom panel, which reinforced with 20 mm thick titanium plate, gives access to oil sump, compressor assembly and turbine assembly. the whole cowling panels can be unfastened by one man in under 20 minutes.

Forward bulkhead, which also function as engine firewall, is fitted with 12 gauge stainless steel plate on the forward side for armour and fire protection and reinforced with channel section 2024-T3 extrusion on the aft side. The forward bulkhead incorporates six forged low-carbon 9Ni-4Co steel thrust mountings and two forged steel wing support hinges

The aft bulkhead is similar in structure to that of the forward bulkhead except with 10 mm thick titanium armour replacing the stainless steel plate facing. The aft bulkhead also incorporates two forged 9Ni-4Co steel wing support hinges and mountings for equipment rack

Empennage

The empennage is a full cantilever structure with a conventionally arranged semi-monocoque horizontal and vertical stabilizers installed as a unit with the aft fuselage subassembly. The empennage consists of four subassemblies: the horizontal stabilizer, elevators, vertical stabilizer and the rudder.

The horizontal stabilizers are constructed as a unit with detachable tips and consists of a forward and aft spar of 7075-T6. Flanged ribs are made of 2024-0 heat treated to 2024-T3 after forming. Stabilizer upper and lower panel is reinforced with 7075-T76 extrusions to resist compression and tensional load and provide a riveting foundation for the 7075-T6 alclad skin. The horizontal stabilizers have no dihedral and are attached to steel trunnion on the aft section of the fuselage, their angle of incidence could be adjusted from 1° down to 2° up in flight using a pair of hydraulic screw jacks.

Elevator subassembly consists of 10 flanged rib and one main spar, a V-section trailing edge and a formed leading edge with cutouts for actuator and hinge fittings. The elevators are statically and dynamically balanced by a cast lead weight attached to the outboard end of their leading edge. Right and left units are interchangeable, and their angular movement is 30° up and 20° down. Elevators have no trim tabs, pitch trim is provided by the variable incidence stabilizer.

Fore and aft motion of the control stick actuates the elevators through a push-pull rod from the base of the stick to the bellcranks fitted on mid-fuselage former. A duplicate set of cables extends from the bellcrank to the hydraulically-boosted actuator assembly aft of horizontal stabilizer trunnion.

The vertical stabilizer is a full cantilever structure and set 3° to the left, rotated about the centerline of the sternpost, to counteract the torque of the engine. It consists of a sternpost and a forward spar of 7075-T6, 6 flanged ribs of formed 2024-0 heat treated to 2024-T3, and covered with 7075-T6 alclad sheet. The skin is stiffened with angle stringers of 7075-T76.

Rudder subassembly is metal-covered with angular movement of 30° each side of neutral position. It is actuated by a horn which is an integral part of the lower hinge fitting, in turn controlled by series of pushrods and bellcranks connected to the rudder pedals. It is dynamically balanced by a cast lead weight attached at the top.

The rudder is constructed around a main spar, 20 flanged ribs, a formed leading edge, a V-section trailing edge and an intercostal beam, all of 7075-T6 sheet. The rudder is hinged to the vertical stabilizer by 3 sealed bearings at the top, bottom and center.

Wing Group

The wing group consists of two full-cantilever structure sections bolted together at the center, each consists of three spars, pressed ribs , main panels, removable tips, split ailerons, flap and armoured radiator cover. Spaces are provided at inboard end for self-sealing fuel tanks, part of which is located in fuselage, and a gun bay in each wing panel accommodates guns, ammunition containers, and chutes. Main landing gear retracts into wheel bay in the inboard leading edge. Wings have dihedral of approximately 5° along 25% chord line and angle of incidence of 1° at the roots, twisting to -5° at the tips to improve low speed handling.

Forward and main wing spar, together with top and bottom wing panel form a torque box structure which acts as the main structural member of the wing. Both spars are made of two 4 mm thick 7075-T6 sheet spliced together with angle flanges riveted along both upper and lower edges for spar caps. Rear spar is made of 7075-T6 sheet reinforced by two 7075-T6 angles. Each wing has 16 pressed ribs made of 7075-0, heat treated after forming to 7075-T6.

Top wing panels are made from 7075-T6 alclad sheets reinforced with 7075-T76 closed-section stringers and bottom wing panels are made from 2024-T3 alclad sheets reinforced with 2024-T351 Z-section stringers. Wing leading edges are made from bent 7075 sheets. There are additional stiffeners in locations where the weapon pylons are attached.

The ailerons are split type, with top and bottom halves that can move independently to act as airbrakes. This scheme provides effective braking without causing aircraft to pitch up or down, a common difficulty with fuselage-mounted airbrakes. The angular movement of top ailerons is 60° up and 11° down, while bottom aileron 11° up and 60° down, maximum split angle is 110° to provide 5° deflection for roll control. Movement of the split aileron is controlled by hydraulically-boosted mechanical combiner assemby

Wing group is attached to the fuselage by four forged 9Ni-4Co steel attachment joints, two on forward fuselage bulkhead and two on aft fuselage bulkhead. The joint is covered by 7075 wing-body fairings to reduce interference drag between the wing and the fuselage.

Powerplant

The heart of Fendrake FG.IV is the Nordenfeld CO4100T.70-23 liquid-cooled, turbo-compounded, horizontally opposing 12-cylinder compression-ignition engine rated at 3,450 bhp. It is 3 metre long, 1.3 metre wide head-to-head and weighs well over one-and-a-half tons. It has bore of 152 mm, stroke of 187 mm and a displacement of 41 litre; the cylinder compression ratio is 8.1 with combined pressure ratio of 31.5.

The CO4100T series turbo-compound engine is a novel design at the time of its conception. The exhaust of the 12-cylinder two-stroke, valveless compression-ignition engine drove a 3-stage axial turbine which in turn drove a 12-stage axial compressor that supercharged the engine up to 613.6 kPa (89 psi) boost pressure at maximum take off power. The turbine shaft, however, also mechanically added power to the engine crank through an infinitely variable gearbox. This configuration result in very powerful yet highly efficient engine with brake specific fuel consumption as low as 198.2 g/kW·h (0.326 lb/hp·h).

The CO4100T key advantage lies in its wide variety of fuel. It can run on fuel oil, biofuel, kerosene or even some types of petrol as long as lubricant is added. This enabled Fendrakes to share fuel with trucks, trains and armoured vehicles which contributes, among others, to its "Flying Guntruck" nickname.

Six vibration-dampening rubber mounting attached the engine to two forged 2024 aluminum alloy engine bearer, stiffened with built up 2024 cross-member, which in turn installed to engine bulkhead through four thrust mountings. Two more rubber mountings secured the turbine assembly to engine bulkhead. The engine directly driven Williams Hydromatic four-bladed, hydraulically-controlled constant speed propeller, 4.3 metres in diameter. Propeller pitch ranged from 25° at the lowest setting to 90° feathered. The spinner is a spun shell of 7075 aluminum which contributes to the plane's streamlining.

Cooling

Engine cooling is provided by single oil cooler and two coolant radiators on inboard wing panel leading edges. Oil flows from bottom of 140l oil tank to oil pump which circulates through engine moving parts, Scavenger pump in sump delivers it to oil tank, either directly through a thermostatic control valve, having a by-pass, or to radiator and then back to tank, depending on oil temperature. The engine cooling system utilizes centrifugal pump to circulates coolant from 180l coolant header tank through engine block water jacket, from which it flows to coolant radiator. Airflow through radiator is regulated by thermostatically controlled outlet flaps. Each coolant radiator has separate circuit and could be isolated from each other in case of damage or leak on one of the radiator.

Fuel

Normal fuel load is carried in three self-sealing fuel tanks. Main fuselage fuel tank holding 1,110 l of fuel oil is installed inside armoured fuselage mid-section between the cockpit and aft armoured bulkhead. Two auxiliary fuel tanks are installed between main and rear wing spar, one in each inner wing panels, holding 620 l of fuel each. All tanks are fitted with baffles to minimize surge and equipped with electrical booster pumps submerged in each fuel cells to prevent vapour lock at high altitude.

External fuel could be carried in 1,000 l or 500 l combat drop tank or 1,500 l non-droppable ferry tanks carried on inboard wing stations

Armaments and Armours

Internal armament consists of two synchronized 15 mm medium-calibre machine guns mounted atop the engine cowling and four free-firing 20 mm autocannons installed in pairs in each outer wing panels. External loads can be carried on six underwing hard points with total payload of 4,500 kg (9,000 lbs).

The 20 mm Autocannons are located in the inboard end of the outer panel with barrels projecting 70 mm from the leading edge and shrouded with aerodynamic fairings. Cannons are mounted on adjustable trunnion posts which permit a wide range of adjustment for the different boresighting patterns. Ejected cases and links fall directly into the airstream through chutes attached below the guns. 320 rounds for each cannons is fed from the magazines, over rollers and through feed chutes into the gun breech, giving them 27 seconds of firing time.

400 rounds of ammunition for the each of the machine guns are strored inside two magazines located behind the engine bulkhead, these magazines can be easily replenished through access doors on both sides of fuselage. The ammunition is enough for 32 seconds.

Sighting is through Mk.23 gyro computing gunsight with auxiliary ring-and-bead sight.

As far as external loads went, there were three pylon under each outer wing. The two outermost pylon could carry a store with a weight of up to 500 kg (1,102.3 lbs) , while each of inboard pylon could handle up to 1,000 kg (2,204.6 lbs) and were plumbed to carry external fuel tanks.

Although it has total effective payload of 4,000 kg (8,818.4 lbs), in practice maximum operational load for Fendrake FG.IV were twelve 250 kg bombs and two 100 kg bombs for a total load of 3,200 kg (7054.7 lbs).

The cockpit and main fuselage fuel tank are protected inside 15 mm thick titanium "belt" armour that covered the area between forward cockpit partition to the armoured aft bulkhead which consisted of 30 mm thick titanium plate armour. The windscreen is made of 5 segment of 35 mm layered perspex panels. The armour gave protections against 15 mm API rounds and shrapnel from 50 mm HE rounds.

The engine and cooling systems also protected from ground fire by 15 mm titanum armour on bottom engine cowling and underwing panels.

Avionics and Other Equipments

Fendrake's electrical system is 24 VDC, single wire, grounded type with current supplied by two 50 Ah battery located underneath the electrical equipment rack. The batteries are charged by a 120 A high-speed generator through a voltage regulator, which maintains the voltage at 26 VDC. A ground power socket is provided on starboard wing-body fairing for maintenance.

Wireless equipment consists of TR.11 VHF communication radio, TI.5C IFF transponder and AI.7 tail warning radar. In addition, AN.10 radio altimeter, RN.9 ADF receiver and RN.11B TACAN receiver provide navigational aid. All wireless equipments are contained inside line-replaceable units fitted on electrical equipment racks behind aft fuselage bulkhead for ease of maintenance.

---