How do the F-35A/B/C variants differ from each other?

The F-35 has three airframe variants: A, B and C. All three variants of the F-35 have been developed with design requirements in mind, resulting in almost different types of aircraft with different performance and operational capabilities. However, the goal of having all three variants with similar performance characteristics and identical advanced avionics has been largely adhered to. The differences between the aircraft variants resulted from differences in the expected operational requirements of the forces. In addition, with the systems, components and parts common to all three variants, it was possible to keep the unit cost of the aircraft as low as possible.

In addition to these three main variants, the F-35 Lightning II has several different configurations and blocks used to indicate different capability and upgrade levels. These configurations and blocks include F-35A: Block 1, 2 and 3, F-35B: Blocks 2B and 3F, and F-35C: Blocks 2 and 3F, which, according to their nomenclature, refer to various upgrades and improvements to the avionics, sensors, weapon systems and other capabilities of each of the variants.

All variants are capable of 1.6 mach, with versions A and C actually capable of slightly higher speeds. All are fifth generation aircraft with avionics (with suitably different programming), sensors and EW/ECM, as well as low visibility capabilities. In general terms, A is known as the Air Force version, B as the Marine Corps version and C as both the Marine Corps and Navy version. Such a generalisation is certainly not 100% wrong, but it is not 100% right either. 

F-35A

The F-35A is the standard F-35 variant used by the US Air Force and most non-US partners and customers. Approximately 70 per cent of all F-35s are produced based on F-35A standards. The F-35A is the classic version with CTOL (Conventional Take Off and Landing). This feature of the F-35A, which can take off and land on runways like normal "Air Force" style runways, is seen as the most important difference between the B and C variants. The F-35A is the variant of the US Air Force (USAF). It is also the main export variant and the base variant for the F-35I Adir, which is specialised for Israel. 

F-35B

The F-35B STOVL (Short Take Off and Vertical Landing) version can take off from short runways and amphibious assault ships such as TCG-Anadolu, which can be considered small aircraft carriers in a sense. It can fulfil the functions of the old AV-8B Harrier II. As an extension of the STOVL system, which requires a complex design, this variant had to compromise on both weapons and fuel quantity. The B variant was developed primarily for use by the USMC and RN/RAF on amphibious assault ships. 

The biggest advantage of the F-35B is that it can take off and land from almost anywhere. Vertical take-off and landing is accomplished with a large lift fan triggered by a driveshaft and thrust vectoring nozzles at the rear. The F-35B, which resembles the F-35A in normal appearance, can be easily distinguished from the other variants by its opening/closing door with the lift fan embedded in the rear of the canopy, its 'hump' or 'humpback' appearance, and especially when the fan nozzle cover is open. Since 2015, this variant, which has been used by the USMC since 2015, is a tried and tested aircraft experienced in operations in Afghanistan. 

F-35C

F-35Cs are the version that can take off and land from short runways. They are aircraft designed to operate from normal aircraft carriers. It is a variant with larger wings than the others. It can be easily distinguished from the A and B variants even by looking at the wing structure. The C variant is designed for conventional aircraft carriers. 

In addition to having a larger wing area, the C's were designed to have folding wingtips in order to have a more compact storage space on the crowded deck of an aircraft carrier. The main reason for the larger wing area of the C version is that the increased wing area (40% larger than the A and B) provides better lift. Many people may think of the wing area size as a plus for manoeuvrability, assuming and accepting that the air-to-air performance of the C's is better due to the smaller turning circle, but in fact the larger wing area was primarily needed to enable lower speed approaches for carrier landings. In a way, we can say that the air-air performance is an added benefit of this need. In addition, the increase in wing area is a factor that increases the amount of fuel that can be carried on the wings in favour of Cs, offering extra fuel storage.

Differences in Wing Structure

While the A and B variants have flaperons (similar to the F-16) integrated with a monolithic wing structure of almost identical design, the C has flaps and flaperons together. This split structure is the result of a corresponding arrangement, which also corresponds to the requirement for the C's wings to be foldable in order to make more room on the crowded deck of aircraft carriers, where a large number of aircraft have to be kept together in carrier operations. If it was only a flaperon structure, it would not allow the wings to be folded. 

The large wing area (40 per cent larger than that of the A and B) also allows for low-speed take-off and landing, increased buoyancy and improved performance of the carrier's launch system (catapult assisted launch/barrier-CATOBAR). The larger wings also allow the F-35Cs to carry more fuel than the others. Compared to the tail hook used on the A and B, the tail hook used on the Cs is more robust and designed to withstand greater forces.

While boom type refuelling probe is used in A model, basket type refuelling system is used in B and C models.

In terms of the angle of attack (AOA) limit during flights, the F-35B has the same 50-degree AoA restriction as the F-35A and F-35C. In general, the F-35 has proven during flight tests that it is capable of reaching even higher angles of attack. According to unofficial data, the aircraft can reach up to 110 AOA in various configurations. However, angles greater than 50 degrees have been limited to 50 AOA by software in order to prevent pilots from accidentally moving the aircraft to uncontrolled higher AOA conditions in air battles, and to avoid unintentionally entering abnormal situations at higher angles of attack.

Differences in the Materials Used in the Airframe Structure

The load stations of the F-35B (STOVL) variant are slightly weaker than those of the F-35A and F-35C. Weaker aluminium structural elements are used in F-35Bs. In the A and C variants, stronger aluminium and titanium structural elements are used. The F-35B uses these weaker (but lighter) structures to meet the weight requirements associated with STOVL flight.

In addition, the nose landing gear of the Cs, unlike the other two, uses a two-wheeled structure.

The F-35Cs entered service with the US Navy and Marine Corps (USMC) in 2019. The USMC has chosen to procure the F-35C variant to replace the F/A-18A/C/D Hornet. Currently, the F-35C is a variant that is used only by the United States Army.

Weapon / Ammunition Carriage Differences

The F-35 has 11 payload stations. Although all three F-35 variants have 11 stations, there are differences in the payloads that can be carried at some stations depending on the aircraft variant. The A and C variants can carry up to 2,500 pounds of payload at the 2/10 and 4/8 stations, while the F-35B can carry up to 1,500 pounds of payload at these stations.

The range and payload of the F-35C are almost identical to the F-35A. The F-35C can only pull about 7 to 7.5 Gs, while the F-35A is certified to pull 9 Gs.

In the bomber role, the F-35A is capable of carrying more payload than the F-35B and can use this payload to hit targets much further away. The F-35C can also carry more payload than the F-35B and can reach greater distances, but since this variant was developed to take off and land from an aircraft carrier, it is not possible to reach larger operational radii due to safety and survivability criteria. However, since the F-35B, which carries a lower ammunition load than the C and A variants, has the flexibility to operate from airfields closer to enemy positions and from aircraft carriers, I believe that it can carry out more critical missions than the other two variants by having a unique bombardment capability when necessary.

In the air defence role, the F-35A, with its operational radius equal to that of the F-35C, can undertake a much more prominent function than either of them. This is because the F-35A's lower weight and fuselage structure, which keeps friction coefficients low, gives the aircraft the advantage of climbing and gaining altitude significantly faster, as well as enabling the aircraft to reach the desired speeds in a shorter time when necessary. Meanwhile, the F-35B is capable of accelerating and climbing more quickly than the F-35C, but has a 165 nmi disadvantage in operational radius.

Use of Machine Gun

The Air Force variant of the F-35A has a 25 mm GAU-22/A internal machine gun in the section where the left wing meets the fuselage, in the protruding section diagonally behind the pilot. Like the Bs, the Cs do not have an internal machine gun mount. However, the B and C variants were intended to be a variant capable of firing a cannon when required by carrying the machine gun pod. For these variants, an optional externally mounted machine gun pod is used.

By the way, the F-35A does not need the gun pod carried by the F-35B and F-35C. In other words, station 6 is not used in the F-35A variant. However, the F-35's gun pod station could serve as a carrier for a "multi-mission pod" slot that could potentially accommodate other systems such as jammers in the future. If this capability is acquired, the F-35A retains the flexibility to integrate new capabilities into this station.

Propulsion System Differences

A and C, like the F-16, have a propulsion system consisting of a single turbofan engine with a converging-diverging variable area exhaust nozzle. The Bs use a rotary nozzle system that can be orientated 90 degrees downwards, which is necessary for vertical landing and sometimes take-off. In addition, on Bs, the engine gases can be directed to rollposts that pass through the wings. This acts as a stabilising thrust for vertical landings and take-offs. The fan system behind the canopy, which I have mentioned before, also provides huge lifting power by utilising the air coming from the engine's low-pressure turbine housing in B's. Due to this system used for vertical take-off and landing in B's, some of the systems of A and C had to be sacrificed. 

The photo below shows an F-35A conducting a ground test of its internal armament, and below it an F-35B carrying an external machine gun pod.

Cost Differentiation

By far the most expensive of the three variants is the F-35B. It is followed by the C and the cheapest is the A variant. The A's cost 68.1 million USD, the most expensive variant B's 98.4 million USD, while the C's cost somewhere in between, at 83.1 million USD. This means that as the aircraft moves away from the classic aircraft (A variant), it becomes an expensive machine.

Weight

The B, which needs to perform short take-offs and vertical landings, is expected to be the lightest variant among the three existing variants. However, contrary to this expectation, the F-35 is the heaviest variant. What is in question here is the empty weight. When fuel and ammunition weights are added, all balances and comparison ratios will undoubtedly change. What makes the B so heavy is the large fan carried behind the pilot, the nozzle system for thrust vectoring and the connecting ducts in between. The C has the most fuel of any F-35 variant, but more weight and overall drag means it has basically the same range as an F-35A. Much of the structure is reinforced with carriers of increased strength, and the landing gear is completely different.

Let's mention it as a footnote. Due to the desire for as much commonality as possible between the F-35 variants, the turbine powering the lift fan on the B has been retained in the turbine module of the engine on the A and C. In the final analysis, the Bs have the heaviest engine, while the A flies with the lightest engine.

Conclusion

In line with the requirements of the US military and other programme partner countries, three variants of the F-35 family have been produced. The F-35A, known as the Air Force aircraft, is the most common variant designed to operate from conventional runways. The Marine Corps F-35B can land vertically like a helicopter and take off from very short distances. It is intended to operate from simple, short-runway military bases and aircraft carriers. The F-35B is used by the US Marine Corps, the UK, Japan, Singapore and the Italian Air Force. The F-35C, which is produced for use on aircraft carriers, is the variant that is only available to the US Navy and Marine Corps. It has not yet been exported to any country other than the USA.