Sylvie Willis
Conformal fuel tanks (CFTs) are additional fuel tanks that are fitted closely to the profile of an aircraft, extending its endurance. They are mounted above the wings to replace the drag of underwing tanks. CFTs have a reduced aerodynamic penalty compared to external drop tanks and do not significantly increase an aircraft's radar cross-section. They also do not occupy ordnance hardpoints, allowing the aircraft to carry its full payload. CFTs are designed to be plumbed into the aircraft, meaning they cannot be discarded in flight. This results in a slight drag penalty and minor weight gain, even when the tanks are empty. CFTs can also impose slight g-load limits, although this is not always an issue.
| Characteristics | Values |
|---|---|
| Definition | Additional fuel tanks fitted closely to the profile of an aircraft |
| Advantage over external drop tanks | Reduced aerodynamic penalty, do not occupy ordnance hardpoints |
| Disadvantage | Cannot be discarded in flight, impose a slight drag-penalty and minor weight gain even when empty |
| Aircraft that use CFTs | F-CK-1D prototype ("Brave Hawk"), F-CK-1C single-seater prototype, F-15B, F-15E, F-16 Block 50/52/60, F-35 |
| Range increase | 20-40% depending on the configuration and mission profile |
| Fuel capacity | 450 gallons (ca. 3,050 pounds, or 2,271 litres) of additional JP-5/8 fuel |
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What You'll Learn
Advantages of conformal fuel tanks
Conformal fuel tanks (CFTs) are additional fuel tanks that are fitted closely to the profile of an aircraft. They offer several advantages over traditional external drop tanks.
Firstly, CFTs have a reduced aerodynamic penalty compared to external drop tanks, resulting in increased fuel efficiency and improved subsonic speed acceleration. This is because CFTs are fitted flush against the aircraft's fuselage, reducing interference drag and allowing the aircraft to cut through the air more efficiently.
Secondly, CFTs do not occupy ordnance hardpoints like external drop tanks, allowing the aircraft to carry its full payload of bombs or missiles. This increases the aircraft's offensive capability and flexibility, especially in defence scenarios.
Thirdly, CFTs can significantly increase the aircraft's maximum flight range and endurance, providing an advantage for long-range or long-duration missions. For example, the F/A-18E/F Super Hornet strike fighters equipped with CFTs can extend their effective striking range, which is crucial for carrier strike groups facing anti-access/area-denial weapon systems.
Finally, CFTs do not significantly increase the aircraft's radar cross-section, making it less detectable to radar systems. This stealth advantage can be further enhanced by utilizing radar-absorbent material (RAM) coatings on the CFTs and the aircraft's airframe.
While CFTs offer these advantages, it is important to consider their limitations as well, such as the weight penalty and manoeuvrability constraints, especially when loaded with large amounts of fuel.
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Disadvantages of conformal fuel tanks
Conformal fuel tanks (CFTs) are additional fuel tanks that are fitted closely to the profile of an aircraft. They are designed to extend the endurance of the aircraft by increasing its fuel capacity. While CFTs offer several advantages, there are also some disadvantages or challenges associated with their use.
One significant disadvantage of CFTs is their permanent nature. Unlike external drop tanks, CFTs cannot be discarded in flight as they are plumbed into the aircraft. This means that even when empty, CFTs impose a slight drag penalty and minor weight gain on the aircraft, without any corresponding benefit. This can impact the aircraft's overall performance, including its speed and manoeuvrability.
The weight and drag penalties associated with CFTs can be particularly noticeable in aircraft with smaller frames, such as fighter jets. In such cases, the use of CFTs may necessitate a fighter escort for protection against hostile fighters or interceptors. Additionally, CFTs may require frequent maintenance, which can be more challenging and time-consuming than with external fuel tanks, as access to certain panels may be obstructed by the CFTs.
Furthermore, integrating CFTs into an aircraft design can be a complex and costly process. It involves significant design, validation, and testing procedures, which may not always provide a clear return on investment. As such, the decision to incorporate CFTs may be influenced by corporate financial considerations, rather than purely technical factors.
While CFTs can enhance an aircraft's range and endurance, these disadvantages highlight the trade-offs that must be considered when employing this technology. Each aircraft platform must carefully evaluate the benefits and drawbacks of CFTs to determine if they align with the specific requirements and constraints of their mission profiles.
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Conformal fuel tanks on the F-15 Eagle
Conformal fuel tanks (CFTs) are additional fuel tanks that are fitted closely to the profile of an aircraft, extending its endurance. They have a reduced aerodynamic penalty compared to external drop tanks and do not significantly increase an aircraft's radar cross-section. They also do not occupy ordnance hardpoints, allowing the aircraft to carry its full payload.
The F-15 Eagle is a dual-role fighter designed to perform air-to-air and air-to-ground missions. All US F-15Es and Strike Eagle export variants, such as the Israeli and Singaporean models, are fitted with CFTs under the wing outside the engine intake and require modification to fly without them. The F-15C entered service with CFT capability, and they were first tested on the F-15B in 1974. Each unit carries an additional 750 US gallons (2,839.1 litres) of fuel, while retaining hardpoints for four AIM-7F Sparrow missiles or bombs.
The CFTs on the F-15 Eagle have some disadvantages. They cannot be discarded in flight because they are plumbed into the aircraft and can only be removed on the ground. As a result, they impose a slight drag penalty and minor weight gain on the aircraft, even when the tanks are empty. They also impose slight g-load limits, although this is not always an issue.
Some F-15E Strike Eagles have been flown without CFTs on air defence missions. The Air Force's first operational Boeing F-15EX Eagle II fighters will also not be equipped with CFTs. However, the omission of CFTs may limit the aircraft's ability to perform air-to-ground missions and fulfill its full potential.
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Conformal fuel tanks on the F-16
Conformal fuel tanks (CFTs) are additional fuel tanks that are fitted closely to the profile of an aircraft, extending its endurance. They have a reduced aerodynamic penalty compared to external drop tanks and do not significantly increase an aircraft's radar cross-section. They also do not occupy ordnance hardpoints, allowing the aircraft to carry its full payload.
The F-16 is one of the aircraft that can be fitted with CFTs. The F-16C/D Block 50/52+, F-16E/F Block 60, F-16 V Block 70/72, and F-16I Sufa export aircraft for Greece, Chile, Israel, Poland, Pakistan, Turkey, Singapore, Morocco, Egypt, and the UAE are all plumbed to carry two CFTs mounted atop the aircraft near the wing root. These tanks can hold 450 US gallons (1,703.4 liters) or 3,050 lb of additional fuel.
The use of CFTs on the F-16 has several advantages. Firstly, they increase the aircraft's range, loiter time, and combat persistence, while also reducing the demand for tanker support. The range increase can be up to twenty to forty percent, depending on the configuration and mission profile. Secondly, the CFTs have a negligible effect on the aircraft's agility at subsonic speeds, although there is increased drag at supersonic speeds. However, the CFTs on the F-16 have very little adverse effect on the aircraft's performance, as stated by Maj. Timothy S. McDonald of the U.S. Air Force.
The use of CFTs on the F-16 also has some disadvantages. Unlike drop tanks, CFTs cannot be discarded in flight since they are plumbed into the aircraft. This means that they will impose a slight drag penalty and minor weight gain on the aircraft even when the tanks are empty. Additionally, CFTs can impose slight g-load limits, although this is not always an issue, as seen with the F-15E which maintained the same maneuverability with CFTs.
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Boeing and conformal fuel tanks
Conformal fuel tanks (CFTs) are extra fuel tanks that are fitted closely to the profile of an aircraft, extending its range and endurance. They are designed to be more aerodynamic than traditional drop tanks, reducing drag and allowing the aircraft to carry its full payload. CFTs are plumbed into the aircraft and cannot be discarded in flight, resulting in a slight weight penalty when empty.
Boeing has been involved in the development and utilisation of conformal fuel tanks for several aircraft models. One notable example is the proposed Silent Eagle model of the F-15E for South Korea and other customers, which incorporates CFTs. The F-15E CFTs are fitted under the wing outside the engine intake and have been reported to provide increased manoeuvrability without g-limitations.
Boeing has also explored the use of CFTs on the Super Hornet, aiming to increase its range and bring it closer to the capabilities of the F-35. However, there have been reports of challenges and setbacks in this endeavour. Test pilots have noted that CFTs can alter the airflow over the fuselage, affecting the lift and handling characteristics of the aircraft, particularly at slow speeds. These changes would require extensive testing and modifications to flight control software, structural loads, acceleration, and aerial refuelling procedures.
Additionally, Boeing's F-16 aircraft, including the F-16C/D Block 50/52+, F-16E/F Block 60, and F-16I export variants, are designed to accommodate the carriage of two CFTs mounted atop the aircraft near the wing root. These CFTs are expected to enhance the endurance and payload capabilities of the F-16.
While conformal fuel tanks offer advantages in terms of increased fuel capacity and aerodynamics, they also present challenges, such as the need for additional testing and modifications, as highlighted by Boeing's experience with the Super Hornet and feedback from test pilots.
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Frequently asked questions
Conformal fuel tanks (CFTs) are additional fuel tanks that are fitted closely to the profile of an aircraft. They are mounted above the wings to replace the drag of underwing tanks.
CFTs increase the endurance of an aircraft and its range. They also do not occupy ordnance hardpoints, allowing the aircraft to carry its full payload.
Unlike drop tanks, CFTs cannot be discarded in flight. They will impose a slight drag penalty and minor weight gain on the aircraft even when the tanks are empty. They may also impose slight g-load limits.
The F-15 Eagle, F-CK-1D "Brave Hawk", F-CK-1C single-seater, Indigenous Defence Fighter II “Goshawk”, and F-16 Block 50/52/60 are some examples of aircraft that use CFTs.
