Rear & Wing Mounted Engines: What are the Advantages and Disadvantages?

It is rare to find rear-mounted engines in commercial aircraft these days.

It’s rare to find rear-mounted engines on commercial airplanes these days. In general, all modern aircraft have wing-mounted engines. However, rear-mounted engines are still fairly common in corporate aircraft. Both of these engine configurations have advantages and disadvantages. We will now examine these advantages and disadvantages.

Wing-mounted Engines

Advantages

• Wing-mounted motors reduce wing bending moments. When the engines are attached to the wings they add weight to the structure, which helps reduce bending loads on the wing due to changes in lift. This means the wing doesn’t have to be that strong, reducing weight and complexity.

• Gravity fuel feeding is possible in the event of fuel pump failure: As wing-mounted engines are below the wings, fuel can be fed from the wing tanks into the engines by gravity if the pumps fail for some reason. 

• The engines are away from disturbed air: With wing-mounted engines, the engine intake is away from the aircraft structure. Thus, the engines can take in clean undisturbed air.

• It is a lot easier to maintain: The wing-mounted engine configuration keeps the engine close to the ground which makes simple maintenance tasks such as oil top-ups very convenient for the maintenance engineers.

Disadvantages

• The aircraft requires longer landing gear: As engines lie low, the landing gear need to be longer to ensure good enough ground clearance.
• The engines are more susceptible to Foreign Object Damage (FOD): Again, as the engines are closer to the ground, in high power conditions they can suck in debris which could damage the engine components.

• It is more difficult to control the aircraft in an engine failure event: Because the engines are farther away from the Center of Gravity (CG), when an engine fails, the aircraft generates a strong undesirable yaw which needs to be countered by the pilot using the rudder. This may also require a larger fin (vertical stabilizer) and rudder design.

• More cabin noise: Wing-mounted engines generate more cabin noise as they are closer to the passenger cabin. 
• It negatively affects the longitudinal (pitch) stability of the aircraft: When engines are mounted below the wings, they are below the CG of the aircraft. Thus, when power is added, it generates a nose-up moment, which reduces pitch stability. This may require a larger horizontal stabilizer design. 
• If the aircraft is to be fitted with larger engines a wing redesign/landing gear redesign might be required: Something we often see is new engines put on an already established aircraft. If the new engines are larger, it may require a redesign of the wing to ensure good enough ground clearance. Another way is to redesign the gear. Both of these add to the cost.

Rear-mounted engines

Advantages

• Cleaner wing: Because the engines are not attached to the wings, the designers have more freedom when designing a wing. They are free to build a much more efficient structure. In wing-mounted engines, the engines discontinue the wing, and this leads to some loss of lift. To recover this lift, complex leading-edge and sometimes trailing-edge flaps are required. Rear-mounted engines solve these issues.

• It is easier to control the aircraft in an engine failure event: The rear-mounted engines are closer to the CG than wing-mounted engines. Hence, when an engine fails, the yaw generated by the live engine is much less which requires less effort from the pilot to control.
• The aircraft can sit low to the ground: As the engines are not close to the ground, the landing gear can be designed shorter which reduces the design complexity. Because the airplane fuselage is close to the ground, the aircraft may not require equipment such as high loaders to load baggage and cargo. In addition to this, the aircraft can be fitted with a simple air stair to board the passengers.

• Less cabin noise: As engines are rear-mounted, away from the passenger cabin, it reduces engine noise and vibration which makes it more comfortable for the passengers. 
• The engines are less susceptible to Foreign Object Damage (FOD): With the engines mounted farther from the ground, there is less risk of the engines engulfing debris. 
• The T-tail design makes the elevator controls more effective: A T-tail means that the tail and the elevator can be placed farther away from the CG. This gives better leverage, giving more pitch control which leads to lower take-off and landing speeds. 

Disadvantages

• T-tail design leads to deep stall characteristics: As the engines are typically placed where the horizontal stabilizer is normally built in, a T-tail design might be the only option. T-tails are susceptible to a dangerous phenomenon known as a deep stall. In a deep stall, the wake from the wing covers the T-tail which makes it ineffective. An ineffective tail means that the pilots can no longer push the nose down to recover from the stall.

• A stronger wing is required: As the wing-mounted engine configuration reduces wing bending moments, with rear-mounted engines this luxury no longer exists. This requires the wing to be stronger, which increases the weight and design complexities. 
• The engine is exposed to disturbed air from the wings and fuselage: In the rear-mounted engine configuration, the engines lie directly behind the wing and most of the fuselage. This means the engine is exposed to the wake from the wing and the disturbed air from the fuselage.

• It requires a more complex fuel system: Because the fuel has to be pumped upwards in rear-mounted engines which are above the fuel tanks, it requires stronger fuel pumps. What this also means is that if the wing tank pumps fail, the aircraft engines may not get the required fuel. This requires standby pumps in addition to the primary pumps for redundancy purposes. 
• The fuselage where the engines are attached must be strengthened: As engines subject the area to which they are attached to loads due to power changes, the point where they are attached needs to be tough enough to withstand the loads. This sometimes requires complex structures between the engines and fuselage to transfer the load, adding to the weight. 
• The engines take away cargo space: This point is specific to corporate jets. As most of these types of aircraft have cargo space behind the cabin, having engines at the rear reduces the space available for cargo. The engines can also make it difficult to load up the cargo.

• The vertical stabilizer (fin) needs to be stronger: Because the horizontal stabilizer has to be attached to the fin, it needs to be stronger to support the stabilizer. 
• Chines need to be designed into the nose landing gear tires: As the engines are at the rear, the water spray when the aircraft moves through wet surfaces can expose the engines to it. Chines designed in the tires of the nose wheel can modify the water spray to prevent this. This makes nose tire design more complex.