Tail-Mounted Engines: Advantages and Disadvantages

[11thHeaven]

I was just struck the other day by the question of why some planes are designed with the engines under the wings, and some are designed with the engines mounted on the tail. What are the relative advantages and disadvantages of such designs?

 

[Enthalpy]

Engines receive a nice flow or air under the wings, so they give their full thrust.

But at the rear, engines re-accelerates the air that the body made slower. Behind the plane, air speed is more uniform, which means less wasted power.

Much drag is produced at the wings and along the body, so pushing the wings transmits less force through the plane than pushing the aft.

If an engine explodes, consequences tend to be worse at the tail than under the wing.

Put together, tail-mounted engines are less fashionable these days. They could thrive again for the improved efficiency, especially at a lifting body or a wing-only design.

 

[Alpha Floor]

Advantages of tail-mounted engines:

- They allow for a neat airflow around the wing, which enhances its aerodynamic properties (better performance, shorter take-off field)
- In case of engine failure, the torque the remaining engine generates is less than if it was wing-mounted, hence a smaller rudder is required.
- Reduced Foreign Object Damage (FOD) while on the ground.Disadvantages of tail-mounted engines:

- They are harder and more expensive to service. Maintenance is an important factor.
- From a structural point of view, wing-mounted engines reduce the bending moment at the wing-fuselage joints when in-flight. On the ground it's the opposite.
- The engine weight is concentrated well aft of the aircraft's CG (unstable inertia, caution placing the payload etc)

 

[Alpha Floor]

Behind the plane, air speed is more uniform, which means less wasted power.

I don't see the connection. You mean less power is wasted on a tail-mounted engine than on a wing-mounted engine?

Much drag is produced at the wings and along the body, so pushing the wings transmits less force through the plane than pushing the aft.

I don't understand this, would you explain it better please?

 

[Aero51]

The pros:
1) Cleaner airflow over the main wing
2) Uninterrupted wing geometry
3) Possible weight reduction due to easier mounting
4) A longer fuselage (good or bad thing depending on design goals) =weight increase
5) Higher landing gear clearance
6) Possible improvement in flow over the tail (no jet engine wake and down wash to deal with)

Cons:
1) Moves CG Farther back - requires long fuselage = weight rise
2) harder maintenance access
3) Restricted to T-Tail design = can very easily stall at high AOA in main wing wake
4) Less uniform velocity profile at jet inlet
5) Positive induced moment on aircraft due to higher mounting of engines
6) Increased Wing Loading
7) Fuel injection into turbine requires more piping and pumps = more weight

 

[Vadar2012]

What about wing mounted engines reduce the maximum deflection of the wings during dynamic loading, since they are placed on a node of the mode shapes. I think that's an important one for design decisions.

 

[Vadar2012]

I kinda worded that badly. Wing mounted engines help change the mode shaped and natural frequency of the wings to a more desireable value. That'd be more accurate.

 

[Alpha Floor]

I kinda worded that badly. Wing mounted engines help change the mode shaped and natural frequency of the wings to a more desireable value. That'd be more accurate.

You mean that a wing mounted engine will increase the speed at which flutter may occur?

While this can be possible, I don't think it's much of an advantage since the lowest speed at which flutter occurs is anyway higher than any design speed.

 

[Aero51]

What about flutter at high angles of attack? Its pretty important when doing model testing, especially if they break...
:/

 

[AlephZero]

While this can be possible, I don't think it's much of an advantage since the lowest speed at which flutter occurs is anyway higher than any design speed.

That is only true once you get the design right

For example in the early days of the 747, there were some issues getting three different engine desiigns to hang off the same wing without any flutter problems.