# What adds more weight to a twinjet? Tail-engines or wing-engines?

Let's say we have a large commercial jet, with exactly 2 engines, capable of holding at least 200 passengers.

It could be a traditional jet like the Boeing 777, where both engines are in the wing. Or it could be like the MD-90 with both engines at the tail.

My question is, which configuration adds more weight? With engines on wings, we must add pylons and strengthen the wing to resist bending, especially when on the ground. With engines at the tail, we must add weight to strengthen the T-tail. There might be some extra fuselage weight at the back for holding up the engines, but I'm not sure.

There may be other weight additions that I don't know about. Ultimately I want to know which config requires more weight.

Note: I'm not asking which is best. Best would require more than one factor. This question is only asking about one factor, weight.

• The greater lateral distance from CG of most wing mounted engines over a tail mounted engine (consider engine out scenarios) means that a certain higher structure value will be required both for the greater thrust asymmetry as well as the greater control required, especially in the rudder. – J Walters Aug 28 '19 at 0:55
• I don't see how one could answer this. Engine placement is decided early on in the design cycle in response to criteria other than weight, and will have effects beyond the engine mounts. How would one then break out the weight that is solely due to the whereabouts of the engine? – CatchAsCatchCan Aug 28 '19 at 1:24
• @ReddHerring How would one then break out the weight that is solely due to the whereabouts of the engine? Seems pretty straightforward to me. All you have to do is ask the question, what stuff is there that wouldn't be needed in the opposite config? I don't see a problem. Pylons are a great example. Maybe there are many tiny components that are config-specific, and the list would be huge? If so i don't need an exhaustive list. I suspect all the major weight stuff is structural anyway, and maybe engine-related like generator size and placement. – DrZ214 Aug 28 '19 at 1:34
• Another way, theoretically, is to find two examples of twinjets, with the same passenger limit, but one has wing-engines and the other has tail-engines. But they would have to be designed in the same era or one of them would prolly be using lighter components. I have not been able to find any examples under this criteria. – DrZ214 Aug 28 '19 at 1:35
• @DrZ214 Well, it's easy then. Design a complete airframe based on one choice of engine location, then do the same design based on the other location. Who would do this? – CatchAsCatchCan Aug 28 '19 at 1:40

## 1 Answer

Tail engines add more weight. Placing the engines in pods underneath the wing relieves the bending moment in flight, allowing for lighter wing root construction. Placing them at the tail requires local beefy frame construction.

Note that the upwards bending moment in flight is the limit design factor:

• In flight all of the weight of the aeroplane times the ultimate load factor of 3.5, and this is the bending moment relieved by the weight of engines mounted at the wing.
• On the ground, the undercarriage supports the weight of the aeroplane at load factor 1, with a smaller moment arm. Way smaller than the ultimate bending load.
• At touchdown, the spring/dampers are dimensioned such that the bending moment at maximum sink rate is below the ultimate design bending moment.

Torenbeek 1 mentions the following on this subject:

Empty weight. The following factors have to be considered:

• A wing structure weight saving is possible with wing-mounted engines due to the mass relief effect on the bending moment on the inner wing.
• Engines placed too far outboard increase the landing impact loads and necessitate a large vertical tailplane.
• Engines at the rear of the fuselage require local "beef-up" and lead to loss of useful space in the tail, resulting in added structure weight and a larger fuselage for the same payload.
• Differences in weïght of the tail surfaces depend on various factors which do not permit a general conclusion.

Summing up, we may say without too much emphasis that the empty weight of a CaraveIle-type layout will typically be 2 to 4% more than that of a comparable design with the engines on the wing.

As mentioned in this answer, at the time of B737 design Boeing had two competing designs from different teams, and the wing mount won. For an Order Of Magnitude check we can compare the wing and fuselage weights as a percentage of OEW for the DC9 and the B737-100. The weight of the wing & fuselage groups as per the table in this answer, the OEW from Wikipedia:

So indeed if we look at the weight only, the underwing configuration wins. The tail configuration compensates with aerodynamic pluses, mainly from the clean wing.

Note that these considerations were of value at the time of DC9 construction, with the narrow engines of the time. A modern 200 pax plane with high bypass turbofans has no good reasons anymore to opt for the tail mount.

1 E. Torenbeek, Synthesis of Subsonic Airplane Design

• Placing the engines in pods underneath the wing relieves the bending moment in flight... Yeah that part I get but I always thought, when the plane is on the ground, the bending moment is increased (as opposed to a wing with no engines on it), so you have to add structural wing-weight anyway. BTW can you tell us who Torenbeek is and put a link to that source doc so we can see exactly what it is too? – DrZ214 Aug 28 '19 at 2:10
• – BowlOfRed Aug 28 '19 at 9:18
• It's not just bending moment. Underwing engines also act as anti-flutter weights (for this reason they are moved forward). 'Clean' wings often have dead-weight balances in their leading edge. On the other hand, underwing engines require longer and heavier landing gear for low-wing configurations. – Zeus Aug 29 '19 at 0:52