The mounting of a jet engine under the wing of an airliner can be a daunting task for turbofan engineers.
Thrust forces generated by gas path momentum flow changes in a jet engine are transmitted by pressure (and friction) forces on stators and struts attached to the engine case. Case engine mounts then transmit the thrust forces (as high as 100,000 pounds thrust on the largest engines) to the wing pylons to pull the plane forward. The mounts must also support the engine weight (as high as 20,000 pounds) and carry nacelle flight loads.
Engine bypass ratios are increasing (12:1 on the new geared fan engines), with fan sizes ever growing (178 inch diameter fan on the new GE9X). Mounting these new engines under a wing can present new challenges.
During the early days of its introduction in the late 1960’s, Boeing’s iconic 747 jumbo jet had engine mount problems. These are examined, together with their solution.
#35 SEPTEMBER 2018
As a window-seated passenger in a large airliner in flight, the view of a mighty jet engine mounted under the wing is an awe-inspiring sight for me - especially during wing-deflecting rough weather. I marvel at the engineering required for the engine mounts to keep the engine safely attached to the wing’s pylon, while transmitting thrust for flight, carrying engine weight and supporting nacelle aerodynamic loading.
Bill Gunston  relates that the D.H. 106 Comet, the first jet liner, in 1947 pioneered the concept of turbojets buried within the roots of its long-chord wing. A year before, Boeing with its Model 450 airplane, had jet engine pods hung on thin pylon struts well below and ahead of the wing’s leading edge. Since then, airliner designers have largely followed the Boeing stratagem of underwing engine mounting.
Thrust forces generated by gas path momentum flow changes in a jet engine are transmitted by pressure (and friction) forces on stators and struts attached to the engine case. Case engine mounts then transmit the thrust forces (as high as 100,000 pounds thrust on the largest engines) to the wing pylons to pull the plane forward. The mounts must also support engine weight (as high as 20,000 pounds) and carry nacelle flight loads. Because of the wide variations in temperatures and loads on engine casings, engine mounts are both fixed and floating, to allow casings to expand and contract freely in both axial and radial directions.
Since aircraft Number 1 had its maiden flight in 1969, Boeing’s 747 was the first jumbo jet. It is the most successful wide-body passenger aircraft yet developed, with over 1,500 produced to date. As a young engineer at Pratt & Whitney Aircraft (now UTC’s Pratt & Whitney) I had some personal involvement with engine mounting troubles with the 747’s inaugural engine, the PWA JT9D .
Less than six months after its maiden flight, it was determined that the JT9D engine case was excessively bending and ovalizing - exhibiting non-circular distortion - under thrust loading that could be as high as 43,500 pounds on takeoff. The ovalizing distortion resulted in turbine and compressor blade rubbing against the interior of the engine case and necessitated power-robbing increases in blade tip clearance gaps. The result was a serious reduction in thrust, and increased fuel consumption, as much as 7 percent above guaranteed rates.
Both Boeing and Pratt & Whitney were essentially betting their net worth on the 747, this the first commercial jumbo jet. At one time, there were 15 four-engine 747 jets sitting engineless on Boeing’s Everett tarmac, representing $360 million - more than $2 billion 2018 dollars - of stranded assets. Getting those planes into the air was an engineering and commercial imperative.
Figure 1 shows a model I constructed to illustrate the ovalization of the JT9D case under thrust loading. The engine case is modeled from a cylindrical plastic tennis ball container, cut up and sectioned as shown.
The bottom is reattached with string to hold a brass weight, simulating engine thrust. The cylindrical portion of plastic container is held in place by a fixed upper attachment (an Allen screw), and a lower floating support point, to simulate the original JT9D engine mounting arrangement.
As one can see, Fig. 1a shows no distortion. When the brass weight is applied in Fig. 1b, ovalization is clearly visible. (Evidence of bending along the container axis is not shown in Fig. 1b and would probably require more careful measurement for verification.)
Pratt structural engineers conducted extensive static JT9D case deflection tests and analysis. They found that if two - rather than one - thrust mounting points were circumferentially located 90 degrees apart at any one axial position on the engine case, the resulting ovalization of each would cancel the other, greatly reducing overall case distortion. This two-point distortion canceling method was very effective, so much so that the two mounting points could be separated by as much as 120 degrees and still yield an acceptable amount of case ovalization reduction.
The Pratt team then devised and designed a Y-shaped titanium tubular thrust frame with arms that were fastened to the compressor intermediate case at two fixed mounts, about 120 degrees apart. The leg of the thrust frame then attached to the rear turbine case mount through an axially sliding joint (to accommodate engine axial length changes) that was rigidly affixed to the pylon. (See Fig. 2).
Subsequent engine tests showed that the new thrust frame substantially reduced ovalization. Maximum thrust could be achieved with little case distortion and engine performance now met fuel consumption specifications. The new thrust frame (which became known as the “yoke” at P&WA) added about 163 pounds of weight to the 8,600-pound JT9D, and required a relocation of several external engine components. But as an addon to the existing FAA certified engine it solved the ovalization problem which was threatening the financial future of both Boeing and Pratt & Whitney Aircraft.
The mounting of jet engines continue to challenge turbofan jet engineers. For proprietary reasons, not much is published in the open literature, but one can go to patent listings to get an idea of the continuing technical activity in engine mounting.
Engine bypass ratios are increasing (12:1 on the new geared fan engines), with fan sizes ever growing (178 inch diameter fan on the new GE9X). Mounting these under a wing is a daunting task!