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Case distortion with subsequent blade tip rubbing and performance loss appeared on the JT9D installation in the Boeing 747 as a result of thrust being taken from a single point on top of the engine exhaust case. Thrust from the rear mount plane was a Boeing requirement.<ref>"Jet Engine Force Frame", US patent 3,675,418</ref> Compared to the 15,000 lb thrust JT3D with its four structural cases the 40,000 lb thrust JT9D made economical use of supporting structure with only three structural cases making a compact lightweight design.<ref>https://nyaspubs.onlinelibrary.wiley.com/doi/abs/10.1111/j.1749-6632.1968.tb15216.x, "Development Of The High Bypass Turbofan", p. 588 'Advanced Structural Concepts'</ref> During flight testing the engines suffered violent surges and loss in performance<ref>"747 Creating The World's First Jumbo Jet And Other Adventures From A Life In Aviation", Sutter, {{ISBN|978 0 06 088241 9}}, p. 187</ref> which were traced to bending of the engine backbone by 0.043 in. at the combustor case and the turbine case going out-of-round which in turn caused blade tip rubs and increased tip clearance.<ref>Flight International,13 November 1969, p. 749</ref>
The three big fan engines introduced in the 1960s for wide-body airliners, Boeing 747, Lockheed Tristar, DC-10, had much higher thrust and size compared to the engines powering the previous generation of airliners. The JT9D and CF6 showed that rotor tip clearances were sensitive to the way the engines were mounted and performance was lost through rotor tip rubs due to backbone bending and local distortion of casings at the point of thrust transfer to the aircraft pylon.<ref>{{Cite book |chapter-url=https://arc.aiaa.org/doi/10.2514/6.1991-2987
The first high bypass fan engine, the TF39, transferred its thrust to the C5 pylon from the rear mount. It was a single point thrust ___location on the turbine mid-frame which locally distorted the casings, causing out of roundness of the turbine stators, increased clearances and a performance loss. The CF6-6, derived from the TF39 had thrust taken for the DC-10 from the front mount plane but also from a single point. This also caused single point distortion and unacceptable performance loss for the airliner. The distortion was reduced by taking thrust from two points which allowed smaller compressor running clearances and better SFC.
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