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Jet wing placement

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Ok, I have a CG/weight and balance question for everyone. Why do jets, especially CRJs and MD90s have their wings so far back on the fuselage? Wouldn't this requires more force on the tailplane, causing more drag and less efficiency? Does it have anything to do with near sonic aerodynamics? 

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1 Answers

  1. John D Collins on Dec 21, 2016

    To be positively stable in the pitch axis, the Aerodynamic Center (AC) needs to be aft of the CG (Center of Gravity). With aircraft that have tail mounted engines, the CG is further aft than for an aircraft with wing mounted engines. Compare the location of the wings on a Boeing 737 with one on a CJ or MD90. The CG has to be located in front of the rear wheels or the aircraft would fall onto its tail when on the ground. If the CG is well forward of the rear wheels, too much weight would be placed on the nose wheel, so typically the CG is not too far in front of the main wheels. For example, on my Bonanza, the CG at full gross can only vary between 82.1 inches and 84.4 inches. The main wheels are at about station 97, only about 15 inches behind the most forward CG permitted.That is about 5% of the total length of my Bonanza. The point is that if estimate the CG to be within 5% of the length of the aircraft in front of the location of the main wheels, the AC must be behind this point. You need a wing that produces an AC located behind the center of gravity to have an aircraft that is stable in pitch.

    IOW, the locations of the engines on the tail force the wing to be further to the rear.

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