Most modern commercial aircraft (e.g. 737, 767, 777, 787, 747-8, A380, etc.) feature inboard nacelle chines to improve flow over the upper surface of the wing at high angles of attack. However, many other similarly purposed aircraft (e.g. DC-8, L-1011, 757, 747-400) do not have nacelle chines. If these chines are so critical for optimum takeoff and landing performance, how are so many planes able to function just fine without them?
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$\begingroup$ Not enough for a full answer but my guess is the savings introduced by the design wasn't enough to offset redesigning existing planes, redesigning the tooling, and retrofitting them onto planes in service. So once they decided it was a good idea, new planes were designed with them but the old ones worked fine without them. $\endgroup$– tpg2114Commented May 28, 2014 at 19:03
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1$\begingroup$ @tpg2114 I would have thought that nacelle chines were literally the cheapest aero modification you could possibly make to an aircraft. They are certainly less involved than a flap redesign or winglets. $\endgroup$– Bryson S.Commented May 28, 2014 at 19:29
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1$\begingroup$ It's entirely certain they are the cheapest but that doesn't always mean they are cheap. Figuring out how to attach them to the nacelle that wasn't designed for them, building the tooling, training the mechanics, performing the modifications, getting the designs certified. All of which cost time and money and the gains may not have been worth going through that for older designs. $\endgroup$– tpg2114Commented May 28, 2014 at 19:32
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3$\begingroup$ For what it's worth, the DC-8s with high-bypass CFM56 engines do have them. $\endgroup$– egidCommented May 31, 2014 at 3:55
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$\begingroup$ The best answer is given by user2168 in this post $\endgroup$– aresCommented Oct 18, 2017 at 19:22
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1 Answer
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The chines help when the nacelle is very close to the wing. When the engine of Boeing's 737 was changed from the slim JT-8D to the higher bypass ratio CFM-56 on the 737-300 and later, the chines were essential to avoid large flow separation. Generally, aircraft with higher landing gear can afford to have more space between nacelle and wing and do not need fixes like chines. But when a new engine has been fitted to an existing design, chances are that some vortex generators are needed to fix the new flow conditions.
For the nitpickers: The 737-100 and 200 had underslung nacelles which left no space between wing and nacelle, but that is aerodynamically similar to a nacelle at a larger distance, because no narrow flow paths are created which produce steep pressure gradients. Only a closely mounted, but still separate nacelle will have these flow conditions which need to be remedied by energizing the boundary layer.
answered May 30, 2014 at 21:58
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$\begingroup$ So what about the 757? It's engines are in relatively close proximity to the wing, yet has no chines. What gives? $\endgroup$ Commented May 31, 2014 at 2:58
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2$\begingroup$ @Bryson S. You are right, but it had the advantage to be designed that way. The 757 was delivered from the start with the PW-2000 or the RB-211, and the nacelles could be optimized together with wing and landing gear in the design phase, a luxury which the 737-300 did not enjoy. $\endgroup$ Commented May 31, 2014 at 5:50
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1$\begingroup$ I have no idea why the C-17 needs them. It has even two per nacelle, and with a design which used those engines from the start. Maybe they were added by Marketing. $\endgroup$ Commented Jun 1, 2014 at 6:58
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1$\begingroup$ @PeterKämpf The 'background to the invention' section of the patent offers a motivation for their use: reduce the noise footprint by steepening the climb after takeoff. The C-17 designers may have been optimizing for something else, such as minimizing the takeoff distance. $\endgroup$– sdenhamCommented Aug 6, 2014 at 12:43