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is this a good answer. the blades rotate, the tip moves faster than the hub. So to make the blades efficient, the blades are usually twisted. The angle of attack of the blades at the tip is lower than at the hub because it is moving at a higher velocity than the hub. the amount of thrust generated is higher near the tips than at the root. Engineers want the thrust to be equivalent on every part of the blade. enter image description here

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    $\begingroup$ Yes that's about right, but it does not fit the format of this site to put the answer imside the question. Ask the question, then answer your own question in an answer. $\endgroup$ – Koyovis Jul 25 '17 at 0:30
  • $\begingroup$ The fan is not to generate thrust. It compresses air. The nozzle generates thrust. $\endgroup$ – user3528438 Jul 25 '17 at 2:41
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    $\begingroup$ The fan most definitely produces thrust. That's a high bypass turbofan we're looking at. $\endgroup$ – Koyovis Jul 25 '17 at 6:11
  • $\begingroup$ Related to the shape: aviation.stackexchange.com/questions/34798/… and tangentially aviation.stackexchange.com/a/26947/4108 $\endgroup$ – Sanchises Jul 25 '17 at 12:12
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    $\begingroup$ You aerodynamics engineers crack me up! I see at least four people commenting on the question and one commenting on the -1 answer who sound like they know enough to provide an actual answer. Yet the person who actually answered got down voted and told he was wrong. Why not answer the question? $\endgroup$ – bclarkreston Jul 26 '17 at 3:50
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I don't think it's accurate to say that the desired thrust is the same along the entire blade. For a large turbofan blade like the one in your image, the outer part is designed to act more like a propeller (efficient for the bypass air) while the inner part is designed to act more like a compressor (efficient for the core). You're not incorrect in stating that the twist does help optimize the angle of attack with the speed of the section, but that's more about not breaking the speed of sound at the tip than keeping thrust constant along the blade.

For any specific engine, though, the blades are highly optimized based on an incredible amount of complex CFD and wind-tunnel analysis for which there is no simple or intuitive summary. The design of a turbofan blade is driven by several factors, mainly

  • aerodynamic efficiency,
  • structural efficiency,
  • noise,
  • temperature,
  • vibrations, and
  • bird-strike resistance.

All of these go into the resultant shape, but you're unlikely to find specifics that are non-proprietary to the manufacturer and available for anyone to analyze. The twist specifically is just another parameter in the extensive, complicated optimization.

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  • $\begingroup$ Peter, I disagree extremerly with the statement that "the outer part is designed to act like a propeller". Do you have a reference for this? I have read many books on gas turbine performance, and never ever have read anythng that suggests this. e.g Mattingly's Elements of Propulsion, or Oates Aerothermodynamics of Gas Turbine and Rocket Propulsion, or Gas Turbine Theory by Saravanamuttoo. Performance modeling of the fan is identical to the LPC or HPC - using a compressor map defining pressure ratios, mass flow, and efficiency. But the rest of your comments are correct. Regards $\endgroup$ – Penguin Aug 2 '17 at 11:43
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    $\begingroup$ @Richard: The concept I'm trying to convey is that the most efficient thing for bypass air is to move a lot of it with as small a change in velocity as possible (the goal of a propeller) and the most efficient thing for the core is to have the air compressed as much as possible. The turbofan blades need to do both, so some efficiency is lost; the twist is one way of mitigating these losses. Compare a propfan, where the functions are decoupled and higher efficiency can be achieved. $\endgroup$ – Peter Schilling Aug 2 '17 at 22:21
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(Hot section) turbine blades are twisted (in the chord wise direction) because they are partially utilising reactive aero mechanical design, and impulse aero mechanical design. (The blade is also twisted in the radial direction due to vortex flow. See the last paragraph of the development section here. This is also mentioned in Frank Whittle's book, Gasturbine Aerothermodynamics. Rolls Royce's book, "The Jet engine" says on page 50: "The reason for the twist is to make the gas flow from the combustion system do equal work at all positions along the length of the blade and to ensure that the flow enters the exhaust system with a uniform axial velocity.")

With fan blades, I am not so sure - I think your answer is along the right lines, (though it's better to say even pressure rise than thrust, although ultimately that pressure translates to thrust), but if that was the only factor at play, I think the blade shape would be a smooth arc from the hub to the tip, and that's not the case. Usually in a gas turbine, the outer fan case diameter is relatively straight, but the hub expands as the air flows though the fan, then into the compressor, the air travels through a bend. So, the air is moving in an axial and radial direction. This maybe an added factor that causes fan blades to be twisted. Fan and compressor blades have the difficult job of making the air flow in the direction of increasing pressure. Flow seperation will cause stall, so care needs to be taken to avoid any local flow disturbances. The air can also choke within a compressor, which is why sometimes air is bleed of the rear stages for stability control. To avoid choking in one region, the air needs to be distributed evenly. (This is more an issue in the booster or HPC than the fan however). Reducing fan noise is another requirement.

Note: Bits in () in 1st para added in subsequent edit, to include feedback in comments.

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  • $\begingroup$ No. While the cross-section curvature of a blade is based on reactive or impulse design, the twist along the blade, which is what the question refers to, is there regardless of the operating principle. $\endgroup$ – Sanchises Jul 25 '17 at 12:06
  • $\begingroup$ Regarding your 'smooth arc' speculation, see the related questions in my comment on this question. $\endgroup$ – Sanchises Jul 25 '17 at 12:14
  • $\begingroup$ Sorry, my first sentence is misleading, and I think that has caused confusion. The first sentence refers only to blades in the turbine (hot) section, as a contextual remark. The rest of the comment is about blades in the cold section - fan, or booster/IPC or HPC. I agree that some fan blades are swept to avoid sonic issues, that results in lower noise and improved performance, as discussed in the related question you highlight. $\endgroup$ – Penguin Jul 26 '17 at 11:48

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