Q: It’s the far future. You are an astrophysicist who leads a clan that must leave the Milky Way for a safer part of the universe. The question is where to go. Aboard your near-lightspeed craft, your chief scientist waves a hand and conjures up a 3D view of a galaxy. The scientist declares the galaxy to be a perfect destination because it took the light from the galaxy 13.6 billion years to reach the spacecraft’s telescope, and the craft’s “bio suspension tank” is certified to keep everyone fresh for 13.8 billion years, providing a nice safety margin. Based on your background in astrophysics, what would you tell the scientist?
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FROM THE APRIL ISSUE: ChatGPT takes on lift
We asked you whether ChatGPT answered accurately when AIAA Fellow Paul Bevilaqua asked the bot to explain how a wing generates lift. Paul reviewed your responses and found none to be entirely accurate, so we invited him to share his critique of ChatGPT.
“ChatGPT is wrong on several points. The air does flow faster over the top of the wing, but not for the reason ChatGPT reports. The two flows that diverge at the leading edge do not reach the trailing edge at the same time. The air goes faster over the top because of the circulation established by the Kutta condition, which says the flows above and below the airfoil must leave the trailing edge parallel to each other. The resulting decrease in air pressure on top of the wing does create lift, but so does the increase in pressure under the wing. Also, ChatGPT reports that a wing must be designed to create ‘smooth’ or laminar flow over the top and turbulent flow under it. In reality, turbulent flow creates drag without increasing lift, so it is desirable to have laminar flow over both the top and bottom of the wing. ChatGPT also did not discover that the circulation creates a momentum change between the flows upstream and downstream of the wing. The air in front is deflected upward, and the air behind is deflected downward. This momentum change provides the reaction to the lift on the wing that satisfies Newton’s Law of Action and Reaction, although it does not create a net downwash. There is a downwash behind the wing induced by the vortices that trail off the wingtips. However, this isn’t the Newtonian reaction to lift because the vortices also induce an upwash outboard of the wingspan.”