PT20J Posted August 1, 2022 Report Share Posted August 1, 2022 Pilots are fascinated with lift -- it keeps us aloft. But, its origin is somewhat mysterious. On one hand, there are the Navier-Stokes equations used by engineers that accurately describe the behavior of fluids in motion and allow prediction of aerodynamic forces, but do not provide much physical insight. On the other hand, there are the popular explanations invoking Bernoulli and Newton that are easy to visualize but incomplete or inaccurate. There are a lot of videos on youtube (some by college professors) that are just wrong. But, here's a very good one that offers a reasonable physical description without the pitfalls created by oversimplification. Skip Quote Link to comment Share on other sites More sharing options...
DonMuncy Posted August 1, 2022 Report Share Posted August 1, 2022 The total effect of that explanation makes my understanding worse. 1 4 Quote Link to comment Share on other sites More sharing options...
DCarlton Posted August 1, 2022 Report Share Posted August 1, 2022 I never enjoyed fluid mechanics. I was more of a solid mechanics guy. His summary and take-away leaves me liking fluid mechanics even less. I suspect the students in the class room were even left confused with his summary. Quote Link to comment Share on other sites More sharing options...
philip_g Posted August 1, 2022 Report Share Posted August 1, 2022 At the end of the day.....I just don't care. 2 2 Quote Link to comment Share on other sites More sharing options...
T. Peterson Posted August 1, 2022 Report Share Posted August 1, 2022 2 hours ago, PT20J said: Pilots are fascinated with lift -- it keeps us aloft. But, its origin is somewhat mysterious. On one hand, there are the Navier-Stokes equations used by engineers that accurately describe the behavior of fluids in motion and allow prediction of aerodynamic forces, but do not provide much physical insight. On the other hand, there are the popular explanations invoking Bernoulli and Newton that are easy to visualize but incomplete or inaccurate. There are a lot of videos on youtube (some by college professors) that are just wrong. But, here's a very good one that offers a reasonable physical description without the pitfalls created by oversimplification. Skip I am neither an engineer nor particularly bright, but my takeaway from this video is that the hand out the window of a speeding car is still the best explanation!! 6 Quote Link to comment Share on other sites More sharing options...
DonMuncy Posted August 1, 2022 Report Share Posted August 1, 2022 I like Wolfgang Langewiesche's explanation best. (I may be praphrasing) "Airplanes go up by pushing air down." 4 Quote Link to comment Share on other sites More sharing options...
Guest Posted August 1, 2022 Report Share Posted August 1, 2022 42 minutes ago, DonMuncy said: I like Wolfgang Langewiesche's explanation best. (I may be praphrasing) "Airplanes go up by pushing air down." I thought that it was all a result of crazed spending. Without which none of us would fly. Quote Link to comment Share on other sites More sharing options...
MooneyMitch Posted August 1, 2022 Report Share Posted August 1, 2022 I, being of simple mind, was taught during my initial flight instruction (many moons ago () about lift, thrust and drag. Without getting technical beyond that, the explanations of lift were (and have been all along) enough for me to recall constantly during flight, losing lift when unwanted was not the thing to do. I’m still here !! Yes, the hand out the window technique I’ve used countless times as a simple method of explaining how horizontal and vertical wings on our aircraft function, both for lifting and turning. Quote Link to comment Share on other sites More sharing options...
Hank Posted August 1, 2022 Report Share Posted August 1, 2022 Lift is proportional to speed. Quote Link to comment Share on other sites More sharing options...
jaylw314 Posted August 1, 2022 Report Share Posted August 1, 2022 This is still my favorite airman's reference: https://www.av8n.com/how/ Quote Link to comment Share on other sites More sharing options...
PT20J Posted August 1, 2022 Author Report Share Posted August 1, 2022 2 hours ago, Hank said: Lift is proportional to speed. Fixed it Lift is proportional to speed squared 1 Quote Link to comment Share on other sites More sharing options...
Cruiser Posted August 1, 2022 Report Share Posted August 1, 2022 It is hard to beat angle of attack for an explanation. Even using the hand out the window example --- if there is not an angle to the direction of motion, nothing moves up or down. 1 Quote Link to comment Share on other sites More sharing options...
EricJ Posted August 1, 2022 Report Share Posted August 1, 2022 41 minutes ago, PT20J said: Fixed it Lift is proportional to speed squared Otherwise it wouldn't be proportional to drag. 2 Quote Link to comment Share on other sites More sharing options...
PT20J Posted August 1, 2022 Author Report Share Posted August 1, 2022 1 hour ago, Cruiser said: It is hard to beat angle of attack for an explanation. Even using the hand out the window example --- if there is not an angle to the direction of motion, nothing moves up or down. Not quite true. A cambered airfoil must have a negative angle of attack for zero lift. 1 Quote Link to comment Share on other sites More sharing options...
DCarlton Posted August 2, 2022 Report Share Posted August 2, 2022 2 hours ago, PT20J said: Not quite true. A cambered airfoil must have a negative angle of attack for zero lift. Isn’t the hand out the window just an example of dynamic pressure and not the reason wings produce lift? one minute of your life below from a quick search … https://www.cam.ac.uk/research/news/how-wings-really-work Quote Link to comment Share on other sites More sharing options...
carusoam Posted August 2, 2022 Report Share Posted August 2, 2022 The fun part of studying fluid flow… It often comes with constraints… 1) Temps are constant… 2) pressures are constant… 3) The geometry of the flow channel is constant… or really well defined… 4) molecules adhere to the surface… ideally, but not in reality… The more complex, the more interesting… with a wing… 5) We have a split line that moves with AOA… 6) We have flow separation as part of an ordinary day… 7) We have a critical angle of attack… where the wheels of lift fall off… 8) And we have a case of too much lift… exceeding Vna and entering a vertical column of air… and a strategy to go with that… 9) We also have a case of changing geometry… ice changes the known flow geometry to something else… Its amazing we don’t need to know much science to make these things work… on most days… Heck… we barely discuss… potential energy, kinetic energy, and chemical energy… But, we have fancy statements like…. you can’t go down and slow down in a Mooney without speed brakes…. The more you know… Go MS! Best regards, -a- Quote Link to comment Share on other sites More sharing options...
T. Peterson Posted August 2, 2022 Report Share Posted August 2, 2022 4 minutes ago, carusoam said: The fun part of studying fluid flow… It often comes with constraints… 1) Temps are constant… 2) pressures are constant… 3) The geometry of the flow channel is constant… or really well defined… 4) molecules adhere to the surface… ideally, but not in reality… The more complex, the more interesting… with a wing… 5) We have a split line that moves with AOA… 6) We have flow separation as part of an ordinary day… 7) We have a critical angle of attack… where the wheels of lift fall off… 8) And we have a case of too much lift… exceeding Vna and entering a vertical column of air… and a strategy to go with that… 9) We also have a case of changing geometry… ice changes the known flow geometry to something else… Its amazing we don’t need to know much science to make these things work… on most days… Heck… we barely discuss… potential energy, kinetic energy, and chemical energy… But, we have fancy statements like…. you can’t go down and slow down in a Mooney without speed brakes…. The more you know… Go MS! Best regards, -a- You smart guys always have to spoil it for the rest of us!!! Lol! Quote Link to comment Share on other sites More sharing options...
carusoam Posted August 2, 2022 Report Share Posted August 2, 2022 11 minutes ago, T. Peterson said: You smart guys always have to spoil it for the rest of us!!! Lol! The cool part… Everyone is smart around here… in many different areas. The hard part… When you see ice building on the wing… The more you know about airplane design…. You really know how bad this can be…. 1) Lift is going away… 2) Plane is getting heavier… 3) thrust is diminishing, until the prop becomes unbalanced…. Then thrust approaches zero, quickly… 4) it’s really easy to see… the prop as a rotary wing… We definitely get to use science when things start working against us… Discussing failures… that’s always going to be challenging… the fun has already been ruined before the discussion even begins… Best regards, -a- 1 Quote Link to comment Share on other sites More sharing options...
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