The physics of airplane flight
Aerodynamic lift, stalls and wing design are revisited here from both practical and theoretical angles, contrasting flat plates with cambered airfoils, the role of flaps, and how horizontal stabilizers and canards affect efficiency and stability. Commenters also debate how best to explain flight—Bernoulli vs. Newtonian momentum change—why powered aircraft arrived only once engines achieved sufficient power‑to‑weight, and whether modern electric VTOL concepts like Lilium are physically and economically viable given current battery technology.
Wing Shapes, Flaps, Stall, and Load Factor
- Any surface at angle of attack can generate lift; cambered airfoils are used because they give more lift, delay stall, and reduce drag for a given lift.
- Flaps increase camber and lift coefficient, lowering stall speed but increasing drag. This is useful for landing and “low‑speed mode,” but can leave planes unable to climb if fully extended.
- Stall depends on both angle of attack and load factor: higher load (e.g., in turns or winch launches) raises stall speed; with very low load (e.g., parabolic flight) you can’t stall.
Stability, Tails, and Canards
- Conventional tails often produce downward force, requiring extra lift from the main wing and thus extra induced drag.
- This is a real inefficiency, but overall efficiency depends on the full configuration; high‑performance gliders still mostly use conventional tails, not canards or tailless designs.
- Some argue canards (and specific VTOL designs) can reduce tail inefficiency; others stress that canards also add drag and complexity, and that static stability rules are the same regardless of layout.
VTOL / eVTOL and the Lilium Debate
- Broad skepticism about electric VTOL economics: battery energy density seen as a hard limiter, small rotors as inefficient, many projects as likely “vaporware” or borderline scams.
- Lilium is singled out: critics cite unrealistic battery assumptions, complex design, demo vs. real product gap, and plunging stock as warning signs.
- Defenders note flying demonstrators, iterative design changes, partnerships, and argue that if any VTOL layout is workable, this might be it—while conceding timelines and ranges may shrink.
Why Airplanes Came Late
- Limiting factors discussed: poor power‑to‑weight of early engines (steam too heavy), weak materials, lack of control/stability understanding, and immature airfoil design.
- Once lightweight internal combustion engines and wind‑tunnel‑driven airfoil work appeared, practical airplanes followed quickly.
Explaining Lift: Bernoulli vs Newton
- Many recall being taught a simplistic Bernoulli story (“air on top goes farther, so must go faster”), which is widely criticized as misleading.
- Several prefer “wing pushes air down, air pushes wing up” (Newton) as more intuitive, but others note it doesn’t fully explain details like stalls or upper‑surface effects.
- Consensus: lift involves coupled pressure, velocity, and flow curvature; simple one‑line explanations invariably oversimplify, and educational materials often confuse rather than clarify.