This was a really good explanation. Ty
Comment on Can someone explain to me how a ventilation fan can suck LESS when on high compared to low?
FuglyDuck@lemmy.world 1 day ago
Fan blades are basically spinning wings or airfoils.
Depending on their design or how expensive they are, they may rely on pushing air rather than aerodynamic effects at low speeds, and they’re always optimized for a specific rpm.
As it speeds up, the aerodynamic flow takes over, with the rotors creating a pressure differential that pulls air through.
As it gets faster and faster, eventually, that pressure differential reaches the next rotor and the entire thing stops being as effective because now the the second rotor is stalled out. (Only they’re all stalled out because any given rotor is both leading and trailing.)
afk_strats@lemmy.world 1 day ago
litchralee@sh.itjust.works 1 day ago
I like this answer. The only thing I would add is that when the fan blades are all stalled, it might seem then that drag and energy consumption should reduce, since there’s not much air moving. But in a cruel twist (fan pun intended) of aerodynamics, the useless spinning of stalled fan blades still causes parasitic drag. So not only does the fan not move air, it’s also consuming more energy than spinning a solid disk of the same moment-of-inertia.
When the engine fails for certain single-propeller aircraft, there’s sometimes a mechanism to lock the propeller to make it stop rotating, since it would otherwise “windmill” in the air and waste the previous kinetic energy that’s keeping the plane aloft. Or so I’m told.
FuglyDuck@lemmy.world 1 day ago
that’s a great addon, thank you.
Side note; that’s why they get noisier, too.
AnAustralianPhotographer@lemmy.world 1 day ago
That could happen with propellors that have Constant Speet Units. (Propellor pitch is able to be changed) The act is called ‘feathering’ . This can happen on multi engine aircraft and reduce the drag of the side with the failed engine. Cheaper propellors are fixed pitch and no means exist to change them.
litchralee@sh.itjust.works 1 day ago
Constant Speed is probably what I was thinking of. And speaking of multi engine failure, you’ve just reminded me of the demise of TransAsia Airways Flight 235 where the right engine feathered itself erroneous, but then the crew misdiagnosed the situation and shut down the left engine. Mentour Pilot made a video on that particular accident.