spork wrote: ↑
Sat Oct 23, 2021 8:25 am
Regis-de-giens wrote: ↑
Thu Oct 21, 2021 3:23 pm
This mechanical link with soil, combined with the calculated blade angle, offers a surface that goes a bit rearward vs the soil...
It's true that the propeller produces a sort of virtual surface that can be thought of as moving backward relative to the vehicle, but not relative to the ground. It's still advancing relative to the ground, but slower than the true wind.
I agree with you, backward relative to wind is true and sufficient to understand and make it work. Relative to soil is just "easier" to catch the principle IMO because it "works" in whatever cart speed; i will try to detail my reasoning.
I also agree on your presentation of distance vs force that is different on wheel and on the propeller ; this provides the "energy conservation" explanation and it works as well ....
... but ...
I still believe (95% sured, personal statement, open to your arguments ), that the propeller surface HAS TO be driven backward faster that the wind. Actually the parameter of the ratio between the cart speed and the propeller "surface" (*) sets the ratio speed vs wind that the cart will ever reach (ideal , in case of no drags nor mechanical friction of the cart. Do you agree ?
for the next part of my message: lets call above case " ratio 1:1": meaning the propeller surface goes backward exactly as fast as the speed and remain at zero speed vs the ground. this is a rigid mechanical link with soil, so it remains true whatever the cart speed.
... so, ratio 1:1 (so backward speed is equal to the ground) is a good particular case where the speed could be infinite if no friction , hence there is no theoretical speed threshold for the cart ... the propeller will offer a surface facing the wind whatever the cart speed and whatever the wind positive value
now imagine that this ratio is lower, say 1:2 , meaning the propeller goes only half way back vs the soil. when cart will reach twice the wind, then the propeller will reach wind speed exactly, and turn "with no thrust in the air", like a Cesna propeller that would have no angle of attack ... cart cannot run faster, ideal threshold is reached, end of the story ...
so to me, having "just" the explanation that the propeller turns in a "lower" wind vs the cart is not complete... it only works until this ratio (speed vs wind) is not reached; and this ratio (*) is set by design but can be changed .
(*) parameter fixed by design, combination of :
- propeller blade angle
- propeller diameter
- ratio of transmission wheel // propeller
- wheel diameter
interesting to know your thoughts about it