Propeller thrust with backwards air flow

[Frodo]

TL;DR

Variation of propeller thrust as air flow varies from backwards to forwards

The Blackbird wind powered car travels directly downwind faster than the wind speed powering it. Typically a 10 mph wind from directly behind it will cause it to travel at about 30mph.

I am trying to analyse how the car accelerates from rest. I know how it travels faster than the wind speed so I do not need that explained.

When the car is stationary the wind coming from behind will act on the "frontal" area and apply a force so the car will begin moving if this force exceeds the resistance to motion.

Once the car moves the propeller rotates because it is driven from the wheels. The propeller is now rotating but the incident wind is from behind. I am interested in what thrust the propeller will be generating in this region.

I am therefore looking for a study showing the thrust produced by a propeller when the incident wind is from behind, and comparing it with the thrust produced when the incident wind is from the front.

I have searched the web and read a number of NACA reports but none tackles this question.

Thank you.

 

[cjl]

When the relative wind is from behind, the prop blades will be seeing a pretty large angle of attack still, so I'd guess they're still basically stalled and it's mostly just being accelerated by bluff body drag. I'd guess the blades don't unstall until it's right around the wind speed. If you actually had a graph of its speed vs time, you could probably figure out when this happens, since I'd guess the acceleration increases substantially when the blades unstall.

 

[rcgldr]

You can see the slow start in this video, and that the propeller starts to make significant thrust a bit before reaching wind speed.

 

[DaveE]

I'm not really the best person to answer this. Except to say that I think you'll find better (at least more) information if you recast the fundamental problem as lift from a wing. Think of a single blade as a wing and, ignore rotation and model it as a small piece of the arc, i.e. linear motion of a wing with some air flow field.

There is also some good stuff out there about how sailboats work. It's all just different versions of the same problem as I see it.