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'The Blackbird problem' has been doing the rounds on the internet ever since Derek Muller uploaded a video of the Blackbird (a propeller-attached car that can travel faster than wind downwind) in the Veritasium channel. I have some very specific doubts regarding steady-state velocity while sailing on land, which I was not able to get clarity from any discussion in the PSE on the Blackbird.

  1. Is the steady-state velocity of a sail car (not Blackbird, no propellers, just a sail and 4 wheels) the same as the wind velocity while traveling downwind?
  2. Does the Blackbird experience aerodynamic drag while traveling slower than the wind downwind? Or is it like 'the drag comes into play only after the Blackbird's velocity becomes higher than the wind speed'?
  3. The Blackbird owners claim that their vehicle traveled 2.8 times faster than the wind downwind. Is this 2.8v the steady-state velocity of the Blackbird downwind in that particular instance? (v is the velocity of wind)
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    $\begingroup$ I cannot answer all questions, but at least for nr 3) the answer is no, it is not a steady-state velocity. It is the maximum recorder over a run about 30 sec long if i remember correctly. See the updated Veritasium video for a mention where the velocity graph is shown from the record run. $\endgroup$
    – Marius Ladegård Meyer
    Jul 2, 2021 at 8:40
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    $\begingroup$ For (1), if you include various friction effects, the car will be slightly slower than windspeed. $\endgroup$
    – Carl Witthoft
    Jul 2, 2021 at 12:04
  • $\begingroup$ @MariusLadegårdMeyer Any idea what exactly is meant by the steady-state velocity of the Blackbird? Because I don't see this machine settling at any speed if we ignore aerodynamic drag. Else, the wind will have to stop blowing, so that the forward thrust by on the propeller will be balanced by the frictional force on the ground. $\endgroup$
    – Curiouser and curiouser
    Jul 2, 2021 at 12:15
  • $\begingroup$ I think you are correct that friction between the air and the body of the Blackbird will become relevant when the velocity exceeds the wind speed. At that point it is not going downwind relative to the air anymore, although it can still accelerate by pushing air backwards. $\endgroup$
    – Marius Ladegård Meyer
    Jul 2, 2021 at 21:59

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The answer to #1 depends greatly on the direction the craft is traveling.

If it is traveling directly downwind, the steady state speed will never reach that of the wind because of real world frictional losses in bearings, tires, etc. If however it is being sailed on a broad reach at the proper angle to the wind, both its speed across the ground and its downwind VMG can be greater than the wind, steady state. (VMG being Velocity Made Good, or the downwind component of your ground track).

http://static1.1.sqspcdn.com/static/f/572109/24503675/1394724903587/Sailing+Yacht+Design+for+Maximum+Speed.pdf?token=5pzNaaWIFu%2F7e3pN5RiQ2e6MmmM%3D

https://www.nalsa.org/Articles/Cetus/Iceboat%20Sailing%20Performance-Cetus.pdf

#2 is pretty straight forward:

Anytime there is relative wind over the chassis of a vehicle there is associated drag. When traveling slower than the wind, this drag on the chassis works to move the Blackbird downwind. Once traveling faster than the wind, the drag on the chassis from what is now a relative headwind is now working to slow the craft down.

#3: In a 10-15mph tailwind, the Blackbird can accelerate to ~3x the speed of the wind and maintain that velocity steady state until you run out of wind, room or interest.

https://youtu.be/5CcgmpBGSCI

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