Women Educators on YouTube – Engineer – Xyla Foxlin

I first came across her channel last year when she made a canoe. It was interesting to watch but her channel was still fairly new so I decided to not feature her here yet and wait how it turns out. But she has made some more crafting and sciencey videos since then, and those that I saw were fairly good so here is her latest, in collaboration with Derek Muller from the Veritasium channel.


  1. rblackadar says

    This is an awesome video, thanks! I’m going to recommend it (and Xyla’s channel in general) to my daughter, an aspiring engineer, who really likes such things.

    In the video, Xyla is careful to say that *some* physicists said her machine is impossible. The title of her video leaves out the qualifier, and that’s my only quibble (a small one). Apparently there were physicists on both sides of this “controversy”. But frankly, I don’t think the correct physics here is at all difficult, and I find it rather shocking that a physics prof went so far as to bet $10,000 that this was all a hoax. Sure, the problem is counterintuitive (even astonishingly so) but it’s not difficult if you actually make the effort to work it out. I wonder, is he going to pay up? Reminds me a bit of the Monty Hall Problem, where a few math professors, who should have known better, got egg on their faces because they talked too much and calculated too little.

    I watched the related Veritasium video some weeks ago and was curious about the engineering that went into the model that it featured — I had no idea that the engineering would turn out to be more difficult than the physics. Kudos to Xyla.

  2. says

    @rblackadar, look at Veritasium’s last video. The professor did admit he was mistaken in the end and paid the $10.000. The concept seems to be so counterintuitive and difficult to understand that even physics professors can struggle with it. Which just proves that even professors are human.

  3. Tethys says

    I enjoyed the video very much, especially the trial and error process that eventually led to success. Bigger wheels are a key design element of my ergonomic garden cart and lawn mower. The difference in the amount of force necessary to move the cart, vs a traditional wheelbarrow is tremendous. None of the force is wasted in lifting or bearing the load, it is only needed to build up some momentum, and steer.

  4. Tethys says

    If you think of wind as a wave, then this vehicle is surfing. I can’t do the math involved in fluid dynamics, but I think the physics analogy is accurate?

  5. rblackadar says

    @Charly — Yes, I was just about to post about the more recent Veritasium, but you beat me to it. Anyone interested in the physics of this should watch that video, for sure. The best sequence for understanding the principle involved comes at the end, where Derek uses a wooden plank to supply the power instead of the wind. Also the explanation Derek gives is much much better than the one in his original video, good enough to convince the professor in fact. Who seems like very much an ok guy, after all.

    I think where the prof went wrong was to overthink it; it’s all too easy to come up with some complex math that is almost right but misses something important. (Like the old canard that bumblebees can’t fly.) When I wrote that this was an easy problem in physics, I was thinking mostly about why it does not violate conservation of energy, nor the 2nd Law either: it seems obvious to me that the wind is doing work on the machine at all times (even faster than wind speed) because the wind is moving at a slower true velocity in the wake of the machine, than it had been moving before. That loss of wind kinetic energy has to be going somewhere — it’s pushing the machine. It’s a bit odd that the slower wind is *upwind* of the machine (in the earth frame) rather than in its lee, as it would be if the rotor were acting as a windmill (which it’s not) but exactly where the wind gets slower is not an issue for the energy balance.

    But the aerodynamic details are not easy at all, and I defer on that to Veritasium and his team of experts. I noted in Derek’s original video that Blackbird has a variable-pitch rotor, and I think that that probably plays some role in its success at least under many conditions. As Xyla discovered, the engineering details are crucial! With fixed pitch, her machine could work well when started at speed on the treadmill (thus demolishing the energy objection) but might very well struggle if started on flat ground from a dead stop. Maybe I’ll try building one, and see!

  6. rblackadar says

    @Tethys — I think it’s a bit different from surfing at least in one respect: a surfer can move faster than the wave while descending, but can’t sustain that speed after reaching the bottom; the wave has to catch up for there to be any increase. Xyla’s machine, in contrast, can go faster than the wind for as long as the wind continues to blow.

  7. Tethys says


    The physics is much easier for me to visualize with a wave of water, since we can’t see the wind.

    In surfing the wave must be traveling at 8 to 12 mph, and the surfer needs to paddle to match its speed in order to catch the wave and ride the cresting edge into shore.

    I note that the treadmill speed necessary for this vehicle to work was 12 mph, and then it catches the windwave and self propels. I suspect you need different numbers to calculate buoyancy on water vs in air, but it seems like both surfer and wind vehicles ride the crest once they match velocities.

  8. Rob Grigjanis says

    Tethys @7:

    it seems like both surfer and wind vehicles ride the crest once they match velocities.

    This is not in any way a wave phenomenon. There is no ‘crest’. It’s a gearing system between two media (air and ground) which have nonzero relative horizontal motion.

  9. Tethys says

    @ rob

    Wouldn’t the relevant physics for both water and wind require fluid dynamics? Wind is a moving energy front without a shore, so I’m wondering if there is a point of equilibrium where the wind vehicle will ride within the wind. If it enters slower wind it would lose velocity.

    I never had the privilege of education in physics or higher math. It does not seem counter intuitive to me that the vehicle can go faster than the wind speed after it reaches a certain velocity. It has to ride the wind, so clearly it can’t go beyond the leading edge of the wind. Waves might not be entirely accurate, but it helps me visualize how the vehicle can exceed the speed of the wind. Surfers use gravity to stay ahead of the wave crest, but they need to match velocity first in order to stand up.

  10. Rob Grigjanis says

    Tethys @9: Yes, fluid dynamics is certainly involved, but forget about waves for a minute. You have a cart on flat ground. On the rear axle is a cog, which engages another cog attached to a rod with a fan on the end. The cogs are engaged in such a way that, if the cart moves forward, the turn of the wheels makes the fan blow backward. If you don’t want to give the cart a push, the wind only has to be strong enough to get the cart moving.

    It’s what happens as the cart’s speed gets close to the wind speed that gave rise to the controversy. It’s worth noting that the arguments by Alex Kusenko, the physicist who made the bet, convinced Neil deGrasse Tyson and Bill Nye that the vehicle couldn’t work. But I consider that a very low bar ;-). Apparently, Sean Carroll was also a witness to the bet, but he was strangely silent on the subject.

    I’ve seen the equations involved (one analysis here), and I’m convinced that the cart can keep accelerating after it exceeds wind speed. But the stuff about what happens just before and after it reaches wind speed lost me in aerodynamics jargon (“air prop swirl efficiency”, “actuator-disk theory”, etc).

    But waves simply don’t come into it, and I think it’s misleading to try and picture it as analogous to surfing.

  11. Tethys says


    Thank you! The paper lays out the equations and relevant coefficients very nicely. The omega section that involves aerodynamics appears to show the pressure differential that is created by the rotor after it matches velocity. They call it hover, I prefer float since it is not flying.

    I compare it to surfing because it is self propelled, and the need to match velocity before the effect can occur.
    Sailing is better as an analogy, and there are instances where you can sail upwind, faster than the wind. You cannot sail downwind faster than the wind because as you increase velocity, the relative wind speed decreases.
    With a specially sized and shaped boat and sail, you can go upwind faster than the wind by in effect, making your own wind.

    sailing close to the wind
    I have mentally leaped straight to a working self powered land vehicle. Dual rotors to provide steering and stability, and a rudder, could make a vehicle that can travel any direction as long as there is enough wind.

  12. Rob Grigjanis says

    Tethys @11: If you actually understand how the relevant coefficients are derived, and the role they play in the problem, congratulations. Not bad for someone with no physics. As someone with a physics background, I just see specialist terms that I suspect would take hours or days to grasp in a useful way.

    Sailing is better as an analogy, and there are instances where you can sail upwind, faster than the wind.

    That’s still a bad analogy. The physics of tacking is the same as the physics of squeezing a wet bar of soap. Most of the force applied is perpendicular to the sides of the bar, but due to the very low friction parallel to the sides, even a small component of the force parallel to the sides can result in significant acceleration in that direction. So for tacking, we’re talking about forces (wind, and keel resistance to the torque) which are at drastically different directions to the direction of motion. “faster than the wind” is as irrelevant in this case as how “fast” your hands are coming together on the bar of soap. What’s relevant is the force applied by the wind on the sails, and the orientation and design of the boat.

    For the ground vehicle with propellers, all the relevant forces are parallel. The vehicle’s design makes clever use of momentum and energy transfer between ground and vehicle, and air and vehicle. There’s nothing clever about a wet bar of soap.

  13. Tethys says

    Sailing is useful because it’s already had all the physics worked out to optimize the dimensions of the boat and the shape and angle of the sails.

    In order for this wheeled rotor to accelerate it needs to be precisely engineered to have all the forces involved in equilibrium so that you can generate enough speed. ( and not fly into pieces because the parts are undersized or cause friction)
    If you meet the requirements of physics, it will generate a zone of low pressure in front of the rotor, hit the hover velocity threshold, and then you can accelerate faster than the wind.

    Water has much more drag than air. This lacks a keel to provide stability and buoyancy but this is merely a fun engineering issue.
    If the rotors were mounted within a suspension gimbal, they could automatically stay aligned with the wind but allow the wheeled base to travel in any direction while still powering the rotor.

  14. Tethys says

    I still can’t trust my nonexistent calculus and trigonometry education, but I think making this pedal powered and adding a flywheel so you could shift the prop to maintain float velocity would then allow the drive wheel/vehicle to travel any direction with very little effort.

    In order to calculate that I’m thinking you need to know how much DDacceleration is possible once you achieve float.

    It’s not that difficult to do differential equations, the difficult part is quantifying the variables correctly, so you can calculate the vectors in the first place. This is why we have engineers, IME, they love running the equations. I’m amazed they can often do the complex math in their heads.

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