Hi there. Rick and I designed and built the Blackbird (with propeller assistance from Steve Morris) and hold the world records together. Your analysis is pretty remarkable in that your simulations and observations are amazingly close to our reality. Your compliment that the Blackbird appears to be set up "near optimum" is appreciated. You should know that I build a massive, gasoline engine powered 'treadmill' upon which we optimized (and broke and broke and broke) the Blackbird before taking it to the desert. Though there were a few issues in our first dirt runs (we'll get to your wheel slip comments), the vehicle performed very close to our own simulations. We did run it in higher and higher winds and at higher and higher speeds until I was begging Rick to slow down in fear of it blowing up (which it eventually did). We approached 60mph several times and regularly took it over 50mph. Of course top speed was not our record goal, but rather a multiple of the windspeed. We were initially working with a $5,000 budget (which included our own custom built ~18ft propeller) so I could spend very little on the chassis. I designed a craft which I felt could be safe at our design point (~25mph ground speed). It was made of wood I beams (TJI) from Home Depot and built up from there with steel, carbon, kevlar and fiberglass reinforcements. Salvaged BMX wheels, etc and other bicycle components you already obviously know about. Some (me) would never call home made propeller tips whipping past the back of the drivers head at over 150mph "safe" in any way. You reference Rick's presentation at the St. Francis Yacht club, so I'm sure you noticed the story of why he was the pilot and not me (recounted at 1hr, 11m and 41s of that video). Our original goal was 2x windspeed. Our thought was that if we could double the speed of the wind it would make it more difficult for skeptics to claim measurement error compared to say 1.1x. Our design point for wind was (as I recall from more than a decade ago) ~9-10mph. Our highest ratio we recorded was around 3.4x, and we often got over 3x, but due to conditions and limited time (and NALSA measurement rules which we found appropriately conservative), we didn't reach that under the constrained record recording conditions. All these numbers fit your simulations very closely. Your observation that an appropriate RC works over a rather broad range of wind velocities was our direct experience. I installed the sprocket cluster just so I would have a range covered, but as I recall we optimized on the treadmill and never moved it after that. Yes, a greater tailwind does deliver more power to the cart, and with a far bigger budget and thus safer and stiffer chassis, we could have raised the bar significantly. As it was, at our top achieved speed, there was something like 600 ft lbs of torque on the prop shaft feeding the propeller blades through a variable pitch hub I designed and machined myself on a mini-mill. As I said, I was literally on the radio begging Rick to slow down at times. I still can't believe it took the abuse it took. As to wheel slip, the first videos you can find of the Blackbird (it was unfaired and called the BUFC ... "Big Ugly fn Cart" at that time) show it with symmetrical rear axle lengths. During our first dirt testing day (see "BUFC blow up run with full audio), Rick exceeded the design point by quite a margin and blew up the top sprocket. He reported an odd "surging" just before the failure (just as you suggested). We closely watched all our video and discovered that he was saved from tipping over (equal and opposite reaction to the aforementioned propeller shaft torque) by one wheel lifting slightly and losing enough traction to create the repeated surge in harvested power. This self limited top speed (and torque reaction) and also hammered the top sprocket (just as you suggested). Before we went out again, I revamped the chassis to extend one axle by several feet which gave the Blackbird it's odd asymmetrical look. BTW, the BUFC version of the craft had a fixed pitch propeller hub (relating to your fixed pitch analysis). It wasn't until Larry Page stepped up with sponsorship that we had the funds to dress up the renamed Blackbird and install the variable pitch propeller hub and controls. As you can conclude, the ability to adjust pitch on the fly doesn't really increase the top performance of the craft, but rather allows it to accelerate faster and 'self-start' in lower winds. If you review the other "BUFC" videos and compare them to the later Blackbird videos, you can see this impact. Overall you did a really great job here. Seeing someone applying good engineering analysis and comparing to actual results was refreshing given the literally tens of thousands of times Rick and I have been called frauds and idiots over the last 15 or so years. I have many, many build pictures if you're ever interested and you might also be intrigued by the upcoming possibilities to truly smash this record given the interest of a certain energy drink company to sponsor such. Composite F1 factory built with truly giant, counter-rotating propellers and generator/motor drive is being discussed. Fun if it were to happen. Thanks and very well done. John "JB" Borton Thin Air Designs team principal.
John, Thanks very much for your detailed and kind comments. You’ve answered some of my lingering questions. This is the first I’d heard of the giant treadmill. Wow! I’d enjoy seeing what it looked like. Also the variable pitch mechanism. I’m assuming that the potential generator/motor design you mention is similar to that in Rick’s yacht club talk (wheels drive generator that drives motor that drives prop). Since it appears that peak performance (cart/wind velocity) is very sensitive to overall efficiency, I wonder if you’ve gotten to overall efficiency predictions for the generator/motor/counter-rotating prop combination.
It is very well done indeed, and the first time (in many years of head-scratching over multiple videos and many well-intentioned but ill-devised 'explanations') that I finally grasped the actual engine that drives the seemingly paradoxical faster-than-downwind phenomenon. Derek Muller's ground-ratchet analog came close, but it didn't quite map into the actual wind-machine at hand. What clinches the deal in this video isn't the explanations, equations and tables, but the parachute -model, which visually and graphically combined well with the literal airscrew-style propeller to show what was my long running suspicion: _there is always *some* element of Blackbird that is moving slower than the wind. I don't know why dozens of youtubers and explainers couldn't have brought this out from the get-go: "You're wondering how it can outrun the wind and still draw power from the wind. We'll tell you right now that there is part of the tailwind that is still moving faster than a part of Blackbird, and hold that thought while we get into the weeds..."
Really wonderfully cogent and complete analysis. It's astounding that this is still even being debated. It points to a fundamental flaw in our thinking, in how easily we can let even well-developed intuitions lead to entrenchment. I took a brief look at this back when it was still a thought experiment, had my thermodynamics boots already pulled on and ready to do battle, right until saw where the untapped differential in the system is. Quite stellar stuff. Thank you.
The wind is always moving forward across the blade in the direction that the blade is turning and the contact point where are the wind meets the blade is always moving down wind slower than the wind
A couple of questions. In the videos showing this demonstrated, slow down the video. At the start, notice the propeller is turning one direction. But when you see the flag pointing towards the rear of the vehicle showing it is traveling forward faster than the wind speed, you’ll notice the propeller is turning the *reverse* direction. Because the *relative* wind is now toward the rear, this is indeed consistent that the forward propulsion is being provided by the wind. But the problem is with the propeller *directly* connected to the wheels shouldn’t that reverse the direction the wheels are turning? I’ve not seen anything of this nature discussed in the demonstrations. That the wheels turn in the same direction would seem to suggest some type of gear conversion that would allow the wheels to turn in the same forward direction regardless of the propeller spin direction. Also, in the small toy models demonstrating it on treadmills you can see there is no such gear conversion being used. This is puzzling. I like your analysis and demo using a parachute analogue. I’m surprised this worked, but I have to say there is a flaw. You see the “parachute” is not moving faster than the wind speed, even if the car is, as the wind is still pushing the parachute *forward*. If the parachute were moving faster than the wind speed the relative wind would push it *rearward*. This is a key distinction from the actual case. In the actual scenario the *relative* wind is pushing rearward against the propeller. In fact I draw the conclusion your parachute analogue would NOT work if the parachute were made to move faster than the wind speed, since then the *relative* wind would move the parachute *rearward*. Here’s a key deduction you can draw from that fact: it is something about the nature of the propeller itself that allows it to work in that case, but not in the parachute case. Here’s what I think that is. Much has been said about this scenario being different than a sailboat tacking into the wind because the vehicle is moving directly downwind, not an angle. But the thing is the propeller, i.e., the sail, is moving at an angle to the wind! This is because the sides of the propeller are *angled*. I haven’t looked at all the video discussions of this phenomenon but I think this fact is not focused on: it really is the same as tacking into the wind with a sail at an angle to the wind direction. If this fact were stated I think the phenomenon would be easier to understand. Robert Clark
I rewatched the videos showing the Blackbird and realize the appearance of the direction of the rotation of the propeller changing was an optical illusion, common with rotating objects. It stays rotating the same direction both before and after exceeding the wind speed.
You may be thinking of the parachute as the propeller analogue. For the parachute concept, the winch is the propeller analogue and the winch moves downwind faster than the wind while the parachute moves roughly at wind velocity. The parachute is simply a means of connecting the winch line to the air (the wind). The cart wheels drive the winch which hauls in the line against the air (the parachute), driving the winch and cart downwind faster than the wind. The parachute can never move faster than the wind since some tailwind is needed to generate parachute drag which reacts winch line tension. But, if the parachute is large enough, it moves very close to wind velocity. Beyond the need for a very large parachute, the parachute and winch concept is impractical since cart travel distance is limited by the length of the winch line as illustrated in the parachute model video clip where the cart quickly overruns the parachute. Still, the cart does move downwind faster than the wind until the cart reaches the parachute. You might think of the air moving through the propeller disk as the line of the parachute model. Now the “line” length isn’t limited so the cart moves DWFTTW without limit.
Thanks for responding. It is a key fact that for wind driven vehicles what’s relevant is the *relative* wind. When the Blackbird is moving faster than the downwind wind speed, the *relative* wind is directed rearward. From the relativity of the situation and the fact the only thing that matters is *relative* wind speed and direction, if from the start, i.e., vehicle starting at 0 speed, the wind is directed *rearward* the vehicle should still move forward. To me this is just as surprising if not more so than the faster than downwind scenario. But I haven’t seen this tested. Do you know if this has been tested? The reason why this should work is because the phenomenon really is the same as tacking into the wind with the *sail* at an angle to the wind. In this case the propeller blades are acting the like sail at an angle. In the sailboat case you move the sail at an angle to the wind and the sailboat goes upwind *at an angle to the wind*, then you reverse the angle of the sail and the sailboat also moves upwind at the reverse angle to the wind. Repeating this process you can move in a zigzag path that amounts to the same as moving *directly* upwind. The propeller on the Blackbird then is doing this action of moving the sail back and forth. BUT there is a key distinction in that the car does not move at an angle. I imagine the reason is the friction of the wheels prevents it from happening. So a key question I would like to also see answered is supposed you have a usual sail-powered car, i.e., no propeller. But we put the car on a track so it could not move sideways. We move the sail side to side as we would usually do for tacking into the wind for a direct head wind. But in this case the vehicle can not move at an angle. In a case like this will the vehicle still be able to move forward?
CORRECTION! I did a TH-cam search and the Blackbird team did demonstrate that it could travel *directly* upwind. Still I would like to see demonstrated a case with just using a sail, no propeller, being moved back and forth with the car restrained to run on a track, so it can only move straight ahead, will it move forward?
Addressing your thoughts about sail driven land vehicles, see the North American Land Sailing Association: www.nalsa.org/ At www.nalsa.org/faq.htm they say: “The slowest point of sail for a land yacht is dead downwind when it sails a little slower than the wind speed. By sailing at 45 degrees off directly downwind the yacht can sail much faster than the wind. The 'velocity made good' down wind is often over twice as fast as sailing directly down wind.” Note that the often contested Blackbird performance is Directly Downwind, i.e., course changes such as gybing (jibing) are not allowed. The downwind “Velocity Made Good (VMG)” is the performance of interest but downwind VMG is the same as cart speed when traveling directly downwind. But, I think that this is moot since, unlike a sail driven vehicle, Blackbird’s performance does not increase traveling off of the wind. If you haven’t seen it yet, Rick’s St. Francis Yacht Club presentation at th-cam.com/video/X6oJpnSJyV8/w-d-xo.html is an excellent summary of analogies offered to explain Blackbird. While Blackbird’s propeller blades are airfoils, I don’t find the sailboat analogy that helpful (except to suggest how propellers work). Blackbird exploits the velocity difference between the ground and wind with the overall mechanical advantage and efficiency of the drive transmission and propeller determining the system’s performance.
Great work. I've been following this topic for many years. I'd love to take Blackbird to the next level. A more automated vehicle that can shift between propeller and turbine mode. Telescopic and dynamic aerofoils to extract energy from crosswinds. Roadways and railways that canalize any available wind, passible and dynamically to optimize energy transfer to a (rail)vehicle. And the absolute pinnacle: extracting the speed differential between surface coundary layer air speed and higher altitude air speed to power a ground effect flying craft. With efficient floating wheels or tracks, a vehicle could "roll" on the water similarly to blackbird and sail DDWFTTW. It will ask a lot from the engineering, but Blackbird shows there is some rooms of inefficiencies to be introduced, as it went 3x wind speed with somewhat rudimentary drivetrain adjustments. What would the ideal scale for a Blackbird competitor be, to optimize power and drag? Mass grows with the cube, wind power with the square, sort of. Would a cart manned by a squirrel manage 4X wind DDWFTTW?
I would say it's that the propellers are always trying to grab More air than the wheels are turning and the air being the open system source. No wind required
You were right on the money - until the "no wind required" part. The cart will in fact come to a stop pretty quickly when the wind dies. This is proven by both theory and real-world results.
@@kdkinen I designed and built the thing and set 4 world records with it. I've given talks on it at NASA, Stanford, Google, and lots of others. That clear enough for ya?
@Rick_Cavallaro Alright well, I cannot argue with that. However, on the treadmill inside your room, there was no active wind only resting air. It was able to climb the treadmill with resting air. Does that not make logical sense why I am saying there is no wind required? Only resting air, And the props are always trying to capture more air than the wheels are turning. Leading to the runaway effect which we all love.
@@kdkinen wind is simply the movement of air relative to the ground. You can move the air over the ground or you can move the ground under the air. Not only can't you tell a difference - there literally is no difference.
It only conflicts in being an order of magnitude more perspicuous. This video outclasses every other video I've seen on the topic over the years, and lays out clearly in the first 10 minutes what people need to know to resolve the vexing paradox.
I only watched the introduction. The plane has his own energy source else it will not travel faster than air. Blackbird has no internal energy source so in order to exceed wind speed directly downwind it will need to have an energy storage device (and it has). Since a very limited amount of energy is stored by the blackbird it can only travel above wind speed for a limited amount of time proportional with the amount of stored energy . Energy in case of direct down wind blackbird version is stored in the pressure differential created by the propeller (air is a compressible fluid).
@@papalegba6796 It has no motor or battery but it can store energy in the pressure differential created by the super large propeller. The treadmill model works the same way. While it is hold with the hand energy is charged by allowing propeller to create a pressure differential using energy from the treadmill. Then when released that pressure differential drops as energy is used to accelerate the vehicle. The acceleration rate drops but not fast enough to see the vehicle slow down and then move backwards as the treadmill is very short to observe that.
@@Tbone139 The amount of time the vehicle drives above wind speed is irrelevant as it is dependent on amount of stored energy and invers proportional with the acceleration rate and frictional losses. So you can design for a low acceleration rate and accelerate slowly for a longer period or you can accelerate faster for a shorter period of time before starting to decelerate. You can check my last youtube video for the equations showing exactly that. A simple way to think at this may be to use a sail instead of a propeller. You will see that a sail vehicle can not exceed wind speed direct down wind because you have no energy storage as with the propeller connected to wheels but if you add an electrical generator at the wheels you will accelerate slower during below wind speed travel as you are storing energy in a battery and then use that energy stored in battery to exceed wind speed for a limited amount of time. Propeller acts both as a sail with variable surface area and an energy storage device due to the pressure differential it creates all explained in my video. The direct upwind version alo uses energy storage but very different as there energy is stored and released multiple times per second and you always have access to wind energy so you can drive indefinitely.
@@electrodacus You claim the time it runs above-wind for is irrelevant, do you further claim it it takes 90 seconds to reach a stable above-wind speed as shown, and doesnt measurably slow down for say, 24 hours before being manually stopped, that 24 hours above-wind was powered by the 90 seconds of acceleration?
@@Tbone139 In the record test Blackbird has done the energy was used up almost fully by the time teste ended as seen by the reduced rate of acceleration. It may have been maybe a few seconds of acceleration at most one minute depending on wind conditions as wind speed was super non constant during that test and propeller had pitch control and no information about that. The energy needed for Blackbird to do that record run was around 6Wh that is about half of what a phone battery contains so almost nothing. You can accelerate fast for 30 seconds and use all that 6Wh or you can accelerate at lower rate for 3 minutes in both cases end speed can be 2x or 3x wind speed and in both cases same 6Wh of energy could be used. Even after energy from pressure differential is used up the vehicle will stay above wind speed but since it decelerates it will start to slow down. It may take a minute or 10 minutes to get below wind speed depending on amount of frictional losses and amount of kinetic energy (based on vehicle speed and vehicle weight). It will be much easier to see all this on the treadmill model where things are way more controlled vs real world where wind speed variation is huge and on top of that driver has control to propeller pitch. The treadmill model has fixed propeller pitch so one variable is out and treadmill speed is constant and air speed is zero. On treadmill model you will see how acceleration rate drops as stored energy is used up.
I would enjoy a long discussion with you , John Borton, and Derek from veritasium
Hi there. Rick and I designed and built the Blackbird (with propeller assistance from Steve Morris) and hold the world records together. Your analysis is pretty remarkable in that your simulations and observations are amazingly close to our reality.
Your compliment that the Blackbird appears to be set up "near optimum" is appreciated. You should know that I build a massive, gasoline engine powered 'treadmill' upon which we optimized (and broke and broke and broke) the Blackbird before taking it to the desert. Though there were a few issues in our first dirt runs (we'll get to your wheel slip comments), the vehicle performed very close to our own simulations. We did run it in higher and higher winds and at higher and higher speeds until I was begging Rick to slow down in fear of it blowing up (which it eventually did). We approached 60mph several times and regularly took it over 50mph. Of course top speed was not our record goal, but rather a multiple of the windspeed.
We were initially working with a $5,000 budget (which included our own custom built ~18ft propeller) so I could spend very little on the chassis. I designed a craft which I felt could be safe at our design point (~25mph ground speed). It was made of wood I beams (TJI) from Home Depot and built up from there with steel, carbon, kevlar and fiberglass reinforcements. Salvaged BMX wheels, etc and other bicycle components you already obviously know about.
Some (me) would never call home made propeller tips whipping past the back of the drivers head at over 150mph "safe" in any way. You reference Rick's presentation at the St. Francis Yacht club, so I'm sure you noticed the story of why he was the pilot and not me (recounted at 1hr, 11m and 41s of that video).
Our original goal was 2x windspeed. Our thought was that if we could double the speed of the wind it would make it more difficult for skeptics to claim measurement error compared to say 1.1x. Our design point for wind was (as I recall from more than a decade ago) ~9-10mph. Our highest ratio we recorded was around 3.4x, and we often got over 3x, but due to conditions and limited time (and NALSA measurement rules which we found appropriately conservative), we didn't reach that under the constrained record recording conditions. All these numbers fit your simulations very closely.
Your observation that an appropriate RC works over a rather broad range of wind velocities was our direct experience. I installed the sprocket cluster just so I would have a range covered, but as I recall we optimized on the treadmill and never moved it after that.
Yes, a greater tailwind does deliver more power to the cart, and with a far bigger budget and thus safer and stiffer chassis, we could have raised the bar significantly. As it was, at our top achieved speed, there was something like 600 ft lbs of torque on the prop shaft feeding the propeller blades through a variable pitch hub I designed and machined myself on a mini-mill. As I said, I was literally on the radio begging Rick to slow down at times. I still can't believe it took the abuse it took.
As to wheel slip, the first videos you can find of the Blackbird (it was unfaired and called the BUFC ... "Big Ugly fn Cart" at that time) show it with symmetrical rear axle lengths. During our first dirt testing day (see "BUFC blow up run with full audio), Rick exceeded the design point by quite a margin and blew up the top sprocket. He reported an odd "surging" just before the failure (just as you suggested).
We closely watched all our video and discovered that he was saved from tipping over (equal and opposite reaction to the aforementioned propeller shaft torque) by one wheel lifting slightly and losing enough traction to create the repeated surge in harvested power. This self limited top speed (and torque reaction) and also hammered the top sprocket (just as you suggested). Before we went out again, I revamped the chassis to extend one axle by several feet which gave the Blackbird it's odd asymmetrical look.
BTW, the BUFC version of the craft had a fixed pitch propeller hub (relating to your fixed pitch analysis). It wasn't until Larry Page stepped up with sponsorship that we had the funds to dress up the renamed Blackbird and install the variable pitch propeller hub and controls. As you can conclude, the ability to adjust pitch on the fly doesn't really increase the top performance of the craft, but rather allows it to accelerate faster and 'self-start' in lower winds. If you review the other "BUFC" videos and compare them to the later Blackbird videos, you can see this impact.
Overall you did a really great job here. Seeing someone applying good engineering analysis and comparing to actual results was refreshing given the literally tens of thousands of times Rick and I have been called frauds and idiots over the last 15 or so years.
I have many, many build pictures if you're ever interested and you might also be intrigued by the upcoming possibilities to truly smash this record given the interest of a certain energy drink company to sponsor such. Composite F1 factory built with truly giant, counter-rotating propellers and generator/motor drive is being discussed. Fun if it were to happen.
Thanks and very well done.
John "JB" Borton
Thin Air Designs team principal.
John,
Thanks very much for your detailed and kind comments. You’ve answered some of my lingering questions.
This is the first I’d heard of the giant treadmill. Wow! I’d enjoy seeing what it looked like. Also the variable pitch mechanism.
I’m assuming that the potential generator/motor design you mention is similar to that in Rick’s yacht club talk (wheels drive generator that drives motor that drives prop). Since it appears that peak performance (cart/wind velocity) is very sensitive to overall efficiency, I wonder if you’ve gotten to overall efficiency predictions for the generator/motor/counter-rotating prop combination.
It is very well done indeed, and the first time (in many years of head-scratching over multiple videos and many well-intentioned but ill-devised 'explanations') that I finally grasped the actual engine that drives the seemingly paradoxical faster-than-downwind phenomenon. Derek Muller's ground-ratchet analog came close, but it didn't quite map into the actual wind-machine at hand.
What clinches the deal in this video isn't the explanations, equations and tables, but the parachute -model, which visually and graphically combined well with the literal airscrew-style propeller to show what was my long running suspicion: _there is always *some* element of Blackbird that is moving slower than the wind.
I don't know why dozens of youtubers and explainers couldn't have brought this out from the get-go: "You're wondering how it can outrun the wind and still draw power from the wind. We'll tell you right now that there is part of the tailwind that is still moving faster than a part of Blackbird, and hold that thought while we get into the weeds..."
th-cam.com/video/azNTzkPhx1s/w-d-xo.htmlsi=Ml27UxOdkDaDKmVF
docs.google.com/document/d/e/2PACX-1vTDmYH5j9dYEpGLH4KU6m6eK20qljAOIdovb82uacTIwMtaSKqJiUxyGGCdvkvibSf5l82EYkec6I5a/pub
Really wonderfully cogent and complete analysis.
It's astounding that this is still even being debated. It points to a fundamental flaw in our thinking, in how easily we can let even well-developed intuitions lead to entrenchment. I took a brief look at this back when it was still a thought experiment, had my thermodynamics boots already pulled on and ready to do battle, right until saw where the untapped differential in the system is.
Quite stellar stuff. Thank you.
Wow, fascinating work.
Can you share the document of the calculations and graphs please?
I am very impressed with your analysis. I guess Rick is the real deal.
The wind is always moving forward across the blade in the direction that the blade is turning and the contact point where are the wind meets the blade is always moving down wind slower than the wind
A couple of questions. In the videos showing this demonstrated, slow down the video. At the start, notice the propeller is turning one direction. But when you see the flag pointing towards the rear of the vehicle showing it is traveling forward faster than the wind speed, you’ll notice the propeller is turning the *reverse* direction. Because the *relative* wind is now toward the rear, this is indeed consistent that the forward propulsion is being provided by the wind.
But the problem is with the propeller *directly* connected to the wheels shouldn’t that reverse the direction the wheels are turning? I’ve not seen anything of this nature discussed in the demonstrations. That the wheels turn in the same direction would seem to suggest some type of gear conversion that would allow the wheels to turn in the same forward direction regardless of the propeller spin direction. Also, in the small toy models demonstrating it on treadmills you can see there is no such gear conversion being used. This is puzzling.
I like your analysis and demo using a parachute analogue. I’m surprised this worked, but I have to say there is a flaw. You see the “parachute” is not moving faster than the wind speed, even if the car is, as the wind is still pushing the parachute *forward*. If the parachute were moving faster than the wind speed the relative wind would push it *rearward*. This is a key distinction from the actual case. In the actual scenario the *relative* wind is pushing rearward against the propeller.
In fact I draw the conclusion your parachute analogue would NOT work if the parachute were made to move faster than the wind speed, since then the *relative* wind would move the parachute *rearward*.
Here’s a key deduction you can draw from that fact: it is something about the nature of the propeller itself that allows it to work in that case, but not in the parachute case. Here’s what I think that is. Much has been said about this scenario being different than a sailboat tacking into the wind because the vehicle is moving directly downwind, not an angle. But the thing is the propeller, i.e., the sail, is moving at an angle to the wind! This is because the sides of the propeller are *angled*.
I haven’t looked at all the video discussions of this phenomenon but I think this fact is not focused on: it really is the same as tacking into the wind with a sail at an angle to the wind direction. If this fact were stated I think the phenomenon would be easier to understand.
Robert Clark
I rewatched the videos showing the Blackbird and realize the appearance of the direction of the rotation of the propeller changing was an optical illusion, common with rotating objects. It stays rotating the same direction both before and after exceeding the wind speed.
You may be thinking of the parachute as the propeller analogue. For the parachute concept, the winch is the propeller analogue and the winch moves downwind faster than the wind while the parachute moves roughly at wind velocity. The parachute is simply a means of connecting the winch line to the air (the wind). The cart wheels drive the winch which hauls in the line against the air (the parachute), driving the winch and cart downwind faster than the wind.
The parachute can never move faster than the wind since some tailwind is needed to generate parachute drag which reacts winch line tension. But, if the parachute is large enough, it moves very close to wind velocity. Beyond the need for a very large parachute, the parachute and winch concept is impractical since cart travel distance is limited by the length of the winch line as illustrated in the parachute model video clip where the cart quickly overruns the parachute. Still, the cart does move downwind faster than the wind until the cart reaches the parachute.
You might think of the air moving through the propeller disk as the line of the parachute model. Now the “line” length isn’t limited so the cart moves DWFTTW without limit.
Thanks for responding. It is a key fact that for wind driven vehicles what’s relevant is the *relative* wind. When the Blackbird is moving faster than the downwind wind speed, the *relative* wind is directed rearward. From the relativity of the situation and the fact the only thing that matters is *relative* wind speed and direction, if from the start, i.e., vehicle starting at 0 speed, the wind is directed *rearward* the vehicle should still move forward. To me this is just as surprising if not more so than the faster than downwind scenario. But I haven’t seen this tested. Do you know if this has been tested? The reason why this should work is because the phenomenon really is the same as tacking into the wind with the *sail* at an angle to the wind. In this case the propeller blades are acting the like sail at an angle.
In the sailboat case you move the sail at an angle to the wind and the sailboat goes upwind *at an angle to the wind*, then you reverse the angle of the sail and the sailboat also moves upwind at the reverse angle to the wind. Repeating this process you can move in a zigzag path that amounts to the same as moving *directly* upwind. The propeller on the Blackbird then is doing this action of moving the sail back and forth. BUT there is a key distinction in that the car does not move at an angle. I imagine the reason is the friction of the wheels prevents it from happening.
So a key question I would like to also see answered is supposed you have a usual sail-powered car, i.e., no propeller. But we put the car on a track so it could not move sideways. We move the sail side to side as we would usually do for tacking into the wind for a direct head wind. But in this case the vehicle can not move at an angle. In a case like this will the vehicle still be able to move forward?
CORRECTION! I did a TH-cam search and the Blackbird team did demonstrate that it could travel *directly* upwind. Still I would like to see demonstrated a case with just using a sail, no propeller, being moved back and forth with the car restrained to run on a track, so it can only move straight ahead, will it move forward?
Addressing your thoughts about sail driven land vehicles, see the North American Land Sailing Association: www.nalsa.org/
At www.nalsa.org/faq.htm they say: “The slowest point of sail for a land yacht is dead downwind when it sails a little slower than the wind speed. By sailing at 45 degrees off directly downwind the yacht can sail much faster than the wind. The 'velocity made good' down wind is often over twice as fast as sailing directly down wind.”
Note that the often contested Blackbird performance is Directly Downwind, i.e., course changes such as gybing (jibing) are not allowed. The downwind “Velocity Made Good (VMG)” is the performance of interest but downwind VMG is the same as cart speed when traveling directly downwind. But, I think that this is moot since, unlike a sail driven vehicle, Blackbird’s performance does not increase traveling off of the wind.
If you haven’t seen it yet, Rick’s St. Francis Yacht Club presentation at
th-cam.com/video/X6oJpnSJyV8/w-d-xo.html is an excellent summary of analogies offered to explain Blackbird.
While Blackbird’s propeller blades are airfoils, I don’t find the sailboat analogy that helpful (except to suggest how propellers work). Blackbird exploits the velocity difference between the ground and wind with the overall mechanical advantage and efficiency of the drive transmission and propeller determining the system’s performance.
Great work. I've been following this topic for many years.
I'd love to take Blackbird to the next level. A more automated vehicle that can shift between propeller and turbine mode.
Telescopic and dynamic aerofoils to extract energy from crosswinds. Roadways and railways that canalize any available wind, passible and dynamically to optimize energy transfer to a (rail)vehicle.
And the absolute pinnacle: extracting the speed differential between surface coundary layer air speed and higher altitude air speed to power a ground effect flying craft. With efficient floating wheels or tracks, a vehicle could "roll" on the water similarly to blackbird and sail DDWFTTW. It will ask a lot from the engineering, but Blackbird shows there is some rooms of inefficiencies to be introduced, as it went 3x wind speed with somewhat rudimentary drivetrain adjustments.
What would the ideal scale for a Blackbird competitor be, to optimize power and drag? Mass grows with the cube, wind power with the square, sort of. Would a cart manned by a squirrel manage 4X wind DDWFTTW?
Fascinating.
Hey,
I am currently doing a research paper on this idea
Can you share the document of the calculations and graphs please?
This would really help me
I would say it's that the propellers are always trying to grab More air than the wheels are turning and the air being the open system source.
No wind required
You were right on the money - until the "no wind required" part. The cart will in fact come to a stop pretty quickly when the wind dies. This is proven by both theory and real-world results.
@Rick_Cavallaro only air is required.
No wind current is required.
Only resting air is required.
That clear enough for ya?
@@kdkinen I designed and built the thing and set 4 world records with it. I've given talks on it at NASA, Stanford, Google, and lots of others. That clear enough for ya?
@Rick_Cavallaro Alright well, I cannot argue with that. However, on the treadmill inside your room, there was no active wind only resting air. It was able to climb the treadmill with resting air.
Does that not make logical sense why I am saying there is no wind required? Only resting air, And the props are always trying to capture more air than the wheels are turning. Leading to the runaway effect which we all love.
@@kdkinen wind is simply the movement of air relative to the ground. You can move the air over the ground or you can move the ground under the air. Not only can't you tell a difference - there literally is no difference.
I feel like this conflicts with Derek’s explanation over on Veritasium
I just re-watched Derek’s explanation in: th-cam.com/video/yCsgoLc_fzI/w-d-xo.html . Could you be more specific about the conflict you see?
It only conflicts in being an order of magnitude more perspicuous. This video outclasses every other video I've seen on the topic over the years, and lays out clearly in the first 10 minutes what people need to know to resolve the vexing paradox.
This was solved recently, a proper mathematical theory was provided
docs.google.com/document/d/e/2PACX-1vTDmYH5j9dYEpGLH4KU6m6eK20qljAOIdovb82uacTIwMtaSKqJiUxyGGCdvkvibSf5l82EYkec6I5a/pubdocs.google.com/document/d/e/2PACX-1vTDmYH5j9dYEpGLH4KU6m6eK20qljAOIdovb82uacTIwMtaSKqJiUxyGGCdvkvibSf5l82EYkec6I5a/pu
I only watched the introduction. The plane has his own energy source else it will not travel faster than air. Blackbird has no internal energy source so in order to exceed wind speed directly downwind it will need to have an energy storage device (and it has).
Since a very limited amount of energy is stored by the blackbird it can only travel above wind speed for a limited amount of time proportional with the amount of stored energy .
Energy in case of direct down wind blackbird version is stored in the pressure differential created by the propeller (air is a compressible fluid).
@@papalegba6796 It has no motor or battery but it can store energy in the pressure differential created by the super large propeller.
The treadmill model works the same way. While it is hold with the hand energy is charged by allowing propeller to create a pressure differential using energy from the treadmill.
Then when released that pressure differential drops as energy is used to accelerate the vehicle. The acceleration rate drops but not fast enough to see the vehicle slow down and then move backwards as the treadmill is very short to observe that.
@@electrodacus This model starts from a standstill and travels DWFTTW for over 3 minutes: th-cam.com/video/LABSbSTIEvM/w-d-xo.html
@@Tbone139 The amount of time the vehicle drives above wind speed is irrelevant as it is dependent on amount of stored energy and invers proportional with the acceleration rate and frictional losses.
So you can design for a low acceleration rate and accelerate slowly for a longer period or you can accelerate faster for a shorter period of time before starting to decelerate.
You can check my last youtube video for the equations showing exactly that.
A simple way to think at this may be to use a sail instead of a propeller. You will see that a sail vehicle can not exceed wind speed direct down wind because you have no energy storage as with the propeller connected to wheels but if you add an electrical generator at the wheels you will accelerate slower during below wind speed travel as you are storing energy in a battery and then use that energy stored in battery to exceed wind speed for a limited amount of time.
Propeller acts both as a sail with variable surface area and an energy storage device due to the pressure differential it creates all explained in my video.
The direct upwind version alo uses energy storage but very different as there energy is stored and released multiple times per second and you always have access to wind energy so you can drive indefinitely.
@@electrodacus You claim the time it runs above-wind for is irrelevant, do you further claim it it takes 90 seconds to reach a stable above-wind speed as shown, and doesnt measurably slow down for say, 24 hours before being manually stopped, that 24 hours above-wind was powered by the 90 seconds of acceleration?
@@Tbone139 In the record test Blackbird has done the energy was used up almost fully by the time teste ended as seen by the reduced rate of acceleration. It may have been maybe a few seconds of acceleration at most one minute depending on wind conditions as wind speed was super non constant during that test and propeller had pitch control and no information about that.
The energy needed for Blackbird to do that record run was around 6Wh that is about half of what a phone battery contains so almost nothing.
You can accelerate fast for 30 seconds and use all that 6Wh or you can accelerate at lower rate for 3 minutes in both cases end speed can be 2x or 3x wind speed and in both cases same 6Wh of energy could be used.
Even after energy from pressure differential is used up the vehicle will stay above wind speed but since it decelerates it will start to slow down. It may take a minute or 10 minutes to get below wind speed depending on amount of frictional losses and amount of kinetic energy (based on vehicle speed and vehicle weight).
It will be much easier to see all this on the treadmill model where things are way more controlled vs real world where wind speed variation is huge and on top of that driver has control to propeller pitch. The treadmill model has fixed propeller pitch so one variable is out and treadmill speed is constant and air speed is zero.
On treadmill model you will see how acceleration rate drops as stored energy is used up.