nechaus
100 kW
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The official NALSA ratified dead downwind record is 2.8 x wind-speed.doctorGONZO said:If you have done 2.5,
Yep, directly upwind carts are pretty common. The directly downwind faster than the wind one seems more counter intuitive, despite being the same thing, just with air & ground swapped.nechaus said:
john says:
July 18th, 2013 at 5:46 pm
The cart is designed to use the energy in the shear plane that exists between the air and ground when there is wind. By leveraging one against the other, speed can be gained at the expense of force. To achieve 3 times wind speed, they had to sacrifice at least 2/3 (I suspect closer to 3/4) of the propeller’s thrust to the ground to provide the torque to turn the propeller.
However, if you add that force into your analysis, you would still probably conclude that the Blackbird can self propel (at appropriate speeds) with no wind. That is because your analysis seems to be ignoring the effect of the rotor’s rotational velocity relative to its linear velocity through the air. This is actually quite important as it switches from Propeller to turbine when the cart’s linear velocity relative to the air becomes greater than the rotational velocity times the pitch. When this happens, the directions of the forces get reversed. Reversing the forces means they are putting the breaks on the cart which gets added to the breaking of the resistive forces.
The rotational velocity of the rotor times its pitch divided by the ground velocity of the cart is a constant determined by the fixed gear transmission. For simplicity, I’ll call this constant R.
For the DDWFTTW cart, the wheel turns the propeller, so through the transmission, the smaller force of the ground on the wheel must overcome the larger force of the air on the propeller. Since power is at best constant through the transmission, this increase in force must be accompanied by a decrease in velocity. So the DDWFTTW cart must have 0<R<1.
With no wind, the ground velocity is equal to the air velocity. So if the cart is pulled (or pushed) with no wind present, R<1 means the air velocity is greater than the rotational velocity times the pitch and the rotor will act as a turbine, and the force will be a breaking force. When resistive forces (air drag, rolling resistance, etc.) are added, they will only add to that breaking.
A similar analysis with any value of R will show that with no wind the cart will always be in a breaking mode (except for R=1 when there are only resistive forces).
A few added notes:
The Blackbird on the salt flats and the cart on the treadmill are essentially the same system. The only difference is that in one case the Earth (the reference frame from which we tend to view things) is the Blackbirds ground frame, and in the other the Earth is the same as the air frame (different from the cart's ground which is the treadmill). An analysis of the cart in the treadmill reference frame is identical to analysing the Blackbird in the Earth frame, and an analysis of the Blackbird in the air reference frame is identical to analysing the cart on a treadmill in the Earth frame.
A push (or pull) start in no wind cannot replace the wind or the treadmill. The carts are continuously taking energy from the system (the shear interface between ground and air). They are not simply getting a push start. They depend on a continuous supply of energy, and without wind they don't have it.
An over unity device (or greater than unity device) refers to its energy usage. If it ends up with more energy than it started with (if its net power balance is positive) then the final energy divided by the initial energy is "over unity" (greater than 1), and it is an over unity device. If these carts could self accelerate in no wind, they would in fact be over unity devices. At the start neither the air nor the ground has energy, and at the end the air would have some energy, and the cart would have more than it started with.
With wind, the air has energy and the propeller is slowing the air down, thus removing energy from the air and transferring it to the cart. It continues to do so past wind speed and only stops when the cart reaches wind speed divided by (1-R). Since 0<R<1, that is greater than wind speed. For the Blackbird, 2/3<R<1, and I would guess R is close to 3/4. Using that guess, the net thrust would be downwind until the cart reached 4 times wind speed, but would not be able to overcome resistive forces past 3 times wind speed for the conditions they had.
http://makezine.com/2010/11/04/downwind-faster-than-the-wind-black/Rick Cavallaro says:
July 18th, 2013 at 7:46 pm
Correct on all counts. The parameter you call R we call VSR (for Vehicle Speed Ratio). And it is in fact around 0.75 for the Blackbird, but can be varied significantly via the variable pitch propeller. That being said, we used a VSR right around 0.75 when our prop pitch was fixed, and used that same value for the design pitch of the prop. Increasing or decreasing the pitch leads to off-design performance which results in a less efficient propeller (at least at the design point).
liveforphysics said:So, if I'm following that assortment of words, it seems you're down to just the point of not understanding how it can harvest energy?
Are you aware that any point you have different speeds, different temperatures etc that this energy difference can be harvested for energy? If so, I'm unclear what argument you're left making.
endlessplane said:Yep, directly upwind carts are pretty common. The directly downwind faster than the wind one seems more counter intuitive, despite being the same thing, just with air & ground swapped.nechaus said:
TylerDurden said:Feedback loop does not imply OU, simply energy is directed back to the source. See overheating motors or runaway serial-wound motors.
re 2.5X
john says:
July 18th, 2013 at 5:46 pm
The cart is designed to use the energy in the shear plane that exists between the air and ground when there is wind. By leveraging one against the other, speed can be gained at the expense of force. To achieve 3 times wind speed, they had to sacrifice at least 2/3 (I suspect closer to 3/4) of the propeller’s thrust to the ground to provide the torque to turn the propeller.
However, if you add that force into your analysis, you would still probably conclude that the Blackbird can self propel (at appropriate speeds) with no wind. That is because your analysis seems to be ignoring the effect of the rotor’s rotational velocity relative to its linear velocity through the air. This is actually quite important as it switches from Propeller to turbine when the cart’s linear velocity relative to the air becomes greater than the rotational velocity times the pitch. When this happens, the directions of the forces get reversed. Reversing the forces means they are putting the breaks on the cart which gets added to the breaking of the resistive forces.
The rotational velocity of the rotor times its pitch divided by the ground velocity of the cart is a constant determined by the fixed gear transmission. For simplicity, I’ll call this constant R.
For the DDWFTTW cart, the wheel turns the propeller, so through the transmission, the smaller force of the ground on the wheel must overcome the larger force of the air on the propeller. Since power is at best constant through the transmission, this increase in force must be accompanied by a decrease in velocity. So the DDWFTTW cart must have 0<R<1.
With no wind, the ground velocity is equal to the air velocity. So if the cart is pulled (or pushed) with no wind present, R<1 means the air velocity is greater than the rotational velocity times the pitch and the rotor will act as a turbine, and the force will be a breaking force. When resistive forces (air drag, rolling resistance, etc.) are added, they will only add to that breaking.
A similar analysis with any value of R will show that with no wind the cart will always be in a breaking mode (except for R=1 when there are only resistive forces).
A few added notes:
The Blackbird on the salt flats and the cart on the treadmill are essentially the same system. The only difference is that in one case the Earth (the reference frame from which we tend to view things) is the Blackbirds ground frame, and in the other the Earth is the same as the air frame (different from the cart's ground which is the treadmill). An analysis of the cart in the treadmill reference frame is identical to analysing the Blackbird in the Earth frame, and an analysis of the Blackbird in the air reference frame is identical to analysing the cart on a treadmill in the Earth frame.
A push (or pull) start in no wind cannot replace the wind or the treadmill. The carts are continuously taking energy from the system (the shear interface between ground and air). They are not simply getting a push start. They depend on a continuous supply of energy, and without wind they don't have it.
An over unity device (or greater than unity device) refers to its energy usage. If it ends up with more energy than it started with (if its net power balance is positive) then the final energy divided by the initial energy is "over unity" (greater than 1), and it is an over unity device. If these carts could self accelerate in no wind, they would in fact be over unity devices. At the start neither the air nor the ground has energy, and at the end the air would have some energy, and the cart would have more than it started with.
With wind, the air has energy and the propeller is slowing the air down, thus removing energy from the air and transferring it to the cart. It continues to do so past wind speed and only stops when the cart reaches wind speed divided by (1-R). Since 0<R<1, that is greater than wind speed. For the Blackbird, 2/3<R<1, and I would guess R is close to 3/4. Using that guess, the net thrust would be downwind until the cart reached 4 times wind speed, but would not be able to overcome resistive forces past 3 times wind speed for the conditions they had.http://makezine.com/2010/11/04/downwind-faster-than-the-wind-black/Rick Cavallaro says:
July 18th, 2013 at 7:46 pm
Correct on all counts. The parameter you call R we call VSR (for Vehicle Speed Ratio). And it is in fact around 0.75 for the Blackbird, but can be varied significantly via the variable pitch propeller. That being said, we used a VSR right around 0.75 when our prop pitch was fixed, and used that same value for the design pitch of the prop. Increasing or decreasing the pitch leads to off-design performance which results in a less efficient propeller (at least at the design point).
There was another self-proclaimed genius here a while back...
The cart is designed to use the energy in the shear plane that exists between the air and ground when there is wind. By leveraging one against the other, speed can be gained at the expense of force. To achieve 3 times wind speed, they had to sacrifice at least 2/3 (I suspect closer to 3/4) of the propeller’s thrust to the ground to provide the torque to turn the propeller.
"(I suspect closer to 3/4)"
endlessplane said:The official NALSA ratified dead downwind record is 2.8 x wind-speed.doctorGONZO said:If you have done 2.5,
By reducing the velocity difference between air & ground (a.k.a true wind).doctorGONZO said:I do not understand how it can harvest energy.
Nope. The velocity difference between air & ground is frame invariant. No matter how fast you move relative to air & ground, you always see the same velocity difference between them.doctorGONZO said:So, " any point you have different speeds, different temperatures etc " is totally dependent on where the observer is, and what is his velocity relative to the system in study.
Yes it is.doctorGONZO said:But, my friend, DOWNWIND is not just a simple swap.
UPWIND: When the cart starts rolling, the GROUND is initially at rest relative to the cart, while the AIR moves back fast relative to the cart.doctorGONZO said:When the car reaches wind speed, the car is then in an envelope of air which is NOT MOVING according to the car's observation of the air around it.
doctorGONZO said:endlessplane said:The official NALSA ratified dead downwind record is 2.8 x wind-speed.doctorGONZO said:If you have done 2.5,
Well I have to ask you, HOW LONG WAS THE 2.8 sustained? An hour? 1.98 nanoseconds?
No, that is not how it works. The cart is not using any internally stored energy for propulsion. That would violate NALSAs rules for sail-craft:doctorGONZO said:Actually, in PRINCIPLE, there is no limit to how fast the flash peak speed can be when the car is draining the prop of its kinetic energy and accelerating past wind speed DOWNWIND.
NALSA said:S2: We found no evidence of any energy storage devices. There has been some discussion on www
discussion groups that the rotating propeller constitutes a form of stored energy that might be converted to
propulsion by slowing it down or changing its pitch. Since the propeller is connected to the wheels with a
constant ratio connection slowing the propeller to harvest some of its rotational energy also slows the
craft. The fact that the craft is required to accelerate during the measurement means, for a fixed gear ratio
craft, that rotational energy is being added to the propeller to speed it up during the measurement period
rather than the other way around.
Since it doesn't use internally stored energy for propulsion, even a few seconds prove steady state.doctorGONZO said:Well I have to ask you, HOW LONG WAS THE 2.8 sustained?
Wrong, he still sees the same true wind (velocity difference between air & ground). You can extract energy from this velocity difference.bobc said:but directly downwind - at 1.0 x the wind speed the observer on the car sees no wind at all
The propeller blades are also not moving directly downwind. They are moving on a continuous helical tack (broad reach). This is visualized here:bobc said:Note the (absolutely marvellous) americas cup racers last year: they went faster than the wind downwind, but effectively by 'tacking' downwind. not by going straight, directly downwind.
The vectors for directly downwind faster than wind work out fine as well:bobc said:Faster than the wind upwind - I have absolutely no problem with this, & all the force vector baloney with the rotor blades spinning etc. holds water in that case![]()
The "car" is at stasis, but the thrust of the prop is not, it exerts positive pressure against the wind.bobc said:at 1.0 x the wind speed the observer on the car sees no wind at all - how do you harvest something from nothing?
TylerDurden said:The "car" is at stasis, but the thrust of the prop is not, it exerts positive pressure against the wind.bobc said:at 1.0 x the wind speed the observer on the car sees no wind at all - how do you harvest something from nothing?
The system uses mechanical advantage by gearing the prop rotation less than the wheels. If the torque (generated) at the wheels is greater than the torque required to turn the prop, thrust is generated.
That has been answered already: You harvest energy from the velocity difference between air & ground. That velocity difference is non-zero regardless of the reference frame.doctorGONZO said:"How do you HARVEST something from nothing?"
You have just answered that yourself:doctorGONZO said:HOW IS AIR THAT IS STATIONARY WITH RESPECT TO THE CAR, PUSHING AGAINST THE CAR?
doctorGONZO said:[The propeller] simply pushes against air molecules within its striking range. And the equal and opposite reaction is to find itself pushed in the opposite direction than where it pushed the air molecule.
In frame of the ground, the propeller removes kinetic energy from the air.doctorGONZO said:The subject is that the car is supposedly absorbing kinetic energy from a mass of air which is stationary with respect to the car.
Smart are the high school kids who passed this semifinal exam for the International Physics Olympiad:doctorGONZO said:I am smart, ...
American Association of Physics Teachers said:Both modes are possible as claimed.
But you have no problem with starting directly upwind?bobc said:I stand by "nothing can possibly work at 1.0x windspeed directly downwind"
You don't have to change anything about the prop as you pass windspeed going downwind. Here is the BB-prototype with a fixed propeller pitch and a fixed ratio transmission, accelerating from rest to above 2x windspeed. Watch at 3:00min:bobc said:You actually want the prop to reverse as you go through,