The wheels drive the prop. The energy comes from the wind . The wind pushes the vehicle forward and rotates the wheels attached to the prop. So the car goes forward faster because the prop is pushing it forward at windspeed plus the speed produced by the prop. It has nothing to do with apparent wind as the apparent wind the car's speed minus the wind's speed and the car's driver should feel wind on his face. It might work in a perfect world but propellors are very inefficent and then the transmission loses.... I still need to piss on this fence.
DDW Faster Than The Wind
- Thread starter Maine Sail
- Start date
It has nothing to do with the apparent wind relative to the car, and everything to do with apparent wind that the rotating propeller sees.
The propeller acts exactly like the propeller on an airplane except it is driven by the wheels. And like an airplane travels faster dead down wind . A plane going 100 mph with a 20 mph tailwind travels 120 mph over the ground.
Hi folks. I'm one of the two primary designers/builders of the
vehicle(s) in question.
I'll try to go back through the posts on the thread and come up with some relevent comments over the next bit, but in the mean time feel free to fire away with any questions you might have.
Basics:
-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).
-- it will take off from a standing start on it's own.
-- we sometimes push it up to speed to save time during testing
-- there's no "null point' at windspeed.
-- It will maintain it's speed at or well above wind speed indefinitely
-- There is no stored energy used to accelerate the vehicle at any time
-- By design, our particular one works best when aimed directly downwind.
-- One can be built to go faster than the wind in any direction
-- A simple gearing change will cause ours to go upwind rather than down
-- It can work on water or land
Our project (www.FasterThanTheWind.org) was sponsored by Joby Energy (wind energy research firm) and Google and associated with the Aerospace Engineering department of San Jose State University here in the Silicon Valley.
As mentioned in the OP our vehicles capabilities have now been verified (they installed 18 recording sensors) and a record ratified by NALSA (www.nalsa.org) at 2.8x the speed of the wind while traveling directly downwind.
There is absolutely no violation of the laws of nature going on here -- and yes, I'm quite happy to provide equations to support the physical evidence.
JB
vehicle(s) in question.
I'll try to go back through the posts on the thread and come up with some relevent comments over the next bit, but in the mean time feel free to fire away with any questions you might have.
Basics:
-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).
-- it will take off from a standing start on it's own.
-- we sometimes push it up to speed to save time during testing
-- there's no "null point' at windspeed.
-- It will maintain it's speed at or well above wind speed indefinitely
-- There is no stored energy used to accelerate the vehicle at any time
-- By design, our particular one works best when aimed directly downwind.
-- One can be built to go faster than the wind in any direction
-- A simple gearing change will cause ours to go upwind rather than down
-- It can work on water or land
Our project (www.FasterThanTheWind.org) was sponsored by Joby Energy (wind energy research firm) and Google and associated with the Aerospace Engineering department of San Jose State University here in the Silicon Valley.
As mentioned in the OP our vehicles capabilities have now been verified (they installed 18 recording sensors) and a record ratified by NALSA (www.nalsa.org) at 2.8x the speed of the wind while traveling directly downwind.
There is absolutely no violation of the laws of nature going on here -- and yes, I'm quite happy to provide equations to support the physical evidence.
JB
welcome to the forum, and congratulations on the record becoming official!
I look forward to reading any further comments you have, I'm particularly curious to see your response to my post explaining how I beleive the apparent wind on the propeller applies (am I missing anything or not).
I have a question though...
it would appear to me that the limiting factor in speed is going to be a combination of 3 things, plus a somewhat related 4th. The three being; the aerodynamic drag of the car, the friction and drag of the transmission from the wheel to the propeller, and the aerodynamic drag of the propeller it's self. At a certain rotational speed the drag of the propeller through the air acting against the wheels by way of the transmission, combined with the drag of the car through the apparent wind equals the force of lift/thrust/forward motion produced by the propeller, and prevents the car from going and faster.
The 4th limiting factor being that as the car accelerates, the apparent wind against the propeller blade moves forward requireing the pitch of the blade to be changed to maintain the optimum angle of attack, and therefore changing the direction of lift produced by the blade from being forward motion of the car, towards being paralell but inverse to the direction of rotation, and causing the blades to slow down.
So, my question is this.
If you were able to significantly reduce the friction and drag mentioned above (most notabley the propeller drag), would it not in theory be possible to acheive a much faster speed over ground by continueing to increase the gear ratio from the wheels to the propeller every time the car stops accelerating in order to rotate the propeller at a higher RPM, and thus maintain the apparent wind against the propeller blade coming parallel to the rotation of the blade, and produce more forward thrust, increasing the car speed far beyond 3.5x the true wind speed?
I also had a question (edit - maybe this is the same as one of the questions Fourpoints asked - just worded a little differently..)
In boating, there is a term called "prop slip" and its given in a percent. With zero prop slip, the prop is traveling at exactly the speed of the boat - ie, if the prop were a screw, the travel of the screw is exactly the boat travel for no slip.
For a boat to have forward thrust, it must have some prop slip - ie, in order to accelerate water backwards in order to propel the boat, there must be some prop slip. For example, if the prop slip is positive 10%, the travel of the prop would be 1.1 times longer than the actual travel of the boat.
So about what "prop slip" do you guys operate at when your going downwind - does this change as your ratio of boat to wind speed increases? I would guess you actually might be always operating with slightly negitive prop slip but really have no idea.
Congratulations by the way..
In boating, there is a term called "prop slip" and its given in a percent. With zero prop slip, the prop is traveling at exactly the speed of the boat - ie, if the prop were a screw, the travel of the screw is exactly the boat travel for no slip.
For a boat to have forward thrust, it must have some prop slip - ie, in order to accelerate water backwards in order to propel the boat, there must be some prop slip. For example, if the prop slip is positive 10%, the travel of the prop would be 1.1 times longer than the actual travel of the boat.
So about what "prop slip" do you guys operate at when your going downwind - does this change as your ratio of boat to wind speed increases? I would guess you actually might be always operating with slightly negitive prop slip but really have no idea.
Congratulations by the way..
Thanks FourPoints. It's been a fun project.
I'll have to go back and take a look at that post.
Correct. You can add the Crr (rolling resistance of the tires) into that mix as well, but perhaps you included that in 'transmission' drag.
Yes -- the system is drag limited. Reduce the drag (from any source) and you can go faster. There is no theoretical upper bound limit on the multiple, but there are plenty of real world limits.
We achieved 3.5x+ wiith a very crude project and very low budget. We believe that 4x is very doable given a reasonable budget. 5x, though tough, could perhaps be done if someone wanted to throw Americas Cup money at it.
JB
Thanks for the kind words.
We built our own instrumented dyno and instrumented the wheel and prop hubs with load cells to optimize our gearing and other. This made it much easier and set us up for success once we drove all the way out to the desert.
http://www.youtube.com/watch?v=5JjXudThU88
I did, I figured it wasn't worth adding to the list because I assume it is relatively small compared to the rest of the items
:naughty: there is still the pesky speed of light...
I wonder if an actual aircraft propeller from a small 1 to 2 seater (maybe an ultralight prop?) would perform any better? Clearly you would need higher RPM than you are currently producing to make them work, but they probably operate more efficiently than the propeller used, and presumably also manufactured. However they are much smaller diameter, so I'm not sure how that would impact your design.
That is why I was careful to state that there was no upper bound on the *multiple* rather than speed.
Create an efficient enough vehicle and one could achieve 1,000,000x and never break the speed limit in front of your local school.
One of the keys to propeller efficiency is disc size. A good example of this is a Cessna 150 vs a Robinson R22 (heli) -- they have similar HP ratings and payloads, however the R22 can hover all day long while the 150 has no hope of ever hanging on it's prop. This is because the R22 has a HUGE disc size in comparison.
Same with our vehicle -- while the prop was made in the garage, it's area is ~10x of most plane or ultralight props. We are getting nicely into the '90s % efficient with our hand made prop while 85% is pretty damn good in the general aviation world.
JB
Interstesting that you bring this up. Nature magazine holds an event every year called "Science FOO".
http://www.nature.com/natureconferences/scifoo/index.html
In their words, they invite "200 leading scientists, technologists, writers and other thought-leaders for a weekend of discussion, demonstration and debate." You don't lobby for an invite and you can't apply -- Nature picks the yearly attendees from their knowledge of the year in science. The invitee list is obviously small relative to the field and it is considered one of the absolute hottest invites in science and technology.
By now you may have guessed that we were fortunate enough to be invited to this year's event which was held just last weekend. We brought the Blackbird and it was set up at the sign-in and entry area for the length of the three day shindig.
There is no preset agenda as in the words of founder Tim O-Reilly "you don't invite smart people over to dinner and tell them what to talk about". We were chosen by the attendees (the attendees decide themselves who gets to present) to present this technology and did so to a standing room only (and literally out the door) crowd.
There are no earth shattering physics involved -- the principles have been in use since the first time a sailboat first made progress upwind, so don't expect Nobels to be handed out. It's merely the clever and counterintuitive application of long established rules of natural law.
JB
When the following video was shot there was no variable pitch installed. This was the vehicle in prototype stage:
http://www.youtube.com/watch?v=EEuAqq8FINw
By the time this video was shot (by a critic) we had installed a variable pitch system.
http://www.youtube.com/watch?v=5CcgmpBGSCI
Variable pitch didn't allow for faster speeds ... only quick acceleration and thus shorter distance to those speeds
There is a chain drive from rear axle to prop shaft, this chain is removed for folding and transport.
JB
There is no stored energy involved in the acceleration of this vehicle, period -- this was one of the unbending rules placed upon us by NALSA for record ratification.
The power transmission method between the wheels and the prop is of the fixed ratio variety -- simply two sprockets connected by a chain. If the props slows down, the wheels slow down and visa versa.
For any energy to be removed from the propeller or other rotating drivetrain component these components would have to be slowed down of course -- but there is no way to slow it down without also slowing the wheels and thus the vehicle itself. NALSA was very careful to inspect the vehicle and make sure there was no way to change this fixed relationship.
Watch the videos ... the propeller continues to spin *faster* as the vehicle accelerates. To go faster, we had to actually *increase* the KE in the rotating components rather than rob from them.
Additionally, people seem to think there must be a LOT of stored energy in the prop -- but they don't realize that each blade only weighs 9lbs and is spinning at about 41rpm at 10mph. Do the math on accelerating a 600lb vehicle (driver included) from 10mph wind speed to ~28mph on the KE in that spinning prop and you'll find it isn't even in the realm of possible (try 1mph). What this means is that even *with* a variable speed transmission, there simply isn't the stored energy anything close to available by even bringing that prop to a total stop.
As an exclamation point to the above, here is a video of the vehicle on the dyno turning about 120rpm (3x the speed of the NALSA record example above). The video shows that when the dyno was placed in neutral and the wheels allowed to rotate freely, the drivetrain's KE can only sustain rotation for only 6 revolutions under *no external load* (that would be ~125ft across the ground)
http://www.youtube.com/watch?v=Od9tV5d2uao
Pretty clear that there isn't much KE there for acceleration use.
JB
I'll get to more of the questions later -- don't want to flood the board. If I've overdone it with responses, someone say so. Not trying to overpost my welcome.
JB
JB
no worries there, every post so far has been welcome (at least in my opnion!)...