Merged DDWFTTW in a “Real” Wind

[profpangloss]

I've only run the numbers for beyond wind speed, since that's the only regime the formulas I know can develop useful numbers for thrust vs power. They don't work for speeds of 0 or less through the air.

Does this mean the numbers show the cart can' get to wind speed? I suspect that is the case but I would like to see some numbers.

 

[profpangloss]

I disagree that "there must be an air mass passing through the propeller from front to back in order for it to be producing thrust." As long as the propeller accelerates air backward, it will produce a forward thrust. If the air is moving forward through the propeller, but the propeller slows that forward motion, it does accelerate the air backward.

The tailwind is not blowing "right through" the propeller; it's being slowed as it passes through. This is all that's required for the propeller to produce thrust.

You may be right, I will look at some thrust formulas and get back to you on this.

 

[69dodge]

43 lbs isn't much! Have you calculated the rolling friction of the cart (650 lbs with rider) moving at 56 mph? Have you considered the force of the resultant headwind blowing at 38 mph, it will be considerable? (Sorry if you have already done all that, I just now tuned in and saw your post and have not caught up on the rest of the thread.)

I haven't taken into account any friction: inefficiencies of the drivetrain, rolling resistance of the tires, or air resistance. These are all important in practice, of course.

But the main reason I did these calculations was to demonstrate to myself (and anyone else who's interested) that, in a wind, even when the cart is moving faster than the wind, the forward thrust of the propeller can in fact be greater than the backward force of the ground on the wheels.

So, the point is, there's no fundamental reason why the cart can't exceed the speed of the wind. There are no basic laws of physics being broken. It's just a matter of reducing friction to acceptable levels.

 

[spork]

I seriously doubt that! A very short chain that is perfectly aligned can achieve theoretical efficiencies of over 90%. The chain on your cart is very long ( I'm guess over 5 meters total length?) and it is twisted. I'l go out on a limb and say it is less than 80% efficient, probably less.

I'll go out on a limb and say you're wrong. These folks reported 98.6% efficiency http://www.jhu.edu/news_info/news/home99/aug99/bike.html

I believe our drive was just over 90% - but I KNOW, our cart went 2.8X wind speed. So I don't really care that much what the exact efficiencies were. I guess I'd love to learn our chain drive gave us less than 80% efficiency. That would give us an easy improvement to shatter our own existing record.

 

[profpangloss]

I haven't taken into account any friction: inefficiencies of the drivetrain, rolling resistance of the tires, or air resistance. These are all important in practice, of course.

But the main reason I did these calculations was to demonstrate to myself (and anyone else who's interested) that, in a wind, even when the cart is moving faster than the wind, the forward thrust of the propeller can in fact be greater than the backward force of the ground on the wheels.

So, the point is, there's no fundamental reason why the cart can't exceed the speed of the wind. There are no basic laws of physics being broken. It's just a matter of reducing friction to acceptable levels.

Sorry, but that is exactly what every believer in perpetual motion says "it's just a matter of reducing friction to acceptable levels". If not for friction EVERY perpetrual motion machine in the world works! Friction is one of the laws of physics, so if you ignore it, and claim the machine works, you are breaking the basic laws of physics.
It does not matter if the forward thrust can theoretically be greater than the drag on the wheels if the other losses make the cart unworkable. Factor in friction and air resistance and see what the numbers say then, please.

 

[profpangloss]

I'll go out on a limb and say you're wrong. These folks reported 98.6% efficiency http://www.jhu.edu/news_info/news/home99/aug99/bike.html

I believe our drive was just over 90% - but I KNOW, our cart went 2.8X wind speed. So I don't really care that much what the exact efficiencies were. I guess I'd love to learn our chain drive gave us less than 80% efficiency. That would give us an easy improvement to shatter our own existing record.

I notice you did not say the cart went 2.8X wind speed Directly down wind. I believe that is the entire point of contention. I have not looked at the link you posted but let me guess that it was a very short chain, compared to what you are using and it was perfectly straight and perfectly aligned. Now that I have said that, I'll go look!

 

[humber]

Ok, I did it for 28 mph in a 10 mph wind. Again, ignoring friction.

If the cart moves at 28 mph, the propeller spins at 133.7 RPM or 2.228 rev/s. The airspeed of the cart is 18 mph, or 26.4 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 26.4/[(2.228)(16)] = 0.7406.

To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0834 and cq = 0.0120.

Then, the forward thrust of the propeller works out to be 61.32 lb, the torque driving the propeller is 141.2 ft-lb, and the backward force of the ground on the wheels is 48.11 lb.

That leaves 13.21 lb to overcome friction and accelerate the cart.

But the very optimistic figures of the submission give the drag load other then the "generator" as 13.4lbs, and that does not take into account rolling tyre losses, but only the surface rolling resistance. Those figures are for 20 mph ( 5mph airspeed) and not 18mph airspeed. It has already failed to get that far, so won't get to 50mph.
What is the power demand using your numbers?

 

[Michael C]

Sorry, but that is exactly what every believer in perpetual motion says "it's just a matter of reducing friction to acceptable levels". If not for friction EVERY perpetrual motion machine in the world works! Friction is one of the laws of physics, so if you ignore it, and claim the machine works, you are breaking the basic laws of physics.
It does not matter if the forward thrust can theoretically be greater than the drag on the wheels if the other losses make the cart unworkable. Factor in friction and air resistance and see what the numbers say then, please.

No. For a perpetual motion machine to work it would be necessary to reduce friction to zero. Here it is just a case, as with any normal machine, of making sure that the friction is under the level that would prevent the machine from working.

 

[humber]

I'll go out on a limb and say you're wrong. These folks reported 98.6% efficiency http://www.jhu.edu/news_info/news/home99/aug99/bike.html

I believe our drive was just over 90% - but I KNOW, our cart went 2.8X wind speed. So I don't really care that much what the exact efficiencies were. I guess I'd love to learn our chain drive gave us less than 80% efficiency. That would give us an easy improvement to shatter our own existing record.

You say it went 2.8WS. Vast difference.

So, what are the prop calculations for 28mph in a 10mph wind, and a rotor AV of 133rpm?

 

[profpangloss]

I notice you did not say the cart went 2.8X wind speed Directly down wind. I believe that is the entire point of contention. I have not looked at the link you posted but let me guess that it was a very short chain, compared to what you are using and it was perfectly straight and perfectly aligned. Now that I have said that, I'll go look!

Oh yeah! A brand new chain in a clean lab environment, perfectly aligned and tensioned and run by computer controlled electric motors. I would say if they didn't get efficiencies in the high nineties, they screwed up. But your chain, hanging out in the wind, very long and twisted isn't going to get anywhere near 90%. Lucky if you get 80%!

 

[profpangloss]

No. For a perpetual motion machine to work it would be necessary to reduce friction to zero. Here it is just a case, as with any normal machine, of making sure that the friction is under the level that would prevent the machine from working.

And how do you know it would not need to be zero in this case if you have not run the numbers? If the losses put the cart in the unworkable range, then the friction would need to be less than zero! Statements like yours do nothing to lend credibility to this, just make it look more and more like internet wooo.

 

[69dodge]

Sorry, but that is exactly what every believer in perpetual motion says "it's just a matter of reducing friction to acceptable levels". If not for friction EVERY perpetrual motion machine in the world works! Friction is one of the laws of physics, so if you ignore it, and claim the machine works, you are breaking the basic laws of physics.
It does not matter if the forward thrust can theoretically be greater than the drag on the wheels if the other losses make the cart unworkable. Factor in friction and air resistance and see what the numbers say then, please.

For a perpetual motion machine to work, friction has to be reduced all the way to zero. Here, it only has to be reduced below a specific positive value. (ETA: Now I see Michael C said this too.)

Initially, I wanted to analyze the situation where the cart moves exactly at the speed of the wind. Then, there would be no air resistance. But I don't know the thrust of the propeller at zero airspeed.

 

[spork]

Oh yeah! A brand new chain in a clean lab environment, perfectly aligned and tensioned and run by computer controlled electric motors. I would say if they didn't get efficiencies in the high nineties, they screwed up. But your chain, hanging out in the wind, very long and twisted isn't going to get anywhere near 90%. Lucky if you get 80%!

A) You're wrong (again)
B) It was efficient enough that it allowed us to set a world record.

 

[69dodge]

And how do you know it would not need to be zero in this case if you have not run the numbers? If the losses put the cart in the unworkable range, then the friction would need to be less than zero!

I don't follow. What losses are there besides friction?

 

[humber]

And now for 56 mph in an 18 mph wind.

If the cart moves at 56 mph, the propeller spins at 267.3 RPM or 4.455 rev/s. The airspeed of the cart is 38 mph, or 55.73 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 55.73/[(4.455)(16)] = 0.7818.

To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0784 and cq = 0.0117.

Then, the forward thrust of the propeller works out to be 230.5 lb, the torque driving the propeller is 550.3 ft-lb, and the backward force of the ground on the wheels is 187.5 lb.

That leaves 43 lb to overcome friction and accelerate the cart.

That is a lot of power.

187.5lb = 843N
56mph = 25.03m/s
Power = 843*25.03 =21,100W = 28.3HP.

 

[humber]

A) You're wrong (again)
B) It was efficient enough that it allowed us to set a world record.

The motion of the "chain tensioner", says you are not even close to the conditions that would allow those lab results to be used.

 

[ynot]

[FONT=Verdana, sans-serif]
[FONT=Verdana, sans-serif]If the propeller and air only interact in an equal and opposite manner . . .[/FONT]
[/FONT][FONT=Verdana, sans-serif][/FONT]
[FONT=Verdana, sans-serif]The only advantage a cart travelling at wind speed has over a cart pushed up to the same speed in still air is that it doesn't have a headwind. Otherwise all other frictional losses are essentially the same for both carts. At wind speed the cart is effectively in still air. How does a propeller spinning in still air develop extra energy to sustain and increase the cart's speed ONLY from an equal and oposite reaction with that air? [/FONT]

 

[humber]

[FONT=Verdana, sans-serif]
[FONT=Verdana, sans-serif]If the propeller and air only interact in an equal and opposite manner . . .[/FONT]
[/FONT][FONT=Verdana, sans-serif][/FONT]
[FONT=Verdana, sans-serif]The only advantage a cart travelling at wind speed has over a cart pushed up to the same speed in still air is that it doesn't have a headwind.
Otherwise all other frictional losses are essentially the same for both carts. At wind speed the cart is effectively in still air. How does a propeller spinning in still air develop extra energy to sustain and increase the cart's speed ONLY from an equal and oposite reaction with that air? [/FONT]

It can't in principle, but the cart is special. Even allowing for that, and using Dodge69's numbers:

"Ok, I did it for 28 mph in a 10 mph wind. Again, ignoring friction.
If the cart moves at 28 mph, the propeller spins at 133.7 RPM or 2.228 rev/s. The airspeed of the cart is 18 mph, or 26.4 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 26.4/[(2.228)(16)] = 0.7406.
To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0834 and cq = 0.0120.
Then, the forward thrust of the propeller works out to be 61.32 lb, the torque driving the propeller is 141.2 ft-lb, and the backward force of the ground on the wheels is 48.11 lb.
That leaves 13.21 lb to overcome friction and accelerate the cart."


Air power at prop = 18mph*61.3lb = 26.4ft/s*61.3lb = 8.0m/s*272N = 2176W =2.92HP
Power at wheel = 28mph*48.11lb = 41ft/s*48.11lb = 12.5m/s*214 = 2675W =3.58HP

The wheel is somehow generating more power than the prop, but the net is 0.66HP.

Using rider and bike weights of 200lbs and 450lbs respectively, and a mechanical efficiency of 85%, the losses are 1.1HP.
It is unlikely that a cart on dirt will have less rolling losses, and certainly more drag. Spork was going to give some numbers for the pylon drag. When he does that, we can include them in the calculation.

 

[69dodge]

The only advantage a cart travelling at wind speed has over a cart pushed up to the same speed in still air is that it doesn't have a headwind. Otherwise all other frictional losses are essentially the same for both carts.

Yes.

Well, also, it's easier to spin the propeller fast enough to get thrust out of it.

At wind speed the cart is effectively in still air.

Yes, but it's moving over the ground.

How does a propeller spinning in still air develop extra energy to sustain and increase the cart's speed ONLY from an equal and oposite reaction with that air?

The propeller isn't just spinning. It's being spun. The cart is moving over the ground. This turns the wheels, which spin the propeller. It takes some force to turn the wheels, but the thrust provided by the spinning propeller is greater than this force. So the net force is positive.

See my calculations, based on figures provided by spork and by ThinAirDesigns. (These calculations are for the cart moving even faster than the wind.)

 

[Michael C]

[FONT=Verdana, sans-serif]The only advantage a cart travelling at wind speed has over a cart pushed up to the same speed in still air is that it doesn't have a headwind. Otherwise all other frictional losses are essentially the same for both carts. At wind speed the cart is effectively in still air. How does a propeller spinning in still air develop extra energy to sustain and increase the cart's speed ONLY from an equal and oposite reaction with that air? [/FONT]

Look at the situation of each cart from the point of view of an observer on the cart:

For cart A, the air is still, but the ground is moving.
For cart B, the air and the ground are moving at the same speed in the same direction.

In both cases we can try to use the power coming from the moving ground to turn the prop and produce thrust. Simple energy calculations (as seen in this thread and in the original downwind discussion) show us that in case A it's possible for the thrust of the prop to be more than the retarding force on the wheels that is necessary to produce this thrust, but in case B the thrust of the prop will always be less than the retarding force on the wheels.