On 30/09/2010 22:43, dennis@home wrote:


"Rick Cavallaro" wrote in message
...
On Sep 30, 1:31 pm, Clive George wrote:

I've got a little car, with 6 wheels - 4 straddling the belt, 2 on
the belt.

Can I arrange a drive mechanism such that my little car can go faster
than the conveyor belt? (no engines, stored energy, etc, just simple
mechanical connections)

(actually, this is all quite like the various cotton reel problems
covered in 1st year physics)

Spool video:
http://www.youtube.com/watch?v=E7vcQcIaWSQ

Under the ruler faster than the ruler:
http://www.youtube.com/watch?v=k-trDF8Yldc

Equivalence of reference frames:
http://www.youtube.com/watch?v=9Yt4zxYuPzI



What is that supposed to demonstrate?
that if you pull the centre along it goes faster because of the gearing
between the centre and the edge?
that's all it does show.

Yes, and that's all it does need to show.

So you agree that yes, I can have that drive mechanism, and it can be as
simple as eg having a a 2:1 chain drive such that the wheels on the
conveyor belt go half the speed of those on the road?

Now with that arrangement, what can you tell me about the forces the
wheels/axles are exerting on a) the road, b) the conveyor belt and c) my
little car? Relative magnitude and direction will do.

On 30/09/2010 22:43, Rick Cavallaro wrote:
On Sep 30, 1:55 pm, Clive wrote:

Go for it.

Here's a simple vector analysis:

https://rcpt.yousendit.com/958723733...d7d26acf72285e

And here's the explanation that goes with it:

Here is the vector diagram for an ice-boat that’s maintaining a course
such that its downwind velocity component is faster than the wind.
I've assumed a boat going 45 degrees downwind at a speed of twice the
wind speed. The ticket is to then compute the necessary L/D of the
sail and keel to make this possible (this would have the boat making a
downwind velocity component of 1.414 times the wind-speed).

Given the wind velocity and the boat velocity, we can easily compute
the apparent wind over the sail. From this we see the direction of
lift and drag on the sail. What we need to do to make sure this
sailing configuration is possible is to insure that "alpha" is small
enough so that the resultant force has a forward pointing component
(relative to the boat). In this case alpha would have to be 45 - 16.3
degrees or 28.7 degrees (or less). That relates to a sail whose L/D is
1.83. Obviously this is easily achievable. However, I've assumed a
keel with infinite L/D in this case. The drawing gets a little more
cluttered if we include the keel's L/D. So I'll try to describe it in
words. I'm going to assume an L/D of 10:1 for the keel (easily done
with an ice-boat). This will trim 5.7 degrees off of my 28.7 degree
budget. That leaves me with an allowable 23 degrees(L/D = 2.35:1) to
achieve this configuration. So with a keel that has a 10:1 L/D and a
sail/boat that has an overall 2.35:1 or better L/D this configuration
can be achieved (we can achieve a downwind component about 40 percent
faster than the wind speed).

Got it. No real need for the numbers, took me a while to get L/D = lift
over drag, and publishing it as a jpg would have been friendlier, but
still it works.

On Sep 30, 3:19*pm, Clive George wrote:

took me a while to get L/D = lift over drag

Sorry about that. As an aero guy I didn't even think to write that
out. I should have.

publishing it as a jpg would have been friendlier

Sorry about that too. .jpgs give you ugly artifacts on high contrast
images. gifs just look better, but either one would work.

Me:
"A" is true (there is usable power in slowing the wind)
and
"B" is true (a prop can be used to slow the wind)
and
"C" is true (slowing the wind provides enough energy to turn the prop)

where is the violation of natural law?


@dennis@home
Because you don't do all three.

Ok, this may be progress. It appears that you have no problem with
"A" nor with "B", but you can't agree with "C" -- you do not agree
that the slowing the wind provides enough energy to turn the prop.

Before I go on, can you confirm that I understand you correctly?
Thanks

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember ThinAirDesigns
saying something like:


Initially the wind merely starts blows the vehicle downwind. Once the
vehicle is moving the wheels provide the input torque to turn the
propeller. The propeller takes wind that was moving at say 10mph
across the ground and pushes back on it until it is only moving say
5mph across the ground. The energy removed from the wind when it is
slowed is the energy powering the vehicle.

Light dawns. If you'd mentioned pusher prop at the start...

My gut feeling is it would be heavily dependent on very low frictional
losses in the transmission of whatever type.
Permanent Magnet bearings? Air bearings need an input.

WD40?

Grimly Curmudgeon wrote:
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember ThinAirDesigns
saying something like:

Initially the wind merely starts blows the vehicle downwind. Once the
vehicle is moving the wheels provide the input torque to turn the
propeller. The propeller takes wind that was moving at say 10mph
across the ground and pushes back on it until it is only moving say
5mph across the ground. The energy removed from the wind when it is
slowed is the energy powering the vehicle.

Light dawns. If you'd mentioned pusher prop at the start...

My gut feeling is it would be heavily dependent on very low frictional
losses in the transmission of whatever type.
Permanent Magnet bearings? Air bearings need an input.

WD40?

Conceptually its easier to think of it like a air paddle steamer car

The wind blows the car along, which drives the paddles which move
backwards against the wind. Therefore the paddle blades are never in
still air when the car is travelling at windspeed. So they can still
extract energy.

On Sep 30, 4:22*pm, Grimly Curmudgeon wrote:

My gut feeling is it would be heavily dependent on very low frictional
losses in the transmission of whatever type.
Permanent Magnet bearings? Air bearings need an input.

To go faster than the wind requires reasonably low losses. As you try
to achieve multiples of wind speed the allowable losses become smaller
at a ridiculous rate. We run in the area of about 3X wind speed. 5X
would be a real challenge.

On Sep 30, 4:28*pm, The Natural Philosopher wrote:

Conceptually its *easier to think of it like a air paddle steamer car

The wind blows the car along, which drives the paddles which move
backwards against the wind. Therefore the paddle blades are never in
still air when the car is travelling at windspeed. So they can still
extract energy.

You're exactly right, but I'm sure that's more intuitive for some and
less intuitive for others. On top of that, you have to describe how
the forward moving paddles don't kill the whole deal. They need to
retract or feather in some way.

On 30/09/2010 23:30, Rick Cavallaro wrote:
On Sep 30, 3:19 pm, Clive George wrote:

took me a while to get L/D = lift over drag

Sorry about that. As an aero guy I didn't even think to write that
out. I should have.

Not that I ever do similar things in other contexts...

publishing it as a jpg would have been friendlier

Sorry about that too. .jpgs give you ugly artifacts on high contrast
images. gifs just look better, but either one would work.

Actually gif published as an image rather than a download would have
been perfectly fine, but never mind, it all worked and I shouldn't be
picky - you're the one putting in the time drawing pictures and
explaining it all to the rest of us :-)

On Sep 30, 4:53*pm, Clive George wrote:

Actually gif published as an image rather than a download would have
been perfectly fine...

I'm not an expert on usenet. On a typical forum I'd post the .gif
image. Can we post images here?

On 01/10/2010 00:59, Rick Cavallaro wrote:
On Sep 30, 4:53 pm, Clive George wrote:

Actually gif published as an image rather than a download would have
been perfectly fine...

I'm not an expert on usenet. On a typical forum I'd post the .gif
image. Can we post images here?

Definitely not :-) This area of usenet is text-only, binaries are out.

I'd just stick it up on a photo sharing website (which is why I thought
jpg on autopilot) - or if you've got your own site, there?

(in case of confusion, when I said image rather than download, I meant
such that the link showed you the pic straight away rather than having
to download - not image within the posting)

makes sense. This was the quick and dirty approach.

Rick Cavallaro wrote:
On Sep 30, 4:28 pm, The Natural Philosopher wrote:

Conceptually its easier to think of it like a air paddle steamer car

The wind blows the car along, which drives the paddles which move
backwards against the wind. Therefore the paddle blades are never in
still air when the car is travelling at windspeed. So they can still
extract energy.

You're exactly right, but I'm sure that's more intuitive for some and
less intuitive for others. On top of that, you have to describe how
the forward moving paddles don't kill the whole deal. They need to
retract or feather in some way.

I'll put them in a housing.

@The Natural Philosopher

I'll put them in a housing.

I'd have to see a sketch of your idea, but I believe you may find that
while a housing might protect them from the relative headwind, it
doesn't keep them from having to 'churn' the air in the housing to a
point of overall system failure.

Again, just a general comment as I'm not privy to your specific idea.

"Clive George" wrote in message
o.uk...
On 30/09/2010 22:43, dennis@home wrote:


"Rick Cavallaro" wrote in message
...
On Sep 30, 1:31 pm, Clive George wrote:

I've got a little car, with 6 wheels - 4 straddling the belt, 2 on
the belt.

Can I arrange a drive mechanism such that my little car can go faster
than the conveyor belt? (no engines, stored energy, etc, just simple
mechanical connections)

(actually, this is all quite like the various cotton reel problems
covered in 1st year physics)

Spool video:
http://www.youtube.com/watch?v=E7vcQcIaWSQ

Under the ruler faster than the ruler:
http://www.youtube.com/watch?v=k-trDF8Yldc

Equivalence of reference frames:
http://www.youtube.com/watch?v=9Yt4zxYuPzI



What is that supposed to demonstrate?
that if you pull the centre along it goes faster because of the gearing
between the centre and the edge?
that's all it does show.

Yes, and that's all it does need to show.

So you agree that yes, I can have that drive mechanism, and it can be as
simple as eg having a a 2:1 chain drive such that the wheels on the
conveyor belt go half the speed of those on the road?


That is easy, nobody has argued against it.
The arguments against have always been deflected onto something else in all
the replies in case you had not noticed.

Like the one about the treadmill demonstrating the required effect..
it doesn't, all it shows is you can take power from the belt to drive a prop
with enough thrust to power the cart faster than the belt.
It doesn't show that you can extract energy from wind and use the same
energy to go faster than that wind.


Even the main video doesn't actually show that, it doesn't even show they
are going downwind, the wind vanes show the apparent wind and not the true
wind so they could be going at an angle and the wind vanes would not show
that.


Now with that arrangement, what can you tell me about the forces the
wheels/axles are exerting on a) the road, b) the conveyor belt and c) my
little car? Relative magnitude and direction will do.

What will that prove that has any relevance to going down wind faster than
the wind?
Is this just another deflection attempt?

"ThinAirDesigns" wrote in message
...
Me:
"A" is true (there is usable power in slowing the wind)
and
"B" is true (a prop can be used to slow the wind)
and
"C" is true (slowing the wind provides enough energy to turn the prop)

where is the violation of natural law?


@dennis@home
Because you don't do all three.

Ok, this may be progress. It appears that you have no problem with
"A" nor with "B", but you can't agree with "C" -- you do not agree
that the slowing the wind provides enough energy to turn the prop.

Before I go on, can you confirm that I understand you correctly?
Thanks

I don't agree that you are slowing the real wind.

@Grimly Curmudgeon

Light dawns. If you'd mentioned pusher prop at the start...

I have no idea how I could have mentioned it any sooner than the very
first sentence of my very first post on this thread -- which is what I
did.

@dennis@home

I don't agree that you are slowing the real wind.

So, you believe that a propeller *can* slow the real wind (you said
"B" was true), you just don't believe that our propeller *IS* slowing
down the real wind.

Is that correct?

dennis@home

I don't agree that you are slowing the real wind.

So you agree that a propeller *can* slow the real wind (you said "B"
was true), but you don't believe that *our* propeller is slowing the
true wind.

Is that correct?

"Grimly Curmudgeon" wrote in message
...
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember ThinAirDesigns
saying something like:


Initially the wind merely starts blows the vehicle downwind. Once the
vehicle is moving the wheels provide the input torque to turn the
propeller. The propeller takes wind that was moving at say 10mph
across the ground and pushes back on it until it is only moving say
5mph across the ground. The energy removed from the wind when it is
slowed is the energy powering the vehicle.

Light dawns. If you'd mentioned pusher prop at the start...

Ok.

So they are travelling at 2.9 times the speed of the wind, no prop
They need to slow the wind as currently they are speeding it up through
drag.
Start prop.. now they have to accelerate a mass of air to greater than 1.9
times the real wind speed before the real wind starts to be slowed.
Does it take more energy to accelerate it to 1.9 times the wind speed than
they get by slowing the real wind down by less than 1?

The claim does not make sense even before you add in friction, turbulence,
etc.


My gut feeling is it would be heavily dependent on very low frictional
losses in the transmission of whatever type.
Permanent Magnet bearings? Air bearings need an input.

WD40?

Does WD40 do magic too?

Anyway as far as I am concerned this thread id dead, other people can
believe it if they want, nothing in the videos explains it.

On 01/10/2010 13:24, dennis@home wrote:


"Clive George" wrote in message
o.uk...
On 30/09/2010 22:43, dennis@home wrote:


"Rick Cavallaro" wrote in message
...

On Sep 30, 1:31 pm, Clive George wrote:

I've got a little car, with 6 wheels - 4 straddling the belt, 2 on
the belt.

Can I arrange a drive mechanism such that my little car can go faster
than the conveyor belt? (no engines, stored energy, etc, just simple
mechanical connections)

(actually, this is all quite like the various cotton reel problems
covered in 1st year physics)

Spool video:
http://www.youtube.com/watch?v=E7vcQcIaWSQ

Under the ruler faster than the ruler:
http://www.youtube.com/watch?v=k-trDF8Yldc

Equivalence of reference frames:
http://www.youtube.com/watch?v=9Yt4zxYuPzI



What is that supposed to demonstrate?
that if you pull the centre along it goes faster because of the gearing
between the centre and the edge?
that's all it does show.

Yes, and that's all it does need to show.

So you agree that yes, I can have that drive mechanism, and it can be
as simple as eg having a a 2:1 chain drive such that the wheels on the
conveyor belt go half the speed of those on the road?


That is easy, nobody has argued against it.
The arguments against have always been deflected onto something else in
all the replies in case you had not noticed.

I'm not doing any deflecting. I'm taking you through it, slowly and
carefully.

Like the one about the treadmill demonstrating the required effect..
it doesn't, all it shows is you can take power from the belt to drive a
prop with enough thrust to power the cart faster than the belt.
It doesn't show that you can extract energy from wind and use the same
energy to go faster than that wind.

Even the main video doesn't actually show that, it doesn't even show
they are going downwind, the wind vanes show the apparent wind and not
the true wind so they could be going at an angle and the wind vanes
would not show that.

Can you stick with my argument, rather than starting your own? You're
introducing confusion into the discussion which isn't helping.

Now with that arrangement, what can you tell me about the forces the
wheels/axles are exerting on a) the road, b) the conveyor belt and c)
my little car? Relative magnitude and direction will do.

What will that prove that has any relevance to going down wind faster
than the wind?
Is this just another deflection attempt?

No, it's a step along the way to explaining it to you.

Can you answer my questions?

dennis@home
Start prop.. now they have to accelerate a mass of air to greater than 1.9
times the real wind speed before the real wind starts to be slowed.

Well there's you problem -- no such thing has to happen.

Let's to back to the skateboard and use your numbers.

Sidewalk traffic = 1ms
Skateboarder = 2.9ms

To start to slow the pedestrian down he need not throw them back at
1.9ms per your assertion. If he DID throw them backwards at 1.9ms,
he would be reversing their direction and they would now be traveling
the OPPOSITE direction on the sidewalk at 0.9ms (1ms - 1.9ms =
-0.9ms).

Now, the skateboards *arms* definitely need to travel 1.9ms backwards
(2.9ms - 1.0ms) before those arms can begin to contact and slow the
pedestrian, but your claim that the weight of the pedestrian must be
accelerated as such is simply false.

Similarly, the propeller need not accelerate the mass of the air
rearward at your claimed 1.9ms before it begins to slow down. If the
wind is merely accelerated rearward by 0.1ms, its speed has been
reduced by 10% (1ms - 0.1ms = 0.9ms) and that energy was harvested by
the mechanism.

ThinAirDesigns wrote:

Alrighty -- following is a simple energy/force analysis for a small
propeller equipped DDWFTTW vehicle. In this analysis, we show that
in a 27.5ftsec wind, and with the vehicle traveling over the ground at
55ft/sec, the retardant force on the wheels needed to drive the
propeller is less than the propeller needs to keep the vehicle at that
speed.

I apologise for my earlier misunderstanding, which came from being
fixated on the boat example being driven by a water propeller using
power harvested from an air turbine. I see that in the case of the
land based car you're discussing it is not using the air screw as a
source of power with which to drive the wheels, but the wheels as a
source of power with which to power the air propeller.

I don't doubt that this is possible, but I believe your analysis below
is wrong. I offer a corrected version below.

A:
1.0 HP = 550 foot-pounds per second
(http://en.wikipedia.org/wiki/Horsepower)
This means that @55ft/sec, 10lbs of force exerted on our chassis will
allow us to harvest 1.0hp from our wheels with a lossless generator
(we'll deal with losses below)

Fine.

B:
1/2HP = 275 foot-pounds per second
This means that at 27.5ft/sec, 10lbs of force can be produced by a
lossless propeller consuming 1/2hp. (we'll deal with losses below)

This is wrong. See below.

C:
If the wind is blowing at 27.5ft/sec and our vehicle is traveling DDW
at 2x the speed of the wind, the vehicle is traveling over the ground
at 55ft/sec and through the air at only 27.5ft/sec.

Fine.

D:
Through the establishment of "A", we know we can pull 1.0 HP from the
wheels of the vehicle if it's propelled by 10lbs of force,

Fine.

and through
the establishement of "B" we can see that the propeller in the
relative tailwind only needs 1/2HP input to produce that same 10lbs of
force.

This is wrong. See below.

E:
We subtract the 1/2hp that the prop needs to produce its 10lbs of
thrust from the 1.0hp that the wheels can produce from that same 10lbs
and you have 0.5hp left over for the system losses.(told you we would
get to losses).

This would be fine if the prop did in fact only need 1/2hp.
Actually it needs a bit more than that. See below.

Do the same calcs on a no wind day and it's easy to see that the
wheels still produce 1.0HP at 55ft/sec, but now the propeller is force
to do work at 55ft/sec

No, this is wrong. See below.

rather than 27.5ft/sec and it now takes a full
1.0HP at the prop to produce the 10lbs of force. This of course means
that there is nothing left for losses and since there are *always*
losses, the vehicle simply can?t motivate itself when there is no
wind.

But look, when there's no wind, and the car is not moving, you can still
claim to be travelling at twice the wind speed. There is no problem,
since two times zero is zero! There will be no losses either! :-)

Now let's get to what's wrong with B and part 2 of D above:

You're using the simple formula that Power = Force * Speed.
This is OK for the wheel situation, but it doesn't apply to the prop,
where the air is being accelerated. More below.

The other problem is that you seem to be arbitrarily using the input air
speed for the prop calc, i.e. the speed of the headwind (aka the apparent
wind, i.e. wind speed minus car speed), which is wrong. To see why it's
wrong, do the calcs for the situation where the car it going at exactly air
speed:

Suppose car speed and wind speed are *both* 55ft/s. The wheels are still
delivering 1hp. The headwind is zero, so you would calculate the power
needed by the prop to produce 10lb of thrust as 0hp. Can't be right. At
0hp the prop won't be turning, and would be producing no thrust.

It would be more correct to use the output air speed. But when I say "more
correct" that just means "less obviously wrong", it's not actually correct
yet. To see why it's not right, let's redo the calcs for your original
example with car speed 55ft/s and air speed 27.5ft/s.

I'm persuaded by your comment elsewhere, regarding the energy balance,
that the optimum acceleration the prop must impart to the wind is such as
to reduce its ground speed to zero. So the prop needs to reduce the wind's
speed over the ground by 27.5 ft/s from 27.5 ft/s to 0 ft/s, and this is
equivalent to changing the wind speed as seen by the car from 27.5 ft/s
coming in to 55 ft/s going out. The air is leaving the back of the prop at
55 ft/s, and if we were to apply your simple formula, the prop would require
a full 1hp to deliver 10lb of thrust. Nothing left for losses, so if the
darned thing works in real life, the formula must be wrong.

How do we fix this? We need to go back to your excellent skateboarder.

He's skating along at 2m/s, going 1m/s faster than the pedestrians around
him who are moving at 1m/s. Suppose he grabs one pedestrian each second,
decelerating them from 1m/s to 0m/s relative to the ground, or
accelerating them from 1m/s to 2m/s relative to himself. The acceleration
required is 1m/s/s. If each pedestrian has a mass of 60kg, the skater needs
to apply a force of 60N to achieve that acceleration. So far, so good.
How much power does the skater need to keep up this rate of work? Well,
if we were to apply the simple power formula of force times speed, we'd
get either 60 W or 120 W depending on whether we used the 1m/s starting
speed or the 2m/s final speed of the pedestrian. Both are wrong. To get
the correct answer we need to get a bit more basic.

Energy = Force * distance. How far does the pedestrian travel during the
second he is being accelerated? It turns out to be 0.5m relative to the
ground, or 1.5m relative to the skater, and so the amount of work the skater
is doing on the pedestrian is 60N * 1.5m = 90J, and if this is done over the
course of one second, the power is 90W. Notice how 90 is equal neither to
60 nor to 120.

How do we apply this to the car? The prop needs to accelerate a certain
quantity of air each second by 27.5ft/s in order to get 10lbf of thrust.
How much air? Ah, curse your antediluvian non-metric units! Having taken
great care to distinguish lb force from lb mass, the simple formula
Force = Mass * Acceleration gives mass = 10 lbf / 27.5 ft/s/s which
comes to about 11.7 lbs of air. Air weighs about 1.7 lb/yd3, so the prop
needs to "pump" about 6.9 yd3/s, and since this needs to leave the back of
the prop at 55ft/s, its effective cross sectional area needs to be 0.38
yd2. But I digress.

Although the prop accelerates the air continuously within its own thickness,
we could instead think of it as a set of arms grabbing a boxful of air each
second, each box containing 11.7lb of air. How far would each box travel
during the second? Turns out to be 1.5 times 27.5ft = 41.25ft.
So each second the prop does work equal to 412.5ftlbf.

That's 0.75hp. What luck, eh? Still comfortably below 1hp.

On 01/10/2010 13:41, dennis@home wrote:

Anyway as far as I am concerned this thread id dead, other people can
believe it if they want, nothing in the videos explains it.

Dennis, if you want to continue looking dim, you're welcome to. But if
you stick with this you might actually learn something. It does require
that you stop assuming it's impossible, and instead look at the actual
physics involved.

On Oct 1, 5:53*am, Clive George wrote:

I'm not doing any deflecting. I'm taking you through it, slowly and
carefully.

I applaud your effort, but I've been through this with too many
Dennis's. I assure you that assistance is futile.


Can you stick with my argument, rather than starting your own?

That's not how this game works. Dennis will NEVER late you take him
anywhere near the answer. When he sees any sign of it, he'll just
make an immediate left-turn and start down some other pointless alley.


Can you answer my questions?

I have one squillion dollars that says no. But it's entertaining to
watch.

On Oct 1, 6:38*am, Ronald Raygun wrote:

I apologise for my earlier misunderstanding, which came from being
fixated on the boat example being driven by a water propeller using
power harvested from an air turbine. *I see that in the case of the
land based car you're discussing it is not using the air screw as a
source of power with which to drive the wheels, but the wheels as a
source of power with which to power the air propeller.

I don't doubt that this is possible, but I believe your analysis below
is wrong. *I offer a corrected version below.

Well, you're making progress. I suspect if you'll continue a rational
discussion with JB he'll be able to convince you that your "corrected"
analysis is in fact wrong.

ThinAirDesigns wrote:
@The Natural Philosopher
I'll put them in a housing.

I'd have to see a sketch of your idea, but I believe you may find that
while a housing might protect them from the relative headwind, it
doesn't keep them from having to 'churn' the air in the housing to a
point of overall system failure.

Again, just a general comment as I'm not privy to your specific idea.

Hey, it was just a way to get the idea across.

Clive George wrote:
On 01/10/2010 13:41, dennis@home wrote:

Anyway as far as I am concerned this thread id dead, other people can
believe it if they want, nothing in the videos explains it.

Dennis, if you want to continue looking dim, you're welcome to. But if
you stick with this you might actually learn something. It does require
that you stop assuming it's impossible, and instead look at the actual
physics involved.

Dennis doesn't do actual physics.

@Ronald Raygun

Hey Ron -- thanks for your detailed response. I'll put together a
reply in a bit -- I've got some important stuff at work I'm into at
the moment.

Later.

Rick Cavallaro wrote:

On Oct 1, 6:38 am, Ronald Raygun wrote:

I apologise for my earlier misunderstanding, which came from being
fixated on the boat example being driven by a water propeller using
power harvested from an air turbine. I see that in the case of the
land based car you're discussing it is not using the air screw as a
source of power with which to drive the wheels, but the wheels as a
source of power with which to power the air propeller.

I don't doubt that this is possible, but I believe your analysis below
is wrong. I offer a corrected version below.

Well, you're making progress. I suspect if you'll continue a rational
discussion with JB he'll be able to convince you that your "corrected"
analysis is in fact wrong.

I'll approach what he has to say with an open mind.

I would point out, though, that my result (of the prop requiring 3/4 hp)
means that 1/4 hp is available to be dissipated on losses and on the
car's air resistance etc. Since we are considering the car as being in
steady state motion, no power is needed to accelerate it.

This spare 1/4 hp has to come from somewhere, and it is from the wind
being slowed down, its kinetic energy being transferred into those
losses. Right?

It is no coincidence that the kinetic energy of 11.7 lb of air at
27.5 ft/s is exactly 1/4 hp s.

@Ronald Raygun
Suppose car speed and wind speed are *both* 55ft/s. The
wheels are still delivering 1hp. The headwind is zero, so you
would calculate the power needed by the prop to produce 10lb
of thrust as 0hp. Can't be right.

But it is right, because at that point in the calculations we are
still describing the scenario using theoretical, lossless components.
We've defined the transmission method as lossless and the propeller
also as 100% efficient.

At 0hp the prop won't be turning, and would be producing no thrust.

If the vehicle is traveling at exactly windspeed, the propeller is
working in what's called 'static conditions' and according to standard
propeller efficiency terms is doing no work (different "efficiency
terms" are used for fans).

In static conditions, a 100% efficient propeller would be infinitely
large and would be turning infinitely slow but would still be
producing 10lbs of thrust at 0hp. Its not unlike leaning a 2x4
against the wall – it can happily exert 10lbs of force on the wall all
day long while needing 0hp to do it. Same with a lossless propeller
in static conditions.

ThinAirDesigns wrote:

@Ronald Raygun
Suppose car speed and wind speed are *both* 55ft/s. The
wheels are still delivering 1hp. The headwind is zero, so you
would calculate the power needed by the prop to produce 10lb
of thrust as 0hp. Can't be right.

But it is right, because at that point in the calculations we are
still describing the scenario using theoretical, lossless components.
We've defined the transmission method as lossless and the propeller
also as 100% efficient.

That's a poor excuse and I don't accept it. We've disregarded losses
only because allowing for them quantitatively makes the calculations
tedious and because we'd need to understand the finer points of
aerodynamics (you might - I admit I don't, but I don't think I should
need to in order to understand the basic energy budgeting here). We
just allow for them qualitatively by observing that the power harvested
by the wheels must exceed that consumed by the prop by a margin
comfortable enough to feed any losses.

The exactly-at-windspeed scenario is a bit of a crazy special case in
that there is no air resistance; if the wheels are also frictionless,
then the car doesn't actually need any power to remain at its speed.
Therefore the presumption that the wheels must generate 1hp and the
prop must therefore provide 10lb of thrust is a bit silly, since
clearly the prop really doesn't need to provide any thrust, and so
it's perfectly all right to run it at 0hp, and our generator will
happily supply 0hp using 0lb of wheel drag.

It's a bit like the skateboarder trundling along at the same speed
as the pedestrians. He still has some thrust requirement to meet
his losses, and he can still accelerate the pedestrians backwards,
but if they are starting out as stationary relative to him, he's
going to need ever longer arms to grab more, they're no longer going
to be coming at him all by themselves, as they did when he was going
faster than they were.

The propeller must have a similar problem. Suppose we introduce an
artificial loss into an otherwise lossless system, for example by
inclining the road slightly uphill. The car will then require
nonzero power to climb the hill at windspeed. The car will still
see a zero headwind, but the prop has to deliver nonzero quantities
of air out of the back of it each second, and at nonzero speed, and
the air has to come from somewhere. Like the skateboarder who uses
up all the pedestrians he can reach, the fan, if it's to keep working,
must suck the air from somewhere, so there isn't really any point at
which its input air speed can be considered zero despite zero headwind.

On 01/10/2010 18:16, Ronald Raygun wrote:

The exactly-at-windspeed scenario is a bit of a crazy special case in
that there is no air resistance; if the wheels are also frictionless,
then the car doesn't actually need any power to remain at its speed.
Therefore the presumption that the wheels must generate 1hp and the
prop must therefore provide 10lb of thrust is a bit silly, since
clearly the prop really doesn't need to provide any thrust, and so
it's perfectly all right to run it at 0hp, and our generator will
happily supply 0hp using 0lb of wheel drag.

Yes, but the freewheeling case isn't interesting. Introducing the wheel
driven prop is what makes it interesting.

You've introduced the assumption that the car must be at steady state -
there's no reason for that to be the case. Drop that assumption, work
with the forces and see where you get.

@Ronald Raygun
That's a poor excuse and I don't accept it. *We've
disregarded losses only because ...

Ron, math doesn't care *why* we've disregarded the losses. Present all
the arguments you wish against my calculations, but arguing that *why*
we ignore losses impacts the math is just silly.

Fact is, at that point in the analysis it's clear were employing a
100% efficient propeller and a 100% efficient propeller will in static
conditions produce 10lbs of force from 0hp just as a 2x4 against the
wall will.

JB

On 01/10/2010 14:56, Rick Cavallaro wrote:
On Oct 1, 5:53 am, Clive George wrote:

I'm not doing any deflecting. I'm taking you through it, slowly and
carefully.

I applaud your effort, but I've been through this with too many
Dennis's. I assure you that assistance is futile.

I know. I played a similar game a few years back with a rather simpler
problem - I persuaded a few, but there was at least one person who
disappointed me, since I thought he ought to have the ability to
understand it - he stuck to saying I was wrong rather than explaining why.

http://groups.google.com/group/uk.re...7c1ab40c8097e3

Can you stick with my argument, rather than starting your own?

That's not how this game works. Dennis will NEVER late you take him
anywhere near the answer. When he sees any sign of it, he'll just
make an immediate left-turn and start down some other pointless alley.

:-)

Can you answer my questions?

I have one squillion dollars that says no. But it's entertaining to
watch.

Dennis would rather carry on looking stupid than admit he's wrong, so
your squillion dollars is safe.

@Ronald Raygun
The exactly-at-windspeed scenario is a bit of a crazy special case ...

small snip

Therefore the presumption that the wheels must generate 1hp
and the prop must therefore provide 10lb of thrust is a bit silly,

You did notice that it was *you* who brought up the "crazy special
case" of exactly at wind speed. :-)

It is your own special case that has brought into play the seemingly
odd (but true) case of the 100% efficient propeller which can produce
10lbs of force on 0hp.

The way the term "efficiency" is applied in any given situation can be
a bit arbitrary. As I previously mentioned, propellers and fans use
the term very differently because they have very different
applications and thus very different definitions of "work". What is
most important is that the the term and it's relevent formula be used
consistently throughout any given. We can't use one definition at 2x
wind speed and another at exactly wind speed. I have been consistent
in my application.

It just so happens that the standard propeller efficiency formula
turns up looking a bit odd (but still valid) in static conditions.

On 01/10/2010 19:25, ThinAirDesigns wrote:
@Ronald Raygun
The exactly-at-windspeed scenario is a bit of a crazy special case ...

small snip

Therefore the presumption that the wheels must generate 1hp
and the prop must therefore provide 10lb of thrust is a bit silly,

You did notice that it was *you* who brought up the "crazy special
case" of exactly at wind speed. :-)

Actually I brought it up a few days ago... but I'm not sure anyone
noticed...


It is your own special case that has brought into play the seemingly
odd (but true) case of the 100% efficient propeller which can produce
10lbs of force on 0hp.

The way the term "efficiency" is applied in any given situation can be
a bit arbitrary. As I previously mentioned, propellers and fans use
the term very differently because they have very different
applications and thus very different definitions of "work". What is
most important is that the the term and it's relevent formula be used
consistently throughout any given. We can't use one definition at 2x
wind speed and another at exactly wind speed. I have been consistent
in my application.

It just so happens that the standard propeller efficiency formula
turns up looking a bit odd (but still valid) in static conditions.


Ron,

imagine a case where in order to generate that 10lbs of thrust at zero
relative windspeed the prop needs half a horsepower. Real losses, quite
achievable - in fact I think TAD did better in real life.

The cart has that 10lbs of thrust, and it's going downwind at exactly
windspeed, and that 10lbs through the wheels at that speed makes a whole
HP. You have half a HP left over to accelerate with!

Andy

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember ThinAirDesigns
saying something like:

@Grimly Curmudgeon

Light dawns. If you'd mentioned pusher prop at the start...

I have no idea how I could have mentioned it any sooner than the very
first sentence of my very first post on this thread -- which is what I
did.

No need to get sarky.
That's my job.

It might never have occurred to you that the post might be missing?

@Grimly Curmudgeon
No need to get sarky.
That's my job.

Here in the US we believe strongly in equal rights. :-)

It might never have occurred to you that the post
might be missing?

No, not once -- since the way I specifically knew that I mentioned it
in the first sentence of the first paragraph of my first post was I
went back in the thread until I found it ... right in the order I
posted it. I just now again went back and sure enough it's still
right there.

There's no crime in you personally missing that post, but if you tell
me you wish I had said something sooner that I opened with, I get to
correct you.

On Oct 1, 5:05*pm, ThinAirDesigns wrote:

if you tell me you wish I had said something sooner that I opened with, I get to
correct you.

If you say something that's true, and needs to be said, I still get to
correct you. That's how we roll here.