DDWFTTW - Directly Downwind Faster Than The Wind

The claim is made that the force on the wheels which produce the torque which turns the propeller does not affect the vehicle's motion. This claim is fundamentally wrong as kerosene states. It is a force acting on the vehicle including the wheels.

 

Thank you for your appreciation, I will try to dispel your doubts. When I originally thought about it I had the same concerns as you.
The wheels of course transfer force to the propeller but generally all the force which spins the wheels is transferred to propel the propeller. From the very beginning of motion. The unbalance according to Newton's second law of motion will accelerate the vehicle.
The force rotates the wheel but does not brake the vehicle (related to ground or air) and this force is used to generate thrust force which moves the vehicle in relation to air masses. It is like a plane: the force from the engine rotates the propeller but does not slow down the vehicle; (it is an external force).
The Only force slowing down a vehicle in relation to ground is rolling friction, and force slowing down in relation to air mass is air resistance.
The initial force is extra force which must be balanced by one of these forces. If the forces are not in balance the object will accelerate. - Newton's second law of motion.

It will not, think about it another way: you have an engine in a car and you start to rotate the wheels, what happen? If force will be bigger than rolling resistance then the car will be accelerating. The engine does not slow down the vehicle. This is the opposite situation but now the ground is our engine.

 

Stop and think the quoted text. Let's take a car accelerating. The engine generates torque at the wheels and the car starts to accelerate (yes assuming rolling and air resistance are beat). Actually lets think of steel train wheels on track so we can assume the rolling resistance is barely there. The torque from the engine creates a force at the contact patch of the wheel which accelerates the vehicle. No magic here F=MA, so mass and force dictate how fast we accelerate - until aerodynamic resistance becomes a factor and so on. We agree I hope?

Now let's disengage the engine ("press proverbial clutch"). Vehicle is rolling nicely slowing only a little as it is impacted by tiny rolling resistance and bit more significant air resistance. Now let's hit the brakes. Now there is a slowing down force - again it affects via torque and ultimately contact to the wheel contact patch. What do brakes do? They convert movement to heat. In this case powered by the ground? Sort of - it is matter of point of reference.
Can we agree that brakes create a force to the vehicle that points towards the back of the vehicle? and that the slowing is transferred at the wheel contact patch?

Now lets get back to the point where we were freely rolling. Instead of brakes lets engage a cement mixer to the axle. It takes a decent chunk of energy to spin wet cement - now the wheels are transferring power from the wheel to do this work. Wouldn't you agree that if we hooked the wheels to operate some very resistive device it would try to slow down the vehicle?

I think the next step is rather obvious. Instead of cement mixer let's put the power pulled from the wheels to do something else: spin the propeller. Same exact thing - the power required by the prop will put a torque on the system causing a force pointing backwards (towards of the rear of the vehicle).

Now how can this force trying to slow it down be used at the same time to push it forward? Because the wheels see much higher speed with the medium they interact with ie. the ground, than the propeller interacting with air.


Here is the example from my youtube comment:

 

Yes we agree, however, remember that mass only slows down the acceleration, but does not make it disappear, if the force balance is positive, the vehicle will start to accelerate even if it weighs a million tons. But it will do it very slowly, of course.

Yes I agree

Yes I agree that if we use the generated force for something like that it will slowing down vehicle.

It is not obvious, If there are no losses (mechanically) and you use this generated force to propel the propeller then it will be balanced. And as long the thrust force will be bigger than the rolling resistance force vehicle will be accelerating. This extra force from ground is not braking the vehicle but it propels the propeller.

There is a mistake in your explanation because you are neglecting the force that will start to brake the vehicle at greater speed in relation to air. The same force that begins the movement relative to ground at the beginning of the experiment. And you are assuming that it will be accelerating indefinitely because (as you said) the ratio will always be positive. But it won't.
I agree that all is a matter of this extra force from ground relative to air masses but you are assuming that this force can accelerate vehicle endlessly- It is a mistake.

I think it would be a good idea to analyze this together from the beginning. Let's first establish what happens at the very beginning of the experiment. We will do it step by step. I think it will be easier to find an agreement. let's analyze this moment (3:23 from my video):

• Wheel is connected to the propeller.
• Air resistance is bigger than rolling resistance. – According to Newton's second law it will accelerate
• Air resistance depends on TAS speed.
• Rolling resistance is constant.
• TAS speed is bigger than 0.
• Ground Speed is 0.
• Vehicle braking relative to the air.
• Accelerating relative to the ground.

Do you agree with all the assumptions?
If yes, what would happen with the vehicle, wheel-propeller mechanism, and how forces will change?
If not what I am missing at this point, is there any force I am missing? (ignoring gravity and mechanism efficiency)
If we find a common language for this point in the experiment, we will go further. It will be easier to understand what is going on with forces later.

 

yes.

Right. And the power taken to spin the propeller is just the same as using that power to operate a generator - or a cement mixer.

That is not correct. But let me clarify that when I said it will brake the vehicle - I didn't mean it would actually slow it down - it tries to but it doesn't cause an actual speed reduction because the trust of the propeller is larger.
This part is the core of your mistake (or our misunderstanding):

It does both. And it has to do both - it takes power to operate the propeller. For the wheels to operate the propeller it needs to exert for on the ground. From that relative movement (wheel to ground) it takes power to operate but it cannot do this without exerting force. You cannot get power from nothing.

It was intentional, not a mistake. Yes if there was no resistance it would accelerate infinitely. At 6m/s there was 6x excess force available (speed ratio of 6:1). That allows for massive losses and very inefficient setup to still operate the vehicle. As we accelerate further the excess diminishes (as the ratio gets smaller and smaller). At some point above all 3 things will set the limit on speed: aerodynamic resistance, rolling resistance and propeller efficiency. When these losses equal the excess a balanced state is achieved.

I don't, see above.

Rolling resistance is meaninglessly small at all points. But yes agree it is the general drag that gets the cart to move in the 1st place.

Yes it is square of the speed. As the cart gets moving and speeds get closer to the air speed the pushing force from drag alone drops towards zero.

sure, as I mention above rolling resistance is so small that it is meaningless in even modestly efficient test vehicle.

yes

yes

if I understand what you mean, yes. It is effectively just a blunt object with drag.

Yes it will start accelerating right away.
[/Quote]Do you agree with all the assumptions?[/Quote]
yes. with notes above.

What you constantly miss is the "cement mixer", The force that starts affecting the wheels as the propeller starts doing work. If when you say "rolling resistance" you include the work of the prop to it then we are far less apart. Rolling resistance to me (and I believe it is generally accepted) refers to losses from tire deformation, rolling friction and alike. Rollin resistance would be the same whether there was a prop or not.
Does your use of rolling resistance include the resisting force from the torque of driving the propeller?

 

I will skip the beginning of your post to clean up the discussion, I hope that those "issues" explain themselves along the way.
It was a screenshot from my video (3:23). I am glad that we have found common ground and we agree at least at the beginning of the experiment.
Rolling resistance is rolling resistance it is constant all the way.
Propeller rotation “cement mixer” creates resistance, but this resistance is what creates thrust. So there will be no losses on motion. Movement of wheels will be converted to movement of propeller. Effect will be the same.

So what happens at the beginning, will the vehicle move or not (relative to ground)? Wheels and propellers (are connected) start to rotate or not? What happens when the propeller will rotate?

 

Absolutely. except those forces are not equal (near the windspeed). The whole point of the leverage is that the wheel operates in speed difference that is much higher (wheel to ground) than where to propeller operates (propeller to air) and that is why smaller force at high speed (at wheels) can be geared to much higher force at the prop. This is exactly what I have explained over 10 years now

Veritasium did the same and they are not wrong. And when the speed goes up the leverage gets closer and closer to 1 thus reducing the excess at and to beat the losses and thus max speed is achieved (naturally also the resistance from air drag and rolling resistance grow with square of speed).

And yes the at the beginning the prop will rotate the very moment the vehicle inches forward. When I say there is braking force - in no way do I mean to suggest that the braking force would be higher than the thrust - or at the very beginning the plain drag. The vehicle will move and prop will start to spin and create thrust.

I have spent tens if not hundreds of hours explaining why the cart does work. In no way do I suggest it wouldn't work.

here are my two videos on the topic.

2010:


and 2021:

 

This is not an argument in discussion that you spent hours on this topic isn’t it? I did not say once that you think it doesn’t work. It is all simple forces and Newton’ second law of motion, no leverage is necessary to work this mechanism. Leverage is misleading argumentation. The key to understanding is not to over-think it! But let's back to our analysis: Do you agree that the wheel and propeller start to rotate and the force of Thrust will appear and when the force of air resistance will drop the force of thrust will grow and the only force braking vehicle at this point will be rolling resistance. Is it correct or not?

 

Start with the very basic equation form of Newton's second law: Mass * Acceleration = Sum of all external forces
The forces acting on the wheels at the contact patches with the ground are external forces. Those forces will include components relating to "rolling resistance" of the tires and components related to turning the shaft which is ultimately connected to the propeller. The entire forces must be included for this equation to be valid, not just a portion of the forces. The user of this equation does not get to arbitrarily decide what external forces to include and what external forces to neglect.

This claim and the associated claim that "no leverage is necessary to work this mechanism. Leverage is misleading argumentation" appear to be at the roof of the fundamentally flawed analysis made by RysiulPL.

The force acting on the wheel is not directly transferred to the propeller. The force of the propeller on the air may be equal to, less than or greater than the force of the ground on the wheels. It does not have to be equal to force acting the wheels less rolling resistance. The force the propeller generates varies with the speed of the air approaching the propeller, the rotation speed of the propeller, the pitch of the propeller and other geometrical factors. This is fundamental to how the device works. The rotational speed and geometry of the propeller must be selected such that for a range of device speeds greater than the wind speed the force of the propeller on the air is greater than the sum of all the other external forces opposing the motion of the device (including rolling resistance and aerodynamic drag). In that situation the sum of all the forces will be positive in direction of motion of the device and the device will accelerate.

 

didn't mean the hours as an argument.

And no I do not agree. Rolling resistance is not relevant in the device.

Rolling resistance - Wikipedia https://en.m.wikipedia.org/wiki/Rolling_resistance

 

I do not understand your objection to my explanation. We assumed 100% efficiency of the wheel-propeller mechanism like Veritasium did so this point should not be a complaint against my analysis. But we can even assume 50% efficiency and this only changes that we will need a bigger initial force for this to work (initial air resistance). The same with propeller efficiency (its construction). The efficiency of the mechanisms is a secondary issue. The only question is how big the difference between ground speed and air masses must be for it to work. If the mechanism is inefficient, it will need a big force to work. If it is very efficient, a small force will be sufficient. It is the same with leverage. It can help with efficiency but on its own it doesn't make it work. You can leverage force but the balance of forces must be positive at first.

 

DCockeye is right above.

In fact with different gearing the propeller would start spinning and make the wheels move the vehicle towards the wind.
The wind flows at start over the static prop and will cause torque on prop axis. Cart is being pushed forward due to blunt drag of the whole setup and that force will _also_ try to rotate the prop jsut the other way round.

the gearing and prop pitch dictates which torque is higher. with different, ahem, leverage the the propeller would act as a turbine and make the wheels spin. This would be a directly upwind device. Just by changing the gear ratio between the wheels and the prop.

but back to downwind cart.
Propeller is not very efficient - especially in broad range of speeds. So a lot goes to waste from the energy taken from the wheels. Thus we need to have clear excess for it to work in practice. Rather crude devices have done it on the treadmil which is a testament to the power of leverage: they work despite clearly significant losses.

 

Rolling resistance is one of two most important forces to overcome by cars to move. Air resistance and rolling resistance.

 

btw:
the fact that the force from the prop is different than the force at the wheels does not mean losses.

 

And we are get to the point where you agreed that leverage help it to work but at first balance of forces need to be positive. as I wrote above:
The efficiency of the mechanisms is a secondary issue. The only question is how big the difference between ground speed and air masses must be for it to work. If the mechanism is inefficient, it will need a big force to work. If it is very efficient, a small force will be sufficient. It is the same with leverage. It can help with efficiency but on its own it doesn't make it work. You can leverage force but the balance of forces must be positive at first.