Downwind Faster than the Wind - Successful Run by manned cart

At the risk of thread drift, perhaps some of you smart physicists/aerodynamists can contribute to what seems to be a related conundrum.

When not sailing I fly model R/C aircraft. Sometimes with a small field, I have diffiulty landing as, even with the engine on tick-over, the 'glide' in exceeds available space. I then cut the engine, I assume to steepen the descent.

'Experts' claim this is wrong and that a slowly turning prop exerts more drag than a stationary one (the 'windmill' forces are not enough to turn over the motor).

I find this counter-intuitive as it seems to me that the engine must be adding some energy to the system thus prolonging the glide.

Any comments - other than the fact that the idiot hasen't got anything better to do on a rainy day?
 
I think I'm beginning to see a possible explanation. There will be a point, well below the point of no apparent wind, when the propellor, with the right pitch and gearing, will be going faster than windmill speed and start producing some forward thrust, whilst there is still a bit of overall aerodynamic drag from the tailwind to push, and turn the wheels. In effect, the propellor gets a forward apparent wind before the tailwind disappears. There must be a finely balanced point where there is little overall forward thrust; a variable pitch prop would make a big difference, as this could give thrust right from the start.
 
tobermoryphil said:
I think I'm beginning to see a possible explanation. There will be a point, well below the point of no apparent wind, when the propellor, with the right pitch and gearing, will be going faster than windmill speed and start producing some forward thrust, whilst there is still a bit of overall aerodynamic drag from the tailwind to push, and turn the wheels. In effect, the propellor gets a forward apparent wind before the tailwind disappears. There must be a finely balanced point where there is little overall forward thrust; a variable pitch prop would make a big difference, as this could give thrust right from the start.
Click to expand...

The real secret to understanding what's going on is to ignore COMPLETELY the apparent wind felt/seen by the cart. The only thing that matters is the apparent wind felt/seen by the blades. As the blades are rotating, the two are VERY different.
 
My take on Keens Ed's video for the benefit of anyone who can't see what's happening.
ie the wind blows the cart along - get it? The fact that the pitch of the prop is reversed explains the difficulty of getting the thing to move initially and, partially, its slow acceleration - so the wind is blowing the cart which is driving the prop counter to the wind. ie if the prop was free spinning it would turn in the opposite direction (viewed from behind) Right- geddit? As the cart approaches wind speed the braking effect of the prop reduces and the cart's inertia is transferred via the drive train to the prop which is now turning in the correct direction relative to the wind. However this is short lived and the cart slows. There is nothing to suggest that it has to be braked to slow it down or that it can sustain constant velocity in excess of wind velocity. Pierrome and supporters seem to think that if you endlessly repeat the same statement it becomes a truth. (are you all vicars by any chance?)
 
well, that answers one of the questions I asked way back - why would the DDWFTTW device only JUST exceed wind speed (as Oracle etc can do 3x windspeed VMG). Seems they plan to get close to 3x as well.

I never understood why anybody who believed Oracle etc could sail DWFTTW doubted you could go DDWFTTW. You have exactly the same conceptual problem, of transitioning from one state (when VMG < true wind) to another (when VMG > true wind) when there would be no apparent 'useful' wind, that the doubters complain about.

MD
 
mobeydick said:
well, that answers one of the questions I asked way back - why would the DDWFTTW device only JUST exceed wind speed (as Oracle etc can do 3x windspeed VMG). Seems they plan to get close to 3x as well.
MD
Click to expand...

That's right. Look at it this way. When it reaches windspeed there is no air drag on the cart/driver, (only rolling resistance), and it can accelerate until these forces balance out the thrust from the prop.
 
mobeydick said:
well, that answers one of the questions I asked way back - why would the DDWFTTW device only JUST exceed wind speed (as Oracle etc can do 3x windspeed VMG). Seems they plan to get close to 3x as well.

I never understood why anybody who believed Oracle etc could sail DWFTTW doubted you could go DDWFTTW. You have exactly the same conceptual problem, of transitioning from one state (when VMG < true wind) to another (when VMG > true wind) when there would be no apparent 'useful' wind, that the doubters complain about.

MD
Click to expand...

The difference between this and BMWO is that it's claiming that it can go dead downwind at twice the true wind speed. BMWO would do 3x the wind speed, but on a broad reach angle.

Now unless this cart is not going dead down wind, I would like to know where it is getting it's energy from once it exceeds the true wind speed. Gears on the wheels, and a big prop are all very well, but once the true wind speed ceases to push the cart, then friction would surely start to slow the thing down again. To say that the propeller drives the cart, and the wheels on the cart drive the propeller is not something that can work for any length of time once the wind that initially pushes the cart ceases to be present.
 
Woodlouse said:
The difference between this and BMWO is that it's claiming that it can go dead downwind at twice the true wind speed. BMWO would do 3x the wind speed, but on a broad reach angle.

Now unless this cart is not going dead down wind, I would like to know where it is getting it's energy from once it exceeds the true wind speed. Gears on the wheels, and a big prop are all very well, but once the true wind speed ceases to push the cart, then friction would surely start to slow the thing down again. To say that the propeller drives the cart, and the wheels on the cart drive the propeller is not something that can work for any length of time once the wind that initially pushes the cart ceases to be present.
Click to expand...
You have to imagine the blade of the propeller as being like the wing on BMWO and how both set an incidence angle to the apparent wind to generate lift and drag. Just as BMWO ends with a down wind velocity component with 3 x wind speed, so the propeller blade can have its axle travelling at 3 x wind speed down wind.

BMWO counteracts the drag forces with its keel. The cart counteracts its drag forces on its rear driving wheels.
 
Pierrome said:
That's right. Look at it this way. When it reaches windspeed there is no air drag on the cart/driver, (only rolling resistance), and it can accelerate until these forces balance out the thrust from the prop.
Click to expand...
I'm sorry but you are missing something. You say there is "only rolling resistance", but the drive train (from the wheels to the prop) will be acting as a brake on the wheels. Having seen the video I am willing to keep an open mind, but something has to be driving the prop, and that "something" is coming from the wheels. And if it is to do any usefuly work in actually driving the prop, it has to be at the expense of the speed of the cart.
Now, if the thrust from the prop can overcome that braking effect on the wheels, with something to spare, then the cart will continue to accelerate. But that is a very big "IF".
 
Woodlouse said:
Now unless this cart is not going dead down wind, I would like to know where it is getting it's energy from once it exceeds the true wind speed. Gears on the wheels, and a big prop are all very well, but once the true wind speed ceases to push the cart, then friction would surely start to slow the thing down again. To say that the propeller drives the cart, and the wheels on the cart drive the propeller is not something that can work for any length of time once the wind that initially pushes the cart ceases to be present.
Click to expand...

Then may I ask, and this is a genuine question: what do you see in this video?



I see the streamers (which are well away from the prop) move from streaming ahead to streaming behind between the 3 minute and the 3 minute 20 mark. They then stream aft until 4:40ish, when the brakes are applied.

So for that 1 minute 20ish period, how fast relative to the true wind do you believe the cart is going? Slower/same speed/faster?
 
Malo37 said:
My take on Keens Ed's video for the benefit of anyone who can't see what's happening.
ie the wind blows the cart along - get it? The fact that the pitch of the prop is reversed explains the difficulty of getting the thing to move initially and, partially, its slow acceleration - so the wind is blowing the cart which is driving the prop counter to the wind. ie if the prop was free spinning it would turn in the opposite direction (viewed from behind) Right- geddit? As the cart approaches wind speed the braking effect of the prop reduces and the cart's inertia is transferred via the drive train to the prop which is now turning in the correct direction relative to the wind. However this is short lived and the cart slows. There is nothing to suggest that it has to be braked to slow it down or that it can sustain constant velocity in excess of wind velocity. Pierrome and supporters seem to think that if you endlessly repeat the same statement it becomes a truth. (are you all vicars by any chance?)
Click to expand...

The first part of what you say is all correct. Then you spoil it all!
From the 2nd video the "short time" it is exceeding windspeed is 1min 36secs. How long would you like? They only have a certain amount of space even on a dry lake, and a ten minute video of it just going downwind would be boring.
If you looked at their website you will see have extensive braking systems. What evidence do you want to see that they put the brakes on? tyresmoke?

Interesting how the argument has changed from "it can never go faster than the wind" to "it does not go faster than the wind for long enough"
 
bbg said:
I'm sorry but you are missing something. You say there is "only rolling resistance", but the drive train (from the wheels to the prop) will be acting as a brake on the wheels. Having seen the video I am willing to keep an open mind, but something has to be driving the prop, and that "something" is coming from the wheels. And if it is to do any usefuly work in actually driving the prop, it has to be at the expense of the speed of the cart.
Now, if the thrust from the prop can overcome that braking effect on the wheels, with something to spare, then the cart will continue to accelerate. But that is a very big "IF".
Click to expand...
Of course, as the cart speeds up, all the resistance forces rise and eventually a stable maximum speed is reached, given sufficient wind to get it going to start with. It's just that what this maximum speed actually is, is not automatically at or below wind speed.

The ultimate limit is significantly reliant on the efficiency of the propeller.
 
It a wet day and I did plan something more constructive BUT:

a bit of time on google has converted me from a mild sceptic into a believer. The most pertinant comment is that DDWFTF (or whatever it is) ia no different to going upwind with a propellor driven car/ boat.The latter is a well proven phenomenen that few doubt.

I can't do proper links but, if you can spare the time the following, explains it all both in laymans terms and the full physics for those that need convincing.


http://www.youtube.com/watch?v=4ZjX_DIosM8
 
davidej said:
. The most pertinant comment is that DDWFTF (or whatever it is) ia no different to going upwind with a propellor driven car/ boat.The latter is a well proven phenomenen that few doubt.

I can't do proper links but, if you can spare the time the following, explains it all both in laymans terms and the full physics for those that need convincing.


http://www.youtube.com/watch?v=4ZjX_DIosM8
Click to expand...

Of course. It's really very simple to understand. What is really very disturbing is that so many who claim numerous qualifications do not seem to understand it.
 
RAI said:
You have to imagine the blade of the propeller as being like the wing on BMWO and how both set an incidence angle to the apparent wind to generate lift and drag. Just as BMWO ends with a down wind velocity component with 3 x wind speed, so the propeller blade can have its axle travelling at 3 x wind speed down wind.

BMWO counteracts the drag forces with its keel. The cart counteracts its drag forces on its rear driving wheels.
Click to expand...

There is no comparison here between BMWO, and this contraption. BMWO broad reaches, so at no point does the the true wind drop to nothing before the apparent wind takes over.

With this, there has to be a point where the true wind stops driving the cart, and at that point friction in the system would have to start slowing it down, instead it carries on speeding up. This is what I don't get. Where is the energy coming from to make this happen?

Keen_Ed said:
Then may I ask, and this is a genuine question: what do you see in this video?
Click to expand...
I confess I see a cart that appears to go faster than the windspeed whilst going dead down wind. But I'm afraid I don't believe everything I see on the internet.

It just so happens that last night I saw Scotty and Captain Kirk beam aboard the Enterprise when it was going at warp speed. But I don't believe that actually happened either.
 
You must log in or register to reply here.

Share this page

Top