The Wind Powered Sail-less Boat

No. I can bring you people all day long that do not agree with this. I'll provide physics and aero professors that will deny it (even though they're dead wrong).

The rest of your post I agree with completely.

You don't even have to be able to do or follow the math. There's ample evidence of well instrumented and well documented ice-boat runs doing downwind VMG of 3X to 4X TWS.

 

spork
You are putting yourself on the vehicle. Not everyone can do this so readily.

Some see an electrically driven treadmill distinctly different to wind blowing down a long road. Damn them for standing back so far but that is how they see the world. So I understand the point of view. They have great difficulty adjusting their frame of reference.

Maybe the lesson here is to show a little figure sitting on the vehicle and give the figure a name so others can relate to the little persons world. A bit like the faster than the ruler clip with the skeptics and informed on the side line.

When you get your full size vehicle going you will have to find a big treadmill to confirm to people that the results are identical.

Rick W

 

That would be fun, and perhaps a great side myth if we get the Mythbusters hooked. Afterall, it turns out the treadmill seems to be as much the myth as the DDWFTTW cart. Perhaps they could use that same stuff they pulled out from beneath the airplane.

 

I recently saw an episode of the mythbusters where they placed a plane on a conveyor belt and started running the belt backwards as soon as the plane started to move forward. The pilot swore it would not take off even though it gathered airspeed. I am glad he is not a pilot I fly with. Had no idea. Some people get really screwed up with frames of reference. He he had taken off and landed he was amazed with the result.

I could not believe that anyone would ever argue over what they proved.

Rick W

 

And Newton wept.

I always get a kick out of people that say "let's ask a real pilot" about some aerodynamics problem. Just as you'd ask a "real bus driver" about how an internal combustion engine works.

As a "real pilot" I can tell you much of the "aero" taught to pilots is absolutely shameful. As a "real" aero engineer, I can tell you what's taught at university is at least a bit better. The only thing worse than the aero taught to pilots, is the aero taught to military pilots.

And it doesn't mean they aren't great pilots. It just means I wouldn't risk a dollar on them getting the "plane on a treadmill" question right.

 

That large length of fabric that they flew the plane off would make a good large-scale treadmill. So they already have something suitable for that part.

Would be interesting to see the vehicle initially move with the belt and then start to travel against its direction of travel.

Rick W

 

I just watched that from youtube. The pilot thought there would be no flow over the wings (= no airspeed). What a stupid idea!

The (just a little) bit trickier questions are:

1. Is there any difference, that could be noticed by the pilot?

a) Is there any difference in the needed runway length?
b) How much more power (if any) is needed for the take off?

Joakim

 

i saw that today on youtube, i find it hard to believe people watch stuff like that without laughing at how ridiculous it is!

the answer to your question with the 1 in 3 gearing- i'm sure its answered somewhere in here but i haven't read all the posts but my immediate thoughts were it was .66m/s and 6m/s

actually ,having said that it can't be that simple.i'll go back for another look!

 

a) none as its airspeed that is required,groundspeed has nothing to do with it.
b) nothing that would be noticed unless the wheel bearing were bad as the only thing the "treadmill" was doing was spinning the wheels at faster than groundspeed, aircraft groundspeed plus vehicle groundspeed

 

Martin
Unless you enjoy the exercise I have provided answers at post #442. I drew the original drawings without the reverse gearing as well so this put a spanner in the works.

I built a little vehicle to show it in practice and placed a clip here:
http://www.youtube.com/watch?v=ZVSjA7Rhccs
The gearing is done with a rubber belt and there is some bounce but you can see it moves faster than the board pushing it.

The demo shown here is somewhat more elegant:
http://www.youtube.com/watch?v=k-trDF8Yldc

I have also made a series of 4 slides that provides a step-by-step understanding of the physics. They are post #4 on this thread:
http://www.boatdesign.net/forums/propulsion/ddwfttw-directly-downwind-faster-than-wind-25527.html

I think Slide 2 is the most important because people trying to understand it have to realise that a propeller can still exert thrust in an air stream that is flowing from behind. Same principle applies to planes with a tail wind. These do not drop out of the sky but just go faster. (Can drop out if there is sudden wind change of course but all these conditions are for steady state or slow acceleration.)

Rick W

 

The wheel resistance does not (mainly) come from the bearings. It comes from the rubber "flexing" losses. Both the bearing and tire rolling resistance can be usually taken as independent of speed, thus the drag due to those will not change, BUT the higher speed means more power. Where is that extra power taken from? How does the plane change the power needed to run the belt? Is this perpetual motion?

How much power from the engine is needed by a car on a belt doing 100 km/h at belt going 100 km/h at the other direction (=stationary)? What about 200 km/h, 100 km/h (= moving 100 km/h forward)? Compare to power need at 0, 100 and 200 km/h on a normal road. How does the car change the power needed to run the belt?

Joakim

 

ok, the physics side of this maybe a bit over my head but the way i'm understanding it is that the only things that matter are the speed of the air over the wings and the thrust from the propellor which seem to me to be independent of the forces on the ground. the treadmill will provide a little more drag in having to spin the wheels faster but thats the only extra power required

in the case of a car on a treadmill it doesn't seem to me to be the same analogy, i can't explain why as i don't have the scientific knowledge.

laymans terms i know, if i'm wrong in my thinking please feel free to correct me.

 

i saw this earlier, i didn't realise the top and bottom wheels were connected,makes a lot more sense now!

 

Thanks for the link, Rick. I agree with the steady state part of slide 2. I need time to think about how to represent the accelerating phase. I'll answer in due course.

This is the best exercise I've found yet for stopping my brain from atrophying. Thanks for your time and patience.

 

You do not need to get too worried about the rate of acceleration. Imagine a very slight tail wind just gradually builds to 3m/s over a period of time. That is only 6kts so not very strong breeze.

This is a key to understanding DDWFTTW. You have to realise that the propeller can apply the same thrust to the moving airstream. It is the fact that the vehicle is on the ground that gets you. If it was an aeroplane with a tail wind you would readily agree that the ground speed was airspeed plus the windspeed. Same thing here really because I have said there is no increase in drag as the speed increases and the prop is already generating enough thrust to overcome this. Partly the reason to start with such a high drag so I did not confuse the story by having to increase it with the speed.
In practice the drag would be much lower than I have shown in either case but we still have another component to get the end result.

Rick W.