Down wind faster than the wind

[spork]

It is important that the prop spins: this is what is giving the extra thrust to get above wind speed.

Right you are.

If the prop was just acting like a sail, the cart would never go above wind speed.

But the prop is acting exactly as a sail on a 45 degree downwind tack.

The prop spins against the direction of the wind.

The prop spins opposite the direction it would if it were being pushed by the wind from behind (as a turbine).

Bernoulli's principle is not important here.

Sure it is. That's one valid way to explain how a wing, sail, or prop develops lift.

 

[spork]

Is this one up on YouTube yet?

http://www.youtube.com/watch?v=dgHBDESd38M&fmt=18


We got everything as balanced as we could, increased the incline so it could just hold it's own, and put small boards under both sides of the belt so it would tend to center itself if it started to wander off the side.

After 1:48 it finally fell off the back of the treadmill. We've been accused of cutting the tape at this point because it proves our cart doesn't work. Of course it was going to eventually fall off the treadmill. It's a coin toss whether it will go off the front or back. If I were trying to hide something I would have cut the tape 2 seconds earlier.

I'm becoming more comfortable with the notion that nothing could ever be done to convince some people. Fortunately, it's not my job to convince them.

 

[Seymour Butz]

The cart is designed with an "advance ratio" of less than 1.0. This means the theoretical distance the prop would advance through still air is less than the distance the wheels would roll with that same single rotation of the prop. This is a critical design parameter for a DDWFTTW vehicle of this sort.

Here's one way of looking at it... imagine we built such a cart that was absolutely perfect - no frictional or drag losses. We could push it 10 mph on a flat surface and it would continue at that speed forever. In the real world we have a 10 knot tailwind on top of that. This tailwind can more than account for the internal losses and drags.

For me the best way to think of this vehicle really starts with the ice-boat on a 45 degree downwind tack. If you accept that the ice-boat can maintain a downwind VMG greater than windspeed on such a tack you're almost there. From there the only question is "how can we wind that downwind tack into a spiral to drive a vehicle straight downwind?" The way to do that is to stick one sail out to each side (i.e. the prop) and let them spiral one another downwind. The one thing left to do is provide the mechanism for constraining them to their 45 degree downwind path. That's exactly what the wheels and transmission do.

The prop is providing thrust - in exactly the same way a sail or wing provides thrust - by being "sucked forward" as you say.

So let's throw some hypothetical numbers in here so I can get it.

Let's say that the prop gives 10 lbs of "suck" to the cart.

The mechanical advantage of the wheels makes the interface with the road see 20 lbs. A boat would see only 10 lbs, since it lacks the mechanical advantage of that the wheels give.

There will be a rolling resistance of 5 lbs.

the aero drag of the prop is 5 lbs.

So the cart's interface with the road "sees" a 10 lb net push.

 

[Seymour Butz]

Sure it is. That's one valid way to explain how a wing, sail, or prop develops lift.

Well, at least I got that part right.

 

[Michael C]

Bernoulli's principle is not important here.

Sure it is. That's one valid way to explain how a wing, sail, or prop develops lift.

A controversial matter. I'll just say that I'm not convinced by the explanation of lift by the Bernoulli principle that was in my old school physics book (air travels faster over top of wing than over bottom). How does this explain how a conventional plane can fly upside-down?

The explanation of lift by action and reaction (Newtonian principles) makes sense to me.

 

[Seymour Butz]

How does this explain how a conventional plane can fly upside-down?

Not all planes can fly upside down. Only the ones with lotsa power can.

The ones that can will have their noses high, reversing the relationship of wing lift. But they are inefficient in this configuration, and need greater power to maintain flight.

They then use their greater power to act more like a missle than a plane by directing their thrust back and DOWN to maintain flight. Whereas in normal flight, nearly all of the thrust is back, with very little down.

 

[Dan O.]

Here is another down wind craft.


A simple weight and gearing on each vane axle causes the vane to make a 1/2 rotation for each full rotation of the main wheel. This causes the vanes to open to the wind when they are below the axis of the main wheel and close when they are above. Since the vanes below the axel are moving slower than the axle relative to the ground, the wind is still abel to push them forward even when the axle (and the average speed of the wheel) is moving somewhat faster than the wind.

 

[spork]

A controversial matter. I'll just say that I'm not convinced by the explanation of lift by the Bernoulli principle that was in my old school physics book (air travels faster over top of wing than over bottom). How does this explain how a conventional plane can fly upside-down?

Way to turn a controversial thread into WWIII

What you were most likely taught was the "equal transit time" theory (which relies on Bernoulli). This says that a parcel of air going over the wing takes the same amount of time as a parcel going under the wing. Sinc the upper path is longer, that air must be moving faster, and therefore has lower pressure.

This is what we aerodynamicists term "Complete B.S."

The reality is that Bernoulli simply says faster moving air exerts less static pressure (if no work has been done). The other reality is that the air moving over the top of the wing (while lifting) typically gets the the trailing edge well BEFORE the corresponding air that goes under.

If we integrate the pressure over the surface of the wing as computed using the observed velocity profile and Bernoulli, we get the lift just fine.

We can also look at it from a Newtonian point of view. Both work just fine.

 

[ThinAirDesigns]

Why the treadmill demonstration doesn’t fairly or credibly represent an outside wind scenario . . .

(1) Artificially stopping the cart moving back wards on the treadmill until the thrust of the propeller is sufficient to do this is essentially the same as artificially pushing the cart over the ground surface up to the speed of the wind in an outside test.

You are correct, it is exactly the same as you state. However:

Notice there is nothing in the claim requiring the device to self start but still at 6:10 of the following video we discuss and demonstrate self starting: http://www.youtube.com/watch?v=4GOlSi-2p3g

So, in summary, the self-starting issue is a non-issue for the following reasons:

A: It's not part of the claim
B: It does and will self start
C: If one did have to give it a shove to get it started, it would not alter the validity of the claim once past the few feet that a pushes influence survives. After all, we don't claim that a surfers motion isn't powered by the waves simply because the surfer must paddle to catch the wave.

(2) Artificially stopping the cart moving forwards on the treadmill beyond when the propeller could do this is the same as artificially pushing the cart over the ground surface beyond the speed of the wind in an outside test.

The above is absolutely false and in fact it's just the opposite of what you state.

Stopping the device from going off the front of the treadmill is the same as grabbing the device in an outside test *after* it has gone faster than the wind, slowing it to below the speed of the wind and letting accelerate to above windspeed again. It perfectly demonstrates the devices ability to go from slower than the wind to faster than the wind repeatedly. Each time, we are robbing the device of energy it's earned fair and square and asking it to start over and earn it again.

How does artificially pushing the cart up to and beyond the speed of the wind prove that the energy of the wind alone can do this?

We never push the device "beyond the speed of the wind". To do so we would have to push on the rear of the device and move it up the treadmill. You will notice that each time my finger (or spork) is shown in the video, it is positioned in such a way that I can *only* push the device back down the treadmill and not pull it up.

To fairly and credibly recreate an outside test on a treadmill the cart should be placed on the moving treadmill and not artificially restricted from moving along it backwards in any way. Unfortunately a very long moving treadmill surface would be required.

That would only be true if we were trying to prove "self-starting" while on the treadmill. We aren't.

We have shown it self-starting on the street and our work there is done.

SPORK - I notice in a clip that you have built a large outside model (that you are testing on a treadmill for some reason). Have you tested this in an outside wind, and if so why haven’t you provided the results?

Our large cart was designed just like a small one was -- treadmill only. On the treadmill we don't have to have RC steering which makes the device much easier to build.

JB

 

[spork]

Not all planes can fly upside down. Only the ones with lotsa power can.

The ones that can will have their noses high, reversing the relationship of wing lift. But they are inefficient in this configuration, and need greater power to maintain flight.

They then use their greater power to act more like a missle than a plane by directing their thrust back and DOWN to maintain flight. Whereas in normal flight, nearly all of the thrust is back, with very little down.

There are plenty of aerobatic planes with fully symmetrical airfoils. They fly upside down just the same way they fly right side up. No more power, no missile behaviour.

They use positive angle of attack to present an airfoil shape to the wind. Explaining why the air goes faster above the wing than below requires the simultaneous solution of the Navier-Stokes equations, and is a topic for a different thread.

 

[ThinAirDesigns]

Not all planes can fly upside down. Only the ones with lotsa power can.

Just not true Seymour. If the aerofoil is symetrical it takes the same power to fly upside down as right side up.

Most planes do not have symetric aerofoils -- many aerobatic planes essentially do. The closer to symetrical, the closer the power needs both ways.

JB

 

[Brian-M]

I admit that I still don't understand HOW this works, because no matter how I think of it, there still must be some torque input to the system. I see the leverage advantage that will minimize that power requirement, but it can never be zero, and I don't see where it's coming from.

Or is the fact that the prop is spinning irrelevant? I think this is what you're saying - the prop is NOT acting as a traditional prop, that is supplying aerodynamic thrust.

The prop is acting like a traditional prop. I think the problem here is partly that there is more than one valid way to think of this, and everyone is providing different explanations to describe how it's working based on which way they're thinking of it. Combining all these different explanations together ends up with one great muddle.

The prop is geared so that when the cart moves forward, the prop spins, pushing the air backward. When the cart is standing still, the wind is trying to turn the prop the opposite direction. When the gearing of the prop is below 1/1, the wind lacks the force to achieve this, and the drag of the wind passing over the stationary prop is enough to push the cart forward, which in turn spins the prop so that it is pushing against the wind.

(If the gearing was above 1/1, the wind would be able to spin the prop of the stationary cart like a turbine, and the cart would progress in the opposite direction to the wind.)

I think it helps if you think of the prop as a wheel pushing back on (and being pushed forwards by) the air.

I've already posted this image before, but look at it again. The blue conveyor belt above the cart represents the air/wind. The cart will move in the same direction as the belt, at twice the speed of the belt. The DDWFTTW cart works in exactly the same way, but with a propeller pushing back on the wind instead of the top-wheel pushing back on the belt.

When the conveyor belt above the cart moves to the right, the cart moves at twice the speed to the right.

If you understand how the above cart travels in the same direction as the belt, faster than the belt is pushing it, then you already understand how the same cart with a propeller can travel in the same direction as the wind, faster than the wind is pushing it.

I take the effort to work out the math behind it here.

 

[Seymour Butz]

Just not true Seymour. If the aerofoil is symetrical it takes the same power to fly upside down as right side up.

Most planes do not have symetric aerofoils -- many aerobatic planes essentially do. The closer to symetrical, the closer the power needs both ways.

JB

Too fast with your answer Turok - that should give you a clue who I am on another site...

Reread the question I answered and see why the response about symmetrical airfoils don't apply.

 

[spork]

Reread the question I answered and see why the response about symmetrical airfoils don't apply.

Not all planes can fly upside down.

True.

Only the ones with lotsa power can.

False. Plenty of low powered planes fly just fine upside down. Including gliders (no power).

The ones that can will have their noses high, reversing the relationship of wing lift. But they are inefficient in this configuration, and need greater power to maintain flight.

True for some planes, false for others.

They then use their greater power to act more like a missle than a plane by directing their thrust back and DOWN to maintain flight.

I can't think of a plane that flies inverted by using any significant portion of its thrust for lift.

 

[ThinAirDesigns]

Too fast with your answer Turok - that should give you a clue who I am on another site...

If your assertion were true, it might give me a clue -- but alas it's not -- there are far too many people on far too many forums making far too many false claims for me to tell them apart and pick you out of the crowd.

Reread the question I answered and see why the response about symmetrical airfoils don't apply.

Ok, here goes:

#1

Not all planes can fly upside down.

(You said nothing to exclude symmetical airfoils in that statement.)

True.

#2

Only the ones with lotsa power can.

(You said nothing to exclude symmetical airfoils in that statement.)

False. Just plain false. Even most sailplanes can fly upside down and it's hard to argue that they are doing it because they have "lotsa power".

#3

The ones that can will have their noses high,

(You said nothing to exclude symmetical airfoils in that statement.)

Check out 1:32 of this video and repeat the above: http://www.youtube.com/watch?v=LyTVCw6LQNQ

#4

But they are inefficient in this configuration, and need greater power to maintain flight.

(You said nothing to exclude symmetical airfoils in that statement.)

Not true -- again this assumes that #2 and #3 are true, and they are false.

They then use their greater power to act more like a missle than a plane by directing their thrust back and DOWN to maintain flight. Whereas in normal flight, nearly all of the thrust is back, with very little down.

(You said nothing to exclude symmetical airfoils in that statement.)

Watch smoke stream trailing Peter in the above and repeat that again.

What you are saying is greatly true for asymetric airfoils, planes with symetrical airfoils don't have the limitations you are placing on them and you said nothing to exclude those planes from your blanket statement.

JB

 

[ynot]

SPORK - We agree using a treadmill is useless for “self start” testing because it isn’t long enough. I think we agree however that it’s okay to use a treadmill to see what happens when the cart is “up to speed”. In other words the treadmill can be used to see if whatever is happening when it is “up to speed” can be sustained. Here is a very simple test for this . .

Tie a string to the center front of your cart and tie the other end to something solid off the front end of the treadmill. So you have your cart at the back end of the tread (on the tread) and a string running up the center of tread to a solid anchor point. The cart can’t move backwards with the moving tread but it CAN move forwards against it. With the treadmill running hold the cart JUST above the moving tread with the string taught then let the cart fall on to the tread. DON”T restrict the cart in any way with your hands. Let it run and see what happens. If the cart moves up the treadmill against the moving tread the string will become slack. If it does this let it run from quite some time to see what happens. I also suggest angling the treadmill so the cart has to travel slightly uphill. You have shown the cart can do this in your previous demonstrations. I would appreciate you doing this test and posting the results. A Utube clip would be good.

I have found some gears so hope to have my test cart finished over the next few days.

 

[Seymour Butz]

If your assertion were true, it might give me a clue -- but alas it's not -- there are far too many people on far too many forums making far too many false claims for me to tell them apart and pick you out of the crowd.


(You said nothing to exclude symmetical airfoils in that statement.)

JB

Good, then I can continue to give you grief over there too.

REREAD THE QUESTION THAT I ANSWERED!!!!!!!!

He asked about conventional planes. Aerobatic planes can hardly be considered conventional.

Aerobatic planes can also fly on knife edge. Conventional planes cannot. So again, hardly worth using them as a comparison.

 

[ThinAirDesigns]

Aerobatic planes can also fly on knife edge. Conventional planes cannot.

Unlike upside down, knife edge IS a question of "lotsa power" rather than one of wing airfoil shape.

So once again your statement is false -- just because a plane has "lotsa power" does not mean it isn't conventional. There are plenty of planes that aren't designed for aero that have better power to weight ratios than those that are. Looking at the time to climb stats from the EAA will prove my point. Aero planes don't dominate.

There are conventional (non-aero) planes that can knife edge. Do your research better (or better yet, just stop making blanket statements that are easy to refute)

JB

 

[Seymour Butz]

Unlike upside down, knife edge IS a question of "lotsa power" rather than one of wing airfoil shape.

So once again your statement is false -- just because a plane has "lotsa power" does not mean it isn't conventional. There are plenty of planes that aren't designed for aero that have better power to weight ratios than those that are. Looking at the time to climb stats from the EAA will prove my point. Aero planes don't dominate.

There are conventional (non-aero) planes that can knife edge. Do your research better (or better yet, just stop making blanket statements that are easy to refute)

JB

I see you saw your mistake about inverted flight for a conventional plane and have decided to gloss over the fact that you didn't read the question I answered before you jumped in and started spewing about acrobatic planes...... Niiiiice.

Also, I think what the OP was about was a plane that could maintain inverted flight. So I think a sailplane, that just trades energy to hold inverted for a short while hardly qualifies. Hell, any plane can fly inverted, as long as they don't mind crashing into the ground.

I never said that a plane with lotsa power means it isn't conventional. Clearly, since the OP was talking about conventional non-symmetrical airfoils, like on the carts, the question was about how a plane with a conventional non-symmetrical airfoil can fly inverted. It takes more power to do that under that circumstance. What you're doing is trying to set up a strawman in order to make it look like you're correct. Tsk tsk....

So what other planes can knife edge then? Did I say that there were no other planes, other than acros, that can knife edge? Nope. Fighter jets come to mind. And there's your high power/weight. Which prove me right. And prove that you again need a strawman to try and make yourself look better.

 

[Seymour Butz]

So let's throw some hypothetical numbers in here so I can get it.

Let's say that the prop gives net 10 lbs of thrust to the cart.

The cart frame sees 10 lbs of thrust

The mechanical advantage of the wheels makes the interface with the road see 20 lbs. A boat would see only 10 lbs, since it lacks the mechanical advantage of that the wheels give.

There will be a rolling resistance of 5 lbs.

So the cart's interface with the road "sees" a 15 lb net push.

Ok, enough of that blather with JB.

Am I getting close here with my above statement.