Sometimes very active discussions about peripheral issues overwhelm a thread, so this is a permanent home for those conversations.
This is also a continuation of previous Sandbox threads (1) and (2) that have fallen victim to the dreaded page bug.
The Skeptical Zone
"I beseech you, in the bowels of Christ, think it possible that you may be mistaken."
keiths:
Alan:
No, it’s incorrect. When the vehicle is traveling at wind speed or faster, it is not “being blown downwind as a ‘bluff body'”. It’s being driven downwind through the air by the propeller. The air resistance is acting against the vehicle’s motion, not in favor of it. The vehicle has a streamlined design in order to reduce air resistance. If the vehicle were simply being blown downwind, higher air resistance would be good, not bad, and the designers wouldn’t have needed to streamline it — but in that case the vehicle wouldn’t be able to exceed wind speed, thus defeating the entire purpose.
Again, to understand this phenomenon, you really need to let go of the idea that the vehicle is being blown downwind “like a ‘bluff body'”. It isn’t.
Yes. When traveling downwind, the propeller always acts as a propeller, never as a turbine. It’s easy to see why this must be the case — just think about what happens when the vehicle is traveling exactly at wind speed.
No, the last part is also incorrect. You quotemined yourself, leaving out the words in bold below:
That’s not right. The key is the air’s motion relative to the ground, not its motion relative to the cart.
Come on, Alan. Quotemining yourself is just as dishonest as quotemining someone else.
You made some mistakes. Please just accept that, acknowledge them, and move on.
I’d say you are both wrong. The key is the air’s motion relative to the propellor blade and this will be a vector of the cart’s air velocity and the velocity of the blade through the air. The effect of this vector is the ultimate source of the power which accelerates the cart until all forces are in equilibrium.
And even at zero relative air velocity of the cart there will be a drag component. The drag acting opposite to the motion of the blades which will tend to slow the propellor down.
No doubt about it. I’ve laboured under the misconception that the wind energy was transferred to the rotor acting as a turbine for quite a while before the penny dropped. Worse, my initial thought (on first stumbling on a discussion on DDWFTTW while on another forum for a completely unrelated reason around the first week of April) was it was a scam and a violation of the conservation of energy. Though as Mark Chu-Carroll fell into the same trap, I don’t feel too bad.
Not clear exactly what you mean here. Certainly the source of energy to move the cart has to be the wind. And for the cart to move, the force from the wind on the cart must overcome resistance. If you watch the Blackbird video, you will see the initial acceleration is very slow until the prop starts to “grip” the wind. In fact, initially, cart designers themselves were unsure whether the “bluff body” effect would be enough to start the cart moving. Test runs used a push from a truck, as they would run out of lakebed before getting to windspeed. There was mention (not sure which team member it was – the site has been down for over a week now) of some kind of retractable fin or sail to speed up initial acceleration but in the end it turned out to be unnecessary.
Well, the only source of power, if the scenario is as apparently demonstrated in the video, is the kinetic energy of air molecules – the wind. Not sure if this conflicts with your statement.
Indeed there will be losses due due friction in bearings and gearing between prop and wheels. The extra energy comes from the turning prop picking up energy from the wind. It is the relative movement of air mass to ground so that the propellor is pulling in moving air while geared to the fixed ground (what I refer to as the cotton-reel effect) that allows the cart to exceed wind-speed.
Hi Alan
The initial movement of the cart will be brought about by the force acting on it by the tail wind or by someone giving it a push. But once it is moving it will be the aerodynamic forces acting on the surface of the blades that propels the cart forward.
Assuming that there is some tail wind then when the airspeed of the cart is zero (ie forward speed is equal and opposite to the wind speed) the airflow which gives rise to forward thrust will be perpendicular to the direction of motion of the cart. So it is not the wind as such but the airflow over the sideways spinning propellor which provides the forward thrust.
And of course there is a bit of feedback loop going on. The faster the wheels turn the faster the propellor turns, the faster the propellor turns the more thrust generated, the more thrust the greater the speed of the cart and hence the faster the wheels turn.
I’ve added a diagam fpr clarity (I hope). I amended this diagram which I copied from the NASA site.
Yes, this is my understanding too: a virtuous circle. Another analogy that helps me is imagining a helicopter hovering. When stationary over the ground in still air, the thrust from the rotor exactly balances the gravitational force due to the mass of the copter. Put the copter in a vertical column of rising air and it will move upwards without need of any additional power from the motor. Apply more power and the copter may climb faster than the rising column of air. And when I talk about a prop or rotor “gripping” the air, the image of a hovering copter hanging from its rotor (a “sky hook”) is the best illustration I can think of.
The problem with you analogy is that you are considering an insignificant detail while disregarding the important aspect. A helicopter alters the amount of lift it produces, not by varying the rotor speed, but by altering the angle of attack of the rotor blades. A column of air rising through the rotor disc will in effect tend to reduce the angle of attack of the blades and so reduce the lift produced. This will more than offset any upward force produced by the column of air. (The vector I was talking about previously is the resultant produced by a parallelogram of motion of the air relative to the rotational movement of the blade and the air produced by the wind, or in this case the upward current.)
It is the aerodynamic forces produced by the differential pressure over the aerofoil surfaces of the blades that allows the vehicle to travel through the air. This is exactly the same way that an aircraft’s wing produces lift.
That’s right, though I mentioned power rather than increased rpm. Increasing angle of attack and lift requires a consequent increase on the throttle. Some helicopter throttle and collective controls are linked.
Well, it was meant to be an analogy drawing attention to the frame of reference. As far as the helicopter is concerned, everything is relative to the air that it is in.
You’re right that prop performance varies depending on airspeed.
I’m not sure what you think we are disagreeing about.
The Natural and the Human: Science and the Shaping of Modernity, 1739-1841
Some folks here might find this book of interest.
Thanks but going to wait for the movie
Skepticize this:
Songs of Their Own – #1 “Black Muddy River” Luther Dickinson & Anders Osborne
keiths, to Alan:
CharlieM:
No, the ultimate source of the power is the kinetic energy of the air moving relative to the ground. The vehicle is just one (very clever) mechanism for exploiting this power source.
Don’t conflate drag on the propeller blades with drag on the vehicle. That’s as serious an error as conflating the lift generated by the propeller blades with the lift generated by an airplane’s wings.
When you’re analyzing the vehicle’s performance, the “lift” generated by the propeller blades isn’t lift — it’s thrust. And the drag on the propeller blades doesn’t manifest as vehicle drag, but rather as a torque.
CharlieM,
Not quite. The force provided by the tailwind does not drop to zero the moment the vehicle starts rolling. It decreases gradually as the vehicle’s speed increases, vanishing only when the vehicle reaches wind speed.
Above wind speed, the propulsive force comes entirely from the propeller.
No, because if that were the case the vehicle would accelerate forever (or until it fell apart). And this would be true even on a still day — just give the vehicle an initial push and watch it go.
This would violate the conservation of energy, so your description cannot be correct.
Alan:
Throwing is a better analogy than gripping.
To grip something is to grab it and hold on. Props and rotors don’t do this.
What props and rotors really do is to throw air in a certain direction, generating an equal and opposite force on themselves via Newton’s third law.
CharlieM:
No, it will increase the angle of attack and increase the lift (just as it does for airplanes), causing the helicopter to accelerate upward. Once the vertical speed of the helicopter matches that of the air column, the angle of attack will be reduced to the original value, where lift = weight. The helicopter will no longer accelerate upward, but it will continue to rise steadily at the same rate as the air column.
Alan’s statement is an accurate description of the long-term behavior:
I agree. The collective control and the engine power control need be linked either mechanically as in older models or electically as in newer models.
Well we are not disagreeing about much. I am trying to be precise, some would say picky, about the relative forces. My reply was not meant to contradict everything you said, I was just trying to clarify matters.
The ultimate power source is the air moving relative to the propellor not the air moving relative to the ground. Granted the initial power source may be the air moving relative to the ground.
I am not conflating the two. The drag acting on the propellor will be as near as makes no difference at right angles to the drag on the vehicle.
The same aerodynamic forces are involved in producing the thrust on the propellor and lift on a wing. The difference between the two is the direction of the weight component. I don’t see where we disagree about the direction of the drag component on the propellor blade.
Well I have never disagreed with this. I ignored the tailwind component as I was focusing on the force which was causing the cart to accelerate.
So you agree that the ultimate power source is the propellor?
The vehicle would not accelerate until it fell apart. It would only accelerate until the opposing forces reached equilibrium.
There are a couple of factors which will prevent the feedback accelerating the propellor without limit.
1. The drag of the propellor is proportional to the square of its speed and so there will come a point where the rotation of the wheels (directly linked to the rotational speed of the propellor) will not be able accelerate as there will not be enough energy to overcome the drag holding it back.
2. Even if it were capable of overcoming drag the speed of the propellor is limited by the speed of sound. After this point the aerodynamic forces will not produce the thrust to accelerate the cart.
What they really do is create differential pressure across their forward and rear (upper and lower) surfaces which creates an unequal force producing the movement.
For anyone reading this who is interested but has no knowledge of aerodynamics hold two sheets of paper vertically a couple of centimeters apart and blow down between them. Before you do so think about which way will the paper will move. Were you correct?
You are right about the angle of attack increasing. I had a senior moment. But I will say that increasing the angle of attack will only produce an increase of lift up to a certain point, after that point the blade will stall and then you are in trouble.
CharlieM:
No, because power is required to keep the propeller rotating in the first place. The source of that power is the wind; that is, air moving relative to the ground.
Not just the initial source, but the only source. If the wind ceases, the vehicle comes to a stop.
keiths:
CharlieM:
That’s not why propeller drag is excluded from the overall drag number. Remember, the propeller is symmetrical and it rotates. The drag on one blade is the same as the drag on the other, but the vectors point in opposite directions. The forces cancel each other out, but since they are offset, a torque is generated, which is why I said
CharlieM:
No, the weight component is always directed downward. In unaccelerated flight, lift equals weight and thrust equals drag.
keiths:
CharlieM:
You can’t ignore the tailwind component. The force causing the cart to accelerate is a combination of the force supplied by the tailwind and the thrust supplied by the prop. Prop thrust is initially very small. If you ignore the tailwind, you will (erroneously) conclude that the vehicle cannot accelerate.
keiths:
CharlieM:
No. You’re confusing thrust, which is a force, with power, which is not.
CharlieM:
keiths:
CharlieM:
Right, which is one reason your description is incorrect. The other reason is that if your description were correct, the vehicle would accelerate in still air if given a tiny initial push. That clearly does not happen, and it would violate the law of conservation of energy if it did.
keiths, to Alan:
CharlieM:
They do both, and those are two sides of the same coin. Because of the pressure differential, the air exerts a net force on the blades. Because of Newton’s third law, the blades exert an equal and opposite force on the air, thus “throwing” it.
CharlieM:
Sure, and an airplane’s wings will also stall if it encounters a sufficiently strong updraft. But Alan’s comment was about a helicopter flying in a steadily rising column of air:
He’s right, and the same principle holds true for airplanes. Both helicopters and airplanes fly relative to the air, not to the ground. If they are flying in a steadily moving mass of air, they will move with the air, regardless of whether the motion is horizontal or vertical.
This is especially important when soaring. In unaccelerated flight, sailplanes sink through the air. The key to climbing is to find air that is rising faster relative to the ground than the sailplane is sinking relative to the air.
ETA: My dad used to tell a story about flying backwards in an Aeronca. The stall speed was only 33 knots, so during slow flight into a strong headwind, you could end up moving backwards over the ground. He said it was a very strange feeling.
The power that keeps the propellor moving comes from the thrust produced by the propellor which is obtained by the propellor moving through the air. The forward motion of the cart is at right angles to this motion.
What do you mean by wind? If the cart has zero groundspeed and zero airspeed then obviously it is stationary. If it has a positive groundspeed but zero airspeed then it is obviously still moving. And it will be the thrust of the propellor which keeps it in motion. This is the point at which the cart moves from experiencing a tail wind into a head wind. Are you saying at this point the cart should be stationary with respect to the ground?
And with these words you contradict yourself. The torque required to turn the propellor must overcome the resistence of the drag on the moving propellors. These drag forces do not cancel each other out but are cumulative as they both oppose the turning motion.
Yes and in a vertically rotating propellor the aerodynamic force produced by the aerofoil section is at right angles to the weight which is what I was saying.
The interesting thing about this cart is that it moves against the air which is in effect a head wind. It can do this because the thrust produced by the propellor overcomes the air resistence tending to push the cart back. The wind speed relative to the cart can only provide a force resisting the acceleration once the cart is travelling faster than windspeed.
But it is the force which gives it the power. The power output is proportional to the force produced.
You are still not getting the thrust produced by the propellor. It is the sideways movement of the propellor which provides the thrust not its forward movement with the cart. This does not violate the law of conservation of energy.
keiths, do you agree that increasing the relative headwind reduces the angle of attack of the prop blades?
Fixed wing aircraft and sailplanes (gliders) achieve their lift by flying forward against the airflow not with it. Thermals will assist the lift forces. It is the forward speed of the aircraft that allows it to climb through any column of air. If the airflow over the wing is too slow no amount of thermal energy is going to hold it aloft. You won’t see sailplanes hovering in thermals they always circle around in the thermal in order to maintain forward momentum.
CharlieM:
Again, you’re confusing thrust (a force) with power (not a force). The distinction is crucial.
The thrust comes from the propeller’s motion through the air, which is linked through the drivetrain to the vehicle’s motion over the ground, which in turn is caused purely by the thrust of the propeller when the vehicle is traveling at or above wind speed. This causal loop is what raises people’s suspicions that this is some kind of perpetual motion machine. It’s not, because although there is a loop of forces, there is not a loop of power. The latter would violate the laws of thermodynamics. The former does not.
While the thrust comes from the propeller, the power comes ultimately from the wind’s movement relative to the ground. The vehicle continuously “steals” energy from the air by slowing it down. In turn, drag and friction continuously steal kinetic energy from the vehicle. To maintain a constant vehicle speed, the energy lost to drag and friction must be continuously replaced by an equal amount of energy “stolen” from the air (by slowing it down).
Air moving horizontally relative to the ground.
Relative to the ground, yes. And there is no wind.
That’s right. Thrust is a propulsive force. For the vehicle to remain in motion, the propulsive forces must equal or exceed the retarding forces (drag and friction). At zero airspeed, the only propulsive force comes from the propeller. The power, however, comes from the wind: air moving horizontally relative to the ground.
No, of course not. Why would it suddenly stop? The physical circumstances haven’t changed drastically. They’re only slightly different from what they were a few seconds earlier, when the groundspeed was a tad below wind speed.
keiths:
CharlieM:
Now you are confusing forces with torques. The prop’s drag forces cancel out, but the torque due to drag is passed on to the rest of the vehicle.
Consider an instant when the prop blades are parallel to the ground. One blade is moving upward; its corresponding drag vector points down. The other blade is moving downward; its corresponding drag vector points up. Because the prop is symmetrical, the vectors have equal magnitudes. They point in opposite directions, so they cancel each other out. There is no net upward or downward force on the rest of the vehicle due to the prop’s rotation.
Torque is an entirely different story. The torque due to drag on one blade is equal in magnitude to the torque due to drag on the other blade. But in this case either both blades are moving clockwise, or both blades are moving counterclockwise. The torques add instead of cancelling each other out, so the prop exerts a torque on the rest of the vehicle.
This fact nearly caused an accident. On one of its runs, the Blackbird achieved a higher than expected speed of 46 mph, and the torque at that speed was threatening to flip the vehicle over. Here’s how Wired describes it:
CharlieM:
No, you were saying that the difference between props and wings is the direction of the weight component:
That’s incorrect. The direction of the weight component is always the same: toward the center of the earth.
keiths:
CharlieM:
The thrust cannot be maintained unless enough energy is extracted from the air, and that in turn cannot happen unless the air is moving sufficiently fast relative to the ground. It is the motion of the air relative to the ground that is the ultimate power source.
No, the power output is proportional to the product of the force and the distance traveled per unit time. A force of 500 pounds acting on a body moving at 10 meters per second delivers twice as much power as the same force acting on a body moving at 5 meters per second.
keiths:
CharlieM:
True, but none of that contradicts my point: you can’t ignore the tailwind component, because if you do, you will erroneously conclude that the vehicle cannot accelerate from zero speed.
CharlieM:
What violates the conservation of energy is the (fictional) process you described:
If that process obtained, then you could get the vehicle to accelerate indefinitely in still air simply by giving it an initial push. That would violate the law of conservation of energy, so we can conclude that your description is incorrect.
The problem is that you failed to include drag and friction in your description.
CharlieM:
Yes, which reduces thrust. The higher effective headwind will also increase drag.
For example, suppose Blackbird is moving at a steady groundspeed of 25 knots in a steady wind of 10 knots. The effective headwind is 25 – 10 = 15 knots.
Now suppose the wind speed abruptly decreases to 5 knots. The groundspeed is still (for the moment) equal to 25 knots, but the effective headwind has increased to 25 – 5 = 20 knots. The higher effective headwind reduces the angle of attack of the prop blades, so the thrust is reduced. The higher effective headwind also increases the vehicle’s drag. Both factors cause the vehicle to slow down until equilibrium is reestablished.
CharlieM:
No, the lift remains the same whether the sailplane is in air that is steadily rising, steadily sinking, or neither rising or sinking. Lift simply matches weight in unaccelerated flight, regardless of whether you are in a thermal.
Adjusting the airspeed at a given angle of attack, in straight flight, simply allows lift to equal weight so that the aircraft doesn’t accelerate upward or downward. Lift equals weight in straight and level flight, in a straight climb, and in a straight descent.
Right, which is why a sailplane in unaccelerated flight must continually descend through the air even when it is climbing relative to the ground in a rising air mass. As I said:
CharlieM:
Right, but the circling doesn’t cause the climb. What causes the climb is the fact that the air is rising faster relative to the ground than the sailplane is sinking relative to the air.
Jonathan Bartett (JohnnyB) is a YEC:
http://www.tulsaworld.com/news/religion/grand-canyon-a-battlefield-in-age-of-earth-dispute/article_a74f9b5f-f14d-54c6-aa38-3d1fb8544188.html
“Jonathan Bartlett, a computer programmer who studied at Oklahoma Baptist University and Phillips Theological Seminary in Tulsa, is a young-Earth creationist.
Bartlett told the Tulsa World in a past interview that he studied the issue carefully after losing his son to a rare genetic disorder.
Belief in an ancient Earth had been his default position, he said, but the closer he looked at it, the more he found arguments for that position to be not as convincing and assumption-free as they seemed, and that a “larger-scale” look at geology supports young Earth creation.”
Take that, all of you who thought that JohnnyB’s judgment might be at all biased by education and desires for immortality.
Glen Davidson
oh for fuck’s sake.
What a moron.
The rest of the article is actually quite interesting.
Good for him. He’s one of the sane ones. Christians aren’t a monolith of moronity.
As a footnote, sailplanes that are trying to proceed cross-country must find one thermal after another. There is a probability that they won’t succeed, and instead end up in some farmer’s field. To go cross-country one goes in a shallow dive from one thermal, seeking the next. The shallower the dive, the slower the cross-country speed is, but the further one can go in order to find the next thermal. and the greater the probability that one can complete the cross-country race.
So this is an optimization problem which involves the characteristics of the sailplane. What does this have to do with evolutionary biology? One connection is that Anthony Edwards first raised the problem in an article he wrote in 1963 in Sailplane and Gliding. It can be read here.
In his day job, Anthony was beginning to publish, with Luca Cavalli-Sforza, some papers that introduced for the first time the major methods for inferring phylogenies. They were the pioneers of parsimony methods and likelihood methods, and they also published on distance matrix methods at about the same time as Walter Fitch did. The other major approach, Bayesian inference of phylogenies, was discussed in a 1970 paper by Anthony. So incredibly important stuff.
But, as we can see, Anthony also kick-started a literature on probabilistic considerations in cross-country sailplane racing.
Joe:
It’s a bit more complicated than that. The shallowness or steepness of a dive is quantified in terms of the angle of the nose relative to the horizon. A shallower dive will increase the gliding distance up to a point (the point at which the lift-to-drag ratio is maximized), but raising the nose beyond that point will actually steepen the descent. Shallower is not always better.
Also, the analysis is complicated by other factors, including wind and sinking air.
Maximizing the L/D ratio allows you to maximize your gliding distance through the air, but in the presence of a wind, maximum distance through the air isn’t the same as maximum distance over the ground.
Also, the air rising in thermals is replaced at the surface by sinking air in the surroundings. To maximize gliding distance, you’ll want to fly faster in sinking air than you would in air with no vertical movement.
It’s fun to see what happens when smart people dip into other fields. Sometimes the results are novel and useful, as in Edwards’ case. Other times they’re a disaster, as with Fred Hoyle.
Take that atheists!
I’d comment further, except I’m an atheist and thus have no reason to care about anything or anybody.
I don’t even know how I managed to care enough to write this.
Glen Davidson
I’d comment, but my ability to comment might be a delusion.
I’m not surprised to see the usual crew of UD mouth-breathers lapping it up, but vjtorley ought to be ashamed of himself:
Should be, but one supposes that he’s a tad suspect now, standing by Swamidass as he has, and so he’s got to fawn over WJM’s miserably stereotyping and mischaracterizing post.
Lie down with dogs…
Glen Davidson