Groundspeed does effect airspeed

[Tom Harper]

The downwind turn, airspeed + windspeed etc arguments treat the aircraft as though it were flying in a huge bottle mounted on a moving flat bed truck. The model circles gently in the bottle uneffected by forces of motion. This is not exactly the real situation.

A sailplane is not the best example because it has a very low coefficient of drag and operates at a high Reynolds number. A typical model is very draggy by comparison. What does drag have to do with it? Consider the basics:

L=W and T=D

These two are not independent. The 'D' term contains the induced drag that is proportional to 'W'. That term constantly refers all flight to the ground.

Consider an SS taking off from the ground. It accelerates into the wind until the equation is balanced - but not quite. It asymptotically approaches a ground speed of airspeed-windspeed. It can never get there because it's drag vector is tied to ground speed through the 'W' term of the equation. It's ground speed will be:

Upwind

GS = WS - (AS * 1 - K)

Downwind

GS = WS + (AS * 1 - K)


'K' being a value, less than one. that is inversely proportional to the weight and induced drag of the airplane.

This is true for both the upwind and the downwind case. It is best illustrated by considering a leaf being carried by the wind. The leaf can never reach the wind velocity. There is always a net flow of air over the surface to carry the 'W' of the leaf. A bubble of air carrying the model has the same problem. The model is tied to the ground through it's weight and proportional drag vectors. When the model is moving in the air mass, it is being blown by the wind. Work is being done.

So, when the model is traveling upwind there is always a small net increase in velocity of the air over the wing. This is integrated over the period of time the model is flying upwind causing the model to gain altitude.

When traveling downwind there is a slight decrease in the flow of air over the wing. This is also integrated over the time the model is traveling downwind causing the model to lose altitude.

In a dowind turn the model must accomodate this change in airflow.

If the model were in a bottle of air carried on a truck it's weight would influence the power required to move the truck whether the model was in the air or resting on the bottom of the bottle.

When a volume of air moves a 5.5 lb model 100 ft in 2 seconds, one horsepower has been expended. If the model rests on the ground no work is required. The difference must be accounted for in the aerodynamics of the model.

[Sparky Paul]

And a balloon? Like one of those thousands of nstrumented weather balloons released around the world, to measure -winds aloft-... Is there a K factor for the reading?

[Ollie]

Tom,
Know about very fast (more than 200MPH) RC sailplanes that are getting lots of energy from wind shear. See:
http://www.charlesriverrc.org/articles/flying/markdrela_ds.htm
http://en.wikipedia.org/wiki/Dynamic_soaring
http://www.sloperacing.com/results/ds-speeds.htm
http://members.tripod.com/douglasturner/id27.htm

[capncrunch]

Tom,

your argument amounts to saying that models fly faster heading east than west.

"upwind" and "downwind" are terms that only make sense in relation to a stationary frame of reference. This is leading you to treat the wind as non constant.

-b

[Tom Harper]

Sparky - The velocity of the balloon is measured from the ground. Yes, there is a correction factor between the velocity of the balloon and the velocity of the air that is moving it.

Ollie - You can extract energy from the wind by accelerating into and out of the boundary layer. We are discussing level flight upwind and downwind. Different discussion.

Crunch - Exactly the point - not east west at all. The wind has to exert a force on the model to move it. Can you explain the aerodynamics of moving a model at 40mph without exerting any force on it.

Let me pose it this way:

1. If the moving air mass moves the model, where is the force applied? Is the force 100% efficient ie no loss?

2. A leaf blowing in the wind is aloft in a moving air mass. Does the leaf ever reach the velocity of the moving air? If so what continues to propel it?

The only way air can move a body is to exert a force upon it. That force is proportional to the dynamic pressure exerted by the air times the area presented by the body. If there is no air flowing over the body there is no dynamic pressure and hence no movement.

Please identify any force that would otherwise account for movement.

[jamessimon]

Tom,

You have made a few errors.

An object suspended in a fluid moving at a constant velocity WILL reach the velocity of the fluid. This is because it takes a force to RESIST the pressure of the flow. The velocity pressure of the fluid will accelerate the object (F=MA) until it reaches the velocity of the fluid. At that point the system is stable. There is no velocity difference between the object and the fluid so there is no friction force or fluid pressure and because the object is moving at a constant velocity it take NO force to keep it moving. Remember that this is for a fluid moving at a constant velocity so that velocity can be zero and the result is the same. Similarly, the direction of that velocity is irrelevant.

[Sparky Paul]

Flags and streamers just hang limp from balloons. There's no motion relative to the air they're in.
Those fancy images of sail powered balloons... wouldn't work.

[capncrunch]

Crunch - Exactly the point - not east west at all. The wind has to exert a force on the model to move it. Can you explain the aerodynamics of moving a model at 40mph without exerting any force on it.

Let me pose it this way:

1. If the moving air mass moves the model, where is the force applied? Is the force 100% efficient ie no loss?

2. A leaf blowing in the wind is aloft in a moving air mass. Does the leaf ever reach the velocity of the moving air? If so what continues to propel it?

The only way air can move a body is to exert a force upon it. That force is proportional to the dynamic pressure exerted by the air times the area presented by the body. If there is no air flowing over the body there is no dynamic pressure and hence no movement.

Please identify any force that would otherwise account for movement.

the air mass is not moving the model at all. the model flies around in it. from the reference frame bounding the air mass, the air is stationary. there is no force that exists between the model and the immediately surrounding air that doesnt exist when theres no wind.

-b

[Tom Harper]

James,

Thanks for the informative response.

Consider a Dandelion seed in a constant velocity wind. The seed approaches the velocity of the wind. If it equaled the velocity of the air mass there would be no relative wind over the seed and it would fall. There must be a small air flow over the seed to keep it in the air. That airflow is provided by the drag of the seed causing it to move slower than the surrounding air.

If you place the seed in a jar and move the jar the seed will not rise. In that case it is part of a stable system.

In your F=MA scenario the wind must exert a force to accelerate an object but no force to sustain it. You propose that at the point where the two velocities match the system becomes lossless. This is only true in frictionless, gravity free, space. That is not the case in the great outdoors of the earth.

No system is lossless. The model is not 'suspended' in a fluid. It is not in a jar. The model rides on a very good air bearing but that bearing has losses. Those losses must be accounted for by a difference in velocity between the model and the moving air mass.

If this is not true then how does the Dandelion remain in the air with no relative wind and what force provides that last little boost needed to overcome losses?

[Tom Harper]

Sparky,

Flags and streamers require a minimum velocity of air to cause them to rise. Balloons fly in (at least in Albuquerque) very low winds. The difference in velocity is too small to raise the streamers.

[Tom Harper]

Crunch,

You are using the model in a bottle scenario. If the air mass were bounded and contained, you would have a closed, stable system. Inthat case what you say would be true.

A model airplane is not in a closed system. For the model to be moved by the air mass there must be a small difference in velocity to account for losses. In an upwind or downwind turn it will make a small adjustment to accomodate those losses.

Two weeks ago we flew a model in dead air and trimmed it for hands off level flight. Last Sunday we flew the same model in a steady 40MPH wind. It was doing straight photo passes upwind and downwind. On the upwind leg it dramatically gained altitude and on the downwind leg it lost altitude.

[capncrunch]

Two weeks ago we flew a model in dead air and trimmed it for hands off level flight. Last Sunday we flew the same model in a steady 40MPH wind. It was doing straight photo passes upwind and downwind. On the upwind leg it dramatically gained altitude and on the downwind leg it lost altitude.


on the calm day, you trimmed it for hands-off level flight *at a given airspeed*. If you were going 20% faster, the model would climb, 20% slower, you'd sink. is that fair to say about this model?

so on the windy day, you were carrying more airspeed upwind and less downwind. people naturally do this when flying models because their frame of reference is outside of the airflow. that's the whole confusing part of this issue.

For the model to be moved by the air mass there must be a small difference in velocity to account for losses.

the model is not moved by the air mass.

-barrett

[HELModels]

Maybe there were terrain effects. You saw what you saw, but why?

[Tom Harper]

Crunch,

Pilotage does enter the equation. Carl's no beginner. He's been flying model airplanes since before they invented dirt.

On the no wind flights the model could be trimmed for level flight, hands off, at about three quarter throttle. You are correct that advancing the throttle would produce a climb.

Last Sunday there was no throttle position that would allow hands off flight both upwind and downwind. The model gained altitude on the upwind leg and lost altitude on the downwind leg.

I have championed the moving air mass argument a number of times on this board. But, after last weekend I don't buy it. There is a wind vector acting on the model.

If you are watching some guy fly a 1.2CI, full symmetrical, fire breathing, super thingy, that vector is small compared to the forces generated by the model. However, if you have a draggy airplane capable of flying in marginal conditions the vector becomes obvious.

Reconsider your statement. You are proposing that once in the air an object can be moved without any force acting upon it. This implies that a Dandelion seed will fly without airflow over it's feathery surface. That would be the case if it moved at the same velocity as the wind. How can that happen?

[Tom Harper]

Storch,

Good point.

We were flying across a mesa and out over an arroyo. If the arroyo edge had an uplift we would have seen it on the down wind leg. If it had a downdraft we would have seen it on the upwind leg. Did not see either.

On the upwind leg the model climbed over the mesa and out over the arroyo.

[capncrunch]

so why are wind tunnel tests accurate to real world conditions?

-b

[Tom Harper]

Crunch,

Not sure what your point is.

A wind tunnel is a closed system with a constrained section.

[Spyda]

Reconsider your statement. You are proposing that once in the air an object can be moved without any force acting upon it. This implies that a Dandelion seed will fly without airflow over it's feathery surface. That would be the case if it moved at the same velocity as the wind. How can that happen?

um, i think your forgetting gravity.

of course a dandelion seed would not 'fly' forever, it would eventually sink and come to rest on the ground. however, it may get caught in the odd thermal and updraft, giving the appearance it is 'flying', but in reality it is always descending relative to the air it is within.

[Tom Harper]

Spyda,

That is probably true. But, if the Dandelion reached the steady wind velocity it would drop like a rock. It does not. It approaches the wind velocity and exhibits aerodynamic characteristics.

[Sparky Paul]

I'd look into the volume/mass of the dandelion seed and comapare that to the equivalent volume of air.

[DRETW]

Thank you Tom,
ted

[BMatthews]

Spyda,

That is probably true. But, if the Dandelion reached the steady wind velocity it would drop like a rock. ....

Sorry Tom but this is all just wrong. You're stuck thinking about the whole picture from your ground bound point of reference. You need to shift that point to the model's, balloon's or dandelion seed's point of reference.

The dandelion seed has a very high drag thanks to the spread of it's design. The terminal velocity of a dandelion seed is based on the effect of gravity on it's mass and this drag. The light weight and high body drag means that the dandelion seed's terminal velocity in a fall is very low. Try dropping one in a still room and you can see it's steady state terminal velocity for yourself. It reacts to the normal gravitational acceleration for only a few 1/10's or 1/100's of a second before reaching its terminal velocity of about 3 inches per second.

Out in the great wide outdoors this means that all the little dust devils and ground turbulence rotors and other wind variations as well as thermals are free to toss it around since it's descent speed is so much less than the vertical speed component of many of these other local and transitory air velocities. The shape is not capable of generating lift. It's purely the relative airspeeds of the local vertical air velocity component compared to the falling speed of the seed.

Same with the balloons. They do not generate lift of any sort. And the only wind you would ever feel if you were riding in one would be very transitory puffs from various directions as the balloon is struck by localized variations created by moving through local turbulence effects, thermals and other such very localized wind effects. The reason the balloon floats is strictly the fact that the total volume and weight of the balloon, basket, ropes and occupants perfectly matches the total volume and weight of the air that would normally fill that volume. It's the same effect that allows an extrememly heavy submarine to remain motionless in a body of water if the ballast tanks are finely trimmed. And it's average speed matches the wind average wind velocity and direction within seconds of being released.

You stress that the drag will prevent the object matching the wind's speed. But for that sort of effect to apply there would need to be something for the object to grab on to so that the drag would be effective. But an object in the air will accelerate until there is no force on the object. And the larger the drag coefficient of the object the sooner that'll happen.

The model in a bottle situation for our airplanes is exactly what we DO have even when the bottle isn't there. My free flight models are trimmed to fly at a very stable speed and attitude. The models react to any variations by nosing up when they speed up or see a faster wind suddenly appear and tend to nose down if they suddenly slow down or see a localized wind gust that results in a lower apparent flying speed. Yet for the most part they fly in their circular path on windy days with a delightfully even speed in their circling flight path while happily drifting downwind. Of course from the ground it seems to be flying very fast on the downwind side of the circle and very slow, stopped or even flying backwards when it's pointed upwind but the attitude of the fuselage doesn't vary enough to see at any point in the circular flight path. That tells me without a doubt that the flying speed that the model is seeing is as even as can be.

Think of the example of a small motorboat with the engine stuck over to one side while running. The boat is running around in circles at a constant speed. Now drop that same boat in a flowing river. The speed of the boat through the water and the size of the circle it's turning won't change but from an observer on the bank it'll look like the boat speeds up a lot when it's facing downstream and that it slows down when it's facing upstream. Stricly points of reference for the observer is the issue here. The motorboat happily runs at the same velocity through the water the whole time.

[jamessimon]

James,

Thanks for the informative response.

Consider a Dandelion seed in a constant velocity wind. The seed approaches the velocity of the wind. If it equaled the velocity of the air mass there would be no relative wind over the seed and it would fall. There must be a small air flow over the seed to keep it in the air. That airflow is provided by the drag of the seed causing it to move slower than the surrounding air.

If you place the seed in a jar and move the jar the seed will not rise. In that case it is part of a stable system.

In your F=MA scenario the wind must exert a force to accelerate an object but no force to sustain it. You propose that at the point where the two velocities match the system becomes lossless. This is only true in frictionless, gravity free, space. That is not the case in the great outdoors of the earth.

No system is lossless. The model is not 'suspended' in a fluid. It is not in a jar. The model rides on a very good air bearing but that bearing has losses. Those losses must be accounted for by a difference in velocity between the model and the moving air mass.

If this is not true then how does the Dandelion remain in the air with no relative wind and what force provides that last little boost needed to overcome losses?

Tom

Unless the wind is blowing up the seed IS falling. There is air moving past the seed in the vertical direction. The force to resist the pressure of the air is provided by gravity. Thus the seed continues to fall. This is independant of what happens in the horizontal direction.

If you place the seed in a jar and suddenly move the jar the seed WILL rise while the jar is accelerated. Try it with a penny. This is the effect of inertia. Unfortunately there is gravity in the vertical direction so you can't get the penny to a constant velocity in the jar in the vertical direction. This is indepentant of what happens in the horizontal direction.

In this you are simply wrong. Acceleration requires a force. Objects in motion stay in motion without any force. When the velocity of the air and the object match there is not loss because there is no friction because there is no relative motion. This is indeed the case in the great out of doors.

A ballon does suspend in a fluid, as does a submarine. A model fly at a constant speed and direction is indeed suspended in the air. There are no net forces acting upon it. Lift = Weight. Thrust = Drag The same effects of air forces then apply equally well to the model. This is easy to see if you consider the case of the wind being across the direction of flight.

The seed is lifted into the air by a current of wind that blows up.

[Tom Harper]

"You stress that the drag will prevent the object matching the wind's speed. But for that sort of effect to apply there would need to be something for the object to grab on to so that the drag would be effective."

That is the point. The model has a ground reference because the force of gravity is converted to a horizontal vector through induced drag.

The Dandelion has a long thread with the seed suspended at the end. This acts like the weighted keel of a boat. It provides a vertical vector that is converted to horizontal by induced drag. What you say about turbulent flight is true. But, seeds will drift across a field in a very light breeze. They do not require turbulence.

There is a basic problem with the model in a bottle theory. An object propelled by the wind cannot reach the velocity of the force that is moving it. To do so would require work without effort. One must, in that case, assume that induced drag does not exist.

For L=W and T=D there must be L and W and for an object that is carried by the wind this can only result from W and D. If the air has nothing against which to work no force is created per Bruce's statement above (and Vogle in Life in Moving Fluids).

The illusion is that since the model propels itself, it is immune from the force of the wind.

That would require that air molecules differentiate between model surfaces and Dandelion seeds. A free flight circling in a light breeze has sufficient mass, and low enough drag, that the effects of the breeze are small. An LT40 flying in a stiff breeze will react to the wind in ways that are noticeable to the pilot. It will tend to turn into the wind on cross wind flight. It will require different trim on upwind vs downwind level flight.

In the case I observed above, the model has high drag and is able to fly in high winds. The effect of the wind is obvious.

The problem with the model in a bottle theory is that the system must be closed. If the bottle is on a huge flatbed truck the wieght of the model is accounted for whether it is in the air or resting on the bottom of the bottle. But the force required to move it is not free. It is exerted by the truck engine.

In an open system a force must be exerted to move an object. A body in motion remains in motion only in friction free space. On earth the weight of an object is reflected as a horizontal vector through induced drag. There must be a difference in velocity between an object and the wind to extract sufficient energy to overcome this vector.

Please explain how an object, however bouyant, can be moved without exerting a force upon it.

[biber]

Please try to distinguish correctly between movenent and accelleration.
For maintaining a movement there is no netto force needed.
For acceleration there is a netto force needed, that is f = m*a.
If you don't stick to the most basic mechanic axioms, then we won't be able to discuss such mechanical issues.

biber

[Tom Harper]

Biber,

Agreed. Please explain how you have successfully moved your equation from frictionless space to an earth bound environment that has losses.

You can nudge a space craft and it will go from Earth to Mars. Nudge the same spacecraft in Alviso and it will not go to Milpitas.

[HELModels]

Imagine you have a glass of water and an insect lands smack dab in the middle on the surface. Stick your finger in the water first and try to remove the insect. You almost get your finger on it and it slips away. You didnt actually exert any force directly on the insect, but it moved away because of surface tension. In air, the dandelion seed is like the insect caught in surface tension and small external air masses are like a finger in the glass. I postulate that the dandelion seed falls to the boundary of an air mass and when another airmass pushes, pulls, then the dandelion seed reflects this.

[biber]

Yes, Tom, there is friction, but the amount friction depends on the speed of motion relative to the medium wich the body is moving through.
The model isn't moving through the ground, nor on it.
It moves through air and airspeed is the only speed counting for forces on the model.
The thing, that makes me so confident in all that, is that such stuff is easy to calculate if you take the well established assumtions, axioms and laws and it's veryfied by the observations in reallity.
I don't see, where your theories can offer that 'features'.

biber

[pmackenzie]

An LT40 flying in a stiff breeze will react to the wind in ways that are noticeable to the pilot. It will tend to turn into the wind on cross wind flight. It will require different trim on upwind vs downwind level flight.



Please explain how an object, however bouyant, can be moved without exerting a force upon it.

Tom,

In answer to your question, "free" objects are accelerated by forces, not moved by them. The force could either increase or decrease the velocity, depending on the direction it is applied.
When having a scientific discussion the exact words used are quite important.

IMO you are approaching this discussion from the wrong direction. You have started off with the assumption that ground speed effects airspeed, and are trying to find enough "science" and hand waving arguments to support that theory.
Instead you should try to figure out why your assumption is incorrect.

Aircraft "react" to gusts, not to steady state wind forces.
The word react implies a response to some change in conditions.
Once a plane leaves the ground there is no mystical connection between them, so the speed relative to the ground is irrelevant.
The only force the earth continues to apply to the model is gravity. It acts toward the centre of mass of the earth, not some point the nearby surface.
The motion of the surface relative to the object is irrelevant.**
In a steady wind a model does not require re trimming for upwind or downwind flight.
It does not tend to turn into a crosswind. Just the opposite. On final as speed is reduced the [i]pilot[i] will have to increase the yaw angle to maintain heading.


The "model in a bottle" analogy is good, whether the model is a paper plane gliding in the cabin of an airliner, a model in some airmass blowing over your flying field,
the same airliner flying in the mass of air moving more or less in lockstep with the earth's rotation, or the entire atmosphere hurtling through space attached to the earth as it orbits the Sun.

Pat MacKenzie
** Newtonian physics - Einstien might have something different to say! But there is no relativistic effects in low speed aerodynamics.

[Tom Harper]

Good points, however you ignore the issues.

1. A model always has a ground reference. It is tied to the ground by gravity. That force appears as a horizontal vector through induced drag.

2. A bouyant object that rises into the air (ie balloon) is acted upon by the moving air mass. It is moved by a force proportional to it's coefficient of drag and the relative wind. It cannot reach the velocity of the wind that is moving it. If there is no difference in velocity there is no relative wind and therefore no force to move the balloon.

3. A self powered object is still acted upon by the force of moving air. The moving molecules do not know the difference between a balloon and a model airplane.

The basic issue here is item 2. Can an object reach the velocity of the air that moves it. Clearly it cannot. It is not possible to move an object without exerting a force.

[biber]

It is not possible to move an object without exerting a force.That is the core of your misunderstanding.
Your statement is not valid.

Grab any physics book and study Newtonial mechanics.

A physiks book I can recommend is that one of Paul A Tipler e.g.

biber

[capncrunch]

Good points, however you ignore the issues.

1. A model always has a ground reference. It is tied to the ground by gravity. That force appears as a horizontal vector through induced drag.


can you work this out in an equation? perhaps in a little more detail than in the OP. or draw some free body diagrams?

I suspect you are trying to account for viscous effects of moving through a (moving) fluid, which do not appreciably affect air at stp.

-barrett

[HELModels]

Viscosity of air is a factor with the dandelion seed, according to my pseudo science.

[biber]

For induced drag there is no ground reference needed.

biber

[Tom Harper]

1. In the case of the balloon:

Assume that the balloon is perfectly bouyant so that it does not bear at all on the air below it or above it. There are no frictional forces. The balloon weighs ~500kg, operates in nominal air velocities of 15mph and has a Cd around 1.2. The initial force on the balloon is proportional to 15^2 * relative wind exerted on the S of the balloon at a Cd of 1.2. As the balloon accelerates to 7.5 mph the force driving it is reduced by a factor of 4. Of course, as this process continues the force driving the balloon slows it's rate of acceleration due to the ever decreasing force driving it. The result is that it asymtotically approaches the velocity of the wind but can never reach it because the driving force will never be enough to reduce the margin to zero.

In the real world the balloon cannot even approach this ideal situation. The balloon is not free of friction. The balloon is always oscillating up and down relative to the wind. This may be due to the physics of the balloon or the turbulence of the air driving it. In that case it bears on the air above and below producing friction. That friction introduces losses that prevent the balloon from reaching the velocity of the air that drives it.

A frictionless balloon would require laminar flow around the irregular gondola and gas bag. That is not the case.

2. Induced drag:

Induced Drag is proportional to: Cl^2/(Pi*AR)

The Cl^2 term is proportional to Weight and the instantaneous flight parameters. Thrust is required to balance induced drag. The result is a horizontal vector that is proportional to the force of gravity acting on the aircraft. Thus it is always referenced to the ground.

Without a ground reference (Cl) there is no induced drag.

[Tom Harper]

Storch,

You are correct. The conditions generally considered for dispersal are terminal velocity (vertical) and time.

[BMatthews]

Gravity is only one force acting on a model in flight. Another way of looking at it is that the model is reacting to it's need to provide lift in whatever direction is needed.

Lets take a model flying inside an atmosphere but without a planet close by. Let's call it a huge one mile across balloon filled with air. The model flies through this air propelled by an engine or motor. Since there's no gravitational force the wings do not need to generate any lift so the wing is flying along at a Cl = 0. Induced drag will also be 0 but parasitic drag will still be there and related to the speed and size of the model. Now turn the model to a new heading. To do so the model will need to roll such that the wings are normal to the plane of the directional change and the model will need to pitch to an angle of attack needed to generate the lift required to acclerate to the new path. This generates induced drag for the duration of the turning.

So what's this have to do with our model's flying near our planet? Simple. The Earth is generating an attractive pull that requires that the model generate lift to maintain the required flight path. In effect what we call lift is actually the model accelerating upwards in a matching force to the gravity accellerating the model downwards. But this is simple coincidence. It does not tie the induced drag to the force of gravity any more than the the need for centripital force that is needed for the model to alter it's direction. It's just that gravity is there all the time and so the model needs to be constantly accelerating itself away from it so that the values match and we maintain level flight. There is no linkage between induced drag and gravity other than this.

[Tom Harper]

Bruce,

You have removed the gravity vector by setting the Cl to zero. in that case there is no induced drag and, since you have described a closed system, the balloon could be moving at a constant rate with no influence on the model.

When you return to earth the Cl is greater than zero and the system is open. The induced drag vector is not vertical - it is horizontal. The model can no longer extract 100% of the energy available from a moving air mass. It cannot be moved at the velocity of the air mass and is therefore influenced by the direction of flow of the air mass in which it is flying.

You have chosen the correct illustration. Most of the scenarios presented in opposition to mine would work fine in space.

[Tom Harper]

Sorry that the title I chose is ambiguous. The topic here is: down wind turn, gain and loss of altitude during upwind and downwind legs and windvaning. All of the stuff that happens because the airplane is not operating in a closed system.

[BMatthews]

The point I was trying to make is that the model still is flying in it's own frame of reference.

It's just that we "usually" fly them in close proximity to the big ball of dirt we live on. To counter the attractive forces of the earth we force the plane to fly in a mode of constant acceleration (the classic lift). But that this constant acceleration is independent of gravit other than being varied based on our demands to alter the direction of travel. There is no solid and required link between the forces of gravity and those that the model generates to counter them. If you take away gravity the model can still fly in the air, generate induced drag and manuever just fine.

The induced drag vector is only horizontal when the model flies normal to the pull of gravity as when flying straight and level. But if we alter how the model is flying, as in a tight pylon turn or during a loop, the induced drag alters it's direction along with that of the model in a manner that is responsive not only to the earth's gravity (just one factor) but also to the needs of the model's required flight path that we input. In the case of a pylon model doing an almost vertical turn, actually a looping maneuver, we see that the force of gravity in this case is a minor one as shown by the 80 to 85 degree bank angle. Again, there is no direct linkage between gravity and lift or induced drag other than the one that we, as the pilot in charge, induce on the model to make it fly in the path of our choosing.

The big balloon of air in space with the model inside it was just a way to indicate a mass of free air that was not in proximity to a planet and to show that a model can generate all of the same forces without any effect from gravity.

Your statements in this thread are a large mixture of some accurate and factual points mixed in with highly erronious assumptions.

The dandelion seed is a case in point. It's a parachute, plain and simple with a drag producing canopy (the radial fibers) and a payload (the seed) connected by the stem (lines). It falls constantly through the relative air around it at its terminal velocity as surely as a combat trooper falling out of a DC3. We only see it going "up" when the local airmass surrounding it has a vertical velocity component for whatever reason that exceeds the downward velocity of the seed. There is just no other forces at work in that system. Certainly there is no lift involved.

One of your correct statements is about the idea that an object (balloon or dandelion seed) in a moving airmass will never truly accellerate to match the average speed of the air mass around it. The idea being that as the speed of the balloon or seed approaches a match that the accellerative force acting on the object that is generated by the drag reacting with the moving air mass approaches zero while the mass of the object remains constant so that we have a situation where the balloon or seed never does actually truly match the moving air. But what that has to do with model flight escapes me. Yes, the object does gain energy from the airmass since it is accellerated from one speed to another by the energy it extracts from the air via the drag but that energy shows up strickly as a new steady state speed.

Lift comes from a shape's interaction with the air. To obtain that lift it needs two things. Speed relative to the airmass around it and a shape that produces a pressure variation.

I suppose that if you want to think of the drag related to the fibers of the dandelion's seed or the canopy of a parachutist as lift there's some room for further discussion but in that case the mechanism for the "lift" being produced is pretty simple. The one for a model in flight is a bit more complex but is the one under consideration.

For a wing the lift is generated by the air RELATIVE TO THE WING moving over the wing. So we come back to the original title of the thread. To generate the lift required the wing must move through the air or the air must move past the wing. In the case of aircraft we force the wing to move through the air. In a wind tunnel we move the air past a fixed wing. Both options produce the same effect. Unless the angle of attack of the wing alters the wing must move at the same relative AIRSPEED to the air around the wing all the time or the model will ascend or descend. Ground speed has nothing to do with this need.

I can't help but wonder if a lot of the misunderstanding in this thread are more about symantics than principles. We're tossing around a lot of terms that it would seem have differing meanings to some us.

[Tom Harper]

Bruce,

Thanks for the clarification. However, the issue is not one of symantics.

It was others who discussed the Dandelion seed in terms of lift. I meant to use it only as an example of an object that does not reach the velocity of the air mass. If in my enthusiasm I wandered, forgive me.

So, we agree that the model cannot reach the velocity of the moving air mass.

If the model does not reach the velocity of the air mass on the downwind leg then the velocity of the air over the wing will be slightly reduced. The same effect will cause the velocity of the air over the wing to increase slightly on the upwind leg. Therefore a model trimmed for level flight in dead air will gain altitude on the upwind leg and lose it on the downwind leg. This effect, though small, is integrated over the time the model travels upwind or downwind. The gain and loss is real and noticeable.

In the example I observed, the wind approached the flying speed of the model. 30-40 mph wind on a 50-60 mph model. The effect was dramatic.

[HELModels]

Storch,

You are correct. The conditions generally considered for dispersal are terminal velocity (vertical) and time.

No, I'm occassionally correct, but out in left field on this. Dandelions have very little to do with what your trying get your finger on. I'm going away for awhile and when I come back, this topic will have taken some unknown turn, but I will have built a new wing for my plane. If I return and jump into some topic with a very remotely relevant reply, then someone please ask me if my wing is finished.

Good Luck All with whatever the topic may be.

[pmackenzie]

Good points, however you ignore the issues.

1. A model always has a ground reference. It is tied to the ground by gravity. That force appears as a horizontal vector through induced drag.

Induced drag has absolutely nothing to do with gravity. Induced drag is a result of the interaction of the trailing vortex sheet with the air approaching a wing. It causes it to meet the section at a slightly different angle. This causes the lift to "lean" back relative to the direction of motion.
As a result even in an ideal fluid with no frictional drag there is a lift induced drag on a 3D wing. It can be minimized by increasing aspect ratio, and optimized by having an elliptical lift distribution.
( Lift is by definition the force acting perpendicular to the free stream flow. Drag is the force in the same direction as the free stream flow)

2. A bouyant object that rises into the air (ie balloon) is acted upon by the moving air mass. It is moved by a force proportional to it's coefficient of drag and the relative wind. It cannot reach the velocity of the wind that is moving it. If there is no difference in velocity there is no relative wind and therefore no force to move the balloon.

What you are talking about here is mathematical concept called an asymptote (http://mathworld.wolfram.com/Asymptote.html) .
Theoretically you are correct. But in practise with real objects the infinitesimal difference in speed between the wind and the object has no practical relevance.
The difference in speed will be smaller than the "noise" that will be present due to natural fluctuations in the wind speed.


3. A self powered object is still acted upon by the force of moving air. The moving molecules do not know the difference between a balloon and a model airplane.

The basic issue here is item 2. Can an object reach the velocity of the air that moves it. Clearly it cannot. It is not possible to move an object without exerting a force.

[BMatthews]

...........If the model does not reach the velocity of the air mass on the downwind leg then the velocity of the air over the wing will be slightly reduced. The same effect will cause the velocity of the air over the wing to increase slightly on the upwind leg.........

When you add RC and a pilot to the equation the various inputs and mis-trims induced by the pilot can show up in a lot of ways. So let's take away this variable and go back to some free flight models.....

Ingnition old timer models are both heavy and yet slow flying. If any example of a model was going to react as you're saying then it should be these. Often the speed of the wind relative to the speed of the model in it's glide is so extreme that the model hovers motionless relative to the ground while pointed upwind. Other than an apparent shift in ground speed they do not show any signs of a nose up and down attitude change that would be related to any such changes in airspeed. And neither do they show any signs of any cyclical gain and loss in altitude while in their circling flight.

I have to admit that over the years I've also wrestled with this old downwind turn issue. But it only seems to show it's head when I'm flying a model that is more aerobatic and forces me to be constantly dancing on the sticks. When I switch to flying my RC gliders the effect does not seem as strong despite the fact that the gliders are slower flying and you would expect the difference in wind to flyingspeed to be more pronounced. When I fly my free flight models or watch my buddies with their large,ungainly and heavy models (typically 5 to 6 ft span and 3 to 5 lbs) flying I see even less evidence of any downwind turn issues. And as I suggested if those models don't show signs of this then I'm pretty firmly convinced that the rise and fall is more related to the pilot induced speed oscillations rather than any aerodynamic or physical related issues.

[biber]

http://www.amazon.com/gp/product/0716783398/sr=8-4/qid=1148766851/ref=pd_bbs_4/102-2564762-8536133?%5Fencoding=UTF8

;)

biber

[Tom Harper]

Bruce,

I have seen ignition competition free flights turn up wind, stop the turn, raise the nose and begin to stall into the wind (Mather field 1947) without resuming the turn. This was caused by weathervaning into the wind after picking up excess speed on the downwind leg of the turn.

I have seen many free flights turn down wind and run straight with the wind, never able to recover the turn.

Neither of these cases would occur if the model in the bottle scenario prevailed.

The inability of the model to equal the velocity of the air mass in which it flys means that a wind vector must be applied to it's surfaces.

I have humored Carl for many years when he claims one of my designs is weathervaning or gaining altitude into the wind. Last week it was obvious I had to figure out why. Carl was right all along. The model is effected by the wind.

[Ollie]

Myth, myth, myth. The myth has longer years than any model or any person.

[Tom Harper]

Biber,

Thanks for the link - lol

If your physics book stops with F=Ma, and never reaches a=Fr/M, I suggest you replace it.

You have my permission to use the link you provided.

[Tom Harper]

Then good Ollie, tell me what force allows the model to reach the velocity of the air mass.

The myth is the model in the bottle theory.

[Sparky Paul]

Tom, pondering on this at the Mall during a tea-break.. I think your conditions are atypical.. too dynamic, so there's no steady state flow to establish what is or isn't trim.
Your mesa reminds me of one of the areas I slope, which has a nice entrance slope, and a long flat area behind it.
In the diagram.. the wind coming in is compressed at A. Right at the lip the speed is much faster than it is out from the slope at B. The wind at C is turbulent, with lots of rotor activity. The wind at D is less turbulent, and at F is about the same as the wind above B.
The plane's response is much more airspeed is required to get thru the compression area at A than is needed to fly at B. In fact, the plane at C or D which fly easily at B might not be able to get to A at all, due to the turbulence and rotors.
At F, to get back over the slope a dive can get it back out there.
For a windspeed-airspeed test setup in the real world, I'd choose a lakebed or a lake surface, with the measuring stuff located where the wind would be constant. Using video or onboard images, upwind and downwind passes, in conjunction with a method of measuring wind speed some distance above the ground to get out of any shear...

[BMatthews]

There's a number of reasons such things happen to free flight models and it's due to how the surfaces are trimmed and not to the air mass it's flying in. For every example of a straight flyer or other anomaly I can show you 20 or 30 that fly just as I described. And as well models that after some warps and adjustments are made go from being a "weathervaner" to being a stable circling flyer.

I've been in air that kept my gliders going up while I was just "weathervaving" too. In fact I got a longest flight of the day award one time at a Boeing electric funfly day by doing just that. But I've been in just as many situations where the model politely comes down while facing into the wind. When you find that it can just weathervane and go up then it's because there's a vertical component to the wind at that location that is going up faster than your model is gliding down.

Anyway, it would seem that this is approaching a state similar to arguing over religion or politics. I'm going to join electrostorch and go build a wing or work on a motorcycle or sumthin'. Cheers and respect regardless....

[Tom Harper]

"Induced drag has absolutely nothing to do with gravity".

Induced drag is calculated for a given set of flight conditions. AR is the most common physical term given but actually that term should be multiplied by a term for planform and another for span wise twist etc.

However: Cl^2/(Pi*AR) will suffice.

The Cl term indicates the coefficient of lift required to meet the flight conditions of interest. Cl determines lift which opposes gravity. The gravity term must be reflected in the induced drag equation. Induced drag is the price you pay for the lift you get. You cannot separate the two.


"Theoretically you are correct. But in practise with real objects the infinitesimal difference in speed between the wind and the object has no practical relevance. The difference in speed will be smaller than the "noise" that will be present due to natural fluctuations in the wind speed."

So we agree in theory. The F=Ma equation is opposed by friction. Friction in a model airplane is expressed as drag (total losses). The air mass must overcome the drag of the airplane to move it. Enough relative wind must flow over the airplane tocreate a force sufficient to cancel the drag force. That relative wind is the difference between the velocity of the air mass and the velocity of the airplane relative to it. The difference is not buried in the noise.

Asymptotes are common in daily life.

[Tom Harper]

Sparky,

Later - gotta eat.

[pmackenzie]

So we agree in theory.
No. What I am saying is this is one of the points that is leading you astray in your reasoning. You keep on bringing it up, but it is irrelevant to the matter at hand.
A capacitor connected across a battery will also never quite be charged to the same voltage as the battery, but after a few "time constants" have passed it can safely be assumed to have done so.
Pat MacKenzie

[Tom Harper]

MacKenzie,

As I said asymptotes are common.

However the capacitor eventually charges. The system becomes static. The current into the capacitor drops to it's leakage value. The moving air mass and the airplane are dynamic. The relative wind can never go to zero.

[biber]

Friction in a model airplane is expressed as drag (total losses). The air mass must overcome the drag of the airplane to move it. Enough relative wind must flow over the airplane tocreate a force sufficient to cancel the drag force. That relative wind is the difference between the velocity of the air mass and the velocity of the airplane relative to it. The difference is not buried in the noise.First of all, does your equation give an expression for a coefficient of induced drag, Cwi, not for the drag itself wich would be a force.

In case of the dandelion the drag is the force that makes it asymptotically approaching the same direction and speed of motion like the air has.
It is thus not the force to overcome, it's not keeping the seed from reaching the air's motion state.
The only force to overcome is not really a force but inertia in this case.

According to the Newton axioms one may chose any frame of reference that is not accelerated itself (such is called an inertial frame of reference).
Then you can do all calculations based on it and forget about any other reference.
That means you can also exchange your frame of reference called ground by the moving one called air mass.
Then you can throw away all groundbased thinking without any problems.
The gravity force vector still is there, but it is (like it was all the time) a free force, free to be shifted, but with fixed orientation.

That link to amazon was not just a joke, I for one have that book in german language and it does not stop at a f=m*a stage, I can tell you...

biber

[adam_one]

"Induced drag has absolutely nothing to do with gravity".

I've been following this thread for awhile… and I can't help to agree with Tom regarding the induced drag and gravity. :)

For the same plane and assuming a level flight, the higher the wingloading, the higher the induced drag will be.
But of course, induced drag also depends on other factors.
The induced drag is proportional to the square of the angle of attack and since a slower airspeed requires a higher angle of attack to produce the same lift, the slower the airspeed is, the greater the induced drag will be.
So, the induced drag is also inversely proportional to the square of the airspeed.

However, after the minimum drag speed is reached, the Total drag (parasitic + induced) starts increasing exponentially.

[pmackenzie]

"Induced drag has absolutely nothing to do with gravity".

I've been following this thread for awhile… and I can't help to agree with Tom regarding the induced drag and gravity. :)
The induced drag statement was mine. Clearly I need to try to expand on it a bit.
Induced drag is a result of 3D airflow and will even exist in an ideal fluid.
It will exist in an imaginary aircraft doing tight loops inside a balloon drifting around at 0G in outer space.
Back here on earth, it will be present all the way around a loop, always opposing the direction of flight no matter which way the lift vector is pointed.
In other words to understand the concept of induced drag, the gravitation attraction of two bodies is not required.

Since it's magnitude is related in part to the total lift, and in level flight lift has the same magnitude and the force of gravity,
Tom is asserting that it somehow "binds" the model to the ground underneath it so that actual ground speed will effect the magnitude of induced drag.

It is this last connection that is a leap of logic. No mechanism exists that I am aware of to make this happen.
The gravitational attraction to a sphere is to the centre of mass. Motion of the surface of the sphere does not come into it.

Before Gallileo people talked themselves into the notion that if an orange and grape were dropped at the same time, the orange being larger would hit the ground first.
I think it is time for Tom to come up with free body diagrams and detailed equations that will predict exactly how much a model will climb going upwind, and descend going downwind.
It is his assertion to prove. The time for talking and hand waving is over.

Pat MacKenzie

[adam_one]

Well, I assumed the discussion was based on facts occurring on earth... :)
Anyway, an imaginary aircraft in outer space has 0 G and so needs 0 lift.
But, it still has a mass and if it starts moving inside a balloon in outer space it will gain inertia, which is related to its mass.
In your outer space example, the inertia would be a force (along with the airspeed and AoA) that would cause the induced drag.

Back on earth, the magnitude of the gravity force depends on the aircraft's mass (weight).

[Tom Harper]

MacKenzie,

The problem here is that I poorly stated the title. Sorry. If I could change it I would. The discussion is whether a model can be moved without a force acting upon it.

Of course, induced drag is felt under any conditions. Since that is obscuring the argument, let's set it aside for the moment.

"I think it is time for Tom to come up with free body diagrams and detailed equations that will predict exactly how much a model will climb going upwind, and descend going downwind."

One of these days I will spend a few hours taking measurements to demonstrate the case. Can't do it today.

For the present, please explain how a model can be moved without exerting a force on it. The urban legend is that the model flys in a bubble of moving air. That the air does not exert any aerodynamic force upon the aircraft - it is simply carried in the bubble as though it were in a jar.

If you accept the urban legend you believe that a model flying at an airspeed of 40 mph in a 40 mph tail wind travels at 80 mph ground speed with no force being exerted external to it. Ground speed is important. If you calculate the amount of work being done you will normally use the ground as a reference. You have moved a mass some distance in a measured time. Because of the presence of the wind You moved the mass twice as far in the time chosen. You have done twice the work. Energy is being extracted from the wind.

My argument is that the extraction of energy requires a difference in velocity between the model and the air mass. There must be a force exerted upon the model.

Perhaps this is not true. It may be that the velocity of the entire air mass is retarded slightly by the presence of the model.

Let's narrow the discussion. How can the bubble move a model without exerting a force on it?

[Tom Harper]

Biber,

Thanks for the link and the recommendation. The book looks interesting. I have a similar one on Technical Mathematics.

So you have accounted for an a1 as acceleration. But there must be an a2 which is an opposite acceleration due to friction. So there is no steady state in which the F vector goes to zero. There must always be a force acting on the model.

Indeed the equation is for a coefficient. But when plugged into the conditions of flight it yields a force. That force accounts for the interface of two surfaces (model and air) bearing a mass (gravity acting on weight) which is friction in the physics book and Induced Drag in the aerodynamics book. Same old stuff.

As MacKenzie points out the induced drag of the wing is felt regardless of attitude. For simplicity let's stay with level flight.

An air mass cannot move a model without exerting an aerodynamic force upon it. There may be another mechanism of which I am unaware. I am willing to be enlightened.

[pmackenzie]

Perhaps this is not true. It may be that the velocity of the entire air mass is retarded slightly by the presence of the model.
This is exactly correct.
When analyzing the "downwind turn" problem from a conservation of energy perspective if you look at the model only it would seem that the system somehow gains energy down wind and losses energy going upwind.
If you try to account for the change in kinetic energy by changing the potential energy through altitude changes flying in any sort of wind would be impossible.
A correct analysis of the system must include the air mass it is travelling in. Since it is so vast the change in speed of the air mass to account for the energy change is minuscule.

An analogy would be taking a step. You move, but to maintain angular momentum the earth must rotate a bit as well.When you come to a stop it moves back.


Let's narrow the discussion. How can the bubble move a model without exerting a force on it?
Stated more precisely, the mass of air in the bubble cannot accelerate the model without exerting a force on it. F=ma always.
And the force the bubble applies to the model is also applied to the bubble by the model. Equal and opposite forces. The bubble also accelerates.
Pat MacKenzie
P.S.
Ever see the movie "Brain Candy"?
I am reminded of this bit of dialogue.
Character A is trying to convince character B not to make public the news that the new drug has dangerous side effects:

A) Like going to the press, wouldn't you agree?

B) No, I wouldn't. We gotta get the word out.

A) Yeah, but you would agree that Paris is the capital of France?
- Wouldn't you agree to that?

B)- Yes, but--

A) Good. Then we're back in agreement.

[Tom Harper]

Pat,

Then, I think we agree that the moving air mass exerts an aerodynamic force on the model. The observable effect on the air mass is small compared to the observable effect on the model.

Did we progress that far?

[pmackenzie]

As long as my agreement does not involve the "observable effect" on the model being a change in altitude, I will agree.
Pat MacKenzie

[adam_one]

Ok folks, let's remember the Newton's First Law of Motion:

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Now for simplicity, one may think of an object floating down a river (I know it's buoyancy) there it should be needed a force exerted by the water upon the object in the beginning in order to get it moving. But eventually the water would carry the object along with the same speed as the water molecules (no friction).

But it's somewhat different with an aircraft downwind, as it also needs airspeed to keep the altitude, but I guess if it has power to keep enough airspeed, the ground speed will eventually be equal to tailwind speed + airspeed...

[Tom Harper]

Pat,

Then do we agree that there is a difference in velocity between the model and the air mass.

[Tom Harper]

adam,

Refer back to #61 - begin at 'Indeed'.

[Tom Harper]

Sparky,

The slope is not dramatic. It is a wind deposited sand.

[adam_one]

Indeed the equation is for a coefficient. But when plugged into the conditions of flight it yields a force. That force accounts for the interface of two surfaces (model and air) bearing a mass (gravity acting on weight) which is friction in the physics book and Induced Drag in the aerodynamics book. Same old stuff.
The friction you refer to (drag) is a result of the airspeed produced by the powerplant.

Let's try another analogy:
Imagine a bowl with water inside a car. When the car starts moving, the water tends to move on the opposite direction due to its mass/weight but after a while the water stands still inside the car despite it's moving with a certain speed in relation to the earth (assuming a constant speed).
Like the car carries the bowl with water, the air mass carries the aircraft…

[Tom Harper]

adam,

That is the model in a bottle concept. The car engine supplies the energy. Different game.

[pmackenzie]

Not really.
Wind just gets it's energy from natural sources. Pressure or temperature differences, coreolis forces, etc.
The energy stored in wind is huge.
Pat MacKenzie

[Tom Harper]

Pat,

But, we agreed in 63/64 that the moving air mass exerts an aerodynamic force on the model. How would that be possible if the velocities were equal?

[adam_one]

adam,

That is the model in a bottle concept. The car engine supplies the energy. Different game.Tom,
That is the same as to say that pressure differences in atmosphere supply the energy that moves the air mass... the same game.
My analogy was to illustrate that the aircraft is carried by the air mass, which may or may not be moving in relation to the earth.

[Tom Harper]

adam,

Go back to post #1. If the model were in a bottle on a huge truck, It's weight would be felt on the bottom of the jar whether it was in the air or at rest. In this case the truck pays for the cost of moving the model - not the air mass.

The airplane is supported by an irrotational vortex that is set up within the air mass. No pressure wave extends to the ground supporting the model. When a model flys over a field, it's weight does not disturb the ground below it.

In your closed system case the weight of the model adds to the total weight of the jar. You can demonstrate this with a fly in a jar on a balance scale.

[pmackenzie]

Pat,

But, we agreed in 63/64 that the moving air mass exerts an aerodynamic force on the model. How would that be possible if the velocities were equal?
I knew you would take my "agreement" to a minor point and see that as support for you theory. See my PS in post number 62.

The velocities are not equal. The plane is flying. Of course there are forces acting between it and the air. That is what I agreed to.

As the plane circles it applies force to the air around it, which in turn pushes on air further away.
The ground and the speed the system is moving relative to it has no effect.
There is no physical force between the moving model and the ground below other than gravity which operates perpendicular to the direction of motion.
It's magnitude or direction does not vary with wind speed, ground speed or airspeed, so it can't alter the flight path of the model as the model circles.

I will say again, the time to make verbal arguments is over. They are of little value.
Do the physics and show the math that supports your claim. Be sure to show that it works from all reference systems.
From the point of view of the pilot on the ground. From the planes point of view. From an observer at the north pole who can see the earth's rotation.
From an observer in space who can see the motion of the earth around the sun.

The most amusing thing about these "downwind" turn discussions is that turning from upwind to downwind is never a problem. It is the turn back to upwind that can lead to a stall or loss of altitude.
This only occurs because you misjudge the speed on the downwind leg and/or try to get the turn done before you drift too far downwind. It is all about perception.
If you keep the speed up on the downwind leg and initiate the turn much sooner than you think you need to it is no problem.

Pat MacKenzie

[pmackenzie]

No pressure wave extends to the ground supporting the model. When a model flys over a field, it's weight does not disturb the ground below it.



The air supports the plane, and the earth supports the air and everything in it. It doesn't matter if it is floating or flying.
We can't sense it because in the case of subsonic flow it is spread over such a large area, and the atmosphere itself is very heavy.
Pat MacKenzie

[Sparky Paul]

Sparky,

The slope is not dramatic. It is a wind deposited sand.
.
That geography is too variable to expect the wind to behave in a steady-state condition.

[Tom Harper]

Pat,

Sorry. I'm not a physicist. Can't do the whole presentation. Sometime soon I'll see what I can do at the flying field.

I used the downwind turn only to relate this discussion to others. I have observed the gain and loss of altitude upwind and downwind and weather vaning when flying crosswind.

My conclusion from all of this discussion is to agree that the model is moved by the air mass that supports it. However, the coupling is not perfect. It cannot be. So the model must lag slightly behind the motion of the air mass.

I'll see what I can come up with to demonstrate it.

[Tom Harper]

Thanks for the discussion.

[Ollie]

Well said, Pat!!!
Well said, Paul!!!
Well said, Biber!!!

[BMatthews]

Tom, I think we can all agree that the air mass movement to drag effect will never be able to accelerate a freely suspended object truly up to the wind speed. But the point that many of us have tried to make is that this effect is below the level of the "noise" that is encountered in windy flying. You're ascribing far too large a role to the effect. A role that it just does not play to any degree that is going to be observable. The comparison that someone made a while back about the charging of a capacitor comes to mind and is probably a lot more realistic.

Your flying field is not a valid place to test or base your nose into the wind climbing effect as a basis for this theory. The diagram and picture show that there is far too much terrain effects to produce a steady horizontal wind effect. If it wasn't coming down then your model was climbing or sustaining altitude in a slope or wave generated vertical component of the airmass. RC handlaunch glider fliers are able to slope soar of less than you have at your field. One that I remember from some time back was slope soaring at altitudes of up to 20 feet off a 5 or 6 foot kick along the end of a field or at the beach.

For any experiment to work or for a theory to be demonstrated you would need to eliminate as many variables as possible. As soon as you have a radio and pilot inputs involved and terrain effects in the wind that cannot be eliminated the experiment or demonstration becomes meaningless.

Getting back to my buddy's and my free flight models at least I showed that the pilot factor has been eliminated. That alone was enough to provide many, many observed flights over more than 2 decades of flying where the speed variations, and therefore the need to accelerate the ground speed, was extreme to the point of being 100% or more on occasion. Yet in all those flights I have never observed any heading related altitude variation within the circling flight that could not easily be a tied to any wind mass and model acceleration effects.

The best thing I can suggest that you try is find a very large flat area somewhere and fly your model up and then trim it into a circling glide. Then watch closely over a few dozen flights of this nature. Even then thermals and other turbulence will kick it around and on some flights where it is grazing the sides of a thermal it may even look like it's going up on the upwind side. But look at it over a large number of flights and you'll see that this whole airmass accelerating the model issue is not something that is apparent. If it's there then it's such a small factor that it's buried far below the effect of the other factors. You're also going to need to allow for the angular situation of your sightlines to the model.

Actually it would be slick if a free flying model of this sort could be set up with a sensitive onboard speed sensor and variometer and compass heading indicator. That would soon answer a few questions. As the compass heading runs around the clock the speed and climb/descent rate could be measured and directly compared over a long period and many flights.

[Tom Harper]

"Actually it would be slick if a free flying model of this sort could be set up with a sensitive onboard speed sensor and variometer and compass heading indicator. That would soon answer a few questions. As the compass heading runs around the clock the speed and climb/descent rate could be measured and directly compared over a long period and many flights."

At this moment Tom, whose real identity is Dr. Vengence, is working in his secret lab to create just such a machine. It will soon rise far above the desert to smite ignorance and vindicate the faithful.

BTW:

Real free flights need one of these -

[BMatthews]

<Cartman voice> sweeeeet...... </Cartman voice>

A Contestor.... and such a clean one to boot! Geez, there's hope for you yet, bro.... :D

My own collection consists of a Super Cyke, that's never been in a plane and a Wahl Brown Jr rep that live in the nose of a Zephyr built by a friend that has his own wings these days.

A couple of months back I found a semi kit for a New Ruler that a buddy gave me some years back and that I'd totally forgotten about. I'm thinking of starting on that but while it'll begin it's life as a free flight I plan on building in the extra wood so that I can easily cut away the control surfaces later on to make it RC assisted.

Some form of big antique cabin job for that Contestor would be oh so nice to see flying around your desert on the calmer days.

[Sparky Paul]

I would expect if there were an objectively measureable effect of wind of a vehicle's speed upwind and downwind, such an effect would be included in the process of speed records, both on land and in the air.
As it is, the speed in one direction is added to the speed in the opposite direction, and the number averaged for the speed.
With all the national and international prestige attached to such records, anything at all that would alter the average speed would be part of the equation.

[vintage1]

I vote a new forum be created: 'Modelling pseudo-science' - for threads like these..

[Tom Harper]

Dr. Vengence is creating a machine that will settle the question once and for all.

We need a jury to evaluate the data.

[Ollie]

"'Modelling pseudo-science' - for threads like these.."
vintage1 says it all.

[Bg~]

Tom,
Have you heard or Zeno's Paradox? Here is a link, but I will summarize:

http://www.mathacademy.com/pr/prime/articles/zeno_tort/

Zeno's paradox says that you cannot cross a room without going halfway. And then at the halfway mark, you cannot finish until you go halfway of the remaining distance, and so on. This reasoning leads to the conclusion that we would never make it to the other side of the room because we always still have halfway more to go! It says that motion is impossible...of course, motion is possible. The resolution to this paradox lies in the time it takes to cover the increasingly small halfway distances...meaning that the closer we get to the other side of the room, the less time it takes to cover half the remaining distances, until it takes no time at all, and thus you cross the room in some finite amount of time. See the link for a more clear explanation.

I think this can be applied to the dandelion situation. You said the dandelion will asymptotically approach the speed of the wind, but never reach the actual windspeed. If you play this reasoning backwards, this means we could never stop once we started moving. Obviously not true, of course. My point is, when you use the word asymptotically, you are actually saying that the speed difference between the dandelion and the wind is getting smaller and smaller, and AT THE SAME TIME the force required to produce the acceleration to close the speed gap gets smaller and smaller. Applying the resolution to Zeno's paradox to this situation, we can see that the dandelion will in fact match the speed of the wind in a finite amount of time.

It all comes down to this: If you accept the dandelion will never match the wind speed, then you must also accept that we would be unable to stop moving once we started. After all, our feet aren't generating any horizontal opposing forces if we are just standing still.

[Aerowhatt]

There is a simple explanation for why most of us observe a climbing attitude when turning upwind and a loss of altitude from turning downwind.

First let's establish two things.

1. A body in a steady homogeneous flow will reach equilibrium at the precise velocity of that flow. There is no drag opposite to the flow until the object is accelerated by some other force, faster than the flow.

2. A neutrally trimed airplane given a small change of attitude in relation to the air it travels through will either climb and continue to climb, or else decend and continue to decend for some distance (before correcting due to decreased or increased airspeed from the accent or decent).

The observed result of turning in or out of the wind differs from the physics because of one factor. When a steady wind blows straight across the ground. The velocity increases as the altitude increases. This is because the farther from the ground the flow is, the less it is affected by the parasitic drag of the ground opposing the flow. Close to the ground the flow is slowed and the flow "piles up" onto itself. This causes the higher altitude flow to develope an upward velocity component in addition to it's much larger horizontal velocity component.

In other words the ground slows down the airflow and the faster airflow upwind has to find it's way up and over the slowed flow nearer the ground.

From our perspective on the ground the angle of attack of the wing is relative to the ground. From the wings perspective, the angle of attack is relative to the airflow, not the ground. In still air there is no difference between the two. In a steady wind reasonably close to the ground the two perspectives diverge from one another.

The relative velocity of the airplane to the air that it's moving through is unaffected by whether it is going upwind or downwind. What is changed is the relative angle of attack of the wing.

When you turn upwind and the angle of attack relative to the ground is held constant. The angle of attack of the wing relative to the flow has increased which produces more lift.

Conversly when the airplane turns downwind and the angle of attack relative to the ground remains constant. The angle of attack relative to the air flow decreases. Yielding a incrimental loss of lift and causing some loss of altitude.

The stronger the wind the more dramatic the effect. The farther from the ground the less the flow is disturbed by the ground and the less apparent is any effect. That's generally why sailplane fliers see it differently than others. They tend to fly much higher.

Aerowhatt

[Tom Harper]

Aerowhatt,

That is an interesting point.

This would not explain weathervaning.

Bg~

Zeno's paradox has a practical limit. The common illustration is that a boy and girl approach each other in the manner described. They may never completely close the gap but they will quickly be close enough for all practical purposes.

Turn this issue around. Consider building a device whose operation requires that there be no differential between it's velocity and the moving air mass that surrounds it. I think the practical margin would prevent it from working.

I should be able to provide test data by the end of the week. Cross wind passes will separate velocity differences from Aerowhatts gradient. If I am wrong I will post a groveling withdrawal of my position. I give good grovel.

However, if I am correct I expect to see some public consumption of crow by multiple individuals.

[Tom Harper]

Bruce,

I scaled up the plans of a New Ruler from the original article in Air Trails magazine. It was my dream plane. I double covered it with silk span and painted it red and white like the one in the article. For power I used my dream engine - Anderson .65.

The thing weighed a ton, but it was so overpowered that it flew well. No need for a dethermalizer though. Took last place at the SAM championships in Las Vegas years ago. The model in my avtr uses the wing with the dihedral removed.

Once read an interview with Henry Struck. He said he only had a worn out Brown for all of his models. So his secret was to build light. The original is a lot like a rubber powered model. I had to beef it up for the Anderson. I've got a Brown Jr. and once in a while I consider building a light weight New Ruler. But I'm too old to go chasing FFs through the cactus.

[Bg~]

... they will quickly be close enough for all practical purposes.


... I think the practical margin would prevent it from working.

So close enough for all practical purposes except for the ones you invent to support your argument? :) Zeno's paradox *is* solved, and not by invoking 'practical limits'. The boy and girl DO meet, you will cross the room in a finite amount of time, and the dandelion will match the speed of the wind...and ground speed does *not* affect airspeed.

[Tom Harper]

We shall see.

Zeno never surveyed a coastline. Practical applications have margins.

[Aerowhatt]

Wearthervaning is simply the inability of us pilots to allow the wind to carry our model downwind at whatever rate it wants to. I think you will find that if you put the pilot on a platform moving in equilibrium with the wind flow you would see no weathervaning whatsoever. It's the pilots perspective in a cross wind that forces him to angle the plane into the wind.

If weathervaning were a true phenomenon wouldn't the plane continue to rotate more and more into the wind? Until it was facing directly into the wind. However, this is not what we observe. What we do observe is a steady crab angle adequate to maintain near zero ground speed downwind. If you try, you can see that we over or under rotate our turns to gain or maintain ground automatically. It takes a lot of dicipline but you can fly crosswind without correcting for the wind. Your plane quickly moves downwind off of the field though

When you do. you get the same thing you get when you row a boat perpedicular to the current. You go downstream at the currents velocity while you cross the river.

If you use a shore reference on either side of the river. It's all but impossible to fight the urge to angle the boat into the current to offset it's motion downstream.

Aerowhatt

[Tom Harper]

Aerowhatt,

Good point. The yaw is simply rudder applied to keep the model from escaping downwind. Sounds likely.

[Aerowhatt]

Sort of, but not even rudder applied.

What I observe is that we over rotate downwind turns and under rotate upwind turns leaving the model faced into the wind some. Then it just crabs across the field (crosswind) maintaining that angle with the wind without any further control inputs.

Aerowhatt

[Tom Harper]

I'm sure that is true, but Carl has attempted to trim the model for hands off flight while flying cross wind. It always yaws into the wind. It's still possible that he is causing the yaw, but he's not convinced. I'll see if we can control the conditions.

[BMatthews]

When I fly my gliders I normally do set up a crabbing flight to carry it across the face of the oncoming wind and I end up using as much control to turn it off the wind as I do into the wind. All the disturbances that alter the model's heading are easily seen to be related to thermal or other turbulence. On calmer days I typically do not have to touch the controls to maintain the crabbing flight across the wind at all if there's no thermals.

[Aerowhatt]

I think it's going to be hard to prove it to yourself from the perspective we have flying RC. You need to be onboard so that your frame of reference is tied to the moving air mass, not the ground.

If you think about it though what are the possibilities for "weathervaneing" reaching a steady state equilibrium. As stated before the airplane could turn all the way into the wind. That"s kind of what they are designed to do. Fly forwards through the air mass except for some very brief and extreme conditions.

The only other reasonable possibility is that the plane would want to orient itself such that it presented an equal amount of resistance forward and aft of the axis of rotation (somewhere between the center of lift and the center of gravity) to the side wind.

In the first case the plane would slowly turn into the wind on it's own without control input. This is not what we observe though so that scenario is unlikely.

In the second case the crab angle would always be the same, independent of the speed of the crosswind. Because as the resistance forward of the axis increased so would the resistance aft of the axis increase equally. They are both dependent on only one variable, the speed of the cross wind. This is not what we observe either though.

What we do observe is that the stronger the wind the greater the crab angle. Except for the new guy at the field who is about to loose sight of his plane downwind.

If it was drag and wind force rotating the plane into the wind. That angle would always be the same angle (for a given airplane) regardless of the crosswinds velocity.

All that said, I am looking forward to reading about your experiment.

Aerowhatt

[HarryC]

Tom, if what you say is true then you had better patent it, because within a few months from now you will be richer than Bill Gates. What you are describing is a perpetual motion machine, objects that overcome their own drag simply by virtue of moving through the air - give an object a push to get it started and it will magically overcome drag and keep going without the need for an engine to push it.

On the other hand you could be wrong.

Harry.