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Old Feb 11, 2008, 08:38 AM
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Upwind/Downwind flying characteristics

Before you start saying "The plane doesn't know if it's flying up wind or down wind" just hear me out. I understand the concept, but something I've noticed is that if the ground speed is near zero (flying into the wind), and I bank down wind, the ground speed is still very low, but now the air speed has dropped substantially. The plane is needs to accelerate (ground speed) to regain it's airspeed. This acceleration takes time and energy, so in that time the plane sinks rapidly until it's airspeed increases.

(edit: kept it short)

Am I seeing this right?
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Old Feb 11, 2008, 08:47 AM
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Phil?...
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Old Feb 11, 2008, 08:49 AM
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No. That is the classic downwind turn myth. Is that short enough?
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Old Feb 11, 2008, 09:29 AM
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Argh. I knew you would say that

But if it's a myth, the plane would not drop 20 feet when I make the down wind turn.

Please explain... I spent an hour experimenting with this over the weekend. I found that if I expanded the leg which is 90 degrees to the wind, and then slightly bear off (to build speed) before entering the down wind leg of the turn, I lost much less altitude.

My conclusion was that I needed to get the plane moving fast (ground speed) to overcome the drop in airspeed when turning down wind.

If it's a myth, why did I need to build speed before turning down wind?

Seems to me this is the equivalent to tacking a sailboat... if you don't maintain speed through the tack you stall.

I am starting to think the Myth applies to flying into the wind or with the wind, but not necessarily the initial turn. Acceleration requires energy, and the plane must physically accelerate to regain airspeed.
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Old Feb 11, 2008, 09:43 AM
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What Phil is saying, or so I think, is that ground speed is one of those relative ideas. The plane flies at it's flying speed in the air it is in. If the planes does not have the speed it needs to flying in it's own micro environment of moving air, then the plane will stall, or loose alttitude until it speeds up to the flying speed it needs to fly in the moving air. That is what we try to judge in the real world because we only see the ground speed as the indicator of the speed of the plane.

Again, in the real world, we are trying to fly the plane to find the area we call the micro environment of the thermal, when we circle. We do not ideally fly in the "box car of air" but rather fly in and out of the micro-environment (the edge) until we find the thermal dimensions and can fly totally in the box. At that time, we then fly the proper turn and bank angles. Flying in the thermal is the easy part, finding the box is the hard part.
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Old Feb 11, 2008, 10:01 AM
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I am following you on that Thermaln2. What I am referring to (may be something different than what you guys are explaining) is this concept:

If I am facing into the wind, holding the plane before launch, I can let go and it'll hover... because it's airspeed is effectively the wind speed.

If I turn it around facing down wind, and let go, it's airspeed is actually a negative number (or lets just say zero). It falls to the ground.

Toss the plane 90 degrees to the wind, and you can build speed before the down wind turn and it does not fall.

A sudden turn down wind without first building additional airspeed creates a similar situation in the sky.

Ok now wrestle it out of my mind if you can.

Perhaps upwind/down wind flying characteristic is not the correct name of the thread.

It should have been called "Maintaining air speed while circling".
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Old Feb 11, 2008, 10:11 AM
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Old Feb 11, 2008, 10:35 AM
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The longer answer

vic20owner is exactly where I was on this issue in the mid 1970s when I first heard of the downwind turn controversy. It is only human to be fooled, initially by this myth. We are ground based creatures and all of our experience is ground based. We naturally find it very difficult to observe flying objects from our ground based perspective and force ourselves to see only airmass related effects and ignore the visual and mental perspectives that come from our ground based observation point.

Just to keep this discussion on topic I will carefully define what I am talking about: I am talking about a glider that is flying in a smoothly flowing air mass. There is no turbulence in this air mass. The air mass is not accelerating relative to the ground. The air mass is simply traveling horizontally across the ground at a constant velocity relative to the ground (this is called wind). There is no vertical movement of the air mass which means we are not talking about thermals or sink.

Imagine a glider flying upwind. Now suddenly turn the glider 180 degrees so that it faces downwind. Now the glider has lost all of its airspeed and actually has a negative airspeed. Of course, the glider will stall, drop its nose and lose altitude until it regains airspeed. It will do this because its airspeed has dropped below its stall speed. So I've just proved that the downwind turn myth is not a myth at all, it is a reality. Right? No. I haven't proved anything yet.

Imagine that same glider flying downwind in the same air mass. Now suddenly turn that glider 180 degrees so that it faces upwind. The glider will then have lost its airspeed and actually have a negative airspeed. The glider will stall, drop its nose and lose altitude until it regains airspeed again. The same thing will happen if you SUDDENLY turn a glider around 180 degrees from the original direction it was flying. It does not matter which direction it was flying relative to the wind direction. The important thing to notice here is the word SUDDENLY and that is the crux of the issue.

Gliders do not turn suddenly. If you attached a maneuvering rocket to the nose of the glider or had an aircraft with thrust vectoring such that you could turn the glider/aircraft suddenly around 180 degrees then you might need to worry about stalling or losing altitude in the turn. It still wouldn't matter if the turn was made from upwind to downwind or downwind to upwind though.

Gliders like all objects with mass, have inertia . To make a glider change its speed or its direction of motion you must apply a force to the glider. lacking maneuvering rockets or thrust or thrust vectoring, the only forces that we have available to change the glider's speed or direction of travel are aerodynamic forces. To turn the glider we must bank the glider's wing and use wing lift to apply a horizontal force to the glider. This horizontal force then accelerates the glider sideways and initiates a circular flight path. We can choose to continue the circular flight path for a long time and just fly circles or we can chopose to level the wings and begin flying in some new direction. The point is that we are using aerodynamic forces to apply an acceleration to the glider that causes the glider to change its direction of motion. The glider can also change its airsped or maintain a constant airspeed depending on what is done with the elevator control. To maintain a constant airspeed throughout the turn, it will be necessary to hold a bit of up elevator relative to the straight flight elevator position. If we hold less up elevator than that, the glider will gain airspeed in the turn. If we hold more up elevator than that, the glider will lose airspeed in the turn.

Hopefully you see now that the direction of the wind has no effect on any of this discussion. When the glider changes its direction of flight, it changes its momentum from one direction to another but it always does this through the use of an aerodynamic force that applies an acceleration to the glider. The change in momentum when turning from upwind to downwind is exactly the same change in momentum as when the glider turns from downwind to upwind. It is only the glider's motion relative to the air mass that matters because it is only the airmass that has any physical contact with the glider and only the airmass that can accelerate the glider or change its momentum. Yes, gravity also applies a force to the glider and can change the glider's momentum but gravity only acts in one direction and is constant in magnitude and acts perpendicular to wind direction so clearly this has nothing to do with how a glider will turn relative to wind direction.

If your plane drops 20 feet during a downwind turn it is because you did not handle the controls in a way to maintain altitude during the turn. Any level coordinated turn requires the proper application of aileron, rudder and elevator controls. When these controls are apllied by a ground based observer then it is very easy for that observer to get things wrong. It is especially easy to get it wrong if that ground based observer expects there to be some effect due to the downwind turn. If you were standing on a moving platform that was traveling with the wind while you were piloting the glider, then you would never see the downwind turn effects and you would never even be able to tell which way the wind was blowing without looking at ground based objects.
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Old Feb 11, 2008, 11:05 AM
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Thanks Phil... I am going to need some time to digest that
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Old Feb 11, 2008, 11:06 AM
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Phil, very good explanation.

Regarding flying in wind vs flying in no wind... Do you think the inertia of the plane acts differently? Dynamic soaring shows that there is a relationship with inertia and ground speed.

Visualize a plane parked into the wind, it has nearly no inertia. Now turn downwind and it must gain a lot of inertia to get its ground speed up, even while maintaining air speed. This sudden gain in inertia must rob energy from somewhere else??? resulting in a loss of altitude?

Now take the same plane, same airspeed, but no wind. The plane flying in the same direction now has a given inertia, and to turn 'downwind' the vector changes direction but the total inertia basically stays the same.

Does this make sense? I know the downwind turn myth has been discussed many times, I have read many of the discussions but to be honest there are still some mystery forces that I haven't figured out to explain the differences of how a plane handles wind vs no wind. I notice IT SEEMS LIKE I can't make as sudden of a turn if flying in wind while maintaining energy. Maybe there is a function of change of inertia over time, obviously the higher the ratio the more energy to be gained or lost. I'm thinking something like the opposite of dynamic soaring is happening when we are turning in wind, changing our ground speed and therefor our inertia.

Edit: for arguments sake assume for my example both planes are flying the exact same turn rate, I argue that even in a slow turn the plane flying in wind has to come up with more inertia (energy) to complete the turn.

The solution for flying in wind has been to ballast up, fly faster and make the upwind turn slower... all support this idea of conserved energy and inertia being effected by the wind.
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Old Feb 11, 2008, 11:25 AM
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Exactly what I am trying to describe also.... effects of inertia and the forces required to change it's direction which in this case is gravity and drift.
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Old Feb 11, 2008, 11:44 AM
db1
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An experiment

Excellent technical description as always Phil.



Here is an experiment to help visualize the lack of effect of wind on an airplane:



You’ll need an assistant, a piece of paper to act as the ground, a circle template to represent the air mass (wind), a pen to represent the plane.



The paper is the ground. If you lay the circle template on the “ground” and use your pen to draw a circle, this represents the circle you would fly on a calm, wind free day.



Now, have someone pull the piece of paper slowly while you hold the template. Draw your circle again, making sure to try to keep the speed of the pen constant all the way around the circle. Make a couple circles. The path drawn gives insight into why we mess up our turns. Very the wind speed and see how that affects the path. Try it. J

db
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Old Feb 11, 2008, 12:35 PM
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Quote:
Originally Posted by db1
Excellent technical description as always Phil.



Here is an experiment to help visualize the lack of effect of wind on an airplane:



You’ll need an assistant, a piece of paper to act as the ground, a circle template to represent the air mass (wind), a pen to represent the plane.



The paper is the ground. If you lay the circle template on the “ground” and use your pen to draw a circle, this represents the circle you would fly on a calm, wind free day.



Now, have someone pull the piece of paper slowly while you hold the template. Draw your circle again, making sure to try to keep the speed of the pen constant all the way around the circle. Make a couple circles. The path drawn gives insight into why we mess up our turns. Very the wind speed and see how that affects the path. Try it. J

db
DB, I understand how flying in wind affects your ground track.

I guess it gets confusing when you bring inertia to all of this, and if it is related to ground speed or airspeed. I'm not really sure what the math proves, need to read about it more... but I suspect the direction of travel, vector, has more to do with it... and in regards to that the change in energy would be roughly the same in wind and no wind, even though the maximum energy on the windy day might actually be more.

In the end I think phil is right and the inertia only has to do with the change in speed, not the total speed.
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Old Feb 11, 2008, 12:51 PM
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So if the plane is flying at 10mph into a 10mph head wind, it's effectively at rest because it's not moving.. instead the wind is moving past it (lets forget the ground... not important here).

If we quickly turn down wind the plane needs to accelerate to compensate for less wind moving over the wings. Obviously this cannot happen instantly and it requires energy to start an object moving which is at rest. The energy is gravity (and of course some drift from the wind itself).

Using gravity to regain air speed costs us altitude.

Maybe I am delusional? I'm still waiting for a reference to the airspeed and accelerometer experiment of down wind glider turns.
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Old Feb 11, 2008, 01:00 PM
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Quote:
Originally Posted by hogfarmer
Dynamic soaring shows that there is a relationship with inertia and ground speed.
Please read the second paragraph of my post again. I will also add to that: there are no wind shear zones involved. The air mass is just a mass of air that moves relative to the ground at constant speed and all parts of the air mass move at the same speed. Dynamic soaring involves the use of wind shear zones, which means that the air is moving at different speeds at different altitudes. To utilize the wind shear requires that you alternately turn and dive/climb into and out of the different wind speed zones (across the wind shear).

We are not talking about dynamic soarring in this thread. We are talking about doing turns in an air mass that just happens to be moving relative the ground.

Quote:
Visualize a plane parked into the wind, it has nearly no inertia.
Words need to be carefully defined and used when discussing engineering and physics principles. All objects with mass have inertia. Inertia is the tendency of an object to resist changes in motion. An object has the same inertia regardless of whether it is moving or stationbary relative to any arbitrary reference.

Perhaps you meant to say momentum. Momentum is described by the mass and velocity of the object. But velocity is not something that is intrinsic to the object itself. velocity must always be described as being relative to something else. An object has velocity relative to some other thing. You, sitting at your computer have zero velocity relative to the floor your chair sits on. You have a very high velocity relative to the center of the earth, this due to the rotation of the earth. You have a much higher velocity relative to the sun. You have a higher velocity still relative to the nearest galaxy outside the Milky Way galaxy. If your computer chair were sitting in the back of an RV that was traveling down the highway, you would have an important velocity relative to the cars that are traveling on the oposite side of the highway. If you wanted to analyze what would happen if your RV swerved across the median into the oncoming traffic, you would need to use the correct reference when analyzing the problem. Your outcome would depend only on your velocity relative to the oncoming cars. It would not matter how fast the earth was spinning or how fast the earth circled the sun because both you and the oncoming traffic are on the same ground reference that is spinning and circling the sun.

If you wish to analyze the motion of a circling glider you need to only analyze the glider's velocity relative to the air mass because it is only the air mass that contacts the glider. The ground's motion below the glider does not affect the interaction between the air and glider any more than the earth's rotation or motion around the sun affects your highway collision with oncoming traffic.
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Now turn downwind and it must gain a lot of momentum to get its ground speed up, even while maintaining air speed. This sudden gain in momentum must rob energy from somewhere else???
I have changed the word inertia to the word momentum in your question because I know that is what you meant. The glider maintains inertia as long as it has the same mass. The glider does not need momentum relative to the ground to fly, the ground is irrelevant to the way the glider flies. the glider only needs airspeed to fly. The glider uses wing lift to apply a force to the glider. This force, supplied sideways via the banked wing, causes the glider to change its direction of travel. The energy for this turn ultimately comes from gravity. The glider maintains airspeed by descending relative to the air mass. The airspeed is used to generate lift which in turn accelerates the glider (a change in direction is an acceleration). There is no mysterious missing energy here. The energy required to keep the glider aloft and to make the glider turn or change its speed is supplied by gravity. If there happens to be a thermal then the glider can use that energy to avoid the need to descend (relative to the ground) while the glider descends through the air to maintain airspeed.
Quote:
I notice IT SEEMS LIKE I can't make as sudden of a turn if flying in wind while maintaining energy.
I'm glad that you used all caps for the word SEEMS. It is the all important fact that you are a ground based pilot that makes a very powerfull optical illusion that can make things seam to be different from what they actually are. This optical illusion also makes people invent some very strange physics to explain what they think they are seeing.
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