Taking off into wind

[baleares]

I understand that once an aircraft leaves the ground it does not"experience" wind and that wind only affects speed relative to ground.

But I am having trouble dealing with this:

If the take off speed is 60kts and there is a headwind of 50kts I think I am correct in saying that I would only need a further 10kts forward motion into the wind to take off. In this situation, I understand it that I would be travelling 10kts with respect to ground and 60kts with respect to the wind.

Surely, this means that when I take off, as I "do not experience wind", I only have 10kts airspeed which would mean a stall?

would really appreciate any help

[Scott C]

Your airspeed is still the same, 60kts, so therefore your relative forward motion is still 60kts.

With a 50kts headwind, your ground speed would decrease to 10kts, so you would still be flying along at 60kts airspeed, but you would only be flying at 10kts relative to the ground.

Say, for instance, you took off then turned downwind. Your airspeed would still be 60kts, but your ground speed would become 110kts as you now have a 50kt tailwind.

Airspeed and ground speed are two completely separate things, so don't get them confused.
Also, if you're planning a flight on a windy day, make sure you do a thorough fuel calculation, as even with full tanks, you may not make your destination!

[baleares]

Thanks for your help but I am still not getting it!

I understood that, when the aircraft is in the air, a headwind, (or tailwind) has no effect on TAS?

I thought that an aircraft in the air is analogous to walking on a conveyor belt.

If the conveyor belt is going forward or backwards it makes no difference as the only thing that matters is your relative speed to the conveyor belt as though it were still? To complete the analogy, the speed of the conveyor belt (wind speed) only affects the speed of the object on the conveyor belt relative the ground?

I'm struggling to understand how the wind can contribute to TAS on the ground (and shorten takeoff) and then once in the air have no effect on it?

Would really appreciate any help.

thanks :)

[fireflybob]

Think of a fish in a goldfish bowl. It has a speed relative to the water as it swims along in it. (TAS) Now move the bowl (WIND) - the speed of the bowl relative to the earth is equivalent to GROUNDSPEED.

Hope this helps!

[The Fenland Flyer]

In your example you have 50 knots of airflow over the wings (and 50 knots on the ASI) one the ground, therefore your airspeed is 50 knots before you start moving.

[baleares]

thanks so much for the replies it really helps!

I totally get the fish in the goldfish bowl analogy (much better than my conveyor belt analogy): The wind velocity is irrelevant once in the air, or only relevant to your ground velocity.

I just don't understand then how a headwind can contribute to TAS on take off (air flow over the wings as stated above) and then be irrelevant to airflow over the wings when airborne.

To me this seems like the headwind being the water in the goldfish bowl during take off and then becoming the goldfish bowl once in the air!

[mary meagher]

Think about a river. Water. Flowing down stream at a speed of ten miles an hour. You are on a raft on the river, floating downstream, what is your ground speed? --- how fast are you traveling past the scenery?

Answer, ten miles an hour ground speed. But how fast is your speed through the water? Zero, right? You and the water are moving together, down a lazy river.

Now turn around and turn on the power, head up stream with a good old Evenrude outboard at a speed through the water of twenty miles an hour.

What is your ground speed past that dock?

Did you work it out? I am sure you had no problem. Well, air is exactly the same as water, only a bit thinner.

So plan always if you can, to do your takeoffs and landings into wind. Assume the wind is twenty miles an hour. Your speed through the air could be sixty miles an hour. What would your groundspeed be at touchdown?

Now get set for takeoff. Full power. Accelerate. Groundspeed is now 40 miles an hour. What is your airspeed?

Few basic aircraft have an instrument that gives you groundspeed, a GPS could do that. What matters to the airplane is the airspeed. Neglect not thy airspeed lest the ground rise up and smite thee!

As happened to that Korean airliner at San Francisco.

[baleares]

I totally get the river / goldfish / walking on a train analogies.

I just don't get how windspeed can add to or subtract from IAS on the ground during takeoff if the wind has no effect on IAS when airborne.

:{

[OhNoCB]

simplifying it slights, TAS is IAS corrected for numerous things (most importantly density).

If you line up on the runway with a steady 50 knot wind, your ASI will read 50 knots and your true airspeed (assuming standard atmosphere, sea level, no instrument errors etc) is also 50 knots. Your ground speed is TAS +- the wind coponent so you have 50KTAS - 50Kts wind speed which gives you 0 ground speed / you are not moving.

If you are struggling with why your TAS is increasing because of the wind, mayb try to think of it this way. You understand that TAS is your speed relative to the air mass you are in? so if the air mass is completely still and you are traveling at 50KIAS (assuming the same standards as before) you are doing 50Kts through this air mass. Now if you are not moving, and the air is moving at 50Kts straight at you, you are still moving at 50Kts RELATIVE to to this air mass.

I don't want to complicate things but if it helps (and you may already know it anyway);

I(ndicated)AS corrected for position/instrument error gives C(alibrated)AS corrected for compressability gives E(quivalent)AS corrected for density gives T(rue)AS

[cats_five]

If you walk into a slight breeze the apparent wind (what you feel) is stronger, if you turn and walk downwind it feels calm. That shows you that apparent wind on the ground depends on adding up the actual windspeed with the groundspeed. Windspeed 3mph, walking into it at 3mph apparent wind is 6mph, walking downwind at 3mph apprent wind is 0mph. The difference is quite easy to feel on your face so long as you know which directions is upwind!

Remember you are adding vectors - speed plus direction. You can draw it out very easily. I would have pointed you to the Wikipedia page but it has a horribly complicated diagram for somthing that is very simple. Sailing websites might have something simple showing how to draw it as apparent wind is important when sailing.

If you are a balloon in the breeze it has no concept of upwind and downwind, it simply gets blown along in the breeze. If the breeze is 10mph northerly you watch the ground sliding past at 10mph as you get blown to the south and there wil be no apparent wind. If you swap the balloon for a plane then your power settings and pitch will provide an apparent wind, but the real wind will still be blowing you south at 10mph.

BTW vector drawings can also show how to deal with crosswinds, and with tides!

[mary meagher]

Dear Baleares, I suggest you stop thinking about it altogether and get your butt into an aircraft. Preferably a glider, for basic training, keeps it simple.
Come back to us after you have had some experience in the air, practiced a few stalls, flown on still days and windy days. There is no substitute for experience. And that does NOT mean flying a computer.

Best regards.....

[BackPacker]

TAS is True AIR Speed. IAS is Indicated AIR Speed. They both indicate how fast the air is flowing over your wings. And it's that flow that you need to go flying. Whether it comes from the propeller pulling you forward through still air, or the wind blowing over your wings at the start of the take-off run doesn't matter.

The difference between IAS and TAS is a complex calculation that has to do with air not being the same density everywhere. But both are speeds relative to the surrounding air and have no relation to the ground whatsoever. And at low altitudes and at temperate temperatures, IAS and TAS are, practically speaking, the same anyway.

You seem to be confusing TAS and IAS with groundspeed, which is the speed you actually make over the ground.

All performance calculations, including stall speed, are indicated as air speeds. Most of the IAS, some of them TAS. You cannot fly at speeds slower than stall speed (at least not for long) but it's perfectly possible to fly at negative ground speeds.

Groundspeed is relevant for two things only. It's required to know your groundspeed (which you get once you apply a wind correction to your IAS or TAS) so you know when you will make it to your destination - if you can make it there at all. And groundspeed is relevant for landings and take-offs, since the runway will not move with the wind. So we need to fly the aircraft so that it does the wind correction for us.

[Heston]

Sit in the aeroplane on the ground, parked with the brakes on, but facing into a 30knot wind. Ground speed zero, air speed 30knots and the ASI will actually indicate that. So what you need to understand to make your confusion go away is that when the wind is blowing, an aircraft that is fixed to the ground actually IS MOVING (relative to the air).


Fly at 50knots airspeed into the same 30knot headwind and the ground speed will be 20knots. The aeroplane doesn't care what the ground is doing, only the air over its wings.


The only time you need to worry about ground speed is when navigating - it doesn't affect the aerodynamics.


How IAS and TAS relate to each other is a different thing altogether.


Hope that helps - and by the way your conveyor belt analogy is perfectly good.

[AirborneAgain]

I'm struggling to understand how the wind can contribute to TAS on the ground (and shorten takeoff) and then once in the air have no effect on it?That is because while on the ground, the aircraft is "connected" to the ground by friction.

So a stationary aircraft on the ground with a 50 kt headwind will have a 50 kt TAS. A stationary aircraft with a 50 kt crosswind will have 0 kt TAS.

Once in the air, the aircraft is free to move with the air mass so the surface wind speed is irrelevant to the aerodynamics of the aircraft.

A different (but important) issue entirely, is that surface wind is critical to the pilot's perception of air speed. A dangerous situation can arise when an aircraft is turning downwind at low height with a strong surface wind. The ground speed will increase which the pilot may interpret as an increase in TAS and pitch up/throttle down to compensate, leading to a stall.

[Shaggy Sheep Driver]

The aeroplane flies in a block of air. Its airspeed is its speed through that block.

The block of air with the aeroplane in it might be travelling over the ground at absolutely any speed, or not. That has absolutely no effect on the airspeed of the aeroplane, but does directly affect the aeroplane's ground speed.

So in the air, a steady wind has NO effect whatever on an aeroplane in flight other than affect its groundspeed.

However, because it affects the aeroplane's groundspeed, wind becomes very important when transitioning from air to ground or vice versa (landing and take off). Groundspeed is only a factor for the pilot to bother about in landing, takeoff, and navigation. But he can ignore it for all else.

[foxmoth]

Another way to think about it is sitting the aircraft on the ground in a wind FASTER than it needs to fly, say 80kts. Put power on for 80 kts, rotate and the aircraft will then stay over exactly the same spot on the ground but in the air ( actually it won't because of lack of pilot skill and variations in wind, but in theory it would), on the ground the Groundspeed is Zero and the airspeed is 80 kts but you have not got flying because the angle of attack is too low (and hopefully it was tied down!) in the air exactly the same applies, but you have increased the angle of attack to get it flying.
When you talk about airspeed here it does not matter really in a light aircraft at or around sea level if you are talking IAS, TAS or CAS, they are so near to each other the difference is irrelevant.
NOT something I would suggest actually trying in real life!:eek:

[dublinpilot]

Imagine a small windmill poking out the top of your wing.
http://previewcf.turbosquid.com/Preview/2013/01/29__07_38_49/ToyWindmill_02.jpg252db42d-a6f4-4212-a76d-15d453d76b25Large.jpg


How fast it's spinning obviously reflects how fast the air is moving past it. This is obviously the same as what the wing experinences.

It's obviously spinning quite fast if you're on the ground pointing into a 50kt head wind. The wing is experiencing the same wind.

Now you use your engine to speed up the aircraft by 10kts so that your airspeed increases from 50kts (standing still on the ground) to 60kts.

What happens to the windmill on top of the wing? It speeds up a little as it experiences the air moving over it just a little faster.

Now the aircraft, having reached 60kts airspeed (50 from the wind, while stationary, plus the 10 extra from the ground run) lifts off. What happens to the wind mill? Nothing changes. It's still experiencing wind passing it at 60kts so it still spins at the same speed.

I hope that helps.

dp

[keith williams]

The reason that you are having problems with all of the statements that you have quoted is that they are not actually true.

Let’s look at them in turn a see how true they are.

I understand that once an aircraft leaves the ground it does not" experience" wind and that wind only affects speed relative to ground.

Let’s suppose that we are sitting on the runway in still air and we need 50 knots TAS to lift off. The nose of our aircraft is tied to a vertical post to prevent it from blowing away. From initial still air conditions a headwind gradually increases until it reaches 50 knots. This produces sufficient lift to raise the aircraft off the ground. We are flying, but our TAS is entirely dependent upon the headwind. If the wind suddenly drops the lift will be less than the weight and so the aircraft will descend back onto the ground. The aircraft responded to the change in wind speed, so it must have been able to experience that wind speed.


I'm struggling to understand how the wind can contribute to TAS on the ground (and shorten takeoff) and then once in the air have no effect on it?

And

I just don't understand then how a headwind can contribute to TAS on take off (air flow over the wings as stated above) and then be irrelevant to airflow over the wings when airborne.

Now let’s imagine that we have taken off and are flying into a 50 knot headwind. Our ground speed is zero, so all of the lift being produced must be the result of the headwind. If the wind suddenly increases to 60 knots our lift will suddenly exceed our weight so we will rise. And our drag will suddenly exceed our that so we will move back wards over the ground. If the wind suddenly decreases the lift and drag will decreases, causing the aircraft to descend and move forward over the ground.


These effects were produced by a change in the wind speed, so the change in wind speed must have affected the TAS. Just think what happens when you are flying in gusty conditions.


I just don't get how wind speed can add to or subtract from IAS on the ground during takeoff if the wind has no effect on IAS when airborne.

In the scenario above the sudden change in wind speed would have produced equally sudden (if brief) changes in the IAS and TAS.


So none of the “truisms” that you have quoted are correct. It would however be more correct to say that once airborne:

1. The aircraft will no long experience the ground.

2. Any steady state wind conditions will not cause any changes in IAS,
TAS, because IAS and TAS are measured relative to the mass of air.

3. Any steady state wind conditions will not cause any changes in lift or
drag, because lift and drag are determined by the relative speed of the
mass of air flowing over the aircraft.


One of the great risks in being an instructor is the fact that repeatedly stating something and having it go unchallenged by the students, reinforces the instructors confidence in the truth of what he/she has just said. But repeatedly saying something does not make it true. You questions in this thread have demonstrated how this process can cause problems for the student.

[foxmoth]

Reading the original question I think the OP was really talking about steady state winds rather than changing ones, what you say here is true, but IMHO actually further complicating it, if you are going to go into this, you also need to add after
If the wind suddenly increases to 60 knots our lift will suddenly exceed our weight so we will rise. And our drag will suddenly exceed our that so we will move back wards over the ground. If the wind suddenly decreases the lift and drag will decreases, causing the aircraft to descend and move forward over the ground. "if nothing is then done to change the power/attitude of the aircraft and with no further wind changes it will, after a short period of time, return to its original (air)speed and settle at its new altitude."

[mikehallam]

I suspect the OP is asking about a vertical wind.

Otherwise known as a wind-up !

mike hallam.

[baleares]

May be this question will help me.

Assuming take off speed is 60kts, when you take off into a headwind of 50 knots, would you take off if you applied 10kts of power or would you still need to add 60kts of power?

(I understand you would take off at 10kts GS.)

[hoodie]

Yes, that's correct.

If the wing requires 60 knots to get airborne, 50 knots is supplied by the wind, leaving a shortfall to be provided by the engine moving the wing through the air by a further 10 knots.

It is the speed of the air flowing over the wing that you're interested in, which can be provided either by wind or by engine power or (as is usual) a combination of both.

[keith williams]

Power required = Drag x TAS

So to fly with a 60 knot TAS you would need what you have described as "60 knots of power".

But before you start the takeoff run, when standing on the runway in the 50 knot headwind you would already have 50 knots of TAS. Assuming that you had zero friction wheels and axles you would need to generating sufficient thrust and to stop you from being blown rearwards.

But because you already have 50 knots of TAS you only need 10 knots of acceleration to reach the 60 knot take-off speed. It is this reduced acceleration requirement that reduces the take-off run.

At the point of take-off the TAS would be 60 knots and the GS would be 10 knots. If by some magical trick (or Star Treck-type transporter system) the ground were to be suddenly removed, the TAS would still be 60 knots. The aircraft would still be flying, and it would need what you have descsribed as "60 knots of power" to continue flying.


As I have said earlier your statement that "aircraft in flight do not experience the wind", is untrue or at best misleading.

It would be more accurate to say that an aircraft in flight cannot distinguish between airflow due to the wind and airflow due to movement of the aircraft. From the point of view of the aircraft and the ASI it is all just airflow. But even this is untrue of modern aircraft with inertial navigation or GPS systems and Flight Management Systems. In these aircraft the inertial velocities are compared with the airspeed velocities to determine the wind speed. But that sort of stuff is for your future studies!

[foxmoth]

Assuming take off speed is 60kts, when you take off into a headwind of 50 knots, would you take off if you applied 10kts of power or would you still need to add 60kts of power?

You will not start moving over the ground until you have applied sufficient power to overcome the 50 kts of airspeed (not quite true - if you do not have brakes on, a smooth surface and insufficient power the wind could blow you backwards!) so to start moving you need more than power for 50 kts, especially as you are still on the ground so have ground resistance as well, so to fly you need power for 60kts PLUS enough to overcome the ground resistance.:ugh:
As someone said, is this serious or are you just yanking our chains?:}

[AirborneAgain]

It would be more accurate to say that an aircraft in flight cannot distinguish between airflow due to the wind and airflow due to movement of the aircraft.An aircraft in flight cannot experience any airflow due to wind as such. It can, however, experience changes in airflow due to rapid changes in wind speed and direction. Because of the limited inertia of light aircraft, this effects is almost always negligible except when flying in turbulence and I don't think it adds to the OP's understanding to discuss it.

[AirborneAgain]

Baleares
Assuming take off speed is 60kts, when you take off into a headwind of 50 knots, would you take off if you applied 10kts of power or would you still need to add 60kts of power?You should not talk about "60kts of power" or "10kts of power". It is misleading as well as physically meaningless.

You should talk about "The power needed to offset the drag of the aircraft at an airspeed of 60 kt" (or 10 kt, as the case may be).

[keith williams]

An aircraft in flight cannot experience any airflow due to wind as such.

So how is that kites can fly?

They have no engines or propellers, so any airflow over them is due to the wind. They are being supported by a lift force that is being generated solely by this wind. Their entire ability to stay aloft is provided by the wind flowing over them.

So in what sense are they not "experiencing" the wind?

I could just as easily say that I cannot experience the wind. If it gets strong enough it will blow me over, but I cannot smell it or see it. I can feel the air moving over me, but then so could the aircraft (if it could feel anything).

It is the use of terms like "aircraft cannot experience the wind" that have caused the OP to become so confused. So how successful was that lesson?

[Shaggy Sheep Driver]

So how is that kites can fly?

Kites aren't aeroplanes. They are anchored to the ground by their string and so, 'feel' the wind in the same way a groundbound object does - so the wind gives them airspeed just as a boat anchored in a current will have speed relative to the water even though it isn't moving relative to the river bed.

Let go the string, however, and they become 'aeroplanes', no longer ground-bound. They'll immediately cease to generate lift as they lose all airspeed and move with the air (the wind) rather than through it, and flop along falling out of the sky some distance downwind of the person who was previously holding the string.

This is all really basic stuff any student pilot knows but a groundling might not... Like others, I suspect a leg-pull here.

[Armchairflyer]

After all it's lift created by sufficient airflow and angle of attack (or at least drag) of some sort, whether kites, gliders, powered aircraft or sharks in a sharknado. The first (unless connected to the ground as pointed out by SSD) and the last only "fly" in an updraft strong enough, so potentially misleading example. Anyway, an aircraft in flight only cares about moving through an air mass with sufficient speed ("airflow due to airspeed"), not whether that air mass itself is moving relative to the ground ("airflow due to wind").

[keith williams]

Anyway, an aircraft in flight only cares about moving through an air mass with sufficient speed ("airflow due to airspeed"), not whether that air mass itself is moving relative to the ground ("airflow due to wind").


Which is pretty much what I said earlier. An aircraft responds to the air flowing over it. It has no mean to distinguishing between airflow due to wind and airflow due to forward motion of the aircraft.

This entire area of discussion is based on using the atmosphere as the frame of reference, then calculating the speeds and forces relative to it. In effect we assume that the atmosphere is stationary and any airflow is purely the result of motion of the aircraft. To do this we reverse the direction of the wind and add its speed to the aircraft. But none of this means that the aircraft does not experience the wind. It simply means that we have simplified the situation by selecting that particular model. But it is still just a model.

If you don't like the kite analogy, consider an aircraft with a TAS that is exactly the same speed of the headwind. If it does not experience the wind it will not be required to generate any thrust or power to maintain the situation. Now just shut down the engine and we will see just how much the aircraft is experiencing the wind.

SSD, like many people you appear to be assuming that because you have been doing something for many years, this must mean that you understand all about it. This is a false assumption.

[Armchairflyer]

... consider an aircraft with a TAS that is exactly the same speed of the headwind.Pardon the question, but: which headwind? For an aircraft in flight there is no such thing as a headwind (or wind from any other direction), these only apply with reference to the ground. And shutting down the engine will make the aircraft descend in the same manner regardless of any wind (and its speed and/or direction).

[shortstripper]

Ok ... Let's throw another old log into the fire!

So, you've powered up, overcome the drag of the 50 knots wind speed, increased your airspeed to the 60 knots needed to lift off and started flying ... But then immediately turn downwind. What happens? :}

SS

[keith williams]

Pardon the question, but: which headwind? For an aircraft in flight there is no such thing as a headwind (or wind from any other direction), these only apply with reference to the ground.

As I said it all depends upon how we choose to model the situation. You have chosen to use the atmosphere as the frame of reference and assumed that the atmosphere is stationary (as I said in my previous post). That's fine but it is just a model.

Now lets' use the ground as our frame of reference and assume that the ground is stationary. Now we can have a headwind and in the situation which I described above the propeller would be providing just enough thrust to prevent us from moving backwards over the ground. Now when we shut down the engine, if we maintain the pitch attitude an observer on the ground would see the aircraft move aft and down. The aft movement would be caused by the headwind. The downward movement would be caused by the reducing airspeed relative to the aircraft. This demonstrates the fact that the aircraft is "experiencing" the wind.

You may wish to argue that we must always use a stationary atmosphere as the frame of reference. But this is not helpful for navigation purposes and systems such as INS and GPS do not do so. If aircraft cannot experience crosswinds why do they drift off track instead of following their noses?

As I said earlier, this whole debate stems from the fact that we choose to use a stationary atmosphere as the frame of reference. That's fine as long as we remember that this is just a modelling choice. But it does not mean that aircraft really do not experience winds in flight. We've just used our model to add the wind speed to the inertial speed of the aircraft, and then called it all TAS.

Many aircraft have been destroyed by turbulence over the years. If the aircraft could not experience the wind, why did they get torn apart? You may wish to argue something along the lines of "Aircraft cannot experience steady state winds, but they can experience changes in wind". To test this argument consider how the aircraft would experience the change. To do this it must experience the initial steady state condition, then experience the new condition, and compare the two conditions.

If this tread really isn't simply a wind up, then I can only conclude that the instructor involved forgot that it was all just a model (probably because his instructor also forgot). The student then went away with some very confused ideas.

[m.Berger]

You have a very busy approach 'cos your base leg will be the turn from downwind to final!

[Armchairflyer]

IMHO that's not a "model choice", it's just airspeed vs. groundspeed. Of course, relative to the earth, any object experiences the wind, and flying objects drift with the wind. Turbulence is another subject, just as waves vs. speed of the current in a river.

[foxmoth]

KW, you have models and ways of explaining it that confuse me - and I understand it - god help your poor students!!:rolleyes:

[Pace]

If you stand in a 50 KT wind holding a balloon then a 50 KT aircurrent will flow over the balloon sufaces.
Let the balloon go and the balloon will tarvel over the ground at 50 kts but there will now be zero wind flow over the balloon sufaces.
Now replace the balloon with a wing :E Go figure

Pace

[baleares]

You will not start moving over the ground until you have applied sufficient power to overcome the 50 kts of airspeed (not quite true - if you do not have brakes on, a smooth surface and insufficient power the wind could blow you backwards!) so to start moving you need more than power for 50 kts, especially as you are still on the ground so have ground resistance as well, so to fly you need power for 60kts PLUS enough to overcome the ground resistance.
As someone said, is this serious or are you just yanking our chains?


Foxmoth - I am not yanking anyones chain! I was just having trouble rationalizing two seemingly paradoxical statements.

Your explanation has actually helped me the best!!! If I picture the aircraft with the brakes off with the required thrust to 'hold' against a 50kt wind so has not to go backwards it really helps me rationalise the fact that the aircraft is then able to fly once airborne or "on the train" or in the "goldfish bowl", "in the flowing river" etc.. etc... (to quote the various analogies)

It was the statement that, "once in the air, an aircraft is not affected my wind (except relative to ground)" which was confusing. It made me picture the aircraft suddenly having the headwind flow of 50knots over the wings removed as it is now "in" the body of air rather than having the body of air flow over it.

Now if I picture the ground removed and the aircraft having the 50knots to "hold against" the wind + the 10knots forwards into it, equaling the required 60knots to lift off I can deal with the aircraft being airborne with 60 knots relative to a now "still" body of air.

Sorry if this does not makes sense, I am very tired!

I really do appreciate all of the responses and help and apologise for being a dimwit. This is a great forum.

:O

btw: The Tramontana are in Mallorca, where I live and the winds are far from mythical!

[AirborneAgain]

KW
As I said earlier, this whole debate stems from the fact that we choose to use a stationary atmosphere as the frame of reference. That's fine as long as we remember that this is just a modelling choice.I would say that most people here have chosen to use the atmosphere as the frame of reference, without assuming whether is is stationary or not.

[foxmoth]

Foxmoth - I am not yanking anyones chain! I was just having trouble rationalizing two seemingly paradoxical statements.

Ok, in that case very happy to have helped, as is often said, there is no such thing as a stupid question, but sometimes when people do not understand you do wonder - always easier actually explaining face to face because you then understand more what they are failing to get a grasp of.:ok:
Hopefully you now understand that the aircraft is in the body of air BOTH on the ground and in the air and in many ways it does not matter where it is.

[Armchairflyer]

It was the statement that, "once in the air, an aircraft is not affected my wind (except relative to ground)" which was confusing. It made me picture the aircraft suddenly having the headwind flow of 50knots over the wings removed as it is now "in" the body of air rather than having the body of air flow over it.Being "in" the body of air doesn't mean that the aircraft loses any speed relative to the air. Yours was apparently a kind of the "plane on a conveyor belt" confusion. The plane does not push itself into the air by spinning its wheels quickly enough ;) (in which case it would indeed lose its airspeed as soon as it gets airborne).

[Dawdler]

Take a look at this Husky taking off into a strony wind. First TO ground run distance?
Husky Short Field Landing and Take-off - YouTube

This may illustrate the effect of windspeed over ground speed.

Edit: I think it is Bob Pooler at the controls

[mary meagher]

All you noisy pilots that require power to get airborne, may be amused by contemplating a maneuver special to certain hill sites, for example, the Long Mynd....

The wind is blowing against the ridge. The glider is moved to the edge of the ridge. A bungee rope is attached to the glider, and six able bodied people attach themselves to the rope, three on each end, (the glider is attached to the midpoint of the bungee rope). Another person is holding the tail of the glider so it won't leave too soon.

The six able bodied people begin to march down the hill, pulling the rope. When there is sufficient tension, the chap on the tail lets go, the glider trundles forward and eventually falls off the hill.

And instead of continuing downhill, is met by the wind blowing up the hill. It ascends triumphantly into the air.

If the wind is strong enough, the glider can then perform the maneuver of

THE VERTICAL CIRCUIT. Turns downwind and base leg not needed. Just fly at minimum sink speed, say 40 knots, the glider then flies backwards over the hill. Reaching the point where a final approach is desired, speed is increased to penetrate the wind, say to 60 knots, and the glider lands straight ahead. This is the rare situation where you can actually GO AROUND in a glider!

I think a bungee launch may be found on YouTube, but doubt if they ever managed to film the Vertical Circuit....

Try that one if you get the chance!

For the information of the earnest OP, the usual methods of launching gliders are by airtow, or by winch. The winch launch is cheaper. But the tow plane can tow your glider to the best thermal, and if you are any good at all, you can then stay up all day....my longest flight in the UK was 8 hours and 53 minutes, traveled a task of 511 kilometers...

That was a very good day indeed.

[foxmoth]

Try that one if you get the chance!
Been there, got the T shirt, and in a power a/c, helped by the fact that the wind at 1,000' was a tad stronger than that on the ground.:ok:

[Flyingmac]

A picture is worth a thousand words?

http://www.youtube.com/watch?v=IPOtDPHjW-Y

[Pace]

That is a good example as looking at the video while the aircraft is stationary and enough wind speed crossing the wings to give lift causes the aircraft to lift as soon as it moves backwards with the wind the aircraft starts settling back down again with a gound speed going backwards.

Pace

[Shaggy Sheep Driver]

SSD, like many people you appear to be assuming that because you have been doing something for many years, this must mean that you understand all about it. This is a false assumption.

Nope. I knew this stuff after my first lesson in a T28 glider. Probably even before that! It's not 'ard!

So, you've powered up, overcome the drag of the 50 knots wind speed, increased your airspeed to the 60 knots needed to lift off and started flying ... But then immediately turn downwind. What happens?

Your groundspeed increases to 110 kts in a sweeping downwind turn. Absolutely nothing else happens (disregarding any wind gradient affecting the lower wing in the turn, and assuming the odd visual effects at such low level don't confuse you into reducing your airspeed and possibly stalling the aeroplane).

Now let's imagine you take off, climb through a cloud layer, then turn downwind. This time you will be unaware of the bizarre visual effects of strong winds at low level because you will be above a cloud deck which is also moving (with the air the clouds are suspended in) at 50 kts over the ground.

Let's assume you are airborne in a hot air baloon above a cloud deck in a 50 kt wind. The balloon has zero airspeed of course, as do the clouds, so as you look out over the side of the basket nothing appears to be moving - not you, not the clouds. Then the clouds disperse... and you have a view out the basket of the ground rushing past at 50 kts!

In my experience it's usually model flyers who 'don't get' this. They remain convinced that their pride and joy experiences a gain of speed on turning into wind, and a loss of speed turning downwind. Just imagine if this were true.... an airliner goes into the hold at FL70 and 190 kts. Does the wind affect the airpeed as it flies around the holding pattern? Of course not! It will affect the ground track, though.

[keith williams]

Let's assume you are airborne in a hot air baloon above a cloud deck in a 50 kt wind. The balloon has zero airspeed of course, as do the clouds, so as you look out over the side of the basket nothing appears to be moving - not you, not the clouds. Then the clouds disperse... and you have a view out the basket of the ground rushing past at 50 kts!

That's an excellent choice of scenario SSD. If we go back to the start of that balloon flight it demonstrates my point very well. I can't imagine anyone would want to launch a hot air balloon in a 50 knot wind, but as you've suggested those conditions let's go with them.

Before launch the basket is tied to the ground. Ground speed is zero but TAS is 50 knots (Produced by of the wind). The balloon would be leaning downwind quite markedly.

We release the tie-downs and the balloon starts to rise. As it rises it accelerates downwind. GS increases and TAS decreases. After a while the balloon is moving downwind at the same speed as the wind, so the TAS is zero and the ground speed is 50 knots. We have gone from a situation in which we had zero GS and 50 TAS, to one in which we have 50 GS and zero TAS.

The only thing that could have caused these changes in TAS and GS is the wind. If the wind accelerated the balloon, the balloon must have experienced the wind. In this case the wind was steady.

[shortstripper]

Your groundspeed increases to 110 kts in a sweeping downwind turn. Absolutely nothing else happens (disregarding any wind gradient affecting the lower wing in the turn, and assuming the odd visual effects at such low level don't confuse you into reducing your airspeed and possibly stalling the aeroplane).

I wondered when someone would answer ... You're right of course, but it was the wind gradient I was alluding too. If you've climbed out of it then great, but with a 50 knot windspeed the gradient will be steep and turning downwind whilst still low enough to be in it could be .... interesting?

SS

[Shaggy Sheep Driver]

There's an interesting article somewhere about the dangers of the downwind turn (pilots have died making such a manouvre). Wind gradient can be a very real danger, but perhaps the biggest is the visual disorientation of the pilot leading to inappropriate control inputs. The other killer in such a situation at low level (so just after take off, for instance) can be the much reduced climb angle relative to the ground when flying downwind in a strong wind.

Also, flying 'down in the weeds' is visually very different to doing the same at a few hundred feet. Add a strong wind, and things can become very interesting.

[Heston]

Students do get confused by this. First they get to understand the aircraft is flying in a parcel of air and that what the ground is doing is irrelevant (Shaggy's "above a layer of stratus scenario"). But then they have to partly unlearn that - when they start learning to land in a wind gradient and are told that it takes time for the aircraft to respond to the changed wind speed so that the air speed (momentarily) changes.


Oh and Shaggy - I've been waiting for you to say "its all in Stick and Rudder", but you haven't yet so I'll do it for you: its all in Stick and Rudder :)

[foxmoth]

Students do get confused by this. First they get to understand the aircraft is flying in a parcel of air and that what the ground is doing is irrelevant (Shaggy's "above a layer of stratus scenario"). But then they have to partly unlearn that - when they start learning to land in a wind gradient and are told that it takes time for the aircraft to respond to the changed wind speed so that the air speed (momentarily) changes.


No, the ground IS still irrelevant, you are not UNLEARNING anything, but learning a new bit, that you are still flying in a parcel of air, but that parcel is changing ( or another way to think about it is that you are moving from one parcel of air to another) and you need to allow for these changes. The ground may the the thing that is causing the change and this is good because this can be anticipated, but there are other things that can cause a change such as CBs, and other weather phenomena and these cannot always be anticipated in the same way.

[Heston]

Foxmoth - of course you're right, I am guilty of not explaining myself properly - the extra bit (that you do have to consider the inertia of the aeroplane) does nevertheless cause confusion at first. This is because the temporary change in airspeed can only be understood by referring to the speed of the aircraft in a frame of reference outside of the air - and that frame of reference is the earth, in other words ground speed. By frame of reference I mean that speed has to be expressed as movement between two things. Airspeed is the movement between the aircraft and the parcel of air it is in. When we talk about wind gradient we are using the earth as the fixed frame of reference (ie the speed between the air and the ground)and we have to use the same frame of reference for the aircraft when we want to describe what happens as the aircraft descends through the wind gradient. This does not mean that the aeroplane somehow magically knows what it is doing with respect to the ground - but we need the ground to anchor our mental description of what is going on. Hence the confusion.


This is what happens when you teach a PhD physicist to learn to fly and he unpicks your explanations...

[glum]

Should we mention that we're already moving at 67,000 miles per hour round the sun and at the equator the ground is doing about 1000mph as the earth spins? :E

[Heston]

Coriolis Effect anyone?