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]

hello, baleares

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!

[dublinpilot]

Imagine a small windmill poking out the top of your wing.



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.