Taking off into wind

[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.
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!!

[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 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.



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.
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.