Flying in the (cross)wind

[Adhika Lie]

Well, this question is not about crosswind landing techniques. But more about correcting for crosswind during a steady straight and level flight.

I read one sentence in the book saying (may not be an exact quotation):

As an aircraft flies in the wind, it is moving together with the wind. Therefore, when an aircraft flies in the wind, it's prevalent that the pilot makes a continuous wind correction to maintain course.

It's kind of difficult for me to accept that air is just like water. Now, when a boat moves in a water stream, it will follow the stream's flow. It will have exactly the same velocity as the water flow. It's very easy to see that. The boat when not rowed will always maintain zero relative velocity with respect to the flow.

But how do I see that in airplane? Let's take an example when the wind is aligned with the flight path, i.e. headwind/tailwind. Well, theoretically when the headwind is strong enough, an aircraft can lift off the ground without any thrust produced by the engine and no relative movement on the ground. This is very easy to see because then there is an airspeed seen by the wing. Meaning that with respect to the air, the aircraft is moving forward. But it's not moving with respect to ground...

Now, we could see the difference. In the boat case, it is not moving with respect to the water, but in the aircraft case, it is not moving with respect to the ground!

Supposed an aircraft is flying straight and level with no wind. Now, supposed the aircraft is subjected to a crosswind of 5 knots from the left while the pilot maintain the heading. I would like to know what actually happens:

1. Does it move to the right 5 knots with respect to ground just like the wind (viz. does it mean that with respect to ground the aircraft is now moving sideways at 5 knots, too?).

Or,

2. Is it because of the weathercock stability it will experience a yaw moment that turns the aircraft's nose into the wind and align itself with the wind and thus changes its heading? If that's the case, how does wind correction angle apply?

I am pretty confused now.. haha.

[Filipe ​Fonseca]

I'm not well explained in flight theory, but we can't forget two physic's threads:

1) Air is a Fluid. A combination of gases. And the water is a fluid too. So, the physics law are the same for both cases (take the Bernoulli's Principle, for example).
2) On High School we learn the vector composition.

http://moon.ouhsc.edu/dthompso/NAMICS/compose.htm

Everything is based in this principle. Especially the operations in crosswinds (size of force, angle between flight path and wind direction and so on...).

Maybe it could help you.

[Peter Bremer]

Or to say it differently:
An aircraft flies in refrence to the air,
if the air moves compared to the ground, the aircraft just moves along; When flying an aircraft does not know about the ground.

For some practical windcorrection calculations during flight:
You will not need the trigonometric calculations or expensive Jeppesen tools.
See my windcorrections story. It is in Dutch so I will try to give a short summary:
1) Determine the maximum WCA to be 60/TAS*windspeed. For this you use the 'wind at altitude' information you get from meteo information in your flightpreparation.
Example wind at the altitude you are flying: 21020, TAS=100 => maxWCA=60/100*20=12 degrees.
2) Use the projection of the wind direction on the horizontal axes of your DG to find a relative number between 0 and 1.
3) Multiply that relative number with the maxWCA to find the WCA you need.
In the Dutch story the example: actual heading 255, projection of wind direction of 210 gives relative figure 3/4 to the left, WCA = 3/4*12 = 9 degrees to the left.


Kind regards,

Peter.

[Adhika Lie]

I know about this vector addition thing. But...

Let's simplify it this way. I want to know when you are actually flying an aircraft:

You are maintaining a heading (say you use a heading hold autopilot which certainly hold your nose direction), and you are on a crosswind. You will always point to the same heading, but how will your groundtrack be? Will it move sideways the same amount that the wind has blown you off? For example if the crosswind is 5 knots, are you 5 miles off the track after an hour?

[Peter Bremer]

I know about this vector addition thing. But...

Let's simplify it this way. I want to know when you are actually flying an aircraft:

You are maintaining a heading (say you use a heading hold autopilot which certainly hold your nose direction), and you are on a crosswind. You will always point to the same heading, but how will your groundtrack be? Will it move sideways the same amount that the wind has blown you off? For example if the crosswind is 5 knots, are you 5 miles off the track after an hour?

Yes, 5 Nautical Miles off track after one hour. The air moves 5 NM in one hour if the wind is 5 kts; Your aircraft which flies in reference to the air which moves in reference to the ground, moves with it in reference to the ground.
That is why you choose your heading in such a way that you fly the correct track. You change your heading depending on your TAS and windspeed/direction, see my earlier post on how to do that.

Kind regards,

Peter.

[Adhika Lie]

Thanks Peter, I've got the vector addition thing.

But I am still a bit confused. If I get it correctly, you are saying that since the wind blows you 5 miles an hour sideways. It means that we are actually "flowing" together with the wind just like in a river. Am I right?

How about if it's a headwind then. Let's imagine a Cessna 172 not attached to the runway and it's being blown off by a 120 knots headwind (just a theoretical condition). Say the engine is off, i.e. no thrust produced. Will the Cessna 172 moves backward (w.r.t the ground) by 120 miles after an hour or will it stay where it is? Because if it follows the flow, then it will move backward at 120 knots as well...

[Filipe ​Fonseca]

Let's imagine a Cessna 172 not attached to the runway and it's being blown off by a 120 knots headwind (just a theoretical condition). Say the engine is off, i.e. no thrust produced. Will the Cessna 172 moves backward (w.r.t the ground) by 120 miles after an hour or will it stay where it is? Because if it follows the flow, then it will move backward at 120 knots as well...

It will, obviously. You're working now with the situation of wind blowing, not the wind outflow.

And, to complete, the aircraft don't make any reaction to reduce speed action. It's just the concept of relative speed.

And the concept of relative speed is located on what Peter said. If you don't make any reaction to avoid wind action, you'll be at the mercy of wind. That's why the idea of Wind Correction is used, with the calculus of WCA as Peter teached.

It means that we are actually "flowing" together with the wind just like in a river.

1) Air is a Fluid. A combination of gases. And the water is a fluid too. So, the physics law are the same for both cases (take the Bernoulli's Principle, for example).

[Adhika Lie]

What do you mean with the wind outflow, Filipe?

[Filipe ​Fonseca]

An aircraft fly basically due to difference of pressure on both sides of wing (up and bottom), provocated by the wind outflow on structure (the relative wind produced by aircraft speed).

If a very strong wind blow on aircraft, the aircraft will not "fly" because of this theory, but only by wind impact on structure. Like a leaf conducted by wind.

These are different conditions.

[Filipe ​Fonseca]

Another thing:

As an aircraft flies in the wind, it is moving together with the wind. Therefore, when an aircraft flies in the wind, it's prevalent that the pilot makes a continuous wind correction to maintain course.

I guess this is incorrect... It should be write like this:

As an aircraft flies in the wind, it is moving together with the wind. Therefore, when an aircraft flies against (relatively or absolutely) the wind, it's prevalent that the pilot makes a continuous wind correction to maintain course.

[Adhika Lie]

There are some flaws in my thought above. Thanks to Filipe and Peter. And I think I am going to correct it here.

Well, surely anything that makes no resistance (for example - thrust produced by the engine in the case of an aircraft) or whatsoever when it is blown by a headwind of 120 knots will surely go backward at 120 miles an hour. Starting from rest, it will speed up to the 120 knot backward movement because of the headwind. Eventually there is no relative velocity between the object with the air. So in the case of a power-off Cessna 172 blown by a headwind of 120 knots. Well, it will certainly have lift but eventually when no other mode of resistance (e.g. thrust) is made by the aircraft, it will fall down. And if we are to look closer of the mechanism, it's the drag created by the relative motion between the body and the air that makes the body "flows" together with the air.

So, still, in order to fly/lift-off, the aircraft still needs to produce power that overcome that drag although it may result in the aircraft not moving with respect to the ground (in the case of Cessna 172 that cruises at 120 knots).

In the crosswind case, I think I should recall again that the wind acts simply just like water. So, if there is a crosswind during cruise, that will surely drift your aircraft sideways, simply because it is moving together with the air. Consider a pure crosswind situation, the forward velocity is simply your velocity with respect to the air. So, it adds up, and "vectorialy" or so to say. Up to this point, we are neglecting the side force or any weathercock stability that may turn the aircraft into the wind. But yes, if we are to look closer again, it is the drag (or in this case side force) that has moved the body together with the wind.

So, in order to maintain a straight ground track, I think there are two possible ways to overcome it.
1. By orientating your aircraft's heading in such a way that the vector component of your velocity that is parallel to (cross)wind direction cancels the wind's speed. In that case, you are flying straight on the ground. I think, this is what is called as the wind correction angle method.

2. By means of rudder application. Rudder can be used to zero the side force. Sufficient left rudder could produce enough side force to maintain the aircraft not drifting to the left when a wind from the right blows, couldn't it? Of course some aileron applications would be necessary to provide the coordinated movement.

What do you think?

[Filipe ​Fonseca]

So, in order to maintain a straight ground track, I think there are two possible ways to overcome it.
1. By orientating your aircraft's heading in such a way that the vector component of your velocity that is parallel to (cross)wind direction cancels the wind's speed. In that case, you are flying straight on the ground. I think, this is what is called as the wind correction angle method.

2. By means of rudder application. Rudder can be used to zero the side force. Sufficient left rudder could produce enough side force to maintain the aircraft not drifting to the left when a wind from the right blows, couldn't it? Of course some aileron applications would be necessary to provide the coordinated movement.

What do you think?

Hum... I'm not a pilot and I'm anxious to read what Peter has to write, but I guess the Wind Correction method is more pratic to make.


Just as a curiosity, the FMC of Boeing's planes shows, the PROG page (if I'm not misguided) shows the Tailwind/Headwind Speed and the X-Wind Speed (crosswind component). It makes the vector composition for you.

[Adhika Lie]

Yes, I am also very interested to know what Peter's gonna say about this.

Practically speaking, I do think that wind correction angle is a much more practical way to do that.

[Peter Bremer]

First about a headwind of 120 kts, for the example to be more realistic I reduce the value to 60 kts, but that has no effect on the principles:
For the example I take a C172 which has a clean stall speed of 47 kts

First as a realistic situation: the aircraft is flying, its indicated airspeed is 90 knots. If it is not flying high its TAS will also be near 90 kts, say 90 kts.
The airplane will fly and stay in the air because it flies with an airspeed larger than the stall speed. The airplane itself does not know it has a headwind of 60 kts. But the pilot or an observer on the ground will see it only flies with a speed in refrence to the ground of 30 kts forwards.
If the headwind would have been 120 kts in this situation, the airplane would still fly because as far as it 'knows' it is moving forward with a speed of 90 kts. But the pilot or an observer on the ground will see it flying backwards with 60 knots compared to the ground.

Than the situation on the ground:
The airplane is standing still on the ground with a headwind of 60 kts and no power. (You might need some blocks behind the wheels to prevent the aircraft being blown backwards on the ground). That means its airspeed is 60 kts. Because that speed is larger than the stall speed it will lift off.
But as soon as it is airborn the drag will cause the airspeed to go down (no energy source) until it is on the ground again at an airspeed lower than 47 knots (suupposing the pilot in the plane triies to keep the airplane in the air). Because at that moment the airspeed will be 47 knots and the wind is still at 60 kts at the moment of stall, not much later the impact,  the airplane will fly with a speed of 13 kts backward compared to the ground.

Same situation with motor power:
As soon as the aircraft lifts off due to the wind induced airspeed and you have enough power with the enigine you can continue flying and even increase the eairspeed to a comfortable value.

Only landing will be a problem: You must land at the end of the run way because at landing speed around 55-47 knots you are moving backward compared to the ground. I doubt that landing an aircraft backward at touch down can be safely done.
A better option then would be to land at the middle of the runway with an airspeed higher than the normal landing airspeed: the same or higher as the wind speed, i.c. 60 kts. In that case your speed in reference to the ground is 0 or soemwhat higher. As soon as you touchdown you have time and runway enough to reduce the execessive energy and power of the engine associated with an airspeed of 60 knots compared to the normal 55-47 knots.

-edit- of course I never experienced this situation in real, but therefore we can use simulator, so I will try. --

In the crosswind case, I think I should recall again that the wind acts simply just like water. So, if there is a crosswind during cruise, that will surely drift your aircraft sideways, simply because it is moving together with the air. Consider a pure crosswind situation, the forward velocity is simply your velocity with respect to the air. So, it adds up, and "vectorialy" or so to say. Up to this point, we are neglecting the side force or any weathercock stability that may turn the aircraft into the wind. But yes, if we are to look closer again, it is the drag (or in this case side force) that has moved the body together with the wind.

So, in order to maintain a straight ground track, I think there are two possible ways to overcome it.
1. ...

2. By means of rudder application. Rudder can be used to zero the side force. Sufficient left rudder could produce enough side force to maintain the aircraft not drifting to the left when a wind from the right blows, couldn't it? Of course some aileron applications would be necessary to provide the coordinated movement.

What do you think?

This is again the same misconception: When the aircraft is flying the wind does not induce a force to the aircraft; Wind just means that the air is moving. The aircraft is moving with it in reference to the ground, but from aircraft point of view, as long as it has no contact with the ground, the wind has no influence on the forces on the aircraft.
The general misconception is caused by the experience that the wind induces a force to a human being in contact with the ground.

That means you can only fly the correct (ground) track by applying a wind correction angle.
Using rudder fr it will not help. It just introduces more drag and a pointing of the nose of the aircraft different from the direction it flies compared to the air. But it will not influence the directon of the aircraft compared to the ground. That can only be done by really flying another heading compared to the air than the required ground track: a wind correction angle.

Kind regards,

Peter.

[Adhika Lie]

This is again the same misconception: When the aircraft is flying the wind does not induce a force to the aircraft; Wind just means that the air is moving. The aircraft is moving with it in reference to the ground, but from aircraft point of view, as long as it has no contact with the ground, the wind has no influence on the forces on the aircraft.
The general misconception is caused by the experience that the wind induces a force to a human being in contact with the ground.

That means you can only fly the correct (ground) track by applying a wind correction angle.
Using rudder fr it will not help. It just introduces more drag and a pointing of the nose of the aircraft different from the direction it flies compared to the air. But it will not influence the directon of the aircraft compared to the ground. That can only be done by really flying another heading compared to the air than the required ground track: a wind correction angle.

But Peter, what makes an aircraft moving with the air in the first place? Isn't it the drag? Let's take some example here:

1. You are moving on a travelator (see picture above). You are standing firmly on it. What makes you move together with it? It's the friction that keeps you in tact with the ground, isn't it? If the travelator moves all of a sudden that it becomes larger than the friction that the travelator can provide, you will not be moving together with it. It will slip.

2. The power-off + headwind case that you have explained very well in the first part of your posting. I think you have made it very clear that even when the airspeed is greater than the stall speed (then lift should be enough to provide the necessary lift), the airspeed will decrease because drag steps in and no thrust overcomes it. It is just like a rock subjected to a headwind, except that an aircraft can lift off. But eventually it will flow with the wind.

So, I think... (this is just a matter of discussion, and from the engineering point of view) an aircraft moves sideway when subjected to a crosswind because there is a drag (or simply a side force) associated with that crosswind. Again, there is no thrust whatsoever that overcomes this side force, so the aircraft is just an object subject to a moving air.

We can also take an analogy of the headwind case. Please forget about the weathercock/directional stability (or any rotation due to the vertical tail). When the crosswind has just started, the aircraft is not moving sideways. So, it will see a sideway airspeed (a sideway speed relative to the air). Drag or sideforce (which is proportional to the airspeed squared) steps in and tries to move the aircraft sideways. Since there is no thrust produced sideways, the aircraft will start to move sideways simply because it is being "pushed" by the wind or, perhaps, actually by the drag/sideforce. As it moves, the relative speed between the aircraft and the air moving sideways starts to decrease and the drag/sideforce decreases as well (again it's proportional to airspeed squared). This keeps on going until finally the aircraft is moving together with the wind (no more airspeed) with the same speed of the wind with respect to the ground - which I think is a very stable condition.

That's why I think that theoretically rudder which can so-called produce counter side-force could reduce the effect of the drag due to the crosswind and be used as a mean of directional control. This is also true when we can have a sideway thrust vectoring. Imagine you can have a propeller mounted sideway. I think the side prop can always "push" the aircraft so that it flies in the desired direction (ground track) without any heading correction.

But of course, Peter has pointed out very well that it is not efficient at all to use the rudder as a directional control in this case. Why? Because pointing your aircraft at a different direction of the wind, you may need quite a large sideforce and the drag resulted from the rudder deflection (just like a wing when it produces lift, it also produces drag) need to be accounted by your thrust.

Does it make sense, Peter?