Some kind of autopilot?

[BigDuke6ixx]

See for yourself, I have no idea why the nose of the plane points slightly to the west and stays there, my best guess is the crosswind is pushing asymmetricly against the airframe and the plane turns the nose into the wind, but that can't be, as laws of physics do not apply in the air? :dunno:

 

I've viewed it and the plane is just heading, with a bit of wandering back and forth, in the direction it started. Tape ends as the heading wanders to the north and starts back to the west. All it does is prove you wrong...again.

[shagrat]

I've viewed it and the plane is just heading, with a bit of wandering back and forth, in the direction it started. Tape ends as the heading wanders to the north and starts back to the west. All it does is prove you wrong...again.

The nose turns to the left and pretty much stays there offset to the flightpath (see the tacview picture), while the plane slowly veers to the right, as I am not correcting. Now the question is why does the nose point to the left and not straight into the flight path, with no rudder input?

[BigDuke6ixx]

The nose turns to the left and pretty much stays there offset to the flightpath (see the tacview picture), while the plane slowly veers to the right, as I am not correcting. Now the question is why does the nose point to the left and not straight into the flight path, with no rudder input?

 

On the track you posted, there's a big disturbance in pitch, yaw and roll right at the beginning, it then settles on a heading of 347, before wandering north, getting to about 359, then, just as it seems to start back west, the tape ends. I see nothing that proves your new theory, nothing at all.

[shagrat]

On the track you posted, there's a big disturbance in yaw right at the beginning, it then settles at 347, before wandering north, getting to about 359 then, just as it seems to start back west, the tape ends. I see nothing that proves your new theory, nothing at all.

There is no "new" theory. On the tacview (see screenshot) you can see after 7 sec. the initial yaw (from the mid-air spawn) has settled.

If you watch the tacview you can see the plane drifting east (as expected with a crosswind from 270°).

If you look at the textbox on the left the difference between the ground track TRK (aka actual flight path) and the heading HDG (where the nose points, also indicated with the blueish vector pointing away from the planes nose) is roughly 3-4 degrees.

 

So please answer my question: why is the nose pointing 3-4 degrees left of the flight path all the time? There must be an explanation, right?

[shagrat]

...oh, and just to be precise: that isn't a temporary difference, the 3-4 degree builds up at the beginning and stays roughly around 4 degrees for the whole 3 min track, so it isn't a "wandering" of the nose.

[BigDuke6ixx]

There is no "new" theory. On the tacview (see screenshot) you can see after 7 sec. the initial yaw (from the mid-air spawn) has settled.

If you watch the tacview you can see the plane drifting east (as expected with a crosswind from 270°).

If you look at the textbox on the left the difference between the ground track TRK (aka actual flight path) and the heading HDG (where the nose points, also indicated with the blueish vector pointing away from the planes nose) is roughly 3-4 degrees.

 

So please answer my question: why is the nose pointing 3-4 degrees left of the flight path all the time? There must be an explanation, right?

 

It spawns with a heading of 347 then wanders about a bit before heading to north. There's no weathervaning into the crosswind on that tape.

[Flagrum]

There is no "new" theory. On the tacview (see screenshot) you can see after 7 sec. the initial yaw (from the mid-air spawn) has settled.

If you watch the tacview you can see the plane drifting east (as expected with a crosswind from 270°).

If you look at the textbox on the left the difference between the ground track TRK (aka actual flight path) and the heading HDG (where the nose points, also indicated with the blueish vector pointing away from the planes nose) is roughly 3-4 degrees.

 

So please answer my question: why is the nose pointing 3-4 degrees left of the flight path all the time? There must be an explanation, right?

One is your heading, which does not change, the other is your course, your track over ground.

 

Remember this pic?

I mirrored it horizontally to match the current situation: your heading is north, but your path over ground is north-east.

 

That is why your nose is always pointing to the left, when following the ground track:

[shagrat]

It spawns with a heading of 347 then wanders about a bit before heading to north. There's no weathervaning into the crosswind on that tape.

Can you answer my question, please? What causes the nose of the plane to turn 3-4 degrees left from the planes course, starting to build directly after the plane settles?

 

I am not even touching the controls, so everything that happens to the plane is only wind, thrust and inertia.

 

P.S. the plane spawns with a heading of 359.8 degrees the 347 on the HSI is the magnetic course...

[shagrat]

One is your heading, which does not change, the other is your course, your track over ground.

 

Remember this pic?

I mirrored it horizontally to match the current situation: your heading is north, but your path over ground is north-east.

 

That is why your nose is always pointing to the left, when following the ground track:

Actually it does change... From 359.8° to 13.3° at the end of the track.

[Flagrum]

Actually it does change... From 359.8° to 13.3° at the end of the track.

I'd suspect the random movements at the in-air start doesn't help to produce precise results ...

 

Could you post the .acmi of this test?

[shagrat]

More important it turns the nose 3° left (357° heading) after 3-4 seconds in flight. Not north after a minute it still flies north with the nose pushed left, then it starts drifting.

Edited June 28, 2017 by shagrat

[David OC]

This comes back to aircraft design and speed of flight and air density right.

 

500 knots in a 737 at 40,000 feet will not be affected by this "directional stability" in the thin air and at this speed.

 

A cessna low at 140 knots in dense air with 20 knots crosswind, will be more affected.

 

Same with the A10 with it's slow speed and barndoor (rudders) in dense air.

Edited June 28, 2017 by David OC

[shagrat]

Tacview recording. Add .acmi to make it work. Forum does not allow .acmi filextension.

 

Tacview-20170628-202255-DCS.txt

[shagrat]

This comes back to aircraft design and speed of flight and air density right.

 

500 knots in a 737 at 40,000 feet will not be affected by this "directional stability" in the thin air and at this speed.

 

A cessna low at 140 knots in dense air with 20 knots crosswind, will be more affected.

 

Same with the A10 with it's slow soeed and barndoor (rudders) in dense air.

You mean after more than a dozen pages of people telling me how stupid I am it boils down to the statement I made in my second post (#28)? :megalol:

[BigDuke6ixx]

This comes back to aircraft design and speed of flight and air density right.

 

500 knots in a 737 at 40,000 feet will not be affected by this "directional stability" in the thin air and at this speed.

 

A cessna low at 140 knots in dense air with 20 knots crosswind, will be more affected.

 

Same with the A10 with it's slow soeed and barndoor (rudders) in dense air.

 

This is a navigation issue. Your 737, cessna and A10, all flying in a 20kt crosswind, all three will be pushed in the direction the crosswind is blowing at 20kt. It's a 20kt crosswind after all!

[shagrat]

Ok, what maybe irritating to the pilots here, is the fact, that in real life airplanes do not spawn in mid-air.

So to get the plane there it needs to travel through the air, and through the crosswind. Of course at that point the inertia will be overcome by the wind for a long time... So in real life you will never notice the forces at work.

What the track/tacview shows is when the inertia kicks in even at altitude it does turn the nose! At slow speeds even more. Vector diagram of the calculated forces ( thrust + wind + inertia reaction ) would show that, I guess.

 

So while you may not notice the nose being turned by the crosswind when you change course, it should still be there...

 

I need some sleep now. See you tomorrow.

[Flagrum]

More important it turns the nose 3° left (357° heading) after 3-4 seconds in flight. Not north after a minute it still flies north with the nose pushed left, then it starts drifting.

I can not say for sure what the cause of this is, but you are not flying level, you are constantly banked a bit to one or the other side. But it almost seems as if this is happening in some sort of oscillation, perhaps induced at the very start.

 

Imo this flight does not prove anything as the aircraft's behaviour is too random...

[BigDuke6ixx]

Ok, what maybe irritating to the pilots here, is the fact, that in real life airplanes do not spawn in mid-air.

So to get the plane there it needs to travel through the air, and through the crosswind. Of course at that point the inertia will be overcome by the wind for a long time... So in real life you will never notice the forces at work.

What the track/tacview shows is when the inertia kicks in even at altitude it does turn the nose! At slow speeds even more. Vector diagram of the calculated forces ( thrust + wind + inertia reaction ) would show that, I guess.

 

So while you may not notice the nose being turned by the crosswind when you change course, it should still be there...

I need some sleep now. See you tomorrow.

 

It won't be there. You really, truly, don't know what you're talking about. This theory of yours will only lead you to more silliness, like the notion some have that a 20kt headwind increases airflow over the wing of a plane flying into it by 20kt. It doesn't, it just reduces the ground speed by 20kt. And if a 20kt headwind was to suddenly appear, it would just produce a transient effect not a sustained one.

[David OC]

This comes back to aircraft design and speed of flight and air density right.

 

500 knots in a 737 at 40,000 feet will not be affected by this "directional stability" in the thin air and at this speed.

 

A cessna low at 140 knots in dense air with 20 knots crosswind, will be more affected.

 

Same with the A10 with it's slow speed and barndoor (rudders) in dense air.

 

This is a navigation issue. Your 737, cessna and A10, all flying in a 20kt crosswind, all three will be pushed in the direction the crosswind is blowing at 20kt. It's a 20kt crosswind after all!

 

You are both right here in different ways, I was confused with using this word weathervane affect, as this is land based, it's more a pendulum affect around the CG or the "directional stability" of the design.

 

The way I see this, at certain speeds and air density, there would be a pendulum affect at the CG to align this unsymmetrical wind current. Until the design can overcome this force with speed and or thinner air.

 

The low slow flying cessna would never overcome this unsymmetrical force or the "directional stability" can not completely fight this force off.

 

.

Edited June 28, 2017 by David OC

[BigDuke6ixx]

You are both right here in different ways, I was confused with using this word weathervane affect, as this is land based, it's more a pendulum affect around the CG or the "directional stability" of the design.

 

The way I see this, at certain speeds and air density, there would be a pendulum affect at the CG to align this unsymmetrical wind current. Until the design can overcome this force with speed and or thinner air.

 

The low slow flying cessna would never overcome this unsymmetrical force or the "directional stability" can not completely fight this force off.

 

.

 

All the crosswind does to a flying plane is push it in the direction its going at the speed its going. It doesn't blow the tail more that the nose. It's an issue for takeoff, landing and navigation.

[Yurgon]

What I wanted to tell FatSlapper was that if you take off and get airborne in a crosswind there is a tendency of the aircraft's nose to turn slightly into the wind and he would need to counter that, [...]

 

Yes, that is all undisputed.

 

Midair start, NO(!) control inputs from my side. [...]

 

Thanks for the track! I guess that answers how aircraft are spawned in relation to the wind. :smartass:

 

After an initial bump, I see the aircraft oscillating in the roll axis at a rate of maybe two oscillations per minute, with a net roll rather to the right.

 

Tacview recording. Add .acmi to make it work. Forum does not allow .acmi filextension.

 

Yeah, the TacView plays for me pretty much like the track did (except that the headings are off because of magnetic variation, to be expected). I see the aircraft oscillating and slowly heading towards 011° true north at the end of the 3 minute recording.

 

I'm completely clueless as to how this is supposed to prove your point.

 

 

Now explain to me, if the laws of physics command weathervaning into the wind, seeing as the vertical stabilizers are more affected by the wind than the nose... why does the aircraft actually turn away from it?

[David OC]

All the crosswind does to a flying plane is push it in the direction its going at the speed its going. It doesn't blow the tail more that the nose. It's an issue for takeoff, landing and navigation.

 

Sideslip angle

 

The sideslip angle, also called angle of sideslip (AOS, AoS, β {\displaystyle \beta } , Greek letter beta), is a term used in fluid dynamics and aerodynamics and aviation. It relates to the rotation of the aircraft centerline from the relative wind. In flight dynamics it is given the shorthand notation β {\displaystyle \beta } (beta) and is usually assigned to be "positive" when the relative wind is coming from the right of the nose of the airplane. The sideslip angle β {\displaystyle \beta } is essentially the directional angle of attack of the airplane. It is the primary parameter in directional stability considerations.[7]

 

In vehicle dynamics, side slip angle is defined as the angle made by the velocity vector to longitudinal axis of the vehicle at the center of gravity in an instantaneous frame. As the lateral acceleration increases during cornering, the side slip angle decreases. Thus at very high speed turns and small turning radius, there is a high lateral acceleration and β {\displaystyle \beta } could be a negative value.

 

https://en.wikipedia.org/wiki/Slip_(aerodynamics)#Sideslip_angle

 

 

velocity vector (Aim point) Flight path etc

 

Offset

 

To longitudinal axis (Line straight along aircrafts body)

 

Angle of Attack (AOA)

Angle of Sideslip (AOS) Cannot remember reading about this on my travels mmm, learn something new everyday.:)

 

 

.

Edited June 29, 2017 by David OC

[Yurgon]

Sideslip angle [...]

 

Fascinating!

 

How does it relate to weathervaning? The quoted Wikipedia article doesn't say anything about the reasons for sideslip angles.

 

To put it differently: If I fly into a headwind, will that affect the Angle of Attack the same way that flying in a crosswind affects the Angle of Sideslip?

[David OC]

I dont like this word weathervaning or weathercocking as it relates to a fix pivot point, an aircrafts cg moves. More like a pendulum affect. Think of the cog when a plane loads fuel and passengers, they make sure the COG is within limits of the design.

 

"The pendulum is subject to gravitational forces that determine its motion. Objects such as weathervanes, arrows, and airplanes are subject to aerodynamic forces, but the same ideas concerning stability apply."

 

Stability

The Basic Concept

http://aero.stanford.edu/e1/stability.html

 

Then read

 

Directional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion. Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation to the oncoming medium (water, air, road surface, etc.) when disturbed (rotated) away from that original direction. If a vehicle is directionally stable, a restoring moment is produced which is in a direction opposite to the rotational disturbance. This "pushes" the vehicle (in rotation) so as to return it to the original orientation, thus tending to keep the vehicle oriented in the original direction.

 

https://en.wikipedia.org/wiki/Directional_stability

 

 

 

.

Edited June 29, 2017 by David OC

[Frederf]

I should amend my previous conclusion about the response from an airplane subject to crosswind suddenly. I said that the heading will respond by changing the heading without changing the course. I.e. the airplane will pivot into the wind changing heading but not changing course.

 

The more careful look at the situation is that the effect will be a mixture of course and heading change. While the heading change is occurring due to yaw stability, there is some lateral acceleration. This lateral acceleration changes the course. The nose position that results in no beta angle to the air flow will not be the crab angle for the initial course but the course that has developed during the time the airplane was responding in heading.

 

An airplane with low yaw stability and low resistance to lateral acceleration will change course due to a sudden onset of crosswind and do little change of heading. Conversely an airplane with high yaw stability and high resistance to lateral acceleration will mostly pivot into the wind with minimal course change.

 

---

 

Back to the steady state situation...

 

You can think about an airplane moving in a crosswind in two ways: that the airplane is subject to two winds simultaneously from two sources or that it is subject to a single wind which is a combination of the two.

 

In the two-winds manner of thinking there is a wind opposite to the direction of motion. For example if the airplane is traveling south to north this is effectively a wind from north to south. Additionally there is a second wind which is the air movement which one would feel if the airplane was not in motion. Examined separately it should be clear that the airplane will be torqued to point directly into that one wind.

 

When both winds are applied at once (forward flight in cross wind) both winds apply torquing forces on the tail to attempt to point the airplane's nose into the direction each wind is coming from. Wind from each is applying a greater force on the tail than the nose at all times (even when beta is zero). The airplane is therefore subject to yaw torque in both directions at the same time by the wind of motion and the environmental wind. These two torques achieve a balance at some heading angle in between pointing at each source of wind. Despite achieving a balance in forces, the crosswind is applying a force on the tail to rotate the airplane forever and always but it cannot do so because there is another wind trying to do the opposite.

 

 

The one-wind interpretation is to consider the wind due to motion and the environmental wind and call it "relative wind." It is mathematically identical to the two-wind interpretation and reaches the same conclusion. When the airplane is crabbed into the crosswind only wind, relative wind, is straight down the airplane longitudinally and there is no yaw torque.

 

 

Deciding which of these two interpretations is correct is unhelpful. They are both valid.