Why's and How's of a Coordinated Turn

[Bearfoot]

Yes, TBH, I am confused as to the direction of yaw during turns.

 

I tried rolling without rudder at low AoA, moderate speed, and as best I can make out it is adverse yaw requiring rudder in the direction of the roll to coordinate. I did not check my wing sweep (on auto) and it possible that fully-swept wings may have different behavior from unswept in this regard?

 

Digging through Natops:

 

11.5.6

 

Since roll control is provided by wing-mounted spoilers and differential stabilators, the aircraft exhibits proverse yaw throughout the flight envelope (yaw in the direction of the lateral stick input).

 

 

11.3.4

 

Large aft stick inputs applied with lateral stick during supersonic rolling maneuvers result in increased adverse sideslip and should be avoided

 

 

11.5.5

Maneuver flap and slat extension delays buffet onset below 0.7 IMN, reduces the intensity of the buffet, reduces the effects of adverse yaw at high AOA through increased positive dihedral effect (roll because of sideslip),

 

11.5.2 Dihedral Effect

Dihedral effect is the tendency of the aircraft to roll in reaction to sideslip being generated. The F-14 exhibits positive dihedral effect throughout the positive AOA envelope (tending to roll away from sideslip), but negative dihedral effect at negative AOA. This tendency is shown by the aircraft response to a rudder input. When right rudder is applied from straight and level flight condition, the aircraft sees sideslip from left and so rolls to the right, or away from the sideslip. Positive dihedral effect is a stabilizing influence in the area of reduced directional stability (high AOA). At negative AOA, dihedral effect is negative such that a right rudder input will produce a left roll. In the PA configuration, negative AOA can be encountered at 1 g flight at the higher limit airspeeds for the configuration.

 

Furthermore the heatblur video on the FM speaks ambiguously about "both adverse and proverse" yaw during rolls:

 

 

and as best I can make out, in the actual example shown, it looks like adverse yaw but I could not tell the speed,AoA, or wing sweep.

 

Perhaps someone smarter, more knowledgable, more experienced than me would be kind enough to have videos showing the different yaw characteristics at different AoA / speeds when uncoordinated?

[Nealius]

Just my ￥1 worth: I've noticed with the wings spread it requires more rudder in the direction of turn, even throughout the turn--assume more adverse yaw. With the wings swept it needs a tap of rudder to start the turn, but my feet are pretty flat (and the ball centered) throughout the turn itself--assume less/no adverse yaw.

 

I have yet to see noticeable proverse yaw. Maybe I just haven't yet gotten into the flight regime where it pops up.

[Bearfoot]

Just ran through a bunch of turns under different conditions (high speed / low speed, high AoA / low AoA, wings swept back / wings auto, SAS on / off).

 

I only see adverse yaw ... except under very slow / high AoA conditions. But even in the latter, initially the nose goes up in the direction opposite of bank and only as it settles into bank does the the nose drop down: i.e., both adverse and proverse yaw, with adverse yaw coming first.

 

But maybe I am doing or reading it wrong?

 

If someone can show proverse yaw that would be great!

[Bearfoot]

Just my ￥1 worth: I've noticed with the wings spread it requires more rudder in the direction of turn, even throughout the turn--assume more adverse yaw. With the wings swept it needs a tap of rudder to start the turn, but my feet are pretty flat (and the ball centered) throughout the turn itself--assume less/no adverse yaw.

 

I have yet to see noticeable proverse yaw. Maybe I just haven't yet gotten into the flight regime where it pops up.

 

I have been trying hard to get myself into proverse yaw, and I have not yet succeeded or if I have, I am too dumb to recognize it.

[Bearfoot]

This is not what my experience is telling me, nor what Natops, Victory205, Wikipedia et al. are explaining. Low or high aoa doesn't the slip occur in the opposite direction of the roll?

 

Would you be able to point out, or reproduce the specific text describing the slip(s)? I am trying to collate info on this, and so far there has been a lot of ambiguity.

[Jester2138]

This is not what my experience is telling me, nor what Natops, Victory205, Wikipedia et al. are explaining. Low or high aoa doesn't the slip occur in the opposite direction of the roll?

 

At high AoA, yes. At low AoA, no. I'm not seeing anything by Victory205 saying otherwise, and he's described the high AoA behavior quite extensively on these forums.

 

The maneuvering flaps extend symmetrically on the F14, so with right stick, you only get four spoilers up on the right wing, TE up on the right stabilator, and TE down on the left stabilator. This, because of vortices, stab TE blanking and rudder blanking, causes adverse yaw from the stabs to the left, resulting in right side slip. Dihedral effect under this condition creates a left roll. The horizontal stabs are far apart (engine cores are 9.5 feet apart) compared to single engine jets and centerline thrust F15's and 18's, so the yaw effects are more pronounced as well.

 

And editing after noticing the simultaneous post above, NATOPS Fight Characteristics is Section IV in the old manuals, and again, the examples you see online reflecting ARI and DFGS that aren't relevant. I don't know where or how those ended up online.

 

https://forums.eagle.ru/showthread.php?p=3796662#post3796662

 

^Read the two posts by Aaron and Victory if you're still confused.

 

The Tomcat gave excellent feedback via buffet on alpha and energy bleed rates. It also buffeted heavily and exhibited wing rock, proverse and adverse roll and needed to be flown using rudder input only with the stick centered laterally for roll at typical air combat turning alpha states. It buffets at one G with the landing flaps down too.

 

https://forums.eagle.ru/showpost.php?p=3746595&postcount=39

 

I have been trying hard to get myself into proverse yaw, and I have not yet succeeded or if I have, I am too dumb to recognize it.

 

Fly <10 units AoA and roll. Watch the slip indicator. Boom, proverse yaw.

Edited September 14, 2019 by Jester2138

[Bearfoot]

Fly <10 units AoA and roll. Watch the slip indicator. Boom, proverse yaw.

 

What I see is just a momentary "spike" of proverse yaw and then followed by adverse yaw as I settle into the turn. Rolls > 60 degrees of bank have very pronounced adverse yaw following the bob in the opposite direction.

 

So, if I roll left, the ball bobs right for just a second or so and then slides down to center with maybe a tad left bias. So at most I need to just briefly tap ever-so-gently a touch of opposite rudder in the beginning, but for the remainder or most of the turn I need rudder in the direction of turn.

 

I tried this with wings swept and wings auto, SAS on/off, etc. etc.

 

Is this what you expect to see when experiencing proverse yaw? I guess I was thinking it would be more pronounced.

[Nealius]

I see the same "spike." I've always attributed it to Newton's first law--the ball is stationary, the plane suddenly moves, of course the ball is going to "move" in the other direction (it's actually just staying where it is in relation to the airframe). When I look out over the nose I do not see such movement as indicated by the ball.

[Jester2138]

Adverse and proverse yaw are caused by the differential drag from the asymmetrical control surface input used to roll. Therefore, yes, they are only present when you are in a roll. If you neutralize the surfaces and continue the turn, the adverse yaw will go away.

 

In that sense, yes both adverse and proverse yaw is present only in "spikes" most of the time. That's how it works.

[METEOP]

Ok lets take a step back for a minute, I think we are discussing two different things:

 

 

- Adverse yaw vs aoa angle

- Rudder usage for turning

 

 

While it is true that various sources discuss adverse yaw at high aoa, they also state that under certain circumstances you also get proverse yaw but are vague as to which parameters are producing said proverse yaw.

 

 

Upon further reflexion, when turning what are we trying to accomplish? We are looking for a level _turn_ not compensating for adverse (or proverse) yaw of the roll...

 

 

I looked for the 'roll induced yaw vs aoa' diagram but could not locate any. Is it relevant?

 

 

For example:

 

 

High aoa, you need to use rudders for 'rolling' and we already agree that rolling (stick) at high aoa induces adverse yaw so if you roll using rudders adverse yaw is not a problem.

 

 

Low aoa, we are _turning_ so if the roll part of the turn induces a bit of proverse yaw that is a good thing, since it will be easyer to stay level in the turn, then apply proverse yaw with the rudders to stay level in the turn.

 

 

So in essence, we are both right just talking about two different things.

Edited September 14, 2019 by METEOP
typos

[Bearfoot]

Ok lets take a step back for a minute, I think we are discussing two different things:

 

 

- Adverse yaw vs aoa angle

- Rudder usage for turning

 

 

While it is true that various sources discuss adverse yaw at high aoa, they also state that under certain circumstances you also get proverse yaw but are vague as to which parameters are producing said proverse yaw.

 

 

Upon further reflexion, when turning what are we trying to accomplish? We are looking for a level _turn_ not compensating for adverse (or proverse) yaw of the roll...

...

 

 

Actually, at least to my reading, this has added a lot more confusion to the issue.

 

We are --- always have been --- talking about a coordinated turn.

 

Which is not, AFAIK, the same thing as a level turn.

 

• A coordinated turn is turn without sideslip.
  
• A level turn is a turn without change in altitude.
  

 

You can have a coordinated turn that is not level.

 

Can you have a level turn that is uncoordinated?

 

Maybe. But either way, I think they are pretty different issues.

 

Now, don't get me wrong --- it took me a while to get a level turn down (on the break, especially) in this bird, and that would be a great discussion to have. But it is a separate discussion.

Edited September 15, 2019 by Bearfoot

[Bearfoot]

Adverse and proverse yaw are caused by the differential drag from the asymmetrical control surface input used to roll. Therefore, yes, they are only present when you are in a roll. If you neutralize the surfaces and continue the turn, the adverse yaw will go away.

 

In that sense, yes both adverse and proverse yaw is present only in "spikes" most of the time. That's how it works.

 

I see the same "spike." I've always attributed it to Newton's first law--the ball is stationary, the plane suddenly moves, of course the ball is going to "move" in the other direction (it's actually just staying where it is in relation to the airframe). When I look out over the nose I do not see such movement as indicated by the ball.

 

Well, then mystery solved. The Tomcat does, indeed, have proverse yaw. And adverse yaw. Often in the same turn. So coordinated flight at low AoA or high speed would typically would involve a only a momentary touch of opposite rudder.

[Nealius]

I find in certain situations that rudder is needed throughout the turn, rather that just a momentary touch. Particularly at slow speeds, I'll need a momentary "strong" touch of rudder, then need a smidge of rudder held throughout to keep the ball centered.

[METEOP]

Actually, at least to my reading, this has added a lot more confusion to the issue.

 

We are --- always have been --- talking about a coordinated turn.

 

Which is not, AFAIK, the same thing as a level turn.

 

• A coordinated turn is turn without sideslip.
  
• A level turn is a turn without change in altitude.
  

 

You can have a coordinated turn that is not level.

 

Can you have a level turn that is uncoordinated?

 

Maybe. But either way, I think they are pretty different issues.

 

Now, don't get me wrong --- it took me a while to get a level turn down (on the break, especially) in this bird, and that would be a great discussion to have. But it is a separate discussion.

 

 

Oops sorry I did mean coordinated turn not level turn so sorry about the added confusion...

[Jester2138]

Ok lets take a step back for a minute, I think we are discussing two different things:

 

 

- Adverse yaw vs aoa angle

- Rudder usage for turning

 

We are discussing both those things. They are related. Because of adverse and proverse yaw, you need rudder when turning (and rolling in general).

 

 

While it is true that various sources discuss adverse yaw at high aoa, they also state that under certain circumstances you also get proverse yaw but are vague as to which parameters are producing said proverse yaw.

 

They're perfectly clear. I've repeated them several times here. You get proverse yaw a low AoA and adverse yaw at high AoA, this is also closely related to wing sweep angle and the switch can be seen around 8-12 units AoA depending on configuration.

 

 

Upon further reflexion, when turning what are we trying to accomplish? We are looking for a level _turn_ not compensating for adverse (or proverse) yaw of the roll...

 

I've only been talking about coordinated flight aka compensating for adverse and proverse yaw. That can be needed in a level turn or a climbing turn or a descending turn or while not turning at all. It's simply needed when you induce roll via lateral stick inputs and asymmetrical control surface activation.

 

High aoa, you need to use rudders for 'rolling' and we already agree that rolling (stick) at high aoa induces adverse yaw so if you roll using rudders adverse yaw is not a problem.

 

No, you would be using only rudder at VERY high AoAs. There is a wide range where you need to use similar rudder and stick inputs to be coordinated during a roll and turn because of adverse yaw.

 

Low aoa, we are _turning_ so if the roll part of the turn induces a bit of proverse yaw that is a good thing, since it will be easyer to stay level in the turn, then apply proverse yaw with the rudders to stay level in the turn.

 

No, you want to be coordinated because it's aerodynamically more efficient and less prone to stalling one wing before the other (i.e. spin). You should use pitch to actually perform the turn as your left vector is rotated, not slip.

 

 

So in essence, we are both right just talking about two different things.

 

Not as far as I can tell.

 

We are --- always have been --- talking about a coordinated turn.

 

Which is not, AFAIK, the same thing as a level turn.

 

Correct...

 

Well, then mystery solved. The Tomcat does, indeed, have proverse yaw. And adverse yaw. Often in the same turn. So coordinated flight at low AoA or high speed would typically would involve a only a momentary touch of opposite rudder.

 

You wouldn't see both in the same turn. Yes, in any speed/configuration/AoA/whatever, in theory the rudder application is only as long as your lateral stick input.

 

I feel like you guys need to go back to basics on what adverse yaw is based on how thoroughly this seems to have confused you.

Edited September 16, 2019 by Jester2138

[Bearfoot]

 

Correct...

 

You wouldn't see both in the same turn. Yes, in any speed/configuration/AoA/whatever, in theory the rudder application is only as long as your lateral stick input.

 

I feel like you guys need to go back to basics on what adverse yaw is based on how thoroughly this seems to have confused you.

 

"back to basics" --- this implies that that we were had the basics at some point previously. I, at least, don't! LOL. Working through this as I go along, on whatever time I have between day job and family, and depending on the kindness/time/experience/understanding of other folks to "correct the sideslips", as it were.

[Bearfoot]

Basics ... with probably the only better looking jet to ever grace the USN inventory

 

Edited September 16, 2019 by Bearfoot

[Jester2138]

I was thinking far more basic:

 

This is adverse yaw. Proverse yaw is exactly the same thing, just the opposite yaw. An aerodynamically simple plane like the Piper he's in only exhibits adverse yaw. The Tomcat exhibits both in different regimes as the plane's configuration changes.

[Bearfoot]

I was thinking far more basic:

 

This is adverse yaw. Proverse yaw is exactly the same thing, just the opposite yaw. An aerodynamically simple plane like the Piper he's in only exhibits adverse yaw. The Tomcat exhibits both in different regimes as the plane's configuration changes.

 

Thank you for this. This was very helpful.

 

For anyone else reading this, even if you understand the fundamental theory of why there is adverse yaw, if you have no formal training or actual exposure to practical flying like me you might find that last bit especially useful just as a practical demonstration of the effect and its correction. This was the "missing piece" for me, and actually seeing it happen in a real demo was the catalyst I needed to bring together lots of vague ideas. The link below is cued to that time.

 

[Callsign JoNay]

Apologies for resurrecting such an old topic, but are we sure the slip indicator is working properly? It seems to work well in straight and level flight, but when rolling the aircraft it behaves in the opposite way I expect it to based on what I'm seeing out of the window.

 

 

In the unlisted video above I put the wings symbol on a mountain peak and roll back and fourth left and right without rudder. You can see that adverse yaw/slip is affecting the nose, but the indicator moves to the same direction indicating proverse/skid. Is this realistic behavior? It's probably one of the reasons why there is so much confusion for how to coordinate the Cat.

 

[GunSlingerAUS]

I'm glad I'm not the only one confused by the indicator! I'm guessing we're reading it incorrectly though, as such an obvious issue would have been picked up long ago. Having said that, so how the hell is the ball meant to be read?

[Spiceman]

It is indicating correctly  You read the ball as “step on the the ball”. So starting around 1:15 when you are flying straight level and step on the right rudder and induce right yaw, the ball is to the left. “Step on the ball”. It’s telling you to apply left rudder to remove the yaw. When you’re quickly rolling left and right you’re giving sort of a fake illusion that the ball is always opposite of where it should be because you’re reversing your turn before the ball settles back from the adverse yaw. Bank left for 20 seconds, then bank right for 20 seconds and post a new video. 

Edited June 7, 2021 by Spiceman

[Spartan111sqn]

5 minutes ago, Spiceman said:

It is indicating correctly  You read the ball as “step on the the ball”. So starting around 1:15 when you are flying straight level and step on the right rudder and induce right yaw, the ball is to the left. “Step on the ball”. It’s telling you to apply left rudder to remove the yaw. 

 ok, but when it is rolling, which ball shall I step? the nose one or the tail one?

[Spiceman]

2 hours ago, Spartan111sqn said:

ok, but when it is rolling, which ball shall I step? the nose one or the tail one?

I’m confused, there’s only one ball. The needle above the ball is a turn rate indicator. It indicates your rate of turn and it is a gyro based instrument. The ball is simply a ball floating in mineral oil and it is a simple inclinometer that indicates the balance between gravity and centrifugal force. When you bank, gravity pulls the ball down and centrifugal force (influenced by application of rudder) pulls the ball up. When gravity and centrifugal force are in balance, I.e. a proper coordinated turn, the ball is centered. 
 

The needle will always move in the direction of your turn. There is a gyro in there mounted vertically and it works on precession. As the plane turns and applies a left or right force on the gyro, the gyro applies a force 90 degrees in the direction of rotation and that is tied to the turn needle. 

 

 

EDIT: I've watched your video again and I think I see more now what you're saying. In the first section, when you go full stick deflection and initiate that rapid roll rate to the left, I believe you may be seeing the effects of fluid compression in the turn coordinator. In a rapid acceleration into a left roll, the fluid in the indicator compresses to the left and the ball moves to the right, or the less dense side of the indicator. As your acceleration into the roll stops, the pressure in the fluid equalizes throughout the fluid. It's a similar odd effect to when you're in a car with a helium balloon, when you turn left, the balloon actually moves towards the left side of the car, to the *inside* of the turn. This is because the air inside the car has compressed to the right, causing the balloon to move left into your turn.  

 

Edited June 7, 2021 by Spiceman

[Callsign JoNay]

Thanks for the reply, Spiceman. (Love your channel and vids btw).

 

Yeah, it seems to behave predictably in straight and level flight, and it's great for zeroing out asymmetry. But it behaves in ways I didn't expect during maneuvers. I've never flown an airplane in real life, but a commercial pilot friend of mine who saw the video thought the behavior was odd, so I thought it was worth asking.

 

Edit: He still thinks it looks odd after I shared your explanation to him, for what it's worth. 

Edited June 7, 2021 by Callsign JoNay