Effect of Density Altitude on Crosswinds

RotorDude said:
This is absolutely correct.
Of course "heading" in this case is the direction the aircraft is moving in, not the indication on the DG, since the latter will have an offset if we are in a slip.
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Heading is what you read on the compass, track is the course over the ground. Always is, always has been. You don't want to get the two mixed up.
 
Silvaire said:
No, I don't think so.

You are approaching the runway, on track in coordinated flight and your nose is 30 degrees off the runway heading to the left because of the xwind.

You now enter a slip by pointing the nose along the runway in preparation for landing.

The wind is still moving laterally in relation to the runway however you are not. So how can you not be moving laterally through the air? :dunno:

This is pretty basic stuff.
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See my post above. You can never move laterally through the air, as defined by the direction the airframe is moving through the air. In your example, you are actually continuing your crab just as before, so your heading (as defined by airframe motion) hasn't changed. The only thing that changes is you have introduced a yaw that causes your DH to have an offset. Your bank is only there to keep that yaw from becoming a turn, i.e. keep your slip attitude in place.
 
dtuuri said:
Yes, a sideslip. I used to have students sideslip from being aligned with the runway lights on one side to the other while maintaining runway heading. Not possible you say? :confused:
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You either did a slipping turn, or turned your flight path first, and then entered a slip. Either way, turning was involved rather than pure sideways travel. If anyone can shoot video of their airplane tracking a flight path in coordinated flight and then moving that flight path in a different direction without changing the nose alignment/heading - without a turning (curved) flight path, in zero wind conditions, I'd love to see it. I think we have different ideas regarding what "sideways" travel means, and another semantics problem.
 
Silvaire said:
Heading is what you read on the compass, track is the course over the ground. Always is, always has been. You don't want to get the two mixed up.
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You are correct that heading read by the DG is what the compass is saying about the position of the nose relative to the Earth.
But if you think in terms of heading relative to the airstream, it's always the direction the airframe is flying. Track over the ground is another issue, which depends your speed and wind.
On short final and flare, as you start slipping, your rudder will change the DG heading, but not the airframe heading, which remains going the same way, and your ground track will also be constant. You introduce a bank to keep that yaw in place, to prevent sideload.
 
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CharlieTango said:
I'm still stuck on this thinking:

I'm in a slipping configuration in order to maintain alignment and track the center-line.

With a 15kt 90* crosswind I do have sideways velocity but relative to the relative wind not relative to the ground. When it comes to aileron input sufficient to track centerline how can the air mass density not play a part? If shear is present and the 15kts changes to 20kts I need more aileron to counter that acceleration.

According to this conversation I should then consider the WCA at 20kts, convert to bank angle and realize that the mass/density of this accelerating crosswind is playing no part. :dunno:
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I think the increased true airspeed/ground speed makes the difference in the control inputs to track. You do agree that higher TAS requires less crosswind correction (or crab for that matter) just by the wind triangle, right? And this is all moot if you don't care about the ground reference (i.e. you're not trying to line up with a fixed runway on the ground). You notice difference in the control inputs, but they are because of increased TAS/GS not directly as a factor of higher DA.

I think the way to do the math is to think of air molecules (mass) hitting the airplane. For straight flight (ignoring wind right now) you fly at a faster TAS to get the same number of molecules hitting the surfaces (and therefore same IAS). When you turn the airframe in a slip, molecules are hitting the "side" surfaces as well because of the slip. Those molecules now are at an angle and their force could be calculated by using vectors. But you are still flying at a higher TAS to get the same number of molecules hitting the airframe to maintain your IAS. So the mass of air hitting the airframe (# of molecules) is the same. Calculate the sideways force in the vector. (I haven't yet nor do I have time to right now, but never forget the higher TAS!)

Now do your normal wind correction triangle at sea level TAS and at high DA TAS. Compare the changes and I think that the higher TAS/GS dominates the control changes required.

Once you touch down, your frame of reference changes and I'm quite comfortable that the density matters. But again, your speeds are now in reference to the ground, not the air. It's the frame of reference changes and perspectives that make this a hard to understand problem.

John
 
I learned something, thanks for the help.

However I live in a place where you see mostly twin turbines and up and not many very light planes and obviously I am now flying a light sport.

The big issue up here is shear, I get gusted from both sides and both ends. For me, most crosswind landings mean flying through colliding winds and in this case I do have various winds impacting my aircraft from various directions. Since that is somewhat typical I tend to see wind with shear as the norm and when that is the case density must matter.

Another case where density matters is when the winds exceed your stall speed. I cannot operate with 40kts + at sea level but can at high altitude.
 
CharlieTango said:
I learned something, thanks for the help.

However I live in a place where you see mostly twin turbines and up and not many very light planes and obviously I am now flying a light sport.

The big issue up here is shear, I get gusted from both sides and both ends. For me, most crosswind landings mean flying through colliding winds and in this case I do have various winds impacting my aircraft from various directions. Since that is somewhat typical I tend to see wind with shear as the norm and when that is the case density must matter.

Another case where density matters is when the winds exceed your stall speed. I cannot operate with 40kts + at sea level but can at high altitude.
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Shears and gusts are places where the air mass relative to you changes, so yes: density matters. As for the stall speed, I think you are again in the realm where higher TAS required saves your bacon.

John
 
whifferdill said:
You either did a slipping turn, or turned your flight path first, and then entered a slip. Either way, turning was involved rather than pure sideways travel. If anyone can shoot video of their airplane tracking a flight path in coordinated flight and then moving that flight path in a different direction without changing the nose alignment/heading - without a turning (curved) flight path, in zero wind conditions, I'd love to see it. I think we have different ideas regarding what "sideways" travel means, and another semantics problem.
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We all should know these terms and agree. A sideslip most definitely can be made from a steady heading and rolled to an opposite sideslip. A yaw string will confirm the lateral motion through the air and a look out the window will confirm the lateral motion with respect to the ground. I think any pilot would agree and I'm highly surprised you and Rotordude have some other idea, one I can't grasp btw.

dtuuri
 
CharlieTango said:
Technically those are turns, now that I get it.
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No you don't. A turn is a change in heading. In a side slip, the airplane is indeed moving forward and laterally in the direction of the slip. If you are slipping into the wind you won't have any lateral movement across the ground because you have compensated for the drift. Did your instructor teach you to slip into the wind when landing in a xwind?
 
vintage cessna said:
No you don't. A turn is a change in heading. In a side slip, the airplane is indeed moving forward and laterally in the direction of the slip. If you are slipping into the wind you won't have any lateral movement across the ground because you have compensated for the drift. Did your instructor teach you to slip into the wind when landing in a xwind?
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A slip is a change in heading (defined as which way the airplane fuselage reference line is pointing, how the DG, mag compass, etc. register will show it). It's not a change in track if you are compensating for the wind.

John
 
whifferdill said:
...If anyone can shoot video of their airplane tracking a flight path in coordinated flight and then moving that flight path in a different direction without changing the nose alignment/heading ...
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No need for a video just start in straight and level coordinated flight then input left aileron while simultaneously inputting right rudder to keep the heading constant. The nose remains pointed in the same direction, you're not turning.

You now have full left aileron and full right rudder, nose still on the same point and you're going to tell us we are not in a slip? That we are not flying "laterally" through the air and that our ground track has not been altered? That this is impossible?
 
jsstevens said:
A slip is a change in heading (defined as which way the airplane fuselage reference line is pointing, how the DG, mag compass, etc. register will show it). It's not a change in track if you are compensating for the wind.

John
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Thanks for the correction. I want to thank everyone, I've learned a lot today.
 
As far as moving laterally through the air mass, or more accurately having a lateral component...

What about those videos taken from a fighter cockpit where one plane "peels off"? You know, maintains heading but raises one wing and moves rapidly away from the camera.

That's what I picture when I think of moving laterally within an air mass.

Anyone have a video like that handy?
 
FastEddieB said:
As far as moving laterally through the air mass, or more accurately having a lateral component...

What about those videos taken from a fighter cockpit where one plane "peels off"? You know, maintains heading but raises one wing and moves rapidly away from the camera.
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That's a simple turn. But jets are moving fast enough that the heading change happens much more slowly than in light planes. It still happens though. They are still turning, like any other airplane - not moving laterally.

dtuuri said:
A sideslip most definitely can be made from a steady heading and rolled to an opposite sideslip. A yaw string will confirm the lateral motion through the air and a look out the window will confirm the lateral motion with respect to the ground.
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But you have not changed your flight path...through the air, or with respect to the ground.

dtuuri said:
I think any pilot would agree and I'm highly surprised you and Rotordude have some other idea, one I can't grasp btw.
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There's nothing I don't grasp about slips. Again, communication problems. And once again, I refer back to my analogy of a car changing lanes without any amount of turning. Of course, cars can't truly do that either. To me, that is sideways travel. Airplanes can't do that. If they can, please send video. I understand slips and slipping turns. I do them every landing. I guess we are simply not communicating. The written word can be difficult. I hope you can at least understand what I'm saying well enough to stop accusing me of cluelessness. I have the sense to understand that we are considering "sideways" movement differently. I'm not accusing you of being clueless. But I understand that folks on internet forums are spring loaded to look for any possible statement that could be misconstrued as something that proves one person is with it, and another person is not, and happily point that out. Keeps the old marbles moving around I guess. Threads that have gone this far rarely accomplish anything, so I'm out and heading to the airport to see if I can figure out those slip things. :D
 
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jsstevens said:
Shears and gusts are places where the air mass relative to you changes, so yes: density matters. As for the stall speed, I think you are again in the realm where higher TAS required saves your bacon.

John
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Just one last question. When air is accelerated, like in a gust, is the air somehow compressed? Is that how the air becomes more dense?
 
whifferdill said:
That's a simple turn. But jets are moving fast enough that the heading change happens much more slowly than in light planes. It still happens though. They are still turning, like any other airplane - not moving laterally.



But you have not changed your flight path...through the air, or with respect to the ground.



There's nothing I don't grasp. Again, communication problems. And once again, I refer back to my analogy of a car changing lanes without any amount of turning. To me, that is sideways travel. Airplanes can't do that. If they can, please send video. I understand slips and slipping turns. I do them every landing. I guess we are simply not communicating. The written word can be difficult. I hope you can at least understand what I'm saying well enough to stop accusing me of cluelessness. I have the sense to understand that we are considering "sideways" movement differently. I'm not accusing you of being clueless. But I understand that folks on internet forums are spring loaded to look for any possible statement that could be misconstrued as something that proves one person is with it, and another person is not, and happily point that out. Keeps the old marbles moving around I guess. Threads that have gone this far rarely accomplish anything, so I'm out and heading to the airport to see if I can figure out those slip things. :D
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It is more than a communications problem, and no one has tried to one up you. People just lose patience after a while. You really just need to hit the books again.
 
vintage cessna said:
Just one last question. When air is accelerated, like in a gust, is the air somehow compressed? Is that how the air becomes more dense?
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Slightly, but the effect on your plane is because of an air mass moving differently than the air around it.

John
 
It's really not a simple turn, since there's no heading change.

Looks like the fighter is cranking in aileron away from the photo ship, but uses rudder to maintain heading. Moves laterally away from the photo ship.

No luck finding a video yet.
 
whifferdill said:
Originally Posted by dtuuri View Post
A sideslip most definitely can be made from a steady heading and rolled to an opposite sideslip. A yaw string will confirm the lateral motion through the air and a look out the window will confirm the lateral motion with respect to the ground.
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But you have not changed your flight path...through the air, or with respect to the ground.
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Yes, complete communication breakdown :rolleyes2:
 
vintage cessna said:
You really just need to hit the books again.
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I obviously offended you by quoting something you said as incorrect. I can make this really easy for you. Throw a model airplane sideways though the air. It turns, it doesn't fly sideways. That's the simple point I'm making about airplanes. But again, some are trying to construe this as my lack of understanding about slips. I guess I can't help that. If you have a book that I can "hit" that describes an airplane flying sideways as I described, I'd be happy to read it.
 
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whifferdill said:
I obviously offended you by quoting something you said as incorrect. I can make this really easy for you. Throw a model airplane sideways though the air. It turns, it doesn't fly sideways. That's the simple point I'm making about airplanes. But again, some are trying to construe this as my lack of understanding about slips. I guess I can't help that. If you have a book that I can "hit" that describes an airplane flying sideways as I described, I'd be happy to read it.
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You haven't offended me, just astonished me. I've already suggested the FAA Airplane Flying Handbook. You can go to FAA.gov and get a free download. When you get it, look up side slip.
 
FastEddieB said:
Looks like the fighter is cranking in aileron away from the photo ship, but uses rudder to maintain heading. Moves laterally away from the photo ship.
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The lateral move is actually using rudder to yaw the nose in the direction you want to go while using aileron to hold the wings level. Otherwise, it makes the next pilot in the formation crazy trying to stay in position as your wings rock back and forth, not to mention looking really bad from outside the formation. But either way, it's a slip. You'd only roll away from the other aircraft if you're splitting off, like when pitching out in the break or detaching a wingman/element.
 
I don't understand why some people here are insisting that you cannot enter a slip without changing your heading, you most certainly can. You can crank your ailerons left and right to the stops and keep the nose on point with the rudder, it's a basic coordination exercise and the result is most definitely a slip, not a turn. You can also do the opposite, let the nose go left and right while maintaining your ground track.

Didn't you guys do these things when you were students?
 
Silvaire said:
I don't understand why some people here are insisting that you cannot enter a slip without changing your heading, you most certainly can. You can crank your ailerons left and right to the stops and keep the nose on point with the rudder, it's a basic coordination exercise and the result is most definitely a slip, not a turn. You can also do the opposite, let the nose go left and right while maintaining your ground track.

Didn't you guys do these things when you were students?
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Yes, both. But don't confuse heading with track. When you keep the nose pinned, you are changing track. When you keep the track constant you are changing heading (moving the nose).

John
 
Sorry if I'm now coming across as a smart ass. I am not an aeronautical genius. But when people state things in direct contradiction to what I have learned from sources like the FAA Airplane Flying Handbook, and my own experience, I am left bewildered. And they say these things with such confidence.
 
jsstevens said:
Yes, both. But don't confuse heading with track.
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As I explained to RotorDude in post #171, heading is what you read on the compass and for aviators there is no alternate definition. The only contested area of confusion that I am aware of is when you are instructed to "maintain runway heading" after take off.
 
Silvaire said:
I don't understand why some people here are insisting that you cannot enter a slip without changing your heading, you most certainly can. You can crank your ailerons left and right to the stops and keep the nose on point with the rudder, it's a basic coordination exercise and the result is most definitely a slip, not a turn. You can also do the opposite, let the nose go left and right while maintaining your ground track.

Didn't you guys do these things when you were students?
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Well, Wx sucks here...

Flight path heading doesn't change, but the direction the nose is pointing sure does. We are cross talking again.

vintage cessna said:
Sorry if I'm now coming across as a smart ass. I am not an aeronautical genius. But when people state things in direct contradiction to what I have learned from sources like the FAA Airplane Flying Handbook, and my own experience, I am left bewildered. And they say these things with such confidence.
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You still haven't acknowledge my simple point and are bent on proving others just don't get it. Go to the hardware store and buy a Guillows balsa glider and try throwing it sideways. It turns. Does the FAA handbook describe aircraft flight 90 degrees to the relative wind? It's a simple aerodynamic fact. No slip can cause the airplane to fly 90 degrees to the relative wind. I'm sure you fully understand this. It is a simple point that is being blown up and out of context by you.
 
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whifferdill said:
Well, Wx sucks here...

Flight path heading doesn't change, but the direction the nose is pointing sure does. We are cross talking again.



You still haven't acknowledge my simple point and are bent on proving others just don't get it. Go to the hardware store and buy a Guillows balsa glider and try throwing it sideways. It turns. Does the FAA handbook describe aircraft flight 90 degrees to the relative wind? It's a simple aerodynamic fact. No slip can cause the airplane to fly 90 degrees to the relative wind. I'm sure you fully understand this. It is a simple point that is being blown up and out of context by you.
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I didn't say it would move 90 degrees. If you slip left, the airplane will have movement forward and to the left. Placing an aircraft into a slip will also change the relative wind. Why don't you stop asking what the FAA handbook says and just read it for yourself. If you won't buy what is in the FAA handbook there is no point in further discussion.
 
vintage cessna said:
If you won't buy what is in the FAA handbook there is no point in further discussion.
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Calm down fella, I don't disagree with anything in the holy FAA handbook. I have read it. This discussion has long outlived its usefulness. If we were sitting around hangar flying in person, it would be much easier for all of us to understand what the other is trying to say. E-talk is just too muddy sometimes, and it can be hard to reign in misunderstandings. And I'm not saying that as if you have reading and comprehension problems. Cheers...
 
jsstevens said:
But they are turning in that example. You can see the heading change.
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That's why I said it wasn't a great example.

But I've see them where the nose stays locked straight ahead.

A slip can be defined as too much bank for a given rate of turn.

It's definitely possible to bank a plane and not turn - not sure why that's even being brought up.
 
whifferdill said:
...Flight path heading doesn't change, but the direction the nose is pointing sure does. We are cross talking again...
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First and foremost, for the third time let's get it straight that heading is where the nose is pointing. Always has been, always is and always will be. There is no alternative definition and there is no such thing as "flight path heading" - that is your track or course.

Secondly: you can most certainly enter a slip with out changing your heading (again - where the nose is pointed) just keep it there with rudder as you apply opposite aileron like I explained earlier.

Thirdly: if you modify your dime store model glider to have an up aileron on the left wing, a down aileron on the right wing and a right rudder deflection it most definitely will fly sideways (in a slip)

Why you insist it won't or that an airplane, regardless of how you manipulate the controls, can't possibly do it baffles me.

This is basic stuff here.

Maybe a picture will help?

Slipping.gif
 
Silvaire said:
Secondly: you can most certainly enter a slip with out changing your heading (again - where the nose is pointed) just keep it there with rudder as you apply opposite aileron like I explained earlier.
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If you enter a slip without changing your heading, you are simply doing a slipping turn to a new flight path of your choice, at which point you can stop the slipping turn such that your new (constant) ground track causes the nose to remain where you want. Your picture of the "side slip" above involves just that. To get from the center of the runway to the edge, you must do a slipping turn, and then stop the slipping turn. The airplane doesn't instantaneously start moving "sideways" along a constant flight path, it simply turns (curved flight path) to a new ground track. The curved flight path change still happens even if not noticed. The flip side is that you cannot slip at a constant flight path without the nose moving off heading back and forth. On the way out today, I shot this. Airplane maintains centerline track, slipping left and right, but the nose moves significantly. I'm sure you understand this, it's just a visual.

 
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whifferdill said:
On the way out today, I shot this. Airplane maintains centerline track, slipping left and right, but nose moves significantly.
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Notice how your nose wanders off heading? Those are forward slips not sideslips.

dtuuri
 
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