Effect of Density Altitude on Crosswinds

CharlieTango said:
Roscoe,

The plane doesn't feel the wind until a side slip is initiated. I used to have a yaw string on my windshield and later one on my wing, it clearly indicated the crosswind in the final stages of landing or at take off prior to crabbing.
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This is just WAY wrong. The airplane feels relative wind in a slip, not the convective wind the earth makes, as seen by the windsock.
 
I think everyone missed that he said -5000ft DA, not 5000ft.

TAS would be lower, air would be more dense...
 
CharlieTango said:
The OP asked if effect of wind increased with decreasing DA and the answer is yes. You are arguing that the wind has no effect because the plane flies in the relative wind. You have to appreciate that at some point the plane flies / lands relative to the runway and the wind has effect, more so at lower DA.
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But what affect does that have on the landing itself and what you do? It's nice to know there's a theoretical package to say that a crosswind changes with altitude (assuming that's the case) but I don't see any practical application to it.

Perhaps you can run a sample set of calculations to illustrate? You know, take an airplane landing in a 15 kt crosswind at a Vref of 65 kts using a crosswind slip with no flaps and calculate the difference in the degree of bank required to maintain the centerline. I'm satisfied from my flight experience at sea level and in the Rocky Mountains that any difference is practically inconsequential. But that's anecdotal. You can show us whether or not it's scientifically/mathematically of any importance.
 
Ron Levy said:
You're ignoring the fact that at the same IAS (which is what you use to manage the plane for landing), your TAS is higher at higher DA, which cancels out the effect on which you are focusing. Further, TAS has nothing to do with stall margin -- it's all IAS, not TAS, since the lift and drag equations include correction for air density.
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The effect that I am focusing on is that the force of a 15kt crosswind upon a landing aircraft at a low DA is less than the same force at a high DA. The effect that DA has on IAS plays no part.

Saying that TAS has nothing to do with stall margin seems really out in the weeds. We all use IAS because we have air speed indicators that show IAS and IAS has the altitude correction built in.
 
RoscoeT said:
This is just WAY wrong. The airplane feels relative wind in a slip, not the convective wind the earth makes, as seen by the windsock.
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Yes the plane feels the relative wind what changes is the relative wind transitions from being on your nose to a wind with a crosswind component.
 
CharlieTango said:
Yes the plane feels the relative wind what changes is the relative wind transitions from being on your nose to a wind with a crosswind component.
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Relative "wind" that the airplane feels as it moves through the air is entirely unrelated to the earth's wind. Relative "wind" shouldn't even be called "wind".

So let me get this straight - you mentioned that you saw your yaw string move during landing in a x-wind. Are you trying to say that the earth's wind (x-wind) caused your yaw string to move? If so, then Houston we have a problem. ;)

You can slip the airplane and make your yaw string move, earth's wind or no wind.
 
CharlieTango said:
The effect that I am focusing on is that the force of a 15kt crosswind upon a landing aircraft at a low DA is less than the same force at a high DA. The effect that DA has on IAS plays no part.

Saying that TAS has nothing to do with stall margin seems really out in the weeds. We all use IAS because we have air speed indicators that show IAS and IAS has the altitude correction built in.
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:sigh: I give up.
 
midlifeflyer said:
But what affect does that have on the landing itself and what you do? It's nice to know there's a theoretical package to say that a crosswind changes with altitude (assuming that's the case) but I don't see any practical application to it.
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If you are confined to a small range of DA it will not do you any good. It was just an honest answer to a question.

I don't find the knowledge useful when actually landing, at that point I just move the controls to intercept the extended runway center-line and then whatever it takes to maintain the center-line and at some point alignment.

I would find an alternate if my sea level destination was reporting a 20kt quartering crosswind gusting to 35 but if the DA was 9,000' with the same winds I might find that worth a try.
 
RoscoeT said:
Relative "wind" that the airplane feels as it moves through the air is entirely unrelated to the earth's wind. Relative "wind" shouldn't even be called "wind".

So let me get this straight - you mentioned that you saw your yaw string move during landing in a x-wind. Are you trying to say that the earth's wind (x-wind) caused your yaw string to move? If so, then Houston we have a problem. ;)

You can slip the airplane and make your yaw string move, earth's wind or no wind.
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The yaw string indicates the relative wind. A southerly crosswind on an easterly landing, when aligned and tracking the center-line will result in a yaw sting moving left.
 
CharlieTango said:
The yaw string indicates the relative wind. A southerly crosswind on an easterly landing, when aligned and tracking the center-line will result in a yaw sting moving left.
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The x-wind isn't doing squat. You moving the rudder and slipping the airplane is moving the yaw string. X-wind doesn't matter. I think we all realize that in a x-wind you must slip the airplane to align with the runway. The same x-wind correction at 0' MSL will be needed at 10K'. But you keep coming back talking about wind force changing with DA and "hitting" the airplane differently. It's not about wind force, but it seems you will not be convinced no matter how many more posters disagree with you.
 
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From the OP"

...Does the effect of wind increase with decreasing density altitude? Suppose there's a 15kt crosswind, but a density altitude of around -5000ft

...Because the air is denser, will the crosswind have a greater effect on the aircraft than a 15kt X-wind at 5000ft DA? Is the effect negligible?

... The question mostly pertains to kicking out on landing...

I'm more interested in the immediate changes in velocity right around the time of landing.
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The OP asked if given a 10,000' change in DA is there more than a negligible difference in the effect of a 15kt x-wind.

The crosswind's effect becomes meaningful only after you transition to flying relative to the runway and the answer to the question is undeniably yes, the effect will increase.

If you guys won't accept that there is a transition or that the force of the crosswind does increase with a 10,000' DA decrease then go ahead and call me clueless. I do understand relative wind and TAS vs IAS but those issues do not address the question being asked.

Another way to look at this is the OP did focus on kicking out when landing and if the speed is slow enough the biggest force we are talking about would be drag due to yawing. In this case the answer remains the same, at 10,000' lower the more dense air would resist that yawing more. The 'kick' would be more force not less.
 
CharlieTango said:
If you are confined to a small range of DA it will not do you any good.
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The example I was using was Ocracoke (4 msl) vs Leadville (9934 msl). Large enough range for the calculations I suggested you give us to help settle the question of whether there is a difference and, if so, to what degree?

I would find an alternate if my sea level destination was reporting a 20kt quartering crosswind gusting to 35 but if the DA was 9,000' with the same winds I might find that worth a try.
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Based on my own experience I would not base my decision on whether or not to try an approach to landing on the density altitude differential. But that's not really the question is it?
 
CharlieTango said:
From the OP"



The OP asked if given a 10,000' change in DA is there more than a negligible difference in the effect of a 15kt x-wind.
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...and the calculations to show the difference are...?
 
RoscoeT said:
The x-wind isn't doing squat. You moving the rudder and slipping the airplane is moving the yaw string. X-wind doesn't matter. I think we all realize that in a x-wind you must slip the airplane to align with the runway. The same x-wind correction at 0' MSL will be needed at 10K'. But you keep coming back talking about wind force changing with DA and "hitting" the airplane differently. It's not about wind force, but it seems you will not be convinced no matter how many more posters disagree with you.
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Yours is post #51 and not everyone disagrees with me. Why make up 100 to 1?

Wind force does change with DA but "hitting' the airplane differently? Where do you get that?

The original question is: Does the effect of wind increase with decreasing density altitude? How can you conclude that it is not about wind force?
 
CharlieTango said:
The crosswind's effect becomes meaningful only after you transition to flying relative to the runway and the answer to the question is undeniably yes, the effect will increase.
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How??? Must you fly a larger crab angle and make a larger slip correction at the lower DA? Isn't that how we "gauge" x-wind strength - how large of a crab angle must be corrected? Crab angle doesn't change because of DA conditions. You still talk as if the airplane "feels" the wind force. Until you free your mind of that fallacy, you will continue to feel that you are the only person on this forum who understands these matters, and that all the rest of us are clueless.
 
RoscoeT said:
So let me get this straight - you mentioned that you saw your yaw string move during landing in a x-wind. Are you trying to say that the earth's wind (x-wind) caused your yaw string to move? If so, then Houston we have a problem. ;)
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When I first read CharlieTango's post with the yaw string, I was about to jump on it as well, thinking it might be a "Stick and Rudder moment".

But rereading it, he was talking about the slip right before touchdown and the slip that may slightly precede a crab into the wind on takeoff.

In his defense, I think he has fully internalized the concepts involved with an airplane in a moving air mass.

Believe me, CharlieTango is no CTLSi!
 
RoscoeT said:
How??? Must you fly a larger crab angle and make a larger slip correction at the lower DA? Isn't that how we "gauge" x-wind strength - how large of a crab angle must be corrected? Crab angle doesn't change because of DA conditions. You still talk as if the airplane "feels" the wind force. Until you free your mind of that fallacy, you will continue to feel that you are the only person on this forum who understands these matters, and that all the rest of us are clueless.
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You wouldn't need a larger crab angle but as I said there would be more drag when you kicked strait requiring more rudder pedal force to accomplish the same correction.

It will take more aileron input to counter drift.

Have you ever landed wing low and on one wheel in order to counter drifting dowwind across the runway? I'll assume you have, what force are you countering?

You can stop with everyone vs me already. How about post #24 and #17?
 
midlifeflyer said:
Based on my own experience I would not base my decision on whether or not to try an approach to landing on the density altitude differential. But that's not really the question is it?
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Lets see:

midlifeflyer said:
But what affect does that have on the landing itself and what you do?
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Personally I factor it into my decision to approach. So yes it was your question. Why so snarky?
 
Whoa, let's just reset and start over here.

Show me the formula for calculating Wind Correction Angle and then explain to me why altitude (or DA) is not anywhere to be found in it?

WindTriangle2.gif
 
CharlieTango said:
You wouldn't need a larger crab angle...
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Good....so this can easily be illustrated. Does it require more rudder force to yaw the nose 30 degrees to the right at low altitude vs. high altitude? I have never noticed a difference. Do you feel there is a difference? If not, then I cannot understand how you maintain that x-wind correction becomes more difficult at lower DA conditions. All x-wind correction involves is slipping the airplane (requires yawing) to correct for some angle of crab.
 
Silvaire said:
Whoa, let's just reset and start over here.

Show me the formula for calculating Wind Correction Angle and then explain to me why altitude (or DA) is not anywhere to be found in it?

WindTriangle2.gif
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The OP asked if given a 10,000' change in DA is there more than a negligible difference in the effect of a 15kt x-wind. Wind correction angle is not the subject.
 
CharlieTango said:
Wind correction angle is not the subject.
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But it is - see my post #61. Wind correction angle sets the amount of correction that must be made to align the airplane w/ the runway in a x-wind. Unless you are saying that it takes more force and control deflection to correct for say a 30 degree crab angle at high DA vs. low DA, then I cannot understand what you mean by "increased wind effect". I obviously disagree that it takes more force to correct this crab angle at high DA vs low DA.
 
RoscoeT said:
Good....so this can easily be illustrated. Does it require more rudder force to yaw the nose 30 degrees to the right at low altitude vs. high altitude? I have never noticed a difference. Do you feel there is a difference? If not, then I cannot understand how you maintain that x-wind correction becomes more difficult at lower DA conditions. All x-wind correction involves is slipping the airplane (requires yawing) to correct for some angle of crab.
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Density plays a part in resisting the yaw. I think there is a difference even if it is air frame dependent and not a great one but what next? Just because you yawed into alignment doesn't mean that you shouldn't have aileron into the wind or even be on one wheel? I counter the crosswind until it is no longer a factor.

To stay on thread, after kicking strait crosswind input is needed to a greater extent at the 10,000' lower DA strip, all other things being equal. We use this technique to keep the upwind wing down and at the low DA it would take less velocity to raise that wing.
 
The only difference between the two altitudes is your ground speed.
 
We're not talking about power, this could just as well be a glider.

The engine makes its max power at sea level and it will decrease as altitude increases but for flight at cruise or on a landing approach the aircraft can obtain a speed at which point the force of the air on the control surfaces and into the pitot tube is equal to what it was at sea level and the indicated airspeed is the same. But the aircraft is moving faster, that's why the ground speed is higher. That's why you do not fly a higher indicated airspeed at higher DA or field elevation.

The difference in the x-wind factor is that in any given number of seconds you have covered more ground.
 
Why do POA threads often become a p!ssing match? :lol:
 
The way I figure, because all forces on the aircraft are either equal or being compensated for via control inputs, given the change in TAS at varying degrees of DA, the 'force' effects are automatically compensated for (thus the differences are negated) whenever an aircraft is acting within an airmass.
 
FlyingIsGood said:
The way I figure, because all forces on the aircraft are either equal or being compensated for via control inputs, given the change in TAS at varying degrees of DA, the 'force' effects are automatically compensated for (thus the differences are negated) whenever an aircraft is acting within an airmass.
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You said it better than any of us.
 
FlyingIsGood said:
The way I figure, because all forces on the aircraft are either equal or being compensated for via control inputs, given the change in TAS at varying degrees of DA, the 'force' effects are automatically compensated for (thus the differences are negated) whenever an aircraft is acting within an airmass.
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At the point of landing there is a transition from 'acting within an airmass' to permitting that airmass to drift by while you maintain a relationship with the runway.

Most posts in this thread ignore this transition and the fact that the crosswind forces only exist after you transition.
 
CharlieTango said:
At the point of landing there is a transition from 'acting within an airmass' to permitting that airmass to drift by while you maintain a relationship with the runway.
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You cannot defeat the laws of physics. The airplane moves as part of the airmass until the wheels touch.
 
CharlieTango said:
...the crosswind forces only exist after you transition.
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By "crosswind forces" I assume you mean the aerodynamic forces resulting from the uncoordinated flight which is necessary when you decide you want your heading and ground track to coincide along the runway. I'm pretty sure everyone gets that. What most of us are trying to tell you however is that the aerodynamic forces at an airspeed of 80 kts are the same regardless of your altitude.

Granted you are moving faster at 10,000 feet to get that airspeed indication. If the forces were somehow less why would the pitot tube tell you they are the same? What we are trying to explain is that it's all relative, it takes x number of air molecules entering the pitot tube per second to register 80 knots and if the air is thinner well then you have to fly faster to collect them but the end result, other than your ground speed, is the same.

Same goes for the forces on the wings and flight control surfaces. If you draw a simple wind triangle the only variable that is different is the ground speed.
 
CharlieTango said:
At the point of landing there is a transition from 'acting within an airmass' to permitting that airmass to drift by while you maintain a relationship with the runway.

Most posts in this thread ignore this transition and the fact that the crosswind forces only exist after you transition.
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The situation you describe exists regardless of DA. When you touch down, you've removed 'flying' from the equation. The lateral force effect of the crosswind is now pushing against the tire/surface connection instead of just moving around an object in-solution within the airmass.

During the transition, the tire/surface connection isn't affected by DA (although one could argue semantics and come up with a mathematically insignificant number where a difference does exist), so the acting forces aren't going to differ to the point where a noticeable change in control input is required to compensate.

Also, remember that TAS is still in play given its relation to DA. Whether flying or touching down, your TAS is different based on DA, so forces are being automatically compensated for throughout the transition.

I imagine there is a crosswind speed where air density creates a noticeable difference at touch down, but I highly doubt that speed is something the vast majority of us would ever experience (much less attempt) during a landing - if that speed even exists.
 
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RoscoeT said:
You cannot defeat the laws of physics. The airplane moves as part of the airmass until the wheels touch.
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True, but the pilot flies in relation to his destination util landing then he transitions to landing relative to the runway.

You are correct but you omit the transition that happens prior to touchdown to remove any side loading.
 
CharlieTango said:
True, but the pilot flies in relation to his destination util landing then he transitions to landing relative to the runway.
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Some pilots choose to "transition" into this slip way up high on final. Slipping to align the airplane with the runway does not all of a sudden cause the airplane to "feel" the earth's wind. You are still moving with the airmass, just slipping through it - no different whether the wind is dead calm or howling. Have not seen a post from you that indicates you understand this.
 
ebykowsky said:
Does the effect of wind increase with decreasing density altitude?
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Both yes and no, I don't know what all the arguing is about. A side slip needed to correct for landing drift requires less yaw angle at higher density altitudes due to higher TAS. But if you are in the habit of using forward slips of a constant yaw angle to lose altitude the control forces will be the same due to the constant IAS (dynamic pressure).

dtuuri
 
CharlieTango said:
I'm thinking about both, until you oppose the crosswind force you are flying through a drifting airmass and drifting along with it. This would be 'the plane can't feel the wind condition'.
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This seems to be the source of the fundamental misunderstanding. The airplane is not opposing the crosswind force until you touch down, and then it's the friction between your tires and the runway doing the opposing. On the approach, you're just flying with and through the airmass. Slipping or crabbing doesn't change the physics, it just changes your direction of flight.
 
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