Effect of Density Altitude on Crosswinds

ebykowsky

ebykowsky

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Hey Y'all!

Been awhile since I've been on here, but just had a question I couldn't find the answer to online:

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? Or is it mitigated by the fact that the air coming straight at you is just as dense? The question mostly pertains to kicking out on landing, more-so than cruise flight. Ostensibly, in equilibrium at cruise, the effects will cancel out; I'm more interested in the immediate changes in velocity right around the time of landing.

Thanks for any insight you can provide!
 
The measurement is taken from the air hitting an object(fan). Wouldn't it have the same effect on another object(airplane)? I'd say the difference is negligible.
 
As long as you fly at the same indicated airspeed, there is no change in the effect on controllability. However, since you'll be at a higher true airspeed, you'll have a smaller drift angle. But on the third hand, the change in drift angle for a 15 knot crosswind and a 70 KIAS approach speed at 5000 DA vs SL (75 KTAS vs 70 KTAS) would be less than 1 degree, so it really wouldn't make any noticeable difference.
 
Yes the crosswind will have a greater effect and it is a meaningful difference.

The crosswind will cause drift or alignment issues that need to be countered. 15kts gives you the velocity but not the mass (density) of the air. Velocity times mass = force.

This concept is evident in the high Sierra Nevada, 100 mph winds are not uncommon yet they seldom cause roofs to blow off, we need 130 mph winds to do damage like 100 mph winds do at sea level.
 
CharlieTango said:
Yes the crosswind will have a greater effect and it is a meaningful difference.

The crosswind will cause drift or alignment issues that need to be countered. 15kts gives you the velocity but not the mass (density) of the air. Velocity times mass = force.
Click to expand...
While true in theory, the practical effect is negligible when looking at a 15 knot crosswind at 5000 DA vs SL.
 
Ron Levy said:
While true in theory, the practical effect is negligible when looking at a 15 knot crosswind at 5000 DA vs SL.
Click to expand...

True in theory and practice. You are talking about drift angle while the effects of the more dense crosswind at the same 15kts is one of 2 things:

1) the amount of drift correction required (amount of aileron input) will be greater.

or

2) the amount of force on the rudder pedal to kick strait from the crab will be greater even if the angel is not.

I generally see ~7,000' of elevation change from my departure to my destination and have learned this lesson the hard way. Early in my career I landed at Palo Alto (sea level) with a nice laminar flow crosswind yet I was overcome by the amount of drift correction needed. Luckily I was flying a Cherokee that could take the side-loading. At that time almost all of my flying was at high attitude fields where the crosswinds are gusty but have less force at the same velocity.
 
I think the device measuring the wind accounts for the density of the fluid in steady state conditions. 15 kts is 15 kts

I did find this counterpoint:

Abstract:

"The effect of air density variations on the calibration constants of several models
of anemometers has been analyzed. The analysis was based on a series of calibrations
between March 2003 and February 2011. Results indicate a linear behavior of both
calibration constants with the air density. The effect of changes in air density on the
measured wind speed by an anemometer was also studied. The results suggest that there can
be an important deviation of the measured wind speed with changes in air density from the
one at which the anemometer was calibrated, and therefore the need to take this effect into
account when calculating wind power estimations."
 
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CharlieTango said:
True in theory and practice. You are talking about drift angle while the effects of the more dense crosswind at the same 15kts is one of 2 things:

1) the amount of drift correction required (amount of aileron input) will be greater.

or

2) the amount of force on the rudder pedal to kick strait from the crab will be greater even if the angel is not.
Click to expand...
Any way you look at it, there is no perceptible difference in what you have to do to handle a 15-knot crosswind between SL and 5000 DA.
 
CharlieTango said:
Yes the crosswind will have a greater effect and it is a meaningful difference.

The crosswind will cause drift or alignment issues that need to be countered. 15kts gives you the velocity but not the mass (density) of the air. Velocity times mass = force.

This concept is evident in the high Sierra Nevada, 100 mph winds are not uncommon yet they seldom cause roofs to blow off, we need 130 mph winds to do damage like 100 mph winds do at sea level.
Click to expand...

Huh? That does not compute. Roofs (and the houses they are part of) are attached to the ground. The more relevant comparison is the roof after it gets blown off and is a free part of the airmass. I'm pretty sure that a 60 knot wind will blow it along at 60 kts whatever the density altitude is.

This sounds a lot like the same old problem people have thinking in terms of the airplane as being "pushed" by the airmass rather than being a "part" of it.

The wind is not "pushing" the airplane, the airplane is flying "in" the airmass. The only time the force (mass X acceleration) of the airmass would be a factor is at touchdown and during the rollout when the airplane is once again on terra firma. And while that may be a valid consideration for computation, I agree completely with Ron that it's negligible.

At least I haven't noticed any more difficulty landing in a 15-kt direct crosswind at Ocracoke (4 msl) than at Leadville (9934 msl). And I guess that's as good a definition of "negligible" as any.
 
All of the aircraft's handling characteristics depend upon the same air density in the same way. Turns and crabs cancel in terms of controllability. However, there are two offsetting and much larger effects, so it's largely irrelevant. One is that crab is a function of true airspeed, not indicated airspeed, and TAS is higher at higher altitudes. The other is that the winds themselves are different. Crosswinds become unsteady very easily in the presence of terrain or uneven heating, both of which are present in spades in or near the mountains (high or not). Deserts also tend to pull the air in especially when it gets warm and especially in the afternoon, and often have terrain at their upwind margin (that's why they are deserts). Which means you cannot plan for the crosswind on more than an order of magnitude scale, even if you're listening to AWOS on short final. It is what it is.

For instance, at Half Moon Bay yesterday, the wind speed varied by more than a knot each minute, from right down the runway to 30 deg off. The highest I saw was 18 knots total. This means you might have a 9 knot crosswind, or you might have nothing. It's also a REAL good bet there is some windshear in that. I had a first timer on board, so I went elsewhere. The terrain around there occasionally makes for some interesting roll at 500 feet. That's a sea level airport, but trust me, you can get some serious windshear just from having rocks around. This is what you care about.
 
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Sleepingsquirrel said:
I think the device measuring the wind accounts for the density of the fluid in steady state conditions. 15 kts is 15 kts

I did find this counterpoint:

Abstract:

"The effect of air density variations on the calibration constants of several models
of anemometers has been analyzed. The analysis was based on a series of calibrations
between March 2003 and February 2011. Results indicate a linear behavior of both
calibration constants with the air density. The effect of changes in air density on the
measured wind speed by an anemometer was also studied. The results suggest that there can
be an important deviation of the measured wind speed with changes in air density from the
one at which the anemometer was calibrated, and therefore the need to take this effect into
account when calculating wind power estimations."
Click to expand...

Velocity is a component of force. Force is at issue when it comes to correcting not velocity.
 
Ron Levy said:
Any way you look at it, there is no perceptible difference in what you have to do to handle a 15-knot crosswind between SL and 5000 DA.
Click to expand...

The difference is in the amount of force required not the technique.

You need to apply enough force to negate the drift or to kick it strait so 'what you have to do' is the same, the amount of force required is the variable.
 
midlifeflyer said:
Huh? That does not compute. Roofs (and the houses they are part of) are attached to the ground. The more relevant comparison is the roof after it gets blown off and is a free part of the airmass. I'm pretty sure that a 60 knot wind will blow it along at 60 kts whatever the density altitude is.

This sounds a lot like the same old problem people have thinking in terms of the airplane as being "pushed" by the airmass rather than being a "part" of it.

The wind is not "pushing" the airplane, the airplane is flying "in" the airmass. The only time the force (mass X acceleration) of the airmass would be a factor is at touchdown and during the rollout when the airplane is once again on terra firma. And while that may be a valid consideration for computation, I agree completely with Ron that it's negligible.

At least I haven't noticed any more difficulty landing in a 15-kt direct crosswind at Ocracoke (4 msl) than at Leadville (9934 msl). And I guess that's as good a definition of "negligible" as any.
Click to expand...

The only time you oppose the drifting air mass that is the crosswind is when you counter drift in a side slip or when you remove the crab, otherwise I agree with most of what you say.

The roof example is to show that wind speed alone doesn't tell the whole story, you have to multiply that velocity by mass to get force. The force at issue does not effect how your plane flies but it does effect how your plane drifts or crabs relative to the runway.
 
MAKG1 said:
Huh?

Crosswind is a component of wind velocity.

Force and velocity are different things.
Click to expand...

Force and velocity are different things, that is precisely what I keep saying.

If you only consider velocity and not density you are oversimplifying. That is why I used the roof as an example. Wind speed alone says a roof that cannot withstand 100mph wind will come off, however this fails to be true at a higher altitude.

The concept is the same as fewer air molecules entering your pitot tube will indicate a slower reading at altitude even at the same speed.
 
Ron Levy said:
As long as you fly at the same indicated airspeed, there is no change in the effect on controllability. However, since you'll be at a higher true airspeed, you'll have a smaller drift angle. But on the third hand, the change in drift angle for a 15 knot crosswind and a 70 KIAS approach speed at 5000 DA vs SL (75 KTAS vs 70 KTAS) would be less than 1 degree, so it really wouldn't make any noticeable difference.
Click to expand...

While I agree Ron, I can tell you from first had experience at high altitude airport take offs and landings it seems "different" and definitely noticeable. I was at Laramie, WY with winds 35 gusts to 45 KNTS at about a 30 degree angle to the runway. Here I don't think I could have taxied, there I could taxi and felt comfortable taking off. I only used 200' of runway. The higher density seems to make the wind "less forceful" even with the same MPH speed. The only reason I tried it was because a CFI and student were doing T&G's. If they could do it, so could I! :eek: :lol:
 
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Geico266 said:
While I agree Ron, I can tell you from first had experience at high altitude airport take offs and landings it seems "different" and definitely noticeable. I was at Laramie, WY with winds 35 gusts to 45 KNTS at about a 30 degree angle to the runway. Here I don't think I could have taxied, there I could taxi and felt comfortable taking off. I only used 200' of runway. The higher density seems to make the wind "less forceful" even with the same MPH speed.
Click to expand...

I was in a flight of 2 SLSA coming home from Page, AZ when I had to land at Tonopah where the winds were over 40kts. I needed more tape on my radiator and over did it and had to land a 2nd time. Neither of us had a problem with the winds. DA was 8,000'

I'm, quite sure we couldn't do this at sea level precisely because of the wind force difference.
 
CharlieTango said:
The concept is the same as fewer air molecules entering your pitot tube will indicate a slower reading at altitude even at the same speed.
Click to expand...

I might agree if we approached at high altitude at the same true airspeed.

But, we don't. And that makes all the difference. We approach so that the effect on the pitot tube is the same as it is at lower altitude. This cancels the density effects on controls -- each of the surfaces sees the same effect and control pressures at a given orientation as it would at sea level.

The one exception is that wind speeds are reported as true.
 
MAKG1 said:
I might agree if we approached at high altitude at the same true airspeed.

But, we don't. And that makes all the difference. We approach so that the effect on the pitot tube is the same as it is at lower altitude. This cancels the density effects on controls -- each of the surfaces sees the same effect and control pressures at a given orientation as it would at sea level.

The one exception is that wind speeds are reported as true.
Click to expand...

Good try, lets explore this thought.

We are landing Mammoth (KMMH) RNWY 09, approaching at 1.3 Vso in my CTSW and indicating 51kts. Winds are 180 @ 20kts.

The above example has a 20kt crosswind component times the less dense air mass resulting in less crosswind force to counter as drift or crab correction.

If I had the same winds at sea level I would have more corsswind force to counter.

The pilot chooses a higher TAS to maintain margin over stall when landing at altitude but nature chooses the crosswind component.

At altitude we land at a higher TAS because there are fewer air molecules to provide lift in the same way that the 20kt crosswind has fewer air molecules and provide less drift force to counter.
 
Geico266 said:
While I agree Ron, I can tell you from first had experience at high altitude airport take offs and landings it seems "different" and definitely noticeable. I was at Laramie, WY with winds 35 gusts to 45 KNTS at about a 30 degree angle to the runway. Here I don't think I could have taxied, there I could taxi and felt comfortable taking off. I only used 200' of runway. The higher density seems to make the wind "less forceful" even with the same MPH speed. The only reason I tried it was because a CFI and student were doing T&G's. If they could do it, so could I! :eek: :lol:
Click to expand...

I agree that taxiing is easier (for me this is theory, I've never landed outside Florida), but that's still in reference to the ground not moving in the airmass independently of the ground.

John
 
MAKG1 said:
All of the aircraft's handling characteristics depend upon the same air density in the same way. Turns and crabs cancel in terms of controllability. However, there are two offsetting and much larger effects, so it's largely irrelevant. One is that crab is a function of true airspeed, not indicated airspeed, and TAS is higher at higher altitudes. The other is that the winds themselves are different. Crosswinds become unsteady very easily in the presence of terrain or uneven heating, both of which are present in spades in or near the mountains (high or not). Deserts also tend to pull the air in especially when it gets warm and especially in the afternoon, and often have terrain at their upwind margin (that's why they are deserts). Which means you cannot plan for the crosswind on more than an order of magnitude scale, even if you're listening to AWOS on short final. It is what it is.

For instance, at Half Moon Bay yesterday, the wind speed varied by more than a knot each minute, from right down the runway to 30 deg off. The highest I saw was 18 knots total. This means you might have a 9 knot crosswind, or you might have nothing. It's also a REAL good bet there is some windshear in that. I had a first timer on board, so I went elsewhere. The terrain around there occasionally makes for some interesting roll at 500 feet. That's a sea level airport, but trust me, you can get some serious windshear just from having rocks around. This is what you care about.
Click to expand...

No argument there, but this just says that other environment effects are present in place where we find lower density air, not as a direct effect of lower density air.

John
 
jsstevens said:
I agree that taxiing is easier (for me this is theory, I've never landed outside Florida), but that's still in reference to the ground not moving in the airmass independently of the ground.

John
Click to expand...

John,

When it comes to landing aligned with the runway you have to transition from moving with the airmass to maintaining the center-line and opposing the drifting of the airmass. It is only at this point, after any crab is removed that you are countering the drift with control input and that input amount required is dependent on both crosswind velocity and density.
 
I tend toward CharlieTango's way of thinking. wind velocity is not what we have to counter, wind *force* is.

On Mars there are often 100mph winds, but the air is so thin it barely stirs the sands.

This remains theory for me though, I have no real high DA experience.
 
CharlieTango said:
At altitude we land at a higher TAS because there are fewer air molecules to provide lift in the same way that the 20kt crosswind has fewer air molecules and provide less drift force to counter.
Click to expand...

I'm trying to work this out in my mind.

Would not a 20kt crosswind have exactly enough "drift force" to move an unsecured plane at 20k perpendicular to the runway, whether at SL or 10,000'?
 
FastEddieB said:
I'm trying to work this out in my mind.

Would not a 20kt crosswind have exactly enough "drift force" to move an unsecured plane at 20k perpendicular to the runway, whether at SL or 10,000'?
Click to expand...

You are in a 20kt airstream regardless. But would not denser air have more mass (thus more force) to counter, and so require more control force to counter?
 
CharlieTango said:
The only time you oppose the drifting air mass that is the crosswind is when you counter drift in a side slip or when you remove the crab,
Click to expand...
I'm still not with you . You "oppose" the drifting airmass every time you go anywhere, even at Flight Level 400 unless your airspeed is zero (as in a balloon).

I think you are still thinking in terms of relative to the ground, not relative to the airmass.
 
bullwinkle said:
You are in a 20kt airstream regardless. But would not denser air have more mass (thus more force) to counter, and so require more control force to counter?
Click to expand...
You mean like if you are flying in a 20 kt headwind, you will need significantly more power to get to your destination at 3,000 msl than at 10,000 msl?
 
CharlieTango said:
The above example has a 20kt crosswind component times the less dense air mass resulting in less crosswind force to counter as drift or crab correction.
Click to expand...

You are still talking like the airplane feels a wind "force" hitting it during a x-wind landing. It doesn't. It moves with the wind. A 20KT wind at 0' MSL vs. 10,000' MSL causes the airplane to move with that wind at 20KT in each case. As has already been noted, the only difference is that your true airspeed (and hence groundspeed) is higher at high DA, requiring less of a correction. It's not because the less dense x-wind doesn't "hit" you as hard.
 
FastEddieB said:
I'm trying to work this out in my mind.

Would not a 20kt crosswind have exactly enough "drift force" to move an unsecured plane at 20k perpendicular to the runway, whether at SL or 10,000'?
Click to expand...

Yes it would and at sea level that force would be greater due to the additional drag from the denser air.
 
RoscoeT said:
Are you related to CTLSi? There is a striking resemblance. :lol:
Click to expand...

That's just a low blow. From hanging out on the CT forums, I can assure you CharlieTango and CTLSi are *not* the same person nor are they related. In fact, I'd probably pay good money to see a cage match between the two!
 
midlifeflyer said:
I'm still not with you . You "oppose" the drifting airmass every time you go anywhere, even at Flight Level 400 unless your airspeed is zero (as in a balloon).

I think you are still thinking in terms of relative to the ground, not relative to the airmass.
Click to expand...

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'.

The OP's question comes into play when you 'correct' for the crosswind drift and at that point, for a given crosswind velocity, density plays a role.
 
CharlieTango said:
Yes it would and at sea level that force would be greater due to the additional drag from the denser air.
Click to expand...

You're having a mental block about x-winds and force. Let me rephrase the issue, which is the same thing, said a different way -

Two airplanes - one is flying along at 20' MSL, with a 20KT wind from its left. Second airplane is flying along at 10,000' MSL, same airspeed, same 20KT wind from its left. Do you think the airplane at 10K flies with less of a crab angle because the wind "hits" it with less force than the plane at 20'? No, the two fly at effectively the same crab angle. It's the reciprocal issue of making x-wind correction during landing. Besides gusts, and unless the airplane is in contact with the ground, there is no wind force "hitting" the airplane.....ever.
 
Geico266 said:
While I agree Ron, I can tell you from first had experience at high altitude airport take offs and landings it seems "different" and definitely noticeable. I was at Laramie, WY with winds 35 gusts to 45 KNTS at about a 30 degree angle to the runway. Here I don't think I could have taxied, there I could taxi and felt comfortable taking off.
Click to expand...
I think if I put you somewhere with a significant crosswind, you would not be able to tell me your elevation by the way the plane taxied or its handling on takeoff. IOW, I think your feelings were basically psychosomatic.
 
RoscoeT said:
You are still talking like the airplane feels a wind "force" hitting it during a x-wind landing. It doesn't. It moves with the wind. A 20KT wind at 0' MSL vs. 10,000' MSL causes the airplane to move with that wind at 20KT in each case. As has already been noted, the only difference is that your true airspeed (and hence groundspeed) is higher at high DA, requiring less of a correction. It's not because the less dense x-wind doesn't "hit" you as hard.
Click to expand...

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.

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.
 
CharlieTango said:
Good try, lets explore this thought.

We are landing Mammoth (KMMH) RNWY 09, approaching at 1.3 Vso in my CTSW and indicating 51kts. Winds are 180 @ 20kts.

The above example has a 20kt crosswind component times the less dense air mass resulting in less crosswind force to counter as drift or crab correction.

If I had the same winds at sea level I would have more corsswind force to counter.

The pilot chooses a higher TAS to maintain margin over stall when landing at altitude but nature chooses the crosswind component.

At altitude we land at a higher TAS because there are fewer air molecules to provide lift in the same way that the 20kt crosswind has fewer air molecules and provide less drift force to counter.
Click to expand...
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.
 
RoscoeT said:
You're having a mental block about x-winds and force. Let me rephrase the issue, which is the same thing, said a different way -

Two airplanes - one is flying along at 20' MSL, with a 20KT wind from its left. Second airplane is flying along at 10,000' MSL, same airspeed, same 20KT wind from its left. Do you think the airplane at 10K flies with less of a crab angle because the wind "hits" it with less force than the plane at 20'? No, the two fly at effectively the same crab angle. It's the reciprocal issue of making x-wind correction during landing. Besides gusts, and unless the airplane is in contact with the ground, there is no wind force "hitting" the airplane.....ever.
Click to expand...

Your two airplanes would be effected the same, they would crab along relative to the ground with their nose in the relative wind and not sense any crosswind. There would be no wind hitting them from the side.

Landing is different even without gusts. If there is a crosswind component and you want to land aligned then at some point you have to transition from flying with your nose in the wind to having wind crossing your aircraft. The OP said his question pertained to effect of crosswind on landing.
 
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