Effect of Density Altitude on Crosswinds

Discussion in 'Flight Following' started by ebykowsky, Feb 19, 2015.

  1. ebykowsky

    ebykowsky Cleared for Takeoff

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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!
     
  2. Gucci Pilot

    Gucci Pilot Pattern Altitude

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    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.
     
  3. poadeleted20

    poadeleted20 Deleted

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    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.
     
  4. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  5. poadeleted20

    poadeleted20 Deleted

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    While true in theory, the practical effect is negligible when looking at a 15 knot crosswind at 5000 DA vs SL.
     
  6. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  7. Sleepingsquirrel

    Sleepingsquirrel Pre-takeoff checklist

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    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."
     
    Last edited: Feb 19, 2015
  8. poadeleted20

    poadeleted20 Deleted

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    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.
     
  9. midlifeflyer

    midlifeflyer Final Approach

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    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.
     
  10. MAKG1

    MAKG1 Touchdown! Greaser!

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    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.
     
    Last edited: Feb 19, 2015
  11. CharlieTango

    CharlieTango Line Up and Wait

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    Velocity is a component of force. Force is at issue when it comes to correcting not velocity.
     
  12. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  13. MAKG1

    MAKG1 Touchdown! Greaser!

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    Huh?

    Crosswind is a component of wind velocity.

    Force and velocity are different things.
     
  14. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  15. poadeleted20

    poadeleted20 Deleted

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    To misquote Galileo, "E pur si non muove."
     
  16. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  17. Geico266

    Geico266 Touchdown! Greaser!

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    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:
     
    Last edited: Feb 19, 2015
  18. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  19. MAKG1

    MAKG1 Touchdown! Greaser!

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    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.
     
  20. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  21. jsstevens

    jsstevens En-Route

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    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
     
  22. jsstevens

    jsstevens En-Route

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    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
     
  23. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  24. bullwinkle

    bullwinkle Pattern Altitude

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    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.
     
  25. FastEddieB

    FastEddieB Final Approach

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    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'?
     
  26. bullwinkle

    bullwinkle Pattern Altitude

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    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?
     
  27. FastEddieB

    FastEddieB Final Approach

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    I'm still working on your Mars thought experiment!

    You didn't happen to have just read "The Martian", did you?
     
  28. midlifeflyer

    midlifeflyer Final Approach

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    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.
     
  29. midlifeflyer

    midlifeflyer Final Approach

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    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?
     
  30. RoscoeT

    RoscoeT Cleared for Takeoff

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    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.
     
  31. RoscoeT

    RoscoeT Cleared for Takeoff

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    Are you related to CTLSi? There is a striking resemblance. :lol:
     
  32. CharlieTango

    CharlieTango Line Up and Wait

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    Yes it would and at sea level that force would be greater due to the additional drag from the denser air.
     
  33. bullwinkle

    bullwinkle Pattern Altitude

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    I did not, but I'm kind of a Mars junkie...do you recommend it?
     
  34. bullwinkle

    bullwinkle Pattern Altitude

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    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!
     
  35. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  36. RoscoeT

    RoscoeT Cleared for Takeoff

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    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.
     
  37. poadeleted20

    poadeleted20 Deleted

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    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.
     
  38. CharlieTango

    CharlieTango Line Up and Wait

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    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.
     
  39. poadeleted20

    poadeleted20 Deleted

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    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.
     
  40. CharlieTango

    CharlieTango Line Up and Wait

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    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.