Effect of Density Altitude on Crosswinds

[Lindberg]

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 plane doesn't ever feel the wind. The yaw string just shows you which direction the relative wind is coming from. Or, in other words, in what orientation the plane is flying. When the yaw string is straight, you are coordinated, i.e. the nose is pointing in the direction of flight. When the yaw string is off to one side, the plane is uncoordinated, i.e. the nose is not pointed in the direction of flight, the plane is flying sideways. That has zero to do with the wind and everything to do with how the control surfaces are positioned. Try slipping down the runway when there's no wind. The yaw string doesn't care, it behaves just the same.

 

[CharlieTango]

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.

When you employ a side slip on final in order to counter the crosswind, or in other words to maintain alignment while you track the centerline you are penetrating the crosswind at the same speed that you are drifting with it. In this phase the fuselage is presented to the relative wind and my yaw string will reflect that.

There is a difference between calm and windy, if you use the same slip when calm you will drift and not track the center line.

 

[CharlieTango]

This seems to be the source of the fundamental misunderstanding. The airplane is not opposing the crosswind force until you touch down,...

You begin opposing the crosswind when you initiative the side slip.

Imagine you are on a 2 mile final with a constant crosswind and you lower your wing, opposing with rudder so that you are tracking and aligned with the runway center-line. The slip is opposing the crosswind component, if you remove the slip you drift off course, if you put the slip back in you maintain the course (center line).

 

[CharlieTango]

Try slipping down the runway when there's no wind. The yaw string doesn't care, it behaves just the same.

You would be crabbing. The objective of a crosswind landing is to ovoid side loading the gear. Add tracking the center line and maintaining alignment and then the yaw string will indicate the crosswind in that condition.

 

[CharlieTango]

The thead asks if a DA lower by 10,000' will produce a crosswind with more effect. When I responded yes many argued that the plane isn't effected by wind and the thread went in the weeds.

A better question might be does a crosswind gust have a greater effect at the 10,000' lower DA? This question removes most of the arguing because Shirley we can agree that the plane is effected by gusts.

To answer my own simpler question, yes, a 15kt crosswind gust has more effect at sea level than at 10,000'. It will require more opposing aileron than if you were at 10k.

 

[CharlieTango]

...

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.

...

This thought has been expressed a couple of times. It is true that some forces are automatically compensated for but the difference in force of the crosswind is not one of them.

 

[FastEddieB]

Again, my stumbling block...

At SL, a 15k crosswind will move a plane 15nm downwind in an hour unless corrected for.

At 10,000', a 15k crosswind will move a plane 15nm downwind in an hour unless corrected for.

What then gives one more "force" than the other?

"In physics, a force is any interaction which tends to change the motion of an object. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described by intuitive concepts such as a push or a pull." - from Wikipedia

 

[RoscoeT]

You obviously are incapable of understanding that when you are one man in a thread arguing with EVERYONE else, you need to stop talking, listen, and put your thinking cap on. I won't hold my breath though.

When you employ a side slip on final in order to counter the crosswind, or in other words to maintain alignment while you track the centerline you are penetrating the crosswind at the same speed that you are drifting with it.

Dude, try to read and comprehend this - You were doing this before the slip - it's called crabbing. The slip changes nothing but your heading - it doesn't change your ground track. All you are doing before and after the slip is aligning your flight path so that as you drift with the airmass, you remain tracking a straight line over the ground.

There is a difference between calm and windy, if you use the same slip when calm you will drift and not track the center line.

Then you do NOT understand slips. If you are flying on a straight path and then apply proper slip inputs, your ground track DOES NOT CHANGE. Only your heading.

The slip is opposing the crosswind component, if you remove the slip you drift off course, if you put the slip back in you maintain the course (center line).

Nope dude, if you're crabbing and tracking the runway, the slip does not change that. A slip does no more to "oppose" a x-wind than a crab does. The airplane does not start "feeling" the x-wind when you slip. You are still drifting with the airmass before and after the slip. All the slip is doing is moving the nose.

We could all keep saying this until we are blue in the face, but I don't think it'll do any good. You will continue to repeat stuff that totally misses the fundamentals. I don't think you will take the hint, though. You are just the one smart guy in a sea of dimwits, it seems.

 

[CharlieTango]

Again, my stumbling block...

At SL, a 15k crosswind will move a plane 15nm downwind in an hour unless corrected for.

At 10,000', a 15k crosswind will move a plane 15nm downwind in an hour unless corrected for.

What then gives one more "force" than the other?

"In physics, a force is any interaction which tends to change the motion of an object. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described by intuitive concepts such as a push or a pull." - from Wikipedia

Eddie,

It takes more force to move an object through a denser media. The air at sea level is less dense than the air at 10,000'.

Imagine air vs water, the amount of force to move your SkyArrow sideways at 10kts through water would be far greater than the amount of force it takes to move it sideways at 10kts through sea level air.

 

[CharlieTango]

You obviously are incapable of understanding that when you are one man in a thread arguing with EVERYONE else, you need to stop talking, listen, and put your thinking cap on. I won't hold my breath though.

That's the third time you claimed that but its not true. I gave you examples the last time.

Dude, try to read and comprehend this - You were doing this before the slip - it's called crabbing. The slip changes nothing but your heading - it doesn't change your ground track. All you are doing before and after the slip is aligning your flight path so that as you drift with the airmass, you remain tracking a straight line over the ground.

The slip adds drag and subtracts efficiency. In the crab you maintained your track by using a heading that resulted in the desired track. In a slip you maintain the heading by opposing the drift. When you transition from crab to slip you change the method used to track the centerline. If you don't side slip enough you will drift with the wind, if you slip to much you will drift into the wind, only if you oppose the crosswind force with the correct amount of slip will you cancel the drift.

Then you do NOT understand slips. If you are flying on a straight path and then apply proper slip inputs, your ground track DOES NOT CHANGE. Only your heading.

I said if you used the same slip you wold drift without the crosswind. If you change your heading you are not using the same slip. The slip should maintain both alignment and track but the slip you are now describing only maintains track not alignment.

Nope dude, if you're crabbing and tracking the runway, the slip does not change that. A slip does no more to "oppose" a x-wind than a crab does. The airplane does not start "feeling" the x-wind when you slip. You are still drifting with the airmass before and after the slip. All the slip is doing is moving the nose.

The slip removes the crab so you don't side load on contact. Once the crab is removed the drift will begin unless you oppose it with a slip or whees(s) on the runway.

We could all keep saying this until we are blue in the face, but I don't think it'll do any good. You will continue to repeat stuff that totally misses the fundamentals. I don't think you will take the hint, though. You are just the one smart guy in a sea of dimwits, it seems.

Thats about 5 times you have attacked me personally.

 

[CharlieTango]

A simpler question than the OP's would be: If you install a 30 knot wind sock at sea level and another 30 knot wind sock at 10,000' and subject both socks to a 30 knot wind what would be the result?

Of course the sea level sock would be strait out indicating the 30 kt wind but the high DA sock would not be due to less force.

Aircraft and pilots have crosswind limitations because at some amount of force the control authority isn't there or the pilots skill isn't there, it is hard to argue that the force isn't there.

Go back to my Tonopah example. The winds were above 'stall' speed for our planes yet we could land, taxi and take off without difficulty. At sea level winds above stall speed would make your aircraft uncontrollable but at 10,000' DA there might be margin left and you can keep it on the ground.

 

[RoscoeT]

Imagine air vs water, the amount of force to move your SkyArrow sideways at 10kts through water would be far greater than the amount of force it takes to move it sideways at 10kts through sea level air.

Since you're still thinking in terms of an airplane "forcing" its way sideways through the air, and you are mentally unable to correct your flawed thinking, we should all just give up. It think most have. We officially crown you the only person on POA who understands groundspeed vs. airspeed, and x-winds. Real complex stuff. Let the rest of us get back to pondering why wheels are round. Wow just wow.

Do you even question why you are alone on the island in this thread?? I'm out on this one, promise.

 

[CharlieTango]

Since you're still thinking in terms of an airplane "forcing" its way sideways through the air...

Why is it called a "side slip"?

In cruise we adjust our heading and crab along the desired ground track. Some pilots crab all the way until touch down and only have to oppose the crosswind in the final seconds and roll out.

Many of us transition to a side slip on final and oppose the drift for a longer time.

The sideways forcing does exist in a crosswind landing.

 

[EdFred]

A simpler question than the OP's would be: If you install a 30 knot wind sock at sea level and another 30 knot wind sock at 10,000' and subject both socks to a 30 knot wind what would be the result?

Of course the sea level sock would be strait out indicating the 30 kt wind but the high DA sock would not be due to less force.

Aircraft and pilots have crosswind limitations because at some amount of force the control authority isn't there or the pilots skill isn't there, it is hard to argue that the force isn't there.

Go back to my Tonopah example. The winds were above 'stall' speed for our planes yet we could land, taxi and take off without difficulty. At sea level winds above stall speed would make your aircraft uncontrollable but at 10,000' DA there might be margin left and you can keep it on the ground.

The difference is, the plane isn't fixed to the ground. The windsock is. A more accurate question would be: If a dandelion seed was in being blown around in a 30kt wind at sea level vs a 30kt wind at 10,000ft how fast would it be moving relative to the ground?

 

[CharlieTango]

The difference is, the plane isn't fixed to the ground. The windsock is. A more accurate question would be: If a dandelion seed was in being blown around in a 30kt wind at sea level vs a 30kt wind at 10,000ft how fast would it be moving relative to the ground?

The original question was if there is more force at the lower DA. I used both the roof and windsock examples to take the plane out of the argument.

Thte dandelion seed would demonstrate the velocity of the wind not the greater force you get with a greater mass at the lower DA

 

[EdFred]

The original question was if there is more force at the lower DA. I used both the roof and windsock examples to take the plane out of the argument.

Thte dandelion seed would demonstrate the velocity of the wind not the greater force you get with a greater mass at the lower DA

Roof and wind sock are both attached to the ground. Try again. Newton can help you here.

 

[CharlieTango]

Roof and wind sock are both attached to the ground. Try again. Newton can help you here.

When you bring the airplane into it a lot of argument ensues. The roof and wind sock demonstrate a fundamental point in the question, namely that the amount of force from an 15 kt wind is greater at the lower DA. Roof and windsock are valid demonstrations of that point.

After knowing that the wind force is greater at a lower DA you can then answer the question about how and if that difference effects the landing.

If you agree that crosswind correction is needed to counter drift at some point in the landing you can then see that more correction is needed at a DA where the wind of the same velocity has more force to counter.

 

[EdFred]

When you bring the airplane into it a lot of argument ensues. The roof and wind sock demonstrate a fundamental point in the question, namely that the amount of force from an 15 kt wind is greater at the lower DA. Roof and windsock are valid demonstrations of that point.

After knowing that the wind force is greater at a lower DA you can then answer the question about how and if that difference effects the landing.

If you agree that crosswind correction is needed to counter drift at some point in the landing you can then see that more correction is needed at a DA where the wind of the same velocity has more force to counter.

F=ma

How fast is the plane accelerating sideways when in a 15kt airmass at 50MSL? How fast is the plane accelerating sideways in a 15kt airmass at 5000MSL? 10000MSL?

The answer is 0.
Let's plug that into the equation.

F = 2400lbs * 0f/s/s

F = 0

The force of the wind on the plane while it is airborne is 0. Only when the plane is on the ground does it "feel" any force from the wind.

Physics. Learn it, love it.

 

[MAKG1]

People seem to have a lot of trouble with that. The PHAK doesn't help with all its descriptions of force.

An airplane moving in any straight line at constant speed sees no net force. This includes climbs and descents at constant speed, but you would be amazed how many people think there is some extra vertical force to make that happen.

With a few exceptions (most commonly turns in non aerobatic flight), aircraft almost always operate in a "terminal velocity" regime, and those are far easier to describe in terms of velocity than forces or acceleration. Even in a steady ("falling leaf") stall.

If you insist on describing forces, you will very quickly run into the problem that drag forces are really nasty to calculate from first principles, especially in uncoordinated flight. Even whether it goes up or down can be counterintuitive; for instance a raised aileron in a near-stall condition can reduce drag.

 

[CharlieTango]

F=ma

How fast is the plane accelerating sideways when in a 15kt airmass at 50MSL? How fast is the plane accelerating sideways in a 15kt airmass at 5000MSL? 10000MSL?

The answer is 0.
Let's plug that into the equation.

F = 2400lbs * 0f/s

F = 0

The force of the wind on the plane while it is airborne is 0. Only when the plane is on the ground does it "feel" any force from the wind.

Physics. Learn it, love it.

EdFred,

Wind-shear is a common component of winds near the ground. When your plane is gusted there is an associated force that is greater than zero. There is a reaction to that force as well.

You are asking a totally new question, how fast does the plane accelerate sideways? The pilot controls the sideways acceleration or drift with aileron input and keeps it at zero.

The question at hand is how the force that is attempting to accelerate the plane sideways, a force that presumably changes with DA, how does this change in force effect the landing?

You seem to be arguing that the plane, when flying, is not effected by wind even in a crosswind landing. If it is not effected why do we lower the wing?

 

[EdFred]

Now you are changing the premise. The premise was steady-state crosswind. Force = 0. No change in acceleration = 0 force.

I am not introducing anything new. You keep talking about force. Force is mass * acceleration. If there is a steady state wind, then there is no acceleration, ergo, there is no force. If you can't understand that simple premise which I learned in 7th grade, then it will be impossible to explain it to you. You simply lack the knowledge to understand introductory physics.

 

[CharlieTango]

Now you are changing the premise. The premise was steady-state crosswind. Force = 0. No change in acceleration = 0 force.

I am not introducing anything new. You keep talking about force. Force is mass * acceleration. If there is a steady state wind, then there is no acceleration, ergo, there is no force. If you can't understand that simple premise which I learned in 7th grade, then it will be impossible to explain it to you. You simply lack the knowledge to understand introductory physics.

I'm not changing the premise just trying to stay on topic. There was a question in post #1 and it is not answered by addressing how fast the plane is accelerating sideways relative to the ground.

It obviously isn't that simple if you can conclude that a steady state wind applies zero force. I can place an object in a stead state wind and the force will effect it.

When you transition from a crab to a slip there is acceleration relative to the airmass that you were drifting with. You heading changes, your speed degrades and you cease to drift with the crosswind component of the air-mass.

 

[midlifeflyer]

I came back in to look for the scientific calculations to show this effect rather than CharlieTango's opinion that it is there. I'm guessing there's no reason to check again.

See you all at the next downwind turn discussion

 

[EdFred]

I'm not changing the premise just trying to stay on topic. There was a question in post #1 and it is not answered by addressing how fast the plane is accelerating sideways relative to the ground.

It obviously isn't that simple if you can conclude that a steady state wind applies zero force. I can place an object in a stead state wind and the force will effect it.

When you transition from a crab to a slip there is acceleration relative to the airmass that you were drifting with. You heading changes, your speed degrades and you cease to drift with the crosswind component of the air-mass.

You don't understand the term force. I can't make it any simpler. I'm done.

 

[vintage cessna]

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.

If an airplane was on the ground on Mars with its' pitot tube pointed into the 100 mph wind, what indicated airspeed would the gauge register?

 

[CharlieTango]

If an airplane was on the ground on Mars with its' pitot tube pointed into the 100 mph wind, what indicated airspeed would the gauge register?

Similar to earth the IAS would be much higher in the Valles Marineris than on top of Olympus Mons, it depends on the DA.

 

[vintage cessna]

Similar to earth the IAS would be much higher in the Valles Marineris than on top of Olympus Mons, it depends on the DA.

Since the atmospheric pressure on Mars is less than 1 percent of Earths', the gauge would probably register just above 0 , even at the lowest elevation on Mars.

A roof on a house won't fly until a certain IAS any more than the wing on your airplane.

 

[Silvaire]

This thought has been expressed a couple of times. It is true that some forces are automatically compensated for but the difference in force of the crosswind is not one of them.

I don't know where you are pulling this pearl of wisdom out of but let's once again go back to basics:

What is the stall speed of your aircraft at sea level?
What is the stall speed of your aircraft at 10,000 feet?

What approach speed would you fly on final at Half Moon Bay?
What approach speed would you fly on final at Telluride?

Why are they the same?

 

[dtuuri]

Similar to earth the IAS would be much higher in the Valles Marineris than on top of Olympus Mons, it depends on the DA.

OTOH, if the plane were correcting for drift in a sideslip on final, the IAS would be higher on top of Olympus Mons because the higher TAS would require less slip and more air would be directed into the pitot tube.

dtuuri

 

[vintage cessna]

Since IAS on approach is the same regardless of DA, landing at 8000 ft will have a higher TAS than at sea level and therefore a higher ground speed. The amount of drift over time will be the same at 8000 ft as sea level but the amount of drift over distance will be less at 8000 ft due to the higher ground speed. It will appear that there is less drift landing at 8000 ft than at sea level.

 

[Silvaire]

Since IAS on approach is the same regardless of DA, landing at 8000 ft will have a higher TAS than at sea level and therefore a higher ground speed. The amount of drift over time will be the same at 8000 ft as sea level but the amount of drift over distance will be less at 8000 ft due to the higher ground speed. It will appear that there is less drift landing at 8000 ft than at sea level.

Exactly. Look at the wind triangle I linked back in post #60. With xwind and airspeed constant and the only difference being increased groundspeed the wind correction angle will decrease. Simple geometery.

 

[vintage cessna]

Exactly. Look at the wind triangle I linked back in post #60. With xwind and airspeed constant and the only difference being increased groundspeed the wind correction angle will decrease. Simple geometery.

Right. And if you play around with an E6B, you will find that the higher the TAS, the less WCA required to maintain a course. So when landing at 8000 ft the amount of crab or sideslip required to maintain centerline will be less than at sea level for any given amount of wind due to a higher TAS. Probably more important to land with zero drift at 8000 ft than at sea level since ground speed will be higher at touchdown.

 

[vintage cessna]

Yes. What remains unclear to me is whether the crosswind component (15 kts) represents 15 kts at sea-level or 15 kts in the rarefied atmosphere. Piqueune, no doubt, but relevant to dispatching the present problem. The wind-triangle solution is elegant and compelling, but the wind component should be measuring the same thing that our ASI is measuring. Put another way, if an airspeed indicator (pitot) was faced directly into the prevailing 15 kt crosswind at 8000 feet, what would it indicate (assume a highly sensitive device, unlike a "real" ASI)?

I would assume wind speed is measured in relation to the ground. Since at 8000 ft your ground speed will be higher than at sea level, as a percentage of the aircrafts speed a 15 kt wind at 8000 ft will be less than the percentage of speed at sea level. An airspeed indicator is not measuring wind but pressure.

 

[CharlieTango]

I don't know where you are pulling this pearl of wisdom out of but let's once again go back to basics:

What is the stall speed of your aircraft at sea level?
What is the stall speed of your aircraft at 10,000 feet?

What approach speed would you fly on final at Half Moon Bay?
What approach speed would you fly on final at Telluride?

Why are they the same?

The ASI is similar to the wing in that at altitude you have fewer air molecules impacting at a higher velocity, in both cases when you have enough mass air flow to produce enough lift to fly you also have enough mass air flow to cause your ASI to read flying speed.

-----------------------------

I reason that the difference in force of the crosswind is not one of the forces that are automatically compensated for because at both high and low altitudes the crosswind is true not adjusted to an indicated number based on air density.

 

[EdFred]

You are already moving in that air parcel. There is no force, ergo there is no pressure. You need to go up in a balloon sometime.

 

[vintage cessna]

Exactly. The point I am trying to make is simply that air at 8000 ft has about 74% of sea-level density. If the 8000 ft parcel of air is moving crosswind at 15 kts, isn't the pressure it produces considerably less (maybe only 74% of the sea-level pressure) than 15 kts at sea level? Such an argument makes your solution more robust given that a reduction in the effective crosswind component is reduced.

It is not the amount of pressure the wind produces, it is the amount of drift that matters. You will drift the same amount over time at 8000 ft as at sea level with a wind speed measured at 15 kts at both altitudes. Since you are moving much faster in relation to the ground at say 60 kts ISA at 8000 ft than at 60 kts IASat sea level, it just doesn't appear that the wind is affecting you as much. Once the wheels are on the ground it is a different story. It would be easier to taxi in a measured 30 kt wind at 8000 ft than a 30 kt wind at sea level. Becuase now you have the resistance of the tires against the ground and there will indeed be less pressure exerted by the wind at a higher altitude.

 

[kgruber]

There most certainly is a force acting on an airframe during a slip. That is why there is NOISE!

 

[vintage cessna]

I understand, Ed. I have no problem with an aircraft moving within a (moving) parcel of air. That's what makes it fun. The question arises in a crosswind landing situation where we attempt to counter the drift of a moving parcel of air in order to arrive with dignity on a stationary runway. There is certainly a force pushing us off the runway centerline, and we counter that force with control inputs that keep the aircraft tracking correctly. The pressure in the context of this discussion pertains to the pressure the wind produces on stationary objects, say an anemometer located adjacent to a runway.

I think you need to see the difference between when the aircraft is in the air and when it is on the ground. When in the air, the aircraft does not resist or push back against the wind, it just drifts with the wind. When you crab an airplane, you are not pushing back against the wind. It would be impossible for an object in the air to push back against the wind, so there is no pressure exerted. On the ground, the surface , due to friction resists against movement of the tires and pushes back. The higher the altitude for a given amount of measured wind, the less pressure exerted on the aircraft on the ground by the wind and so less push back by the surface against movement of the tires.

 

[vintage cessna]

I'm totally on with your concept of drift, but as you clearly state, a wind has more force, or pressure, at sea level than at 8000 feet. I am only saying that the crosswind component of the wind triangle is actually less than the nominal velocity because of density effects - the same thing that makes it easier to taxi in a 30 kt wind at 8000 feet.

By the way, I did some research on anemometry, and it seems that altitude corrections are in order for proper wind speed measurements. The wind-triangle solution calls for input of "true airspeed," that is corrected for altitude and temperature. The leg called "windspeed" is not so well-defined. I am seeking to more accurately characterize the wind vector as it pertains to crosswind landings at altitude. Keep up the good work

The TAS input is for for the aircraft. I'm fairly certain wind speed is measured in relationship to the ground.

 

[vintage cessna]

In an attempt to answer the OP's question, a 15 kt wind at -5000 and a 15 kt xwind at +5000 DA, the effect would be the same. If you consider the"effect" to mean the amount of drift over time. At -5000 DA, the amount of crab or side slip would be greater. But at + 5000 the ground speed will be higher. So a screw up at + 5000 could be much worse since you would be traveling faster.