Wind "Force" At Different Altitudes

Geico266

Geico266

Touchdown! Greaser!
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Geico
Does a 30 MPH wind at sea level have more force that a 30 MPH wind at 10,000'. :confused:

Thinner air, same velocity, the force exerted on a stationary object would be less right? :confused:

Should not the cross wind component of airplanes have an altitude factor? :dunno:

Just sayin.....


(We switched coffees this am.;))
 
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sigh, this thread is going to be long.


Yes, denser air has more 'force' than thiner air. A flag flapping at sea level in 30kts would flap harder than a flag flapping at 60,000' in 30 kts.

BUT, for a plane that is supported by the airmass the crosswind is the same. Flying 100 kts TAS at sea level on a course of 090 with the winds from 180 at 30 kts will result in the exact same correction to maintain that 090 course as flying 100 kts TAS at 15,000 with the same winds.

Except, your true airspeed would be faster (normally) at 15,000' so it could be argued that the wind would have less effect on you there. If you flew 100 kts indicated at sea level in the above condition and then flew 100 kts indicated at 15,000 you would notice that you would need less crosswind correction at 15,000'. Why? Because you are going faster (true airspeed) and the crosswind is less of a component of your overall speed.

Enjoy the next 10 pages...
 
Equivalent airspeed (EAS) is defined as the speed at sea level that would produce the same incompressible dynamic pressure as the true airspeed at the altitude at which the vehicle is flying. An aircraft in forward flight is subject to the effects of compressibility. Likewise, the calibrated airspeed is a function of the compressible impact pressure. EAS, on the other hand, is a measure of airspeed that is a function of incompressible dynamic pressure. Structural analysis is often in terms of incompressible dynamic pressure, so that equivalent airspeed is a useful speed for structural testing. At standard sea level pressure, calibrated airspeed and equivalent airspeed are equal. Up to about 200 knots CAS and 10,000 feet the difference is negligible, but at higher speeds and altitudes CAS must be corrected for compressibility error to determine EAS.:popcorn:
 
I'm assuming you are referring to crosswind component for the purpose of landing. I would believe you are correct that DA would be somewhat relative (no pun), but at first blush the significance would be smaller than the accuracy of measurements anyway.

Plus, the biggest effect would be in transitioning between different regions of wind speed/direction, right?
 
All you guys in Denver have it easy! :D
 
Geico266 said:
How can that be when landing? :dunno:
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Until the wheels touch you are supported by the airmass. It makes no difference if your are 1 inch off the ground or 10 miles.
 
Captain said:
Until the wheels touch you are supported by the airmass. It makes no difference if your are 1 inch off the ground or 10 miles.
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I understand the air mass movement. I get that, but when landing in a cross wind you are applying force to overcome the energy of the wind pushing the plane sideways. If there is a difference in the force of the wind then there would be a difference in the energy needed to overcome the wind.

If there is a difference in "force" then there must be a different in the amount of correction needed at different altitudes right? I'm not arguing with you yet ;), I just need to understand.

The amount of correction may be very slight, even imperceptible, but in theory it is still correct right? :dunno:
 
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A force needs a resistance to measure that force. Take the flag flapping. The flag has inertia and gravity pulling it down. The 30 kt wind at sea level has x number of air molecules slamming into the flag disrupting it and causing it to wave in the breeze. At 15,000 there is a fraction of x number of air molecules slamming into the flag. Fewer molecules means it flaps less with the same 30 kts.

But an aircraft in flight has no resistance to the wind. A balloon at sea level in 30 kts travels at 30 kts. A ballon at 15,000 feet in 30 kts travels at 30 kts. This is because there is no resistance to the airmass. An airplane is the same when it's suspended / supported in that airmass. No resistance to the movement of that airmass.
 
Captain said:
But an aircraft in flight has no resistance to the wind. A balloon at seal level in 30 kts travels at 30 kts. A ballon at 15,000 feet in 30 kts travels at 30 kts. There is no resistance to the airmass. An airplane is the same when it's suspended / supported in that airmass.
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Agree. But landing in a cross wind requires energy to overcome the cross wind force. :dunno:

Those Denver pilots do have it easy! :lol:
 
Take it to an absurd extreme. Let's compare sea level and 100,000 feet.

At sea level you fly your approach at 60 kts indicated and that results in a 60 kt true airspeed. If you land on runway 9 with the winds out of the south at 30 kts then you will have to crab into the wind with a heading of 150'ish. You cant touch down with that much crab so you will have to transition to a slip to counter the wind. A 30 kts crosswind likely exceeds the limit of your plane but you can see how you would have lower the right wing and align with the runway with rudder.

Now lets bump the whole thing up to 100,000 feet. You fly your approach at the same 60 kts indicated. But at that altitude you need about 2,000 kts true to get 60 indicated. So you are coming in at 2,000 kts and the crab required to compensate for 30 kts direct crosswind would be a degree or two at most.

Using this you can see that folks in Denver do correct less for a given wind as a result of a faster true airspeed approach. But in going from sea level to 5,500 feet the difference isn't noticeable, at least not to me. What I do notice is the faster ground speed. Landing at TEX at 120 indicated you really notice the ground going by faster than normal. Not the degree of crab for a given wind. I just crab as needed and don't notice difference between set angles and altitudes.
 
Captain said:
Take it to an absurd extreme. Let's compare sea level and 100,000 feet.

At sea level you fly your approach at 60 kts indicated and that results in a 60 kt true airspeed. If you land on runway 9 with the winds out of the south at 30 kts then you will have to crab into the wind with a heading of 150'ish. You cant touch down with that much crab so you will have to transition to a slip to counter the wind. A 30 kts crosswind likely exceeds the limit of your plane but you can see how you would have lower the right wing and align with the runway with rudder.

Now lets bump the whole thing up to 100,000 feet. You fly your approach at the same 60 kts indicated. But at that altitude you need about 2,000 kts true to get 60 indicated. So you are coming in at 2,000 kts and the crab required to compensate for 30 kts direct crosswind would be a degree or two at most.

Using this you can see that folks in Denver do correct less for a given wind as a result of a faster true airspeed approach. But in going from sea level to 5,500 feet the difference isn't noticeable, at least not to me. What I do notice is the faster ground speed. Landing at TEX at 120 indicated you really notice the ground going by faster than normal. Not the degree of crab for a given wind. I just crab as needed and don't notice difference between set angles and altitudes.
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:idea:

I get it now! Thanks! :yes:

The reason I bring it up is I could feel the difference taxing and taking off in Laramie, WY. 7,300 MSL. Winds were 35-45 mph according to ATIS, but it just didn't feel like winds at 1500' that I am use to. Just seemed a little weird to me that 40mph winds went knocking me around more that's all.
 
Captain said:
A force needs a resistance to measure that force. Take the flag flapping. The flag has inertia and gravity pulling it down. The 30 kt wind at sea level has x number of air molecules slamming into the flag disrupting it and causing it to wave in the breeze. At 15,000 there is a fraction of x number of air molecules slamming into the flag. Fewer molecules means it flaps less with the same 30 kts.

.
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Agreed....... BUT......

The pull of gravity at 15,000 feet is less .. so the flag will wave more freely by that factor alone..:yes:;)
 
N801BH said:
Agreed....... BUT......

The pull of gravity at 15,000 feet is less .. so the flag will wave more freely by that factor alone..:yes:;)
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And!! Would change depending on the cycle of the moon! :eek:

Oh crap! Now I have something else to worry about. :mad2:
 
That's still not correcting for a force, merely a directional component correction. The only relevant velocity is True Airspeed (relative to aircraft movement and the ground). IAS is relevant only to airfoil response.

The only time, in landing, that force comes into play is when the tires are on the ground and must resist lateral movement since no crab is in effect
 
Captain said:
snip.. So you are coming in at 2,000 kts and the crab required to compensate for 30 kts direct crosswind would be a degree or two at most.
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Great analogy. But, would you also agree that to get that degree or two would still require perhaps full rudder or more?
 
skidoo said:
Great analogy. But, would you also agree that to get that degree or two would still require perhaps full rudder or more?
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Nope. The rudder would feel 'normal'. Remember, your airspeed indicator is a molecule counter. That's why 60 indicated works regardless of altitude. When there are enough molecules going down the pitot tube compared to the static port to register 60 kts then there are enough air molecules going over the wing to fly and the rudder to be effective.

Also remember that as a plane accelerates down the runway to take off the rudder is the first 'wing' to fly, followed by the elevator and finally the wing flys and the whole contraption goes up. Landing is the opposite, the wing stops flying first, followed by the elevator and the rudder is the last 'wing' to remain effective. In fact, often just the prop wash alone can make a rudder effective.
 
N801BH said:
Agreed....... BUT......

The pull of gravity at 15,000 feet is less .. so the flag will wave more freely by that factor alone..:yes:;)
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A flag at 15,000' does weigh less than a flag at sea level. But you would need a million dollar scale from NASA to be able to measure the difference.
 
N801BH said:
Wait..........

The Sierra Lima fleet do not blow prop wash over the rudder...;):rofl::wink2:
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Stay in Beta too long on the landing roll and they do! But otherwise you are correct.
 
If the density altitude cause thinner air to exert less force, then I would think that would also be accounted for by the wind measuring device.
In the same was that IAS readings decrease with lower density, and prop efficiencies decrease with density, the wind measurement device would account for lesser "force" (at least in part) by reduce efficiency at the measurement device (just like a prop, but in reverse).
 
Understand that the "true airspeeds" both 'forward' and 'crosswind' are both higher proportially at higher altitudes. Yes, things are different landing at higher altitudes. Takeoff and landings distances are longer (compounded by decreased engine performance), but crosswinds aren't a problem.
 
PilotAlan said:
If the density altitude cause thinner air to exert less force, then I would think that would also be accounted for by the wind measuring device.
In the same was that IAS readings decrease with lower density, and prop efficiencies decrease with density, the wind measurement device would account for lesser "force" (at least in part) by reduce efficiency at the measurement device (just like a prop, but in reverse).
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I'm no expert on wind measuring equipment, but I would think that they would account for this. If they report 30 kts at any altitude then I would expect a helium balloon released at the source to be EXACTLY 30 nm away after an hour given steady and continuous winds over that hour.

There simply has to be some sort of calibration of the wind vane to correct for thinner air at altitude...and also for non-standard pressure at all locations. Like I said, I'm no expert...but that's my guess as to how it works.
 
Captain said:
I'm no expert on wind measuring equipment, but I would think that they would account for this. If they report 30 kts at any altitude then I would expect a helium balloon released at the source to be EXACTLY 30 nm away after an hour given steady and continuous winds over that hour.

There simply has to be some sort of calibration of the wind vane to correct for thinner air at altitude...and also for non-standard pressure at all locations. Like I said, I'm no expert...but that's my guess as to how it works.
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I don't think the cup-type anemometers used in ASOS setups have much altitude sensitivity - they measure speed directly, unlike the pitot systems in our aircraft which measure pressure.
 
chucky said:
I don't think the cup-type anemometers used in ASOS setups have much altitude sensitivity - they measure speed directly, unlike the pitot systems in our aircraft which measure pressure.
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Actually, I found out differently..

A few years back I bought a Davis Weather station,( Weather Monitor II ).

It was shipped and advertised as factory caliberated.. I set it up at my private airport here in Wyoming.. The wind speed that was displayed seemed too low, so I mounted the cup type amemometer on a 10 foot pole attached to the bed of my truck.. I wanted it up and out of the way of any wind currents that would flow off the windshield and give a false reading.. I then went out on a perfectly calm day and drove a mile or so down the road using my dash mounted 196 for exact speed indications. I did a 180 and drove back the same path.... Did that 5 times and adveraged all the readings and the cup type anemometer was reading too low.. ie... I was going 30 mph and it displayed 24 mph... I called Davis and had a long and friendly chat with an engineer there.. Since they are in California and at sea level they never really considered the high altitude complications.. My airport is at 8000MSL....

Lucky for me,.. and them, their unit has the ability for the user to change the "cal" numbers in the unit to compensate for errors.. I made a few more round trips with the truck test rig and finally nailed the sweet spot on the caliberation.... It has been reading perfectly ever since.....

The engineers explaination to me was since the air is less dense, the cups are not spun as fast as at sea level and caliberation is needed at higher elevation installations... Live and learn..:yes:
 
Hmm. It might be friction losses - the friction would be roughly independent of altitude (some temperature sensitivity, though) but the wind force does change. I guess there would need to be some calibration. At any rate, to answer the original question, the NOAA anemometers are supposed to read true wind speed within their stated accuracy (+/- 2kts or 5%, whichever is greater). Each ASOS is probably calibrated individually after it is installed.
 
N801BH said:
The engineers explaination to me was since the air is less dense, the cups are not spun as fast as at sea level and caliberation is needed at higher elevation installations... Live and learn..:yes:
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Now see, when I first posted that, you guys though I was smoking crack......
Even a stopped clock is right twice a day.
 
PilotAlan said:
Now see, when I first posted that, you guys though I was smoking crack......
Even a stopped clock is right twice a day.
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Now Alan... We all didn't know you actually quit smoking crack..:):D;)..


Jus kiddin......... Please step away from the pipe sir...:rofl:
 
N801BH said:
Now Alan... We all didn't know you actually quit smoking crack..:):D;)..

Jus kiddin......... Please step away from the pipe sir...:rofl:
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Looks like I picked the wrong week to stop sniffing glue.....
 
Not for nuthin, but I think I may qualify for the 'gloat camp' too.



Captain said:
I'm no expert on wind measuring equipment, but I would think that they would account for this. If they report 30 kts at any altitude then I would expect a helium balloon released at the source to be EXACTLY 30 nm away after an hour given steady and continuous winds over that hour.

There simply has to be some sort of calibration of the wind vane to correct for thinner air at altitude...and also for non-standard pressure at all locations. Like I said, I'm no expert...but that's my guess as to how it works.
Click to expand...
 
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