Weathervaning (in the air)

Jaybird180 said:
To answer Alfa's question in terms Roscoe can understand:

That big thingie at the back of the aeroplane that sticks way up in the air...whatever that thing is called...when the wind blows, it hits the side of it...and it doesn't like it, so it shys away from it and it makes the front of the plane...where the twirly thing is...it makes that thing go straight.
Click to expand...

Yes, the thingy at the back of the plane will cause the plane to 'weather vane' into the relative wind...the wind caused by the twirly thingy at the front dragging the whole ship through the air.

But 'wind' as described is the movement of the air over the ground. 'Wind Direction' the the movement of that air as it relates to a compass.

There is nothing that causes a plane to deviate from it's course through the air and align itself with the movement of the air as it moves over the ground unless there is contact somehow with the ground (wheels for a plane, rope for a blimp).
 
Jaybird180 said:
To answer Alfa's question in terms Roscoe can understand:

That big thingie at the back of the aeroplane that sticks way up in the air...whatever that thing is called...when the wind blows, it hits the side of it...and it doesn't like it, so it shys away from it and it makes the front of the plane...where the twirly thing is...it makes that thing go straight.
Click to expand...

I guess it is kinda like when you are on an airliner flying along at 36,000 feet. Your row is empty and you decide to move from seat 18A to seat 18F. You put on your backpack and start walking at a 90 degree angle to the direction of flight. You cannot help being spun to the left as all that wind catches the side of your backpack. After all, the airplane is going 500 miles per hour or better so the air in cabin must be going that fast also; it is a moving airmass.
 
Last edited:
Jaybird180 said:
To answer Alfa's question in terms Roscoe can understand:

That big thingie at the back of the aeroplane that sticks way up in the air...whatever that thing is called...when the wind blows, it hits the side of it...and it doesn't like it, so it shys away from it and it makes the front of the plane...where the twirly thing is...it makes that thing go straight.
Click to expand...

:rofl::rofl::rofl:
 
Jaybird180 said:
To answer Alfa's question in terms Roscoe can understand:

That big thingie at the back of the aeroplane that sticks way up in the air...whatever that thing is called...when the wind blows, it hits the side of it...and it doesn't like it, so it shys away from it and it makes the front of the plane...where the twirly thing is...it makes that thing go straight.
Click to expand...

If a dirigible is tethered to the ground and the wind blows, the tethering will keep the dirigible from moving with the airmass, so it will probably weathervane. While that wind is blowing, and the tethers are released, the dirigible will begin to move with the airmass. Once moving with the airmass, what force is there to affect a change that makes the "big thingie at the back" push one way or the other?
 
Jaybird180 said:
To answer Alfa's question in terms Roscoe can understand:

That big thingie at the back of the aeroplane that sticks way up in the air...whatever that thing is called...when the wind blows, it hits the side of it...and it doesn't like it, so it shys away from it and it makes the front of the plane...where the twirly thing is...it makes that thing go straight.
Click to expand...

One needs to be clear on the difference between wind as the term is normally used, i.e., movement of the air relative to the ground, and relative wind, i.e., movement of the air relative to the aircraft.
 
Okay, one more example. You are at 10,000 feet in smooth air flying a heading of 090. You know from reading the winds aloft forecast, that the wind is out of 360 at 50 knots at your altitude. That means, it is coming strong from your 9 o clock. Do you have trouble holding your heading of 090 because the plane wants to weathervane and turn left, into the direction the wind is coming from? No, you fly straight and level with the DG on 090 and it stays there.

If you were on the ground, and had a strong wind coming from this direction (your 9 oclock) the plane would want very badly to weathervane, turn to the left, and face into the wind. But aloft, it flies the direction you point it... because planes don't weathervane in the air!
 
When you are flying straight and level, you are flying inside the moving pocket of air...but that air is only moving relative to the ground...there is no weathervaning because there is no counteracting force. Simple as that. A fixed object like a weathervane turns into the wind because it is fixed to the ground...so relative to the weathervane, the wind is moving.
 
dell30rb said:
If you ride in a hot air balloon you might get the idea. You can be at 500agl moving at 20kts over the ground with zero apparent wind. Kind of a strange feeling.


In the balloon, when you land in a breeze, it feels like there is no wind. As in, if you lit a big fat cigar, the smoke would go straight up above your head. When you hit the ground and stop, you will feel wind on your face, and your cigar smoke will go blowing away.
Click to expand...

I'm with you sofar

dell30rb said:
If you were on the ground, and had a strong wind coming from this direction (your 9 oclock) the plane would want very badly to weathervane, turn to the left, and face into the wind. But aloft, it flies the direction you point it... because planes don't weathervane in the air!
Click to expand...


Why?
 
Ok..

The cigar smoke goes straight up, because the balloon moves with the air. On the ground, you feel wind on your face, and your cigar smoke blows into your friend's face. This is because there is now "wind" pushing on you and your cigar smoke. We call "wind" the strength and direction of the air moving relative to the ground.

In the plane, when you are flying straight and level at 100 knots, with a 50 knot wind coming from your 9 o clock, the only forces the plane "feels" is the straight and level at 100 knots.

Lets do a 360 degree steep turn at 10,000 feet with the wind blowing like that. Does your airspeed change as you go through the turn? No, though when you are heading south your groundspeed is 150kts, and north, 50 knots. We measure "wind" as the air moving in a speed and direction relative to the ground. But the airplane does not care what the wind is doing relative to the ground. Because the ground is not touching the airplane. (unless its sitting on its wheels)


I can't really do any better.. if ya cant wrap your head about it, just accept that there is no "weathervaning" in an aircraft. Try my example for proof.. climb up on a day with strong winds aloft to some high altitude, then see if the plane flies normally, or consistently wants to turn in the direction the wind is coming from.
 
Jaybird180 said:
Originally Posted by dell30rb
If you were on the ground, and had a strong wind coming from this direction (your 9 oclock) the plane would want very badly to weathervane, turn to the left, and face into the wind. But aloft, it flies the direction you point it... because planes don't weathervane in the air!
Click to expand...

Why?
Click to expand...

When the airplane is on the ground with wind coming from the side, the motion of the air relative to the airplane exerts force on the side of the airplane, while the ground exerts an opposing force through the wheels. There is more wind force on the vertical stabilizer than on the rest of the airplane because of its size, so the net wind force is centered behind the wheels, thus creating a moment arm that makes the airplane tend to turn into the wind.

When the plane is airborne the ground is not able to exert any force on the airplane, and the entire plane is free to move sideways along with the air mass, so the air doesn't exert any sideways force on the airplane either. Thus, the forces that made the airplane tend to turn while it was on the ground no longer exist.
 
cleared4theoption said:
When you are flying straight and level, you are flying inside the moving pocket of air...but that air is only moving relative to the ground...there is no weathervaning because there is no counteracting force. Simple as that. A fixed object like a weathervane turns into the wind because it is fixed to the ground...so relative to the weathervane, the wind is moving.
Click to expand...


An even simpler summary: To weathervane, we need TWO points of force. A weathervane like you see on old barns is fixed to a post so it can pivot; that's one point. The arrow's tail is the other, since the wind strikes it and causes a side force that rotates the arrow around the fixed post to line up with the wind.

An airplane on the ground has two points: the main landing gear, and the vertical tail. A strong wind can weathervane it. In the air, the gear is of no use whatever and the wind cannot force the airplane to turn into it. The whole airplane will move sideways, if the wind is from the side, and we will see the airplane crabbing, not weathervaning. The relative wind is still straight on the nose. This whole thing is why pilots are taught how to figure the crab angle necessary to counter the wind so that we get where we want to go. Once we establish that heading, it stays put without any rudder pressure, which proves that the airplane will not weathervane in flight.

This whole argument is related in a small way to the "deadly downwind turn" myth that persists to this day.

Edit: Now I see that someone beat me to it...

Dan
 
Jaybird180 said:
What is this myth that you speak of?
Click to expand...

I'll tell ya. Some people believe that when you're flying upwind there is a chance of losing lift and maybe even stalling by turning the plane around and flying downwind.

I like to use extremes to make a point so... say I'm flying an ultra light at 50 kts straight into a 200kt headwind. When I make a 180 degree turn won't I fall out of the sky while my plane tries to absorb a 200 kt tail wind?


The answer, of course, is no. The planes wing has no idea how fast the airmass is moving across the ground. It doesn't care. The plane is always moving INTO the air at the same speed. It's speed over the ground may vary greatly, but that's not the point or the question.
 
If this is not true, why compensate on final for gust factor? Logic would seem to indicate one should always fly 1.2-1.3 Vs0
 
Jaybird180 said:
If this is not true, why compensate on final for gust factor? Logic would seem to indicate one should always fly 1.2-1.3 Vs0
Click to expand...

I think you would do yourself a service if you take a stab at figuring it out yourself. IMO. Why would someone carry extra speed landing in a gusty condtion? Let us say you are landing RWY 9 and the wind is 090 at 16G30.

Wind Gust
Is a sudden, brief increase in speed of the wind. According to U.S. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. The duration of a gust is usually less than 20 seconds.

Discuss.
 
Last edited:
Does a boat adrift in a river point up stream? Assume no wind.

Does a water skier who needs fwd motion to stay up fall when the boat turns around from upstream to down stream?
 
alfadog said:
I think you would do yourself a service if you take a stab at figuring it out yourself. IMO. Why would someone carry extra speed landing in a gusty condtion? Let us say you are landing RWY 9 and the wind is 090 at 16G30.

Wind Gust
Is a sudden, brief increase in speed of the wind. According to U.S. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. The duration of a gust is usually less than 20 seconds.

Discuss.
Click to expand...
The advice of carrying extra speed is oft repeated here; sounds like you don't agree with that either???
 
alfadog said:
No, I do agree. Why? Think it through as if you are landing.
Click to expand...

I'm saying there's a contradiction somewhere. Seems there is no reason to add airspeed for gusts, :idea:unless it's due to inertial damping.
 
Jaybird180 said:
I'm saying there's a contradiction somewhere. Seems there is no reason to add airspeed for gusts, :idea:unless it's due to inertial damping.
Click to expand...

Tell you what, when you can get the plane to change directions fast enough going downwind to match the delta V of the gust...
 
Jaybird180 said:
I'm saying there's a contradiction somewhere. Seems there is no reason to add airspeed for gusts, :idea:unless it's due to inertial damping.
Click to expand...

You are correct that inertia is the reason for adding airspeed in gusty conditions. When the air mass accelerates or decelerates due to a gust, the inertia of the airplane prevents it from instantaneously matching the acceleration or deceleration of the air mass, and that time lag causes an immediate change in the relative wind.
 
Jay, when you stand on a moving walkway in the airport and face sideways, is there any tendency for you to spin (yaw)? Now imagine you are off to the side of the walkway standing on the carpet and facing the moving walkway from outside of it. It is moving left to right. You reach out and grab the moving handrail with your left hand and try to keep your feet in one place. What happens? You "weathervane".
 
Jaybird180 said:
I'm saying there's a contradiction somewhere. Seems there is no reason to add airspeed for gusts, :idea:unless it's due to inertial damping.
Click to expand...

Perhaps I should add the smilie that says "Got it".
 
Jaybird180 said:
The advice of carrying extra speed is oft repeated here; sounds like you don't agree with that either???
Click to expand...

The gust factor's influence is instantaneous. Turning from upwind to downwind is not, and therein is is a world of difference.

If you are a pilot, take a pilot friend as a lookout and fly on a day with fairly strong, steady wind. Put on the IFR hood or foggles, and put the airplane into a steady, rate-one turn for several minutes. Keep the power and altitude constant, keep the ball centered. See if the airspeed changes or if you can feel any acceleration or deceleration or sideways movement as the airplane turns into and out of and across the wind.

You won't see anything change, but the airplane's path over the ground is a loopy affair that carries you far downwind.

Dan
 
Dan Thomas said:
The gust factor's influence is instantaneous. Turning from upwind to downwind is not, and therein is is a world of difference.

If you are a pilot, take a pilot friend as a lookout and fly on a day with fairly strong, steady wind. Put on the IFR hood or foggles, and put the airplane into a steady, rate-one turn for several minutes. Keep the power and altitude constant, keep the ball centered. See if the airspeed changes or if you can feel any acceleration or deceleration or sideways movement as the airplane turns into and out of and across the wind.

You won't see anything change, but the airplane's path over the ground is a loopy affair that carries you far downwind.

Dan
Click to expand...

Obviously the downwind turn can't have any effect when the pilot is under the hood and making standard rate turns. However, if the pilot is flying visually and trying to make a nice rectangular track across the ground, then the plane will be crabbed into the wind on crosswind and base. That means that the pilot would have to make a greater than 90 degree turn when turning from crosswind to downwind, or from downwind to base, which could induce him to bank more steeply, making a stall more likely.

Of course, a solution to that is to consciously limit one's bank angles in the pattern.
 
jaybird,

Have you taken a class in ambivalence from Henning? I can't believe after all the examples and all the different analogies presented here, you still don't get it. Take a balloon ride and smoke a cigar. Maybe that will help... Other than that I think you are purposefully keeping a ridiculous argument going longer than necessary....:crazy:
 
Palmpilot said:
Obviously the downwind turn can't have any effect when the pilot is under the hood and making standard rate turns. However, if the pilot is flying visually and trying to make a nice rectangular track across the ground, then the plane will be crabbed into the wind on crosswind and base. That means that the pilot would have to make a greater than 90 degree turn when turning from crosswind to downwind, or from downwind to base, which could induce him to bank more steeply, making a stall more likely.

Of course, a solution to that is to consciously limit one's bank angles in the pattern.
Click to expand...

Moose stall. Killed a lot of folks.
 
Palmpilot said:
Obviously the downwind turn can't have any effect when the pilot is under the hood and making standard rate turns. However, if the pilot is flying visually and trying to make a nice rectangular track across the ground, then the plane will be crabbed into the wind on crosswind and base. That means that the pilot would have to make a greater than 90 degree turn when turning from crosswind to downwind, or from downwind to base, which could induce him to bank more steeply, making a stall more likely.

Of course, a solution to that is to consciously limit one's bank angles in the pattern.
Click to expand...

Turning in such wind produces the illusion of a slip or skid, depending on whether you're turning into the wind or out of it. If you are turning out of the wind, it looks like you're slipping and the temptation is to add rudder, which actually causes a skid and there's the danger. Skidding turns, especially when descending and already slow, can be deadly.

Watch that ball. It knows what's happening.

Dan
 
FlySince9 said:
jaybird,

Have you taken a class in ambivalence from Henning? I can't believe after all the examples and all the different analogies presented here, you still don't get it. Take a balloon ride and smoke a cigar. Maybe that will help... Other than that I think you are purposefully keeping a ridiculous argument going longer than necessary....:crazy:
Click to expand...
Go back and read the thread. We were done already.
 
If you're interested in what may be the cause of some of my confusion, Sporty's "Tip of the Week" video on Aircraft Stability describes weathervaning being a function of the side of the aircraft and the size of the vertical fin. Yes, they used the term weathervane.

The video is posted in the recent tips section.
 
alfadog said:
You've not told us what force or combination of forces would induce the blimp (or the airplane) to turn from its given path in a homogeneous body of moving air in order to align itself upwind in reference to the ground. E.g. airmass (wind) is from 180 at 20 kt. Blimp is moving 090 at 10 kt. Why would it turn?
Click to expand...

Tell you what. I'll play with it for him. The only way I can think of getting it to "weathervane" in flight is to be flying crosswind and then remove all trust.
 
KSCessnaDriver said:
Tell you what. I'll play with it for him. The only way I can think of getting it to "weathervane" in flight is to be flying crosswind and then remove all trust.
Click to expand...

After reviewing the link Jaybird refers to, I can see how the unfortunate choice of words to explain Yaw stability may have induced some confusion...

Select the " aircraft stability" link. The segment starts around 2:00 or so.

http://www.sportys.com/source/slashpages/videodemos/tipOfTheWeek_archive.cfm?catalog=PilotShop
 
Last edited:
You must log in or register to reply here.
Back
Top