Weathervaning (in the air)

gismo

gismo

Touchdown! Greaser!
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Last week I met the pilot/owner of a two seat powered parachute and I think he might even be a CFI-SP for that category. He insisted that his aircraft will "Weathervane" directly into the wind if you don't touch the "steering" controls while still in the air and that as a result it can be relied on to head directly into the wind by itself for landing in a field. I tried to explain that the craft has no reference to the wind direction unless some portion is in contact with the ground and although he agreed with that he still insisted that the weathervaning tendency was there. I offered to bet him $100 that he was wrong and he actually accepted in front of mutual friends although I wouldn't really take his money if and when he's proven wrong. I suspect that what he's actually experienced is the ease with which a pilot of a slow moving aircraft can visually sense the wind direction and is able to subconsciously steer the ship directly into the wind by such cues. The really unfortunate side of this is that it appears that he's' teaching this untruth to others.
 
Funny thing.

Back in the olden days, I used to fly model rockets a lot. And being cheap, I would use flame retardant crepe paper for wadding and tape it to the shock cord so I could re-use it. So, as the rocket came down the crepe paper would give an indication which way the rocket was gliding under the parachute (and glide they did - about a 1:1 glide ratio).

For reasons I don't understand, they would spend most of the time gliding into the wind.:dunno:
 
Capt. Geoffrey Thorpe said:
Funny thing.

Back in the olden days, I used to fly model rockets a lot. And being cheap, I would use flame retardant crepe paper for wadding and tape it to the shock cord so I could re-use it. So, as the rocket came down the crepe paper would give an indication which way the rocket was gliding under the parachute (and glide they did - about a 1:1 glide ratio).

For reasons I don't understand, they would spend most of the time gliding into the wind.:dunno:
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Odd, my rockets always drifted with the wind. How did your crepe paper indicate direction?
 
In something as slow as a powered parachute, I have to assume there is not an instantaneous response to shifting wind.

Perhaps he is referring to a momentary delay until the PP reaches the prevailing wind direction and velocity?
 
The chance he/she can fly this demonstration without ( "if you don't touch the "steering" controls while still in the air and that as a result it can be relied on to head directly into the wind by itself for landing""") is questionable... :yesnod::yesnod::dunno: IMHO
 
roncachamp said:
Odd, my rockets always drifted with the wind. How did your crepe paper indicate direction?
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The motion WRT the ground is downwind.

The motion WRT to the air (as indicated by the streamer flying off to one side of the 'chute) was more upwind than down.

Don't know why.

I would upload a video, but I can't figure out how to attach a VHS tape.
 
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I thought something had to be in contact with the ground in order to truly 'weathervane'.
 
Ask the guy if you should add airspeed when landing with a tailwind.
 
Let'sgoflying! said:
I thought something had to be in contact with the ground in order to truly 'weathervane'.
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That's exactly correct. In the air there's no difference between being in a stationary body of air with the ground moving underneath vs in an airmass that's moving over the ground. And if you can grasp that concept it's fairly easy to recognize that the ground moving under the air shouldn't have any effect on your direction relative to the air since all is still where you are.

There is a possibility that in a rapid descent, a change in the wind vector with altitude could impart a rotational torque on something with different aerodynamic properties over it's vertical extent but my challenge specified level flight so that's not going to be a factor.
 
Capt. Geoffrey Thorpe said:
Funny thing.

Back in the olden days, I used to fly model rockets a lot. And being cheap, I would use flame retardant crepe paper for wadding and tape it to the shock cord so I could re-use it. So, as the rocket came down the crepe paper would give an indication which way the rocket was gliding under the parachute (and glide they did - about a 1:1 glide ratio).

For reasons I don't understand, they would spend most of the time gliding into the wind.:dunno:
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There may be an explanation involving a difference in wind direction between the chute and the rocket/streamer.
 
Most aircraft (Airplanes, gliders and powered parachutes, and even to a lesser extent rotorcraft) tend to want to turn themselves into the RELATIVE wind. This is because they're designed to be stable and coordinated. In the case of a powered parachute which doesn't have much forward speed of its own, that will appear to be more into the wind than a powered aircraft that can maintain 100 knots TAS.
 
gismo said:
There may be an explanation involving a difference in wind direction between the chute and the rocket/streamer.
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Wind shear. The wind strength decreases near the ground. That could cause an angled streamer if it was long enough. Would only take a knot or two difference in windspeeds to angle it.

Dan
 
flyingron said:
Most aircraft (Airplanes, gliders and powered parachutes, and even to a lesser extent rotorcraft) tend to want to turn themselves into the RELATIVE wind. This is because they're designed to be stable and coordinated. In the case of a powered parachute which doesn't have much forward speed of its own, that will appear to be more into the wind than a powered aircraft that can maintain 100 knots TAS.
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Never, ever experienced that. It takes a force couple to cause weathervaning, and it isn't present when off the ground. The CG has nothing but inertia working on it and the gear has no reference at all. A small gust could cause a brief couple between the CG and tail, but that's it. Gravity works only in the vertical. The earth has no influence on an aircraft horizontally.

Dan
 
flyingron said:
Most aircraft (Airplanes, gliders and powered parachutes, and even to a lesser extent rotorcraft) tend to want to turn themselves into the RELATIVE wind. This is because they're designed to be stable and coordinated.
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Yes the tail feathers on an airplane tend to align the fuselage with the relative wind but that has absolutely nothing to do with the motion of the air over the ground. Aside from gusts and wind shear, the relative wind is solely the product of the aircraft's movement "relative" to the airmass it's flying through. E.G and airplane flying on a due north heading at 100 Kt with "winds aloft" coming from the east at 100 Kt will see the same realtive wind as it would if there was no wind or if the wind was from the west.
In the case of a powered parachute which doesn't have much forward speed of its own, that will appear to be more into the wind than a powered aircraft that can maintain 100 knots TAS.
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I'm not sure what you're saying here. It's true that the crab angle generated by flying through an airmass moving at an angle relative to the aircraft's heading will be more significant and visually obvious when the speed of the aircraft is low relative to the airmass's speed over the ground but again that has no influence on the aircraft's heading unless the pilot intervenes.
 
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I could easily envision a powered parachute weathervaning on a very gusty day, with every gust having the tendency to turn it (inertia combined w/ the whole relative wind thing).

However, I doubt that they'd be flying on such a day.
 
timwinters said:
I could easily envision a powered parachute weathervaning on a very gusty day, with every gust having the tendency to turn it (inertia combined w/ the whole relative wind thing).

However, I doubt that they'd be flying on such a day.
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I was thinking gusts when i first read the title.
 
IF (big if) ..the flowing air is glassy smooth, the gliding airplane would probably feel no sideward push on it's fuselage/vertical stab,...but the wind usually doesn't blow that way. It blows in little pushes and gusts that do actually push a bit on the side of the airplane, and gradually, it will come around into the wind.

I know. I never though about it either, but, then I read an article, and checked it out in really windy conditions and yeah, I think it does.
 
The problem with a gust that might cause a bit of weathercocking is the fall in windspeed between gusts that lets the airplane's CG's inertia (it has been accelerated a bit) carry the nose back to where it had been before the gust as the fin gets pushed from the other side. If there really was weather cocking, we'd always be having to turn out of the gusty, turbulent wind as we're going cross country. Instead, we just find the tail wagging back and forth.

Arguments like this start to sound like the guys who built the old inertial-motion machines that tried to move using weights that shifted back and forth. There is no net gain. Many folks have tried to create locomotion or levitation using inertia and it doesn't work. What you gain in one direction has an equal cost in the other. Newton's Third Law.

Dan
 
I am not yet convinced enough yet (haven't really experimented with it) as to whether or not an aircraft will fully 'weathervane', but I am somewhat of an expert when it comes to driving large ships.

FWIW, for those that think an object must be fixed to the ground to weathervane, consider this - a ship or boat when travelling through the water has a pivot point that is not co-located with its CG. That pivot point is located on the longitudinal axis and is found up near the bow (actual locatation depends on the speed through the water). In a ship, if you have a current from the side (or wind for that matter) and leave the rudder alone, the ship will turn into the current/wind all on its own.

If a ship will do it travelling through a body of water, I don't see why an aircraft won't.
 
Fearless Tower said:
... If a ship will do it travelling through a body of water, I don't see why an aircraft won't.
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A ship can certainly weathervane into the wind; in this scenario the water is serving the role of the ground, i.e. providing resistance that prevents the ship from accelerating the craft until its velocity matches the wind's velocity, thus eliminating the relative wind.

But in a steady-state condition (i.e. ignoring variation in wind velocity, ignoring the transient effects during liftoff), a plane's relative wind is solely the result of its airspeed and the movement of the air relative to the ground is immaterial. The same is true of a ship with respect to a constant, even current, though the ship's situation is complicated by the interaction of both wind _and_ water.
-harry
 
mantakos said:
A ship can certainly weathervane into the wind; in this scenario the water is serving the role of the ground, i.e. providing resistance that prevents the ship from accelerating the craft until its velocity matches the wind's velocity, thus eliminating the relative wind.

But in a steady-state condition (i.e. ignoring variation in wind velocity, ignoring the transient effects during liftoff), a plane's relative wind is solely the result of its airspeed and the movement of the air relative to the ground is immaterial. The same is true of a ship with respect to a constant, even current, though the ship's situation is complicated by the interaction of both wind _and_ water.
-harry
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Believe it or not, wind and water actually act very much in the same way. As I mentioned abover, the ship will turn into the current or wind and by current, I am talking about the direct that the body of water the ship is travelling in is moving. The difference is in the level of the effect:

Our general rule of thumb is that 1 kt of current is equal to 25 kts of wind. So, if the ship is travelling in the open ocean with no current and there is a 30 kt wind blowing from the side, the ship WILL turn into that wind.
 
Fearless Tower said:
I am not yet convinced enough yet (haven't really experimented with it) as to whether or not an aircraft will fully 'weathervane', but I am somewhat of an expert when it comes to driving large ships.

FWIW, for those that think an object must be fixed to the ground to weathervane, consider this - a ship or boat when travelling through the water has a pivot point that is not co-located with its CG. That pivot point is located on the longitudinal axis and is found up near the bow (actual locatation depends on the speed through the water). In a ship, if you have a current from the side (or wind for that matter) and leave the rudder alone, the ship will turn into the current/wind all on its own.

If a ship will do it travelling through a body of water, I don't see why an aircraft won't.
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To make this example work, will a submarine weathervane into a current?

John
 
jsstevens said:
To make this example work, will a submarine weathervane into a current?

John
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Actually, that is a good point and maybe I've been on shore duty too damn long. The more I think about it, the more I'm thinking the ship itself doesn't really weathervane into the current, but will weathervane into the wind if the wind is significant.

I need to get back on a ship....powerpoint staffwork is EATING MY BRAIN!
 
Fearless Tower said:
Believe it or not, wind and water actually act very much in the same way...
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Certainly, but the combination of the two doesn't act the same as one of them acting alone, because the two influences have different speeds and directions.
As I mentioned abover, the ship will turn into the current or wind and by current, I am talking about the direct that the body of water the ship is travelling in is moving.
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If that was true, then a submarine would need to exert a continual torque to prevent itself from "weathervaning" in order to continue traveling in a straight line, and the same would need to be true of an aircraft, a bird, a fish, floating wood, etc.

It would mean that any non-powered object that we see floating in water would necessarily have to be aligned with the current, it would never be possible to see something floating in water "sideways". In fact, we would be able to determine the direction of current by throwing a piece of wood into the water and seeing which way it pointed. Does that match your personal observations?
So, if the ship is travelling in the open ocean with no current and there is a 30 kt wind blowing from the side, the ship WILL turn into that wind.
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Yes, we agree on that point, much like a plane sitting on the ground will weathervane into the wind. Again, the water and ground serve to resist the wind's ability to push the craft until the craft's velocity matches the wind's velocity, thus zero'ing out the relative wind. You need two forces to produce this, a pusher (wind) and a resister (ground/water).
-harry
 
mantakos said:
Certainly, but the combination of the two doesn't act the same as one of them acting alone, because the two influences have different speeds and directions.

If that was true, then a submarine would need to exert a continual torque to prevent itself from "weathervaning" in order to continue traveling in a straight line, and the same would need to be true of an aircraft, a bird, a fish, floating wood, etc.

It would mean that any non-powered object that we see floating in water would necessarily have to be aligned with the current, it would never be possible to see something floating in water "sideways". In fact, we would be able to determine the direction of current by throwing a piece of wood into the water and seeing which way it pointed. Does that match your personal observations?

Yes, we agree on that point, much like a plane sitting on the ground will weathervane into the wind. Again, the water and ground serve to resist the wind's ability to push the craft until the craft's velocity matches the wind's velocity, thus zero'ing out the relative wind. You need two forces to produce this, a pusher (wind) and a resister (ground/water).
-harry
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Go back and read my response to jsstevens....I corrected myself.
 
Another potential theory...

How does one steer a powered parachute? I just cruised around a few powered parachute websites trying to find this answer but struck out. IF one steers by dipping the chute into the direction of the turn then, again, I could see how it may be able to weathervane into the prevailing wind...

Okay, assume the chute is flying perpendicular to the wind. The "basket" hangs beneath the chute. Assume the "basket" has more vertical surface area than the chute itself does (and I think this is a safe assumption) then the wind hitting the side of the "basket" would push it leeward of the chute because it has more surface area for the wind to act upon. This would cause the chute to dip the into the wind since the "basket" is hanging beneath and being pushed to the leeward side. This dip would cause the chute turn into the wind...again IF one turns a chute by dipping.

Weathervane...
 
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timwinters said:
Okay, assume the chute is flying perpendicular to the wind. The "basket" hangs beneath the chute. Assume the "basket" has more vertical surface area than the chute itself does (and I think this is a safe assumption) then the wind hitting the side of the "basket" would push it leeward of the chute because it has more surface area for the wind to act upon. This would cause the chute to dip the into the wind since the "basket" is hanging beneath and being pushed to the leeward side. This dip would cause the chute turn into the wind...again IF one turns a chute by dipping.

Weathervane...
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Except, dear bro, the chute is travelling along with the wind, like a balloon or any other aviating thing, and feels no wind from the side. None at all. Its path is over the ground at an angle to its heading.

Remember navigation in groundschool: we have to figure track first, then find the upper wind, and get actual heading from that in order to keep the airplane flying the direction we want. We wouldn't have to do that if the machine insisted on following the track line on the map regardless of any wind, would we? Then we'd have wind on the side of the aircraft.

But we don't.

Dan
 
timwinters said:
Another potential theory...

How does one steer a powered parachute? I just cruised around a few powered parachute websites trying to find this answer but struck out. IF one steers by dipping the chute into the direction of the turn then, again, I could see how it may be able to weathervane into the prevailing wind...

Okay, assume the chute is flying perpendicular to the wind. The "basket" hangs beneath the chute. Assume the "basket" has more vertical surface area than the chute itself does (and I think this is a safe assumption) then the wind hitting the side of the "basket" would push it leeward of the chute because it has more surface area for the wind to act upon. This would cause the chute to dip the into the wind since the "basket" is hanging beneath and being pushed to the leeward side. This dip would cause the chute turn into the wind...again IF one turns a chute by dipping.

Weathervane...
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Once more, there's no difference between the ground moving under still air and air moving over the ground from the perspective of anything in the air. If your explanation doesn't hold up in the ground moving/still air reference frame it won't work in the air moving over the ground one either.

I've tried to consider the effects of wind direction varying with altitude and haven't found anything that would cause the craft to point into the wind at either of the altitudes (canopy or airframe). One question I have is whether the canopies used for LS 2-place powered parachutes have any static stability in the vertical (yaw) axis. IOW if such a chute were attached to a symmetrical weight (e.g. bucket of cement) and it was rotating rapidly about the vertical axis during it's descent would the rate of rotation slow down and stop in a reasonably short period (e.g. within one rotation) or would it pretty much continue to rotate as it descended (presumably in a spiral)? If there is some static yaw stability then a difference in wind direction/speed between the altitude of the canopy and the altitude of the airframe would impart a torque on the connection (risers?) between the airframe and the canopy. Said torque might cause a rotation but I can't see how the rotation would result in the airframe aligning with the movement of the air over the ground. The ground does have some influence on the wind shift with altitude. Generally baring a surface front, the wind direction will become more westerly and less strong near the surface so if the upper wind was from the south the wind at the airframe ought to be more southwesterly than the wind at the canopy but it seems like any wind delta induced torque ought to apply the same turning moment regardless of the airframe's heading.
 
I may have actually witnessed this particular bet take place...
I would throw two things out:
1. Is weather-vaned being used two different ways between the two parties in the bet?

2. Could the weight distribution, and the rigging of the chute play into the behavior of the craft give the wind.

I'm not sure if this is the case, but I figure I'll throw it out there. If the rigging were to be set in such a way that the forward leaning tendency of the craft were to point the nose down, causing forward motion, which is offset by the blowing wind pushing against the chute, could that not cause a landing as the LS guy is describing by saying they steer into the wind? Keep in mind, unlike a rocket, or a bucket with a chute attached to it, this craft will have been moving in 3 axis leading up to the "engine failure"

Clearly this is a bet that need settling when the weather is nice enough for it, or we all want an excuse to go out west of Minnetonka and check out some cool ultralight flight...
 
Very interesting conversation. I have had the odd occasion to observe vessels at sea from the air both underway and without power. In my experience a vessel drifting without power in wind almost always stays broadside to the wind. Sea anchors were invented ages ago to hold the bows of ships in distress into the wind, and by extension the prevailing seas, to provide a stable ride and to keep the vessel from broaching to. If there is significantly tall superstructure at one end of the hull or the other I might expect the wind to point the vessel, but the rules of stability generally preclude such a design. The area of the hull, waterline to main deck is always the greatest sail area of a vessel. When conducting hoists by helicopter off a disabled vessel we always attempted to get the crew to stream a sea anchor to keep the bow into the sea and wind to reduce rolling and all the attached dangers. Only vessels at anchor will weathervane, as they are attached to a fixed point.
 
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bjohnsonmn said:
1. Is weather-vaned being used two different ways between the two parties in the bet?
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That's possible.

2. Could the weight distribution, and the rigging of the chute play into the behavior of the craft give the wind.
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Only if the weight being distributed is ferrous metal and there's a really powerful magnet sitting on the ground.
 
Just remember that in theory, there is no difference between theory and practice, but in practice there is...
 
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