Effect of wind on aircraft size
Drift angle is simply the steady state angular difference between track and heading. End of.
Transient effects due to gusts and turbulence may be influenced by wing loading, momentum and gust response.
Lightning Mate, I understand your frustration!
Transient effects due to gusts and turbulence may be influenced by wing loading, momentum and gust response.
Lightning Mate, I understand your frustration!
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes
on
0 Posts
This has already been answered several ways, but I will try one more.
The aircraft's inertia is proportional to its weight, which equals Lift in steady flight.
It has various kinds of speed stability designed in, and the restoring forces are significant fractions of the Lift force.
This means the restoring forces are significant fractions of its weight, and and so the correcting accelerations are significant fractions of "g" (9.81 m/s/s or about 20 kts/sec).
Imagine a trimmed aircraft with centred controls flying in zero wind.
Now imagine a 10 kt wind starts to blow from the left.
Because of dihedral, you should soon get something like a 1/4 of the Lift force accelerating the aircraft to the right.
That's a 1/4 of the weight. Which will accelerate the aircraft at 1/4 G, which is about 5 kts/sec acceleration.
So 2 seconds after the wind started to blow, the sideways airspeed has gone.
Inertia (or Momentum if you prefer) is relevant for a couple of seconds after the wind changes, but not at all after that.
The bottom line is: aircraft are designed not to fly sideways.
Now imagine a 10 kt wind starts to blow from straight ahead.
Precisely because the aircraft has inertia, it will not instantly change ground speed, but it will instantly gain airspeed.
If the aircraft is travelling at 90 kts, the Lift will increase by about 25% (airspeed squared counts) and gravity will slow it as it climbs.
After a few seconds the airspeed and Lift have returned to normal again, but the groundspeed has changed.
The bottom line is: aircraft wing Lift depends on airspeed, not groundspeed.
I have not worked the numbers in detail, because I am trying to get a point across, not write a book.
Over a one hour flight, there may be a few seconds of transient adjustment to changing wind conditions, and the detailed aircraft behaviour over those few seconds will depend on the aircraft's inertia.
Those few seconds are of no significance.
The aircraft flies in the air, and the air moves over the ground. End of!
The aircraft's inertia is proportional to its weight, which equals Lift in steady flight.
It has various kinds of speed stability designed in, and the restoring forces are significant fractions of the Lift force.
This means the restoring forces are significant fractions of its weight, and and so the correcting accelerations are significant fractions of "g" (9.81 m/s/s or about 20 kts/sec).
Imagine a trimmed aircraft with centred controls flying in zero wind.
Now imagine a 10 kt wind starts to blow from the left.
Because of dihedral, you should soon get something like a 1/4 of the Lift force accelerating the aircraft to the right.
That's a 1/4 of the weight. Which will accelerate the aircraft at 1/4 G, which is about 5 kts/sec acceleration.
So 2 seconds after the wind started to blow, the sideways airspeed has gone.
Inertia (or Momentum if you prefer) is relevant for a couple of seconds after the wind changes, but not at all after that.
The bottom line is: aircraft are designed not to fly sideways.
Now imagine a 10 kt wind starts to blow from straight ahead.
Precisely because the aircraft has inertia, it will not instantly change ground speed, but it will instantly gain airspeed.
If the aircraft is travelling at 90 kts, the Lift will increase by about 25% (airspeed squared counts) and gravity will slow it as it climbs.
After a few seconds the airspeed and Lift have returned to normal again, but the groundspeed has changed.
The bottom line is: aircraft wing Lift depends on airspeed, not groundspeed.
I have not worked the numbers in detail, because I am trying to get a point across, not write a book.
Over a one hour flight, there may be a few seconds of transient adjustment to changing wind conditions, and the detailed aircraft behaviour over those few seconds will depend on the aircraft's inertia.
Those few seconds are of no significance.
The aircraft flies in the air, and the air moves over the ground. End of!
Last edited by 24Carrot; 9th Dec 2012 at 14:45.
Join Date: Nov 2010
Location: Tamworth, UK / Nairobi, Kenya
Posts: 616
Likes: 0
Received 0 Likes
on
0 Posts
guys nobody out here is actually answering my question.
My question was "Why is aircrafts momentum, size and shape not taken in consideration while determining drift angle"
can some one please give me a simple explanation.
My question was "Why is aircrafts momentum, size and shape not taken in consideration while determining drift angle"
can some one please give me a simple explanation.
Momentum (mass/speed), size, shape are only factors when the forces acting on the object change.
You can take a few physics courses at university to get the complete explanation, in order to understand static and dynamic forces, but I'll try to give a brief explanation.
Acceleration (or deceleration) in any direction is affected by Mass (weight), but an object that is at a constant Speed has no acceleration, so Mass is not a part of the equation.
The force equation is F=MA where F is force, M is Mass, and A is acceleration.
Speed is calculated as V=D/T where V is velocity, D is distance and T is time.
When speed is to be calculated as a result of acceleration, V=Vo + AT where Vo is the initial velocity, A is acceleration and T is time.
If the forces on a aeroplane change (wind speed or direction changes or power changes or control changes), then the speed (and thus ground track) will change and this change will be a factor of Mass.
However if the forces do not change, then the ground track (Velocity) will NOT be affected by Mass (weight)
In a moving wind-mass (block of wind) the if the aeroplane were 'stationary' in the wind it would be travelling the same speed and direction of the wind.
To make things simple, let's say that the wind is blowing toward 090 at 20 knots.
And to continue to make things simple, let's say the plane is pointed 000 and is travelling at 100 knots.
As the plane moves toward 000 at 100 knots within the block of wind it is also moving toward 090 at 20 knots as a result of the wind moving across the ground.
Weight doesn't come into it at all.
If the plane weighs 1000kg it will be going to 000 at 100 and to 090 at 20.
If the plane weighs 9999kg it will be going to 000 at 100 and to 090 at 20.
"Isaac Newton defined inertia as his first law in his Philosophić Naturalis Principia Mathematica, which states:
The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line."
mike hallam (with thanks to Wiki.)
The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line."
mike hallam (with thanks to Wiki.)
Last edited by mikehallam; 10th Dec 2012 at 10:13.
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes
on
0 Posts
There seem to have been a lot of very good responses to a pretty minimal input from jehan. Perhaps it is time to put the boot on the other foot.
Jehan, perhaps you would care to explain why you think weight, momentum, or aircraft shape should have any effect on drift angle.
Let us take darkroomsource's example.
Let us have two aircraft in a crosswind: same speed, same location but vertically separated, and with different weight and shape.
You propose that, because of momentum or shape, they require different drift angles. Let us suppose they make no wind corrections, so I take it they must drift, but drift at different rates? At any rate they cannot remain one above the other if they require different drift angles, and are not getting them?
So which of the following do you believe:
a) One of the two 100kt aircraft gains on the other, because of its momentum or shape.
b) One of the two aircraft on the same 000 heading flies more sideways than the other, because of its momentum or shape.
I am sure that any suggestions as to the physics behind all this will be heartily appreciated by all.
Jehan, perhaps you would care to explain why you think weight, momentum, or aircraft shape should have any effect on drift angle.
Let us take darkroomsource's example.
Let us have two aircraft in a crosswind: same speed, same location but vertically separated, and with different weight and shape.
You propose that, because of momentum or shape, they require different drift angles. Let us suppose they make no wind corrections, so I take it they must drift, but drift at different rates? At any rate they cannot remain one above the other if they require different drift angles, and are not getting them?
So which of the following do you believe:
a) One of the two 100kt aircraft gains on the other, because of its momentum or shape.
b) One of the two aircraft on the same 000 heading flies more sideways than the other, because of its momentum or shape.
I am sure that any suggestions as to the physics behind all this will be heartily appreciated by all.
Last edited by 24Carrot; 10th Dec 2012 at 11:02.
Join Date: Nov 2014
Location: Melbourne
Posts: 37
Likes: 0
Received 0 Likes
on
0 Posts
I know its a bit late but I don't think anyone ever said...
Because the drift is caused by the air moving, not the plane(s) moving. Therefore all aircraft supported by that air will be influenced equally.
Because the drift is caused by the air moving, not the plane(s) moving. Therefore all aircraft supported by that air will be influenced equally.
