Originally Posted by
JPerrone
HarryC, post 209
Well that is certainly food for thought!
If there is NO sidewind against an aircraft, that would imply
- All the force vectors linked in post 189 are wrong;
- The force vectors referenced in 201 are wrong;
And if the wind is always aligned with the aircraft, with no side wind, then the following scenario would have to be true.
- Lets start the plane flying North, into a North wind. Airplane and wind are perfectly aligned.
- The plane turns a little, lets say 15 degrees. The apparent (not true) wind is still aligned with the aircraft
- At 90 degrees, the true wind is straight from the side; but the frame of reference makes the wind still aligned with the airplane, and it should still be coming from in front on the nose
- Let now turn a total of 120 degrees. The wind is coming from behind the plane. Is the apparent wind still coming from in front of the plane?
- Let's complete the turn, at 180 degrees the plane is going in the same direction as the wind. Seems like the true and apparent wind and aircraft are aligned, but now the wind is on the tail.
For there to NEVER be a crosswind, somewhere around 90 degrees, the apparent wind would have to switch from on the nose to on the tail...
It could happen I guess.
Any force applied to an object will result in acceleration of that object.
If you believe there is a constant force being applied to the side of an aircraft when flying in a cross wind, that means there is a constant acceleration of the aircraft in the direction of the wind. This is not true. Acceleration is completely different to "movement"
Lets assume a steady constant unchanging wind from the north of 10 Mph.
The aircraft accelerates along the runway, takes off and is immediately influenced by the wind. Whatever direction the wind is coming from, it will apply a force to the aircraft. The aircraft will accelerate in the direction of the wind until it reaches equilibrium in the moving air mass. (within a few seconds of take off)
The aircraft is now moving wholly within the moving air mass and there is no further acceleration from the wind (IE no further force applied to the aircraft by the wind) regardless of the direction of the aircraft heading.
The only forces the aircraft now experiences is from it own movement within that airflow, because it is no longer being accelerated by the wind.
This principle is so easy to demonstrate in many ways.
Close your eyes and walk east. (with the earth's rotation)
Now close your eyes and walk west (against the earths rotation)
Can you tell the difference? Can you feel any force being applied by the rotation of the earth?
NO.. because from the moment you were born, you have been moving "with" the rotation of the earth.
Your speed matches the speed of the rotation of the earth, so you cannot feel any force regardless of which direction you walk.
The same concept applies to aircraft and explains why an aircraft does not experience a side force when flying in a cross wind.
Because it is already moving with the wind since shortly after take off.