AeroPlus wrote:
“… flying in a crosswind”. You can land with a crosswind but flying in a crosswind …
you’ve out-“pedanticized” me!
Airborne_Again wrote:
That’s exactly right
No, the air is a fluid, the aircraft is not. Lift on the aircraft is the total sum of the pressure on the wing perpendicular to the skin for the whole wing (the whole aircraft in fact). The airplane has no idea if it moves air up or down, or if there are vacuum cleaners on the top making low pressure and blowers on the bottom making high pressure.
AeroPlus wrote:
It is an old book written over 60 years ago, but I don’t think it is considered outdated and the principles of flight haven’t changed.
)
IMO there is no easy way of understanding lift, it’s causes. You have to understand fluid momentum, viscosity, shear layers, fluid velocity gradients (more or less what this cross wind exercise is about) and many other things, Newtons 3rd law and fluid rotation included. Some aspects are easy enough, like pressure and Newton, but they explain nothing, they are only the end effects. A good start is to try to understand fluid momentum, and the difference between sucking and blowing: you can blow a small piece of paper from meters away, but you cannot suck the same piece even from a few cm away, given the velocity of air through your mouth is the same in both cases.
LeSving – how good are you with X-Plane? Somebody who knows it thoroughly could probably fairly easily set up a simulation of the lift of a typical GA (parallel / slab) aerofoil, versus the angle of the airflow.
You don’t get more pure headwind; that remains at 70kt. But you get more wind, if slightly off-axis, and I think 73kt at 16 deg off-axis does produce more lift than 70kt on-axis.
5% more airspeed is about 10% more lift.
The amount of lift is not relevant to the take-off point, It is purely an act of faith, the ASI sayeth 
Peter wrote:
I reckon it will make you reach the takeoff point a bit sooner. The takeoff AIRspeed itself won’t change.
The cumulate drag of the crosswind during the roll cannot be discarded. If the ASI was a perfect device that only measured the on-axis velocity, or dynamic pressure, then what you describe would not happen. The ASI is not perfect of course, however I don’t think it is fair to assume, how it would respond without some testing.
That is a misconception from the start. The principles of flight have existed long before humans existed. Our understanding of how flight works has most definitely changed in the last 60 years and is still changing, being refined and detailed. 60-year-old book “explaining” flight in popular terms: throw it away (In my humble opinion of course )
Interesting how easily you dismiss this book as the Stick and Rudder book written by Wolfgang Langewiesche is still considered relevant. It was the first exact analysis of the art of flying ever attempted. When the book initially came out, some of its contents were considered highly controversial. However, in recent years, its formulations have become widely accepted to this day. I think that it is still the leading think-book on the art of flying today. It was last renewed in 1972 by the author. I have not come across a critique of this book that stated it as obsolete.
Airborne_Again wrote:
A crosswind won’t affect the takeoff run or the takeoff speed, but as soon as the aircraft is airborne it will weathervane into the wind and then gain some extra airspeed. So the performance of the first 50 feet or so of climb is slightly improved.
That’s how I see it, and it can be improved even more by actually turning into the wind head on giving a jolt of added speed. More improvement is done by also using the runway diagonally. But I also think Peter has a point. Lift is proportional to the square of the airspeed, while the difference in angle is pure geometric effect (linear). A wing is a 3D object with span, cord, AR. Slanting it slightly into the wind doesn’t necessarily decrease the effective area or AR (heading into the relative wind), it could increase both. The propeller will also help the leeward wing. It seems to me it’s more to this than what immediately meets the eye.
Peter wrote:
LeSving – how good are you with X-Plane? Somebody who knows it thoroughly could probably fairly easily set up a simulation of the lift of a typical GA (parallel / slab) aerofoil, versus the angle of the airflow.
He he. This is probably one of those things I wouldn’t trust X-plane to do correct, nor any other flight sim for that matter, not all the details. They are all a bit too “mechanical”. Some of it can be done in X-Plane though, and one is to only look at the lift vs angle as you describe.
LeSving wrote:
IMO there is no easy way of understanding lift, it’s causes.
I’m not trying to explain what causes lift, I’m just saying that if there is an upward (lift) force on the wing, there has to be a downward force on the air.
AeroPlus wrote:
After take-off it will weathervane into the wind but with no increase in airspeed. The ground speed changes but not the airspeed :-)Right. I meant the component of airspeed along the axis of the aircraft, which is the one that produces lift.
Airborne_Again wrote:
I’m not trying to explain what causes lift, I’m just saying that if there is an upward (lift) force on the wing, there has to be a downward force on the air.
OK, but that is also a bit wrong actually (and a common misconception, even done by NASA when doing popular explanations of thrust and lift). The air is not a body as such, it’s a fluid, a continuum. Newton’s third law is about forces between objects, it’s not really about accelerating a fluid or any dynamics at all. It’s the second law that must be used: F = ma, because the force applies to the aircraft, while the fluid (the air) is being accelerated.
So, the total lift and drag force is equal to the total mass of air being accelerated is a correct description. Also the total lift and drag is equal to the sum of all the pressures acting on the airframe, is also correct. On that boundary (the skin of the aircraft) Newton’s 3rd law can be used of course, since there has to be an opposing force to the lift and drag, but that opposing force IS the pressure, the molecules of air touching the skin, thus by using the 3rd law you only end up in a circular argument which don’t explain what happens to the fluid continuum the air.
