How comes lift is less than weight on take-off? [duplicate]

If $$lift=weight * cos(angle)$$ this means that lift is less than weight during takeoff.

Could someone please explain me why it is so

• where did you hear that lift>weight during takeoff? Apr 6, 2015 at 14:05
• Why does the figure say L=W but also L=W*cos(α)? Anyway, the rest of the weight in the cos(α) case is supported by thrust.
– fooot
Apr 6, 2015 at 14:13
• must be an old book - the plane looks like the classic de Havilland Comet. Also, this airplane should fly straight, because $\alpha$ is the angle of attack, not the flight path angle. That would be $\gamma$. Apr 6, 2015 at 18:59

Indeed, what puzzles a lot of people is that excess lift (excess as in more than we need for weight) it not what makes an aircraft climb.

Of course, a take-off is a dynamic situation which doesn't really call for a balance of forces and momentum, so the situation is a bit more complicated to explain, but let's try a thought:

Imagine it this way: If lift > weight you will accelerate 'up-'wards (ie. balloon style). Air will come not only from the front moving backwards, but also from above moving below. The relative air moving past your wing is now coming from front and slightly above. This decreases your angle of attack and thus decreases lift, usually up to the point where lift = weight again.

If lift< weight, the opposite happens (ie. accelerate downwards) and air will move from below the wing towards up. This increases the angle of attack, increases the lift, and again lift will match the weight.

It's basically a statically stable system, where the only really stable position ,if you think about, is when lift equals weight (in straight-level flight, mind you. For climbing and descending you have the formula stated in your question).

If you want to climb, increasing lift is not the way to go !

• If you want to continue climb/descent/level flight, increasing lift is not the way to go.
– fooot
Apr 6, 2015 at 14:33
• Wing lift is not the only force with an upward component during climb - thrust will make up the difference, which is (1-cos$\gamma$)$\cdot$W. Apr 6, 2015 at 19:01