Something I noticed while flying - while on takeoff it feels as if the plane is tilting upwards even though the plane is clearly still on the ground with all sets of landing gear.
Does the nose gear extend immediately upon acceleration to perhaps generate more lift? Or maybe it's just an illusion caused by the acceleration. (I've only noticed this on large aircraft - like for example: Airbus A320 or Boeing 737.)
No, the gear doesn't actively extend, and any passive change to the strut extension during takeoff is likely to be imperceptible.
What you are perceiving during takeoff is the somatogravic illusion. It is a physiological sensation, not a physical change. Rapid acceleration, as during a takeoff roll, causes the inner ear to report what your brain assumes is your head tilting back.
The otolith organs in your inner ears sit in fluid and detect the tilt of your head. Acceleration and pitching up (or tilting back) cause the exact same movement of the otoliths. Because the otoliths are pushed backwards by the fluid, just like they would by you tilting your head back, it feels like a climb.
When an airliner is taking off, you have limited outside references to the horizon and your inner ear takes over the task of orienting you. You might notice it in smaller aircraft if you closed your eyes; in very small aircraft, the acceleration may be too slow to trigger the illusion.
I'm assuming you are referring to the 'tilting backwards' that occurs when the aircraft begins its takeoff run. There is of course a much more pronounced tilting backwards when the nosewheel actually lifts off the runway, a few seconds before the main wheels come off.
This tilting at the start of the takeoff run is actually a sensory illusion. It's caused by the acceleration of the aircraft. The forward force on you from the seat is combining with the upward force (resisting gravity) makes it seem as though the aircraft is tilting, when in fact it isn't. You would get a similar effect in a fast-accelerating sports car; however in a car you have a clear view of the outside world which means your mind can understand that this is acceleration, not a tilt.
You can check this in a number of ways: 1) if you watch a plane taking off from outside, there is no noticeable tilt of the plane, nor any noticeable change in the nosewheel extension - certainly nothing like as much as the tilt appears to be from the inside 2) Looking at the outside world through the window will confirm that the plane is not tilting 3) This effect occurs on planes with engines both above and below the wings, which you would not expect if it were an actual tilt.
I will assume you refer to the initial acceleration when you speak of "while on takeoff", since after the rotation the answer would be quite tautological.
Let's be clear, the nose gear does not "extend" anywhere during take-off since its damper/shock absorber is not an active component, but rather a passive one.
The effect you are feeling is due to the acceleration and the inertia of the aircraft: the engines of a large aircraft sit below the Center of Mass which will incur a rotational force to the aircraft, lifting the nose up a bit. The weight will be "sitting" more on the main gear rather than the nose, you have a similar effect on a car or, more notably, on an accelerating motorcycle.
You also have the reverse effect when decelerating: the weight will tend to stress more the nose gear rather than the main gear, the same way a motorcycle tends to "sit" on the front wheel when stopping.
On aircraft with the engines up high the rotational force will be reversed until the elevators have enough airflow to counteract it.
