What is the altitude of the nose vs tail in airplane flight? Suppose you have an airplane flying level on a long flight. Now suppose you place one altimeter sensor in the tail, and one in the nose. Suppose you record and store these values independently (perhaps take 10 readings per second.)

And then suppose you graph this data.

This is the graph i'm looking for. Has anyone ever seen data like this?

In other words, i'm looking for experimental data which has logged a comparison of the altitude of the nose, vs. the altitude of the tail, (several times per second) on a long flight.

The intent is to study how the pitch of the aircraft is constantly in flux -- e.g. by turbulence, wind speed changes, the curvature of the earth, and (of course) by the pilot's inputs themselves, etc.
Yes that data exists , it is recorded during flight test data gathering for full flight simulators. The transducers used are not two bars altimeters, but direct gravitational angle finders and gyro's for increased frequency response. Sample rate was 30 Hz in the olden days, now 100 Hz or higher.

A full set of measurements consists of correlated sets of


pitch, roll, yaw angle measurements from dedicated mounted transducers; 
angle of attack and sideslip, dynamic and static pressure, magnetic course, and all other parameters recorded by the aircraft instruments;
stick and control surface positions from dedicated mounted transducers.


The aircraft data sets are sold for a lot of money, your best bet would be to try to get your hands on flight data measurements for an older type aircraft or a type that is not produced anymore.

Next time you fly, you could even measure this yourself by using the transducers in your mobile phone, which has acceleration, rotational velocity and angle transducers, plus a set of linear GPS co-ordinates.