I asked my instructor and this was his answer!
The Nefarious Skyhawk Gas Gremlin !
Fact: When a high wing plane (such as a Cessna 172 or a Cessna 150) is flown on a training flight with the fuel selector in the "Both" position (C150 fuel valves are always on the "both" position - even when "off") with a primary flight student at the controls, the plane will almost always land with more fuel in the right tank than in the left (but the very same plane may occasionally land with more fuel in the left tank with a slightly more experienced student, or - eventually - with equally balanced fuel tanks).
"Why is this?" you ask. "How can this be?" you demand to know! I ask all my students this, but I try not to give them the answer. The answer is completely obvious once they have flown for a while. Once they understand the most fundamental basics of flight, they will be able to tell me the secret of "The Nefarious Skyhawk Gas Gremlin". (But once in a while a CFI is compelled to point out the obvious).
WHAT IT IS NOT:
It is not because of a design flaw in the Cessna.
It is not because of an obstruction in the fuel line or the vent (while that is possible, this rare obstruction problem is not germane to the nearly universal "Gas Gremlin" phenomenon).
It is not because of turns and banks during flight.
An aircraft can do steep turns all day long; loops, wing-overs, and barrel rolls, ad infinitum, with the fuel selector in the "both" position, and never have a fuel imbalance. Yet a simple training flight will usually result in more fuel in the right tank than the left.
WHAT IS THE CAUSE?
Hint, the answer is related to the "native call of the CFI in his/her natural environment".
The cause is revealed, not by the trusty aircraft mechanic, nor by the psychic aviation consultant, but by the most simple indicator found in all airplanes: The slip/skid ball. The force that causes the ball to be pulled to the right side of the curved glass tube during takeoff, climb, and power-on stall practice, is the very same force that is pulling the fuel from the left tank into the right wing tank.
This is also the origin of the "native call of the CFI in his/her natural environment" ... "RIGHT RUDDER! RIGHT RUDDER! RIGHT RUDDER! ... STEP ON THE BALL !!!"
Note: this unbalanced fuel condition will also be caused when even the most experienced pilot is practicing side-slips and forward slips. But this is OK, and elicits only a silent smile from the CFI.
JT Rairigh, CFII, MEI, Helicopter, A&P/IA