Tailplane in Ground Effect or Pitching Moments?
As LGB says, there are many factors involved and I for one would be the first to admit that I am uncertain about which may be dominant. Having said that I offer the following comments to create some discussion on the contribution of the effect of the change in pitching moments due to change in lift during rotation and liftoff.
We accept that in flight the mainplane of a conventional (stable) aircraft produces a strong downward pitching moment due to the lift distribution over the mainplane. To counteract this and control the aircraft around the pitch axis the tailplane produces a balancing downforce and adjustment of the elevator angle allows the pilot to refine the precise amount to give effective pitch control.
Obviously at zero airspeed prior to commencing the takeoff roll the mainplane is producing zero lift and zero pitching moment. And also obviously the tailplane is also producing zero downforce. As rotation speed is approached at zero pitch attitude the mainplane has started to produce a small amount of lift and therefore negligible pitching moment. However with the tailplane correctly trimmed for takeoff it will start to produce significant downforce on the end of a long moment arm and this can cause the nose wheel to become light and skippy. (Different aircraft types recommend varying techniques on the use of elevator during the takeoff roll.)
At the rotation speed, the stick force to initiate the rotation about the main wheels (aft of the cg) is normally quite low. As the pitch attitude approaches around 8-10 degrees the mainplane generates enough lift to overcome the weight of the aircraft and liftoff is imminent. Coincident with the generation of this lift by the mainplane a nosedown pitching moment is also generated by the fore/aft lift distribution. This centre of pressure is also aft of the cg. Therefore it is logical that this would require an increase in backpressure on the stick to create the corresponding increase in tailplane downforce to balance the new and additional pitching moments (either to maintain the desired pitch rate or maintain the pitch attitude).
Standing by to be corrected…!
The following discussion applies to Airbus brethren only:
We might be careful about jumping the gun in applying the observation of the reduction in rotation rate to Airbus operations, The Airbus A330/340 FCOMs specifically warn about increasing backstick at the point of, or immediately after liftoff due to the increased possibility of tailstrike directly resulting from this action. However, at liftoff after a V1 engine failure at heavy weight (in the A330 in particular), the increase in nosedown pitching moment can easily cause the main wheels to skip along the runway or even the aircraft to settled back to terra firma sometime after liftoff (especially embarrassing after passing the end of the runway or after gear retraction!).
Practically it does seem that some very small increase in sidestick backforce may be required to ensure that the pitch rate does not tend towards zero degrees/sec before the target pitch attitude is achieved especially at high weights. Shortly after airbourne the flight control laws change from ground law to normal law (auto pitch trimming) and the stick pitch force to maintain a selected pitch attitude is zeroed. This changeover from a sidestick backforce required to zero backforce can cause a tiny pilot induced pitch bobble that may be noticeable during engine-out work at high weights where performance margins are critical and demand that very accurate pitch attitudes need to be flown.
Perhaps the experienced and skillful Airbus driver would guard against the possibility of tailstrike by following the FCOM guidance and accurately flying the recommended pitch rate to the recommended engine-out pitch target of 12.5 degrees.