Forward pressure on take off & Downwash
 

Hello everyone ,
I'm new to this forum and would like some insights on these two questions that I've been having trouble finding answers for. My questions are specific to the aircraft that I fly currently, a boeing.

1. Light forward pressure is required on control column during take off till 80kts. I understand that it's to increase nose wheel effectiveness. What could be the implications or adverse effects of maintaining this pressure till rotation ? Something to do with control surface deflection and the drag generated because of that ?

2. After rotation ,passing 10 degrees of pitch attitude, extra backward pressure is required to continue rotation to the target pitch attitude. Someone told me that it's beacsue fo a phenomenon known as Downwash. Could someone here please she's some light on it.

Thanks a lot in advance.

As airspeed builds, the rudder becomes more effective therefore nosewheel steering is less required/desirable. Your elevator is also becoming more effective, amplifying the forces on the nosewheel. This may result in unnecessary force on the gear and increased scrubbing of the tyres. There will also be a drag penalty, as you mention, though it’s only a small control deflection so I suspect it will also be small.

I’ve not heard the term “downwash” applied to this phenomenon but I understand it to be the horizontal stabiliser entering ground effect. This causes an upward force (or absence of the downward force you’re trying to create) on the tail plane which results in the pitch rate reducing.

After takeoff; could be reducing ‘ground effect’, where the aircraft pitching moment changes nose down with increasing altitude; ≈ less air trapped under the wing ≈.
Depends on aircraft type, size, wing / tail characteristics, control system, etc.

As to 1), flypaddy nailed it. With higher speed, steering should transition to the rudder anyway. Pushing down on the nosegear with elevator is then not only unnecessary, but can strain its structure (which is lighter than the main gear).

It can even begin to produce "wheelbarrowing" or porpoising (although that is more common with lighter aircraft) - pitch-down force strong enough to lift the main gear right off the ground.

Always treat the nose-gear like a fragile straw.

As to 2), without getting into the various mechanisms of lift (which will immediately lead to 20 pages of thread drift ;) ), the net effect of the main wing is to push air downwards. That is its downwash. This can be seen in extreme form with helicopters' rotary wings - but fixed-wings do it also.

Normally, the tailplane or horizontal stabilizer of most aircraft is slightly above the wing and out of most of the downwash. But at rotation - and afterward, in nose-high climb - when the nose goes up, the tail goes down, and is now closer to the downwash coming off the wing's trailing edge (enhanced at takeoff by flap lift). And that changes the airflow pattern over, and the forces and effectiveness of, the horizontal stabilizer/elevator.

Hi Aviator_8998
“After rotation, passing 10 degrees of pitch attitude, extra backward pressure is required to continue rotation to the target pitch attitude.”

That effect is due to the proximity of the tail to the runway. Around 10 degrees of pitch, the airflow felt by the tail is parallel to the runway surface - hence the angle of attack of the elevator is reduced (by the aircraft’s pitch attitude). More backward pressure is required to continue the rotation rate. After lift off, once clear of the ground, the airflow over the tail is restored to its normal direction - hence less back pressure is then needed.

Thanks a lot everyone for your replies. Appreciate the help.