What?

 

The negative lift of the tail balances the "nose heavy" airplane about the center of lift and the fact that the main wing airfoil causes increasing lift with increasing speed (flaps in cruise position), generally nose down trim is used to lessen the negative lift of the tail as designed when in the neutral cruise position.  That is, the AOA of the main wing is reduced by reducing the negative AOA of the horizontal stab, thus pitching the nose down.  If the negative lift of the tail was disturbed or made less effective, the nose would pitch down.  Remember that when you pull on the stick, the upward movement of the elevator increases the negative lift of the horizontal (AOA of upside down wing), thus raising the nose (and, by association, the AOA of the main wing is also increased).  Also remember that the main wing was installed with a positive angle of incidence and the horizontal stab with a slight negative angle.

 

In a 200-300 series Lancair, the flaps cruise position is in reflex to lessen the lift at cruise (AOA is reduced, drag is reduced, pitching moment is becomes more nose up requiring reducing the neg lift of the HS and also reducing the drag).  One feature this provides is the strong nose down pitch trim available by extending the flaps slightly (taking them out of reflex).  An interesting point of difference between Legacy and 200-300 series is the fact that if the horizontal stab effectiveness to hold the nose up were reduced by opened canopy turbulence during the takeoff climb, nose up pitch could be added by moving the takeoff flaps (10 degrees down - out of reflex) to the full reflex cruise position (7 degrees up).

 

The Legacy has Fowler flaps and, as such, may not have as much of a dramatic pitching moment as do the flaps on the 200-300 series.  Perhaps some Legacy pilot can describe the actual change in pitch trim as the plane speeds up or slows down - with and without some flaps.

 

Embrace our differences!

 

Grayhawk

 

How true Dennis (about getting it backwards).

Paul Miller

Legacy N357V

On 10-Feb-09, at 11:32 AM, Dennis Johnson wrote:

The tail provides negative lift in level flight, pushing the nose down, not up.  If the open canopy blanketed the tail, I think the nose would have pitched up, not down.  

 

I point this out for two reasons:

 

1.  It's an important concept that all of us pilots sometimes get backwards and that can lead us to make incorrect decisions  with possibly bad outcomes.

 

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