Aerodymanic Tail Down Force for you CFIs

M

meritflyer

Well-Known Member
Reviewing my commercial material, I came across tail down force and its association with longitudinal stability.

(Its kickin' my as$)

Does tail down force occur b/c the manufacturer pre-sets the stabilizer to a negative angle of incidence or is there another explanation for it (the more basic, the better at this point of review).
 
A pilot controls lift with two things...airspeed and angle of attack. Focusing on AOA, as it increases, lift increases. The tail is designed with a natural negative angle of attack. This causes the tail to create lift in the opposite direction of the wing, which is the tail down force.Angle of incidence is 'twisting' or 'washout' of the wing that causes it to stall at the root first as opposed to the tip. The wing has a slightly higher AOA at the root than the tip, so it reaches citical AOA first.
 
This is also the reason why a tail heavy a/c is either more efficient, or can pick up a little speed in cruise. Putting more weight aft of the CG reduces the need for tail down force, creating less overall drag!!
 
meritflyer said:
Does tail down force occur b/c the manufacturer pre-sets the stabilizer to a negative angle of incidence or is there another explanation for it (the more basic, the better at this point of review).
Click to expand...

How much pain would you like?

A conventional configuration aircraft (i.e. not a canard) will pitch nose down for two reasons:

1. CG is forward of center of lift.
2. Center of lift on upper and lower airfoil surfaces are displaced, resulting in a nose down twisting force.

Both #1 and #2 must be countered by a balancing force if the aircraft is to maintain level flight. This balancing force is provided by the tail on a conventional configuration (i.e. not canard) aircraft. Yes, the manufacturer does provide a negative AOA at the horizontal stab so that this down force is "dialed in", so to speak. Any difference between "dialed in" and required for current flight is provided by trim and elevator.

BTW, #2 is usually more trivia than any DE can stand.
 
In order to maintain stability, there must be a tail down force on the stabilizer, because when the aircraft is stable the cg is forward of the center of lift. an example in unaccelerated flight (ignore the dots in the diagram):

(main wing lift) = -[(weight) + (stabilizer lift)]

...............(main wing lift)
.......................A
.......................|
.......................|
............====================
..................|........................|
..................|........................V
..................V...............(stabilizer lift)
..............(weight)

Also, to prevent a pitch moment:

(weight)*(x_cl - x_cg) = (stabilizer lift)*(x_stab - x_cg)

where x is measured from some arbitrary point (typically at the front of the a/c).

In the case of an unstable airframe (some fighters and most missiles), the cg is aft of the cl, which requires a positive lift from the stabilizer. This means that less overall lift is produced (since we are not "losing" lift from the stabilizer) which means less overall induced drag, and higher efficiency.

This is all from memory so if anyone sees anything wrong feel free to correct it.
 
angle of incidence has is infact, as first stated, the angle between the cord line of the wing and the longitudinal axis of the airplane. most wings are mounted at a small positive angle of incidence
 
To tack on to Hattrick's post: The tail doesn't necessarily have to be mounted with a negative incidence angle to produce a downward aerodynamic force...(Calling it a downward "lift" just sounds wrong). Due to the flow of air over the wing the air hits the tail at a downward angle anyway. They call this "downwash". When you come into ground effect, in addition to improving the effectiveness of the wing and therefore increasing lift and decreasing drag, the ground also reduces the amount of downwash angle on the tail. As a result, more back elevator is required to establish the same pitch. The effect is most pronounced when the wing and the tail are at about the same height, so you won't notice it as much (or at all) on T-tail type designs, especially T-tails and low wings like many business jets.
 
frog_flyer said:
why do md80s elevators always appear to be pulled up?

is that really the neutral position for them?
Click to expand...

Gravity.

They're controlled by control tabs only. They'll flop around in the wind just like a cessna. When an aircraft in front of us powers up, sometimes you hear/feel a big "BOOM" at it flops around against the stops.

Unless, of course, it's an MD-90 with a hydraulic pump running.
 
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