How does an aircraft tailplane work?

  • How does an aircraft tailplane work? Pranav

    How does an aircraft tailplane keep the aircraft stable, and prevent it from tipping over? Also, how does the lift generated by a tailplane compare to that generated by the wing?

  • A wing with a conventional aerofoil profile makes a negative contribution to longitudinal stability. This means that any disturbance (such as a gust) which raises the nose produces a nose-up pitching moment which tends to raise the nose further. With the same disturbance, the presence of a tailplane produces a restoring nose-down pitching moment, which may counteract the natural instability of the wing and make the aircraft longitudinally stable (much the same way a windvane always points into the wind).

    The tailplane does not produce any lift. You could say it produces a 'Negative Lift'. The reason many early aviators were killed is because the tailplanes produced lift in order to help the plane fly which would result in unrecoverable nose-up tailplane stall. Most modern aircraft are designed so that when the airflow decreases, the effect/momentum produced by the tail surface is decreased as to prevent the previously mentioned condition

  • The wings(which has a aerofoil cross section ) produce lift (basically a force that acts opposite to weight) which act at a distance from center of gravity (C.G) so the force gets transferred to C.G. as a force and moment(in clockwise direction) which lead to pitch up movement

    To balance that moment tail are used the tail produce lift (small compared to that produced by wings) so if we transfer it to C.G. an force and moment (since it produce less lift it should be placed far from C.G) this moment acts in anticlockwise direction thus neutralizing the moment due to wings ...Thus making the aircraft stable ...

  • The absolute value of the lift generated by the tailplane varies and depends on the phase in which your plane is in that moment:

    Takeoff (flaps extended): high drift
    Climb (no flaps): mostly lift (not much)
    Cruise (no flaps): drift
    Landing (flaps extended): high drift

    Due to the fuel consumption the weight of the plane reduces while flight. This may change the position of your center of gravity and this in turn will affect the absolute value of your lift/drift. Usually |drift| increases, in other words, while flight the lift of the tailplane decreases.

    Some words to stability: Just think about equilibrium of moments.
    The center of gravity is near the main wing. The high lift of the main wing is very near to c.o.g., the drift of the tailplane is rather far away from it. The sum of all moments equals zero, they will balance the plane if there are gusts etc.

  • For conventional designs, the tail is composed of two parts: the horizontal tail and the vertical tail. They play a role in the trim and the manoeuvrability of the aircraft but at different levels. The horizontal tail is mainly used for longitudinal stability (and trim) while the vertical tails used for the lateral stability (and trim).

    About Stability

    It is possible to talk of stability only after having defined an equilibrium point around which the stability is studied. An aircraft is in equilibrium if the forces and moments it experiences are balanced. Using a simple model for the longitudinal analysis, it can be decomposed in three relations called the trim equations. In order to keep it simple, it will be assumed here that the angle of attack and the flight path angle are zero. (Note that the same reasoning can be achieved with non-zero values but the equations then become quite messy.)

    Longitudinal Equilibrium

    These three equations are:

    $$L=mg$$ $$T=D$$ $$M=0$$

    where $L$ is the total lift, $mg$ is the weight of the aircraft, $T$ is the thrust, $D$ is the drag and $M$ is the pitching moment around the centre of gravity of the aircraft. The second equation won't be studied further since it does not help to understand the role of the horizontal tail and its influence. Looking at the following picture, one can see that usually, the centre of gravity and the point where the lift applies (called the aerodynamic centre) are not the same. This means that the lift generated by the wing creates an induced moment around the centre of gravity that one should add to the already intrinsic pitching moment due to the main wing (usually a pitch down moment for conventional airfoils).

    Longitudinal Stability

    Knowing that, it is possible to rewrite the two equations of interest including the contributions from the main wing and from the horizontal tail.

    $$W+L_t=L_w$$ $$M_0+bL_t=aL_w$$

    From these equations and the figure, it appears that the horizontal tail is used to generate a lift which induces a moment helping to balance the moments equilibrium and thus prevent the aircraft to spin on itself (pitchwise).

    Drawback and Solution

    From both the figure and the equations it turns out that the lift contribution from the tail is usually negative, meaning that more lift from the main wing is needed to keep a trimmed (or balanced) aircraft. This drawback can be overcome by the use of a canard configuration instead.

    Lateral Stability

    The same can be done for the lateral equilibrium and stability but there it is the vertical tail that is used. It is symmetrical so that there is no yaw induced and if there is some side force experienced, it will create a moment in order to reduce the side-slip angle.

    Comparison of Lift Created by the Tail and Main Wing

    For a trimmed configuration, it is easy to see that the lift created by the main wing is more or less the one created by the tail plus the total weight of the aircraft, which gives an idea of the difference between the two forces.

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