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  • 08. Tail assemby surfaces


    8.1 Airframe basics

    The empennage

    A monoplane has a single wing,  or a left and right  pair of wings — the port and starboard mainplanes. A biplane  has two sets of wings mounted one above the other.  The engine and propeller may be in front of the wing (a 'tractor' configuration) or behind the wing (a 'pusher' configuration). It must have some sort of pilot/passenger seating (usually enclosed in a cockpit or pod, which will have either tandem or side-by-side seating and is referred to as the 'occupant zone'); fuel tank(s); and a  rigid structure mounting the engine/propeller, the wheels or undercarriage, and the cockpit, so that their weight is supported by the wing main spar(s)  and the propeller thrust reaction is transmitted to the body and wings.




    The photograph of  Mick McCann's 'Breezy' shows a basic   high-wing monoplane, pusher-engine configuration with tandem pilot/passenger seating, nose-wheel undercarriage and an open-frame, welded tubular-steel fuselage — the aft part of which is upswept, so that the aircraft's attitude in pitch can be adjusted during take-off and landing without the tail striking the ground. Also the arrangement keeps the rear stabilising and control surfaces within the energetic airflow of  the propeller slipstream. The term fuselage is derived from an old French word meaning a tapered 'spindle' used for manually weaving yarn. The 'Breezy' has no refinements for comfort —  or for drag reduction. The fuel tank is not discernible in the photograph but is small and close to the engine.


    Attached to the rear fuselage are the horizontal stabiliser and elevators, plus the vertical stabiliser or fin and the rudder,  together forming the pitch and yaw stabilising and control mechanisms  — the tail assembly or empennage.   The latter term is derived from a French word meaning to feather an arrow; maybe that is why some people refer to the empennage as the 'tailfeathers'. The horizontal stabiliser and elevators are referred to as the 'tailplane'. 


    Moments and couples

    moment.jpgThe moment of a force  or the torque is a measure of   the rotational effect produced by a force acting about  —  or with respect to  —  a  fulcrum, axis, centre of mass (cg) or aerodynamic centre.  Its magnitude is the product (in newton metres) of the force (N) and the length (m) of the arm (the leverage) from the  pivotal point to the line of action of the force. The moment will act in a particular direction, for example, as we saw in the 'Aerofoils and wings' module, the  pitching moment of a cambered wing produces a nose-down torque.


    The forces generated by the tailplane control surfaces are dependent on the stabiliser area, the control surface area, the length of the tail arm to the cg, the control surface deflection  and the airspeed. Only deflection and airspeed are controlled by the pilot.


    Two equal and opposite forces acting parallel to each other, but separated,  form a couple. The rotational effect or moment of a  couple   is the product of  one force and the perpendicular distance between them. The ailerons, for example, form a couple when deflected.


    8.2 Tailplane

    Horizontal stabiliser

    The existence of the wing pitching  moment makes the wing inherently unstable. To overcome this problem, it is necessary  to couple it with another aerodynamic moment  about the lateral or pitch axis — opposing the wing pitching moment — that  will balance that  moment at an airspeed selected by the designer.  The moment of a force is   the arm length multiplied by the force; so the longer the tail arm,  the smaller the aerodynamic force required. The standard solution is to extend the fuselage rearwards so that  a horizontal stabiliser can be mounted at a distance from the cg; note the Breezy's very long tail arm – between the cg and the small horizontal stabiliser.  The horizontal stabiliser is usually a  lift-generating surface — or 'plane' — mounted  so that the aerodynamic force it generates acts in the opposite direction to the lift from the mainplane, i.e. generally downwards.  The plane could incorporate a cambered aerofoil with the cambered surface underneath, or perhaps  a symmetrical aerofoil, or even just a flat plate —  as the Breezy's appears to be. The symmetrical aerofoil and the flat plate would both be mounted at a negative incidence to produce the downward force.  The end result is that the net pitching moment of the mainplane and tailplane couple is zero at a particular geometric aoa of the main wing; that aoa would equate with a speed selected by the designer —  usually the designed cruise speed or perhaps the engine-off glide speed. The fuselage may also produce pitching moments that must be balanced by the stabiliser.


    As the horizontal stabiliser  is usually designed to produce negative lift, then the wing must fly at a slightly greater aoa to provide  additional lift, so that the net aircraft lift balances weight.



    The pilot must be able to initiate and hold aoa changes for airspeed adjustments, manoeuvres (accelerations) in the pitching plane (pull-ups, turns, push-downs) and adjustments of aircraft attitude relative to the airfield or alighting area surface during take-off and landing.   The elevators —  hinged to the trailing edge of the horizontal stabiliser so that they may be deflected up or down — are the control surfaces that enable controlled changes in  wing aoa.  Elevators are aerodynamically similar to the  ailerons, but move  in unison rather than differentially.


    The elevators are linked, via control rods or cables, to forward/backward movement of the control column, so the pilot can, in effect,  increase or decrease the camber of the stabiliser–elevator combination.  Camber changes will alter the magnitude and direction of the aerodynamic reactions generated by the stabiliser–elevator, and the changed forces impart a pitching moment in the longitudinal plane.  This pitching moment rotates the aircraft about its lateral axis, initiating the change in wing aoa. Once the new aoa is established, the pitch moment returns to zero and the aircraft will hold that aoa — provided the elevators are held in the deflected position by the pilot or a trim device — thereby controlling airspeed for a given power setting. Backward movement of the control column raises the elevators and the aircraft's nose  pitches up; forward movement lowers the elevators and the aircraft's nose  pitches down. The force able to be exerted via the elevators is the most significant control force. The  'up' and 'down' terms in pitch are not relative to the horizon but to the original flight path in the aircraft's longitudinal plane.


    A stabilator is an 'all-moving' or 'all-flying' tailplane combining the horizontal stabiliser and elevator  providing similar force with a lesser deflection, thus less drag.  Sometimes used in higher speed light aircraft but rarely in very light aircraft.  There may be some net advantages in mounting the stabiliser and elevators in front of the wing — a canard — but such arrangements are rather rare amongst very light aircraft.


    8.3 Vertical stabiliser and  rudder

    Because of drag and other effects, aircraft perform much better if their longitudinal axis is accurately aligned, in plan view, with the flight path.   If unaligned, the aircraft velocity will have both  a forward component and a  slight lateral component, and the relative airflow — the flight path — will not be aligned with the longitudinal axis. Such bodily sideways (translational) movement along the lateral axis  is called slip or sideslip  or skid. The skid term is generally associated with excess 'bottom' rudder and skidding out in a turn, as a road vehicle might. 


    Thus, some means is required to ensure that if the horizontal direction of the relative airflow  is changed (i.e. the aircraft acquires slip because of a minor disturbance)  then the aircraft will automatically yaw — rotate itself about its normal axis —  to realign its longitudinal axis with the airflow, so that   the sum of all the lateral moments —  fore and aft of the cg —   equals  zero. 


    The long-established means is to use a fin, or vertical stabiliser, mounted at the rear of the aircraft,  that has an  aerofoil section — usually symmetrical — or is just a flat plate.  The fin applies the restoring moment to realign the longitudinal axis with the airflow.  That moment does not realign the aircraft with its original flight path; after restoring alignment with the relative airflow, the aircraft may be aligned with a  different flight path, depending on the amount of original displacement.


    The fin is often angled away from the aircraft's longitudinal axis by a few degrees. This offset creates an  aerodynamic force that compensates for the rotating propeller slipstream applying a force to one side of the fin.


    The rudder is the control surface hinged to the fin and is the lateral plane equivalent of the elevators; though the rudder is operated by the pilot's rudder pedals rather than the control column. Pressure on the left pedal causes the rudder to deflect to the left, so that the fin/rudder act together as  a cambered aerofoil to produce an aerodynamic force that pushes the tail to the right — and consequently the nose swings  left; i.e. the  aircraft  yaws left. (Yaw is an old nautical term associated with the motion of the sea swinging the bow off-course.)  The amount of yaw, at a given airspeed, is dependent on the degree of rudder deflection. (But, of course, it is primarily dependent on the tail moment arm and rudder area.) The aircraft will continue yawing if the rudder deflection is held by the pilot, but as the aircraft turns (i.e. it is rotating about its normal or vertical axis while moving forward), the wing on the outside of the turn must be moving slightly faster than the inner wing and thus generates more lift. The increased lift will raise the outer wing and the aircraft will enter a banked turn, but will tend to  skid out because the bank angle will not be correct for the turn. Only one bank angle will produce the desired radius or rate of turn for a particular airspeed. 


    Note the Breezy's small fin with its relatively  large rudder.  The pilot's feet are on  the pedals  linked to the rudder and he is holding the control column —  linked to the ailerons and the elevators —  with one hand. The other hand is probably holding the engine throttle lever. The rudder initiates yaw about the normal axis; the ailerons initiate roll about the longitudinal axis;  the elevators  initiate  pitch movement about the lateral axis.


    8.4 Control balance

    Aerodynamic balance

    Aircraft designers try to impart a good 'feel' to the controls so that the pilot finds they  are not too 'heavy' or too 'light' to operate through most of the speed range. So, the elevators and rudder are usually fitted with some sort of aerodynamic balance, which puts part of the control surface forward of the hinge line. Such devices might be inset hinge balances, leading-edge balances or control horns that reduce the hinge moments needed to deflect the control surface.


    Mass balance

    Control surfaces  need to be hinged near the leading edge, the centre of their mass will be well aft of the hinge line; i.e. the mass of the control is not statically balanced. That, combined with the necessary elasticity of aircraft structures,   leads to a  control flutter problem.  This might occur with mass unbalanced control surfaces at any speed, but particularly with ailerons at high speed. Flutter has the potential to lead to structural failure. The prime solution to the mechanical unbalance and the flutter problems is for the manufacturer to accurately balance the  mass of the control surface by  inserting weights forward of the hinge line usually within the hinge insets or the control horn. This — known as mass balance — increases the stability of the control surface and ensures that accelerations don't deflect the control surface.


    Notes for scratch-builders
    • You may occasionally come across the terms tail volume and tail volume ratio. The horizontal tail volume is the surface area of the horizontal stabiliser plus elevators multiplied by the length of the moment arm of the horizontal stabiliser measured from the wing MAC quarter chord to the horizontal tail MAC quarter chord. The horizontal tail volume ratio or tail volume coefficient is the tail volume divided by the product of wing area and wing MAC. Tail volume ratio is usually in the range 0.35 to 0.45 for minimum aircraft and 0.45 to 0.55 for aircraft of composite construction — when the units of measurement are feet. The higher the coefficient value, the more stable the aircraft. There is a similar equation for the vertical stabiliser and rudder, but the divisor is the product of wing area and wing span. Such ratios are of interest to an aircraft designer, as there is a linear relationship between tail moment or tail area, and stability — doubling the tail moment or the tail area doubles the static stability and quadruples the dynamic stability. The aspect ratio of the tail affects tailplane pitching moments; for a given area higher aspect ratio produces less induced drag and hence the lift component of the aerodynamic force is higher.


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