Let's pick up with the tail unit — the empennage — Page 28, Lesson 31

Let's pick up with the tail unit — the empennage. I want to walk you through the different designs and the structural principles that govern them. First, let me finish the thought on flutter, because it directly affects how we design these surfaces. Flutter must not occur within the normal flight operating envelope of the aircraft. That's a hard requirement. We prevent flutter by mass balancing control surfaces — that means we alter the moment of inertia of the surface, which changes its period of vibration. In practical terms, we move the control surface's centre of gravity closer to the hinge line. That's the key: the closer the C of G is to the hinge, the less tendency the surface has to oscillate. Now, a poorly maintained aircraft is a real hazard here. Excessive control surface backlash — that's play or free movement in the linkages — or excessive flexibility in the structure can mean flutter occurs at speeds below the limit airspeed. So maintenance standards directly protect against flutter. For the mainplanes themselves, we can also prevent flutter by using the engines as mass balances. We mount them on pylons forward of the wing leading edge. That forward placement acts as a counterweight, damping the wing's tendency to flutter. Now let's move to the stabilizing surfaces. The empennage — that's the tail unit — comes in several designs: conventional, T-tail, H-tail, and V-tail. Look at Figure 1.25 to see these configurations. The tail units provide, in most cases, the longitudinal and directional stability, and the means of longitudinal control. Some aircraft instead get their longitudinal stability and control from foreplanes — those are canards, mounted forward of the wing. Let me break down the surfaces. The horizontal surfaces are known as the tailplane or horizontal stabilizer. They provide longitudinal stability by generating upwards or downwards forces as required. The vertical surface — the vertical stabilizer or fin — generates sideways forces as required. For control: longitudinal control comes from the elevators, or from a moving tailplane. Directional control comes from the rudder. Both the tailplane and the fin are subject to both bending and torsional stresses — bending from the lift forces, torsion from the twisting loads. Structurally, the tail unit components are generally smaller versions of the mainplanes. They use the same construction: spars, ribs, stringers, and skin. On some aircraft, they may also be sealed to provide fuel tanks — particularly those used for longitudinal and/or mach trim. That's a clever use of the structure. They also use the same basic materials: aluminium alloys, composites with honeycomb structures, or high density expanding foam for control surfaces. The foam and honeycomb give greater stiffness at lower weight. Take a look at Figure 1.26 to see the empennage layout. So the key takeaway: the tail unit mirrors the wing's structural philosophy — spars, ribs, stringers, skin — but on a smaller scale, and it must handle both bending and torsion while providing stability and control.