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Unit 9 · Lesson 9-3
Load Paths, Mounts and the Secondary Air System
1h 35m1300 XP
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Load Paths, Mounts and the Secondary Air System

Load-bearing frames and force transmission, statically determinate mounts and thrust links, seals and the secondary air system, and the cost of bleed

Amsterdam, 4 October 1992
Five minutes after take-off from Amsterdam, El Al Flight 1862, a Boeing 747 freighter, loses its No. 3 engine. It separates from the right wing, shoots forward, falls back and strikes the No. 4 engine, tearing it off as well. The aircraft crashes into apartment blocks in the Bijlmermeer: the four people on board and 43 on the ground are killed (El Al Flight 1862). A 747's engines hang from fuse pins designed to break cleanly under an extreme overload, so that an engine can go without tearing open the wing and its fuel tank. These pins had fatigue cracks before they failed. Every load an engine makes has to reach the wing through a handful of parts like these; this lesson follows the path.
The course deck lists what the load-bearing system must achieve: enough stiffness and strength; deformations that stay coordinated under uneven heating, so the clearances between rotors and stators survive; good vibration behaviour; and reliable work over a long life. Loads from the rotors' bearings cross the gas path through load-bearing frames — struts or vanes that carry them to the outer casings. Engines in service use 2–4 such frames; the one that carries the main mounts is the main frame. In turbojets the casings mostly share the load between inner and outer paths (a hybrid scheme); turbofans mostly carry it in parallel paths.
💡Mounts
The mounts are where the engine meets the aircraft. Main mounts carry thrust, weight, gyroscopic moments in manoeuvres and inertia loads, and sit near the centre of gravity, on a strong, stiff and cool part of the casing; auxiliary mounts carry part of the weight and moments. Mounts should be few, for quick engine changes, and only one point is fixed: the others must let the engine grow as it heats. Six constraints — three forces, three moments — make a statically determinate mount: its link forces follow from equilibrium alone, whatever the engine's thermal growth (course deck).
Derivation
Six links, six equations
Link jj acts at point pj\mathbf p_j along unit vector dj\mathbf d_j with unknown force fjf_j. Equilibrium of forces and of moments about any origin, for an external load (F,M)(\mathbf F, \mathbf M):
∑jfj dj=−F,∑jfj (pj×dj)=−M.\sum_j f_j\,\mathbf d_j = -\mathbf F, \qquad \sum_j f_j\,(\mathbf p_j \times \mathbf d_j) = -\mathbf M.
Six scalar equations in six unknowns, Af=−[F;M]A\mathbf f = -[\mathbf F; \mathbf M], with columns [dj;pj×dj][\mathbf d_j; \mathbf p_j \times \mathbf d_j]. If AA has full rank the mount is statically determinate; if not, some load has no path.
📐Mount loads
A 4000 kg engine on two vertical links and a side link at the front, a vertical pair and a side link at the rear, and thrust links (Lab 1):
  • Take-off: the thrust links carry the whole 134 kN; the verticals share the weight and the thrust's moment.
  • 3 g pull-up at a pitch rate of 0.2 rad/s: the fan's gyroscopic moment, 27.6 kN·m, appears as ±11 kN in the side links.
  • Fan blade out, 400 kN sideways at the fan: the rear verticals see −700 and +719 kN — the case that sizes the mounts.
💡The secondary air system
Not all the air that enters the core makes thrust. The secondary air system takes air from the compressor to cool the turbine (Lesson 6-4), to pressurise the seals of the bearing chambers so that oil stays in and hot gas stays out, to balance the rotors' axial loads (Lesson 9-1), to control tip clearances, and to supply the aircraft with bleed air. Every kilogram of it has been compressed, at a cost in fuel, and the seals that meter it — labyrinths, brush seals — decide how much is lost.
m˙=Cd A p0R T01−(pN/p0)2N−ln⁡(pN/p0)\dot m = C_d\,A\,\frac{p_0}{\sqrt{R\,T_0}}\sqrt{\frac{1 - (p_N/p_0)^2}{N - \ln(p_N/p_0)}}

Martin's formula for a labyrinth seal of N teeth and clearance area A between upstream pressure p0 and downstream pressure pN.

📐Labyrinth leakage
A 300 mm seal at take-off, from compressor-delivery pressure (3.11 MPa) into a cavity at half of it (Lab 2):
  • Clearance 0.5 mm, 8 teeth: 0.59 kg/s — 1.2 % of the core air.
  • Rubbed open to 1.0 mm: twice as much. Matching the new seal's leakage with more teeth would take 35 of them: clearance matters far more than teeth.
📐The cost of bleed, and the 787
The Boeing 787's architecture is bleedless: electrically driven compressors replace bleed air for the cabin (the 787). At cruise, for 0.5 kg/s of cabin air per engine (Lab 3):
  • Bled from the compressor: TSFC +2.86 % — that air carries 232 kW of compressor work, much of it then thrown away cooling it for the cabin.
  • From an electric compressor fed by the engine's generator: 46 kW off the shaft plus its own intake's ram drag: TSFC +1.08 %.
All... must be built with due reference to durability, convenience, and beauty.
– Vitruvius, De architectura, Book I (Wikiquote)
⚠Qantas 32: a bearing chamber's oil
On 4 November 2010 an A380's Trent 900 suffered an uncontained failure of its intermediate-pressure turbine disc. The cause was a fatigue crack in a stub pipe feeding oil to the HP/IP bearing structure, made too thin on one side by a misaligned counterbore: the oil leaked, caught fire, and the disc failed. All 469 people on board survived (Qantas Flight 32). The secondary air and oil systems are part of the structure's integrity.
💡Where you will use this
Mount and load-path analysis is the same for any heavy machine hung from a structure — engines on pylons, rotors on helicopters, generators on ships. Unit 10's geared fans and open rotors bring new loads: gearbox torque reactions and, for open rotors, blades with no case around them at all.
📚Further reading
  • Rolls-Royce, The Jet Engine (5th ed., Rolls-Royce plc, 1986), on internal air systems and engine mounting.
  • H. I. H. Saravanamuttoo et al., Gas Turbine Theory (7th ed., Pearson, 2017).
  • El Al Flight 1862 and Qantas Flight 32 on Wikipedia.
↓But there is a problem
The engine is complete: it makes thrust, matches its parts, holds its rating, changes speed safely, and holds together. The engines being designed now look different — three streams, geared fans, blades with no case, hydrogen in the tanks. Why, and what do they change?