Relative Positions of Inlet to Exhaust Nozzle
The relative positions of the inlet and exhaust nozzle of each engine can affect throttles-only .ight control. The ram drag vector acts through the centroid of the inlet area along the .ightpath and, thus, rotates with respect to the airplane geometric reference system as angles of attack and sideslip change. The gross thrust vector usually acts along the engine nozzle centerline and thus maintains its relationship to the airplane geometric reference system. Ram drag can be a signi.cant percentage of gross thrust, particularly at low power settings where ram drag may approach the magnitude of the gross thrust.
In the pitch axis, positioning the inlet above the engine nozzle centerline is bene.cial so that an increase in throttle, which increases ram drag and gross thrust, will result in a nose-up moment. This bene.t is the case for the B-2 and for center engines on the B-727 and L-1011. If the inlet is below the engine nozzle centerline, an increase
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(b) MD-11 simulation, gear down, .aps up, center engine idle. Figure 9. Concluded.
in thrust causes an undesirable nose-down moment; the F-16 and F-18 are examples of such a con.guration. Podded engines typically have the inlet and nozzle closely aligned and would have a nearly neutral effect, although as angle of attack increases, this effect would become more favorable.
In the yaw axis, the principles are similar. The desirable geometry is to have the engine nozzles outboard of the inlets so that an increase in thrust results in a favorable yawing moment. Unfortunately, this geometry is not the case for many .ghter airplanes with inlets outboard of the engines.
Normal .ight control system operation masks the inlet-nozzle effects to such a degree that crews may be unaware of the effects. In addition, all but the highest .delity simulations usually neglect these effects. For the F-15 PCA .ight test, these effects were quite signi.cant (ref. 1). For the MD-11 airplane, with podded engines, these inlet-nozzle effects are small.
Trim Speed Control
After the normal .ight control surfaces of an airplane are locked at a given position, the trim airspeed of most airplanes is only slightly affected by engine thrust. However, speed can be changed in several ways. In general, the speed needs to be reduced to an acceptable landing speed; this requires developing nose-up pitching moments. Methods for developing nose-up pitching moments include moving the CG aft, increasing the thrust of low-mounted engines, or decreasing the thrust of high-mounted engines. Extending the landing gear often results in decreased trim speed because an increase in engine thrust is required to maintain the desired .ightpath. Lowering .aps increases lift, which would tend to reduce trim speed. But .ap extension often results in a nose-down pitching moment; so the overall effect is con.guration dependent.
Trim speed is also affected by changes in weight. As weight is reduced (such as by burning or dumping fuel), assuming that the longitudinal CG remains constant, the lift remains constant, and the airplane tends to climb. To maintain level .ight, the throttle setting must be reduced until lift and weight are again in balance.
On the MD-11, several ways to control speed are available; .aps and stabilizer trim require hydraulic power, which may not always be available in an emergency. The center engine can be used as a moment-generating device to change angle of attack, and hence to control speed; increasing center engine thrust has a strong nose-down pitch effect and increases the trim speed. Starting with all throttles equal, increasing thrust on the wing engines and decreasing thrust on the tail engine to idle thrust reduces speed by 20 to 30 kn. Lowering the landing gear with the alternate gear extension system (which leaves the wheel wells open and the landing gear doors exposed) reduces speed an additional 15 to 17 kn in part because of the increased thrust required on the wing engines (if the tail engine is not used) to maintain a desired .ightpath. Weight reduction also reduces trim speed.
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本文链接地址：Development and Flight Test of an Emergency Flight Control System Using Only Engine Thrust on an MD-11 Transport Airplane(16)
