PPRuNe Forums - View Single Post - Thrust on during flare...Q for AIRBUS test pilots...
Old 26th Mar 2014, 00:00
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FullWings
 
Join Date: Dec 2003
Location: Tring, UK
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I think this discussion is becoming heated because we are talking about a specific situation, i.e. within seconds of the flight path intersecting with the ground.

In such a situation, we are relying much more on the short term effects of control inputs rather than the eventual steady state you would get in unaccelerated flight. This is a dynamic situation.

If you wish to reduce the rate of descent before touching down (a usual requirement), you need to apply a significant force for a period of time to the mass of the airframe such as to reduce the component of its velocity normal to the plane of the runway. How to do this in a jet aircraft...?

1) Add more thrust. As common airliners in a normal approach attitude have a thrust vector almost tangential to the surface, most of the applied force will go towards accelerating the airframe in the direction of flight. On an aircraft with underslung engines, a secondary effect will be a pitch-up moment but this will be mostly countered by the FBW on aircraft like the 777 and modern Airbuses. We are left with the other secondary effect of thrust, which is to increase the groundspeed over time (and hopefully the airspeed with it). The increase in lift generated from a few knots of extra airspeed (a few %) is not enough to significantly reduce the RoD during the time available before touchdown.

2) Increase the angle of attack. On a typical airfoil, the coefficient of lift could go up by c.25% for a c.3deg increase in AoA. This would result in a significant normal acceleration, reducing the RoD markedly over a short space of time.

On a non-FBW, low engined jet, applying thrust has the secondary effect of raising the nose, if not countered by the elevator. At this point the pilot has ceased to control the pitch through normal means and is relying on that secondary effect to provide an appropriate AoA to reduce the RoD. It is a technique but not one that I would recommend due to the extra delay in the feedback loop and the possibility of adding significant unwanted speed just before touchdown.

On the 777, adding more thrust just drives you faster towards where you were going before. Manual go-arounds often catch out people the first time when converting from conventional types as you have to deliberately pitch the aircraft positively into a go-around attitude after pressing TO/GA. Eventually, the 777 will pitch up of its own accord as the built in speed stability kicks in but it is a rather lengthy process and involves significant height loss before climbing.

I suppose the difference in the two methods near the ground is that by using pitch for RoD and power for airspeed, you are directly controlling all the important variables: the flightpath (touchdown point) and speed (runway used to stop). By relying on secondary effects, you may have to accept a significant deviation in one in order to make the other what you want.
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