Let's pick up right where the flowfield changes around the aircraft start to… — Page 98, Lesson 113

Let's pick up right where the flowfield changes around the aircraft start to bite. When you're in ground effect, the downwash behind the wing is reduced, and that changes what the tailplane sees. The tailplane is sitting in that downwash, and with less downwash, the effective angle of attack of the tailplane increases. That's the key: the tailplane's down load is reduced, which produces a nose-down pitching moment. So entering ground effect tends to push the nose down. Now, there's a second effect that's easy to overlook. Because the flowfield around the whole aircraft changes, the position error changes. Position error is the difference between the pressure the static port actually senses and the true ambient pressure. In the majority of cases, when you enter ground effect, the local pressure at the static port increases, and that causes both the ASI and the altimeter to under-read. So your instruments are telling you you're slower and lower than you actually are. Now let's flip it around and look at leaving ground effect. The effects of climbing out are generally the opposite of entering. So consider an aircraft climbing out of ground effect while maintaining a constant CL and a constant IAS. As it climbs out, the CL will reduce, and the induced drag, CDi, which is the thrust requirement, will increase. To maintain that same CL, the aircraft will need an increase in angle of attack. The downwash increases again, and that generally produces a nose-up pitching moment. So the pilot's pitch input may need to be reduced — less elevator back-pressure. The position error changes again, but in the opposite direction: in the majority of cases, the local pressure at the static port decreases, causing the ASI and altimeter to over-read. Here's the dangerous one. It is possible to become airborne in ground effect at an airspeed and angle of attack which would, after leaving ground effect, cause the aircraft to settle back on to the runway. That's why it's vitally important that correct speeds are used for take-off. And because of that nose-up pitching moment, you can get an inadvertent over-rotation and a tail strike. So the very thing that helps you get airborne in ground effect can bite you the moment you climb out. Let me show you the downwash picture so you can see what the tailplane is dealing with. Now let's pull the whole chapter together with the summary. Three major factors influence the production of the required lift force. First, dynamic pressure, which is your IAS. Second, pressure distribution, which comes from the section profile and the angle of attack. Third, wing area, which is S. To provide a constant lift force, each IAS corresponds to a particular angle of attack. And the angle of attack at CLMAX is constant — that's the angle of attack at maximum lift coefficient. A higher aircraft weight requires an increase in lift force to balance it, so an increased IAS is needed to provide the greater lift at the same angle of attack. As altitude increases, a constant IAS will supply the same lift force at a given angle of attack. So your IAS is your friend here — it's the thing that stays constant with altitude for a given lift. Now, wing thickness. A thinner wing will generate less lift at a given angle of attack, and have a higher minimum speed. But a thinner wing can fly faster before shock wave formation increases drag. And a thinner wing requires high lift devices to have an acceptably low minimum speed. So there's a trade-off: thin is fast, but you need flaps and slats to make it landable. The Lift/Drag ratio is a measure of aerodynamic efficiency. And contamination of the wing surface, particularly the front 20% of the chord, will seriously decrease aerodynamic performance. That's the leading edge region — keep it clean. Finally, wing tip vortices. They decrease overall lift production. They increase drag. They modify the downwash, which changes the effective angle of attack of the tailplane — that's the thing we started with. They generate trailing vortices which pose a serious hazard to aircraft that encounter them. They affect the stall characteristics of the wing. And they change the lift distribution. The sudden full effects of vortices, or their absence, must be anticipated during take-off and landing — because that's exactly when you transition into and out of ground effect, and the vortices suddenly appear or disappear. Let me show you the increased effective angle of attack picture so you can see how the tailplane reacts.