Lift 5
5 Lift
Conclusion
Increasing downwash (G to D) gives a decrease in tailplane (effective) angle of attack and decreasing downwash (D to G) gives an increase in tailplane (effective) angle of attack.
It is necessary to understand the effect of changing downwash on tailplane angle of attack, but it is vital to understand the influence of downwash on aircraft pitching moment.
Entering Ground Effect
Consider an aircraft entering ground effect, assuming that a constant CL and IAS is maintained. As the aircraft descends into ground effect the following changes will take place:
•The decreased downwash will give an increase in the effective angle of attack, requiring a smaller wing angle of attack to produce the same lift coefficient. If a constant pitch attitude is maintained as ground effect is encountered, a “floating” sensation may be experienced due to the increase in CL and the decrease in CDi (thrust requirement),
•Figure 5.15 & Figure 5.26. The decrease of induced drag will cause a reduction in deceleration, and any excess speed may lead to a considerable “float” distance. The reduction in thrust required might also give the aircraft a tendency to climb above the desired glide path, “balloon”, if a reduced throttle setting is not used.
•If airspeed is allowed to decay significantly during short finals and the resulting sink-rate arrested by increasing the angle of attack, upon entering ground effect the wing could stall, resulting in a heavy landing.
•The pilot may need to increase pitch input (more elevator back-pressure) to maintain the desired landing attitude. This is due to the decreased downwash increasing the effective angle of attack of the tailplane, Figure 5.23. The down load on the tail is reduced, producing a nose-down pitching moment.
•Due to the changes in the flowfield around the aircraft there will be a change in position error which may cause the ASI to misread. In the majority of cases, local pressure at the static port will increase and cause the ASI and altimeter to under-read.
Figure 5.26
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Lift 5
Leaving Ground Effect
The effects of climbing out of ground effect will generally be the opposite to those of entering. Consider an aircraft climbing out of ground effect while maintaining a constant CL and IAS. As the aircraft climbs out of ground effect the following changes will take place:
•The CL will reduce and the CDi (thrust requirement) will increase. The aircraft will require an increase in angle of attack to maintain the same CL.
•The increase in downwash will generally produce a nose-up pitching moment. The pitch input from the pilot may need to be reduced (less elevator back-pressure).
•Position error changes may cause the ASI to misread. In the majority of cases, local pressure at the static port will decrease and cause the ASI and altimeter to over-read.
•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 It is therefore vitally important that correct speeds are used for take-off.
•The nose-up pitching moment may induce an inadvertent over rotation and tail strike.
Lift 5
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5 Lift
Summary
Three major factors influence production of the required lift force:
•Dynamic Pressure (IAS).
•Pressure Distribution (section profile & angle of attack).
•Wing Area (S).
5 To provide a constant lift force, each IAS corresponds to a particular angle of attack.
Lift
The angle of attack at CLMAX is constant.
A higher aircraft weight requires an increase in lift force to balance it; 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.
A thinner wing will generate less lift at a given angle of attack, and have a higher minimum speed.
A thinner wing can fly faster before shock wave formation increases drag.
A thinner wing requires high lift devices to have an acceptably low minimum speed.
The Lift/Drag ratio is a measure of aerodynamic efficiency.
Contamination of the wing surface, particularly the front 20% of the chord, will seriously decrease aerodynamic performance.
Wing tip vortices:
•Decrease overall lift production.
•Increase drag.
•Modify the downwash which changes the effective angle of attack of the tailplane.
•Generate trailing vortices which pose a serious hazard to aircraft that encounter them.
•Affect the stall characteristics of the wing.
•Change the lift distribution.
The sudden full effects of vortices or their absence must be anticipated during take-off and landing.
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Answers from page 77 |
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CL |
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1.532 |
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CLMAX |
150 Knots |
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5 |
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Lift |
0.863 |
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200 kt |
STALL |
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0.552 |
250 kt |
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300 kt |
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0.384 |
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ANGLE OF ATTACK ( DEGREES ) |
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Figure 5.27
a.How many newtons of lift are required for straight and level flight? 588 600 N.
b.Calculate the airspeed in knots for each highlighted coefficient of lift. As above.
c.What is the lowest speed at which the aircraft can be flown in level flight? 150 kt.
d.What coefficient of lift must be used to fly as slowly as possible in level flight? CLMAX
e.Does each angle of attack require a particular speed? Yes.
f.As speed is increased, what must be done to the angle of attack to maintain level flight?
Angle of attack must be decreased.
g.At higher altitude air density will be lower; what must be done to maintain the required lift force?
Increase the True Airspeed (TAS).
h.At a constant altitude, if speed is halved, what must be done to the angle of attack to maintain level flight?
Increased so that CL is four times greater.
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