9

extend farther out on the wing. Since the eﬃciency of a

wing increases with length, this gives increased eﬃciency .

One ca v eat is that winglet design is tricky and winglets

can actually b e detrimental if not prop erly designed.

XI I. GR OUND EFFECT

Another common phenomenon that is misundersto o d

is that of ground eﬀect. That is the increased eﬃciency

of a wing when ﬂying within a wing length of the ground.

A lo w-wing airplane will exp erience a reduction in drag

b y 50% just b efore it touc hes down. There is a great

deal of confusion about ground eﬀect. Many pilots (and

the F AA VFR Exam-O-Gram No. 47) mistak enly believe

that ground eﬀect is the result of air being compressed

b et ween the wing and the ground.

T o understand ground eﬀect it is necessary to hav e an

understanding of up w ash. F or the pressures in volv ed in

lo w sp eed ﬂigh t, air is considered to b e non-compressible.

When the air is accelerated o ver the top of the wing and

do wn, it must b e replaced. So some air must shift around

the wing (below and forw ard, and then up) to compen-

sate, similar to the ﬂo w of w ater around a canoe paddle

when rowing. This is the cause of upw ash.

As stated earlier, upw ash is accelerating air in the

wrong direction for lift. Thus a greater amount of down-

w ash is necessary to comp ensate for the upw ash as well

as to provide the necessary lift. Th us more work is done

and more pow er required. Near the ground the up wash

is reduced b ecause the ground inhibits the circulation of

the air under the wing. So less do wnw ash is necessary to

pro vide the lift. The angle of attac k is reduced and so is

the induced p ow er, making the wing more eﬃcient.

Earlier, w e estimated that a Cessna 172 ﬂying at 110

knots m ust divert ab out 2.5 ton/sec to provide lift. In

our calculations we neglected the upw ash. F rom the mag-

nitude of ground eﬀect, it is clear that the amount of air

div erted is probably more like 5 ton/sec.

XI I I. CONCLUSIONS

Let us review what we ha ve learned and get some idea

of how the ph ysical description has given us a greater

abilit y to understand ﬂight. First what hav e we learned:

• The amoun t of air diverted by the wing is propor-

tional to the sp eed of the wing and the air density .

• The vertical velocity of the div erted air is prop or-

tional to the sp eed of the wing and the angle of

attac k.

• The lift is prop ortional to the amoun t of air di-

v erted times the v ertical velocity of the air.

• The p ow er needed for lift is prop ortional to the lift

times the vertical velocity of the air.

No w let us lo ok at some situations from the physical

p oin t of view and from the p ersp ective of the p opular

explanation.

• The plane’s sp eed is reduced. The physical view

sa ys that the amount of air div erted is reduced so

the angle of attack is increased to comp ensate. The

p o wer needed for lift is also increased. The popular

explanation cannot address this.

• The load of the plane is increased. The ph ysical

view says that the amoun t of air diverted is the

same but the angle of attac k must b e increased to

giv e additional lift. The p ow er needed for lift has

also increased. Again, the p opular explanation can-

not address this.

• A plane ﬂies upside down. The physical view has

no problem with this. The plane adjusts the angle

of attac k of the inv erted wing to give the desired

lift. The popular explanation implies that in verted

ﬂigh t is imp ossible.

As one can see, the popular explanation, whic h ﬁx-

ates on the shap e of the wing, may satisfy man y but it

do es not give one the tools to really understand ﬂight.

The physical description of lift is easy to understand and

m uch more p ow erful.