Page 90 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 77

Wings 77

Choosing the correct wingtip is a matter of compromises, which
include aerodynamic performance, structural loads, manufacturing,
and (possibly most important) marketing. In the early days rounded
tips were simple to build because all the builder had to do was bend a
rod from the leading edge around to the trailing edge. Since loads are
small at the wingtip, little internal structure was needed. Many of the
pre-WWII airplanes have rounded wingtips. But, squared-off tips are
aerodynamically more efficient. The squared-off wingtip better restricts
the passage of high-pressure air from the lower surface to the upper
surface. Any high-pressure air leaking to the upper surface
The Boeing 777 was designed
leads to lower aerodynamic efficiency. Thus, most of today’s
with folding wingtips. No
airplanes have simple squared-off tips.
customer ever ordered them.
Winglets
Today, many aircraft sport winglets, which are wingtips turned vertically,
as shown in Figure 3.18. While the winglets point up on wings, it should
be noted that winglets on horizontal stabilizers point down since the
horizontal stabilizer pulls down. Winglets go one step further in pre-
venting the passage of high-pressure air from flowing around from the
lower surface to the upper surface. In essence, they provide a block. The
result is that the wing can carry a finite amount of lift all the way to the
tip. As we discussed in Chapter 2, the efficiency of a wing increases with
length. Winglets increase the effective length of the wing and thus
increase the wing’s efficiency without increasing its length. Winglets
have become the golden child of all airplane sales representatives.
However, winglets do come with disadvantages. The net effect of
the winglet is roughly equivalent to laying the winglet flat, which
would result in extra span and wing area. So, one could achieve the
same wing performance by doing exactly that, laying the winglet flat.
The added tip loading requires a stronger and heavier wing. So,
winglets cannot be retrofitted to existing airplanes without changing
operating conditions to lower the wing loading. But, on a new design,
winglets can help make the wing more efficient by reducing the
induced power required for a given lift, just as extra span would.
Nevertheless, one of the biggest reasons for the preponderance of
winglets on today’s business jets is that they are considered very sexy.