Download TE 336-Engineering II

-DRAG

Energy is neither created or
destroyed…visited earlier in the semester.

How does a car as an energy system lose
energy?

FRICTION: Where does friction exist in a car?

Apply this to the simple technology system of
a dragster.

The friction is not totally lost, it leaves the
system, yes, but in the form of heat.
-DRAG

Thrust is created by the jet engines and
must overcome the retarding force
known as drag/wind resistance.

There are three main categories of drag
– induced, interference and parasite.

Total Drag = D + D + D
ind
inf
par
-DRAG

Induced drag is that drag which is attributed
to the production of lift.

The magnitude of induced drag decreases as
the airspeed increases.

Water skiers experience this phenomenon
when being pulled by a boat. Starting at rest,
and mostly submerged, the skier feels a
massive amount of drag while being pulled
through the water. As the speed increases,
the skier skims over the surface of the water
and the drag is significantly reduced.
-DRAG

Because of the high speed at which jet
transport aircraft cruise, induced drag makes
up less than one-quarter of the total drag.

Parasite drag is made up of three
components.
– Skin Friction
– Profile Drag
– Interference Drag
-DRAG

Skin friction is the resistance air
molecules encounter as they flow over
the aircraft's surface.

Notice the smooth seams of the
aircraft's skin; this design minimizes the
skin friction.
-DRAG

Profile drag, also called form drag, is
caused by the air pressure in front of
the aircraft.

Fast moving aircraft are designed to be
sleek to minimize the frontal area and
the profile drag.
-DRAG

The third component is interference
drag.

If the aircraft's components disrupt the
air's smooth, or laminar, flow, the cost
is paid in interference drag.

The landing gear is raised into the body
of the aircraft after takeoff to decrease
this drag.

Passengers can feel the airflow smooth
when the landing gear is retracted.
-DRAG

Interference drag is also demonstrated by
cross winds.

As a body travels through motionless air, the
resultant drag is the fluid particles hitting
against exposed surfaces of the object.

If there were a cross wind, that portion of the
total drag factor would be interference.

The velocity in which the cross wind
“interferes” with the object causes greater
drag.
-DRAG

The thin air at high altitudes reduces
parasite drag.

Flying at 40,000 feet, the air has onefourth the density as at sea level.

If jet transport aircraft could fly as high
as space shuttles, they could shut down
the engines and coast, since no drag
exists if there is no air around the craft.
-DRAG

Induced drag decreases and parasite drag increases
with airspeed.

Total drag is the sum of the induced and parasite
drag components.

What happens in the wind tunnel?

The slowest speed an aircraft can fly is known as the
stall speed.

Total drag is very high at the stall speed because of
the induced component. As the airspeed increases,
the drag decreases to a minimum point.

As the airspeed continues to increase above this
point, the parasite drag causes the total drag to
increase.
-DRAG







Total Drag = DInd + DInf + DP.
Principle of Parasite drag is such that Parasite Drag
quadruples as Wind Speed doubles.
So, when you get your dragster to the wind tunnel,
you need to test at 25 mph, record the drag.
Turn wind speed up to 50, record drag.
If initial drag is 5 grams, then the second reading is
20. If there is a breach of this principle then there is
something wrong.
Where is the additional drag coming from?
Examine our equation: Total Drag = DInd + DInf +
DP
-DRAG


So it must be coming from induced drag.
Induced drag is that drag which is attributed
to the production of lift.

How did I get lift?

Angle of attack.

Make sure the bottom of your dragster is
parallel with the ground.

By reducing your angle of attack and
controlling the shape of your dragster, you
minimize the production of lift.
-DRAG

Aircraft designers are making airliners
more and more efficient.

Reducing the total drag of an aircraft
allows it to travel faster and use less
fuel.

Aeronautical engineers know that air
resistance is a real drag!