SmokeDog's
Note: In previous lessons we’ve presented 2 of the classic
“4 forces of flight”, drag and lift. We will now
move on to thrust, the force which will move our aircraft
forward.
Note a concept
presented in the first diagram on our “How
an Airplane Flies” page. The thrust arrow opposes
drag. This concept is not totally accurate. As we stated in
the “New This Week” article “Need More Engine
Power? Supercharge it!”, In a climb, the thrust arrow
[term  vector] points up slightly. For the purposes of this
lesson, we will ignore this fact.
Those arrows presented
in previous lessons which represent forces are termed “vectors”.
A vector is simply a representation of both direction and
quantity. If you represented a force vector of a person throwing
a ball, weakly at a 45 degree angle up from the ground, you
would draw a vector representation as a line, perhaps 1 inch
long, aiming up at 45 degrees to level. To represent the force
of a person throwing a ball very hard, straight up, you would
draw a vector representation as a line, perhaps 2 inches long,
aiming up. And so … the concept of vectors is simple,
a representation of both direction and quantity. Should you
advance in the study of physics, you will learn that the power
of vectors comes from adding (or subtracting) vectors from
each other in order to determine the resulting motion in terms
of both force and direction.
Thrust
vs. Speed from Car Engines
In this lesson
we will use the thrust and opposing drag vector in order to
determine the resulting force. When you start moving in a
car the resulting force is forward. In the following figure
we assume that the car’s engine will produce constant
thrust, no matter what its speed. (The real thrust varies
with engine RPM, so we are assuming that the car has a variable
speed, constant RPM drive as found on modern hybrid autos.)
The term acceleration
is defined as a change in speed. Acceleration will occur whenever
opposing forces are not equal. You can quantify the amount
of acceleration by subtracting the drag vector line length
from the thrust vector line length.
Once the drag vector
length equals the thrust vector line length, there is no more
acceleration. The car has reached “equilibrium”
speed. You’ve just learned a new term, equilibrium.
Equilibrium exists when all force vectors are balanced. They
cancel each other out.
Note that at 30
MPH the drag vector is only 1/4 the length of the drag vector
at 60 MPH. We just want to reinforce the fact that parasitic
drag increases as the square of velocity. Review the parasitic
drag curve in the next figure. While cars have other forms
of drag, parasitic drag is the major drag factor opposing
speed.
The next figure
is our goodole parasitic drag curve with a constant thrust
line added. The equilibrium speed will be where the drag line
meets the thrust line.

Speed
vs. Parasitic Drag Curve plus a constant power line. Given
constant power, a car will accelerate until power = thrust.
This is termed a point of "equilibrium". 
Thrust
From Aircraft Engines
Piston Engines
Driving Propellers:
The thrust produced
by an aircraft piston engine is illustrated in the following
figure. This figure is for a 260 HP Lycoming brand engine
turning a “constant speed” propeller. A constant
speed prop changes the bite of its blades (term – blade
pitch) in order to keep the RPM and resulting horsepower the
same, no matter what the airspeed. The horsepower may be constant,
no matter what the airspeed, but the thrust produced drops
off quickly with airspeed. This is due to a problem of propeller
efficiency, not engine efficiency.

Illustration
of thrust produced by an aircraft piston engine driving
a constant speed propeller. Thrust is excellent at low
airspeeds but drops off quickly with increasing airspeed. 
Jet Engines:
The next figure
shows the thrust curve of a representative jet engine. The
power drops off as airspeed increases but not as quickly as
the piston engine. At approximately 500 MPH the power decrease
levels off. This is primarily due to the fact that the jet
engine has a big mouth. Air rams into this big mouth to help
increase the pressure needed to produce power.

Illustration
of thrust produced by a jet engine. Thrust reduction curve
is much flatter than the curve for a piston engine/propeller
combination. 
Our last figure
shows both the piston engine and jet engine thrust curve superimposed
onto the aircraft total drag curve. The more powerful jet
engine should be placed further up the chart, but for the
purposes of this lesson, the figure illustrates that, assuming
equal piston power vs. jet power, the piston engine gives
more thrust (and acceleration) at low airspeeds while the
jet engine gives more thrust at higher airspeeds.

Illustration
of piston engine and jet engine thrust (shown in blue)
superimposed over the aircraft total drag curve. The equilibrium
points for top speed are shown as red dots. Which engine
will give better low speed acceleration? Which engine
will give a better top speed? 
We have discussed
thrust, lift and 2 forms of drag. But there are four forces
in balanced flight. Next lesson we will begin a discussion
of weight.
(continued
next week)
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“Simple
Aerodynamics"
Part 5
copyright
1984, 2004, Sublogic Corporation 
