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Guest Lecturer
"SmokeDog" Stu Moment
Stu Moment is an air show performer who also participates in many aviation education activities for kids and adults.
SmokeDog's Airshow Site

“Simple Aerodynamics"
Part 5
copyright 1984, 2004, Sublogic Corporation 

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 good-ole 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