EGH167 Hands-on Lab

## Lab 4: Aerodynamics And Propulsions

#### Introduction

##### Background

Airplanes are just one type of machine that uses pressure and propulsion in order to function. The design of aircraft and spacecraft centers on a detailed understanding of pressure and how it is related to other engineering concepts. Lift, drag, engine propulsion, and speed measurements are all based on pressure and its use.

##### Purpose

The purpose of this lab is to familiarize you with the properties of propulsion, Thrust, and Impulse.

##### Basic Principles

In this lab write-up, we will cover some basic principles behind:

1)            Pressure in flight,

2)            Propulsion,

3)            Thrust, and

4)            Impulse.

##### Lab Experience

The lab experience will encompass:

1)            Using data acquisition to collect data from a model rocket engine, and

2)            Building Airfoils.

#### Aerodynamics: Basic Concepts

##### Pressure in Flight

An airfoil is a device that alters the air pressure around its surface, and in doing so causes a net force to act on the object. As air blows across an airfoil (which is the term for the shape of an airplane wing) the air on the top surface of the wing is made to move faster than the air on the bottom surface. The speed of the air is inversely related to pressure; as speed increases the pressure decreases. The pressure on each side of the wing is pushing on the wing, since the pressure is less on the top surface of the wing, the air pushes down on the wing less than the air pushes up on the wing thus producing a net force pushing the airfoil up.

Racing cars use this same principal, but in reverse, cars get better traction if they are heavier, but they go slower if they are heavier. To solve the traction problem of light, fast cars, an airfoil is used to push the car down onto the track, and in doing so, simulate a heavier car.

The aerodynamics of airfoils and other related devices are a great deal more involved than the simple explanations stated. The characteristics of the flow are important: turbulent flow (erratic and choppy) or laminar (smooth) create much different results. Also the surface properties, the chemical makeup of the air, and many other factors influence the performance of the airfoils.

##### Propulsion

Propulsion puts rockets in space, moves jet airliners, and also causes garden hoses to flop around aimlessly when no one is holding on to them. Propulsion causes objects to move by building up pressure and then expelling the pressure to create a thrust that pushes an object along.

A rocket engine burns fuel and creates high pressure gases which are forced out a narrow opening. When the hot, high pressure gasses are expelled, they are at a high rate of speed and while the gases are pushed out, the gases are pushing back on the rocket (equal and opposite forces). In a sense, it is this force pushing back on the rocket that causes the rocket to move. (Actually this can be quite complex, and we've somewhat simplified it here.)

When we talk about propulsion, we are often interested in discussing just how much force our rocket (or jet) engine can develop, and just how much work it is capable of doing. This is just the same as when we talk about a car, and wonder what its engine size is (which we assume is related to acceleration) and its fuel tank size and fuel economy (which we assume is related to total distance it can travel between fill-ups).

##### Thrust

Thrust in propulsion is analogous to the engine size in a car. Thrust is defined as the amount of propulsive force generated by the engine. For example, a jet engine might be capable of generating 50,000 pounds of thrust, which is the equivalent of pushing on the airplane with a force of 50,000 pounds (that's a lot!).

Thrust is often expressed in the form of a thrust curve. You can see an example of a thrust curve in the figure above (left side). The thrust curve describes how an engine generates force over time. Often, a thrust curve also shows the average thrust, which is the average force generated by the engine over the time it was firing (above, right side). In calculus, you will (or already have) learned that the area under the thrust curve will be equal to the area under the average thrust line (as shown above).

##### Impulse

Impulse is analogous to the range between fill ups for a car (this is stretching the analogy very thin!). It is defined as the total change in momentum for the system.(Remember that momentum=m*v ... m=mass, v=velocity) It turns out that impulse can also be calculated by integrating the area under a thrust curve. This is a very important result because if we know the impulse a rocket can generate, we can then figure out how much it's momentum will change. Note that if the rocket is being acted on by gravity, we must take gravitational forces into consideration.

#### LAB EXPERIENCE

Make sketches of equipment used in class; include them in your lab write-up.

##### Rocket Engine

A data acquisition system is used to collect data in order to obtain the thrust over time curve of a given model rocket engine.

q          Record the specifications of the model rocket  engine to be tested.

q          Attach an igniter to the model rocket engine as directed.

q          Place the model rocket engine on the test stand.

q          Connect the igniter to the ignition circuit.

q          Make sure that the sensor is connected to the signal conditioning. module, and that the power supply is ON and properly set to 20 Volts.

q          The data acquisition program should be started at the same time the model rocket engine is ignited. Stop the program as soon as the engine runs out of fuel. The data is saved automatically as an ASCII file.

q          Copy the file onto a diskette for future analysis. Use the given calibration chart  to convert raw data (volts) into Thrust (Newtons).

NOTE:  the data is acquired at a rate of is 100 samples/second.

Question:

1.      How high would the rocket have traveled if it were launched straight up? Be sure to state your assumptions and show your work.

##### Airfoil Design

Make two airfoils, the one you have a model for, and one you design yourself. Follow the instructions on the following page to make the airfoil. Mount your airfoils on a pan balance. Observe the flow characteristics of the airfoils. Measure lift (in grams) for at least three different  angles of attack. Tabulate your results.

Questions:

2.      Comment on what you discovered from lab. From your observations, what are the characteristics of a good airfoil?

3.      If by increasing your angle of attack you can increase the lift force on the airfoil, why do you think airplanes do not typically fly at high angles of attack?

#### LAB REPORT

##### Format

·  Lab reports must be done in Teams

·  Follow given lab report format.

·  Maximum 4-5 pages (including figures and tables)

##### General Guidelines

·  Cover page: Include your names, course section, lab experiment, and date.

·  Follow standard lab report format.

·  Include a brief introduction/background.

·  Describe the experiment. Show a sketch of the test setup. Use labels.

·  Write a program to calculate average thrust, impulse, and delay time for your model rocket. Use the calibration chart to convert the raw data (volts) into Thrust (Newtons).  Include equations and sample calculations. Document your program.

·  Tabulate your results and compare to those supplied from the manufacturer.  Use consistent units.

·  Write a program to generate a graph of the rocket thrust force (Newton) over time (seconds) using the acquired data.  Document your program.

·  Make sketches of the two airfoils showing the angle of attack. Indicate the difference in lift between the two designs.

·  .Include Analysis of Results/Conclusions.

#### APPENDIX: BUILDING AN AIRFOIL

##### Procedure

Building an Air FoilUse this procedure for the provided design.  Follow the same steps when making your own design.

##### Step One
1. Design an air foil and make a pattern for it.  The template is given.
2. Lay the pattern on the balsa wood and cut out five (5) pieces using an exact-o-knife.

Hint: By taking advantage of the edges of the balsa wood, gluing will be easier. All pieces must be identical.  Smooth the edges of all the pieces.

##### Step Two

1. Lay out a strip of paper (4" x 10").
2. Glue down the bottom of all the air foil pieces 1" apart.

Hint:    Place the small end of the air foil at the edge as shown here.

1. Wait a few minutes to allow glue to set.

##### Step Three

1. Place glue over the top edge of the air foil pieces.
2. Bend the strip of paper over the top.

Hint: Glue down a small section at a time and wait for the glue to dry as you work your way toward the end.

##### Step Four

Seam must be on the lower end here!

Your completed air foil should resemble this drawing. Make sure the glue is dry before testing.