Constructing a wing

[phil813]

Hello Science Buddies,
I am planning on doing the project "The Effects of a Propeller's Chord Length on Its Efficiency." In the project, instead of buying the kit, i have decided to just make my own propeller instead. However, I am aware that many factors such as material, and the way it is constructed can affect my output. So, my question is, do any of you guys have tips on how to effectively construct the propellers and adjust their chord length (ex: materials used, and process)? Thanks!

[rmarz]

phil813 - I've never experimented with this project, so I don't know what the results are when following the procedure as described. As it appears that only one variable is being evaluated, chord length, you can approach the task of creating a test bed pretty simply. You might construct a propeller hub with two narrow slots cut at at fixed angle of pitch (referred to as twist, in the Science Buddies project). The procedure states that the diameter is fixed, only chord length varies. You could cut blades from 1/16" balsa wood and insert them into these slots to create your test propeller. Start with a wide blade (long chord) and trim it narrower for each successive test. There is no mention of a airfoil in this design, so your propeller is a fairly simple affair. I don't see any reason that this wouldn't provide you with a design to only evaluate the effect of chord length in this project. As you mentioned, there are many other variables in propeller design, you could do some research on the subject and perhaps add some other hypotheses to your experiment.

Rick Marz

[phil813]

o i see. thanks!. I have another question. Regarding the efficiency of the propellers in the end, how can I plug in a formula to give a percentage of how efficient the propeller is in the end?

[rmarz]

phil813 - This experiment was only evaluating one variable, chord length. It is difficult to demonstrate anything except a comparison of different chord length models you used and show the relative performance of each one. There is not sufficient information of two important pieces of data, like input power compared to output power that would enable an efficiency calculation. Perhaps you could construct a table of the relative findings and measurements to show the differences that chord length had on measuring rotational speed (voltage or current on the multimeter). You could assign one design as a baseline and calculate the performance of others as being better or worse than your standard baseline.

Rick Marz

[phil813]

Alright! It's okay then. But, regarding the science behind this, I've been doing research on how thrust is generated and there are two different explanations. Some say that lift and thrust is generated due to the difference of air pressure on the top and bottom of the propeller (bernoulli's principle), while others say that's wrong. Do you mind telling me which one is to be believed or is there still no definite answer? Thanks!

[bradleyshanrock-solberg]

LIft is generated by the difference in air pressure above and below the wing, although the exact interactions involve turbulent flow, which is way beyond even high school physics in theory, although the practical results can be measured at your level of education.

Thrust is entirely different, that is what propels the airplane and causes airflow over the wing. Thrust is usually generated with a propeller or a rocket.

[bfinio]

Hi phil813 -

Like bradleyshanrock-solberg said, the actual explanation of how lift is generated gets pretty complicated. As you discovered, there are some conflicting explanations out there. One very common misconception is that "the top of the wing is curved, so air has to flow faster over this longer surface than it does over the straight (and therefore shorter) bottom surface. Faster air has lower pressure, so due to Bernoulli's Principle, the pressure below the wing is higher than the pressure above the wing, generating lift." The first part of this statement is incorrect. As a simple example, imagine holding a flat piece of cardboard out the window of a moving car. It will still generate lift, even though the top surface is not curved. This NASA site has some good explanations debunking the "incorrect" theories of lift, if you want to read more:

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

Finally, to clear up confusion about lift vs. thrust as mentioned in bradleyshanrock's comment: these terms are used in reference to the body of an airplane, like in the diagram here:

http://www.grc.nasa.gov/WWW/k-12/airplane/lift1.html

So a propeller generates forward thrust that moves an airplane, and a wing generates lift that keeps the airplane in the air. In your case you ARE measuring thrust from the propeller.

[phil813]

Thanks for the replies! I understand that thrust and lift are different but just to make sure, are they both created in similar ways? If not, may someone please explain how thrust is generated in terms of this project.

[bradleyshanrock-solberg]

You can create thrust all kinds of ways, air pressure is only one.

For example, if the engine on an airplane suddenly shuts off, it is possible (depending on the design of the plane) to still generate lift by using gravity...you go into a dive, which causes air to flow over the wings allowing you to keep control of the craft. (This is also a technique sometimes used to come out of a stall. Military pilots are very aware of this, going back to the days of biplanes).

A glider also uses this principle...jump off a cliff with a hang-glider and you get lift but the only "thrust" providing velocity is coming from gravity (ok, wind and updrafts can change this but you get the idea).

Another way is to simply tow the airplane. In WWII, D-Day, there were gliders towed by normal aircraft, and they "flew" just fine until released. When you run with a kite to get it to lift off when there isn't much wind, that's the same thing. You're just tugging at it, providing motion.

In a rocket aircraft, such as the X-15 the thrust is caused by exhaust gases from the rocket itself, not air displaced by a propeller.

Finally, if you make a paper airplane and toss it, that's another way of generating thrust. Carrier aircraft use this principle, using a catapult to launch them from the deck to get enough airspeed for their wings to provide lift.

Thrust is anything that will move the aircraft, with or without displacing air. Many forms of thrust work equally well in a vacuum, and some work underwater. Lift is when the thrust does more than move the aircraft like a cannon-ball, when the forward motion causes air to interact with the wing in such a way as to cause the entire structure to want to rise.

Throw a paper airplane and then throw a rock. One has lift, the other does not. Both have thrust.

One of the most interesting things I learned in my aerodynamics class was that baseball would be a very boring game if the ball wasn't designed just as it is, stitches and all. The spin on the ball, combined with the interaction of stitches with the air allows pitchers to throw all those crazy curve-balls, sinkers, fastball vs knuckleball etc. One reason softball pitchers can do far less with the ball is that it has different properties...a baseball behaves more like a frisbee or boomerang, where a softball behaves more like a simple rock.

[phil813]

thank you for all the insightful facts! I have a couple questions. For an average airplane or boat, is the motor what causes the propeller to generate thrust? If so, do you think that testing this project with a thrust meter would be better than just measuring the voltage produced? Also, do you think Newton's Third Law contributes to the thrust generated by a propeller ( sorry I keep asking you about thrust, I am just confused about how to explain thrust with air displacement if the Bernoulli principle is partially wrong)? Thanks!

[bfinio]

Hi phil813,

I apologize for some of the confusion - I just now read the procedure of the project you are doing in detail. There is a very important distinction to make between what you are doing in this project and how a regular airplane works.

In an airplane, the motor causes the propeller to spin, which generates thrust (so your statement "the motor causes the propeller to generate thrust" is correct, as long as we're talking about propeller-driven aircraft and not rockets or jets or gliders or baseballs, or any of the other things bradleyshanrock mentioned). Newton's third law would apply - the propeller pushes back on the air, so there is an equal and opposite reaction pushing back on the propeller, which generates thrust and pushes the aircraft forward. IF that's what you were doing in this project (using a motor to spin the propeller), you would want to measure the thrust produced, as you guessed.

HOWEVER, in this project, you are aiming a fan at the propeller, causing it to spin. Since the propeller is connected to a motor, this generates electricity, which you can measure with a multimeter. The propeller is acting more like the blades of a wind turbine in this case. Wind makes the blades of a wind turbine spin, generating electrical power.

I hope you can see the difference between the two processes. In one (airplane propeller), you take power from a motor, use it to spin a propeller, and the spinning propeller moves air (generating thrust). Reverse that process and you get a wind turbine - the air is already moving, which pushes on the turbine, causing it to spin a motor (generating electricity). Even though the two processes are reversed, the principles that determine a "good" shape for the propeller are similar (google some pictures of wind turbines and airplane propellers - you'll probably notice that they look pretty similar). So, while technically this project should probably be called "Efficient Turbine Design" and not "Efficient Propeller Design", you can still do the procedure as it's written.

I hope that helps clear things up instead of creating more confusion. Please let me know if you have more questions.

-Ben

[phil813]

Thanks for the reply and raising my awareness on that. Now knowing that this project technically works better for wind turbines than airplane propellers, would you say that using a thrust meter to measure the reverse effect (motor pushes propeller and propeller moves in accordance to air displacement and causes thrust ) be a better option, because i kind of want to change the project so that it test thrust and is as a similar as an airplane model (with an engine and propeller). Do you have any tips on how to make a thrust meter if so? And also, I know i keep bothering you guys with the same question but do you know any good sources to find out how thrust really is generated? Based on what I understand, the principles of lift applies to thrust as well, except thrust is the horizontal, not vertical movement. If that's the case, that means that with the incorrect theories of lift, an explanation for thrust would be incorrect as well. Since my teacher wants me to be as scientifically correct as possible, I am trying to find an answer that truly supports how thrust is generated. Thanks!

[bfinio]

Hi phil813,

I will try to break down answers to your separate questions.

1. If you want to actually drive the propeller with a motor, you will need to connect a battery pack to the motor to make it spin. If you can send me a link to exactly what type of motor you bought, I should be able to tell you what kind of battery pack you need.

2. What you need for a "thrust meter" will probably be referred to as a "force sensor" or a "load cell". This is a scientific instrument kind of like a scale, except it can measure force in a horizontal direction (unlike a scale, which depends on gravity and can only measure force in the vertical direction). HOWEVER, I think one of these might be kind of hard to find and expensive for a science project. You could make your own very simple "thrust meter" by using a kitchen scale. Place your propeller on a kitchen scale so it is facing upward, and measure the difference in mass when the propeller is spinning and when it is off. Just as an example (I am just making up the numbers here as an example, I'm not sure what you'll actually measure): Say you measure that the propeller has a mass of 500g when you put it on the scale. When you turn the battery pack on and the propeller spins, the reading drops to 400g. That means the propeller is generating 100g of thrust. (note that this method is not 100% accurate, there is something called "ground effect" that will affect the reading when the propeller is very close to the ground, but this shouldn't matter for purposes of a science project)

3. Finally, as for the explanation of lift - you are correct that the same explanation for how an airplane wing generates lift applies to how a propeller generates thrust. Technically, both wings and propellers are referred to as "airfoils", an aerodynamic surface that generates forces when it moves through the air. Whether you call it lift or thrust just depends on which way things are facing. For example, think of the blades on top of a helicopter - they form a propeller, but it's pointed "up" so we say it generates lift - as opposed to a propeller on an airplane, which is pointed sideways so we say it generates thrust. The principles in both cases are exactly the same.

The best resources I can find to explain in detail are the NASA pages I linked earlier. They have several pages explaining common "incorrect" theories of lift:

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

And this final page explains the argument between the "Bernoulli" explanation (that lift is caused by pressure difference between the top and bottom of the wing) and the "Newton" explanation (that lift is caused by the reaction force from the wing deflecting the air downward).

http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

An important quote from the page: "The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force."

So the important part for your project is NOT to use the "equal transit time" theory as an explanation, because that is incorrect. It IS correct to say that "there is lower pressure on the supper surface of the wing than on the bottom surface of the wing, which generates lift". It is NOT correct that air particles that arrive at the front edge of the wing at the same time have to reach the back edge of the wing at the same time.

I hope that helps. I'm not sure what grade you're in, but the reading level of those NASA pages is pretty advanced. I wouldn't sweat it too much - I think even some high school (and college!) physics and engineering teachers get this explanation wrong.

[phil813]

If I use the motor that was for the initial project (small 1.5-3 V DC motor) would that work?

[bfinio]

Yes. For a 1.5-3V motor you will want a 2xAA battery holder like this:

https://www.sparkfun.com/products/9547

[phil813]

do you have any step by step guidelines on how to construct a simple thrust meter? ( I have my design set but I don't know how to work with the electric setup)

[bfinio]

Hi phil813,

Unfortunately we do not have a step by step guide for that, since it was not part of the original project. You can see my previous suggestion for using a kitchen scale to measure thrust. Can you ask your science teacher or another adult for help with this part?

For the electric setup, your motor and battery pack should each have one red and one black wire. Connect the red wires together, and connect the black wires together, and that will make the motor spin. We do have several projects that show how to connect motors to batteries, for example:

https://www.sciencebuddies.org/hands-on ... s/brushbot

https://www.sciencebuddies.org/science- ... p014.shtml

In those projects the motor has a cork attached to it (which makes the robot vibrate and buzz around), in your project it will have a propeller attached.

[phil813]

O alright, but all the materials I need for the thrust meter are wood and nails for a testing station, motor, aluminum to hold the motor on the stand, scale, and battery? How do I control the output of electricity so that each propeller is tested with the motor generating the same amount of electricity?

[bfinio]

Hi phil813,

Your general description of how you build the test stand sounds correct, but it's important to distinguish whether you are talking about a KITCHEN scale or a BATHROOM scale. A kitchen scale is much smaller and measures smaller weights. Since the propeller and motor for this project, I think a bathroom scale will be way too big. So, you might need to build a much smaller support for your motor - for example out of popsicle sticks and glue, instead of wood and nails.

As for the motor - if your tests take a relatively short period of time (say, less than a minute each), then you will not need to do anything special. If you let the motor run for a long period of time, you will want to use fresh batteries for each test, because the batteries will start to drain.

One clarification on wording - remember that the motor does not generate electricity in this case. The electricity is supplied by the battery pack, and makes the motor (and propeller) spin.

[phil813]

Thanks for the reply. I have been doing research on different thrust meter designs and have eventually found a design that seems suitable for this project. Here's the link to take a look at it.

http://rcexplorer.se/projects/2009/08/t ... est-stand/

For this design, he added a watt meter.. do you think it's necessary to add the Turnigy watt meter to my thrust meter to prove to the judges that each test was run with the same input of electricity each time..

[bfinio]

phil813 - it is really up to you whether you want to use the watt meter. I would say that if you do each test with fresh batteries that should be "good enough" for your average science fair. If you are really aiming for winning a prize or moving on to a higher-level fair then you might want to do more measurements.

[phil813]

Thanks for the info! For the design, how do I connect the kitchen scale and the thrust meter? ( does it need to be screwed on?)

[bfinio]

Phil - based on the design you linked earlier, the kitchen scale does not need to be screwed to the thrust meter. It looks like the 2x4 on top needs to be free to pivot, so it can push down on the scale (so you wouldn't want to attach it to the scale), and the scale is just sitting on a piece of plywood that the rest of the structure is also attached to. As long as the scale doesn't wobble around a lot, you are probably fine.

[phil813]

o thanks! so I should get 2 2x4 wood pieces and screw them together, and screw on smaller wood pieces to allow it to stand still. The wood pieces that are on the scale and holding the motor must be securely screwed right and not free to move around a little? If I'm using the propellers that the initial project recommended do you know any good spinners/hubs i could use to hold it in place on the motor.

[bfinio]

Hi Phil - unfortunately I am not really that familiar with model airplane parts, so I don't know what you could use to attach propellers to the motor.

In the link you sent earlier, it actually doesn't look like the 2x4's are screwed down to the base, but I can't tell for sure. They are probably large and heavy enough to just stay in place.

That brings me to another important point - I believe the motor you are using is much smaller and less powerful than the one used in that link. You might want to reproduce a much smaller version of the thrust meter, using tinier pieces of wood (for example, stuff you can get at a craft store) than 2x4s.

[phil813]

o.. if i want to change my motor to mimic his… do you have any websites that i could buy the motor?

Here's the battery i am considering on buying- http://www.hobbyking.com/hobbyking/stor ... _Pack.html
but i can't find a motor to match that… o.. i also have a question.. what does a speed controller do? I saw that he added one and assumed that a speed controller allow you to control the electric input

[bfinio]

Phil - the battery you linked to is rated for 11.1 volts. That will NOT work with a motor that is only rated for 1.5-3 volts, so you would definitely need a bigger motor to match. I am really not familiar with batteries and motors that are used in remote control aircraft, or websites that sell them. If you really want to use those, I would recommend searching for a hobby/RC store in your area, and asking about it in person at the store. They also might be able to tell you about speed controllers, which I assume control the RPM of the propeller.

Like I said before, I can recommend this smaller motor and battery pack because I know they will work together:

https://www.sparkfun.com/products/9547

https://www.sparkfun.com/products/10171

Those will be sufficient to do a science project, but they are not the same as you would use on a real model airplane. However, that is ok for purposes of a science project. Scientists and engineers build "models" of larger things to test all the time. For example, engineers will build smaller models of huge things (like airplanes and bridges) because they are much easier to test in a laboratory.

Ultimately it's up to you - if at all possible, I'd try to find a hobby/RC store in your area where you can ask questions, or get an adult (like a parent or your science teacher) to help you figure this out in person. There's a limit to how much useful advice I can provide over the internet.

Thanks,

Ben

[phil813]

I've been doing research and gathered and built the necessary stuff for my thrust meter. But, my question now is, where on the propeller do I determine where the chord length is, because the propellers width changes across

[bfinio]

phil813 - I'm not enough of an aerodynamics expert to know the answer to that for sure. My guess would be either:

a) measure the maximum chord length at the thickest part of the propeller, or

b) measure the chord length at multiple evenly spaced points along the propeller, and then calculate the average.

I would do some googling about how chord length is defined and you should be able to figure it out.

[phil813]

Thanks for the reply. From some thorough research I found that I need to find the average of the root and tip chord. Anyway, I realized that by using a thrust meter, though I may get different thrust readings, chord length and thrust don't really affect each other but rather, chord length affects efficiency of propellers. So, my question is, how do I tie thrust with propeller efficiency? Is propeller efficiency defined as thrust over watts consumed? Does drag come into the equation?