How accurate is my wind tunnel for testing RC plane wings?

[trailogy]

Hi everyone,

I am new and I hope I'm putting this in the right section. I am currently a Sophomore in High school and I am building a wind tunnel. It's not for a class but rather its is like an independent study. I have been into planes pretty much all my life and I am hoping to get a pilots license in the near future. In the meantime I am tiding myself over with some projects with airfoils with RC planes and to do that I want to build a wind tunnel.

I have looked all over the internet for information on this to try and not post and I have found one thread on this forum but it didn't answer all my questions. Hence I am here :)

I am following this guide: http://www.tririg.com/docs/CogganLSWT.pdf to base my wt off of. I have built the contraction cone and the test section so far. My major question is if I were to build this for testing RC plane wings how accurate would it be? I have researched the Reynolds number and I know that it is the relationship between viscous forces and inertial. The wind tunnel is basically a closed pipe so how should I go about testing velocity and quality of the flow? Are there any websites that you suggest I should look into? I am currently looking for aluminum honeycomb to straighten the flow and I am using window screens as a way to eliminate smaller turbulence.

Anyways, there is a lot of info but I would appreciate any help even if its just a small tip. Thank you so much!

- Ben

 

[Spinnor]

What do you see yourself doing in 10 years? Do you hope to become an aeronautical engineer?

My guess is there is free software that will do a much better job of testing airfoil designs then a home built wind tunnel? Here is a start,

https://www.grc.nasa.gov/www/k-12/freesoftware_page.htm

From, https://www.google.com/search?safe=...j33i22i29i30k1j33i21k1j33i160k1.0.41lpapKCOPk

As a student you might get a discount on some not free software.

The computer age is here, get with it!

If you want to build something, build some model aircraft, fly them and break them. Knowing how to build and repair stuff can not hurt you as a future engineer.

Good luck!
Signed, Debbie Downer.

 

[russ_watters]

I did this project, but was in Junior high, so mine was of lower quality...

...you can measure airspeed easily with a homemade pito-static tube and manometer (and Bernoulli's equation).

 

[boneh3ad]

My major question is if I were to build this for testing RC plane wings how accurate would it be?

That depends on the accuracy of your instruments. It certainly won't be as accurate as a commercial force/moment balance, but you could get some pretty good measurements in a suitably-designed homemade tunnel. You will need to characterize your tunnel (free stream flow quality/regularity) and your instrumentation to get an idea of how accurate your measurements are.

I have researched the Reynolds number and I know that it is the relationship between viscous forces and inertial. The wind tunnel is basically a closed pipe so how should I go about testing velocity and quality of the flow?

You would first want to test that your test section has a uniform velocity across its cross-section and then characterize how that velocity changes along its length. If you want to avoid it changing, your test section walls will have to diverge somewhat to account for boundary-layer growth. You will also want to know what sort of free-stream fluctuations are occurring in your facility. A Pitot tube (as @russ_watters mentioned) is likely your most useful instrument here. You could do it as he suggested or buy a fairly cheap pressure transducer you can read on a computer as well.

You might want to see if a local library has a copy of Low-Speed Wind Tunnel Testing you could use for greater detail, though it's likely going to be a little over your head at this point. It's not a cheap reference so buying it is probably a bit overkill.

WMy guess is there is free software that will do a much better job of testing airfoil designs then a home built wind tunnel?

Your guess would be a poor one. There is a reason that the Air Force and NASA and Boeing and Lockheed and the rest all still pay big money to operate wind tunnel facilities. Computer simulations simply cannot handle everything. They are particularly bad at calculating drag.

If you want to build something, build some model aircraft, fly them and break them. Knowing how to build and repair stuff can not hurt you as a future engineer.

Do you mean stuff like... wind tunnels? Fact: the workforce operating wind tunnels in the US is retiring at an alarming rate. There will be a pretty large need for good people to continue running them.

 

[Spinnor]

Your guess would be a poor one. There is a reason that the Air Force and NASA and Boeing and Lockheed and the rest all still pay big money to operate wind tunnel facilities. Computer simulations simply cannot handle everything. They are particularly bad at calculating drag.

So are you saying that a student homemade wind tunnel will give better results then the following software,

http://www.hanleyinnovations.com/vf50.html

 

[boneh3ad]

When it comes to drag, yes, provided the wind tunnel isn't total trash. It's not like the link you provided is some high-powered, sophisticated CFD code. It's also unbelievably overpriced considering you could get XFOIL to do the same thing using the same vortex panel method for the low price of "on the house."

 

[anorlunda]

A high school sophomore and already struggling with engineering concepts. Good for you @trailogy .

So, one thing that all engineers in all fields need to learn is to state your requirements before thinking about solutions.

Are you trying to make the best model plane and the wind tunnel is just a tool?
Are you interested in making an efficient airfoil for the wing?
Is the wind tunnel project itself a matter of interest to you, or just the plane?
Is it only the usefulness of the the wind tunnel of interest, or is it learning about the factors that make one design better than others?
Perhaps you are interested in instrumentation and how to measure the characteristics of your wind tunnel?
Perhaps all of the above?

An airplane designer can make a career designing the best airplanes.
A wind tunnel engineer can make a career designing the best wind tunnels.
A software engineer can make a career designing software tools to be used by other engineers.
A theorist can make a career studying the physics of air flow.

Do you see why requirements come first? The real answer depends on what the real question is.

I am a boater. I learned many years ago that boat projects, boat building, and boat maintenance are as much of an attraction to many boaters as actually being out on the water.

 

[jrmichler]

Building a wind tunnel is a great project. Good for you.

Try to avoid paralysis by analysis. Build it and play with it before trying to to fully characterize it. Fly a pop can, a paper airplane, and random other objects before studying the smoothness of the airflow or making actual measurements. When you are ready to start making actual measurements, then study about the effects of Reynolds number on airfoils.

And it you end up building a wind tunnel and using it only enough to see it work, you will have had an excellent learning experience.

 

[trailogy]

Wow, I am so grateful of all your suggestions! It honestly really brings me a lot of joy seeing that there are people who really just want to help each other out.

The computer age is here, get with it!

I looked into software but found that I really wanted to build one as I think if I could get it accurate enough it would be a good learning experience for younger students at my school and it could be more "hands on" per say. I also built a hotwire foam cutter to cut out foam for RC planes and I wanted to take a section of that to put into the wind tunnel as there are minor deficiencies in it so I can see that (I am thinking now that this wind tunnel might not be as accurate to test that/ if it is even worth it)

Your guess would be a poor one. There is a reason that the Air Force and NASA and Boeing and Lockheed and the rest all still pay big money to operate wind tunnel facilities. Computer simulations simply cannot handle everything. They are particularly bad at calculating drag.

I didn't know that the software was bad at calculating drag. Good to know! Thanks!

...you can measure airspeed easily with a homemade pito-static tube and manometer (and Bernoulli's equation).

I bought an anemometer to put in the wind tunnel. From what I have read it does the same thing but in different ways. I also wanted to calculate the wind speed before building it (Its kind of late because I already built the test and contraction :D) to see the range of airflow velocity (or range of reynolds numbers, not sure which is more important).

That depends on the accuracy of your instruments.

My balance system is homemade and its mechanical. I am good with fusion 360 so I designed some mounts so the drag force and lift force can be measured on two separate electronic mass scales used in my chemistry class. Next year I plan to improve this with pressure sensors so I can do it on the computer and make real time graphs. Though I also feel like that the flow quality is a major part because if it isn't good than wouldn't the readings be false?

see if a local library has a copy of Low-Speed Wind Tunnel Testing you could use for greater detail

I live next to a college so I will see if they have it. I saw this on another resource and it looks way more complex than what I am with my current math level (Algebra 2 Honors) Right now the stuff we are going over is actually helping with this project.

A high school sophomore and already struggling with engineering concepts. Good for you @trailogy .

Thanks! I try my best to understand :)

Are you trying to make the best model plane and the wind tunnel is just a tool?
Are you interested in making an efficient airfoil for the wing?
Is the wind tunnel project itself a matter of interest to you, or just the plane?
Is it only the usefulness of the the wind tunnel of interest, or is it learning about the factors that make one design better than others?
Perhaps you are interested in instrumentation and how to measure the characteristics of your wind tunnel?
Perhaps all of the above?

My original goal was to find out an efficient wing to do FPV RC stuff with so I can fly for longer. However I quickly realized that the wind tunnel itself was very hard to build so I switched and I wanted to build an accurate wind tunnel to understand the physics behind flow quality in the wind tunnel. I found this was beyond my learning. I tried watching videos to help with this on youtube (Integrals, Derivatives and matrix math). Thank you for your tips. Sometimes I jump the ship and find myself lost. :)

Try to avoid paralysis by analysis. Build it and play with it before trying to to fully characterize it. Fly a pop can, a paper airplane, and random other objects before studying the smoothness of the airflow or making actual measurements. When you are ready to start making actual measurements, then study about the effects of Reynolds number on airfoils.

I think this is what I am leaning towards now :) Thanks!

Thank you all for these tips! I found an undergrad thing on this project and I have been doing some calculations based on it. Here is a link. (PDF) https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=0CEcQFjAH&url=http://www.asee.org/public/conferences/8/papers/3461/download&ei=h7c8U4jsFurP2wW0oIHABQ&usg=AFQjCNHoYSiWqTeH6chJn7bR-XKObIjfwg&sig2=zHb8geC3JOg8nSq-h_yzbw&bvm=bv.64125504,d.b2I&cad=rja
I'm a little stuck right now on where to go next but I will google on how to see the flow quality of my wind tunnel and see if I have the ability to do it. A couple of specs: built of wood (high friction probably), contraction cone is not based off a polynomial rather it is linear from 2 feet to 1 foot (there are numerous papers out there explaining the effects but for simplicity I built it this way). Are there any tips right off the bat of what I should do to make it better? Thank you everyone!

- Ben

 

[boneh3ad]

I didn't know that the software was bad at calculating drag. Good to know! Thanks!

Indeed. Lift is fairly easy to calculate. Drag has many components, and in particular, viscous drag is very difficult to calculate accurately.

I bought an anemometer to put in the wind tunnel. From what I have read it does the same thing but in different ways. I also wanted to calculate the wind speed before building it (Its kind of late because I already built the test and contraction :D) to see the range of airflow velocity (or range of reynolds numbers, not sure which is more important).

What sort of anemometer? The word anemometer simply means a device to measure wind speed, so a Pitot static probe is an anemometer. A constant-temperature hot wire is an anemometer. A cup anemometer is an anemometer. For wind tunnel testing, you will want something that can measure the velocity with minimal disturbance to the surrounding flow.

Your velocity, Reynolds number, and Mach number will all be important. You will be able to calculate these things as soon as you have a diffuser and fan system designed (or rather can select those designs based on your desired test section conditions).

My balance system is homemade and its mechanical. I am good with fusion 360 so I designed some mounts so the drag force and lift force can be measured on two separate electronic mass scales used in my chemistry class. Next year I plan to improve this with pressure sensors so I can do it on the computer and make real time graphs. Though I also feel like that the flow quality is a major part because if it isn't good than wouldn't the readings be false?

The readings wouldn't be "false", but they would have the effects of poor flow quality wrapped up in them. You'd need to account for that.

I live next to a college so I will see if they have it. I saw this on another resource and it looks way more complex than what I am with my current math level (Algebra 2 Honors) Right now the stuff we are going over is actually helping with this project.

If the college has an engineering program, then it likely will have a copy of that book. You are correct, though, in assuming it will be at a fairly high math level. That's why I suggested that buying it would be a bit much. Checking out a copy to even just thumb through would give you an idea of where this is all headed, though, and might provide some guidance on the sorts of math courses to take in the future if you want to continue pursuing this long term.

 

[trailogy]

What sort of anemometer?

I have one that looks like this, (a little smaller as I knew it would disturb flow and it is also placed downstream) with the mini fan that generates electricity to find wind speed.