Home built wind tunnel?

[Not]

Hi all,

Does anyone know what it would take to build a wind tunnel capable of producing 100+ mph airflow? It doesn't have to be big, just a chamber size of about 10" for testing rotor blade designs.

-Not

[Sparky Paul]

$60,000???
It takes more than a refrigerator shipping crate and a ceiling fan.

[Bob Reynolds]

Find someone with an old copy of Air Trails magazine. A long time ago they had a artical on how to build a smoke tunnel. Mine worked very well until a rainstorm took it out. No, I don't know where my plans for it are.

[BMatthews]

I'm not sure about the power needed for the speed but if you plan on any meaningful data from your tunnel then you really need to do some research into the thickness of the boundry flow, flow straightening and turbulence damping velocity gradients from the walls inwards (related to the boundry flow) and a host of other stuff to arrive at a tunnel that will actuallly tell you something of worth. All this stuff is critical even if all you want to do is provide a smoke trail flow off a wingtip shape or something similar. For truly useable data your 100 MPH tunnel would end up costing thousands and take up most of your house in size to provide the motor, fan, flow straighteners, turbulence damping and finally provide for some instrumentation that can give you data you can use for anything.

It's also one of those areas where there is no point in doing it with half measures. For testing the differences between your blade profiles the differences will be very small between shapes. Such differences can easily be lost in the airflow turbulence and poor instrumentation arrangements of less than full out efforts.

While it's a noble effort you're far better off to study what is available out there on the present airfoil sections, learn the Reynolds number ranges your blades operate in and use the existing data to make your selections.

Also helicopter rotors are a very special case of wing application. Near the hubs are slow enough that the blade is just along for the ride, the middle to outer part does the majority of the work and the tips are overdriven and provide a lot of turbulence which is drag. Have you noticed that a lot of the new full sized heli's use Schumann style or similar tips?

[Berdie]

Not,

That's quite fast for testing rotor blade designs you're going to have all sort of problems making this wind tunnel but I gave it the calculation a quick shot and assuming that the air is going at 100mph and you have a square 10" cross-section the opening of your wind tunnel is going to have to be 50"x50" and you need a fan that'll blow air at 4mph.
In theory you'll probably need even more than 4mph because it's not a perfect wind tunnel. One last observation is that you won't be able to get any useful information out of the wind tunnel anyways using a conventional fan, the flow will be swirling in one direction which your rotor blade will have to cancel out. The swirling motion also has a nasty habit of either strengthening or weakening vortices as well shed from the tips your rotor blades.

[Not]

Thanks for all of your inputs,

You all certainly know much more about wind tunnels than I do, because you mentioned terms that i never heard of :)

I realize now that i didn't give enough info in my first question, so here goes:

I'm not looking to test different airfoils, I'm looking for more coarse data, like the amount of raw lift that a given blade size/area will produce at differing aoa. All of the data that I plan to gather, will only be used to compare my different blades to each other, not the rest of you all, who actually know what you're doing. :)

Long story short, what I really needed to know was if I start with a fairly high powered blower and forced its thrust through a small opening, would the velocity of the thrust increase by a calculable and useable amount? And seems like Berdie answered it. (Although, I'm having trouble extracting his formula).

I'll worry about stopping the air from swirling once I get it to moving that fast :confused:

-Not

[geoffbeneze]

I'm not looking to test different airfoils, I'm looking for more coarse data, like the amount of raw lift that a given blade size/area will produce at differing aoa. All of the data that I plan to gather, will only be used to compare my different blades to each other, not the rest of you all, who actually know what you're doing. :)

Long story short, what I really needed to know was if I start with a fairly high powered blower and forced its thrust through a small opening, would the velocity of the thrust increase by a calculable and useable amount? And seems like Berdie answered it. (Although, I'm having trouble extracting his formula).


Part of it is actually defining your goals (as you've done). The other is deciding if you're looking for absolute data, or comparative data. As long as you're comparing Part A to Part B, both of which you have in hand, you CAN do it without great problems.

I've seen a number of examples of early 1900's wind tunnels. Rectangular boxes with HAND CRANKED fans on the end.

A number of years ago, my godson and I did a science project where we built a tunnel using 16" (?) metal AC duct (round). A box at the end carried four vacuum blowers with a gridded difuser in front of them. I used two tripper pull scales to measure life and drag on a variety of 8" sections of wings with various air foils.

In terms of comparative data, it was a great success both as a science project and learning experience.

[Not]

Part of it is actually defining your goals ...

Now you're talkin! How was the coffee it made? :D

-Not

[geoffbeneze]

Now you're talkin! How was the coffee it made? :D

-Not

It didn't make any, but it was certainly mostly FUELED by coffee (G)

[BMatthews]

Oh well. that's different. In that case why not make up a simple tower style rotor dynamometer out in your backyard and test real rotors in open air? Since this is just for lift to power and direct comparison the setup, build and testing will be quite easy. And if you think of it it's a more direct real world scenario.

First off you want a nice strong motor that'll turn your rotor and that you can do some good measureing on. Woodworking router motors are ideal for this and can be had for cheap in used condition. The old non plunge types are crazy cheap and common. Then get yourself a good quality 1200 watt dimmer for a speed control.

The RPMs at max are up around 10 to 12K so it's close to the engines and this says you'll want to use similar gearing. Power is also similar with 1 and 1 1/2 HP routers being fairly easy to find.

Make up a tower about 8 to 10 feet high for the testing. 8 to 10 feet would probably be ideal but you'll need to add a hinge to allow it to swing down for rotor changes. The motor will drive a shaft up to the rotor head. The motor will also be mounted on a slideing and pivoting frame with some form of scales or calibrated spring scales added to allow lift and torque measurements. Similarly you'll want some form of tachometer for rotor RPM I would imagine. You may want an amperage measurement on the motor as well but the power required can be inferred by the torque value required to spin the rotor. For the rotor head an older outmoded heli unit that allows you to alter the pitch but with locked pitch and roll motions will let you alter the pitch to test the rotors.

In use you can swap blades on the rotor head, swing the unit back into place and run a test to obtain rpm, torque and lift for various angles of attack. Swap blades, run a second set of tests and it should be apparent from this which is more effective, efficient and anything else you want to find.

Certainly a rotor dynamometer of this sort will tell you a lot more, in less time and more directly than playing with short fixed sections in a bad wind tunnel will.

And if any nosey neighbours start looking oddly at you just tell them you're designing the perfect ceiling fan.... :D

[Not]

Not a bad idea...

[BMatthews]

On second thought make it a horizontal rig with the rotor set up like a big prop. Just be sure you mount the testing shaft and rotor head high enough that you have a good foot or more of clearance from the tips to the ground so there's no ground effect interference.

I suggested the tall tower to keep the rotor wash away from the ground but a horizontal option would do that better and not look so odd to the neighbours... :D

[Sparky Paul]

A research quality wind tunnel, and a horizontal whirling test rig from 50 years ago.... :)

[Not]

so this design pulls the air across the wing instead of pushing it?

[Sparky Paul]

so this design pulls the air across the wing instead of pushing it?
.
Yes...it's standard to have the fan behind the model, to keep the flow over the model free from the very disturbed air coming from the fan.
A major problem with model airplanes in test are the relatively small forces involved, both lift and drag, with drag very small in most instances. To measure this requires a very sophisticated friction-free balance, which of itself is expensive to develop.
The modern tunnels generally use an air pressure rake that moves across the tunnel behind the test model to measure the disturbance in the flow created by the model. This disturbance is then translated into lift and drag.. and is another expense.
Qualitative tests, relating one model's effects to another without determining any hard values for either model, rather than a definitive quantitative test is the most practical use of an low-cost tunnel.

[Not]

.
Yes...it's standard to have the fan behind the model...

Thank you! You have restored my faith in the forum :)

-Not

[hoppy]

Instructions for building a wind tunnel:
http://www.theplanpage.com/Months/2306/tunnel_files/tunnel.pdf

[hoppy]

Perhaps an AC squirrel cage blower might give those velocities. ??? Cheap source for a lot of air:)

[Not]

Perhaps an AC squirrel cage blower might give those velocities. ??? Cheap source for a lot of air:)
Is that a real thing or are you just skrewing with me? :)

[hoppy]

Is that a real thing or are you just skrewing with me? :)

That's that round thing in your AC system that blows air through your house.

I know some SCB's can do 60mph..... perhaps by narrowing the outlet, higher velocities can be obtained.

What mph you can get out of a used AC SCB is unknown to me but they do throw a lot of air. Here is what it looks like: http://www.alpinehomeair.com/viewproduct.cfm?productID=453055496

[Not]

Oh! You mean a hampster wheel looking blower thingy! I've seen those! :) That's the same design as the main lift fans in full scale hovercraft...
I never realised that they moved air that fast...

Thanks!

[C@T]

Hello,
this does look like a little bigger project to me.

let me add my first ideas and constrains:

- 160mph does translate to an airspeed of ~ 45m/s
- 0,25 square meter of test area would need ~ 12 m3 airflow
- driving this will requiere a driving power of ~ 1000 W
- the diameter of the fan (prop type) will be ~ 1m diameter

In addition to that you will get a free of cost hurricane turning your workshop in a diasteraerea all in a sudden !

This does lead to a closed loop design, putting the vent underneath, above or behind the measurement chamber, also for noise constrains.

Of the shelf vents in my point of view would not be the right choice, for you need to adjust rpm to set your airspeed.

This would be a nice application for one of those kW brushless outrunners (see power systems section looking for "Powerditto" or "Olmods Motor build") and a big prop, but you can also use three or four smaller units, making up for the same aerea covered.

That´s what a CAT beeing arround for a while has to add to the subject.

[C@T]

Have a look at those links:

http://www.audi.com/de/de/neuwagen/technologie/forschung_entwicklung/windkanal_zentrum/windkanalauswahl/aeroakustik_wk/aeroakustik_wk.jsp

http://www.audi.com/de/de/neuwagen/technologie/forschung_entwicklung/windkanal_zentrum/windkanalauswahl/aeroakustik_wk/messstrecke360.jsp

This gives a good overview and will also work when scaled down.

[BMatthews]

Good one on the fan being on the rear Paul. I'd forggoten that one.

However I disagree on the measuring setup being equal to the task of even seeing and evaluating the very small differences that will occur between two airfoils. The differences are going to be VERY small and suitably sensitive equipment will be required with some very clever and low friction force transfer linkages. So the tunnel itself may not be that bad to build but I'm pretty sure that the measuring apparatus and linkages are going to be a nightmare for any home shop type other than a VERY dedicated person with a lot of time on their hands.

There's also something else to consider Not. Once you run the tests and get some numbers what are you going to do with them that isn't already given by presently available data or that can't be generated by the decently accurate Xfoil available with a nice interface via Profili2? There's still going to be a lot of number crunching and chart analyzing to come up with a new choice for a rotor blade.

My own feeling is that a rotor dynamometer is still a more usable testing rig for your needs Not. First off it's more directly applicable because you're testing real blades in real operating conditions. As a bonus you can transfer the blades directly to your helicopter for flight testing. And if the idea is to avoid the math and physics involved in blade designing then this would be an important factor. Windtunnel data is only direct up to a point and then you need to massage the data to obtain something you can use in the real world.

BTW, did you ever see the data on blade sections for Autogyro models that was done and reported in one of the magazines a while back? I can't remember the exact setup but they used open air real rotors as well. The methods used there would be similar to your needs other than they were testing autorotation and you want powered rotors. So a few mods would be needed to add the power train but the basic principles are the same. Good lift with low drag. Their final findings of the sections they tested was that the Selig 6062 or 6063 was the hot setup. In any event it would be well worth finding and reading. Can anyone tell us what magazine and issue that was?

[C@T]

@ BMatthews

Have you ever had a close look at a prop, running under full load in stationary conditions.

You will realise a mayor bend forward under those conditions and also torsion of the blades also, using a strobo light.

That is what will not happen under real flight conditions. Thats why props used for powermeasurement are made out of solid alloy profile. But you can even make those bend and break within the rpm limit by just accelerating too fast, if you motor has enough power and low inertia.

So for my best guess a wind tunnel is much closer to reality, even if there is still a lot to consider such as keeping the environmental parameters constant, ...

Have a look what the guys with the ducted fans do already.

[BMatthews]

Yes I have, when I was a kid we used those old white nylon Top Flite props. Also you should see the wave action going on with the soft white Grish props on a pulsating PAW 19 deisel engine. It's enough to make you reach for the body armor... :D

For me the real world flexing is why I think the dynamometer option would be the best in this case. It not only can test airfoils used on identical blades but also changes to the blade planform and tip design that would not be that easy to test in a tunnel environment.

About the only thing I can think of where the tunnel would be better would be for detecting the stalled condition. But even there I think the stall will be apparent when you see the motor current suddenly jump for a very small pitch angle change.

I'm also thinking that with some thread tufts taped to the top of the blade in conjuction with a strobe light you could actually see where on the blade the stall occurs by the sudden fluffing around of the threads. The ability to see effects like this directly as opposed to inferring them from tunnel data could shorten the study time on a given blade design. Simlarly that same strobe light in conjunction with some chordwise lines on the blade could be used to "freeze" the rotor in view and by comparing the lines on the blade to a fixed outer reference you could see if the blade is twisting unduly. That could lead to further study on the pros and cons of very rigid vs more flexible blade design. Rigid isn't neccessarily the best option in some cases.

Not, these days with superbright white LEDS readily available you can make a lovely strobe light that is triggered once or twice per rev from a pickup or contact on the rotor shaft that sets off a short pulse of voltage to the LED's. A bit of shade over the rotor would be enough to let you study these effects I suspect.

And I repeat about the advantage of being able to actually transfer the rotor blades from the dynamometer to the heli and obtain direct performance results so that study and practical results can support each other either positively or negatively for little effort.

I'll grant you that a tunnel can do the fussy analytical work better but I still think that for less effort a dynamometer could be built along with the instrumentation for about the same cost and the results would be closer to the real world.

And lets not forget that with a dynamometer blades from various heli's could be mounted, with suitable adapters of course, and tested to gain a foundation of performance figures to base new developments on and better guage the progress of the design work.

And finally there's just a helluva lot less math involved with a direct measurement dynamometer than with the wind tunnel option where the speed and drag findings would need to be used to calculate the differences along the entire blade length. I suspect there's some integral calculus in there somewhere.....

[Sparky Paul]

The standard flash on a digital camera stops this Graupner 6x3 folder running several thousand rpm.
A much slower turning rotor would be easy.

[jrb]

Here a shot of my propeller test stand and some test data using some motors I found at the Axman surplus store.

The motor mounted on the front pod is synchronous; running at 3600rpm regardless of the prop mounted; well at least for those I ran w/o blowing the 15amp breaker in my home’s fuse. That’s a 22x11 mounted on it in the photo.

The other motors are universal (series wound) and work very well with a dimmer switch; though they are happier with my Variac (limited to 10amps, would like more!). I turned a 14x10 @ over 6000rpm with the big universal by plugging it directly into the wall outlet rather than going through the Variac.

As previously stated; it far easier to generate rather than to analyze data.

[Not]

Wow! You guys are waaaay smarter than me! You must have HUGE heads :)

Seriously though, thanks!

-Not

[BMatthews]

Hey, it's a lot more fun than doing MENSA crossword puzzles... :D

....hat size 42 and swelling by the moment.....

[Not]

Hey, it's a lot more fun than doing MENSA crossword puzzles... :D

....hat size 42 and swelling by the moment.....

I'm so jealous! I have cerebellum envy :D

-Not

[JustinONE]

Maybe if you take a trip to a local junkyard you might be able to fine a monster turbo charger off of a diesle tractor trailer, they are HUGE! I'm pretty confident they will put out some serious airflow. If you can't find one there, maybe your local turbo repair shop can help, they might even donate it for the cause.

Then adapt it to a small 4 cylinder engines' exhaust manifold, so that you can spool it up!
Would make a fun project, GO for IT!!!

[Berdie]

Small thought on the subject... What's wrong with doing a static thrust test? No need for any fans to have air flow over the motor. (Though this doesn't relly apply) I know Rolls-Royce (large civil aircraft engine manufacturer) often tests their engines for performance data statically first, and get most of their information this way. I don't know if this applies to model airplanes or not (just recently got into modelling, but studying aerospace engineering). The main problem I think is that the motors will make no appreciable difference to the flow through them, maybe even stall the tips of the propeller. In a static thrust test the differences due to the propeller are much easier to measure because they are larger. ( I think the eqn is T=To-kv^2, where To is the static thrust) since the speed of the model won't be very high you could say that the the thrust while moving is approximate to the static thrust.

Oh Not, just in case you're still curious the equation for estimating the speed and areas in a wind tunnel is A1*v1=A2*v2, A is the cross-section area, v is the velocity at that area.

[JustinONE]

Check this out, http://www.lerc.nasa.gov/WWW/K-12/gallery/ryans_tunnel/tun1.html
this can't be too difficult to build. I only wonder how do they measure drag at certain airspeeds?

[Not]

Check this out, http://www.lerc.nasa.gov/WWW/K-12/gallery/ryans_tunnel/tun1.html
this can't be too difficult to build. I only wonder how do they measure drag at certain airspeeds?

If a kid can do it, I can at least do one half as good! :)

Seriously though, the way he has the scale connected to it, is EXACTLY, the way I was planning to do mine... Spooky...

As far as drag, I would figure that a similarly rigged spring scale could probably do the trick...

-Not

[JustinONE]

your gonna need some sensitive measuring equipment.
The only prolem with the kids design is that the wind tunnel is square, it really should be round, because the corners of the box are creating turbulence, something you don't need after all that effort. another mod would be to get some honeycomb plate too straighten the airflow before it gets near your part that is being tested, less turbulance more accuracy.

[geoffbeneze]

your gonna need some sensitive measuring equipment.
The only prolem with the kids design is that the wind tunnel is square, it really should be round, because the corners of the box are creating turbulence, something you don't need after all that effort. another mod would be to get some honeycomb plate too straighten the airflow before it gets near your part that is being tested, less turbulance more accuracy.

These are matters of detail rather than "problems" to be overcome (assuming one knows the details (G).

Measuring equipment can be accomplished by using (rifle) trigger scales. Weighing in grams or oz will give all the data an amateur scientist needs. Such scales can be purchase from any good gunsmithing outlet. I'd recommend checking with Brownell's Inc (brownells.com) as a starting point.