Prop Static Test Wind Tunnel

[gjohnson]

Dave,

I'll start asking you all my wind tunnel questions here. I'll have to figure out a better format than a JPG for posting a drawing. Hopefully you can follow it.

Since my wind tunnel will only be for indoor and micro motors it doesn't have to be large. I was planning to fold up a square tube from a large sheet of corregated cardboard (per your suggestion). I'll mount that on a couple of boards to raise it up over about a four foot length of plywood to allow mounting various pieces of equipment off of. This way the entire unit can be picked up and moved off my workbench. I think between four and five feet of overall length should be sufficient (it was for the Wright Bros, I think).

The grid (per your suggestion) will be florescent light grid, with about 1-inch squares. I'm wondering if I should stack two of them to get longer vanes? Did you say something at one time about a screen mesh to help straighten out the airflow? Also, should I put another grid in front of the prop that generates the airflow to help keep it from swirling the air in front of it?

Should I position the motor to be tested in approximately the middle of the tunnel? Or, should I position it further forward?

I like your idea of a weighted flag to calibrate wind MPH. But, I was wondering about cutting a hole in the side of the tunnel and inserting the Kestrel wind meter through that. The problem with this is it is not in the center of the tunnel. How far in would it have to protrude to get good measurements.

For the tachometer, I was planning to mount it on a narrow post, with a clear plastic window allowing me to read it from outside the tunnel.

The pivot for the test stand will come through a slot in the bottom of the wind tunnel. A small door will allow removing the motor for prop changes.

My biggest quandry is about the motor to create the wind. For starts I'm thinking of putting one or two large GWS props on my 14.4v cordless drill. I'd like a more permanent solution. I think you said you once used the motor from a 110v AC drill. But, since the drill may not have a continuously variable speed, I'd need a rheostat. Where to get one? I do have an old drill. Or, what about taking a Speed 400 or larger can motor, and an ESC and receiver and using a spare transmitter to vary motor speed? Sounds kind of wasteful, but might be easier than trying to go the rheostat route. What are your thoughts.

Regards, Gordon

[Dave Robelen]

Hello Gordon,
You are off to a good start. As far as the tunnel shell, there are a couple of additions needed to the geometry. First, the entry of the tunnel needs to be enlarged much like a trumpet bell except for your work it can be a simple matter of keeping the square crossection but tapering it open until the entry is about 4X the area of the test chamber. This is where the screen and straighteners should go, and yes a double layer of grids would be much more effective. The incoming air should "see" the grid first then the window screen directly behind it.
Back at the rear of the tunnel you need to create a transition section that goes from the square chamber to a round section surrounding the prop. Ideally, the test chamber crossection should be a bit smaller than the area at the drive prop. This helps get the airspeed up without the drive prop working so hard. Also without the transition, a round prop disk in a square tube will have enormous losses.
If you can hook up with a variable speed AC motor, it is likely that a decent light dimmer will provide all of the control you need.
The Kestrel meter should be placed near the center of the tunnel near where the test props will be placed. In simple tunnels like this you can expect a velocity gradiant across the chamber. If you calibrate a vane against the meter initially, the vane may not be at the same actual velocity as the meter but it will still provide an accurate indication as long as it stays in the same place.
When you are set up to take data you will need to provide some tare weights on the scale arm for each airspeed to account for the drag of the motor and mounting system. (prop off of course).
Till next time, Dave Robelen

[Dave Robelen]

Hello Gordon,
This is sort of a PS to the above post, but there is another attractive option for the drive fan. I have had good success with the "box fan" type of fan units. The 20" size works very well. Considering that you will need only two or three velocities for the prop testing, the switch on these fans should do the trick. Since they are regulated by the 60 hz freq. they tend to land on the same RPM regularly.
To answer another of the questions, I would place the test prop about midway between the front and rear of the test section. Prior to running prop tests, it would be useful to place a few light narrow mylar streamers in the test area to verify smooth reasonably straight flow.
Regards, Dave Robelen

[gjohnson]

Dave,

Thanks for all the details. I had wondered about the bell shape and what I would do around the drive prop (thought about just putting a round cardboard tube inside the square tunnel, but might make lots of turbulence). Sounds like the crossection of the bell should be about double the test area because of the 4-times area rule for doubling the diameter of a circle.

Now I'm thinking more of making the tunnel in cardboard "stages" with transition cones to connect the stages. I'll draw that up and post. Last night I did the drawing with my four-year old, who decided he wanted to learn drawing on the computer. I hurried it up while listenting to "dad, I want a turn."

The screen behind the grid, is it just regular household window screen? Won't this make it really hard to get much wind speed? Is there some other screen I should get that is a bit more porous? Again, do you think I need a grid immediately in front of the drive prop?

In terms of a box fan, would that eliminate the need for a round section, or did you mean to take the fan and motor out of the box fan? A thought just occurred to me. If I had a grid in front of the fan or prop, I could adjust the air speed around the three fan speeds by inserting pieces of paper in front of the grid, similar to how a Semi Truck has canvas flaps over part of the radiator to restrict flow in cold weather. That way if I want 10mph and I'm getting 12mph I could slow the air flow down. Alternately, I like your idea of a household dimmer switch. I could also wire one of these up in the power cord going to the box fan. I think this would allow me to fine tune the speed. Or, should it go between the fan's three-way switch?

I think you are saying that I can have the Kestrel flow meter or a measurement arm (with a flag and weight) at the side of the chamber as long as I know what their corresponding reading in the center of the chamber would be. So, since the Kestrel can probably only protrude about 2.0 to 2.5 inches into the chamber, for each speed on the box fan, I should take a reading in the center and then a corresponding reading at the side. I might find that 8mph at the side corresponds to 10mph at the center. So, if that's the case, why not do this calibration and dispense with the measurment arm and just have the Kestrel poking out the side where I can read it and a table for converting measured (at the side) into equavalent at the center flow speed? I also like this because the Kestrel, or measurement arm is towards the side and out of the main flow. The only question I have is the Kestrel crossection appears to be about 2 inches across, will this cause more turbulence than a measurement arm and interfere with the prop being tested?

You wrote:
When you are set up to take data you will need to provide some tare weights on the scale arm for each airspeed to account for the drag of the motor and mounting system. (prop off of course).

I think you are referring to the 90-degree arm with the test motor on it? My digital scale has a zeroing feature. So, for my static tests so far, I have always had a lead weight on the arm, which causes a certain amount of weight on the scale. But, if that weight is pushing down on the scale when I turn it on, it zeros out and measures "incremental" weight. So, I thought I would use the same technique in the tunnel. Once the tunnel was running at the desired speed, I just turn on the scale, it zeros out, and then I turn on the test motor and start taking readings. Does this make sense to you? I can probably select a single lead weight sufficient for the highest wind speed and just leave it up to the scale for all speeds.

One final question, for now (I feel like Columbo). Do you have any idea how much larger the test chamber should be than the largest prop size to be tested?

Now, where do I find box fans in New England in the middle of the Winter? :p

Gordon

[Dave Robelen]

Hello Gordon,
OK, I will try to remember all the questions (they were all good ones). First, to clear up the entry cone, the crossection area of the entry should be at least 4X the crossection area of the test area. The reason the screen does not pose a blockage problem is because the airspeed is 4X slower at this point. Bringing in a large volume of air and accelerating it is the method of getting the velocity more constant across the test chamber.
The concept of moving the Kestrel meter is fine as long as you know what the correction factor is.
The blades of a fan should still be in a round enclosure faired into the chamber otherwise there will be large losses in airspeed and uneven velocities across the test area. The same goes for putting blockage in front of the fan, this will propagate up stream and cause uneven flow. Controlling the speed of the fan will be more useful. Also, when testing props the absolute speed is less critical than knowing what you have at several points to develop a curve.
When I was discussing the need to compensate for the drag of the motor, etc, I understood that you can tare out the weight of the rig. The idea is that when the air is flowing you need a method to bring the reading to zero without the prop. I have a set of weights with one for each airspeed for this purpose. It would go something like this, zero the scale with the prop off. Turn the air on and add weight to the arm until it is at zero again for that airspeed. Leave this weight on, add the prop, rezero and take the data point with the wind and prop running.
As for finding a fan in New England in the winter, it is presently 21 deg this AM and our fans are in the attic;)
I probably missed something, so let me know where to go from here.
Regards, Dave Robelen

[gjohnson]

Dave,
Now I understand about the tare (what does that word mean?) weights. This means for a given motor I first figure out the approprite weights without prop for various speeds, then put the prop on ONCE, and then proceed with tests. It's all clear now. I wonder if I should invest in a set of calibrated weights rather than hacking up lead weights for model airplanes?

So many subtleties ... but oh so fun.:)

Gordon

[Dave Robelen]

Hi Gordon,
A few more thoughts on your rig. To keep interactins from corupting the data the mount inside the wind tunnel should be designed so that the prop is directly above the vertical arm. This way the weight of the prop coming and going will not alter the apparent force. I realize the this will involve a jog in the vertical arm but I belive it would be worth it.
On another area, I found that I needed to mount a set of tail feathers behind the motor and prop to keep them steady in the airstream. Without the tail the prop would tend to go into sort of a "whirl mode" and shake the daylights out of the mount. My tail is really simple, a light tubing boom, and stab + fin cut from foam picnic plates. The fun goes on---
Regards, Dave Robelen

[gjohnson]

Dave,
I picked up the last couple items I need. The florescent light plastic grid comes 24 inches wide. So, unless I want to start patching a larger grid together, this implies an intake area 24x24 inches square, and a quarter sized test area 1/4 that or 12x12 inches. For testing smaller props I think this might be ok. But, what about, say, if I find I want to test a very high pitch 9-inch prop for indoor floater use? Would the prop be coming so close to the walls that there would be interference? How much space do you think I need around a prop size? This will help me as I plan how big to make this thing. One thing I'm trying to keep in mind is I can't forsee myself testing a motor larger than a DC5-2.4, or maybe at most a DC1717 (I'll leave the park flyer 280 and 400's to you)

Regards, Gordon

[Dave Robelen]

Hello Gordon,
There are a couple of possibilities. First, it is well to consider that there will be some "wall effect" over a range of sizes. Second, if the main application of testing larger props would be with the "floaters", then the airspeeds would likely be lower and this reduces the interference. For strictly propeller work I would increase the chamber to 14" X 14" and to smooth the flow, use two layers of window screen seperated 1" behind the grid. Also, making the angle fairly shallow on the intake will promote smooother flow. For your application, this ought to get the props in decent flow.
Regards, Dave

[gjohnson]

Dave,

Thanks. I had wondered if for low speed props the wall effect would be less, now I know. Would there be any dissadvantage to putting one screen in front of the grid and the other behind it -- kind of a sandwich? Also, by stacking two of the grids and gluing together this will result in a grid thickness of about 3/4 to one inch (not sure right now). Is this sufficient thickness.

I'll draw up a CAD plan and post it. I had been thinking of making the angle shallow, but I just don't have a feel for how shallow I can make it. It's a function of how long I make the tunnel. So, I have to play around with the dimensions a bit first.

Regards, Gordon

[Dave Robelen]

Hello Gordon,
All of the screens should be behind the grids. Even 3 layers of grid would not be excessive. The concept is that the grids will straighten the flow and the screens will smooth it. The intake opening being larger than the chamber will provide the constant velocity across the chamber. Wall friction is the reason for a velocity gradient across a constant section tube, so we pull in extra air to compensate. Also the flow will be slower at the entrance, which makes the screens more effective and less draggy there.
Cheers, Dave

[gjohnson]

Dave,
Earlier in this thread you laid out how to estimate flying speed, and then how to determine pitch speed required. In the thread for your Pixel, you mention that because it is a bipe, you estimate it's effective wing area to be 80% of its true wing area. So, for estimating the flying speed of a biplane, say a Fokker D-VII:D , what adjustment do I do? If I reduce the wing area in the computation, that will increase the flying speed. But, we all know bipes fly slower.

On a related note, can you recommend a primar or basic reference book that has some of these things in it? Or are these just things you've worked out and "know". Thanks for putting them down here, as I'm using them more and more.

BTW, years ago in college I took a class on the physics of flight, which satisfied a science requirement. I worked hard for my "B", but had not taken calculus, let alone matrix algebra, at that point. So, some (a lot?) of it was somewhat over my head, and I memorized what I had to for exams. Maybe now I can get more out of a reference book.

Regards, Gordon

[Dave Robelen]

Hello Gordon,
You pose some interesting points. For example "we all know that bipes fly slower" Do they? The main parameter that I know of that influences the minimum speed is wing loading. The airfoil is also a player to a lesser extent. The only reason a bipe could fly slower than a monoplane would be if the net wing loading were lower due to a greater area for a similar weight.
My method for calculating the minimum flying speed involves the following formula.
Wing area(sq.ft)Xairfoil factor X .002378 (the constant for sea level air). This is divided into 2X the weight in pounds, and the square root of that is the speed in feet/second. Divide by .682 for MPH.
I have been using .6 as an airfoil lift factor for the thin curved balsa wings. This seems to work out about right.
That business about using 80% of the area for a bipe is dealing with the mutual interference between the two wings.
Working with the Pixel I have found that props that are geared 4.2-1 and have a pitch speed on a fresh 3-cell battery of about 1.5 times the predicted flying speed produce very reasonable results. I am running a 5.4" dia X 5" pitch prop on my 4.2-1 Toytronix M-20 and the cruise is at about 75% throttle.
My source for these formula are the result of years of doing these same calculations on the job at NASA . I have never seen a single text that does a reasonable job of providing performance calculation methods for model airplanes. I wish there were one.
Regards, dave

[gjohnson]

Thanks Dave,

How about a primer article for RC Microflight?

Maybe I should have put "we all know" in quotes about bipes. I've read it often enought that everyone says it as if it's a universal truth. I had wondered if it was the interaction between wings that resulted in the 20% effective reduction. Now I now.

Did you get my private message about gears? If not, please email me at gordonjohnson@attbi.com

Regards, Gordon

[Dave Robelen]

Hello Gordon,
That's strange about the message. I found your message last night and sent you a reply by e-mail. Perhaps one way or another we will get this thing together. Just to be certain, I will repeat the E-mail.
Dave

[gjohnson]

Dave,
You say you use 0.6 for a lift coefficient for thin curved balsa wings like the Pixel. What would you suggest for a Kolibri/Stubenfiege type of thin curved mylar wing with no ribs?

Gordon

[Dave Robelen]

Hello Gordon,
These are all estimates that are open for refinment. I would work with .5-.6, mostly because of the very low aspect ratio and thin airfoil. If the aspect ratio is increased a bit and curved ribs are used this could go up to .6-.7. When you know the area, the airspeed, and the weight, you can twist the airspeed formula around and solve for C sub l. This is much easier done indoors in still air flying across a measured distance and timing the runs. When you have a speed in feet/second you are most of the way there.
Another point, these numbers are for a case of cruising just short of the stall, the higher the airspeed, the lower the lift coefficient.
Regards, Dave
PS The mail came, thanks