Aeronautical Engineer's advice needed

Pierson

I'm trying to help out my Dad with a project he's been working on for a while. We haven't discussed·it recently, but I'd like to do something for Christmas.

He's an aero engineer, and has been building model airplanes for over 60 years. He's interested in airfoils, and to satisfy an urge, he built a NASA Baals wind tunnel. The problem is instrumentation. He wants to use electrical strains guages with an amplifier. He so far hasn't come up with an interface solution.

My idea is to use an H48C and a Stamp to measure forces on the airfoils that he's testing. I'm just trying to find out from you guys if I'm in the ballpark with my idea. I have a little schematic I'm trying to upload into this document that will explain it better than I can type it. I hope that it at least comes through as an attachment.

Pierson

I think I may be in over my head. An objection raised was that any 'shift' in the center pole would change the angle of attack of the airfoil.

tperkins

AFAIK you need to measure the x, y, and pitching moment forces. That's without looking at tip and root flow effects, and assuming a constant chord, no sweep, and no washout. Also, this assumes the airfoil is fixed so it can't move--no dynamic effects.

To cover all that, you'd need reflective bits that are bitmapped for photogrammetry resolving about 6 degrees of freedom.

As it is, you need to measure three.

Assuming an airfoil of 0 sweep and constant chord how are you mounting it? I don't see a sting.

Post Edited (tperkins) : 11/18/2006 4:14:13 PM GMT

Pierson

tperkins said...
AFAIK you need to measure the x, y, and pitching moment forces. That's without looking at tip and root flow effects, and assuming a constant chord, no sweep, and no washout. Also, this assumes the airfoil is fixed so it can't move--no dynamic effects.

To cover all that, you'd need reflective bits that are bitmapped for photogrammetry resolving about 6 degrees of freedom.

As it is, you need to measure three.

Assuming an airfoil of 0 sweep and constant chord how are you mounting it? I don't see a sting.

As I said, I'm in a little bit over my head. The way it's set up now is there is a wing·section with no tips on it. It is mounted on a tube that goes through·the center of the wind tunnel and a hole in the wing section. The·section can be manually pivoted to change the angle of attack. It can also rotate/change yaw. So, it doesn't necessarily move, but it can be.

I have no idea what you're talking about·with reflective bits, bitmaps, photogrammetry. Help, please?

tperkins

So if you are only going to measure the forces in the x and y directions and the moment about the pitch axis, how many ozs or pounds (and oz.inches) do you think the airfoils can generate? Your Dad should be able to give you a close idea of the maximum forces.


Photogrametry is where you measure the distance between known points in a photograph and compare them to unknowns to determine unknown distances. In a wind tunnel where the model is free floating, it can be used to find the forces on the model. Motion capture (without resolving the forces involved, just displacements) is what the Hollywood people call it.

Yours, TDP, ml, msl, & pfpp

AIman

Pierson,

I was a chapter president for the EAA (Experimental Aircraft Association). They either know how to get stuff like this set up or can point you to resources that can or will help. The EAA is specificly set up for people who want to build or work on their own airplanes and I know that the BEDE series on aerodynamics goes into that stuff. Granted the series is about a dozen tapes and costs a bit...

Anyway, the link to the EAA home page is http://eaa.org/
If you can't find what your after by browsing the website email·or call headquarters and they will do what they can to help you. Their library is rather exentsize (sp?) ·and even carried a few documents on the flying saucer that Hitler was supposed to have been working on.

You can also surf through aircraft spruce, they are a major supplier of parts to people in the EAA. The link is http://aircraftspruce.com/

Pierson

AIman,

Thanks for the advice. Dad has been a member of EAA and gone to Oshkosh every year for over 25 years. I believe he first saw the Baals wind tunnel at a NASA pavillion. I have done some internet research but not come up with anything helpful as far as instrumentation. The two that I remember was a graduate student who used some Lego Mindstorms parts, and her advisor asked that if I ever came up with a better solution that I send him my results. There was also a company that does aerodynamic testing on model rockets using the Baals windtunnel, but I never got a serious response from them.

I believe that what info/advice Dad got up at Oshkosh was to contact Omega.com. I'm looking at Omega's 'The Pressure Strain and Force Handbook', a sort of bible for this sort of stuff. A lot of good educational stuff, and also a catalog. But the sensors and the software are very, very expensive. I was looking for a cheaper alternative.

I'm going to ask him to bring back some pictures of Hitler's flying saucer next year. If the EAA owned one, it would be immaculately restored and on display in their museum.

AIman

I don't think the EAA owned the actual saucer, but they were very helpful in getting me docs on it for a research project.

Heres what I can pass on and this is based on a full airplane model. For only an airfoil adjust things accordingly.

Use·four tri-axis acceleramoters to measure roll, pitch, yaw and to·collect data on g's. One acceleramoter goes on each wing tip, one in the nose and one in the tail. This configuration gives more exact data because of the locations, the sensativity to·movement·and g's. Put a BS2 in the center of gravity so nothing gets affected and put in your controls for the airplane just like it were an RC airplane. In short the BS2 controls everything with nothing exposed on the outside.

Next hook in a wireless transmitter - Parallax sells ones with 500 ft ranges so they would probably be note worthy.

The BS2 transmitts data to external computer source - for recording only - while it maintains a straight and level attitude via the use of accelerometers. To change the program simply open the cover on the model, hook up your cable and download the next program.

For sensors - Parallax has several touch sensors. Mount these internally against the skin so that pressure changes can be monitored on top, bottom, and leading edges. For trailing edge use a sensor to monitor the pressure on the push rod moving the aleron. If so desired an IR can be mounted on top bottom, leading edge, trailing edge and both exterior edges of any model, this gives measurments to each of the locations within the wind tunnel and by tracking changes, you can also get things like rate of climb, accuracy of acceleramoter changes, turn and bank·etc.

All told, without the wireless the Basic Stamp can do most of this. With the wireless you get immediate data.

I think I spend to much time at the airfield...

AIman

Oh ya, one more thing. When downloading data by whatever method, down load each sensor individually. It give much more to play with and with the Stamp maintianing straight and level flight you will get better results then if you had a model to fly BECAUSE the Stamp can react faster then we can.

I would personally build a seires of programs as follows:

Straight and level Flight
30 degree turn
Level to 30 degrees and back
Climb up to half of stall
Climb to stall angle
Descend to half of VNE
Descend to VNE

And, of course, combinations for the fun of it.

John Abshier

To characterize an airfoil as you vary the angle of attack you want to measure two forces, lift and drag, and one torque, the pitching moment. Strain gages come to mind, but may be more trouble than you want. Ignoring the complexities of the complete problem for now. What if we just want to measure the up or verticle force. If the verticle support is free to move up against spring pressure, we now have a displacement to measure. From the displacement, we can get force by Hooks law. A linear pot could transform a displacement into a variable resistance. If there is a college nearby, read up on process control. A good library may have or be able to get OLD aeronautical engineering books that may show how measurements were made 50 years ago. These may be better adapted to cheap, home approaches than the current state of the art. I just looked at my old textbooks. Unfortunately, I was in structures. The one picture of a wind tunnel I found had the model supported by two verticle supports which went to balance scales.

tperkins

The degree to which a opaque plate moves between an LED and a photodiode can provide noncontact measurement of displacement and rotation. I suspect you will want to have the airfoil generate forces against the least resistance possible, because I suspect your forces will be low in absolute terms.

Yours, Tom P.

Pierson

All,

Thanks for your ideas. They show ingenuity.

Looking at the old school books is a great idea. I'm sure Dad still has his college texts.

Keep the ideas coming!

bubblehead

Maybe you can find/hack an old notebook computer that has a stiff-stick type cursor control devices (they look like an pencil eraser embedded in the middle of the keyboard. They basically use strain gauges to determine which direction you are trying to move the 'eraser'. One of them should get you strain in two dimensions, so you may need several. I know Thinkpads had them, my current Dell does to.

Some of the buttons on a PS2 control pad are force sensitive. I have no idea how they work or what kind of resolution you can get from them.