Stress on model aircraft wing and breaks

Hello everyone,

I recently had a catastrophic failure on a model glider, it is launched from a weather balloon so the stresses involved are quite high.

We believe it may have been a wing failure, but the impact zone was so great it is difficult to tell :)

With this in mind does anyone have a cheap/simple suggestion to measure the stress on the wing (we send live telemerty via radio) through the arduino system. I have heard that you can measure resistance through a wire but I don't know an awful lot about electronics.

It would be appreciatted if anyone had some suggestions, bearing mind it has to be light and cheap.

I look forward to your suggestions

Nathan

You could use a flexiforce sensor glued to the wing, then wire it to an analog input on the arduino to send the data out over an xbee radio, like the ones found here...http://www.sparkfun.com/categories/143

thank you for the link and heads up, but they are really quite expensive and onlup to 7.75" in lengh, what I was hoping for is to run a a thin copper cable throughout the superstructure of the glider and some how measure its resistance on a set voltage from the anolog port. Then when the the wing stresses the resistance should change and if it broke then you would loose any voltage enabling you to see if the wing of fusalarge for instance had failed.

Any more ideas would be welcome remember weight and low cost are essential :)

Now, this might sound like a stupid question, but, I'm going to take that chance and ask it anyway.

what I was hoping for is to run a a thin copper cable throughout the superstructure of the glider and some how measure its resistance on a set voltage from the anolog port. Then when the the wing stresses the resistance should change and if it broke then you would loose any voltage enabling you to see if the wing of fusalarge for instance had failed.

What possible good would it do you to know, during the flight, that the wing had failed?

Typically, strain gauges are used to measure the forces on things, before they are deployed to production. If you know what forces the glider needs to withstand, you can see what those forces (or smaller ones) are doing to the structure, BEFORE you put the glider in the balloon. If the forces are too large to be handled by the glider structure, you can't change them after the glider is broken.

With respect, for the same reason that NASA have monitoring in all different parts of the shuttle. Our data shows us a lot of things but not if the superstructure failed, just in case for future flights I would like to know this and we can then present this to the manufacturer. The impact was so great it was difficult to assert if a wiling failure occurred or the ground impact caused the break.

The glider is designed to withstand the forces expected in flight but we feel on this occasion the polystyrene structure may have been weakened either in production or by UV light over time.

We are I guess developing a black box recorder of sorts...

Is it possible to do what I am asking?

Is it possible to do what I am asking?

Sure. Strain gauges are still the way to go, in my opinion. You have some idea what maximum strain ought to be seen. You can broadcast/record the actual strain. If the measured strain (greatly) exceeds the expected strain, failure of a joint can be reasonably expected to follow.

Strain gauges are not expensive, and weigh next to nothing. You might (probably will) need to send the output through an op-amp before going to the Arduino, but an op-amp should be able to boost data for several strain gauges at the same time. Op-amps are pretty small/light, too.

Check this instructable out:

http://www.instructables.com/id/Make-Conductive-Glue-and-Glue-a-Circuit/

Also check his website:

http://www.inklesspress.com/conductive_glue.htm

Note how he talks about the glue changing resistance with flexing? There you have a potentially inexpensive flex sensor that can be painted on to the fuselage of your plane. Now, I am only reccomending this if price is an issue, if this is a hobby device, etc; you'll have to do some experimentation to see if this will work for you (or if you can change the formula to make it better for that purpose, rather than the purpose of electrical traces). There also might be issues with temperatures (especially low temperatures) causing problems with such a "glue"; that again will require experimenting.

Good luck, and I hope this helps (or at least inspires?)...

:)

At the really, really cheap end of things, you could draw a line up the wing and back with conductive ink and have the arduino keep an eye on it and inform you if it goes open circuit.
That would at least tell you if the wing failed.

I would look into the construction of the wing, rather than look to measure the forces. It should be possible to make a very strong wing light enough to carry your glider safety. It is possible that flutter is the cause of the failure - a torsionally stiff wing would prevent that from happening. If your wing is polystyrene foam, a skin of thin balsa (1/32") would have a big impact on the stiffness of the wing without adding weight. How big is your model?

[quote author=Paul J link=topic=59433.msg474476#msg474476 date=1309040802] I would look into the construction of the wing, rather than look to measure the forces. It should be possible to make a very strong wing light enough to carry your glider safety. It is possible that flutter is the cause of the failure - a torsionally stiff wing would prevent that from happening. If your wing is polystyrene foam, a skin of thin balsa (1/32") would have a big impact on the stiffness of the wing without adding weight. How big is your model? [/quote]

I agree with this. Add some carbon fiber rods to your wings. Use is extremely common for R/C aircraft. If you show pictures of your wing, I can help here.

There are many strategies which can be employed to significantly strengthen your wings without adding significant weight. An extremely common approach is to grab some fabric, or more preferably, some fiberglass. Epoxy it over your wing halve joints. If you can spare the weight, insert several carbon fiber rods between the two halves, before you laminate the joint. Next, make some small routes the span of your wings just deep enough to embed/glue some carbon rods the span of the wings. If you expect very high stresses, two should be considered; one near the leading edge and one about mid chord. If your wing has no dihedral, considering spanning the entire wing with single pieces of carbon fiber rod.

Also, if you have control surfaces, flutter might also be a significant event here. Pictures and performance window expectations would really help shed some light on things. Control surface flutter can easily induce control surface failure. It can also cause the entire airframe and wings to suffer violent shearing forces as it rolls, pitches.

Also, if you are creating the aircraft, rather than buying outright, I can help you with some cheap, light, readily available materials which are extremely strong.