Strain gauges to measure wing tip twist

[aa2105]

Hi
for an experiment i need to measure the twist of the wing (a plate with a foam covering) at the tip using strain gagues (i have 2 full wheatstone bridges; one to measure bending and one to measure torioanal strain) which are located close to the root. i have calibrated these using known loads at the tip and measurign the displacement. ie. when i run the wind tunnel and get a voltage reading from the srain gauges, i can correlate that to a tip displacement (load was applied at elastic/flexural axis). (not sure how to calibrate the torsional gauges just yet...apply torque and measure twist perhaps...). for now assume its calibrated.

how can i use these the bendign strain results and use them to get the twist at the tip?

thanks

 

[dik]

I don't know about aircraft, but if you require strain gauges, then you might be better served by using them to measure the change in length of something attached to a 'rigid' base and the component of the wing. You might also consider LVDT's for linear measurement. I wouldn't normally use a strain gauge for that type of application.

Dik

 

[rb1957]

i'm sure you Could use a wing root strain gauge result to determine wing tip twist, but it would involve a lot of assumptions ... to the point of being meaningless ?

what is the spanwise loading ?? center of lift ?? etc

i would suggest a stereo-scopic camera setup ... it'd be neat if you could arrange for two cameras (one ahead of the wing, one behind) to both view both extremes of the wing tip what you'd want would be vectors to the tips. maybe, if this is too difficult, try to recreate the tip position on the ground based on the in-flight pictures, then measure it.

 

[SAITAETGrad]

I don't have anything to add in terms of your air loading- but I once determined the centre of twist on a rotor blade of constant cross-section on a CNC mill bed. One end was rigidly clamped while the free end was set up with LVDTs at the leading and trailing edge. The CNC machine head was used to apply a tared displacement on the rotor blade at points along the chord. It was quite effective.

Of course this doesn't help you with determining the torsion under your air load distribution but it may trigger an idea for you.

rb has a good idea of a stereo-scopic camera set-up. Always nice if you have unlimited resources. But if you can get a view perpendicular to your wing cross-section, one good industrial camera should suffice (I think?). Using LVDTs at the tip as Dik suggested is probably the easiest method.

 

[graemew]

I agree with rb1957.
In a wind tunnel the aerofoil section will generate lift according to its incidence, which if it twists, will be a variable.
I would suggest a photographic approach as well. Use small wire uprights at the LE and TE of the tip with a small knob or cross bar at each upper end and use a fixed known camera geometry relative to the tip.
Calibrate the rig out of the tunnel using known tip torsional and bending deflections against scales and protractors taking photos as you go, and then when you do your test, take photos and using a bit of good graphics software (I use Adobe Photoshop Elements since my son showed me how to use it properly) do some photographic measuring.
It will be quite accurate. You can enlarge the photos so that you can see .005" or less at the tip.

 

[aa2105]

thanks for the tips but unfortunately i have very little time and equipment to play around with.

using a camera is also another way i was going to use to get the tip deflections at the trailing and leading edges (and hence twist). basically i plan to take a picture of the wing tip (end on, so looking at the cross section) with zero speed (u=0) and then without moving the camera take a picture some time later with a higher tunnel speed (say u = V). of course, the wing will have displaced upwards by a bit. i then plane to superimpose the picture at u=V on the one at u=0 and get the tip displacements.

but im not sure of a program that can do this .... does anyone know of a program (preferably free!!) that i can use to superimpose two pictures and then measure distances within the pic?

thanks in advance .

 

[rb1957]

you've got windows in the tunnel, right ? (so you can see the test piece under load).

whilst i'm sure there's some sort of graphics analysis program that can "mess" with the before and after pictures, i see the camera as being a distance measuring device (you could use a laser, but probably a camera is cheaper?). i'd set up (and lock down) two tripods for the L/E and use the camera to focus on the L/E to measure the distance, and a little geometry and you've got your answer. maybe it's enough to mark places to set up the tripod ... all you're doing is establishing a baseline for your triangulation.

alternatively use the picture to deflect the wing (outside of the tunnel) where it's easier to measure the displacement.