Beam deflection of composite landing gear

[thechainrule]

Hello,

Im looking to see if anyone has any insight on measuring the beam deflection of of a composite beam for the main landing gear of a UAV (during a landing event). The aircraft weighs roughly 350 lbs and incorporates a wish bone style rigid main gear which is made from a carbon composite.

Ideally i would like to setup something up in such a way that I could get both the left and right independently. My initial thoughts were to use a piezoelectric strip on each leg similar to a lab I performed in school but I have forgotten how to accurately do that. Am I on the right path, have any how tos on how to setup a piezoelectric system or know of a better way to accomplish what Im looking for?

Any help is much appreciated!

Thanks!

 

[YoungTurk]

Strain gauges? Laser displacement sensor?

Do you have somewhere you can stow data acquisition gear?

 

[MikeHalloran]

You can get a one-time indication of the maximum deflection by having something frangible extend from the fuselage toward the ground. Gauge the deflection by the degree to which the object is bent or abraded.



Mike Halloran
Pembroke Pines, FL, USA

 

[Kwan]

two accelerameters (one near axle, one near gear to wing attach). integrate twice

 

[amorrison]

Fuselage mounted video camera pointing at the beam.
Print out the max displacement video frame.
Get out the ruler.

 

[GregLocock]

"two accelerameters (one near axle, one near gear to wing attach). integrate twice"

won't work, unless you do a lot of work. Even then, it won't work.

Stringpots are the way we do it, for most suspension work, and cameras are the we way do it for crash. We also use frangible (like it!) foam or modelling clay to work out how close things are to touching.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[thechainrule]

Thanks alot! Yeah Im looking into string pots now. Iorginally started with the simple frangible device idea, which I may end up falling back on.

Some updates to the problem though

beam is aluminum 6061-t6
ideally i would prefer a portable solution something that I could mount to the aircraft and fly with.

There are 4 places on the beam that I am really interested in observing the stress at, so ideally i would like it if I could make a device using those thin film strain gauges I would prefer too. I found an all in one package that was pretty small in size but the cost is damn near 40K and emits RF signals, that will never get approved.

I remember doing a lab like this in school that could not have cost more than $100 to build. Just some resistors, wire, and a dc power supply. Relative error was around 3-4%.

 

[mudflaps]

Aluminum landing gear isn't composite. I assume it tapers in plan but has constant thickness. The taper makes calculations difficult, however deflection should be straight line as long as you don't exceed the elastic limit of the material. My suggestion is to suspend the plane slightly above ground and measure from a line between the wheels to the bottom of the body. Then set it on the wheels and measure again. I assume you know how much weight the wheels carry, so you can calculate a spring constant from what you know at this point. What you don't know is how many "G's" you pilot is going to get from a crappy landing. I suggest you buy a whole lot of props!