Hi Guys,

 

 

I was reading a spec for a commercial UAV and it stated that the design was capable of +/- 6G. I also noted that some catapault launchers generate a 15G load on launching.

 

Does anybody have an idea of the kind of G forces you see in hobby UAV flying? I was wondering what kind of loads the foam wings could take. Do you think a 6G loading on the wings necessitates making wings from carbon fibre or similar?

 

 

Ollie

 

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I use simple rules:

30% of material strength at 10g loading on all three axis

Simple fast easy math good baseline.

We have reduced weight (minimullay) by going to 30% at 6g;  however we developed fatigue cracks in certain areas.

So there is no easy answer. depends on material, application, loading, service life, and what you had for breakfast. :) 

 

Hi Ehren,

 

Thanks for your reply. I am trying to make an airframe for a UAV using plastic parts coated in a thin nickel alloy. The plastic parts are upto 10 times stronger and stiffer when coated.

I decided to design the wing as a load bearing spar with a seperate leading and trailing edge, this is the easiest for me to make.

I have plenty of ideas on the design of the spar etc but what I am struggling with is what loads to put into the model to validate it....Im using Solidworks to do the analysis.

Very roughly speaking if the total weight of the uav is 7Kg and it is seeing + 6g is the calculation:

7x6x9.81 divided by 2 (for half the wing ) = 206 Kg load

In Solidworks I can apply the load over the top surface of the wing spar and then analyse the deflection and stress. I can then look at the structure from the other axis. Does this analysis technique seem suitable to you or would you do it another way?

I am doubtful that the material I am making will survive such forces unless I can strengthen it with CF rod or perhaps some kind of additional CF spar that fits inside the wing. I was trying to design something that needs no tooling to manufacture.

 

Ollie

 

 

 

Doh!

 

Just realised I got the calc for the g load wrong it should:

 

7x6x9.81 divided by 2 (for half the wing ) = 206 N or 21 Kg

 

This a much more achievable figure for my plastic/metal wing

 

 

Ollie

 

 

Two comments:

On using metal covering over rapid prototype core.  That is a useful technique. I've used it as a way to make master plugs for an air intake assembly. We made carbon molds off the chromed plastic plugs then heat treated the molds to use in the autoclave. Worked great but the master plugs were destroyed in the process. Just used recently as a display for old engine. This is an SLA plug that was chromed.

Works fine but I've never used it for a part under load. Your wings will definitely look cool.

 

On solidworks- I use it every day.

The simulation is pretty good I have some extremely accurate simulations compared to the actual event. On the other hand I have some that are way off.  I suggest building test pieces and compare them to the simulation results. Doesn't have to be a full size or full load. In fact as I get closer to the max material strength the shape of the mesh in the FEA model can affect the results.  So I don't go there.  I'm also not sure how well SW does on secondary stress of composite sandwich. I would check the buckling load of the compression skin.

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