Non-Destructive Load Test -- Wing

[Wylbur]

I am thinking about some aircraft designs, and what concerns me is how one can determine if a wing (as designed and built per plan) can handle a 4G load.

What I really want is 3.5G, but 4G will give a margin of error.

Suppose that one decides to take a design and modify it to be a twin. In this case the weight of the plane will increase by about 700lbs (after removing the engine from the front, building engine nacelles, extra battery, etc.).

So if the original aircraft was based on a 2,500 MTOW (Max T/O Weight), and now it will weigh about 3,200, how can one determine if the wings can handle 12,800 lbs (w/o destructive testing)?

I realize that the landing gear and associated mounts will have to be beefed up. I'm just trying to figure out a few things not being a BSME (Bach. Sci. Mech. Engineer).

Thanx in advance
Wylber -- a Wrong Brother ;)

[Matt Gonitzke]

A complete re-analysis of the entire structure is the only way. You're designing a completely new airplane at that point. Turning a single into a twin is going to change stability and control aspects (for example, the vertical stabilizer may not be large enough in the event of an engine failure) and many other things.

[Wylbur]

Matt: Before we go there, I am not putting the engines on the wings.

Let me ask my question another way. When one looks at the weight on a wing for wing loading purposes, how does one determine the fulcrum? Now how does one determine the "spread" of the weight on the wing? From that, does one calculate the strength of the main spar for the weight, or is it spread among the main and sub spars?

I'm not a mechanical engineer. I'm trying to learn from others who have done all of this.

Regards,
Wylber -- a Wrong Brother

[WLIU]

"I'm not a mechanical engineer."

I will suggest that you are about to become one if you want to do the calculation for your purposes. There are a number of books that provide general explanations of how wings are loaded, and then you get into beam calculations for your specific wing spar. I will offer the info that a wing is not loaded evenly along its span but rather the stress is high at the root since all of the length of the wing out to the tip acts on the root, and then the stress gets lower as you move down the spar towards the tip. A wing strut changes the calculation.

Time to hit the books.

Best of luck,

Wes

[Mike Switzer]

You need a book on Mechanical Engineering Static Calculations. Mine is "Engineering Mechanics - Statics" by Hibbeler - it was OK but there may be some better ones out there. (This one was published in 1983)

It is basically vector math that you should have had some of if you had high school physics, but you really need a good understanding of Trig to be able to do all of the calculations.

Look on Alibris or Amazon, you should be able to find a used book cheap. Also, books published after the mid 80s may be mostly metric. If you are an old fart like me & prefer English units, get an older book.

[Matt Gonitzke]

In addition to Mike's suggestion, there's a few more books you'll want that are more specific to aircraft structures. Aircraft Structures by David J. Peery, (1950 edition or 2011 reprint of 1950 edition...avoid the 1980-something edition) and Analysis and Design of Flight Vehicle Structures by E.F. Bruhn.

Statics is the basis for all of the calculations, but it's not enough to actually go and perform all of them.

[Wylbur]

Thanx for the info. I don't think this should be so hard. Others have been down this road, one should be able to stand on their shoulders... But, I will get to this... I just have to finish my Commercial SEL this summer, while handling all the Honey Does.

[Mike Switzer]

In addition to Mike's suggestion, there's a few more books you'll want that are more specific to aircraft structures. Aircraft Structures by David J. Peery, (1950 edition or 2011 reprint of 1950 edition...avoid the 1980-something edition) and Analysis and Design of Flight Vehicle Structures by E.F. Bruhn.

Statics is the basis for all of the calculations, but it's not enough to actually go and perform all of them.

Without the basics Bruhn might be a bit hard to follow. I bought a copy (well, actually 2 copies, a very early one & a recent edition) sometime in the past couple years when I started working on my design. I hadn't done that type of work regularly for a few years & I had to do some "remembering". :)

[Matt Gonitzke]

That's why I said "in addition" ;)