# Q: How solid does a wing need to be?

• posted

Just a question out of curiosity: How do you determine how much force a wing needs to be able to withstand? I'd imagine it's some function of the planes weight vs. angular displacement capability or somesuch? I've googled around for any info but pretty much came up with nothing useful. What I'm curious about is mainly a way to figure out if a wing will withstand the forces exerted on it during flight.

Thanks a bunch as usual :)

Jenni

• posted

Wow, that's a good question. Everything I learned about designing wings I learned from building magazine plans designed by other people. I've had only a couple of cases where the wings failed in flight due to flight loads. In one case I used dinky plywood as a center spar joiner and it failed during a loop. In another case a really lightweight .020 powered plane's balsa strut failed at a glue joint during a loop.

Over the years I have noticed that 40 sized planes tend to use spars made of 14" spruce or 3/8" hard balsa generally. Assuming that the center joint is strong, these wings typically stand up to just about anything besides a severe crash. There are other general sizes that apply to 049, 20 or 60 size, etc. I've adopted these generalities as a rule of thumb, and it always seems to work.

But of course you specifically asked how you would determine the flight load capabilities of a given design. Well, I think the only way to do it would be to build a prototype, place the wings between two chairs with the support around midspan on either side, then pile weight on the center section until it breaks. Then you divide the weight by the weight of the airplane. This will give you a rating, i.e. "This plane will withstand 12 Gs." I believe that's the way it's usually done in full scale aircraft. They do a lot of design, engineering and math. Then they build one and destroy it.

• posted

Hi

It could be argued that most - if not all - model aircraft are over-engineered anyway. For example, even in a full bore vertical "arrival" (thankfully not very often) I have noticed that whilst anything forward of the wing is usually reduced to matchwood, anything rear of the trailing edge is usually completely intact. This would suggest that there is ample scope for making the rear of a model much lighter - no bad thing anyway. Likewise, when wings fail in flight, it is invariably at the centre section, indicating that the spar could easily be reduced in size without compromising strength - I must admit that personally I always use 1/4inch spruce rather than 14inch (just being pedantic) 8^)

Some years ago, I was in an airport standing by the gate waiting to board my plane when I overheard two pilots talking about the - then newly introduced - Boeing 777. According to one of these pilots, to test the wings, they bolted a complete 777 wing down at the centre section, then applied a load at each wingtip to see how far it would bend. Apparently, it was well past the vertical before it failed when, because of all the composite material in the construction, it literally exploded.

Personally, if I was staring out of a plane window and the wings were approaching anywhere near vertical I would be panicking a bit!!!!!

Regards

KGB

• posted

The problem I'm having is this: measuring what a wing can withstand should be easy using the method you mentioned. But I've got somewhat of a hard time trying to remember physics class and calculating the actual G forces exerted on the wing. All I can think of is that there are two apparent forces working on a wing: Inertia and air pressure. I'd imagine in straight flight (no matter the actual angle with the exception of free-fall) the force ought to be more or less 1G. But I go all cross-eyed trying to figure out what happens when I pull, say, a loop with a radius of, say. 5 meters. Or a roll. There are in essence two things I don't know:

1. What determines how tight my aircrafts maneuvers can be. I'd imagine that is pretty much a matter of control surface size, but the actual details are a mystery to me.
2. What formula gives me the forces exerted on my wing.

Thank goddess so far none of my wings have failed, so it's really more of a "hmmmm wonder how..." kind of question. Not to mention, good to know :)

• posted

I guess in that case I could consider all my wings adequate so far. They don't bend - they can't. They could theoretically warp, I have to check that when I'm back home from work.

So when I understand what you're saying correctly then I should really have ample warning before the wing fails?

• posted

The answer to number 1 has more to do with wing loading and lift than control surface size. If you find the smallest loop you can make and then increase your elevator size and throw, you'll find that instead of a smaller loop your plane will do a high speed stall. That's why I build most planes lighter than they were designed. They're more fun to fly because they don't stall out of a turn and they can turn around quickly.

#2 is a trickier question, and I'm not sure there's an easy answer. The force exerted on the wing is lift in one direction and weight in the other, and you would have to know how much acceleration you were generating in a turn or loop to be able to multiply it. The only way I know of to do this is to install a G meter in the plane. There's also the bending force of drag and the negative G when you do an outside loop or landing. Of course a wing can be stronger in lift than landing, and most of them are.

In a practical sense, when you build a plane you can tell pretty quickly whether it's adequate or not. The most important factor is that you want it to be rigid so that the aerodynamic characteristics will not change in flight. Your biggest clues are flutter and warping. You can usually tell when that starts happening. It's kind of like when you don't put enough rubber bands on your wings. You're flying along and all of a sudden everything changes. I had a couple of planes like that. I designed them for slow flying, but when I installed too big of an engine they would start wandering around and drastically changing their flight characteristics under severe load.

• posted

The other way around. Engineers design the wing based on the strength of the materials, their arrangement, attachment and anticipated loads and load cycles and fatigue modes and a bunch of other stuff, and then they'll load a wing to confirm their numbers. They don't eyeball something together and bust it to get a G loading; it's not necessary and costs a lot of money and time fooling around like that. They don't build a bridge and then break it to make sure it's strong enough, do they?

Dan

• posted

Years ago Nova did a documentary on the 747. They showed footage of a wing stress test that was very impressive. The 747 wing could be bent up

28' as measured from the wingtip in a normal flight configuration. The main spar exploded spectacularly when the tip hit 28 feet.

g.

• posted

... | > But of course you specifically asked how you would determine the flight | > load capabilities of a given design. Well, I think the only way to do | > it would be to build a prototype, place the wings between two chairs | > with the support around midspan on either side

Alas, that's only an approximation of the load on the wing in flight. The reality will be different.

| > then pile weight on the center section until it breaks. Then you | > divide the weight by the weight of the airplane. This will give | > you a rating, i.e. "This plane will withstand 12 Gs."

... of course, other parts of the plane may fail before you reach 12 G's. Your wing supports may break off first, with a lawn dart hitting the ground, then a really strong wing flutters down later, probably undamaged.

And having the wing break is not the only possible problem. If it bends a lot but doesn't break, that'll affect how the plane flies. I've got a spad that the wings bow a whole lot in a loop. It doesn't break, but it makes the plane fly like ass. Some carbon fiber fixed that up, but if I ever crash it the plane won't be so resilient.

On the bright side, most wings bend before they break, so if you're paying attention and see your wings bend, you know to ease off the stick somewhat. Alas, this is most noticeable on large aspect ratio planes like gliders, and not always so obvious on planes with smaller wings.

| 2. What formula gives me the forces exerted on my wing.

I believe it's relatively complicated. If you were in a loop, and knew the radius of your loop and your airspeed, you could calculate the force there easily enough (but actually knowing these values might be hard), but to go beyond that, especially without flight testing, will require a whole lot of math.

The usual way that most models deal with this is to make really really strong wings. Sure, they're heavier, but it's better than the alternative. And really, they're not that much heavier if done right.

I'll bet the dynamic soaring guys know a whole lot more about this than most of us -- they fly their gliders in loops, over and over and over, often well over 100 mph -- the world record is 302 mph.

• posted

yes, they do...or at least as an engineering student that was what we did.

How else do you arrive at the material strengths in the first place?

Wings? for a lightweight free flight 2-3g is enough. For a sports aerobat 4-6g, for a high speed glider maybe 10-20g.

In general if you take the speed of the model, and work out the lift at stall for the wing at its highest speed, and then translate that into g force, you will be somewhere near what you need.

Beacause its not possible to force the model into a higher force mode than te stalled wing will achieve.

needless to say many models are not stressed that high..wing folding is fact of life in many.

• posted

In article , The Natural Philosopher wrote: | Dan_Thomas snipped-for-privacy@yahoo.com wrote: | > On Jun 3, 10:53 pm, Robert Reynolds wrote: ... | > They don't eyeball something together and bust it to get a G loading; | > it's not necessary and costs a lot of money and time fooling around | > like that.

In the case of full scale planes, they generally do both.

They design the wing, with a target strength and other factors in mind, then build it (perhaps a mockup first?) and see just how strong it really is, and how it handles things up until it fails.

They have a really good idea of what to expect before they break it, but the costs of failure are just too high (people die!) to skip that step. Though I'll bet the computer simulates things so well nowadays that actually breaking a wing is mostly a formality -- but still one they don't want to skip.

• posted

Well I don't know about that. Your center section joint may fail without warning, like a couple of mine have done. But the wings themselves will typically give you warning before failure. You may think that it was radio interference, though.

• posted

I didn't say everything was eyeballed and designed by trial and error. I said they design it using known parameters, then they test it to make sure. New airplane designs are always tested.

Obviously complete bridges are not treated the same as aircraft. But you can be sure that there is a laboratory somewhere testing all of the steel and concrete that goes into the bridge.

• posted

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