How do aeroplane wings support the forces and engines?

[Andrew123]

I was just wondering how these amazing engineering feats support the loads on their wings? Specifically the larger passenger aeroplanes.. such as the 737 and 747. The wings must undergo an extreme number of forces of great magnitude.. how do they not snap off? How the they support their own weight, the fuel and the big engines? Cheers!

 

[mgb_phys]

When they are flying they aren't suporting their own weight so much as supporting the plane.
The aerodynamics push the wings up - the wings are attached to the body, the body is lifted up. Thats one of the reasons for putting engines and fuel in the wings, it's the easiest place to handle the weight.

Connecting the wings to the body is a big bit of engineering, in fact since on most large planes the wing spar goes through the body, you could argue that the plane structure is the wing with a body sitting on top.
From an engineering point of view a wing is very similair to a bridge.

 

[rcgldr]

During testing of a design, the aircraft is fixed into place, and the load on the wings is increased until they snap. I've read that most commercial aircraft are speced for about 3.5g's and wings are tested to 150% of this, but this will result in permanent deformation, like bent wings and wrinkeled surfaces. General a stress test continues until it breaks the wings.

Video of a wing stress test on a 777, it breaks at 154% of design load.

777 wing stress test youtube

The wings, although mostly hollow have a very strong, but flexible structure, including the attachement to the fuselage. Generally the wings are coupled to each other independently of their coupling to the fuselage. In model aircraft, a wing "spar" is used to couple the wings.

Some radio control models, mostly specialized radio control gliders made up of carbon / kevlar fiber, can handle 40g's or so.

 

[mgb_phys]

The wings are statically tested to 150% of their maximum design load. I'm notsur eif the design load would include 3.5G - that's a lot of force!

The perhaps surprising thing is that the wings are tested by pushing them UP. Looking at a plane on the ground you think of the wings as big heavy weights hanging down from the body, but in fight they are the lifting surfaces lifting up the heavy body.

 

[russ_watters]

Whoa, Jeff, that is veeeeery cool. And amazing that they were so close on their design limit calculations.

Andrew - one thing to remember about such things is that we gain intuitive knowledge of how things work through perceptions in our daily lives. You've driven over enough bridges, looked at enough cranes, seen enough houses being built to get an intuitive feel for how much structure is required to keep them from falling apart. But your perceptions fail you here in two ways:

-Safety factor: For a plane, in the above example 150% - for bridges, I think it is 200% (or even more). So they have more than twice as much support steel as they really need (considering that quite a bit of what a bridge needs to hold up is its own weight). But I bet you'd be terrified if you went over a bridge that had less than half as much steel as you were used to seeing!

-Strength to weight ratio: Bridges, tall buildings, stadiums, etc. are all made of steel (and concrete). Steel is heavy and has a relatively poor strength to weight ratio. Most older airliners are made of aluminum, which has a strength to weight ratio of 2-3x that of steel. Newer planes include some titanium, which has a strength to weight ratio of 3-4x that of steel and carbon fiber, which has a strength to weight ratio 20-40x that of steel.

The F-15 and Mig-25 are remarkably similar in configuration and physical size, but the F-15 is mostly made of aluminum and the Mig-25, being built for the sole purpose of high-speed intercept of the SR-71 and XB-70, was made of nickel and steel to resist the heat of high speed flight. As a result, the empty weight of the Mig-25 is almost twice the empty weight of the F-15 and other performance characteristics (g-load) suffer accordingly. Ironically, the Sovs didn't have the money to buy titanium they themselves dug out of the ground (titanium resists heat even better than steel) - so they sold it to the US (through intermediaries) to build the SR-71!

 

[rcgldr]

I'm not sure if the design load would include 3.5G.

Oops, make that 2.5 g's.

http://en.wikipedia.org/wiki/Load_factor_(aerodynamics)

Not covered by the Wiki article, the F-16 can handle sustained 9g's, not sure about the pilot, typically it's done for one full 360 degree turn at an air show.

Other aerobatic aircraft can handle 10g's but this is only done for brief periods.

Radio control helicopters have thrust to weigh ratios over 5 to 1 both positive and negative. Example video (action starts about 25 seconds into the video, after engine is warmed up while the guy spins the heli):

rcheli.wmv

 

[Danger]

Excellent video, Jeff. Thanks.

 

[Andy Resnick]

That video is awesome. Thanks for posting it.

 

[Cyrus]

Whoa, Jeff, that is veeeeery cool. And amazing that they were so close on their design limit calculations.

Andrew - one thing to remember about such things is that we gain intuitive knowledge of how things work through perceptions in our daily lives. You've driven over enough bridges, looked at enough cranes, seen enough houses being built to get an intuitive feel for how much structure is required to keep them from falling apart. But your perceptions fail you here in two ways:

-Safety factor: For a plane, in the above example 150% - for bridges, I think it is 200% (or even more). So they have more than twice as much support steel as they really need (considering that quite a bit of what a bridge needs to hold up is its own weight). But I bet you'd be terrified if you went over a bridge that had less than half as much steel as you were used to seeing!

-Strength to weight ratio: Bridges, tall buildings, stadiums, etc. are all made of steel (and concrete). Steel is heavy and has a relatively poor strength to weight ratio. Most older airliners are made of aluminum, which has a strength to weight ratio of 2-3x that of steel. Newer planes include some titanium, which has a strength to weight ratio of 3-4x that of steel and carbon fiber, which has a strength to weight ratio 20-40x that of steel.

The F-15 and Mig-25 are remarkably similar in configuration and physical size, but the F-15 is mostly made of aluminum and the Mig-25, being built for the sole purpose of high-speed intercept of the SR-71 and XB-70, was made of nickel and steel to resist the heat of high speed flight. As a result, the empty weight of the Mig-25 is almost twice the empty weight of the F-15 and other performance characteristics (g-load) suffer accordingly. Ironically, the Sovs didn't have the money to buy titanium they themselves dug out of the ground (titanium resists heat even better than steel) - so they sold it to the US (through intermediaries) to build the SR-71!

Did you read the book skunk works?

 

[russ_watters]

Did you read the book skunk works?

My favorite.