A question about wing loading

[enginesrus]

I watched a video about the 787 wings being tested. It said they went 153 or so percent of design load, so what would that be in "G" load?

[Ron Blum]

Minimum required G load for part 25 airplanes (transport category) is 2.5G. So, 2.5 X 1.53 would be 3.83G (my guess). Maximum G is not required to be >3.8G. So, 3.8 X 1.53 = 5.81 (highly unlikely). Flaps other than UP requires a minimum of 2.0G (test to 3.0G is required).

Bottom line: 3.83G

PS. Regulation is 25.337. Google “14 CFR 25.337”, and as Paul Harvey used to say, “You will get the rest of the story.”

[Bill Berson]

25.303 mandates the safety factor of 1.5.
First the designer chooses the "design limit load" of say around 2.5.
Then multiply by the safety factor of 1.5 to get 3.75, which is called the "ultimate load" and used for the actual test.

I assume they went to 1.53 safety factor just to be sure.

[enginesrus]

Scary low G's, a good bit of turbulence would exceed that I'd think.

[Ron Blum]

Scary low G's, a good bit of turbulence would exceed that I'd think.

It’s funny you mention that. Gust loading (loading due to vertical velocity of the air) is actually calculated, too. The bigger (faster and higher wing loading) airplane, the less it effects them. IOW, a 20 knot vertical gust will greatly effect a C172 with a low wing loading of 10 lbs./sq. ft. and flying at an airspeed of 100 knots versus an airliner with a high wing loading of 150 lbs./sq. ft. and an airspeed of 200 knots.

Bottom line: smaller airplane wing structure is often defined by gust loads and not maneuvering loads. Maneuvering loads are what are published in the AFM/POH limitations section (i.e. +3.8/-2.3)

[jedi]

I watched a video about the 787 wings being tested. It said they went 153 or so percent of design load, so what would that be in "G" load?

I suspect the 153% was a test to failure. It amazes me how Boeing can design a wing and then test to failure and it consistently comes out between + 3% and - 0%. Great structural engineers.

What is more amazing is that they almost never come off in flight.

Well they are not always that great. The 787 failed a load test and wings needed rework. The rework added weight and the aircraft was already in production. Airlines refused the overweight airframes and flight museums got some modern airliners.

[martymayes]

It said they went 153 or so percent of design load so what would that be in "G" load?

Need to know what they used as a design load factor. No evidence that they used the FAR minimums cause there's a lot of other factors involved.

[martymayes]

It said they went 153 or so percent of design load so what would that be in "G" load?

Need to know what they used as a design load factor. No evidence that they used the FAR minimums cause there's a lot of other factors involved.

I do know those 787 wings bend like crazy!

[Ron Blum]

I do know those 787 wings bend like crazy!

Wing bending is good, and it is often designed into wing structures. The reason that wing bending is good is that it gives a smoother, more comfortable ride to the cabin/fuselage/passengers.

It is easier for larger airplanes to have a smaller deviation from the design goal simply because of the material thicknesses and processes involved. For example a small GA airplane or homebuilt made out of aluminum may have to change a skin thickness from 0.032 to 0.040 - a significant percent material thickness gain and resulting strength gain. On the other hand on a larger airplane, the skin typically ranges from tip to root and the skin is either chemically milled, shot peened or machined within small tolerances … producing a much tighter tolerance to design.

For a similar example in a composite airplane, a small airplane may have a 2core2 skin (required for a buckling failure not strength). A 3core3 layup would be ~50% heavier. In a larger airplane and because the layup would be thicker, the 1 or 2 ply layup delta would be much lower … and the failure mode may not be buckling, either :|

[David Lewis]

It was not understood that flexing wings were perfectly alright until the DC-3 came out. The skin thickness was less than anything that had been tried before, and Douglas staff, including the structural designer, Vladimir Pavlecka, toured the country reassuring the public and airline crews that wings flapping up and down, was something you needed to get used to and no cause for alarm.

[Ron Blum]

:thumbsup: That’s cool! I learned something new.