Why a wing shears a steel column at speed

Same wing. Same steel.
2,500× the force.

Steel has a fixed breaking point. The force arriving at it does not: it grows with the square of speed. Below the line the wing loses. Above it, the steel does.

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Is aluminum harder than steel? No. And it never mattered. Shear strength is a pressure, not a contest between materials. Steel shears at about 30,000 psi no matter what is touching it. Machine shops cut it with a jet of water and sand.
Can the wing deliver 30,000 psi? That is the only question, and speed is its only real input. At 10 mph the aluminum buckles first and never gets close. At 500 mph it arrives with ten times more than the steel can hold.
What this column takes before it shears  1,869,210 N
Wing at 10 mph7,859 N · 1,767 lb
Wing folds
Wing at 500 mph19,647,659 N · 4.42 million lb
Steel shears
1 kN10 kN100 kN1 MN10 MN100 MN

Every step across this scale is ten times the step before it. That is the only way both bars fit on one screen — the bottom bar is 2,500 times the top one, not five times.

Photographs, not simulations

Soft things go through hard things every day

None of these is disputed by anyone, and none of them needed an aeroplane. In each one, something everybody agrees is weak arrives fast at something everybody agrees is strong. Speed is the only thing any of them has going for it.

The fan of a JT8D jet engine after a bird strike, with every blade bent backwards.

A bird folded every blade in this fan

Feathers, meat and hollow bone, against a certified engine running at full power. Look at the blade tips: not one of them is straight any more.

JT8D fan after a bird strike · Plenumchamber, CC BY-SA 3.0, via Wikimedia Commons

The canopy of an F-16 after a bird strike, cracked across its width with the bird's remains smeared on the outside.

A bird broke this canopy from the outside

The red is what is left of the bird. The cracks run the full width of the canopy, and the fighter is the hard object in this photograph.

F-16 canopy after a bird strike · U.S. Air Force, public domain, via Wikimedia Commons

A waterjet nozzle cutting a shaped profile through thick steel plate.

That is sand, and it is cutting steel

The stream carries ordinary garnet sand. Poured onto the plate it does nothing at all. Carried fast enough it cuts straight through. The sand never changed — only its speed did.

Abrasive waterjet · WARDJet, CC BY-SA 2.0, via Wikimedia Commons

The actual arithmetic

Two equations. One of them has a v² in it.

What the column can hold

F = τ × A

τ = 207 MPa   steel shear strength
A = 9,030 mm²   column cross-section
F = 1,869,210 N

What the wing delivers

F ≈ m × ÷ d

m = 140 kg   the piece of column in the path
d = 0.356 m   how deep that column is
v = the only thing that changes

v = 4.4704 m/s(10 mph) 7,859 N 0.4% of the limit
v = 223.52 m/s(500 mph) 19,647,659 N 1,051% of the limit

Same equation. Same column. Same wing. Only v changed.
And the ratio holds no matter what you think of my constants, because they cancel: fifty times the speed is 50² = 2,500 times the force, exactly. You can halve every number above and 500 mph still clears the limit five times over.

The wing does not have to survive. It has to deliver. It carries that pressure for 1.6 milliseconds. It is confetti either way, and so is the column.

Your hand cannot push a nail into wood. A hammer does it instantly. The hammer is not harder than your hand.

Filmed proof, and its limits

A fighter jet, 480 mph, into 12 feet of reinforced concrete

In 1988 Sandia drove a complete F-4 Phantom, both engines included, into a reinforced concrete target at 215 m/s — about 480 mph. Watch what is left of the aircraft.

An F-4 Phantom on a rocket sled, intact, moments before striking a concrete target.
Before

A whole aeroplane, wings level, engines aboard. The pale block ahead of it is the target, and it is the last moment anyone sees it clearly.

The F-4 Phantom at the moment of impact, forward fuselage already destroyed.
Contact

The front of the aircraft has already stopped existing. The rest of it has not yet found out.

Moments after impact: a large cloud of debris and dust where the aircraft was.
After

There is no aeroplane in this photograph. The target is somewhere inside that cloud.

Close-up, slow motion, silent, 9 seconds. The target is the pale block, in frame and intact before anything reaches it. The film ends inside the dust cloud, which is where the camera run ends. Rocket-sled test, 19 April 1988, Sandia National Laboratories, run for the Muto Institute of Structural Mechanics, Tokyo.

Destroyed and harmless are not the same word. An aircraft being pulverised is delivering millions of pounds while it is pulverised. Those are the same event, not alternatives.

One honest warning about this film, because it will be used against you. The target barely moved, and that was the design. Sandia built it 3.66 metres thick and essentially rigid, and states the test was “not intended to demonstrate the performance (survivability)” of any particular concrete structure. A target built to break would have measured nothing. They read the force off instruments against time, not off the wreckage — and found the major force came from the engines. So this is not a film of a wing cutting a column. It is the answer to the one objection that matters: an aluminium aircraft that disintegrates on contact still arrives with millions of pounds behind it.

The six things people say next

"The wing would be destroyed."

It was. Completely. It still broke the columns on the way in. A thrown wine glass shatters and the window still breaks.

"The photo shows a pole winning."

At walking pace, over about a tenth of a second. Everything has time to flex and share the load, so the wing is the weak one. That flips with speed.

"That's a wing against one pole."

That is the wingtip, the last few feet. No engine, no fuel, thin skin. The strong part is the wing box at the root, holding 15 tons of fuel. Sandia's test found the major impact force came from the engines, and both of those went in too.

"It was reinforced steel beams."

Hollow box columns, 14 inches square, about three feet apart. Four inches thick at the ground, a quarter inch up top. Both planes hit up top.

"Soft can't beat hard."

A 10 lb goose, meat and feathers, destroys a titanium jet engine at 300 mph. It is why engines are certified by firing bird carcasses into them, and it is what put Sullenberger in the Hudson. A 767 is about 29,000 geese at once.

"Then where's the video?"

Nowhere, and that is expected. Wings hit things at taxi speed constantly and it gets filmed, which is where every photo like this comes from. At 400 mph a jet is at altitude, nowhere near a pole. Nobody has ever been able to run that test on purpose.

Why time is the hidden variable

At 500 mph the column is standing alone

A column in a wall is strong because it is bolted to everything around it. Push slowly and the whole building leans in and helps. That help travels through steel as a wave, about three miles per second, and it needs time to arrive.

80 ms at ramp speed: the signal crosses 400 metres, the full height of the tower. The building helps.

1.6 ms at flight speed: it crosses 8 metres. Eight columns know. The rest of the building has not found out yet.

Same physics, easier to accept

Waterjet cutting

A jet of water and garnet sand cuts steel plate for a living. Nobody finds it suspicious, because everyone has seen it.

Watch →

Workshop test

Paper saws wood, because spinning fast is what makes it rigid. Sitting still it is limp.

Watch →

Purdue University

The engineers' own model: the skin peels off instantly. Fuel and engine shafts do the damage.

Watch →
Every number above, and where it came from
QuantityValueTypeSource or method
Impact speeds443 / 542 mphPublishedNIST NCSTAR 1, north and south tower
Perimeter column14 in boxPublishedNIST NCSTAR 1. Hollow, ~3 ft on centre, plate ~0.25 in at the impact floors, up to 4 in at the base
Aircraft mass at impact~130 tonnesPublishedNIST, Boeing 767 with ~10,000 gal of fuel aboard
Steel shear strength (τ)207 MPaDerived0.6 × 50 ksi yield, the standard shear approximation for structural steel
Column cross-section (A)9,030 mm²DerivedFour plates, 14 in × 0.25 in
Column section mass (m)140 kgDerivedA 2 m length of that section at 7,850 kg/m³
Sandia sled test480 mphPublishedSandia National Laboratories, 19 April 1988, into 3.66 m of reinforced concrete
Wave speed in steel~5,100 m/sPublishedStandard longitudinal wave speed for structural steel
Contact time80 / 1.6 msDerivedColumn depth ÷ speed: 0.356 m ÷ 4.5 and ÷ 224 m/s

F ≈ m v²/d is an order-of-magnitude scaling estimate, not a finite element model. It answers one question: why the same steel survives one collision and fails the other. The constant in front is arguable, the v² is not, and NIST and Purdue reach the same place with the full models.