
Why Airplane Wings Oscillate in Turbulence - miralabs
http://www.wired.com/2015/11/the-physics-of-why-airplane-wings-oscillate-in-turbulence/
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asmithmd1
In my mind he didn't address the fundamental reason why the wing visibly
oscillates while most other structures that you trust your life to like
buildings, bridges, and cars do not. Airplanes are designed with a 1.5 factor
of safety

A factor of safety is applied to a design after every load the structure will
be subjected to is calculated, they multiply by 1.5 to be sure the structure
will be safe. 1.5 may sound conservative but it is the smallest factor of
safety someone normally encounters, spacecraft and fighter jets might use 1.2
while cars often use 3; buildings and bridges use 5. Airplanes are used much
closer to the strength limit of their materials so we see them flexing.

Of course everything deflects under load - my strength of materials instructor
illustrated this by analyzing how much an anvil compresses when a fly lands on
it; answer: less than can be measured.

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Dwolb
My question would be, "What's 1?". I don't know if 1 is the mean, 1 standard
deviation, 2 standard deviations, etc.

I'd be really worried if 1 was "A fully loaded plane of 180lbs adults with two
checked bags of 50lbs each flying in clear blue skies with no wind."

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sound_of_basker
Unless you have a background in solid mechanics, this is difficult to explain.

On the outset, there are different failures possible, tensile, warping, etc.

Under a load distribution, a structure would fail at some point depending on
the intensity of the load. It could be tensile or due to bending moment. Call
that L. Now, you would add flitches or increase say, the thickness of a plate
to withstand upto 1.5L. That is a design with a 1.5 times factor of safety.

~~~
Dwolb
My question pertains to the testing load. If different industries structure
their tests differently, it doesn't make much sense to compare safety factors.

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vacri
On a tangent, one thing that _slightly_ freaks me out a bit is that on the
ground, the wings hang from the plane (wings bend down); whereas in the air,
the plane hangs from the wings (wings bend up).

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joosters
Well, the wings of the plane are the only bit that actually _flies_ , so you
can think of the fuselage as just hanging on to the wings for dear life...

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WalterBright
That's actually incorrect. What keeps the airplane flying is gripping the
armrests. The proof is simple - when the airplane dips, grip the armrests
harder. The plane will recover.

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vlehto
I've been wondering why passenger planes and military cargo planes have so
different layout?

So far my best guess is that it's related to landing gear. Landing gear makes
significant portion of the total weight, you want to keep it short and
compact. Engines are the heaviest things in passenger planes, so landing gear
is close to engines.

[http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137...](http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137447b.jpg)

With cargo plane the heaviest part of the plane is the cargo.

[http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-a...](http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-
an-low-level-flight-cargo-plane-wallpapers-wallpaper-256058768.jpg)

It seems bit weird that one of the most dictating thing to airplane layout is
not really related to flying itself. But I could be wrong on this one. Does
anybody know better?

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lotsofactors
Mostly, the high-wing and landing gear configuration on many cargo aircraft
designs is intended to facilitate a low loading floor and the ability to load
and unload the aircraft on rough fields without special ground equipment (e.g.
by means of a ramp integrated with the the rear doors of the aircraft like a
C-130 or with a deployable forward ramp and kneeling front gear like the
An-124.) Another benefit is providing extra ground clearance for the engines
to reduce foreign object ingestion.

Everything else flows from this. The wing spar and structural supports needed
in a cantilever monoplane (i.e. not a biplane with a truss structure) take up
a fair amount of room. By moving these structural elements to the top of the
aircraft, it's possible to get a low floor and still hang engines from pylons
beneath the wings. The wings themselves will be angled downward from root to
tip (known as anhedral) to partly counteract the pendulum stability caused by
having a center of gravity so far below the center of pressure.

Passenger aircraft do not usually use this configuration because it is heavier
- the sides of the fuselage must be built stronger to support the load,
compared to a low-wing configuration. Weight costs performance costs fuel
costs money, so you don't build a large aircraft like this unless it has to
operate at fields without ground equipment.

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vlehto
There are lot of bombers with high wing (B52, almost anything several engines
from WWII). About half of all propeller driven passenger planes have high
wing.

>Unlike the An-124, the An-225 was not intended for tactical airlifting and is
not designed for short-field operation

It still has high wing. And U2 has high wing. It's probably the plane with
most carefully calculated weight ever.

If it was inherently heavier, this would make no sense.

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lotsofactors
> There are lot of bombers with high wing (B52, almost anything several
> engines from WWII).

Most bombers dispense ordnance from their undersides, ideally near the center
of gravity; limiting the structural members needed in those parts simplifies
things.

> About half of all propeller driven passenger planes have high wing.

These are designed not to require ground vehicles for loading and unloading.
Propellor ground clearance is also an issue for the larger types.

> Unlike the An-124, the An-225 was not intended for tactical airlifting and
> is not designed for short-field operation > It still has high wing.

The An-225 carries cargoes of unusual bulk to unusual places. Being able to
get these cargoes on and off the aircraft without some sort of lift has
advantages.

Low-wing freighters, which are all variants of passenger types these days,
almost always carry palletized cargo of standard dimensional units (See
[https://en.wikipedia.org/wiki/Unit_load_device](https://en.wikipedia.org/wiki/Unit_load_device))
- this allows for standardized ground handling equipment for most loads.

> And U2 has high wing. It's probably the plane with most carefully calculated
> weight ever. > If it was inherently heavier, this would make no sense.

The U2 is not a high-wing aircraft, it is a mid-wing aircraft with its spar
roughly running through its center of gravity. This is actually the most
efficient and aerodynamically cleanest configuration, and many high-
performance aircraft use this arrangement. The main problem with it for
passenger or cargo use is that the wing structure ends up interfering with the
middle of the fuselage volume near the center of gravity, which tends to be
valuable space in a transport aircraft.

~~~
vlehto
Why should I believe you. Do you design airplanes?

~~~
zwiteof
Because his answers are accurate. While weight and aerodynamic performance are
very important, aircraft design is inherently multi-disciplinary and requires
tradeoffs depending on the design requirements.

~~~
vlehto
With no sources or authority. While my answer seems logically just as good to
explain this stuff.

Seems like the argument is won purely by asserting certainty. But I'm not
objective of course.

~~~
zwiteof
> While my answer seems logically just as good to explain this stuff.

It sounds logical if you don't have a background in aerospace, but otherwise
it's relatively inaccurate. For example:

> So far my best guess is that it's related to landing gear. Landing gear
> makes significant portion of the total weight

Landing gear makes up roughly 3% of the total takeoff weight. Hardly
significant compared to fuel and cargo/passengers. [1]

> you want to keep it short and compact.

This is certainly true from a structural standpoint.

> Engines are the heaviest things in passenger planes, so landing gear is
> close to engines.

Compared to a person, yes. Compared to the total cargo/passengers, not really.
The 787 MGTOW is ~500,000, of that, the two GE GEnx-1B engines weight about
26,000 lb combined.

The landing gear is "close to the engines" in your example picture, but this
is because you typically place the main landing gear such that it is near the
center of gravity. The nose gear only supports 8-15% [2] of the aircraft
weight to make steering possible while taxiing. Some commercial aircraft have
tail mounted engines such as the MD-80
([https://upload.wikimedia.org/wikipedia/commons/2/25/Allegian...](https://upload.wikimedia.org/wikipedia/commons/2/25/Allegiant_MD-80_at_McCarran_International_Airport_\(6334695369\).jpg)).
The wing (and landing gear) are indeed further aft since the CG is moved back
further due to the engine placement.

In addition, comparing the number of wheels is a red herring for a commercial
jet v. a cargo plane. The CG location relative to the wheelbase will be
remarkably similar in both cases. However, military cargo planes often operate
out of poor and/or shorter airfields. This limits the amount of weight you can
put on each wheel if you're landing on asphalt rather than reinforced
concrete, so you have more wheels with less load per wheel to keep from
sinking into the ground. In addition, more wheels allows you to slow down
quicker since you can spread out the braking action.

> With cargo plane the heaviest part of the plane is the cargo.

The cargo/passengers are a significant portion for commercial transports as
well. Again for the 787, you've got around 100,000 in cargo/passengers (about
20% mass fraction). The C-17 carries 170,000 lbs of cargo with a 585,000 MGTOW
giving a mass fraction of 29%. Not too surprising they have a higher mass
fraction there since they're not adding any parasitic mass for things like
passenger comfort.

lotsoffactors' comments on cargo loading/unloading considerations and the U-2
being a mid-wing aircraft are correct as well.

Sources: Aerospace Engineer and Raymer's Aircraft Design textbook (basically
the bible of aircraft design).

[1] Chapter 15 of Aircraft Design: A Conceptual Approach (3rd Edition) by
Daniel Raymer

[2] Chapter 11 of Raymer

~~~
vlehto
Thanks. Now I actually learned something. I've been into airplanes since
little kid, but never heard of that Raymer book. I probably have to pick it
up.

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zwiteof
The article's explanation is a bit simplified. It sounds like his explanation
is more akin to a body freedom flutter response where the aircraft's short-
period mode is coupled to the structural response. What he observed is
probably closer to a more classical flutter response with the wing's bending
and torsion coupled. A gust hits the wing, which increases the load, this
increases the bending deformation which can also induce torsion causing a
twist in the wing. At too high of a speed (the flutter speed), the response is
unstable and can quickly become catastrophic. At normal speeds, the
oscillation will tend to die out as the restoring force of the structure and
the damping from the structure, aerodynamic loads and controls causes the
response to die out. Interestingly, you can go past the first flutter speed of
an aircraft with a properly designed control system (aeroservoelasticity)
meaning you can get away with a lighter (read, more flexible) wing structure.

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LoSboccacc
On a tangential note plane wins are quite strong themselves

Here's Boeing wing being stress tested:
[https://m.youtube.com/watch?v=sA9Kato1CxA](https://m.youtube.com/watch?v=sA9Kato1CxA)

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Gravityloss
The lift varies since the aircraft flies fast through regions where the air
has different velocity.

Think about a column of ascending air, and the plane flying quickly through
that. All else being equal, the plane will accelerate vertically until it is
traveling vertically at the same speed as the air column (if the column is
large enough).

That's why flying low and fast is extremely bumpy and taxing for aircraft.
Lots of air speed variations.

