Why Airplane Wings Oscillate in Turbulence | Hacker News Reader

Why Airplane Wings Oscillate in Turbulence

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21 pointsmiralabs39 comments

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.

Being close to failure isn't a requirement for visibly bending. For example, see wire rope or cloth. Anything very long and thin can flex a long way without much strain on any individual part of it.

Low FoS yes, but even if you doubled it to 3, you'd still see the wing tip moving - only half as much.

Truck trailers are also very soft in torsion. See an empty one go over a big bump and it'll visibly twist. Not close to failure, just not very stiff.

Or a ship in heavy waves - https://youtu.be/oiqofDBsFL0?t=60 . But not close to failure.

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."

"1" is what the plane would be expected to experience during normal operation, where that includes flying through a storm, crosswinds, etc. How did they determine this figure? By years and years of analyzing successful designs and crashes (not-so-successful designs).

Just to add a word, it would be the maximum expected there, so would be for a plane loaded to the maximum takeoff weight doing the most aggressive maneuver in the worst weather (or really, most adverse external conditions).

Is this 'the maximum ever observed in a real setting'? Could it be 'the maximum inputs ever recorded on earth and then used in my calculation'?

It's the maximum load you would expect to see in the life of the aircraft

Sorry about this, I'm struggling a bit. Isn't the "load observed over the life of an aircraft" just a statistical distribution? Therefore a maximum doesn't make a lot of sense. It would make more sense to talk about standard deviations away from the mean unless the maximum was "maximum ever recorded in our dataset"

First, I'm not doing structural design, so maybe someone will wander by with a better answer. Anyway, it's probably overly simplistic, but passing the load test would involve surviving the maximum expected load with zero damage. Implying that lesser loads would also never do damage.

In reality, cyclic fatigue is also considered.

This seems like a nice discussion that shows the complexity:

http://www.coe.montana.edu/me/faculty/cairns/Introduction%20...

Suppose you have a bridge. You must design it so it can handle the maximum weight expected. You must not design it with a simple Gaussian distribution in mind.

For example, a small bridge in a seldom used road in the countryside might have a few cars per hour. That's a load of a couple of tons per car. But cars density isn't randomly distributed. If one is slow, then there might be a lineup of 4 cars in a row, and that's more likely then a simple calculation based on 1-(1-p)^4 where p is the single car density.

Then twice a day there's the school bus. And there's the occasional cattle truck, and semi, and anhydrous tanker. So you might have a school bus with a couple of heavy vehicles behind it, as a possible maximum design load.

The odds of this can't be described with a simple Gaussian, so standard deviations make no sense.

Of course, there's no need to go crazy overboard and design all bridges to handle a convoy of M1 Abrams tanks. That's why rural bridge might have a posted weight limit of, say, 10 tons, with only 1 truck allowed at a time. Even then, some people will push the limits, which is why there's a safety factor.

For example http://blogs.mcall.com/roadwarrior/2014/08/wehrs-mill-bridge... describes a wooden bridge which had a 10 ton limit, until some dumbass fuel tanker weighing 38.2 tons went through. Now the rated limit is 4 tons because of the structural damage.

So it's not "the maximum that was ever recorded in the dataset of all rural bridges" but "the maximum expected for this bridge", along with efforts to restrict higher loads.

Yea you've got it mostly right. It's based on a statistical distribution derived from actual flight data.

yeah...planes have a concept of maneuvering airspeed, where you can fully deflect control surfaces without damaging the aircraft. but higher than that you can cause structural damage. even below that if you don't just deflect but dynamically go back and forth just right.

https://en.wikipedia.org/wiki/American_Airlines_Flight_587

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.

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

1 is the max load one would ever expect to see in service.

That will vary by aircraft certifying authority.

For the FAA part 23 certified aircraft, the basics are here (for Federal laws, these are written in fairly plain English, but you need to hop around a bit at times): http://www.ecfr.gov/cgi-bin/retrieveECFR?n=pt14.1.23#se14.1....

For FAA part 25 certified aircraft (transport category aircraft): http://www.ecfr.gov/cgi-bin/text-idx?rgn=div5;node=14%3A1.0....

If the wings were stiff, the forces that would normally deflect the wings would be transferred into the cabin.

No, the forces are transferred to the fuselage in any case. If they weren't, the wing wouldn't be bending relative to the cabin, it would just experience rigid body motion instead.

Not quite true. As long as the frequency of the dynamic load isn't too slow relative to the natural period of the wing, then not all of that force has to be resisted by the wing connection. That's why wings are flexible, why we put shock absorbers in cars, and why flexible buildings can be designed for lower seismic loads.

But buildings and bridges do oscillate. Architects just try to keep it below the level of human perception because they know people would freak out. The fact that we do not often see them oscillate has more to do with design and psychology than physics.

Structural engineer here: Buildings and bridges do not use 5, give us a little credit

We mostly assume that materials are 90% as strong as they say they are, that the building weighs 1.2 times as much as it really does, and that stuff in the building weighs 1.6 times as much as it really does. The total factor of safety is something like 1.5.

The reason that buildings don't vibrate (noticably under service loads) is that people would get queasy, so we design things for stiffness as well as strength. Aircraft are more space and weight limited, and no one is sitting on the end of the wing, so the engineers mostly just care that everything will stay attached.

If a wing is too stiff, you get one kind of flutter, too soft, and you get another. The idea is to be midway between those two cases. Flutter (dynamic instability) will rip the wings off. An awful lot of design effort goes into avoiding flutter.

Blow on a stretched rubber band, and it'll vibrate, too.

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).

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...

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.

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...

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

http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-a...

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?

Rough fields - high engines.

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.

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.

> 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) - 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.

Why should I believe you. Do you design airplanes?

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.

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.

> 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...). 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

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.

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.

On a tangential note plane wins are quite strong themselves

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

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.

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