Big plane vs small plane in turbulence

[mkqq]

Hello all,

I am unable to find a definitive answer to this question anywhere online, and even pilots themselves seem like they don't know the answer.

The question is if a small plane and a very big plane went through the same area of turbulence/wind, does the small plane feel the effect PROPORTIONALLY GREATER when comparing the mass of both planes?

Personally I think is the answer is while the small plane will feel more turbulence, it is NOT proportionally greater.

I've taken off in a small 2-seater that weights around 750kg total in very windy conditions and it is bumpy, but not a "rollercoaster". I've also sat as a passenger in a 737 taking off in very windy conditions and it also bounces around a fair bit, but maybe not as much.

If it was proportional, then the small 2-seater would feel like the wildest rollercoaster ride, and you would feel nothing in the 737 as it weights almost 100 times more.

Can someone provide a more definitive answer to this question?

[Jan Olieslagers]

While mass certainly is a factor, it is not the only one. I always understood wing loading weighs in heavily, too. How relevant they are in the total sum is beyond me; people like GtE or Pilot DAR may have more to tell. But since several factors are at play, there can never be a single one that has a proportional effect.

[mkqq]

I think the area of the wing makes a difference too. The 737 obviously has much bigger wings which get exposed to more air movements, where as the smaller 2-seater has very small wings and will not "catch" all the rough movements around it.

[Colibri49]

This answer is based only on experience and instinct. Therefore it is highly subjective and begging to be "shot down in flames". Turbulence is usually a mixture of various "patches" of disturbed air, some rising, some falling and some swirling.


Assume that the small 'plane and the big 'plane have the same wing loading The very big one however has wings which are more likely to span across several of these patches, which would partially cancel each other out. The small aircraft would encounter such patches individually and thus feel the full effect of each one.


Goodness! You beat me to it. By the time that I had composed my reply, you had already come to the same conclusion.

[The Ancient Geek]

It is about momentum.
Turbulence is a motion of the air through which the aircraft is flying and if the aircraft had zero mass it would move with the motion of the air. The wings are not part of the problem, they are still moving through the same air.
The aircraft has mass and velocity therefore it has momentum which will resist any change of motion. A passenger in the Boeing will experience less motion than the passenger in the Cessna because the greater momentum of the Boeing will resist the change of motion dv/dt more. Both passengers experience a G force proportional to the rate of change of motion.

[Shaggy Sheep Driver]

It's about wing loading. The heavier the wing loading, the less bouncing around. The lovely Chipmunk has a relatively light wing loading and in rough air gives a very lively ride. The Yak52 has a heavier wing loading for a broadly similar aircraft size and is much less disturbed by turbulence.

[piperboy84]

My Maule has relatively light wing loading compared to other spam cans and is very adept at picking up every bit of turbulence that's available. Think: unbalanced washing machine on high spin cycle.

[3wheels]

“The question is if a small plane and a very big plane went through the same area of turbulence/wind, does the small plane feel the effect PROPORTIONALLY GREATER when comparing the mass of both planes?”

Just imagine you are in a choppy sea in a rowing boat or aboard the Queen Mary...there’s your answer.

[paulthornton]

Whilst we are on the subject, and apologies for the slight thread drift, I was thinking of turbulence and how it acted on an aircraft a lot yesterday morning.

At the time, I was failing to sleep on a 744 due to the fact that the world was bouncing around like an out-of-balance washing machine at FL340 so naturally my sleep-deprived brain started to think about physics.

I was trying to answer my mental question of "Where should I sit to minimise the movement caused by this". I was close to the nose and wondered if there might be a significant pitch component as well as the general up-and-down motion. My instinctive answer was that the minimum effect of the turbulence would be at the centre of lift - but then you have the problem of the moving air mass doing different things at different places in something the size of a 744.

Anyone with better aeronautical credentials than me care to comment? Was the correct answer to have another drink and ignore the infernal shaking?

Paul.

[pulse1]

Airspeed must surely be a critical factor in how turbulence "feels". I have been thrown around quite a lot in light aircraft but I have never experienced the violent bumps I've occasionally felt in the faster and bigger stuff. I also remember reading that wing shape has quite a big effect on gust response although that may only be relevant to fast jets at low level.

The boat analogy above should also consider a speed boat in the same choppy sea. Up to a point, a heavier hull which is designed to cut through the waves would be more comfortable at higher speeds. Continuing the boat analogy I was a little apprehensive the first time I went on a big cruise ship. Although I had sailed small boats for years and never been sea sick, I was unsure about big boats. What really surprised me as we steamed slowly out of Southampton, was that I could feel the ship move as we passed through the wake of the I.o.W ferry.:eek:

[Curlytips]

Yes, slight drift, but as you must know, if you turn left when you get on, the seats nearer the nose tend to be thrown about less, but just as importantly, are more comfortable and service is much superior! Ask your nearby cabin crew to steady your g & t if you must.......

I've spent years traveling (in both ends of 744s) and can verify that the further back you go, the more the tail "wags".

[B2N2]

If we go to a model for a moment:
Turbulence is caused by sections of air moving up and sections of air moving down.
So we have a heavy aircraft at speed transitioning through a zone with an alternating updraft and downdraft.
From a physics perspective we could have an airplane of sufficient weight and speed that momentum would prevent the aircraft from being displaced. We would feel no “bumps”.
We could transition the same are with the same m/sec up and downdraft in a slow light airplane and it will “ride the wave” like a peanut shell in the ocean.
Proportionally different I’d say.
Unless I’ve got my physics wrong.

[Cough]

I've flown on windy days in light aircraft and also heavy aircraft.

I'd say one factor is what constitutes a windy day in a heavy aircraft grounds a light aircraft...

So its unlikely that you have actually compared the two accurately!

[mary meagher]

Flying a single seat modern glider with a 15 metre wingspan, bumpy air below cloud base is usually indicating a thermal. Rising air may continue up into the cumulus cloud, and if it is interesting weather, the bumps may form cu nimb, or a thunderstorm, with tops up to 36,000 or so.

But the nastiest bump I ever met in my glider was in clear air over Scotland, near Aboyne, climbing in wave, at about 15,000 feet (using oxygen, of course). Evidently a system of weather traveling from the Atlantic, heading easterly, encountered a higher mass of clear weather moving from the Scandinavian area, heading west. I really thought the glider would break apart, it was so violent a wind shift. Changed my mind about going any higher that day.

I think any pilot meeting that wind shift would not have enjoyed the experience, whatever the size of his aircraft. Curlytips, have you ever met this kind of weather in a Boeing?

[Curlytips]

Not sure whether as violent as you experienced, but CAT has shook me up on several occasions. Crews do report it to help those behind.......

[Genghis the Engineer]

The critical factors are lift curve slope and wing loading. Let's concentrate on wing loading.

Roughly speaking, gust response is inversely proportional to wing loading. So double the wing loading, you halve the response. Big metal tends to have a much higher wing loading than light metal, and so responds much less.

At the same time, we also need to think about inertia and g-limits.

Big aeroplanes have much higher inertia, so as the forces due to turbulence are applied, they tend to accelerate more slowly. This means that the lighter aeroplane will move more - a further impact, but at the same time the movement relieves internal stresses on the aircraft.

Also g-limits, a typical light aeroplane has g-limits of about 3.8g, and a glider somewhat more. A typical airliner 2.5g.


So, in the big jet you will get, and feel much less response than in the little aeroplane. On the other hand, for the same response, the big jet is more breakable, whilst the little aeroplane is more likely to be put (temporarily, one hopes) out of control.

G

[B2N2]

Well either way I can tell you from experience that moderate turbulence reported by a 737 is no fun in a Citation CJ2 let alone a C172.
Anything reported by an aircraft significantly bigger then you, pay attention.

[fujii]

Wouldn’t it be inertia and not momentum?

[Genghis the Engineer]

Whilst I used "inertia" and others "momentum" you can pretty much treat the terms as interchangeable for the simple explanations we're all using.

G

[fireflybob]

For the same type if you're flying at lighter weights turbulence feels worse but is potentially less dangerous in terms of airframe strength etc. At higher weights the turbulence might feel less but is potentially more damaging.

As has been said previously the feel of turbulence is subjective. Years ago they put experienced pilots in a simulator and asked them to rate the level of turbulence i.e. light, moderate, severe. Quite often when pilots reported light or maybe moderate the turbulence was severe and exceeding the Flight Manual G limits. Also on other occasions thew reverse applied i.e. they reported severe turbulence when recorded G was well below the AFM limits. It's to do with the frequency of the turbulence as well as the perceived amount.

[mary meagher]

So comparing a Boeing with a 60 metre wingspan to fit in the usual hangar, with an Eta glider, having a wingspan of 30.9 meters from wingtip to wingtip, the Boeing is not that much bigger, just a lot heavier!

[tescoapp]

before I had operated into Vargar Fareo's I thought I had experienced moderate turbulence........

After VArgar I realised I had only experienced light chop.

[Genghis the Engineer]

As has been said previously the feel of turbulence is subjective. Years ago they put experienced pilots in a simulator and asked them to rate the level of turbulence i.e. light, moderate, severe. Quite often when pilots reported light or maybe moderate the turbulence was severe and exceeding the Flight Manual G limits. Also on other occasions thew reverse applied i.e. they reported severe turbulence when recorded G was well below the AFM limits. It's to do with the frequency of the turbulence as well as the perceived amount.

I can't help suspect that has a lot more to do with the quality of the simulator than it does with the perception of the pilots, in the particular instance.

G

[Jan Olieslagers]

before I had operated into Vargar Fareo's

You got me curious there, but I could not find such a place on any map... Must be a very exotic and/or elusive destination!

[tescoapp]

****e spelling which auto correct doesn't pick up

have a look at the brief

https://aim.naviair.dk/media/files/h5jsmfx5f1o/EK_AD_2_EKVG_en.pdf

[scifi]

If you take the full range of aircraft to include model aircraft, then I can say that the smaller they are, the more wind gusts can disturb them, especially in bank angle.


The smallest airplane I have ever flown weighs just about one ounce, think of it in terms of a paper bag with a very thin carbon fibre framework. It is impossible to fly this model in any wind over 3 knots. Even when flying indoors, if someone opens a door the draft will cause it to quickly roll over.


So I suppose the Moment of Inertia is the proportional factor that you are looking for, and this increases with Mass and aircraft size.
.

[tescoapp]

its wing loading and speed

lift varies with the square of speed and directly with the coefficient of lift.

low speed and low low Cl then not much increase or drop in force.

Loads of both and its a huge change in lift.

[Jan Olieslagers]

@tescoapp: thanks, I had a kind of suspicion there...
None like the Brits, though, for mangling up nouns proper! :)

[B2N2]

Truthfully it doesn’t take a pilot to understand a little plane experiences “bumps” differently then a big airplane....
:}

[Crash one]

I once considered that the air surrounding a model aircraft was effectively more dense than the air around a full size aircraft, I did say "effectively" not actually.
The analogy of the model ship looking as though it is sailing in thick oil rather than throwing up fine spray.
The big aircraft must fly faster to maintain lift.
The wing loading (area--weight) doesn't consider relative (effective) air density, the density is considered constant.
Maybe I'm all wrong, the instructor I mentioned it to told me to bugger off!

[Maoraigh1]

What about the scale of the air movement?
Alternating lift and sink, repeated, (wave?) when flying a Pa28, would possibly appear as turbulence in something with more inertia and much more speed?
What throws a Jodel about would cancel out over the wingspan of an airliner?

[tescoapp]

your getting into none dimensional numbers which are used for making comparative models Crash one.

The number in aviation is the Reynolds number.

And the one for boats is called the froude number.

Unless your instructor was an Engineer of some form they won't have a clue.

or maybe they were and they thought you were about to launch into a discussion about Navier-Stokes equations.

[Crash one]

I have heard of the Reynolds number but no one has explained how to apply it.
So building a model just by scaling it down isn't going to work.
Anyway how can you scale the density of air? Depressurise the test flight building?

[tescoapp]

For a model to be comparable you need to match the Re number density is a bit difficult to change but I suppose you could heat/cool the fluid. Also change the pressure in a closed system.

They tend to change the velocity of the fluid.

For wind tunnel stuff you need to match the Euler number as well.

[Genghis the Engineer]

I have heard of the Reynolds number but no one has explained how to apply it.
So building a model just by scaling it down isn't going to work.
Anyway how can you scale the density of air? Depressurise the test flight building?

That is exactly what we used to do when I worked in high speed wind tunnel testing back in the early 90s. It was a very slow and expensive process.

The use of non-dimensional variables exists throughout engineering analysis, and there are all sorts of variables in various applications. The two most pilots are likely to familiar with are Cl and Cd; Reynolds Number is another, mainly used for matching wind tunnel results to full scale flight results - particularly the Cl.v.AoA and Cd.o.v.AoA curves are likely to be a function of Re, although don't change much (usually) within an order of magnitude change. Two orders, and it starts to become quite important.

The formulae for gust response are a function of lift curve slope and wing loading. Lift curve slope in turn is a function of Reynolds Number (and Mach Number, another nondimensional variable of-course).

G

[Forfoxake]

See BBC website for this video:

"Plane tries to land in Austria storm... takes off again"

Just shows that even the big ones can have trouble landing in turbulence!

PS If there was any power to kill at the actual touchdown, it might have worked out but going around and back to Frankfurt was almost definitely the right decision.

[mikehallam]

To muddy the waters, there's the squared and cubed scale factor.

Thus a half scale a/c has 1/2 length, span etc., but only 1/4 the area and 1/8 the mass.

In practical terms e.g. a 2/3 "Spitfire" replica has 4/9 (44%) wing area but merely 8/27 (30%) the weight of the real thing, all other things being equal.

Different construction materials, and a full size 1:1 space for the P1 make more differences when comparing "the same" shape with the original a/c.

mike hallam

[Crash one]

Thank you Gengis.
I'm no mathematician, so much of it is going over my head.
Asking a gliding instructor once for the formula to calculate lift, he said "Half Ro V squared". Yes all very well, what units? Pounds? Kilogrammes? Miles per hour? Feet per minute? Not very helpful. It took me a while to guess at the speed of light?
No wonder my car doesn't fly like bird at 60!

[scifi]

Hi Guys, I think we need to consider the two different types of turbulence...

The first type is lift related, and is the type that makes you bump your head on the cabin ceiling. That can be explained by how much more Cl you have left before the wings stall, or the speed of the downdraught.

The other type of turbulence is caused by the rotor effect of say, flying into your own wake turbulence when doing tight turns with the wings already near their stall speed. This can soon turn you upside down and spit you out of your radiused turn. These rotors can also occur behind very large airfield buildings, and behind mountain ranges. I am just thinking that an airplane with full tip-tanks would be less susceptible to this type of turbulence than the same aircraft with the fuel in its inboard tanks. It all depends upon the distribution of Mass, and it's distance from the centre of gravity.
.

Hi Crash One, If you use the same units in the lift equations you will get the correct answer (use all metric, or all imperial.)
There is a simpler way of finding out the lift. As an example... K8 Glider Lift=350Kgs, Cessna 172 Lift=1050Kgs, Airbus =60,000Kgs.... It is simply equal to the aircraft's weight, in level flight. You can also work out the drag force easily... K8 glider has a 35:1 L/D. Therefore Drag = 10Kgs.

As for your car not flying at 60mph. It would if it drove off a cliff. hi hi.
.

[Forfoxake]

Hi Crash One, If you use the same units in the lift equations you will get the correct answer (use all metric, or all imperial.)
There is a simpler way of finding out the lift. As an example... K8 Glider Lift=350Kgs, Cessna 172 Lift=1050Kgs, Airbus =60,000Kgs.... It is simply equal to the aircraft's weight, in level flight. You can also work out the drag force easily... K8 glider has a 35:1 L/D. Therefore Drag = 10Kgs.


As for your car not flying at 60mph. It would if it drove off a cliff. hi hi.
.


That's not flying- it's gliding very badly!


Not much affected by turbulence though....

[Crash one]

That's not flying- it's gliding very badly!


Not much affected by turbulence though....

Isn't that a ballistic parabolic curve?

As for the lift thing.
Yes, my Emeraude weighs 600kg, lift is produced at 40 knots.
That is the answer, not the equation that Claude Piel, bless his cotton socks, used to get that answer!!

As for the K8 I thought it had the LD of a pair of pliers, and penetration a bit less than me at 77years.

[PaulisHome]

Oh, good - a physics problem.

Let's get rid of all the ideas that 'the momentum' of an aircraft somehow reduces its susceptibility to turbulence. Momentum has no effect on how an aircraft moves given an external force. Mass, on the other hand does (ie inertia). But that's not the same as momentum.

To first order:

1. Take the equation for lift (proportional to v^2, wing area, CL). Again to first order, CL is proportional to angle of attack.
2. In level flight lift = mass * gravity
3. Now consider what happens when your aircraft moves from air that's not going up or down into air that is (eg a thermal). Effectively, the angle of attack changes, and so the lift from the wing changes, accelerating the aircraft vertically. That's your turbulence.

Solve the equations above, and you find that the extra 'g' experienced by the aircraft is proportional to velocity and wing surface area, and inversely proportional to mass - or if you like proportional to velocity and inversely proportional to wing loading.

That all makes intuitive sense. For otherwise identical aircraft - if you fly faster you get more 'g' from turbulence (hence manoeuvring speeds), and if you add weight you get less (at higher speeds, the angle of attack is less, by a factor of V^2; the change in angle of attack due to the thermal is less too, but only by a factor of V). Likewise for aircraft of equivalent weight and speed, the one with the bigger wing is effected more, since it starts off with a lower angle of attack.

Of course, in the real world, the linear assumption about lift and angle of attack doesn't quite hold, but I think it's good enough for this exercise.

As a sanity check, putting a few numbers in the formulae above, suggests that a B747 with a wing area of 525 m^2, max weight of 400,000 kg, flying at 200 kts (100 m/s), has a vertical acceleration due to a vertical thermal about a fifth of my glider (wing area 11.6 m^2, weight 500kg, speed 30 m/s).

Paul

[Crash one]

Thank you Paul.
If I had a degree in aerodynamics I'm quite sure that would have made perfect sense.
Unfortunately I don't. I work by numbers, therefore 600 kg times the coefficient of lift equals 600kgcl. Which means nothing of any sense.
If the coefficient of lift had a number, it would help.
600 times 3 = 1800. That's easy.
Next!

[PaulisHome]

If the coefficient of lift had a number, it would help.
600 times 3 = 1800. That's easy.

The co-efficient of lift is dimensionless.

Lift = 1/2 * density of air (rho) * velocity squared * wing area * Coefficient of lift.

Lift is in Newtons (a force, which has units kg*m/s^2)

density is kg/m^3
velocity squared is (m/s)^2
area is m^2


Multiply those last three together and you get (kg/m^3)*[(m/s)^2]*m^2
or kg*m/s^2 - the same units as lift, so the CL is dimensionless.

If you look up graphs of CL vs angle of attack you find that it's varies with the wing design, but is typically around 1.0 at typical angles of attack (5-10 degrees).

See https://commons.wikimedia.org/wiki/File:LiftCurve.gif

https://commons.wikimedia.org/wiki/File:LiftCurve.gif


Paul

[wiggy]

I think you've got to be careful about generalising about big v small...one of the smallest fighters back in the day was the F104 but it generally had a ride that was smooth as glass at low level on a windy day vs. even slightly bigger stuff with slightly lower wing loading.

[Crash one]

Thank you Paul.
That makes sense.