Big plane vs small plane in turbulence
 

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?

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.

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.

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.

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.

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.

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.

“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.

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.

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:

744 experience
 

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".

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.

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!

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?

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.......

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

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.

Wouldn’t it be inertia and not momentum?

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

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.