[Geezer (OP)]

Why does an aeromobile get kicked around as soon as it enters a nice puffy white cloud?

[Airthumbs]

This is an educated guess Geezer so don't quote me on this......  The Sun is heating up the top of the cloud and the temperature difference causes convection and movement of air within and around the cloud.

I know of a Hang glider pilot who was unfortunate enough to take off in not the best weather conditions.  Due to the updraft underneath a large cloud he was sucked into the cloud.  After a few minutes ice had formed all over the aircraft and he became unconscious due to Hyperthermia.  Fortunately the glider was able to land itself eventually and he survived, although somewhat battered and bruised! I understand that in some larger clouds wind speeds can reach upto 400mph.

Also you may notice on a clear sunny day with white puffy clouds in the sky that occasionally there is a gust of wind, this is caused by the clouds drawing air into them as they pass over and is something that Hang glider pilots are very aware of. Of course there are other reasons for this gust such as thermals and changing weather patterns but clouds cause it too.

[Marinthe]

It just their normal shape.... It happens during the natural process...

spam deleted

[Geezer (OP)]

Yes,yes! Thanks Airtumbs. That could be it. Mind you, I've noticed it even seems to happen when the Sun is very low on the horizon, so maybe something else is going on too.

[RD]

Turbulence (cloud not necessarily formed)

[Geezer (OP)]

Yes, yes! Thanks RD. It could be turbulence, but when it happened the other day, I noticed that the clouds didn't seem to be turbulating or anything. Everything was perfectly smooth until we actually flew into the clouds.

I should add that we were on a final approach at the time and I'd estimate we were at around 8,000 feet. It was around 8 pm and there were scattered showers in the area.

[Geezer (OP)]

I agree with Marinthe. I also think that it's just a natural process..

Do you now. You wouldn't be thinking of spamming us too, I hope.

[Monox D. I-Fly]

After a few minutes ice had formed all over the aircraft and he became unconscious due to Hyperthermia.

Wait, if ice had formed all over the aircraft how did he get Hyperthermia instead of Hypothermia?

[evan_au]

Why does an aeromobile get kicked around as soon as it enters a nice puffy white cloud?

I too have noticed that an aeroplane can be flying smoothly through clear air, but if it descends through a layer of clouds, the ride does become a bit rougher.

I'm not talking about the updrafts and downdrafts in a thunderstorm - pilots and air traffic control normally steers the plane around these. 

This seems to be more of a slight deceleration/acceleration motion, and if you are by the window, you can feel the correlation between entering and leaving the wisps of cloud outside the plane, and the bumpiness of the flight.

Could it be that:
- the wisps of cloud contain tiny droplets of water in the liquid phase (which scatters the light, making a visible cloud)
- while the adjacent volumes of clear air only have water in the gas phase
- If you heat up clear air to (say) 1000C in a jet engine, it will expand by a similar amount regardless of the humidity
- While if you heat up tiny liquid water droplets past 100C, they will expand by a factor of over 1000
- The tiny water droplets make up a small fraction of each cubic meter of air, but could they produce slightly higher engine thrust as you enter a wispy cloud? And that slightly increased thrust would fade away when you reenter clear air?

I seem to recall the Harrier jump-jet using water injection into the engines when they needed more thrust for vertical landings?

[alancalverd]

"Puffy" clouds are very active. As warm, moist air rises, it expands and cools. At some point it cools below the dewpoint and water (around 5 - 10% of the mass of air) condenses to form clouds. The energy released by condensation is, as you recall from your schooldays, over 2000 joules/gram, and it all has to go somewhere,. This means that the rising air is now much warmer, hence much less dense, than its surroundings, so a "forming" cumulus cloud contains a hugely accelerated updraft of anything up to 40 kt, with an associated downdraft at the edges. This is great fun in a glider where you can circle slowly in the updraft and reach enormous heights (up to 30,000 ft in a good Texan bubble) but seriously exciting in a powered aircraft as you bash straight through it at 250 mph.

In really strong thermic conditions the rising damp air will reach a level where the ambient temperature is well below freezing, so the water undergoes another phase  transition and forms ice crystals. If these aggregate to hailstones, they will fall through the cloud until they melt and get sucked up again, thus forming the natural Van de Graaff generator inside a thundercloud.

In order to fly at constant altitude (and thus avoid other traffic in the cloud) you - or your autopliot -  have to put the aircraft into a descent attitude when you hit the updraft. If you don't throttle back at the same time, the plane will indeed fly faster and the whirly bits will do likewise. My brain is just recovering from 3 hours in pretty little cumuli yesterday. Probably looked charming from the ground, but I was rowing really hard to stay within 100 ft of assigned level.

It's more complicated in an aircraft with variable-pitch propellors, descending through cloud to land. As you descend, you also need to slow down. Reducing the prop pitch initially makes the engine (especially a turboprop) speed up to match the propellor airflow to the present airspeed, the you reduce engine power - same as changing gear and using engine braking on a car descending a hill. Different aircraft and engines have different approach characteristics but this is typical of small commuter turboprops.

Water injection turns excess exhaust heat into steam, which has a lower molecular weight than oxygen, nitrogen or any combustion product, so increases exhaust velocity and reduces exhaust temperature - better than an afterburner for landing on a ship without melting the deck.

[evan_au]

hugely accelerated updraft of anything up to 40 kt, with an associated downdraft at the edges

I can understand that a light aeroplane will be badly affected by vertical movement of air. Even jet passengers sometimes get that sinking feeling in the stomach as the plane enters patches of rising or falling air.

However, my recollection of climbing or descending through wisps of cloud in commercial jets is that the horizontal acceleration/deceleration seemed to be simultaneous with entering or leaving the visible fingers of cloud. This makes me think that this is not due to the control-loop response of an autopilot or human pilot reacting to deviations from glideslope, but something more fundamental about the power of an engine in cloud/fog.

Reading some more about water injection, I see that moisture in the air intake reduces the burning temperature in the engine - and this probably reduces its efficiency and thrust. This would produce a more immediate effect than the autopilot detecting the plane being above or below the glideslope(?)

[alancalverd]

Six of one and half a dozen of the other! Civilian jet and turbprop engines respond rather slowly to power demand changes compared with piston engines, but jet aircraft are generally slipperier than small prop planes, so respond more quickly to pitch changes, and modern autopilots are exceptionally quick to respond, as anyone who has sat in the back seat of a 380 will know - it waggles up and down like a dolphin's tail to maintain GPS cockpit altitude to +/- 1 foot!

You are right. Harrier water injection precedes combustion to reduce combustion chamber temperature during extended periods at max power and to cool the intake air to increase its density. 

I can't think of any case where adding water increases engine power with all else being equal, so if there is a positive effect in cloud, it must be due to evaporative cooling increasing the oxygen density.

[evan_au]

If the engine thrust varied on entering cloud, that should be visible (on a commercial jet) by detecting whether the engine pulls forward or drags back when entering or leaving cloud.

Time for some on-the-spot investigation - I just selected my seat for a flight this week; unfortunately, I couldn't get a window seat with a view of the engines   . Anyway, I got a window seat further back, and I'll try to determine whether the plane slows down or speeds up when entering cloud.

But I can't guess what the weather be like on my flight path in 2 days time... (except to say that Sydney weather is generally less cloudy than London weather).

[alancalverd]

I just found this

"...the real innovation behind Boeing’s smoother ride technology lies, not in the aircraft’s mass, but in its electronics and software. To maintain stability, Boeing engineers use a combination of technologies like those in an automobile’s stability control system. Sensors around the aircraft measure changes in angular velocity and pressure distribution. Wind gusts that cause yaw, pitch or roll, for example, are detected and recorded by gyroscopic sensors. Similarly, vertical and horizontal forces on the craft are measured by accelerometers. At the same time, pressure sensors detect pressure distribution changes around the skin of the airplane through a selected (but unspecified) number of static air intake ports.

All of the sensor data is then sent to central processing units around the airplane. Boeing engineers won’t say how many or what kind of microcontrollers process that mountain of sensor data, but big commercial aircraft typically have more than a hundred electronic control modules, usually governed by a few master controllers. During flight, the controllers take in the data, apply proprietary software algorithms and determine what course of action to take. Using fly-by-wire techniques, the controllers then send signals to electric motors that actuate the rudder, elevators, spoilers, ailerons and flaperons. As a result, the aircraft autonomously actuates the control surfaces it needs to correct its own inertial reaction to the wind gusts. "

There are further complications as you set the aircraft up for final descent from a high altitude cruise - about the only time an airliner will fly through an active cloud. You may deploy spoilers to reduce lift, but they also create drag so you may need to increase engine power to maintain your cruise speed - a matter of traffic control as well as aerodynamics. An upward gust will increase your angle of attack, and if you are flying in the critical corner of the flight envelope, you need to increase thrust to prevent a stall, particularly if you were descending with low power. As some Boeing operators have discovered, a modern airliner is as aerodynamically efficient as a glider when it flies in the same critical region, but is a lot more complicated and unforgiving.

Not sure you could tell much by looking at the engine cowlings, but have a good flight anyway!

[Rasull]

This air, and the air it pushes above it eventually cools to the point that the moisture in it condenses. That creates puffy cumulus clouds. ... So clouds are bumpy partly because of rising air and partly because of the way that bubble of air breaks up inside them.