Inside The Plane

What is Wing Mist?

While I started a series a little while ago on the inner workings of an airplane, I think it might be time to pick back up on the intricacies of aircraft, how they work and more.

Hundreds of millions of passengers take to the skies every year, and most people are not that familiar with the craft they are flying on or how any part of it works. Nowhere was this more apparent to me than during my last flight from London to Chicago, where I was upgraded to business class for no cost. After a quick taxi to the runway, our 777-200 lined up for takeoff. Outside of the aircraft was a fairly typical London afternoon… so in other words it was wet, and the air was full of moisture.

Hearing the engines spool up, I prepared for takeoff. Lumbering forward, our giant plane gained speed and rotated into the air. As the aircraft pulled back, a stream of mist came off the top of the engine near the support strut. As soon as this began to happen, from the back of the business class cabin came the voice of a surprised woman. She was worried, asking what was happening to the engine. While she was surprised and seem slightly worried, she didn’t freak out, most likely in part due to the rest of the passengers paying no attention to it.

Personally, I was a bit surprised to hear this from a business class passenger. While I know some inexperience travellers sometimes use the cabin, usually business class is filled with very experienced travellers who have seen this happen many times. It does make a point to me though that there are people who don’t have my background in Aerospace Engineering, who have no idea what's going on outside that window.

If you travel long enough, you will see this quite often. Not only does this happen over the engine, but more commonly over the entire wing. Usually the air needs to be quite moist in order for this to happen, but it is something you will see pretty frequently.

So what is happening here?

Mist clouds forming over a wing, in an engine or sometimes over the body of an aircraft, is due to a process called the Prandtl-Glauert singularity, or the Prandtl-Glauert effect. In essence, we are creating a cloud over the wing due to the moist air. When air flows over a wing, it flows faster over the top surface than the bottom, this is how a plane lifts into the air. When the aircraft pulls back on an accent, or pulls back to increase lift during a landing, the air over the wing sees an immediate and increased reduction in air pressure.

The rapid reduction in pressure over the wing causes moist air to rapidly lower in density. When this occurs, the temperature of the air over the wing drops for a split second. Temperature drops cause the moist air to condense into a cloud. This is very much how a normal cloud forms, water rises in the air until the temperature lowers to a certain point, and then the vapor condenses into a standard cloud. The same thing is happening on the wing, just rapidly and due to a quick pressure drop in the air.

So the next time you’re flying into or out of a rainy city, take a look out the window and see if you can catch a cloud forming on the wing. It’s not dangerous in any way, but to those who don’t know what it is, it can be startling. No need to fret, everything's shiny captain.

Inside The Plane: How A Jet Engine Works

Those who aren't aware already, my background is in Aerospace Engineering, it’s what my degree is in, though my focus in college was on Astronautics, we spent a good deal of time on air breathing aircraft. Entering this field was a product of my massive obsession with airplanes when I was young. After college I was fortunate enough to work on the Boeing 787-8, 3 Dassault FalconJet aircraft, and then the Boeing CH-47 Chinook helicopter. Since then I have moved in the scientific field with nuclear research, but my obsession with airplane still remains.

Many people seem very interested in how planes work, and from the outside they seem to be big tin cans with lots of controls and a number of big engines that somehow work together to fly hundreds of people through the upper reaches of the atmosphere. They may be far more complicated than you realize, and the level of effort that is devoted to safety when building these giants is nothing short of amazing. So in this new series of posts, I’ll go through the parts of an aircraft, and how  those work. Look forward to seeing these once a week or so.

This week we’ll take a look at an aircraft engine. There are a few different types of engines that can be used to power an airplane into the sky. There are two types of engines people tend to be very familiar with, the propeller engine and a jet engine. Each of these has various versions and even combinations of the two, though from the outside these engines look much the same. Today we’ll take a look at the turbofan engine, this is the standard that is used on most airliners today.

At the core of every turbofan is a turbojet engine. These are the type that are often seen on fighter jets. The only difference between the two is the massive bladed fan that sits on the front of a turbofan engine, you probably have seen the big fan blades on an airliner’s engine. There is a very good reason for these fan blades.

Just like when you drive your car to work every morning, in the airline industry, fuel efficiency is key. When the sheer volume of gas that an aircraft goes through is taken into account, efficiency can mean the difference in $1000’s of dollar per flight. This is where that big fan comes into play. It turns out that the most efficient engines are ones that move the greatest volume of air, this means that the bigger the fan on the front, the more efficient the engine is. Naturally there are limits on this due to forces involved, size of the aircraft and a few other factors. In the fan world, bigger is better.

If you look to the top photo you can see the fan pointed out to the left. Part of the air goes into the core, and part flows around the outside of the core. Air flowing around the core is called the bypass air, and the higher the bypass ratio of air to the core air, the more efficiency we get out of the engine typically. The core, however, is where all of the work is being done. Part of the incoming air is funneled to the compressor stage, point out in the photo as low and high pressure compressors. In order to efficiently burn all this air being pulled into the engine, it must be at a very high pressure, in the order of 60 times higher pressure than the outside air. This is very important up at high altitude where the air is much thinner.

Once air has been compressed by many compressor blade stages, the fuel is added and ignited. As the combustion happens it continues to move back to the turbine stage. The turbine is what helps power the whole engine. As the air combusts, it expands and accelerates out the back passing over turbine blades. These blades are connected to the compressor and fan through a long shaft at the center of the engine. So as the air turns the turbine, the turbine turns the compressor and fan, pulling in more air. The exhaust exits the core and joins the fan air in thrusting the aircraft forward.

That is the general overview of an aircraft engine and how it works. Bypass air makes up something on the order of 80% of the thrust an engine produces, where the core makes up the other 20%. As we continue developing more advanced turbofans, the bypass ratios have increased for efficiency sake, the combustion is more complete also for higher efficiency. Boeing 787 engines are part of an effort to decrease fuel expenditure, hitting a whopping 20% decrease in fuel usage on the 787.

For myself, the most impressive engine to date is the GE90 that was developed for the Boeing 777. The most powerful air breathing engine ever developed, capable of producing 115,000 pounds of thrust. This engine would be capable of lifting a Boeing 737 straight up in the air… and they strap 2 of these onto each 777 plane.

If you have any ideas for what you’d like me to cover next week, let me know in the comments.