Memes

Eagles may soar, but weasles aren't sucked into jets.

Gentle Readers,

There are many “Memes” in the English language that are bone-crushingly ignorant and cause your humble correspondent to worry for the long term health and survival of the Republic. These memes often drive me more than a little buggy, but since I have never wanted this blog to be of the political variety, I choose not to discuss the popular phrase “Economic Stimulus Package”.

No, instead I wish to continue with the aviation theme I have established here and to discuss the phrase: “the birds got sucked into the engines!” The reporting of what I am coming to call the ‘Hudson Glider’ has caused me to attempt to use my humble blog to attempt to set the record straight.

The physics involved in the impact of one of our avian cousins upon the intake of a modern day jet are highly complicated and – I think – fascinating. If you stick with me through this discussion, I hope you will agree.

First, let’s discuss the design and layout of a ‘gas turbine engine’. As you can see from the link, a turbofan jet engine has 5 sections: the inlet, compressor, combustion, turbine and exhaust nozzle section.

( ‘Suck, Squeeze, Burn, Blow and Go’ as Air Force student pilots are encourage to memorize…..)

The important section for our discussion is the inlet and compressor section where the air is sucked in and squeezed down to be efficiently combusted. These are the sections that bear the brunt of an avian impact.

The compressor section is a massive construct of whirling metal blades spinning at an awe-inspiring rotation rate. For comparison, you only infrequently exceed 5000 revolutions per minute (RPM) on your car’s engine. If you have an electric fan blowing a refreshing breeze around your home, I doubt seriously it ever exceeds 2000 RPM. Now compare this to a jet engine operating up in the range of 20,000 RPM.

Back to your car, please consider your tires. Your tires are spinning nowhere near the rate at which a jet engines spins but if you do not have your tires carefully balanced, they will vibrate your car in annoying and often unsafe ways. The compressor section of a jet engine is hundreds of times more massive than your tires and spinning at a much faster rate. If the compressor is not perfectly balanced it will vibrate excessively and dangerously.

Let’s consider for a moment what would happen if one of these compressor blades were to come loose - bad things happen quickly. The blade initially flies outward and backward due to centrifugal forces and the airflow. If it is ‘contained’ inside the engine it probably knocks loose more compressor blades aft of it which knock loose more blades and you have a runaway chain reaction that in a blink of the eye ‘shells out’ the engine. Some of the blades not knocked loose come loose anyway because of the awe-inspiring out-of-balance vibrations. Pieces of hot smoking metal exit the aft of the engine and you no longer have a jet engine producing thrust but a mere smoking hulk of metal hanging below the wing.

(Let’s not discuss what happens if the blade is not contained and some shrapnel escapes and flies outward from the engine….)

Put that notion at the back of your mind now and let’s turn to the physics of impacts.

What we wish to discuss is ‘inelastic’ impact physics where the total mass and energy of the flying object is totally absorbed by the body impacted. It will be instructive to turn our attention to artillery, specifically the cannon balls of the sailing ship era. A common cannon ball size was the ten pounder which is roughly the same weight as a Canadian Goose.

Consider what would happen if you fired one of these cannon balls at a jet airliner.

(An empty jet, of course. No imaginary passengers were harmed during this thought experiment.)

The cannon ball leaves the muzzle of the cannon at a subsonic speed. I have not researched the exact speed but I assume it is in the range of 300 to 600 mph. The ball would probably penetrate the thin aluminum skin of the aircraft then travel through the fuselage and exit the other side, its speed not much diminished. Very little of the kinetic energy of the ball would be transferred to the aircraft. Now consider if the ball were designed to shatter upon impact causing the ball to remain contained in the jet and to ricochet around the interior. All of the kinetic energy of the cannon ball would be transferred into destructive work on the interior of the jet.

A bird’s fragile body is much like this effect. When the bird impacts upon the jet engine, all of the kinetic energy is totally transferred into destructive work upon the inner workings and hidden mechanisms of the engine.

Next, let’s consider a Canadian Goose, flying along at the leisurely speed of 20 mph or so. If one of these things were to hit you it would be much like being hit by a ten pound bean bag thrown by your 10 year old niece. Not much damage would occur.

The destructive kinetic energy of a bird / airplane impact comes from the speed of the airplane. It is much more accurate to say the airplane hits the bird. At the point our Heroes of the Hudson hit their birds they were up in the vicinity of 250-300 mph – the maximum speed we are allowed to fly below 10,000 feet altitude. This is low order cannon ball speed.

Back to my thesis statement as I began this discussion: there was very little ‘sucking’ going on during these impacts. The only sucking that occurred was after the bird entered the intake of the jet - which is less than 5’ long. The bird traveled this last leg of his final journey in a split second and impacted the compressor blades. If the birds hand been 10 feet away in any direction, they would have hit the airplane somewhere other than the engines - or missed completely. The engines most emphatically do not reach out and suck things into them while you are in the air.

(On the ground is a different kettle of cliches....Jets will most definitely suck things up on the ground.)

No, what really happened was that fate fired a cannon ball at their engines. At this point, the bird was basically a 300 mile per hour frangible cannon ball and the runaway chain reaction damage I described above occurred.

The rest of the story we already know.

At some later date, esteemed readers, we will turn our attention to the meme: “We sat on the runway for hours!”.

I remain,

Dad / Geoff