So I know the simplified theory and basic principles here, Newton’s 3rd law, air compression and sudden exhaust and all, but it boggles me how this principle can lift, even move, 350 tons flying objects.
I get how it works but I need some kind of metaphor to understand and wrap my mind around the forces and amount of thrust at play here
In: Engineering
I think you are confusing a turbojet engine with a turbofan engine. The latter has a big fan that works like a propeller and is driven by a gas turbine. Also an engine does not lift a plane. It pushes it forward. The wings are generating lift.
Little correction from the title, a turbo jet can’t move a 350 ton plane, a turboFAN can
The fan of the engine moves 80-90% of the air, powered by the small powerful core
A hand fan might keep your face cool but a massive ceiling fan can cool a room.
it’s all about mass of air moved with the help of energy dense fuel
It’s not the engines alone lifting the plane, it’s not like a rocket where it’s pure force to break the bonds of gravity.
The Bernoulli effect is how planes fly. The engines need to provide enough power to get the plane moving quickly on the ground, and then the shape of the wing provides the lift.
Due to the shape of the wing air moves faster over the top than under the bottom, this creates a pressure differential where the air pressure above the wing is lower than under it, which creates lift. The engines just have to provide enough energy to keep the plane moving quickly enough that the air pressure below the plane keeps it in the air.
The engines are not doing much to lift the plane. They just need enough power to overcome the drag forces not the entire weight of the plane.
If you look at a typical passenger plane during take off they have a thrust to weight ratio of 0.2 to 0.3 typically. So the engines are only provided enough thrust to lift 1 third of the planes weight. And most modern jetliners have 2 engines so each could only lift 1/6 of the aircraft weight.
Also the type of engine effects how much trust it can typically produce. You original questions asks about turbojets but those are a specific type of engine that isn’t commonly used anymore. Modern passanger planes use high-bypass turbofan engines, which are more effecient at sub sonic speeds. They make most of their trust just by moving lots of air through the fan. Less than 10% of the air going in goes through the turbo core to power the engine. The rest just gets blown out the back.
So your typical short haul airliner (737, A320) is burning about 10 gallons of fuel per minute. That’d drain your car’s fuel tank in about a minute and a half. That’s probably faster than your gas station can pump it! Not only that, but the engines are extremely efficient compared to piston engines. I think there’s sometimes a challenge to understanding that massive scale of a plane, but when it burns fuel at something like 300 times the rate of your car and probably triple the efficiency* you’re looking at the power of a thousand or so cars. I might be off a bit, but that’s a lot of power available.
* Making this up, but I think I’ve heard 25% efficiency for cars and 80% for planes.
A couple things.
1) The engines throw a LOT of air backwards at very high speeds. And that ends up being a lot of mass. Its a lot more mass than you think.
2) For planes, it isn’t lifting it straight up, but along a ramp. So less than 350 tons of force is required to lift the plane, as long as the plane also travels forward a large distance.
3) It doesn’t take much force at all to just “move” an object. A single person can pull an entire jumbo-jet. It does take a lot of force to oppose other forces, like friction and drag. But air doesn’t resist that much at low speeds, and at higher speeds that resistance is all the engines have to fight for level flight. They don’t have to ‘lift’ the plane directly.
All of these answers about how the wings lift the airplane are 100% correct. However, they don’t really address the fact that turbomachinery is just insanely powerful for its size compared to anything else we’re used to dealing with on a daily basis. Turbine engines spin really, really, really fast, convert a LOT of fuel into a LOT of heat very quickly, and move a LOT of air. If you’ve ever been in the presence of any kind of operating turbine engine, even at idle, they are absolutely terrifying.
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