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Comment by tus666 | Hacker News Reader

At what point does a prop become a fan?

A 777 engine has 22 fan blades, twice this thing.

> At what point does a prop become a fan?

Linguistically, a prop becomes a fan when you anchor it to something immovable, or attach it to something movable but without any intention of propelling it using the prop.

Fan - try to move the air around, but not the thing it's attached to.

Prop - try to propel the thing it's attached to

Edit: I realize that might sound ambiguous given terms like "turboprop" and "turbofan" both of which are on planes.

The turboprop uses air to speed up propellers to move the plane. A turbofan uses just accelerated exhaust from the engine, and the fans are just a means of feeding the engine combustion material (air)

I am not sure this is really correct - turbojets (as in fighter jets) sound more like what you describe, turbofans if I remember correctly, get most of the thrust (>70%?) from bypass air around the combustion, and very little from actual combustion thrust. It just also feeds it's own combustion chamber air to keep the fan part going. Perhaps it is a fan if enclosed? Vs. a prop if not?

Most modern fighter jets have turbofan engines as well.

>Perhaps it is a fan if enclosed? Vs. a prop if not?

Stipa Caproni

Dear Lord! Half of the lift is due to the repulsion Earth feels from it.

That's not true about a turbofan. A large part of the thrust is actually from the fan itself. The bypass air that never goes into the combustion chamber produces most of the thrust.

"A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.":

https://en.m.wikipedia.org/wiki/Turbofan

The relevant conceptual distinction seems to be whether the aerodynamics of the blade(s) are conceptually independent, or interdependent. Better explained here: https://en.wikipedia.org/wiki/Propeller_(aeronautics)#Aircra...

Back in the 80s there were experiments run with propfans -- propellers with many stubby blades that offered near jet-like performance while consuming half the fuel. They never got anywhere because propfan engines were large and noisy and the dropping price of oil made the fuel economy concerns less urgent.

A think a lot of the noise was because the blades were counter-rotating. The size limitation was to be compatible with tail-mounted engines like the ones used in DC-9's and 727's.

> what point does a prop become a fan

I don't know, because I think the whole purpose of a prop is to be a fan.

No quite. Fans == move air, relative to the thing the fan is mounted to.

So a jet engine has fans, because it is moving air into and around the engine, but not using them to move itself around directly (It uses the jet of air created by the thermal expansion from the burning fuel and compressed intake air to do that, mostly).

Propeller == moves something through air (or another media like water) directly.

Semantics, mostly.

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