reversers and a/c speed

[PBL]

No, it's not a trick question, bearfoil. It's a despairing question.

Let me explain. You can generate lift in an airfoil by blowing air across it.

I hope we can agree on this.

Suppose you attach an airfoil to a fuselage with pilot. Can you lift it vertically? Yes, by blowing air across the airfoil.

How much air? Enough.

Can you generate "enough" air from devices mounted in front of the wing? Theoretically, yes. Try the kinds of solid-rocket motors used to power the Space Shuttle.

Can these be attached to the aircraft, or must they be physically separated? Doesn't matter.

Let's go back to ships.

Mount a rotating cylinder vertically on a ship. Mount a keel under the cylinder to keep the ship from moving perpendicular to its bow. Mount a fan on the ship, let's say on the left side of the ship, which blows air across the cylinder perpendicular to the bow.

Will the ship move?

In which direction will the ship move?

My problem at the moment is that I seem to be discussing aerodynamics with people more interested in repartee than they are interested in physics or aerodynamics. Further, the repartee is not very entertaining. What a waste of time!

Except for lomapaseo's intervention, which I missed first time around (sorry!)

Loma, consider. A mass of air is accelerated towards the engine inlet duct (by sucking). Some of it goes in the duct, some of it goes around. But it all gets stopped (relative to the aircraft); the stuff inside by plates, the stuff outside by encountering reverse-velocity air (formerly the stuff inside).

You have to put energy in to stop all that air, but stopped it gets.

Now, that air had momenttum. All that momentum goes somewhere (conservation of, and so on). Where does it go? If you have designed things cleverly, it goes into force. In which direction? The same: negative-x (that is what conservation of momentum means). That negative-x direction force is experienced as braking by the airplane+occupants.

Now, exactly how which bit of air gets momentum-reduced, and by how much, is a question no one here has yet answered. Some people don't care; they can stick with the story above. I care, and I'd like to find an answer. I will find it, but I don't think any more through this discussion.

PBL