Harrier Forward Flight and Hover

How does a Harrier fly? How does it hover?
- question from Lucy

My friend and I are arguing about the Harrier. He says it hovers like a helicopter when I think it can only hover by basically pushing off the ground.
- question from Nick Lengyel

The mysteries of the Harrier's flight characteristics are explained in a previous question we answered on its hover capability.

The Harrier is able to hover by directing its thrust through four separate nozzles that can be rotated between the horizontal and vertical, as shown below.

V/STOL thrust-vectoring arrangement on the Harrier

When the nozzles are horizontal and point backwards, the aircraft flies forward. When the nozzles are vertical and point downward, the Harrier can fly up or down or hover in a stationary location.

Nick's question really comes down to a matter of perspective. Based on the way it is worded, the question would seem to be asking whether or not flying vehicles pull themselves up off the ground or push themselves off the ground. From a pure physics point of view, it makes no difference! As Newton's third law of motion states, for every action there is an equal and opposite reaction, so both arguments would be acceptable. As my high school physics teacher used to say, one could state that "my fist punched my little brother's stomach" or "my little brother's stomach punched my fist" and both would be equally valid. Somehow my parents never bought that argument.

But to truly answer this question, we need to take a closer look at the mechanisms by which the Harrier and a helicopter achieve hover. The helicopter is able to hover by aerodynamic means. The helicopter rotor rotates, and as it does so, it generates an aerodynamic lift force. Meanwhile, the Harrier directs its vectoring nozzles downward to generate a propulsive lift force. While both methods produce the same effect of a lifting force, the means are fundamentally different. If we were to use the hovering vehicle as the point of reference, it would clearly follow that the helicopter pulls itself upward because of the aerodynamic force acting on its rotor while the Harrier pushes itself upward because of the thrust force acting on its nozzles.

Flow visualization of the Harrier in hover

The above image illustrates the Harrier "pushing against the ground," as it were. The red streamlines indicate the high-velocity exhaust exiting through the downward-deflected nozzles to produce the thrust needed to keep the aircraft airborne.
- answer by Joe Yoon, 10 November 2002

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