I have heard that Harriers use downward jet exhaust to do VTOL Maneuvers and I was wondering if someone could explain how that works?
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The Harrier achieves vertical lift by having four thrust-vectoring nozzles positioned on the sides of the fuselage, coupled with one turbofan engine. Here's how the nozzles look on the actual fuselage:
Image via Alan Wilson on Wikimedia Commons, CC-BY-SA
And here's a diagrammatic view of how the engine is coupled to these nozzles:
Public domain image via Fama Clamosa on Wikimedia Commons
When vertical lift is desired, the pilot moves a lever next to the throttle in the cockpit and sets the desired angle (which is itself a function of speed and phase of flight). The pilot may additionally dump water from a reserve tank into the engine exhaust to provide extra lift.
When configured for vertical flight, the Harrier faces a number of challenges:
Disrupted airflow into the engine intake: not only is less air being rammed into the engine as would be the case during forward flight, but the air is now moving perpendicular to the engine's axis.
Extremely limited control authority on all three axes: with almost no airspeed, there's no real yaw, pitch, or roll control, and the vectored thrust nozzles don't include differential thrust (as far as I know).
Newer platforms like the F-35B incorporate wingtip nozzles for improved roll authority and fine, automatic rear nozzle vectoring to provide a safer, more controllable experience, and are further augmented by computers to relieve the pilot from managing all these extra features.
High fuel burn: the Harrier only provides enough vertical thrust to lift itself, a little bit of its fuel, and only the lightest armament configuration in most conditions, typically with the throttle nearly at full power. Water can be dumped into the exhaust to provide some extra thrust, but this reserve usually lasts 2-3 minutes. This is all caused by the engines having to provide all the lift, unassisted by the wings.
Little or no computer assistance, especially when compared to modern counterparts like the F-35B. Pilot error in the Harrier's VTOL regime had the potential of very quickly becoming catastrophic, between the dulled input responsiveness and high pilot workload.
In general (though definitely not always), Harriers would perform vertical landings on underway ships rather than at airfields. The 15-30kn of forward motion was enough to get the wings to generate some lift, mitigating a handful of the issues I listed above.
For more information, please see the Wikipedia page on the Harrier family.
