Naked Science Forum
Non Life Sciences => Technology => Topic started by: syhprum on 25/01/2018 13:09:36
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When a fighter aircraft is launched either by a steam catapult or more recently by a magnetic device the pilot is subjected to quit high g forces is this sufficient to impair his performance at this critical phase of the operation and what is done to alleviate this.
What is the power generated by the catapult and what acceleration is achieved.
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Figures suggest about 3-4g for a steam catapult.
A worked solution here: https://pumas.nasa.gov/files/03_23_02_1.pdf
For a first-hand account of what it is like, see: http://omegataupodcast.net/144-flying-from-aircraft-carriers/
People who have flown in commercial airlines will be aware of the illusion that the aisle is tilted upwards, even when the aircraft is accelerating horizontally down the runway.
Now multiply that by the forces experienced by a pilot launched from a steam catapult, and the pilot would have the impression that his plane was flying upwards at a steep angle, risking a stall. This has led to some pilots steering into the water after launch from an aircraft carrier.
See: https://en.wikipedia.org/wiki/Aircraft_catapult
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Assume stall speed about 100 kt = 600,000 ft/hr = 10,000 ft/min = 166 ft/sec. Suppose flight deck is 500 ft long. Then
v2= 2as
so 1662 = 27556 = 2 x a x 500
∴ a = 27.556 ft/sec2, just under 1g
This would work for a WWII piston-engine plane on a small carrier but the catapult piston needs room to slow down again, so 1.5 g is a minimum to ensure that you get to flying speed without wrecking the ship. Modern fighter-bombers need closer to 200 kt to take off with a full load. The good news is that modern flight decks are longer, but even the Nimitz class is only around 1000 ft so 2 - 3g may be needed.
When operating well offshore, carriers turn into wind at "full ahead" for flying operations: the additional 30 - 60 knots of headwind makes life easier, particularly for recovery, but "all the runway behind you is useless" so there's little temptation to reduce catapult power for a few seconds' temporary comfort, and the next sortie could well be at close quarters in a dead calm.
Forward acceleration to 2 or 3g doesn't present too many problems to an experienced aviator, and having reached flying speed whilst being dragged "controls neutral" in a straight line by the catapult, the next phase of acceleration and climbout under engine power and manual control is (relatively) civilised.
There can be a disconcerting effect if the pilot is reclined. Feet-first acceleration feels like negative g. This isn't a great problem in large aircraft where the pilot is essentially upright, but it can be disorienting in a winch-launched glider where the initial attitude is very nose-up: the temptation is to correct the apparent negative g by pulling back on the stick, which breaks the cable with the glider in a stalled attitude, with unpleasant consequences.
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People who have flown in commercial airlines will be aware of the illusion that the aisle is tilted upwards, even when the aircraft is accelerating horizontally down the runway.
Have you noticed that the rear of an airliner fuselage is chamfered upwards and fitted with a damn great skid? Once you have reached VR (curiously known as "rotation speed" - I wonder why?) you do indeed have to tilt the aisle upwards to avoid hitting the trees and generally to point the plane towards your 40,000 ft target. The skid is there to guard against excessive enthusiasm on a short runway. The sensation differs between propeller and jet aircraft, which have different acceleration profiles and optimum climb attitudes.
The sensory illusion is used in full-motion flight simulators. If you tilt the "cockpit" upwards but maintain the "level" visual display through the window, all the occupants think they are accelerating horizontally. Add another attitude tweak and gradually lower the visuals, and it feels like climbing to cruise altitude. A perfectly balanced turn is simulated by tilting the cage upwards and rotating the visuals, and any skid or slip in the turn, by tilting the cage sideways. The eye/ear/baroreceptor interaction is fascinating on the ground, and potentially dangerous when actually airborne in zero visibility or with a mountainous horizon.
There is a school of thought that, whilst simulators are great for type-conversion and suchlike, they can't simulate sustained manoeuvers above about 0.5g so commercial pilots need occasional aerobatic refresher sessions to appreciate envelope limits and recovery from serious upsets.