A gravity knife is a knife with a blade contained in its handle, which opens its blade through the force of gravity. This mechanism of opening is fundamentally different from the switchblade, which extends its spring-propelled blade automatically upon the push of a button, switch, or fulcrum lever.
The gravity knife uses a button, trigger, or fulcrum lever to release the blade from both the open and the closed positions, and may use a side-folding or telescoping (out-the-front, or OTF) blade. While most military gravity knives utilize a locking blade design, other types may not mechanically lock open but rely instead upon friction to wedge the blade's rear section against the handle's interior. Factory-made gravity knives have various types of buttons, triggers, and fulcrum levers, which usually are used to release the blade from both the open and the closed positions.
Design and operation
Flieger-Kappmesser / Luftwaffe Gravity Knife (LGK)
thumb|right|German or air force [[paratrooper knife]]
One of the most recognizable gravity knives is the World War II-era (), which utilizes a telescoping (OTF), gravity-propelled locking blade. First produced in 1937, the often so called () was initially issued to German flight crews and paratroops, primarily to cut a trapped parachutist from his rigging in case he landed with a tangled parachute, or became entangled in trees or in the water with the shroud lines. The spike was used to untie knots while packing the parachutes. Luftwaffe air crew members used the knife to cut themselves out of their harnesses or cut through the aluminum hull of the aircraft after a crash landing. Though not intended for use as a fighting knife in the first place, the LGK was introduced to the 1st Skijäger Division and SS units on the Eastern Front to be used primarily as a close combat weapon from 1944 on.
The uses a sliding blade inside a metal grip frame, which was initially fitted with smooth wood scales. The blade is relatively blunt spear-point, and the profile is flat ground, tapering to a utility edge. To open the blade, the user points the knife downwards while flipping up the fulcrum-style operating lever, allowing gravity to draw out the blade to its fullest extent. Releasing the lever locks the blade into position. The LGK may also be opened by flipping the blade release lever while flicking the wrist holding the knife, causing the blade to extend. The LGK was also equipped with a folding marlinspike or awl. Primarily intended for untangling rope knots, it was also used as a prying tool or to fix jammed weapons. The spike does not lock when opened and was never intended to be used as a combat weapon.
There are two principal types of wartime-era with 10 known variations. The Type I LGK (three manufacturers, five WWII variations) has wood scales (handle), was made from 1937 to 1943, and, unlike successive models, has no takedown capability. The Type II LGK (two manufacturers, five WWII variations) is the same knife, but with takedown features, and was produced from 1943 until 1945 and then again from c. 1950 until 1965. Under the initial wartime contract, George Ibberson & Co. made 500 gravity knives for issue to the SOE and other special forces. These Sheffield gravity knives had smooth wood or textured plastic scales. Still, they were otherwise identical in features and operation to the Type I , with a locking gravity-deployed blade and a folding rigging spike or awl. In the hands of British SOE agents, the Sheffield gravity knife was considered to be a secondary combat weapon. In addition to the knife blade, SOE close combat instructors found that the folding rigging spike was useful for silent killing of sentries by opening the carotid artery on the neck.
'False gravity' and non-gravity knives
Some folding or telescoping knives that can open their blades by force of inertia or gravity were not intended or advertised by the manufacturer to do so. Knives that will lock their blades open, but do not have any closed position lock device, are referred to as "false" gravity blades.
thumb|right|upright=1.4|Folding "false" gravity knife
The illustration shows the internal parts of a 1960s Japanese import 'false' gravity knife. Other knives may be considered 'false' gravity knives, including certain lock-back and liner lock folding knives that will not lock a blade in both open and closed positions.
After the passage of the Switchblade Knife Act in the U.S. in 1958, it was discovered that some U.S. businesses were importing folding stiletto knives without spring mechanisms that did not have a pronounced blade heel (internal surface) in the pivot area allowing the longer bladed models to flick open quickly.
Other
thumb|Conversion knife
The 'conversion' gravity knife is a standard folding knife fitted with after-market modifications. The knife pictured does not lock closed, and is thus classed as a 'false' gravity knife. It can still be opened manually without touching the toggle switch. The addition of the toggle, which is attached to an internal bellcrank-arm, will lift the lockback mechanism to allow the blade to swing more easily. Local jurisdictions including cities and counties may also restrict the ownership or carrying of gravity knives.
In a few jurisdictions, such as Colorado and New York City, courts have periodically attempted to classify ordinary lock-blade folding knives with a blade that may be opened by centrifugal force (normally, using a flicking motion of the wrist) as a gravity knife, thus making the knife's owner subject to the same criminal penalties imposed for illegal possession of a gravity knife. The New York statute banning gravity knives has been declared "unconstitutionally vague" by a federal District Court in March 2019. New York State legalized the possession of gravity knives on May 30, 2019 but New York City retains its ban on public transport.
See also
- Butterfly knife
- Rampuri
- Pantographic knife, often incorrectly referred to as a "Paratrooper Knife"