Envelope

Envelope (pronounced on-vuh-lohp or en-vee-lope (non-U))

(1) A flat paper container, usually having a gummed flap or other means of closure and used to enclose small, flat items (especially letters) for mailing. 

(2) Something that envelops; a wrapper, integument, or surrounding cover.

(3) In biology, a surrounding or enclosing structure, as a corolla or an outer membrane.

(4) In geometry, a mathematical curve, surface, or higher-dimensional object that is the tangent to a given family of lines, curves, surfaces, or higher-dimensional objects.

(5) In the radio transmissions of a modulated carrier wave, a curve connecting the peaks of a graph of the instantaneous value of the electric or magnetic component of the carrier wave as a function of time.

(6) The fabric structure enclosing the gasbag of an aerostat or the gasbag itself.

(7) As an idiom, in pushing the envelope, to stretch established limits, as in technological advance or social innovation.

(8) In music, the shape of a sound which may be controlled by a synthesizer or sampler.

(9) In computing, the information used for routing a message that is transmitted with the message but not part of its contents, the best known example of which is the blind carbon-copy (bcc) in eMail.

(10) In astronomy, the nebulous covering of the head or nucleus of a comet; a coma.

(11) In civil engineering, an earthwork in the form of a single parapet or a small rampart, sometimes raised in the ditch and sometimes beyond it.

(12) In engineering and design, the set of limitations within which a technological system can perform safely and effectively.

(13) In aviation (of dirigibles), a bag containing the lifting gas of a balloon or airship; fabric that encloses the gas-bags of an airship.

(14) In electronics, a curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.

(15) In computing, the information used for routing a message that is transmitted with the message but not part of its contents.

(6) In music, the shape of a sound, which may be controlled by a synthesizer or sampler. 

1705: From the Middle French and Old French envoluper, the construct being en- (from the Old French en-, from the Latin in-, a prefixation of in (in, into)) +‎ voluper (to wrap, wrap up).  In Italian, the derivation was viluppare, from the Old Italian alternate goluppare (to wrap) from the Vulgar Latin vlopp (to rap).  The Proto-Germanic wrappaną and wlappaną (to wrap, roll up, turn, wind) came from the primitive werb (to turn, bend), akin to the Middle English wlappen (to wrap, fold) and ultimately the Modern English lap (to wrap, involve, fold).  The modern wrap is derived from the Middle English wrappen (to wrap), the dialectal Danish vravle (to wind, twist), the Middle Low German wrempen (to wrinkle, distort) and the Old English wearp (warp).  The French enveloppe, is a derivative back-formation of envelopper (to envelop).  Envelope is a noun; the noun plural is envelopes.

Pushing the envelope

The phrase pushing the envelope is from the lingo of test pilots, whose job is among the most dangerous of their profession.  It entered general usage following the publication of the late Tom Wolfe’s (1930-2018) book about test pilots and the early US space program, The Right Stuff (1979).  The envelope in the phrase is a mathematical construct, what is called the "flight envelope" of a given aircraft: combinations of speed, altitude, range and stress that are considered the limits of an airframe’s capabilities and so-named because usually it's graphically represented in the shape of the familiar DL envelope.  Within the envelope formed by these parameters, the airframe is structurally sound; beyond those limits, perhaps not and that’s what test pilots do, verify the safety of the aircraft within those limits and pinpoint possible points of failure if the envelope is pushed too far.  Although big, fast computers now make the parameters of the envelope more predictable and the job of the test pilot less dangerous, structural failures during test flights continue to happen.

Lockheed F-104 Starfighter.

Few airframes have operated within such a tight envelope as the Lockheed F-104 Starfighter, introduced into service in 1956 as a single-engine, supersonic interceptor, built for the United States Air Force (USAF) but used by many nations.  Best thought of as the manned missile by which it was referred to by many, it had a radical wing design, a very small, straight, mid-mounted trapezoidal.   After the German research undertaken during World War II (1939-1945) became available, most jet fighters had used either swept or delta-wings, a compromise between speed, lift, maneuverability and internal space for fuel and equipment.  Lockheed sophisticated wind-tunnels and primitive computers however determined the optimal shape for high-speed supersonic flight was small, straight and trapezoidal.  An extraordinary achievement of manufacture as well as design, the wing was so thin and sharp it was a cut-hazard for ground crews and protective guards were fitted during maintenance.

The F-104 was the first combat aircraft capable of sustained Mach 2 flight, its speed and climb performance impressive even by today’s standards.  However, there was a price to be paid, take-off, stall and landing speeds were high as was the turn radius, combat pilots referring to low-speed turns as “banking with intent to turn".

The flight envelope, note the DL envelope shape.

The safety record was infamously bad.  Of the 916 delivered to the West-German (FRG) Air Force, 262 crashed, gaining it the nickname witwenmacher (widow maker) and some of those grieving widows sued Lockheed, receiving judgment in their favor.  In USAF service, the write-off rate was 30.63 accidents per 100,000 flight hours.  By comparison, the rate for the Convair F-102 Dagger was 14.2 and for the North American F-100 Sabre, 16.25.  The F-104's two nicknames, "manned missile" and "widow maker" may be thought of as cause and effect.

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