Of course the new vectored thrust (and LOTS of it) and computerized canards and stuff make the modern day "cobra" stunts possible, but they are derivatives of good old manual hand-flying (with zero computerized flight controls) that some of the old "good moves" were. You had "controlled" out-of-control flight departures in your bag of tricks, in addition to things such as PMC, which, as Hoser alludes to, is not a departure at all -- you never "stall" the aircraft since you operate at and below zero "angle of attack" (AOA) during the very slow speed maneuver.
The AOA is the angle at which the RELATIVE airflow (straight up, level, 60 degrees nose up/down, whatever) is attacking the wing while smoothly flowing over it. Once that angle keeps increasing due to increased pitch control forces, it reaches a certain point (due to the design of the wing), when the smooth liftie-creating airflow is instantly, ABRUBTLY, disrupted (turbulent flow) and lift on the wing goes to zero.
Two things can increase lift on the wing -- faster airflow, and/or increasing the AOA which is the angle at which the airflow hits the wing -- like your hand out the window of a car at 60mph. An airplane can "stall" at ANY speed -- even at really fast speeds --as long as "stall AOA" pull is reached where the air can no longer smoothly flow over the wing. At higher airspeeds, of course tight hard turns result, meaning much higher "g" forces. Still, high speed "stall" can still occur if enough "g"/pull is applied until that SAME stall AOA is reached.
The airplane doesn't stall as a function of speed -- it stalls as a function of a specific AOA -- only. The higher the speed, though, the more "pull" (read "g") is required to reach that same AOA.
The flip side is an airplane can fly at any speed -- even well BELOW the civilian mind's "stall speeds" approaching zero -- as long as the current AOA is below stall AOA. For example, in a pure vertical hammerhead stall, one is not actually stalled, but still "flying" the airplane at zero AOA while STILL controlling the plane of rotation with the tiny amount of airflow over the wingtips which are still "moving." The wingtips are moving through the air via the airplane's "mass" rotating about the yaw axis due to last second rudder kick while a little airspeed still remained while decelerating to zero.
So, the lift on the faster (outboard relative to rotation) wing wants to roll the airplane on its back, so, even though airspeed is showing near zero, slight opposite airleron control is required to prevent that. So, one is "flying" the airplane at near zero airspeed via aileron at the top of the hammerhead since the wingtip is STILL moving through the air. (Some can be controlled at minus airspeeds; Shawn Tucker does it in tailslides at airshows all the time, and we used to have that big stab on the F-14 control a pure vertical nose in contest tailslides also!). Anyway, one can be in full control, at zero AOA, zero airspeed, and NOT "stalled". IOW, the rotational energy of the airplane's mass is still being "controlled" with rudder and aileron inputs to that tiny airflow over them, with pitch/elevator input keeping us weightless by pushing to zero AOA.
So, it is the ANGLE at which the "wind" is meeting the wing chord line that detremines stall; NOT the speed of the airflow over the wing (which only increases TOTAL lift; just as increased AOA also does). Typical "stall speed" thinking for a X,000 lb airplane means constant altitude (i.e. the wing creates exactly X,000 lbs of lift) sitting in your seat at exactly 1"g" like now at your computer with the engines pushing exactly enough thrust/power to counteract drag for a constant airspeed. Then pull power and one starts slowing down and decreasing lifties on the wing due to slower airflow (Bernoulli's principle). To maintain constant altitude (relative wind on wing is parallel to the ground), then, slowly increasing the nose angle up (higher AOA) relative to the "ground parallel" wind for level flight increases the lift via that increased AOA so that X,000 lbs of lift is still generated so that level flight is still maintained -- but at that slower airspeed/increased AOA.
This increasing pitch attitude in level flight (relative wind parallel to the ground) will continue as speed decreases until the angle of the "relative wind" to the wing is so high the airflow suddenly separates, becomes turbulent, and lift goes to zero (STALL). That angle is stall AOA, and was from a slow, constant altitude, 1"g" stall. Also, I could "pull" at much higher speeds (with resultant nose pitch rates) and still make that SAME angle between the "relative wind" and the wing MAC (mean aerodynamic chord). The stall will occur at that exact same angle as in the 1"g" case. It just occurs at higher speeds and higher "g"s because we are now TURNING (instead of just staying level at 1"g" counteracting our weight vs. gravity).
So, one could be 70 degrees nose up, at any speed, and if one pulls hard enough, one can reach stall AOA and stall. If I do not pull during that nose high deceleration, but I instead push forward as I slow down (approaching a tail slide condition), I keep the AOA near zero and stall AOA never occurs. The airplane can be controlled without stalls at VERY slow speeds if elevator control is precisely used to keep it near zero AOA (basically weightless) conditions and away from postive (or negative) stall AOA's.
Once at very slow airspeed, however, It takes VERY LITTLE "pull" on the elevator/stabilator at those very slow speeds to go from low AOA to stall AOA in a split second. The opposite is true at high speed. It takes a lot of pull force and/or effort (in old normal hand flown jets) zinging along at, say 400 knots to even get close to stall AOA, cuz "g" force limits from the centrifugal forces of the resultant turn are reached before stall AOA can be reached.
Anyway, this low speed, low AOA maneuvering is key to a maneuver such as the PMC. One is fully "in control" and not "departed" from controlled flight (although "controlled" high AOA "departures" are also used to great advantage in other scenarios -- as Skogs mentioned, for example, in getting that fast roll rate out of the F-14's normally "glacial" roll rate). IMO, such moves could be called the non-computerized, hand-flown "cobras" of their day!!
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发表于 2010-1-9 21:57:57
发表于 2010-1-9 22:00:25
楼主
大多数新鸟都有这个毛病啊~~~
发表于 2010-1-9 23:17:37
发表于 2010-1-10 00:41:34
