Vg Diagram
The flight operating strength of an aircraft is presented
on a graph whose vertical scale is based on load factor.
[Figure 4-47] The diagram is called a Vg diagramâ€”velocity
versus G loads or load factor. Each aircraft has its own Vg
diagram which is valid at a certain weight and altitude.
The lines of maximum lift capability (curved lines) are the
first items of importance on the Vg diagram. The aircraft in
the Figure 4-47 is capable of developing no more than +1 G
at 62 mph, the wing level stall speed of the aircraft. Since the
maximum load factor varies with the square of the airspeed,
the maximum positive lift capability of this aircraft is 2 G at
92 mph, 3 G at 112 mph, 4.4 G at 137 mph, and so forth. Any
load factor above this line is unavailable aerodynamically
(i.e., the aircraft cannot .y above the line of maximum lift
capability because it stalls). The same situation exists for
negative lift flight with the exception that the speed necessary
to produce a given negative load factor is higher than that to
produce the same positive load factor.
If the aircraft is .own at a positive load factor greater than
the positive limit load factor of 4.4, structural damage
is possible. When the aircraft is operated in this region,
objectionable permanent deformation of the primary structure
may take place and a high rate of fatigue damage is incurred.
Operation above the limit load factor must be avoided in
normal operation.
There are two other points of importance on the Vg diagram.
One point is the intersection of the positive limit load factor
and the line of maximum positive lift capability. The airspeed
at this point is the minimum airspeed at which the limit load
can be developed aerodynamically. Any airspeed greater than
this provides a positive lift capability sufficient to damage
the aircraft. Conversely, any airspeed less than this does not |
provide positive lift capability sufficient to cause damage
from excessive flight loads. The usual term given to this speed
is "maneuvering speed," since consideration of subsonic aerodynamics would predict minimum usable turn radius or
maneuverability to occur at this condition. The maneuver
speed is a valuable reference point, since an aircraft operating
below this point cannot produce a damaging positive flight
load. Any combination of maneuver and gust cannot create
damage due to excess air load when the aircraft is below the
maneuver speed.
The other point of importance on the Vg diagram is the
intersection of the negative limit load factor and line of
maximum negative lift capability. Any airspeed greater than
this provides a negative lift capability sufficient to damage
the aircraft; any airspeed less than this does not provide
negative lift capability sufficient to damage the aircraft from
excessive flight loads.
The limit airspeed (or redline speed) is a design reference
point for the aircraftâ€”this aircraft is limited to 225 mph.
If flight is attempted beyond the limit airspeed, structural
damage or structural failure may result from a variety of
phenomena.
The aircraft in flight is limited to a regime of airspeeds
and Gs which do not exceed the limit (or redline) speed,
do not exceed the limit load factor, and cannot exceed the
maximum lift capability. The aircraft must be operated
within this "envelope" to prevent structural damage and
ensure the anticipated service lift of the aircraft is obtained.
The pilot must appreciate the Vg diagram as describing the
allowable combination of airspeeds and load factors for
safe operation. Any maneuver, gust, or gust plus maneuver
outside the structural envelope can cause structural damage
and effectively shorten the service life of the aircraft. |