Every aircraft’s operational manoeuvrability is limited by several fixed and variable factors.
Typical V-n diagram
The aircraft limitations shown on the V-n diagram are:
1. The lift boundary limitation. The ‘lift limitation’ on turning performance refers to that portion of the flight envelope in which the aircraft is limited in angle of attack because of aerodynamic stall, some form of α limits, or other factors such as aileron snatch, wing rock buffet, pitch up, yaw divergence, tracking limit etc rendering further lift unusable. Every point along the lift boundary curve, the position of which is a function of gross weight, altitude, and aircraft configuration, represents a condition of CLmax or angle of attack (α) limit. It is important to note that for each configuration, CLmax occurs at a particular αmax, independent of load factor(n), i.e., an aircraft stalls at the same angle of attack and CL in accelerated flight,i.e n>1, as it does in unaccelerated flight, n=1.
All aircraft can be flown to the lift boundary limitation in level flight in the low speed portion of the flight envelope. By combined diving and turning manoeuvres (Wind Up Turn), this limitation maybe explored through a large portion of the airspeed range. Tracking limit, aileron snatch, wing rock, buffet, pitch up, yaw divergence or limiting angle of attack all represents outer limits of the usable lift. The test technique and associated theory will be elaborated in PART-3 of this series.
2. The structural limitation. Structural limitation is normally due to the aircraft limit load factor, which is defined as the load factor where permanent structural deformation may take place or to the ultimate load factor, which is defined as the load factor where structural failure may occur. Normally, the ultimate load factor is equal to approximately 1.5 times the limit load factor and is a property of the materials from which the aircraft is constructed.
All aircraft, regardless of design or weight, will achieve the same rate and radius of turn when maintaining the same velocity and load factor. Thus, when the limit load factor is reached in flight, test can be discontinued, and the rate and radius of turn can be calculated for that portion of the airspeed range in which limit load factor can be maintained. Even among high performance aircraft, there is only a small portion of the flight envelope in which limit load factor can be maintained in level flight, although it can be achieved in manoeuvres such as dives and pullouts through a much larger portion of the envelope.
3. The q limitation. ‘q’ limit is the maximum dynamic pressure the aircraft can withstand or the maximum flight velocity. This is a design feature and not a subject for investigation through flight tests.
‘Corner speed is defined as the minimum airspeed at which the maximum allowable g can be generated. At corner speed, the aircraft can attain its maximum turn rate’. Below this speed, if you attempt to pull more “G”, the aircraft may enter buffet and stall at its aerodynamic limit.
Although a V-n diagram is published for most aircraft, the information contained in it does not include the aircraft thrust capability which is necessary to determine ‘sustained’ manoeuvrability, called the ‘thrust boundary’.
Thrust Boundary is defined by the maximum ‘g’ (i,e n) that can be sustained in level flight at a given speed with a given engine setting. Unlike the other (lift boundary) this definition presupposes a comparatively static situation in which speed, height and n are all steady. In stabilized level flight, thrust and drag considerations will be the limiting factors through a large portion of the flight envelope. For combat flying, this is the limitation on sustained turning ability without loss of energy. Note here that the thrust boundary is mathematically defined whereas the lift boundary is more subjectively defined. The test technique and associated charts will be elaborated in PART-3 of this series.
Another limitation which must be considered for a manned aircraft is the physiological limitations of the human pilot. Although physiological limits have nothing to do with the V-n diagram directly, g limits on the human body can be thought of in the same terms as g limits upon the aircraft. If the pilot can withstand greater g-loads than the aircraft, he must always be aware not to exceed the aircraft limitations. If the aircraft can withstand greater g-loads than the pilot, the pilot must always be alert to the possibility of gray-out or black-out when pushing the aircraft to the boundaries of the flight envelope. Naturally, this physical limitation will vary with the individual pilot.
In the next pert we shall look at flight test/investigation of manoeuvring performance.
This is the second part of a four part tutorial.…
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