The ability of a pilot to predict the performance of an aircraft is based on the information supplied by the manufacturer (even for experimental aircraft). These are the operating limitations and show the boundary within which the aircraft must be flown and is often referred to as the flight or performance envelope. It is expected from the pilot that he/she operates the aircraft safely, thus within this performance envelope.
The operating speeds and limitation are represented graphically in a diagram, known as the VG diagram (some use VN) and it shows the relation between acceleration (g) and aircraft speed (V). We also will find the maneuvering speed (VA) on this diagram.
This is an useful method for showing the limitations with regard to speeds and acceleration. It is most often used during flight testing and certification and valid for one aircraft at (usually) maximum weight.
The VG diagram shows the amount of positive or negative lift the airplane can generate at a certain speed. At the same time it shows the maximum G the aircraft can sustain.
The curved lines indicate the maximum positive and negative lift before the airplane stalls at a given speed. This positive line curves up to maneuvering speed (VA) where the maximum G is reached (3.8 G, normal category) and then horizontally to maximum speed (VNE). Hover over the diagram to the right to see more detail.
Another important speed we can see is normal stall speed (VS), it is shown by following the one (1) G horizontal where it intersects the positive curved line and then vertically down to the speed axis. It is the beginning of the green area.
Maximum structural cruising speed (VNO) can be seen where the green area stops. The area between VS and VNO is the normal operating range of the aircraft shown in green on the airspeed indicator. Caution range is yellow and starts at VNO (when in rough air do not exceed this speed, remain in the green) and ends with VNE, red line. Never ever exceed this speed at any time!
Beyond VNE the control surfaces may begin flutter and even break off eventually if the flutter persists, with terrible results.
An important and critical speed is maneuvering speed. Defined as the speed where you can use full and abrupt control movement without causing structural damage due to overload. Any access load will result in a stall and at speeds below VA you will not reach maximum load, the aircraft just stalls.
Note that if, airspeed is higher than VA and you exceed maximum G limits then you will have created an overload situation and structural damage may occur. This can happen when flying in turbulence, mountain areas or when descending into a shear zone or turbulent layer caused by high winds over low level obstructions.
Remember that this VA speed is determined with the whole aircraft in mind and not only the wings. The engine mount, for example, must also be designed to handle these forces without damage or even the complete engine falling off.
Which would create a very interesting weight and balance problem...
VA is set for MTOW or MAUW. At weights below this the aircraft is able to accelerate quicker thus generating more G's at the same speeds. From this we can conclude that VA must reduce with lower weights.
Conclusion: when you reduce aircraft weight you need to reduce lift to maintain within the designed load limits set by the manufacturer. And to reduce lift you need to reduce airspeed.
For actual speeds you need to check your aircraft manual for limitations on lower all up weights.
Again, in turbulent conditions reduce airspeed to VA or below, thereby keeping actual weight in mind as maneuvering speed is given for maximum weight.