This diagram illustrates the variation in load factor with airspeed for maneuvers. At low speeds the maximum load factor is constrained by aircraft maximum C_{L}. At higher speeds the maneuver load factor may be restricted as specified by FAR Part 25.

The maximum maneuver load factor is usually +2.5 . If the airplane weighs less than 50,000 lbs., however, the load factor must be given by: n= 2.1 + 24,000 / (W+10,000)

n need not be greater than 3.8. This is the required maneuver load factor at all speeds up to V_{c}, unless the maximum achievable load factor is limited by stall.

The negative value of n is -1.0 at speeds up to V_{c} decreasing linearly to 0 at V_{D} .

Maximum elevator deflection at V_{A }and pitch rates from V_{A} to V_{D} must also be considered.

Loads associated with vertical gusts must also be evaluated over the range of speeds.

The FAR's describe the calculation of these loads in some detail. Here is a summary of the method for constructing the V-n diagram. Because some of the speeds (e.g. V_{B}) are determined by the gust loads, the process may be iterative. Be careful to consider the alternative specifications for speeds such as V_{B}.

The gust load may be computed from the expression given in FAR Part 25. This formula is the result of considering a vertical gust of specified speed and computing the resulting change in lift. The associated incremental load factor is then multiplied by a load alleviation factor that accounts primarily for the aircraft dynamics in a gust.

with: a = (dC_{L}/da)

U_{e} = equivalent gust velocity (in ft/sec)

V_{e} = equivalent airspeed (in knots)

K_{g} = gust alleviation factor

Note that c is the mean *geometric* chord here.

The FAA specifies the magnitude of the gusts to be used as a function of altitude and speed:

Gust velocities at 20,000 ft and below:

66 ft/sec at V_{B}

50 ft/sec at V_{C}

25 ft/sec at V_{D}.

Gust velocities at 50,000 ft and above:

38 ft/sec at V_{B}

25 ft/sec at V_{C}

12.5 ft/sec at V_{D}.

These velocities are specified as equivalent airspeeds and are linearly interpolated between 20000 and 50000 ft.

So, to construct the V-n diagram at a particular aircraft weight and altitude, we start with the maximum achievable load factor curve from the maneuver diagram. We then vary the airspeed and compute the gust load factor associated with the V_{B} gust intensity. The intersection of these two lines defines the velocity V_{B}. Well, almost. As noted in the section on design airspeeds, if the product of the 1-g stall speed, V_{s1} and the square root of the gust load factor at V_{C} (n_{g}) is less than V_{B} as computed above, we can set V_{B} = V_{s1} sqrt(n_{g}) and use the maximum achievable load at this lower airspeed.

Next we compute the gust load factor at V_{C} and V_{D} from the FAA formula, using the appropriate gust velocities. A straight line is then drawn from the V_{B} point to the points at V_{C} and V_{D}.

1) The lift curve slope may be computed from the DATCOM expression:

where b is the Prandtl-Glauert factor: b = sqrt(1-M^{2})

and k is an empirical correction factor that accounts for section lift curve slopes different from 2p. In practice k is approximately 0.97. This expression provides a reasonably good low-speed lift curve slope even for low aspect ratio wings. The effect is an important one as can be seen from the data for a DC-9 shown below. The maximum lift curve slope is about 50% greater than its value at low Mach numbers.

2) Recall C_{Lmax} may vary with Mach number as discussed in the high-lift section.

Check at all altitudes, weights, loading distributions.

Include pitching rates and pitch accelerations (dq/dt):

maximum elevator deflection at V_{A}

Checked maneuver with dq/dt = 39 n (n-1.5) / V rad/sec^{2} or lower if not possible

For loads use this dq/dt at speeds from V_{A} to V_{D} combined with 1-g loads

also check: dq/dt = -29 n (n-1.5) / V combined with the positive maneuver load from V_{A} - V_{D}

Tail load due to gust can include full downwash and K_{g}-factor.