# What is Vg in this VG diagram?

There are two vertical bars beyond Vne labelled: @800#-Vg = 150MPH and @840#-Vg = 147MPH. These lines represent Vg at two different weights.

Can you tell me what Vg is, where it's officially defined, and how it's calculated?

• Obviously not Vglide
– rbp
Mar 9, 2015 at 14:44

It is the maximum speed in gusty weather. Aircraft designers and certification authorities make assumptions about the maximum vertical gust an aircraft will encounter and design the strength of the wing spar for it. Here 24 feet per second are assumed.

Depending on the mass of all parts not creating lift (fuselage, vertical tail, pilots, equipment ...) the load on the wing spar varies with the aircraft mass. For that reason, lighter aircraft can afford to enter a gust at a slightly higher speed. Here, the 800 lbs glider may fly up to 150 MPH, while the 40 lbs heavier glider should stay below 147 MPH to suffer the same wing spar bending loads when flying into this 24 fps gust.

Flying faster means flying at a higher dynamic pressure and at a lower angle of attack. Entering a gust will increase the angle of attack by the ratio between gust speed and flight speed. Since dynamic pressure is proportional to the square of flight speed, a linear wing lift increase due to higher flight speed will remain when entering a vertical gust. Also, at lower speed the angle of attack might be high enough that the angle of attack increase due to the gust will stall the wing, putting a natural limit on the load increase. Flying faster will remove this load limitation.

The maximum flight speed in gusty weather v$_g$ is a limit speed just like v$_{NE}$, but applies only in specific weather conditions (admittedly those which are most suitable for glider flying). While v$_{NE}$ might be determined by a minimum flutter margin, a formula in the certification requirements or structural loads in maneuvers, v$_g$ is determined by the maximum load factor the aircraft can tolerate.

• Is this a structural limitation rather than aerodynamic, like Vne? How would someone use Vg in practice?
– rbp
Mar 9, 2015 at 16:42
• Yeah, I'm unclear on how this relates to Vne, especially since the 800lb Vne is is 7.5mph slower than the 800lb Vg.
– egid
Mar 9, 2015 at 16:48
• At the end of para 2, I believe you mean 24 fps, not 24 fpm. Either that, or you meant minute at the end of the first para, not second. Mar 9, 2015 at 17:49
• @FreeMan: Thanks, yes, it is fps. A 24 fpm gust would be a boring affair. Mar 9, 2015 at 18:47
• @egid: Yes, makes me wonder myself, since v$_{NE}$ is the maximum speed in calm air. I guess this is to show where the theoretical v$_g$ is, but v$_{NE}$ bites first. I am more used to v$_B$ for this limit, but when you look at the v-n diagram, v$_g$ is where v$_B$ should be. Mar 9, 2015 at 19:06