What is Va Maneuvering Speed?
It is the speed at which the airplane will stall before exceeding its design speed limit-load factor when one flight control is deflected fully in one direction during flight in smooth air.
What are the Speeds?
C172 R-Model* C172 S-Model*
1600lbs - 82 kts 1900lbs - 90 kts
2000lbs - 92 kts 2200lbs - 98 kts
2450lbs - 99 kts 2550lbs - 105 kts
How is lift changed by the pilot?
You can change the lift by either increasing/decreasing the velocity or changing the Coefficient of Lift, a.k.a. increasing/decreasing Angle of Attack (AoA) of the airplane.
Back to the Basics
Some basic understanding at first. Think about when you are flying and trying to maintain a constant altitude, you pull out some power and you start going slower, and you notice you have to pull back on the yoke a little in order to maintain that altitude. Now if you were to increase the power and start going a little faster, you would notice yourself pushing forward on the yoke in order to maintain altitude. This occurs because of the amount of lift that is being created by the wing increase and decrease with speed and AoA changes. You trade one for the other when changing pitch and power settings.
With that in mind, remember now that the airplane stalls upon reaching the Critical Angle of Attack (CAoA). And for the Limit Load Factor (LLF), think of it as an angular distance the AoA must travel before the wing reaches its load limit.
So Why does it Change?
Now, why does the speed change with weight? Take the lighter aircraft and just imagine it in flight, at that weight and at that speed. That airplane would be at the specific AoA that the manufacturer intended on in order to stall the airplane at CAoA before reaching the LLF (See Figure 1).
Now add some weight to the aircraft, and you will notice it start to descend. This is because the aircraft requires more lift to be produced in order to maintain the same altitude.
Thinking back, you can only change the AoA and speed to either produce more or less lift. You want to keep the AoA at the same angle, so you increase your speed to compensate for the added weight. The same goes for a lighter aircraft, that would require less speed to compensate for the lighter weight.
What if I Exceed it?
If you were to fly faster than the maneuvering speed, you would experience the following. Your speed would be increasing therefore lift would increase and you would start to climb. Thinking back, you can only change the AoA and speed to either produce more or less lift. In this scenario, we’re demonstrating being at a faster speed so we won’t change speed. So in order to maintain the same altitude, you would have to lower your nose (decrease AoA) which would further you away from the CAoA and make it more likely to reach the LLF (See Figure 2). In this scenario you would break the plane before stalling it.
The Va Formula
Check your aircraft Pilot Operating Handbook for these speeds. If you want to calculate it for a specific weight, you can use the following formula.
Va (At Max Gross Weight) x √ (Current Weight / Max Gross Weight)
In Conclusion (SparkNotes)
Staying at or below maneuvering speed protects the aircraft from reaching its limit load factor by stalling it before it breaks (See Figure 3). If you go above the maneuvering speed, you risk breaking the aircraft with an abrupt control input. You can change the angle of attack and speed to either produce more or less lift. And with maneuvering speed, you are aiming to maintain the manufacturer determined Angle of Attack, therefore you change the speed to compensate for changes in aircraft weight.
*- Refer to your aircraft specific Pilot Operating Handbook (POH).
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