If you’ve been flying for a little while, you’ve probably heard of maneuvering speed, like if your CFI tells you to make sure you’re below that speed before attempting maneuvers in the practice area.
But why is this speed important? What happens to the airplane both above and below this speed if we start making maneuvers. Let’s look at how stall speed, load factor, and angle of attack play a role in this.
When we’re in equilibrium, our lift equals our weight. So if our airplane weighs 2000 pounds, we are producing 2000 pounds of lift. The ratio of lift to weight, which we get by simply dividing lift by weight, give us our load factor.
Load factor is the total load on an aircraft’s structure, as a multiple of its weight. So at equilibrium here, we have a load factor of 1, expressed as 1 g as in g force. Incidentally this is the same g force you and I are experiencing sitting down right now. We feel our bodies’ weight as the same amount of pounds we read when we step on the scale.
In normal, straight and level flight in relatively smooth air, we feel the same thing. We’re at a load factor of 1g, we don’t feel heavier or lighter in our seat at all.
In order to generate this amount of lift to maintain this equilibrium, we have to fly at a certain angle of attack at a certain airspeed. Recall the lift equation for how these factors interact.
Angle of attack is the angle made by the relative wind we’re flying through, and the chord line of our wing. Let’s say here it’s 2 degrees. Now let’s say at this 2 degree angle, we need to maintain about 110 knots, with a relatively high cruise power setting.
If we want to slow our aircraft down, in order to keep the same amount of lift, we’ll have to trade away airspeed and raise our angle of attack. Reducing power and pitching up allows us to enter slow flight. We’re now at a 10 degree angle of attack, but we’re slower, lift hasn’t changed, so our load factor stays at 1g.
If you’ve done slow flight before you know that you don’t feel particularly heavier or lighter as you slow the aircraft down.
Now if we pull the power back and slow the airplane down some more, maintaining lift requires us to continue increasing angle of attack. Eventually our angle reaches 18 degrees, the critical angle of attack for our aircraft. Notice our speed is below the green arc, we’ve reached Vs1, the stall speed in this configuration. The aircraft will stall.
A well designed and properly loaded aircraft will pitch down in a stall, decreasing angle of attack and breaking the stall. We increase power and return to our original situation.
Now, let’s start pitching the airplane up, while leaving power and airspeed alone. We only need 2 degrees of angle of attack at this speed to maintain sufficient lift. If we pitch up 4 degrees, doubling this angle, we can expect to see our lift double as well to 4000 pounds. This one to one relationship isn’t a hard and fast rule, as there are many different types of wing design, but for our Cessna, it works pretty well for simplicity sake.
So doubling our angle of attack has doubled our lift, and consequently has doubled our load factor to 2. We would do this by abruptly pulling back on the elevator control in flight. Can you picture what your body would feel like in that scenario? You’ll be pulled into your seat, and feel twice as heavy as you normally do.
Let’s add another 2 degrees to the angle of attack and we get a load factor of 3gs. Another 2 degrees and we’re at an 8 degree angle of attack, and we’re now experiencing 4 times our normal weight. This is quite a lot. The astronauts on the space shuttle launch typically felt 3gs, and most of us start to lose color vision at 4gs as the blood gets pulled down from our heads into our legs.
4gs will make the airplane unhappy too. The engine mounts holding the engine assembly to the cowling, and the floor board propping up our seats start to warp and fail under this much weight.
The Cessna POH lists a maximum flight load factor in the limitations section, it’s 3.8gs loaded in the normal category. So pulling 4 gs like we’re doing here, we risk structural damage to the airframe.
So abruptly pulling back on the controls maybe isn’t the best move in this scenario. And by the way, it’s not just an increase in lift from something we do we need to watch out for, but a strong gust of wind from beneath us could increase our angle of attack to 8 degrees and cause excessive loads too.
But what if we’re slower? Let’s reduce power, pitch up, and slow down to just above 90 knots. We’re maintaining equilibrium so lift has stayed at 2000 pounds and load factor is still 1g. This isn’t the abrupt pitch up maneuver we just tried before.
Now, in order to maintain lift, we need a higher angle of attack, 6 degrees. Let’s see what happens if we pull back on the stick abruptly now. First, we’ll double the angle of attack to 12 degrees, giving us a load factor of 2 gs. Next, we’ll triple it, we’re at 3gs, but we’ve also reached an 18 degree angle of attack, our critical angle. 3gs is within the load tolerance stated in the POH. If we pitch up any more than this, we won’t overload the aircraft, because it will stall.
And when it stalls, it’s not producing much lift at all to create these excessive forces. So in a way, the stall has saved us from ourselves. We were flying slow enough that the aircraft has stalled before reaching critical load factors.
So by slowing down to this speed, we are able to pull back abruptly, or experience a large gust of wind, and not worry about causing damage to the aircraft before we experience a stall. The limitations section of the POH also lists the speeds where this is possible.
We’re looking for maneuvering speed, Va. At our current weight of 2000 pounds, it’s 92 knots. Notice that it’s lower at lower weights.
The remarks say don’t make full or abrupt control movements above this speed. What this means in detail is that they’ve tested the aircraft’s ability to withstand loads at this speed using a single, full deflecting of one control surface, for example by pulling the elevator all the way back. It doesn’t mean they’ve tested continuous back and forth inputs or inputs on more than one axis such as elevator and rudder at the same time. This is cold comfort to the American Airlines flight 587 which crashed on takeoff from JFK in 2001 after expereienceing wake turbulence and vertical stabilizer failure from back and forth rudder inputs.
So at this weight, our maneuvering speed is 92 knots, right here.
But let’s say we’re lighter, why does maneuvering speed go down? Let’s drop a 400 pound heavy load out of the aircraft, so that now we’re just 1600 pounds.
Our maneuvering speed drops as the book says to 82 knots. Meanwhile, our load factor increases, it’s 2000 pounds of lift divided by 1600 pounds of weight for a load factor of 1.2. We can reduce lift to return to 1g by reducing angle of attack, from 6 degrees to 4 degrees.
While still flying at around 90 knots, above our current maneuvering speed, lets pull back on the controls abruptly and see what happens. We’ll quadruple our angle of attack from 4 to 16 degrees, getting us a load factor of 4. We’re able to reach these loads without stalling since we’re below the critical angle of attack here. So the aircraft is at risk of breaking up.
Let’s try this again, only now we’ll reduce our speed to the Va of 82 knots. In order to do so, we’re flying at a higher angle of attack, 6 degrees. We’re closer to a stall already. Now if we pull back hard, we reach that critical stall angle before loading up the aircraft more than 3 gs.
Although maneuvering speed is one V speed not displayed on the airspeed indicator, it’s one of the more important ones to know about, not just for the safety of your flight, but as a way to gain an understanding of how stall speed, load factor, and lift and weight interact in different phases of flight.