Maneuving Speed

Overview

In the February 2006 issue of AOPA'a magazine, Ron Machado discussed the importance of Maneuvering speed when encountering turbulence.  I have tried to simplify this explanation using some graphic images in this article.

Some simple facts about Va

  • Definition
    • That speed at which an aircraft will stall before high enough stresses are imposed on the airframe to cause structural damage.  At or below maneuvering speed will also allow you to make substantial changes to the control surfaces of the aircraft before structural damage will occur.  Be careful - this is limited to yoke-type control changes and not necessarily the rudder.
    • Each aircraft has its own published maneuvering speed which is based on the weight of the aircraft and can be found in the aircraft's owners manual or its pilot operating handbook
  • As aircraft weight increases, Va increases
    • This is because of the Va equations shown below
  • As altitude increases, Va increases
    • This occurs because speed must be increased to generate the same amount of lift because of the less dense air

Maneuvering speed is dependent on an aircraft's weight

Maneuvering speed is not constant, it is dependent on the aircraft's current gross weight (this includes everything that contributes to the aircraft's current weight).  There is a relatively simple calculation to obtain an aircraft's current maneuvering speed

Stalling speed is dependent on an aircraft's weight

Stalling speed is also constant, it is dependent on the aircraft's current gross weight (this includes everything that contributes to the aircraft's current weight).  There is a relatively simple calculation to obtain an aircraft's current stalling speed

Why Worry about Va?

Why is Maneuvering speed so important when encountering turbulence?  Let's first take a look at the angle of attack of an aircraft when moving through the relative wind.

Angle of Attack


Figure 1

To maintain straight and level flight, you have to establish an angle of attack that will allow the same amount of lift to be produced by the wings at different air speeds.

  • At  high air speed (the plane at the top of Figure 1), the angle of attack can be lower in straight and level flight because the amount of lift being produced by the wings is greater because of the higher air speed through the relative wind

  • At lower air speeds (the plane at the bottom of Figure 1), the angle of attack must be greater in straight and level flight because the amount of lift being produced by the wings is less as a result of the lower air speed through the relative wind.  The higher angle of attack increases the lift of the wings at these lower air speeds.

Stalling the aircraft

As you probably know, an aircraft will stall at a predetermined angle of attack each and every time.  This angle of attack is different for each aircraft design and is crucial to this entire discussion.  For the sake of this discussion, let's say that our little red Cessna stalls at an 18 degree angle of attack.


Figure 2

As shown in Figure 2, our High-Speed Cessna is flying straight and level at a three Degree angle of attack.  To stall the aircraft, we have to increase the angle of attack to eighteen degrees.  The difference in this angle of attack, obviously, is 18/3 = 6 times the straight and level angle of attack.

Our Low-Speed Cessna is flying straight and level at a six degree angle of attack.  To stall this aircraft, the angle of attack, again, needs to change to eighteen degrees.  To do this, you need to increase the angle of attack from three to eighteen degrees or 18/6 = 3 times the original angle of attack.

As can be seen, our slow Cessna only needs a to change it's angle of attack by 3 times as much where the fast Cessna needs to change its angle of attack by 6 times as much.

G's

G's, or the force of gravity, is determined by the amount of force that the earth pulls on any body whether in flight or at rest.  When you are sitting in an aircraft that is flying straight and level, the G-Force is one (1) since your weight is the only thing pulling you down to the ground.  What keeps the airplane in flight is the lifting force of the wings compensating for the gravitational force of the earth (it's amazing that the combined weight of the earth cannot overcome the simple lifting force of two small airplane wings - that's a different discussion about the theory of relativity that we won't go into here).

To increase the number of G's imposed on a person or an aircraft's airframe, you have to make some sort of modification to the attitude of the aircraft.  For this discussion, let's suppose that a change from three to six degrees (doubling the angle of attack) will also double the G-forces on the aircraft from 1 - 2.  A tripling of the angle of attack from 3 to 9 will triple the G-forces on the same aircraft to 3-G's.

So, if you pull back on the yoke suddenly or if turbulence causes the pitch of the aircraft to go from three to nine degrees, the aircraft will experience a 3-G force on its airframe, an increase of 3 times the normal G-force.


Figure 3

As shown in Figure 3, Our low-speed Cessna, when turbulence hits, goes from a 6-degree angle of attack to 18 degrees of pitch.  The total G's imposed on our low-speed Cessna, because of this change, is 3 G's (3 x 6 = 18 degrees, thus 3 x 1G = 3 G's). 

Our high speed Cessna in Figure 3 is in a different situation.  It is flying straight and level at 3 degrees when it encounters the exact same turbulence.  Our fast Cessna sees a change in it's angle of attack from 3 to 18 degrees  or a change in pitch angle of 6 times the original pitch angle.  This change will cause the G's imposed on the airframe to increase by a factor of 6 (1 x 6 = 6 G's). 

A normal Cessna will probably only have a G-factor limit of 3.8 G's, so our fast plane probably won't make it home because the G-factor imposed will destroy the airframe before the airplane stalls.  Our slower Cessna will stall thus reducing G-factors on the airframe before the G-factor gets too high.  Our slower Cessna is coming home in one piece.

Other forces on an aircraft

Pitch isn't the only thing that can cause structural damage on an aircraft.  There have been cases where an aircraft is below maneuvering speed during heavy turbulence where over-use of the rudder caused this control surface to be ripped off of the aircraft.  During heavy turbulence it is important to understand all of the forces that are being placed on an aircraft and what damage can be done on all axis of the plane.

Conclusion

So, the slower you travel through the air, the higher your angle of attack during straight and level flight, thus the greater change in angle of attack can occur without getting too close to your airframe G-limits.  This is why maneuvering speed is important when flying through turbulent air.  You'll fly slower, you'll get home later, but you'll get home.  Gentle changes to controls is also critical to avoid any other types of control surface or airframe damage during heavy turbulence.  Keep calm and be gentle on the aircraft during heavy turbulence is a rule everyone can live with.

All graphics on this web page are copyright Komanetsky Avitaion, LLC

This page was last modified on 12/03/2006