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3 Answers

Airspeed limitations

Asked by: 12710 views Aerodynamics, General Aviation

I was going through the POH for a cessana 172rg and noticed for maneuvering speed decreases as the weight goes down and was hoping someone could shed some light on this.  I would have thought as the weight goes down, your maneuvering speed would increases since there would not be as much of a stress on the aircraft.  

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3 Answers



  1. Jon Moore on Oct 25, 2012

    Think of maneuvering speed as a load factor limit expressed as an airspeed limit. To get an idea what maneuvering speed is in the first place, look at the chart on page 4-33 of the PHAK: the Vg diagram. Maneuvering speed is labeled as the intersection of the stall speed curve and the load factor limit, which is a horizontal line. Flying an airplane around normally, you will be operating close to the 1g horizontal line. Operating near 1g, if you increase back pressure suddenly, your airspeed doesn’t change instantaneously but load factor instantaneously increases. The concept of maneuvering speed is that if you are operating below Va and you suddenly pull back on the yoke, you will stall before exceeding the load factor limit. If above Va, you will exceed the limit before you stall.

    To answer your question, you need another piece of information that is not found on the Vg diagram. You need to know that as the airplane’s weight decreases, stall speed decreases. You know this to be true at 1g from your experience doing landings and stalls. Hear me now and believe me later when I say this is true at all load factors, and that the gap between stall speed at a given weight and stall speed at max takeoff weight widens as load factor increases. Knowing this, you can see that maneuvering speed will be lower at lower weights because the stall speed curve will intersect the load factor limit at a lower airspeed.

    By the way, what is load factor? It is nothing more than lift provided by the wing divided by aircraft weight.

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  2. Lucas on Oct 25, 2012

    Many students have this same problem, (I remember I did as well).
    We actually answered a similar question with a video because its practically impossible to explain the concept with words alone.

    If you missed it here is the youtube link:

    http://www.youtube.com/watch?v=oEHJXTlu6B8

    Lucas
    Passfaaexams.com

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  3. Brian on Oct 25, 2012

    I will offer a slightly different approach. The magnitude of a moment (a force that causes a rotation, such as pitch) is determined, in part, by the acceleration. How much acceleration depends on the force’s magnitude and the amount of time that force is applied.

    The magnitude is predetermined, for simplicity, by how much control throw we have. This is something that, again for simplicity, we will assume we have no control over. We apply maximum stick back and that gives us the same force magnitude every time. Really this varies with speed, but this is a complexity we need not worry ourselves with.

    The other parameter is time. In the case of this discussion the amount of time we can apply our force (the pitch up) is determined by the distance we are from stall. That is, how many degrees of AOA can we go before we stall? Regardless of everything else changing, this is one thing that doesn’t change. If a 6 degree change in AOA will result in us reaching our maximum load factor then we need to be within 6 degrees of our critical AOA.

    Let’s give it some arbitrary numbers just for the sake of discussion. Let’s say our critical AOA is 16 degrees. We just said the most AOA change we can have without breaking our airplane is 6 degrees. If we travel a distance of 7 degrees from our critical AOA then we will apply the force (pitching back) for too long, the acceleration will be too great, and we will break our airplane. We can then conclude that whatever speed gives us 10 degrees AOA (six from critical) will be a safe speed to fly.

    —

    With this new knowledge we can now look back to what happens to our flying AOA, at a given speed, if we change weight? Jon already explained this and your question on load factor in a turn parallels this topic.

    That is, as we increase weight, or load factor, if speed remains constant, we need a greater AOA to keep level flight. It is for this reason that higher load factors, which are synonymous with adding weight to the airplane, result in a higher stalling speed.

    I realize this can be incredibly confusing, but please don’t be deterred. Instead ask some more questions, it will help me refine my presentation.

    —

    Another way to remember, if math doesn’t scare you, consider this:

    Va = Vs sqrt (n) where ‘n’ is load factor.

    So if you remember stall speed increases with weight, and recall this formula, it is clear Va will follow suit.

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