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Vg changing with weight

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Aerodynamics

I know Vg changes with weight. Aircraft weight decrease, Vg (and other V speeds) decrease. My understanding of why so may be flawed, so I am wondering why does Vg changes with weight? Is it because a lower AOA is required to produce the best lift/drag ratio?

 

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2 Answers



  1. KDS on Oct 03, 2020

    Someone did a nice job of writing on that subject here:

    https://aviation.stackexchange.com/questions/13137/what-is-vg-in-this-vg-diagram

    and if you want to read more, try this:

    http://www.aviationchief.com/operating-flight-strength-v-g–v-n-diagrams.html

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  2. Brian on Dec 18, 2021

    In case you’re asking about Glide speed and not Load Factor: L/D max occurs at a specific angle of attack.

    To answer your question simply: No, it is because the AOA must remain at a specific value to achieve best glide. Best glide AOA is a fixed value. Changing weight means flying at a different speed to keep the AOA the same. Here’s how this looks mathematically:

    L = 1/2 V2 CL S p where AOA can be substituted for CL in this situation because it is the only portion of CL that you have control over and Lift can be substituted with Weight since they are equal in all unaccelerated flight. With a bit of mumbo jumbo we can rewrite our basic lift formula to:

    AoA Best Glide = Sqrt ( 2W / V S p)

    All we care about here is the relationship between AOA, Weight, and Speed (V for velocity). Written simply as:

    AOA for best glide (a constant) = Weight / Speed

    If AOA must stay the same and weight increases, what happens to speed?

    Example: if 10 AOA = 20 Weight / 2 Speed and we increase weight from 20 to 30, what must happen to the speed so that we still get 10 AOA?

    10 AOA = 30 Weight / ?

    Answer: 3 Speed – your speed had to increase also.

    Side note: Learning to manipulate the Lift formula in this way, rearrange it, substitute variables that are similar and analyze how they play off one another can really help solidify your understanding of why you do what you do. Worth memorizing? Probably not. Worth spending a couple hours on someday at a desk…I think it helps convince the brain that the techniques you use are rooted in solid ground rather than a swampy marsh of opinions. Those 5 variables juggled about can explain almost every proper and improper flying technique you’ll ever come across and why they do or don’t work as advertised.

    ————————

    On the off chance you’re question is about Load Factor or if your brain can take a bit more number pain read below:

    Full deflection of the controls generates a specific rate of onset for G load and rate of change for AOA. For simplicity, let’s say that pulling to the aft stop gives a 1G load and 1 Degree AOA per second increase. You’re always starting this at 1G meaning at 1 second you’ll be 2G, 2 seconds 3G, and so on.

    Stall angle of attack is a specific value, let’s say 15 degrees. Let’s assume we start at 100 knots in two different aircraft:

    Heavy airplane – 100 knots – AOA = 12 degrees: If we pull full aft what, at what G load would we stall?

    Light airplane – 100 knots – AOA = 10 degrees: Same question?

    If we use our 1 degree per second rule from above the heavy plane would take 3 seconds to reach stall and the light airplane would take 5 seconds.

    If we use the 1G per second we chose then the heavy airplane will stall at 4G and the light one at 6G. Since ~4G is the load factor limit in most planes you’re flying the heavy plane was near the limit at 100 knots while the light plane was not and must fly slower to increase it’s AOA.

    Thinking of Va more as an AOA flown, rather than a speed flown, will help you wrap your head around how/why the speed changes inversely with the weight.

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