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Old   May 16, 2007, 11:16
Default Lift/Weight Ratio
  #1
Simon Nava
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What is the ideal Lift/Weight Ratio an airplane should have? Of course it should be greater than 1, but having a value of 20 for example (and I'm going beyond for purposes of my question)

Is it really true to say "The more the better"?
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Old   May 17, 2007, 10:24
Default Re: Lift/Weight Ratio
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OPS
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First of all, Lift/Weight ratio (L/W) not necessarily be greater than 1 always. L/W varies during the course of flight (from take off to landing). During take off, L/W > 1, at cruising at a constant speed at the peak height L/W = 1 and during gliding (downward flight ), L/W < 1.

An airplane is designed to operate at the maximum lift coefficient (this helps to maintain a stalled speed when L/W = 1) . Further, L/W is a function of the square of the plane velocity. Hence an airplane with low L/W (high weight) should fly at higher speed compared the airplane with high L/W.

OPS

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Old   May 17, 2007, 12:24
Default Re: Lift/Weight Ratio
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Simon Nava
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Thank you for the reply. I undertand that the L/W ratio varies during the course of a flight. What I was referring to is the L/W Ratio during take off. When it comes to designing an aircraft, what is the most desired L/W ratio that designers would love to have, for example in a 1000 Lbs aircraft?
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Old   May 17, 2007, 23:05
Default Re: Lift/Weight Ratio
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OPS
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An airplane is designed for the maximum weight it can carry with some extra factor of safety. The same airplane can have considerably different weight between its two flights (no passenger, 1 passenger, 100 passenger etc. ). Once the load carrying capacity of an airplane is fixed, the rest design is done to maximize the lift coefficient (Lc). The value of Lc is limited and it start decreasing beyond certain values.

Hence, e.g. if the MAXIMUM lift of 10,000 Lbs can be generated by a given design for 1,000 Lbs plane, then the ideal L/W = 10. Depending upon the requirement and instruction from the control room, a pilot adjust withing this IDEAL L/W ratio during take off.

If you know the airplane parameters like wing span, surface area etc, MAXIMUM lift and hence IDEAL L/W ratio can be calculated experimentally.

OPS
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Old   May 18, 2007, 09:57
Default Re: Lift/Weight Ratio
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Simon Nava
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Thanks!
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