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I'm calculating the lift of a Cessna 172 and I don't understand the numbers given for weight. The maximum landing weight is 2550 pounds which is nearly 1134kg. But does that mean 1134 Newtons (weight) or 1134 kilograms (mass)?

Since weight = mass x gravity, I think the weight of the C172 is in Newtons even though it's written using a unit of mass. Am I correct?

Pondlife
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wonwoo
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    No, but the person specifying weight in pounds could have been more explicit. It is pound-force (as opposed to pounds as a unit of mass), and you convert 1 Newton to 0.2248 pound-force. Somehow the insights of Isaac Newton or Galileo Galilei never found their way into the English language or the imperial system. Use the metric system - it is better. – Peter Kämpf Dec 04 '15 at 07:12
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    @PeterKämpf *mutter* The English language copes just fine with the concepts of mass and weight, and the system of units you're referring to is American customary units, not Imperial (the two agree on the pound but have different tons and different liquid measures). – David Richerby Dec 04 '15 at 09:12
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    As I understand your question, I believe you to be technically correct. However, remember that U.S. aircraft manufacturers typically publish weight limitations in both pounds and kilograms even though the first is a unit of force and the latter a unit of mass, and use a simple constant to convert between the two. Boeing uses 0.45359237 pounds per kilogram. This is a simplification that is workable because the variation in gravity between the surface of the earth the max altitude of aircraft is negligible for the practical purposes of aircraft weight limitations. – Terry Dec 04 '15 at 09:27
  • Sorry! I made a mistake in question. To be specific, the C172's maximum landing weight is 2550 pound which is nearly 1134kg. Then, does that 1134 means 1134Newton(weight) or just 1134KG(mass)? – wonwoo Dec 04 '15 at 10:57
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    The question has an obvious answer: the mass of the C172 is about 2500 lbs, 1130 kg. This is what is sometime referred as weight in layman terms. If someone asks you your weight, usually you provide your mass for the answer and you don't use Newtons. In aviation MTOW is actually the maximum mass. – mins Dec 04 '15 at 12:01
  • @Terry, pound may be a unit of either mass or weight, the later being called force-pound if the distinction matters. – Jan Hudec Dec 04 '15 at 14:04
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    @mins, So you mean the weight is 1130 X 9.8 kgm/s^2 ? – wonwoo Dec 04 '15 at 14:19
  • @wonwoo Yes, or about 11085 N (+/- some N depending on where you live). Same thing. – JulianHzg Dec 04 '15 at 16:20
  • Correct, weight is the force created on a mass by the acceleration of gravity. Force = mass × acceleration, and is expressed in Newtons (N). The weight of a C172 is about 1130 kg × 9.81 m/s² = 11,085 N (or 11,085 kg⋅m/s² if you prefer) – mins Dec 04 '15 at 18:13
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    99% of everything involving weight is technically incorrect ! 99% of the time, it's about mass to be correct. That's another lost battle over life. You never ask someone his mass, nor read weight in newtons on a can of beans, and that's because on earth, weight vary depending on altitude while mass remain constant. Using weight instead of mass was a language error from the beginning, not only in english, american, but french, spanish, etc. Too late to change that back. – Karl Stephen Dec 06 '15 at 15:21
  • Hmm... This seems like a situation where the pound unit is actually correct and the kilogram unit is incorrect. It's the force that matters (because that's what the wings and/or landing gear have to counter,) not the mass. A pound is a unit a force. A kilogram is not. However, assuming the aircraft are operated near the surface of Earth, a kilogram is proportional to a force, since the Earth's gravity is a reasonable approximation of constant, for this purpose anyway. – reirab Jun 03 '16 at 17:00
  • Possible duplicate of What is the difference between dry operating weight and dry operating mass? – os1 Jun 04 '16 at 14:51
  • Unless you're planning on flying your 172 on Mars (spoiler: it won't work), does it really make an actual difference? – Vikki Oct 11 '19 at 05:29

4 Answers4

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The maximum landing weight given here is actually the mass, not weight.

In general, the various 'weights' given for aircraft are masses, not weights. i.e. they are in kilograms, not Newtons.

This is simply an extension of our everyday usage. As @mins pointed out, if someone asks your weight, you tell your mass (70 kg or whatever), not the weight itself (~686N).

aeroalias
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    This is correct - since Gravity is (near enough) constant on/near Earth (~9.81 N/kg), mass can be used in lieu of weight - as long as you specify units to avoid ambiguity, anyway. When non-scientists talk about weight they almost always mean mass. – Jon Story Dec 04 '15 at 15:42
  • Actually, the actual limit is weight, because it is about the induced drag needed to generate corresponding amount of lift and the thrust needed to overcome that and those are all forces. However, there is a general habit of quoting mass for weight and on Earth it works fine. – Jan Hudec Jun 06 '16 at 20:48
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On Planet Earth, 1kg of mass exerts 9.81 Newtons of force (weight). Because this is true anywhere on planet Earth to within a tiny fraction of a percent, we use kg as a convenient measure of weight, even though pedantically it is a measure of mass.

If you are thinking of landing your C-172 on the Moon, Mars, or some other locale where G is different from 9.81 ms-2, then you'll have to convert kg to Newtons of weight. Otherwise, you should consider kg a unit of both mass and weight.

abelenky
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Reiterating the point made by @aeroalias, the "weight" that FAA/manufacturer talks about is really "mass" in the pure physics world. However, in the real world, imagine you have a big enough scale to hang the airplane to. The MTOW or Max Take Off Weight would be the reading on that scale. So, if you are weighing your passengers' baggage, use a reliable scale. You can then use it directly in your weight and balance calculations.

So, don't worry about conversions. Just use the real world "weight" in Kgs or Lbs as the case may be as long you are flying on earth.

NullReference
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  • The FAA weight is the force of the mass on Earth. Divide the weight by 32 to get the mass in SLUGS. – user3344003 Dec 04 '15 at 17:00
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    "in the real world, imagine you have a big enough scale to hang the airplane to", large aircraft are weighted using scales below the wheels actually, see How does a commercial airliner measure its weight/mass? – mins Nov 01 '21 at 18:38
  • @user3344003, why, (like mins in another post) are you bringing slugs into the discussion? No pilot has ever computed their aircraft weight, and then decided to convert the unit of measure to slugs. What would be the point? Does the POH display weight limits in slugs? Why is this even a thing?!!! – Michael Hall Nov 03 '21 at 01:16
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Aircraft are weighed on scales. Attempting to measure the mass of a loaded aircraft isn't practical. Clearly, the figure indicates weight.

Jim Perris
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  • The other answers state the opposite. Would you happen to know something the others don't? – Federico Dec 15 '17 at 06:34
  • @Federico, I'm not sure whether you are referring to my two initial postulates or my conclusion. While the plane's mass and acceleration ultimately determine the landing load, in practical applications, the starting figure for calculating landing weight is one directly measured by scales placed under the landing gear. – Jim Perris Dec 22 '17 at 21:28
  • Everyone says that the figure indicates weight, you are the only one claiming it does not. Moreover, even if direct measure is impractical, there are well established estimation methods. – Federico Dec 22 '17 at 21:55
  • Jim Perris: "Clearly, the figure indicates weight." @Federico: "...you are the only one claiming it does not." (indicate weight) I am missing the miscommunication... – Michael Hall Nov 03 '21 at 01:09