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-   -   Dimensional Analysis on Lift Equation not working out? (https://www.pprune.org/tech-log/614632-dimensional-analysis-lift-equation-not-working-out.html)

aviationluver 22nd Oct 2018 18:26
Dimensional Analysis on Lift Equation not working out?
 
Hello,

I am showing that the equation for lift on a wing is
L=1/2 Cl (air density) (Area of Wing) Velocity^2

However, the dimensions aren't working out.

L is kg ; Cl is dimensionless; Air density is kg/m^3; Area is m^2; Velocity is m/s

If you work the dimensions it does not work out. You get L in kg and kg m on the other side.

What am I missing?

Thanks.

infrequentflyer789 22nd Oct 2018 19:13
Originally Posted by aviationluver (Post 10289673)
Hello,

I am showing that the equation for lift on a wing is
L=1/2 Cl (air density) (Area of Wing) Velocity^2

However, the dimensions aren't working out.

L is kg ; Cl is dimensionless; Air density is kg/m^3; Area is m^2; Velocity is m/s

If you work the dimensions it does not work out. You get L in kg and kg m on the other side.

What am I missing?

Thanks.
First you are missing (1/s)^2 from the velocity term, this gives kg*m /s^2

Second, you are missing that lift is a force so it is in Newtons which is also (e.g. from F=ma) = kg*m/s^2

Sorry you didn't discover new physics, maybe next time :)

aviationluver 23rd Oct 2018 19:52
Just realized that. No need for the insult.

Derfred 24th Oct 2018 11:11
Originally Posted by aviationluver (Post 10290637)
No need for the insult.
Is this the new generation of pilots?

I need to start sorting out my affairs.

FakePilot 24th Oct 2018 12:01
Originally Posted by infrequentflyer789 (Post 10289728)
First you are missing (1/s)^2 from the velocity term, this gives kg*m /s^2

Second, you are missing that lift is a force so it is in Newtons which is also (e.g. from F=ma) = kg*m/s^2

Sorry you didn't discover new physics, maybe next time :)
I get the joke, like "new math".

VintageEngineer 1st Nov 2018 13:53
To add to the tongue-in-cheek ‘new physics’, I’m glad Newton got there first. Imagine force’s being measured in Aviationluvers or Infrequentflyer789s

Vessbot 1st Nov 2018 14:02
I had this one landing last trip that I think was because of New Physics...

Chris Scott 1st Nov 2018 14:55
Originally Posted by VintageEngineer (Post 10298875)
...Imagine force’s being measured in Aviationluvers or Infrequentflyer789s
Didn't realise Newton was a grocer... ;)

Crabman 6th Nov 2018 11:33
Not sure if he was a grocer but I always think of apple's when I think of Newton.

Back to topic, the OP isn't the first person to have a "brain fart" and mix up mass and force in an equation.

India Four Two 6th Nov 2018 12:33
I was confused when I was first saw that the dimensions of a force are M.L.T^-2

However, it’s obvious when you look at F = m.a


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