Lift and Drag formulas on a plane

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Discussion Overview

The discussion revolves around the physics of lift and drag as they pertain to the design of a remote-controlled (R.C.) airplane. Participants explore the relevant formulas and the variables involved, as well as the practical considerations for building an effective aircraft.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant presents formulas for lift and drag, seeking confirmation of their correctness and clarification on the variables involved.
  • Another participant acknowledges that the formulas can yield lift and drag but emphasizes the importance of understanding their derivation and the context in which they apply.
  • A different participant suggests that scaling down an existing aircraft design may be a more practical approach for creating a successful R.C. airplane, rather than starting from scratch.
  • One participant corrects the formula for the drag coefficient, indicating that it should involve the aspect ratio (AR) of the wing rather than the area (A) and suggests that the provided lift coefficient formula is an approximation not suitable for wing design.
  • There is a suggestion that a comprehensive understanding of aircraft design requires more than just the provided equations, recommending the use of specialized literature.

Areas of Agreement / Disagreement

Participants express differing views on the applicability and accuracy of the lift and drag formulas presented. There is no consensus on the best approach to designing an R.C. airplane, with some advocating for empirical measurement and others for theoretical understanding.

Contextual Notes

Participants note that coefficients of lift (Cl) and drag (Cd) depend on various factors, including airfoil shape and design specifics, which are not fully addressed in the initial formulas. The discussion highlights the need for additional context and knowledge when applying these equations.

Who May Find This Useful

This discussion may be useful for hobbyists interested in building R.C. airplanes, students studying aerodynamics, or anyone exploring the principles of lift and drag in aviation.

cjacobson22
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I am going to make a R.C. airplane and need help understanding the physics behind it. I have found these formulas for lift and drag.

Lift=Cl*(r*V^2)/2*A
Cl=2*pi*AoA

Drag=Cd*(r*V^2)/2*A
Cd=Cd0+Cl^2/(pi*A*R*e)

Now I have two questions. First, are these the correct formulas for finding lift and drag on an airplane? Second, what are all of the variables? I think I know some of them, but I would like to be sure so please include them.
 
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These formulas do give lift and drag, but you can't just apply them without any knowledge of their roots unless you are just going to do it on te wings, and even then it would be a pain to check your answer.

In particular, Cl and Cd (coefficients of lift and drag respectively) depend on a lot of things. They are well know for many airfoil shapes, but that's about it as far as you are concerned.
 
Your best shot at making a decent RC airplane is to take an existing aircraft and scale it down. Then take it out and the road with some force gauges and measure lift and drag. Once you have reasonably cambered wings it will fly. But if you're designing something new, you got a few problems on your hands.
 
cjacobson22 said:
I am going to make a R.C. airplane and need help understanding the physics behind it. I have found these formulas for lift and drag.

Lift=Cl*(r*V^2)/2*A
Cl=2*pi*AoA

Drag=Cd*(r*V^2)/2*A
Cd=Cd0+Cl^2/(pi*A*R*e)

Now I have two questions. First, are these the correct formulas for finding lift and drag on an airplane? Second, what are all of the variables? I think I know some of them, but I would like to be sure so please include them.

Here are the variables.
Cl=lift coefficient
V=velocity
r=density
A=area
Cd=drag coefficient

The equation for Cd should be pi*AR*e, not pi*A*R*e. AR is referring to the aspect ratio of the wing.

You could use the equations for lift and drag. Your equation for Cl is really just an approximation for the lift coefficient of an airfoil and should not be used for a wing. Anyways you will need a lot more than those equations to design an airplane. I would suggest finding a good aircraft design book and going from there.
 

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