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  • Author Author: phoenixcomm
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Flight Simulator 101 or back to college - part 4: Forces

phoenixcomm

Forces?! What Forces? There are 4 forces that act on an aircraft: image

  1. Lift: This is a force generated by the difference in the speed of air across the uper and lower wing surface. The modern lift equation states that lift is equal to the lift coefficient ( Cl ) times the density of the air ( r ) times half of the square of the velocity ( V ) times the wing area ( A ).
    L = .5ClrV2A Lift is a dynamic pressure as noted by Bernoulli's equation:
    P1 + ½ρv12 + ρgh1 = P2 + ½ρv22 + ρgh2
  2. Weight: This is combined load of the aircraft itself, the crew, the fuel, and the cargo (bombs, missiles, baggage, etc).
  3. Thrust: This is forward force produced by the engine(s). It opposes or overcomes the force of drag. As a general rule, it acts parallel to the longitudinal axis
  4. Drag: The force on an object that resists its motion through a fluid is called drag. When the fluid is a gas like air, it is called aerodynamic drag (or air resistance). Aerodynamic drag comes from anything that is part of the aircraft, props, engine nacelle(s), fuselage, wings, bombs, missiles, landing gear or and their doors, and antennas, and probes. There are two types of drag that we talk about: 
    • parasitic drag: The three types of parasite drag are: form drag, interference drag, and skin friction.
    • induced drag: is a result of an airfoil developing lift.

    We use a formula derived from Bernoulli; where  Drag ( D ) is equal to half of density of air ( ρ ) times the coefficient of drag ( C ) times the aera ( A ) times the speed  of the aircraft ( v2 )  
    D = .5ρCAv2

 

So what does all this mean? Well Lift must be more than weight (mass) and thrust has to be more than drag but basically the last one is always a losing propasion, that is the faster you go, you have to have more thrust but remember that pesky v2, well thats sort of like chasing a dragons tail. image So when you are flying level all the forces are balanced, why not check with Sir Isaac Newton's First Law of Motion as described in "Principia Mathematica Philosophiae Naturalis" in 1686.

"Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impress on it".

And please dont forget the Newton's Second Law of Motion:

"Force is equal to the change in momentum ( mV ) per change of time. For a constant mass, force equals mass times acceleration."

F = ma we will use this as well.image

But here is the rub. The forces are angular measurements, or a vector and they are quantifiable via a transducer called accelerometers.  There cost is very expensive, single channel units, but lucky the world turns, thing change and we win. Kind of sounds like Moore's Law doesn't it.

image

 

 

 

image keep tuned in more to come

Cris ~

Parents

  • Hi Chris,

     

    Are you going to adjust mass for the loss of fuel burned?

     

    You could add a lot of realism by going through an air refueling cycle to teach operators that they need to obey fuel management goals.

     

    Just a thought,

     

    DAB

  • in reply to DAB

    I have to work on the aerodynamic model of my bird this month.. after 10 years+ it is still only a bad guess.

    and my model does adjust mass for fuel burned or if you drop ordnance or fire a gun, or open the doors for wheels, etc. as well as the vectors. BTW that will also change your CG image

     

    I have several tanks and they are electronically monitored (cap sensors) and pump fuel to other tanks automatically to hold the CG...

     

    Cris

    have you looked at www.phoenixcomm.net/~phnx2000 thats the plane

Comment
  • in reply to DAB

    I have to work on the aerodynamic model of my bird this month.. after 10 years+ it is still only a bad guess.

    and my model does adjust mass for fuel burned or if you drop ordnance or fire a gun, or open the doors for wheels, etc. as well as the vectors. BTW that will also change your CG image

     

    I have several tanks and they are electronically monitored (cap sensors) and pump fuel to other tanks automatically to hold the CG...

     

    Cris

    have you looked at www.phoenixcomm.net/~phnx2000 thats the plane

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