# Computing Flight Forces. Im getting so upset

• Gibbon
In summary: Stokes equations. You can find a lot of information on this on the internet, but a good starting point is this article:In summary, Andy is looking for help understanding how to apply forces to a 3D model in order to make it act like a plane, but is having difficulty understanding physics concepts. He says he has read about the lift and drag equations many times, and knows the book "Physics for Game Developers." However, he is having trouble applying these concepts to his project because he does not have a good understanding of math. He has attempted to refresh his memory with algebra and trigonometry, but has not yet had any success. He has also tried to learn about airfoils, but does not know where to start
Gibbon
Hi, ok.. i know I am probably expecting way to much off you guys here but its my only last resort before i give up!

Ill explain.. I am a avid aviation fan as well as an avid game developer whilst also being a fulltime software developer. So i program in delphi at work and what i want as a hobbiest, mostly Lua with C++.

My recent project i am trying to achieve for fun, learning and hobbiest reasons is a "realistic" flight simulation. I am using leadwerks engine (so the 3D part is done), that uses the "Newton game dynamics" library, so the physics calulatiions are done.

And so.. as silly as it sounds all i need to do in order to get a basic 3d model to act like a plane... is give the model the correct forces in order for it to move and act like a plane.

I say its silly because... i cannot seem to do it. And i think its because i am having problems understand the equations needed for this.

I fly planes myself, Rc planes and Rc helicopters i have done for years, I've built my own RC plane designs and I've built all my model helicopters that i fly. So.. i do undertsand flight and the physics... this is why it upsets me! As I am a programmer and flyer yet cannot seem to put these two together!

I know its not easy, but a lot of the work has been done for me.

So.. i am after in computer terms the equation needed for lift, drag and trust.

I know the equations.. i must have read them 100 times now, i also have the book "Physics for game developers" which explains it in there also.. but my lack of math knowledge let's me down. I've refreshed my mind with algebra and Trig as i am very serious about givving this ago, and honestly won't be wasting your time.

Ok, so what i know already...

I know that the lift equation is :

lift = CL*(r*V)/2*A
CL = 2 * pi * AoA

Break down:
V = ? (i don't know how to work out instant velocity in programming)
AoA = is fine, i can get the local angle in degrees or radians.
CL = 2 *... ETC : What is the 2?

Drag = totally confused me because: D = .5 * Cd * r * V^2 * A (thats fine its the Cd)
Cd = Cd = Cd0 + Cl^2 / ( pi * Ar * e) : Ok so what is Cd0 and what e (efficiency factor)?

So.. I am stuck. I have at the moment... a 3d model that i can generate a velocity (by just uping a value) which generates a force from front of the model making it move.

I also seem to successfull give my model lift by using the lift equation and obviosuly changing the AoA of my model, but then once up, it really doesn't act very plane like.

I can roll, pitch and yaw the model by adding a torque force according to each local axis.

This is done currently with gravity turned off, as when i turn it on i cannot seem to get the model off the ground as it seems the only lift comes from AoA and as I am on the ground the AoA is 0!

Does anyone (and again i know its not simple else eveyrone would be doing it) understand "where i am" with this, and possibly suggest any good advise or good "simplish" articles i can read that may help.

Andy

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Coefficient of lift versus AOA varies with the airfoil and Re (Reynolds number):

http://www.ae.illinois.edu/m-selig/uiuc_lsat/s1223/s1223_liftcm.html

http://www.mh-aerotools.de/airfoils/hdipolar.htm

What you need are the "polars" for the airfoils you use for the main wing and elevator. Do a web search for airfoil polar, and you get a number of hits.

For takeoff from ground, you need to include the lift component from the elevator, which pitches the aircraft up, increasing AOA of the main wing, which then produces lift.

Hi jeff, thanks for the reply, will have a good look into those articles, all helps!

Just on the note of CL, how does this work, or sorry... how can i make this work in programming? Could i have an array of values that represents points of a aerofoil and then somhow calculate the CL at all points...

And also.. i might being blind but, where in the Cl equation is it reading any aerofoil information?

Thanks
Andy

Gibbon said:
Just on the note of CL, how does this work, or sorry... how can i make this work in programming?And also.. i might being blind but, where in the Cl equation is it reading any aerofoil information?
The process of creating Cl versus AOA graphs or tables requires using fluid dynamics and some simplified form of Navier Stokes equations.

http://en.wikipedia.org/wiki/Fluid_dynamics
http://en.wikipedia.org/wiki/Navier-Stokes_equations

The Navier Stokes equations normally can't be exactly determined, so they are approximated using various method. You can get xfoil for free:

http://web.mit.edu/drela/Public/web/xfoil

You'll need to do some type of curve fit to turn the table of AOA and CL values into an equation, or just use the table directly and do linear interpolation. Note that second curve you see on those Cl charts is usually the pitching moment, the torque genrated by the wing. To simplify things, you can assume Re is constant (ignoring air speed affect on CL), or you can deal with the equivalent of a 3d table, although the polar generators generally only run a small set of Re numbers.

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Ok, thanks again. Excellent information and although over my head at the moment ill have a good read!

Do most flight simulators use this much information in them? I am not doubting it, asking as a real question? I never know like how much detail a simulator has got...

Is the level of simlation I am trying to achive here.. basic.. mid level or complex?

and the airfoil value your referring to, is that in placement of the "2" i question about in CL? EDIT: Hmm maybe not... Ill read!

Thanks
Andy

Click on the links section from FMS:

http://n.ethz.ch/student/mmoeller/fms/index_e.html

Do most flight simulators use this much information in them?
The high end ones like RealFlight generate 3d polar equations or the equivalent based on airfoil data, and use real time numerical integration to do the flight stuff. It pretty complex. The link above is to the FMS free flight simulator, and it's fairly complex. Link to RealFlight, one of several commercial RC flight sims:

http://www.realflight.com/new/index.html

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Yeh i have FMS and RC reflex (which i think is fantastic).

Ill keep reading and having a ago.

thanks again.

Andy

## What is computing flight forces?

Computing flight forces is the process of using mathematical equations and computer simulations to predict and analyze the forces acting on an aircraft during flight. This includes lift, drag, thrust, and weight.

## Why is it important to compute flight forces?

Computing flight forces is crucial for designing and testing aircraft, as well as understanding the performance and safety of existing aircraft. It also helps pilots and engineers make informed decisions about flight maneuvers and adjustments.

## What factors affect flight forces?

Flight forces are affected by a variety of factors, including air density, airspeed, angle of attack, wing shape and size, and the weight and shape of the aircraft. Other external factors such as wind and turbulence can also impact flight forces.

## How is computing flight forces done?

Computing flight forces involves using complex mathematical equations and computer programs to simulate and analyze the forces acting on an aircraft. These equations take into account various factors such as air pressure, velocity, and surface area to calculate the forces on each component of the aircraft.

## What are some applications of computing flight forces?

Aside from designing and testing aircraft, computing flight forces has many practical applications. It is used in the development of flight simulators, weather forecasting, and even sports equipment design. It also plays a critical role in the study of aerodynamics and fluid dynamics.

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