Aerodynamics... Drag question.. Please help a bit.

[Snailman]

Nice day to you all, fellow modders) I need a bit of help, with physics actually)

 

What I need is a simple formula how to calculate a realistic drag value for certain shapes (payload related)...  Me old boozer with sponge brains))))

 

So... there is drag that comes from air resistance, in the front of the object... the drag from friction, if its a long object with bug surface (even if small diameter), and a drag - don't know how it is called - made by the air sucked in behind the object if the shape is not so "streamlined"..

 

Well all three is important but what I need most is to know how to calculate the drag in front of the object.. (both subsonic and supersonic)  for a cylinder (flat surface in front), a cone headed object, a hemisphere headed object, or a nicely pointed, curved missile head. Given the diameter (area) mass and the shape... is there anything else needed?

 

I wonder shall I add both the front and the friction drag?

 

thanks for your help!

[MigBuster]

um afaik there is:

 

Drag caused by lift is called Induced Drag

Drag caused by the airframe and pylons is called Parasitic drag

 

Total drag = Induced drag + Parasitic Drag

 

Dont think drag from simple shapes will tell you anything - aircraft are not simple shapes - they have gaps and notches all over them.

 

Any formula would probably have drag and lift coef variables - these are generally unknowns for airframes and estimated as far as I can see (although no expert)

 

Never looked into it but there might be air tunnel app you can get where you can put a 3D object in?

 

Also if you use debug in the TW games you will get a total drag figure - although wont be dead on might give you an idea - (well for good flight models)

Edited January 22, 2014 by MigBuster

[Snailman]

Oh... sorry I should have been more specific. it's not about the plane, but the payload objects itself. Weapons... Missiles.

 

So what would be the drag of a missile with spherical head or conical or pointed.. or a flat cylinder like bomb... and retarded/para bombs use the drag value multiplied by the drag multiplier. But it can be as high as 100 if the base value is nothing

[MigBuster]

You would want to include the pylon and take account of where the store is on the wing 

 

For example a wingtip missile adds no drag (Drag Index = 0 )

 

But the same missile on a further inboard pylon will add more overall drag even though its the same shape  (i.e. DI = 22)

 

Flight manual supplements should have drag index for stores and related performance drops specific for that airframe only.

 

Again a virtual wind tunnel might show you levels of Induced / Parasitic drag on each part of the wing/ store / pylon - which will differ vastly depending on speed, altitude, temperature - and that's just level flight - it all changes as soon as you bank or turn.

[Snailman]

Uh... how to tell .. i need only the bomb and the missile ... during flight. without plane)  I dont know the name of it in english... drag coefficient maybe?

 

something like this  http://en.wikipedia.org/wiki/Drag_coefficient    but its too difficult for me, the scientific language

Edited January 22, 2014 by Snailman

[MigBuster]

The real thing in flight should have the same aerodynamic principles as the aircraft - but  i assume you need this for an in game object? (the weapons parameters look more simplified)

[Snailman]

The real thing in flight should have the same aerodynamic principles as the aircraft - but  i assume you need this for an in game object? (the weapons parameters look more simplified)

 

Exactly... I need to modify aerodynamics of weapons actually. And much depends on drag coefficient

[Wrench]

oh, man. what a drag!!

 

(sorry, somebody had to say it!!)

[MigBuster]

Exactly... I need to modify aerodynamics of weapons actually. And much depends on drag coefficient

 

If you see here:

 

http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html

 

The values shown here were determined experimentally by placing models in a wind tunnel and measuring the amount of drag, the tunnel conditions of velocity and density, and the reference area of the model. The drag equation given above was then used to calculate the drag coefficient. The projected frontal area of each object was used as the reference area.

 

 

I'm fairly sure the actual values need to be determined through physical experiments - but they could be published somewhere  - or for this game they might be estimated in relation to similar shapes.

 

A comment from someone who does FMs would be usefull.

oh, man. what a drag!!

 

(sorry, somebody had to say it!!)

 

[Fubar512]

All the series is concerned about with weapons is (as you've mentioned earlier) subsonic and supersonic coefficients, and maximum coefficient of lift..  

 

I can tell you from my research that at least 75% of the CD is determined by the frontal shape and presented aspect of the object, irregardless of length to width ratio or the shape of its terminal end. Note the difference between a full and half-sphere in the diagram below:

 

 

 

 

And every shape creates some amount of lift, whether it's positive or negative (down force). This is expressed in-game by the weapon's CLmax value, and this is where the object's planform (length and width) come into play.

[Snailman]

Huh)) Well... that's something.. I can do something with)))) 

 

Well... Soooo if I understand it correctly,  for a round nosed object... like a  Sidewinder, I should use the sphere or the hemisphere shape and the diameter value and that's all? 

 

What if a hemisphere is on the front of a long cylinder?  :omg:

[Toryu]

No. The missile may have a blunt nose, but it is a lot more streamlined than just a half-shere as it is longer and enjoys a pretty good fineness-ratio. Thus, there is less pressure-drag associated with the missile than with the half-sphere itself.

 

You also can't just take over the values given by some table, without checking it's Reynolds-number. The C_D given above for the sphere is fpr Re-numbers smaller than 1.7*10⁵, which is not exactly, where aviation takes place (rule of thumb: >10⁶). At those Re-numbers, the sphere would - for example - have a C_D of 0.09 to 0.18. That is due to different flow-characteristics between turbulent and laminar boundary-layers concerning spherical or round objects. In layman's terms: The turbulent flay manages to stay attached longer, creating less pressure-drag - although turbulent boundary-layer itself (e.g. over a long, flat plate) creates more friction-drag than laminar-flow.

 

If you're into that topic, visit the following page, have a coockie and drink some coffee. Not very simple, but highly interesting:

http://scilib.narod.ru/Avia/DAC/dac.htm

[Snailman]

oooo craaap.     well thanks to you guys. I try to solve some simplified stuff...  with the data asked by the game.

[Toryu]

Patient young pandawan has to be