Model Flow Over Bombs: Rotation & Drag Coefficients

In summary, the fins on a bomb can either help to keep it flying on a predictable path, or cause it to spin. Some bombs, like the GBU-54, have fixed fins while others, like the GBU-12, have stability only fins. With rotation, drag is increased, which may be desirable for some missles. Extrapolating to air-to-air missiles, some may use rotation to help minimize distance and velocity. For example, the AIM-9X Sidewinder uses rotation to help it fly in a tighter arc.
  • #1
gm3
4
0
I'm working on a project that will find the coefficient of drag on bombs with various added drag features by modeling flow over the bomb at various angles of attack using CFD.

The problem is that I'm not sure if bombs with fins rotate while they are falling due to gravity, and if so, how I can relate the velocity of the flow to the rotation rate of the bomb.

Since the fins on a bomb are flat and not curved like an impeller, do they simply keep the bomb flying on a predictable path?
 
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  • #2
Same purpose as an the feathers or fins of an arrow. They keep the nose pointing in direction of travel and prevent the arrow ( bomb ) from tumbling head over heal. In this way drag is minimized along the streamlined surface of the projectile, travel distance ( especially for the arrow ) and velocity would be maximized upon reaching the target. Thus time from release to impact would be minimized which could be a desirable feature as could velocity upon impact.

A weather vane works upon the same principle.
 
  • #3
Excellent. Thank you.
 
  • #4
Some bombs are meant to spin as they fall towards their target. Others not; it all depends on the bomb and its purpose. Here are some examples:

Fixed fins, stability only: Paveway IV ( GBU-12 added GPS ) and the GBU-54
Read more: http://www.defencetalk.com/forums/air-force-aviation/bomb-aerodynamics-12499/#ixzz30VtqO1Nu

Fins that cause the bomb to spin:
https://share.sandia.gov/news/resources/news_releases/wind_tunnel/#.U2LllGdOVgU

Grid fins (or lattice fins) are a type of flight control surface used on missiles and bombs in place of more conventional control surfaces, such as planar fins. These fins increase the aerodynamic drag.
http://en.wikipedia.org/wiki/Grid_fin

Hundreds of images of various types of bomb fins:
https://www.google.com/search?q=bom...6JseMyASFhoCYAw&ved=0CGUQsAQ&biw=1093&bih=528

A technical paper on the subject:
JOURNAL OF THEORETICAL
AND APPLIED MECHANICS
48, 1, pp. 27-44, Warsaw 2010
MODEL OF GASODYNAMIC CONTROL SYSTEM FOR
GUIDED BOMBS
http://yadda.icm.edu.pl/yadda/eleme...c/httpwww_ptmts_org_plglebocki-zug-2010-1.pdf
 
  • #5
gm3 said:
I'm working on a project that will find the coefficient of drag on bombs with various added drag features by modeling flow over the bomb at various angles of attack using CFD.

The problem is that I'm not sure if bombs with fins rotate while they are falling due to gravity, and if so, how I can relate the velocity of the flow to the rotation rate of the bomb.

Since the fins on a bomb are flat and not curved like an impeller, do they simply keep the bomb flying on a predictable path?

gm3 said:
Excellent. Thank you.

So after reading the great responses and links, can you summarize when a ballistic bomb design would be better served with rotation versus not?

And can you extrapolate to air-to-air missles, and why some may use rotation? Which ones do?
 

1. What is the purpose of studying model flow over bombs?

The purpose of studying model flow over bombs is to better understand the aerodynamics of bombs in order to improve their accuracy and efficiency during deployment. This can also aid in the development of more advanced bomb designs.

2. What are rotation coefficients and why are they important?

Rotation coefficients are numerical values that describe how a bomb rotates in flight due to aerodynamic forces. They are important because they affect the bomb's stability and accuracy during deployment.

3. How are drag coefficients calculated for bombs?

Drag coefficients for bombs are typically calculated through wind tunnel experiments or computational fluid dynamics simulations. These methods involve measuring the drag force on a model bomb and dividing it by the dynamic pressure of the air flow.

4. How do rotation and drag coefficients affect bomb performance?

Rotation and drag coefficients directly impact the stability and accuracy of a bomb during flight. A high rotation coefficient can cause the bomb to spin out of control, while a high drag coefficient can slow down the bomb's forward motion and decrease its range.

5. Can the results from model flow experiments be applied to real bombs?

While there may be some differences in the aerodynamics of a model bomb and a real bomb, the results from model flow experiments can still provide valuable insights and help improve the design and performance of real bombs.

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