drag
02-05-2006, 06:41 PM
#2
Senior Member
Join Date: Oct 2003
Location: Winnipeg,
MB, CANADA
Posts: 322
Likes: 0
Received 0 Likes
on
0 Posts
RE: drag
It is not easy to calculate the total drag of any object apart from a few trivial cases. There are so many variables in a complete airframe - skin friction, turbulence around various projections, vortices at wingtips, fin, stabilizer, angle of attack, airspeed, air temperature, relative humidity - to name just a few.
Visit [link=http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12.htm]http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12.htm[/link] for a discussion of the math in both lift and drag. Note that the innocuous little term "CD" in the drag equation is the source of all the problems. The coefficient of drag is where all of the variables are hidden, and it is hard to know just how each of them should be factored in. You can see the calculated CD values for some simple shapes at [link=http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12G8.htm]http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12G8.htm[/link].
If you have access to a wind tunnel, you can get an experimental estimate of total drag at various airspeeds, but most modelers don't have such tools at their disposal. I suppose that if you were really serious, you could probably get a reasonable estimate of the total drag force with a home-built wind tunnel, but you would need to ensure a nearly laminar airflow at typically 50-100 mph over the entire frontal area of your model. Not a trivial design and construction challenge.
Visit [link=http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12.htm]http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12.htm[/link] for a discussion of the math in both lift and drag. Note that the innocuous little term "CD" in the drag equation is the source of all the problems. The coefficient of drag is where all of the variables are hidden, and it is hard to know just how each of them should be factored in. You can see the calculated CD values for some simple shapes at [link=http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12G8.htm]http://www.centennialofflight.gov/essay/Theories_of_Flight/coefficients/TH12G8.htm[/link].
If you have access to a wind tunnel, you can get an experimental estimate of total drag at various airspeeds, but most modelers don't have such tools at their disposal. I suppose that if you were really serious, you could probably get a reasonable estimate of the total drag force with a home-built wind tunnel, but you would need to ensure a nearly laminar airflow at typically 50-100 mph over the entire frontal area of your model. Not a trivial design and construction challenge.
03-13-2006, 12:54 AM
#4
Member
Join Date: Mar 2006
Location: , KS
Posts: 41
Likes: 0
Received 0 Likes
on
0 Posts
RE: drag
don't forget that a major portion of drag will be induced drag (drag induced by lift) which is easy to calculate. C_D_i=C_L^2/(pi A e). A=b^2/S where b is span and s is wing area. C_L is the Airplane lift coeff (approx by wing lift coeff, not airfoil c_l). use e=.8 probably.
