Expert: Paul Soderman - 10/31/2011

Question
QUESTION: Sir,

  I want to know,is the design of a UAV is the same as any aircraft model. S= (390*W)/[Cl(max)*V(square)].Is this the same formula we use to get the wing area in case of UAV? and b=square root of (AS) for wing span.
This is what we were tought in our Aircraft design. But trying for  a UAV of 20-30 lbs. It does not seem to work. Please correct me if I am off the track.
Thank you

ANSWER: Saurav - Your equation for wing area is correct, but only for one flight condition.  The value of Cl is usually not Clmax but is the Cl required for the aircraft to achieve level flight at velocity V.  In other words, Cl is less than Clmax unless you are flying right at stall speed (Clmax).  And your equation is for sea level conditions.  At altitude, you need to include a density ratio sigma = rho/rho(sea level) as follows:

S = 390 W/(Cl sigma V^2)

The 390 results from the odd use of area in ft^2,weight in pounds, and airspeed in miles/hour. (See Perkins and Hage: Airplane Performance Stability and Control if it is still available.)  You might be better off to use the basic equation with metric units:

S = 2W / (Cl rho V^2)

Your equation for wing span as a function of aspect ratio is correct.  Yes, a UAV is an aircraft so all equations for aircraft will apply.
Paul

---------- FOLLOW-UP ----------

QUESTION: Sir,

Thank you for the answer. In addition to this I want to know, about computer simulation that we use to check the fluid flow and strength.

I mean we were taught AutoCAD, to design the three views and the exploded view of an aircraft, where we choose and calculate all the parameters, required. From Landing distane to ground loads and all.

However, I want to know, which software are primarily chosen by the professional aircraft design engineer except ACAD. Apart from Anderson's Design and performance can you suggest the best book to understand all the concept form basic to advance of Aircraft Design.

Answer
Aircraft engineers have many tools to help them do their job.  Typically, they might have a panel method such as the one at:  http://www.mh-aerotools.de/airfoils/javafoil.htm  to do the basic wing/body design and performance.  NASA engineers often use similar codes developed in house, but I forget the names of them.  There are commercial codes available for aerodynamic performance.  In addition to aerodynamics, you need to consider things like range, stability and control, engine power required, climb performance, etc.  For that I would turn to various books such as:  Perkins and Hage:  Airplane Performance Stability and Control.  (Unfortunately, it was written in the days of feet and pounds.)  Another good one is:  B. Etkin:  Dynamics of Flight, Stability and Control.  H. Ashley:  Engineering Analysis of Flight Vehicles might help, though it is a little advanced.  And of course a good aerodynamics book would help.  I use Kuethe and Schetzer:  Foundations of Aerodynamics. For aircraft drag I use S. Hoerner:  Fluid-Dynamic Drag. A design engineer would also consult the numerous NASA and AIAA papers on aircraft design and performance. I do not know of one book or computer code that covers the entire design procedure. Best of luck.
Paul