Wing drag vs Total A/C Drag

[rbogie]

Hi, I'm trying to help my teenage daugter with a science fair project. We want to take the plotted Lift/Drag curves of three different airfoils, use the lift and drag formulas (from 2D airfoil L/D graphs) convert to 3D actual wings, and see how close we can come to the actual stall speed and max Vne speeds for 3 different airplanes (Cessna 172, Lockheed C-130, Boeing 747). The thing is while the lift equation is for the wing alone (and its a reasonable assmuption that only the wing creates lift so therefore the lift equation represents the whole airplane), it appears that the drag equations only calculate drag for the wing alone. If the drag equation only calculates for the wing, how can we approximately calculate the fuselage drag? I'm thinking that there could be a fraction of total a/c drag is due to fuselage/empennage and the remaining fraction of total drag is from the wing. The total drag at max Vne is equal to the total thrust available, subtracting off the wing drag should leave the fuselage/empenage drag. As a crude example: Total thrust for C-130 equals, say, 25,000 lbs which would equal the total drag. At this point I don't know what speed we are at, if I assume that 1/3 of the total thrust is for wing drag (ie 8,333 lb) then I can work backwards to find the Velocity. Has anyone seen such a proportion of Wing drag to Total drag? It would probably be adjusted by the fuselage slenderness ratio, and Mach no. and air density.

Second note: The C-172 wing uses a NACA 2412 airfoil which I can get the L/D graphs for (I have the book "Theory of Wing Sections"), and the C-130 uses NACA 64A318 airfoil. But I don't understand the Boeing 747 wing. I looked it up and its listed as "BAC 463 to BAC 468" Does this mean it changes from root to tip? What does a BAC 463, 464, 465, 466, 467, 468 look like and how can I find L/D data of this wing?

 

[GregLocock]

You could make the project more complex by estimating the skin friction drag for the body and tail surfaces.


So far as cookbook number go, they'd be easy to find if only someone could invent a way of finding information on the interweb.

Luckily someone sent me this URL

http://www.centennialofflight.gov/essay/Theories_of_Flight/drag/TH4G6.htm

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rbogie]

Thanks Greg,

The URL you gave showed the drag contributions for the different parts of a ME-109, and if the stats from the ME-109 are any indication then my crude guess of 1/3 total drag = wing drag; was a reasonable approximation. I just never saw anything like that in my design classes. Everything I've found starts with some previously done wind tunnel testing to find a Cd0 (or Cdmin). Even on Aerospaceweb.org <

http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml>

they give a chart for Cd0's for a number of representative airplanes.
I wanted this to be a calculation based on going into this "cold" with no prior wind tunnel testing or baseline values of the "target airplanes" ahead of time. Only with the charted values for the airfoils.
If I can't find a way to do that then we'll be forced to use the formula Cd = Cdmin + Cl^2/(pi X A R e) and take a value for Cdmin off the chart on the Aerospaceweb.org

I'm still waiting to see if any Boeing engineers can tell me about the BAC 463-468 airfoils on the B-747.

 

[rb1957]

wing lift also has a contribution from the horizontal tail; the horizontal tail lift is down, so wing lift is higher than the weight of the plane; if you want a guess say 10% of the airplane weight.

try airpalne design books ... Raymer "Airplane Design: A Conceptual Approach" or Torenbeek "Synthesis of Subsonic Airplane design" ... these give rules fo thunb for estimating these factors.

aslo i think you're being a bit optimistic in getting data on three cross-sections of a wing ... you might be ok on the cessna (the high wing means that the wing section doesn't change much with the fuselage intersection (and the design issues of the landing gear).

you should be able to make a reasonable estimate of the airplane stall speed (CL data is reasonably available). Vne on the other hand combines so many factors (engine performance, propulsor performance, airplane drag) that i think it'll be hard to come close.

good luck

 

[rbogie]

I've got a book on Acft design "Fundamentals of Acft Design", 1975 by Leland Nicolai and so far all I've found is the equation for Cd that I wrote above. You're probably right about HStab lift, but I've usually seen it assumed to be 0 just to simplify the calculations.

I'm beginning to think that I should drop the whole B-747 part of the analysis, since 1. I'm thinking that Boeing is keeping the airfoil data proprietary, 2. It may involve more than one airfoil--which would make things complicated, 3. The wing is swept which adds a slight amount of more complexity to the equations (daughter is only 15 and she hasn't even gotten to Trigonometry yet), 4. We all know that the B-747 operates around M .85 so the wing has locations on it that are at M .90-1.2 and that adds compressibility to the equations.

It may be better to stick with three different speed ranges of the same basic style of airplane ie low-mid subsonic, propeller driven, with straight wings, and airfoils that are listed in the NACA charts.