Lift-dependent drag estimation

In summary, the pilot estimates that the drag of their small two-place airplane is 0.043, and calculates the induced drag based on the lift, air density, wing area, aspect ratio, and applicable Oswald number. They have doubts about the lift-dependent drag, so they calculate 1/3 of the induced drag as the lift-dependent drag.
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I'm no engineer, but just a pilot trying to estimate the drag of my small two-place airplane from the relevant formulae.

My plane is quite draggy, and from comparison with other aircraft data, I have taken Cd,0 as 0.043. Apart from this, I calculate the induced drag at a given speed from the lift, equal to the weight in S&L flight, air density, wing area, aspect ratio and applicable Oswald number (I've taken 0.73). So far, so good, but I have doubts concerning the lift-dependent drag. In a solved problem in Anderson's Aircraft Performance and Design, the author estimates (for a Gulfstream) that lift-dependent drag as 1/3 of the induced drag.

Should I take that figure for my calculations? I don't feel sure, since 1/3 is a lot...

Thanks in advance...
 
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  • #2
I had the impression that induced drag and lift dependent drag mean essentially the same thing. In an ideal case, at the best lift to drag ratio (best glide ratio) which occurs at a specifc speed, and the induced drag is 1/2 the total drag. However most powered aircraft travel at "cruise speed" which is well above the speed associated with best lift to drag ratio, so most of the drag is parasitic / profile drag.
 
  • #3
rcgldr said:
I had the impression that induced drag and lift dependent drag mean essentially the same thing. In an ideal case, at the best lift to drag ratio (best glide ratio) which occurs at a specifc speed, and the induced drag is 1/2 the total drag. However most powered aircraft travel at "cruise speed" which is well above the speed associated with best lift to drag ratio, so most of the drag is parasitic / profile drag.

Well, yes, but -in general- the induced drag coefficient calculated by Cl^2/pi*e*AR corresponds only to the lift-vortex drag. Since the profile drag varies with the attitude of the plane, there is a complementary profile drag that is the lift-induced drag, which can be estimated as a fraction of the latter. My query is about the magnitude of that fraction...
 
  • #4
There are variations in the definition of induced drag, but the concept is easiest to visualize using an ideal wing that from the wing's frame of reference, diverts a relative flow (the mass of the affected air per unit time) by some average angle downwards without changing the speed of the air, so no change in energy from the wings frame of reference. (From the air's frame of reference energy is added to the air, resulting in downwards and some forwards flow). The horizontal component of velocity of the flow is decreased by 1 - cos(angle of deflection). Assuming lift equals weight, then you can used the idealized formula for induced drag based on the average angle of deflection:

induced drag = lift (1 - cos(θ)) / sin(θ)

The issue then is how to determine the average angle of deflection. I don't know if there is a simplified approximation for this based on an aircraft and it's wing parameters.

However, getting back to my previous post, if the best glide ratio and corresponding speed are known, then assume that 1/2 the drag is induced drag. Lift always equals weight, so the two components of drag equal 1/2 the weight x drag / lift (drag/lift = 1 / (glide ratio)). Then for the best glide ratio parameters, let V0 = speed of best glide ratio, Di0 = induced drag at best glide ratio and Dp0 = parasitic drag at best glide ratio. Then for approximation, Di ~= Di0 (V0/V)^2, and Dp ~= Dp0 (V/V0)^2, and Di/Dp ~= (V0/V)^4.
 
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1. What is lift-dependent drag estimation?

Lift-dependent drag estimation is a method used to calculate the aerodynamic drag on an object based on its lift characteristics. It takes into account the lift force generated by the object and how it affects the surrounding air, resulting in an estimation of the drag force.

2. How is lift-dependent drag estimation different from other drag estimation methods?

Lift-dependent drag estimation differs from other methods because it specifically considers the lift force generated by the object. Other methods may only look at the shape or size of the object, while lift-dependent drag estimation takes into account the lift coefficient and angle of attack.

3. Why is lift-dependent drag estimation important in aerodynamics?

Lift-dependent drag estimation is important because it allows for a more accurate prediction of the aerodynamic drag on an object. This information is crucial in designing efficient and streamlined aircraft, as well as understanding the performance of various objects in fluid environments.

4. What factors are taken into account in lift-dependent drag estimation?

Lift-dependent drag estimation takes into account the lift coefficient, angle of attack, and the shape and size of the object. Other factors such as air density, viscosity, and speed may also be considered depending on the specific method used.

5. How is lift-dependent drag estimation used in practical applications?

Lift-dependent drag estimation is used in various practical applications, such as designing aircraft, spacecraft, and other aerodynamic objects. It is also used in sports equipment, such as cycling helmets, to reduce drag and increase speed. Additionally, it is used in wind tunnel testing and computer simulations to optimize the design and performance of various objects.

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