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In a cruising flight, lift is equal to the aircraft weight (L = W). Thus,
(3.18)
The maximum aerodynamic efficiency is
Equations (3.19) and (3.16) reveal that the following factors play an important role in air vehicle aerodynamic efficiency: (1) aircraft zero lift drag coefficient (CDo) and (2) induced drag correction factor (K). Furthermore, due to Equation (3.6), the Oswald span efficiency factor (e) and the wing aspect ratio (AR) are also impacting the aerodynamic efficiency.
As a conclusion, in order to increase the air vehicle maximum aerodynamic efficiency, one must: (1) decrease the aircraft zero lift drag coefficient and (2) increase the wing aspect ratio. High aspect ratio wings (long and slender) are conducive to good range and endurance. Short stubby (low AR) wings generate low aerodynamic efficiency (i.e., penalize the length of time‐on‐target during reconnaissance missions), but are good for highly maneuverable fighters.
The wing aspect ratio for the MALE UAV General Atomics MQ‐1 Predator (see Figure 11.1) wing is about 22.5, while for the HALE UAV Northrop Grumman RQ‐4 Global Hawk (see Figure 1.4) wing is about 25. The wing aspect ratio for the small UAV AeroVironment RQ‐11 Raven (see Figure 18.11) is 3.7.
Example 3.1
A MALE UAV with a turbofan engine has a zero‐lift drag coefficient of 0.021 and an induced drag factor of 0.04. Determine the maximum aerodynamic efficiency of the air vehicle.
Solution:
from (3.16)
from (3.19)
Questions
1 Define aerodynamics.
2 Name primary forces that act on an air vehicle.
3 Name elements that make considerable contributions on air vehicle’s aerodynamic features.
4 What is the primary aerodynamic function of a wing?
5 Name two aerodynamic forces.
6 The aerodynamic forces of lift and drag are functions of a number of factors. What are they?
7 Define lift.
8 Draw the side‐view of a wing at an angle of attack. In the figure, illustrate: (1) total aerodynamic force, (2) lift, (3) drag, and (4) name and the location of the forces.
9 Define Mach number.
10 List the flight regimes, when the airspeed is compared with the speed of sound. Briefly define each one.
11 What are the two most important parameters of an airfoil?
12 Provide at least five geometric parameters of an airfoil.
13 Briefly compare the patterns of air pressure over a two‐dimensional airfoil and a three‐dimensional wing.
14 What do NACA and NASA stand for?
15 Define streamline.
16 What is the thickness‐to‐chord ratio of the NACA airfoil 23021?
17 What is the lift coefficient of the NACA 23021 airfoil at an angle of attack of 10 degrees when R = 3 × 106?
18 What is the maximum lift coefficient of the NACA 23021 airfoil when R = 3×106?
19 What is the minimum drag coefficient of the NACA 23021 airfoil when R = 8.9 × 106?
20 What is the drag coefficient of the NACA 23021 airfoil when the lift coefficient is 0.2 at R = 8.9 × 106?
21 What airfoil is used on the wing of the Northrop Grumman RQ‐4 Global Hawk?
22 What airfoil is used on the wing of the General Atomics MQ‐1 Predator?
23 Compare the static pressure of the top and bottom surfaces for a positive cambered airfoil at a positive angle of attack. Draw a figure.
24 Why is a vortex generated at each wing tip?
25 Briefly describe how the lift is generated on a wing.
26 Draw a typical drag polar and discuss its characteristics.
27 The drag coefficient is the sum of two terms. What are they?
28 Write the drag polar equation.
29 Define aspect ratio.
30 Discuss how to convert the “infinite wing” (i.e., airfoil) lift coefficient to the lift coefficient of a real wing.
31 What is the most important element of drag introduced by a wing at high angles of attack?
32 What is the significance of an elliptical lift distribution over the wing? Discuss.
33 What can be concluded from Equation (3.9) about induced drag?
34 Discuss characteristics of the boundary layer of a fluid flowing over a surface.
35 Name three regions of the boundary layer.
36 How is mathematically Reynolds number expressed?
37 What is the typical Reynolds number for small UAVs?
38 What
