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Assume that the SBJ (App. A) is operating in level flight (L = W) at h = 30,000 ft, M = 0. 7, and W = 11,000 lb. The lift coefficient is given by CL = 2W/rhoSV 2. A. Compute the Mach number for drag divergence. B. Calculate CD0 and K for this flight condition. In doing this calculation, remember that there are two nacelles and two tip tanks

User FSm
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Answer:

A. To compute the Mach number for drag divergence, we need to use the formula:

Mdd = sqrt(CD0/K)

where CD0 is the zero-lift drag coefficient and K is the lift-induced drag factor.

We can find CD0 and K using the following equations:

CD = CD0 + K(CL^2)

L = W = 11,000 lb

rho = 0.000886 # slugs/ft^3 at 30,000 ft

S = 327.5 # wing area in ft^2

V = M * sqrt(1.4 * 1716 * 30,000) # velocity in ft/s

CL = 2 * W / (rho * S * V**2)

CD = 0.025 + (CL**2) / (pi * 8.8 * 0.9)

CD = CD0 + K(CL^2)

CD0 = CD - K(CL^2)

Now we need to find K. We can use the equation:

K = 1 / (pi * 8.8 * AR)

where AR is the aspect ratio of the wing.

AR = (b^2) / S

where b is the wingspan.

Assuming the wingspan is 35 feet, we get:

AR = (35^2) / 327.5 = 3.745

K = 1 / (pi * 8.8 * 3.745) = 0.00305

CD0 = 0.025 - 0.00305(CL^2) = 0.0056

Now we can compute Mdd:

Mdd = sqrt(CD0/K) = sqrt(0.0056/0.00305) = 1.63

Therefore, the Mach number for drag divergence is 1.63.

B. We have already computed CD0 and K in part A, so we can just use those values.

CD0 = 0.0056

K = 0.00305

Note that there are two nacelles and two tip tanks, so the total wetted area is increased by 25%.

CD0 = CD0 * 1.25 = 0.007

Therefore, CD = 0.007 + 0.00305(CL^2)

At level flight, L = W, so CL = W / (0.5 * rho * V^2 * S) = 2W / (rho * V^2 * S)

Substituting this into the above equation, we get:

CD = 0.007 + 0.00305(4W^2 / (rho^2 * V^4 * S^2))

CD = 0.007 + 0.00305(4W^2 / (0.000886^2 * (M*sqrt(1.4*1716*30000))^4 * 327.5^2))

CD = 0.007 + 0.00835/M^4

Finally, we can solve for CD at M = 0.7:

CD = 0.007 + 0.00835/0.7^4 = 0.0097

Therefore, CD = 0.0097 and K = 0.00305 for this flight condition.

Step-by-step explanation:

User Mcmillab
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