Introduction You have designed a launch vehicle to insert a satellite into orbit . You now need a means to transport that satellite from your factory to the launch site. A cargo jet aircraft is the most likely candidate, though it may have to be unusual in size and shape because of the weight and shape envelope of the satellite design. Speed is not critically important but range is. Since the aerospace industry has become truly an international business it would be advisable to have a delivery aircraft with the ability to fly across the Atlantic Ocean, say NewYork to London or New York to Paris. This requires a range of about 2500 nautical miles. That is also roughly the coast-to-coast distance across the U.S., so delivery from within the U.S. to a coastal launch site would probably be convenient as well. This aircraft design portion of the project is a very simplified version of the preliminary design process which will be undertaken by some of you in AE420. It requires only algebraic calculations. In several cases, rule-of-thumb estimations are used to avoid having to do analysis which is beyond the current technical background of the typical first year college student. These estimations are explained at the fundamental concept level when they are presented.
The general nature of any type of design project is summarized in Figure 1. The specific sequence of steps to be followed in this project is depicted in Figure 2. The equations presented in the following discussion presume that the aircraft is subsonic. Supersonic flight over land is not currently allowed in any heavily populated country because the sonic boom irritates people on the ground.
Identify need Define problem in detail Search for data Design requirements specified Alternative solutions Analysis Decision Describe details of chosen solution Communicate solution Revise design and iterate
Figure 1. The Design Process
SPECIFICATION PAYLOAD GROSS WEIGHT WING AREA PLANFORM SHAPE CENTER OF GRAVITY TAIL SURFACES DRAG ESTIMATE ENGINE SELECTION FUEL LOAD CONFIGURATION DRAWING Figure 2. Sequence of Aircraft Design Calculations
Details of the Design Calculations 1. FIND THE PAYLOAD THE AIRCRAFT WILL CARRY (1) Wpayload = (170) N + Wcargo
where Wpayload (lbs) = weight of people and cargo to be carried N = number of passengers and crew 170 = weight of one person (lbs), specified by Federal Aviation Regulations Wcargo (lbs) = weight of the spherical mirror satellite 2. ESTIMATE THE GROSS (TOTAL) WEIGHT OF THE AIRCRAFT Gross weight, WG, is the sum of empty weight, WE, payload weight, WP, and fuel weight, WF. (2) WG = WE + WP + WF
A precise calculation is quite complicated and can only be done in later design stages as the structural design is completed. General statistically derived values as shown in the table below are used in preliminary design. Type of aircraft Empty WE piston .57 WG .55 WG .55 WG .50 WG weight, Fuel WF Eq. 3 Eq. 3 Eq. 3 Eq. 3 weight,
Light aircraft, engine Light aircraft, turboprop Small transport Large transport
Aircraft are designed with large variations in range, so fuel weights are harder to describe with simple averages. Plotting the data from 21 aircraft illustrates a trend that we can use as an estimate. Curve fitting a straight line to the data gives the equation below. (3) WF = [0.15 + 3.33 x 10-5 (R – 1000)] WG
Example: For a large transport aircraft with a range of 5000 miles we would estimate WG as WG = WE+ WP + WF = .50 WG + WP + .283 WG or WG (1-.50-.283) = WP ⇒ WG = 4.608 WP 3. FIND WING PLANFORM AREA REQUIRED TO MEET DESIRED LANDING SPEED Lift, L = WG, at any time the aircraft is in level flight. An important definition, which will be rearranged and used to calculate the wing Planform area is:
(Definition - 4)
Lift coefficient, CL =
L 0. 5ρV 2 S
where ρ = air density (.002377 slugs/ft3 at sea level standard (SLS) conditions, good to...