Lecture- Traditional Aircraft Design
Aircraft Design: a distinct discipline in aeronautical engineering
Designer must be well versed in all disciplines “The intellectual engineering process of creating on paper a flying machine that either meets certain requirements and performance objectives, or explores new concepts, technologies and innovations”
Three phases of design: Conceptual:
First step in the design process In response to a certain design goal (requirements or exploration) Overall (fuzzy) shape, size, weight, performance of airplane configuration Basic drivers are aerodynamics, propulsion, and performance Some, but not much, consideration for stability/control, cost, no detail design
Anderson’s 7 Intellectual Pivot Points Requirements Weight-first estimate Critical performance parameters: Clmax L/D T/W W/S Configuration Layout Better weight estimate Performance analysis Optimization AE 3310 Performance
Raymer’s Design Wheel Sizing Trade Studies Design Analysis
Requirements
Design Concept Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Next 2 Phases of Aircraft Design
Preliminary Design: major design features locked in, only minor changes allowed. Substantial analysis begins to take place, including CFD and wind tunnel tests. By the end of this phase, the manufacturer will decide if the program is a “go” or “no-go”. Detailed Design: Precise and detailed decisions are made. By this time, the aero, propulsion, structures, etc are locked in. Only very subtle details are left to make the design a realistic vehicle capable of being manufactured.
AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Requirements
Often put forth in a document called an RFP (Request for Proposal) Outlines basic design requirements and goals Basic mission or mission profile Examples: payload and type of payload range/loiter requirements cruise speed and altitude field length for takeoff/landing FAR 23- normal, utility, aerobatic, commuter fuel reserves FAR 25- transports climb requirements maneuvering requirements Certification base (experimental, FAR 23, FAR 25, military) Can be specific or vague
Initial Weight Estimation WTO = WOE + Wf + WPL
AE 3310 Performance
WTO
Takeoff Gross Weight
WOE Wf WPL
Operating Empty Weight Mission Fuel Weight Payload Weight
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Further,
WOE = WE + Wtfo + Wcrew
WE
Empty Weight = manufacturer’s empty weight + fixed equipment weight
Wtfo
Weight of all trapped (unusable) fuel and oil
Wcrew
Weight of crew
For initial weight estimation, we want to estimate WTO, WE and Wf Step 1: Determine mission payload weight passengers and baggage cargo military loads (bombs, ammunition, etc)
Rules of Thumb: passenger aircraft: 175 lbs per person 30 lbs luggage per person, short to medium flights 40 lbs luggage per person, long flights AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Crew Weight: passenger: military:
175 lbs per crew member, 30 lbs baggage 200 lbs per crew member, no baggage
Step 2: Guess a likely value of take-off weight Base this guess on existing similar aircraft
Step 3: Determine mission fuel weight, Wf Wf = WFused + WFres Wfres are the fuel reserves required for the mission. As a fraction of WFused As a requirement for additional range to an alternate airport As a requirement for loiter time AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
To find Wfused, use fuel fraction method Break down mission into a number of mission phases and calculate fuel used in each phase based on simple calculations or experience.
Fuel Fraction: for each phase is defined as the ratio of end weight to begin weight Phase 1: Engine Start and Warm Up W1/WTO Use Handout Table 2.1 Phase 2: Taxi W2/W1 Use Handout Table 2.1
Phase 3: Takeoff W3/W2 Use Handout Table 2.1
AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Phase 4: Climb to Cruise and Accelerate to Cruise Speed W4/W3 Use Handout Table 2.2 and Use Breguet’s Endurance Equation (you will need to assume appropriate values for L/Dclimb, specific fuel consumption, time to climb [or rate of climb])
E = 550ηpr cV E=
1 ct
L D L D
ln
ln
W0 W1
(prop)
W0 W1
Phase 5: Cruise W5/W4 Use Handout Table 2.2 and Use Breuget’s Range Equation AE 3310 Performance
Be careful using these equations. You must use consistent units!
(jet) R = 550ηpr L D c R= V ct
ln
W0 W1
L ln W0 W1 D
(prop) (jet) Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Phase 6: Loiter W6/W5 Use Handout Table 2.2 and Breguet’s Endurance Equation Phase 7: Descent W7/W6 Use Handout Table 2.1 Phase 8: Landing, Taxi, and Shutdown W7/W6 Use Handout Table 2.1
Now calculate the mission fuel fraction: Mff = multiply all phase fractions together WFused = (1 - Mff) WTO WF = (1 - Mff) WTO + WFres AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Step 4: Calculate Tentative WOE WOEtent = WTOguess - WF - WPL
Step 5: Calculate Tentative Value for WE WEtent = WOEtent - Wtfo - Wcrew Wtfo can be as high as 0.5% for some airplanes, often neglected at this stage
Step 6: Find Allowable Value for WE For most aircraft of a defined type, there exists a linear relationship between log10WE and log10WTO. Use existing graphs or use Handout Table 2.15. If you don’t have graphs or Handout Table 2.15, create your own by plotting WE and WTO of similar aircraft and regressing the curve yourself. For completely new aircraft, try and extrapolate existing data, or use non-traditional techniques (GT advanced design sequence in graduate program) AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
This linear relationship looks like this:
Regression Curve: Assumes WE was designed to be lowest possible for best cost/performance, so each point represents “state of the art”.
Empty Weight, lbs
log10 scale!!! Gross Takeoff Weight, lbs
Regression Equation: inv log10 {[log10WTO - A]/B} AE 3310 Performance
A and B are the regression constants (slope and intercept) of the line. Use Handout Table 2.15 for values for types of aircraft, or plot your own regression curve and estimate your own A and B.
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Step 6: Compare WEtent and WE Compare the two values. Make an adjustment to WTOguess and repeat Steps 3-6. Continue until the two values agree with each other to some tolerance (0.5%)
AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
T/W vs. W/S Sizing Plot
T/W and W/S are two of the most important parameters in aircraft design and performance. Earlier we have seen how often the two are interrelated, and we understand the performance tradeoffs and interrelationships. “Good” estimates of T/W and W/S are critical in conceptual design. In general, want the lowest T/W and the highest W/S, while still meeting all other criteria. This establishes the “design point”
Example: if we chose the design point shown, we would have to achieve a landing CLmax of about 2.0 (maybe with high lift devices), and a takeoff CLmax of 2.0. We would meet all of our climb requirements, and our T/W would be 0.35 with a W/S of 62 psf. Because we have already estimated our weight, we can then calculate the thrust required (which sizes our engine) and the wing area required (aerodynamics).
AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
In addition to range, speed, and endurance criteria, aircraft must also be designed to meet performance objectives, such as stall speed takeoff field length landing field length cruise speed (or maximum speed) climb rate (all engines operating AEO and one engine inoperative OEI) time to climb to some altitude maneuvering The parameters that have a major impact on these performance objectives are: Wing Area, S Takeoff Thrust, TTO, or Takeoff Power PTO Maximum Required Takeoff Lift Coefficient with flaps up, CLmax(clean) Maximum Require Lift Coefficient for Landing, CLmaxL We now use methods and equations to estimate these parameters for a range of W/S and T/W. Then, it usually follows that the combination of these factors that produce the highest wing loading and the lowest thrust loading which still meet all performance requirements will result in the airplane with the lowest cost and lowest weight.
AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Sizing to Stall Speed Minimum stall speeds are often put into the requirements. They can also be found in the FAR’s.
FAR 23:
single engine; Vstall < 61 kts at WTO multi engine < 6000 lbs; Vstall < 61 kts at WTO unless they meet a climb requirement can be met flaps up or down
FAR 25:
no minimum stall speed requirements
For a given Vstall: Vstall =
2(W/S) ρ∞ CLmax
establishes a maximum W/S for a given CLmax
CLmax is a function of: 1. wing/airfoil design 2. flap type and flap size 3. center of gravity location AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Sizing to Takeoff Distance Requirements Normally takeoff requirements are given in terms of takeoff field length, either specifying the ground run or the total length W S
sTOG ∝
ρh ρ0
sTOG
TO
W P
TO
= TOP23
CLmaxTO
= takeoff ground roll distance
TOP = takeoff parameter, a simple metric, referring to the appropriate FAR W P
= power loading for a prop aircraft (most FAR23 aircraft are props) TO
CLTO AE 3310 Performance
=
CLmaxTO 1.21 Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
sTO = 1.66 sTOG
sTOG = 4.9 TOP23 + 0.009TOP232 sTO = 8.134TOP23 + 0.0149TOP232 These relationships are all based on regressed data and correlations. So, first determine if sTO or sTOG should be used. Example: Requirements in RFP are stated as STOG < 1000 feet and STO < 1500 feet
at 5,000 ft altitude, standard atmosphere
Using sTO = 1.66 sTO G, then sTO that corresponds with the first requirement is sTO < 1660 ft. Note this is larger than the second sTO requirement. so the second sTO dominates. Thus, we use
sTO = 8.134TOP23 + 0.0149TOP232 Now solve for TOP23 AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Now use W S
TO
W P
TO
= TOP23
CLmaxTO
ρh ρ0
Make a table. For each CLmaxTO, plot W/S vs. W/P CLmaxTO
W/P
AE 3310 Performance
W/S
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
For FAR 25, use: W S sTOFL ∝
ρh ρ0
TO
CLmaxTO
= TOP25 T W
TO
sTOFL = 37.5TOP25 For military, use: usually sTOG is specified.
K1 sTOG
AE 3310 Performance
=
ρ CLmaxTO K2
T W
W S
TO
- µg TO
-0.72CD0 Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
(jet)
K1 =0.0447
(prop) K2 =0.0376
K2 = 0.75
K2 = lp
(5+λ) (4+λ) ρh ρ0
NDp2 PTO
λ = bypass ratio 1/3
lp = 5.75 (constant speed props) lp = 4.60 (fixed pitch props)
T=P N = number of engines NDp2 PTO µg
AE 3310 Performance
= propellor disk loading = ground friction coeff.
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Sizing to Climb Requirements In order to size for climb, we must have an estimate for the drag polar. Roskam has one method for this, or use experience or another method.
Let G = (T - D) = climb gradient = ratio of vertical and horizontal distance traveled W Rearranging a familiar equation:
W S
=
T W
-G
+
T W
2
- G - 4CD0 πeAR
2/ (qπeAR)
This relates a required climb gradient to W/S and T/W. Also note that the quantity under the square root must be positive. In words, this says that the T/W must always be greater than the required climb gradient.
AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
Lecture- Traditional Aircraft Design
Sizing to Maneuver Usually maneuver requirements are specified through load factor specification at some speed and altitude. Rearranging some familiar equations:
If nmax is specified, use T W
=
qCD0 W S
+
W n2 max S
πeARq
If turn rate is specified at a given speed, use
nreq’d =
V∞W g
1/2
2
+1
then plug this nreq’d into first equation to get T/W, W/S relationshop AE 3310 Performance
Dr. Danielle Soban Georgia Institute of Technology
