Writing of the second edition of Aircraft Engine Design began as soon as the first edition was published in 1987. The AIAA Educational Series and editorial staff provided substantial support for the publication of the second edition.
Nomenclature (Chapters 4-10 and Appendices)
J = ratio of water flow to crossflow m o m e n t u m flow or dynamic pressure, Eq. k j, k-j = forward, reverse rate constant for reaction j t h, Eqs. MFp = static pressure mass flow parameter, Eq. nci = corrected mass flow rate at station i, Eq. Nci = corrected engine speed at station i, Eq. 5.24) N/4 = high pressure spool rotation speed NL = low pressure spool rotation speed.
Engine Cycle Design
1 The Design Process
- Designing Is Different
- The Wheel Exists
- Charting the Course
- The Atmosphere
- Compressible Flow Relationships
- Looking Ahead
- Example Request for Proposal
- Mission Terminology
In short, the material in this textbook provides a realistic presentation of the entire design process. And this is what happens in the case of the AAF engine design as this handbook unfolds.
2 Constraint Analysis
Concept
Excess current is required to increase the aircraft's energy altitude, and the rate of change is proportional to the amount of excess current. The additional drag is normally confined to the intake and exhaust nozzle surfaces, but under unfavorable circumstances can be found anywhere, including adjacent fuselage, wing and tail surfaces.
In this case the limitation curve is obtained from Eq. 2.12) including the location of the minimum Tsr/Wro. CONSTRAINT ANALYSIS 29 where kro is a constant greater than one (generally specified by the relevant flight regulations) and VSrAU~ is the minimum speed at which the airplane flies at CL m,x, then.
V ,ALL (W O)
Preliminary Estimates for Constraint Analysis
Before a constraint analysis can be made, preliminary evaluations of the aerodynamic characteristics of the airframe and thrust transfer of the installed engine are required. Typical ranges for CL max divided by the cosine of the sweep angle at the quarter chord (Ac/4) are shown in Table 2.1, which is taken from Ref.
Cmin
Example Constraint Analysis
Chapter 1 air-to-air (AAF) aircraft Request for Proposal (RFP) performance requirements that could limit the aircraft's allowable take-off thrust load (TsL/Wro) and wing load (Wro/S) are listed in the table . 2. El. The calculations and data required to construct each boundary in the full AAF constraint diagram are contained in Sect.
This often happens when the RFP is the result of thoughtful consideration and balancing of requirements. The same calculations can be performed and the boundaries plotted much more quickly using the Constraint Analysis section of the AEDsys software included with this textbook. An additional benefit of the software is that it provides a checklist of the quantities required to calculate any chosen restriction limit.
A conservative estimate of ~L for the A A F is obtained by evaluating C o at 0.8 of the landing lift coefficient (CLmax/k2o) and d by assuming (Con - IZBCL) = O.
3 Mission Analysis
Concept
The next material could very well be called the "thermodynamics of flight". because it largely has to do with how the engine's thrust work is used by the aircraft. From a mathematical point of view, a good integration of Eq. 3.3) requires knowledge of the behavior of thrust-specific fuel consumption and "instantaneous thrust load". 3.5) shows that u is the fraction of engine thrust dissipated, so (1 - u) must be the fraction of engine thrust invested in mechanical energy.
Note that when T = D + R and u = 1, all the thrust work is dissipated, and the type A analysis yields no useful results.
- Case 5: Constant Altitude~Speed Cruise (Ps = O)
Otherwise, the integration of Eq. 3.10) can be achieved by breaking the bone into several smaller intervals. Special care must be taken in applying Eq. 3.20) because h and V are truly independent of each other until the flight path is chosen. Also, as already noted, the term [(Co + COR)/CL](~/Ot)(Wro/TSL) represents the fraction of the engine thrust that is dissipated, and {1 - [(Co + CDn)/ CL] (~/oO(Wro/Tsl.)} is the fraction invested in mechanical energy (i.e. potential plus kinetic) or energy height, where u can be obtained from Eq. 2.20 and M is an average value during take-off.
This situation can be treated almost the same as case 5, except that L = n W. The duration of the rotation can be shown, with the help of Eq. and CD is calculated from Eq. Because the turnaround time is usually a small fraction of the mission, the values in Eq.
Aircraft Weight and Fuel Consumption Data
A preliminary estimate of the ratio of aircraft empty weight (WE) to take-off weight (Wro) is required to solve for the aircraft take-off weight as indicated by Eq. MISSION ANALYSIS 71 must be estimated for each phase/segment of the mission to determine its weight fraction [see Eq. The installed thrust specific fuel consumption of an aircraft engine varies with Mach number, altitude, engine type and throttle conditions.
The following TSFC models (expected for advanced engines in the 2010 era and beyond) are provided to aid these preliminary estimates.
Example Mission Analysis
Because the takeoff payload fraction (Hip~WTo) for fighters is usually in the range of the AAF value of 0.111, it shows that it is quite a workhorse. In addition, the results of the mission analysis provide a better estimate of the takeoff and landing weight fractions (see Table 3.E3). If the difference were greater, a reconsideration of the pressure load would be appropriate, but this small change does not warrant reselection.
The AAF's RFP mission is flown in this section to find the weight fraction of each mission segment and stage.
Supe -
The weight fraction of this mission segment is calculated according to the minimum time-to-climb path depicted in Fig. The initial, final, and average Mach number of each interval and h = 30,000 ft are used in the climb/acceleration weight fraction calculation method of Sec. The initial, final, and average Mach number of each interval and h = 30,000 feet are used in the climb/acceleration weight fraction calculation method of Sec.
Applying this equation to the subsonic portion of the minimum fuel-to-climb trajectory in Fig.
4 Engine Selection: Parametric Cycle Analysis
Concept
First, analysis of engine performance at out-of-reference conditions, commonly referred to in the literature as "off-design" cycle analysis, cannot begin until the reference point and engine size have been selected in some way. A reasonable expectation after completion of the parametric cycle analysis is that the most promising type of cycle has been identified and the possible scope of each design choice has been bracketed. To provide the most powerful tools for parametric cycle analysis, this chapter will develop relationships for one of the most complex and flexible engine cycles known: the mixed flow, afterburning, two-spool cooled turbofan engine with venting and power extraction.
From this point on, the nomenclature has shifted entirely to the pushing nomenclature of the symbol table and Ref.
Design Tools
The subroutine FAIR has been developed to calculate the temperature dependent properties given the fuel/air ratio f and one of the following properties: T, h, Pr or ~p. For the case of the calorically perfect gas, we assume a reference value of zero for the enthalpy at an absolute temperature of zero. It should also be taken into account that for the case of the calorically perfect gas the relations of Sec.
For the calorically perfect gas, the entire r component, except that of the burner and backburner, becomes ratios of the total temperature.
Finding Promising Solutions
To use the calculated results, initial targets for S and F/rho must be set as described below. Clear objectives can be set for S because of the initial expectations already established by the mission analysis. Since the physical size of the engine (ie design rho) is unknown at this point, there is no set target for F/rho.
A good general rule here is to focus on increasing F/rho for those legs that formed the boundary of the solution space in the constraint analysis.
Example Engine Selection: Parametric Cycle Analysis
The parametric computer program O N X accompanying this textbook was used to study 60 different design point combinations of the design parameters Zrc, zrf or Tt4 and Tt7 for the three flight conditions shown in Sec. The required uninstall specific fuel consumption (S) is plotted to allow easy comparison with the estimated value of the uninstall fuel consumption. Consider the sensitivity of thrust specific and thrust specific fuel consumption to changes in flight conditions and engine design choices for a mixed flow turbofan engine whose component performance design values match those of the printout shown in Sec.
Fortunately, this insensitivity is not typical of all results of the analysis.
5 Engine Selection: Performance Cycle Analysis
Concept
In the absence of actual component hardware in a preliminary engine design, simple component performance models are used to provide preliminary estimates of engine performance. In the next section, the relationships for determining the engine performance of the engine cycle, whose parametric performance (reference point) was discussed in Chapter 4, are developed. Now the task is to develop design tools in the form of equations and solution procedures of them that will allow the determination of the component and therefore the performance of the engine in any condition.
Design Tools
The total enthalpy ratio of the ideal turbine (rand) follows directly from its definition. 4.9d) can be solved for the enthalpy ratio of the high pressure turbine (rtn). The total enthalpy ratio of the ideal turbine (~Li) follows directly from its definition: 4.9e) can be solved for the low pressure turbine enthalpy ratio (rtD. For a given value of the Mach number at station 6 (M6) and the constant values of the area ratio ( A 4 . 5 , / A 6 ) and the turbine efficiency (rhD, there is only one set of total properties of low pressure turbines (rtL, rCtL) that satisfies both Eqs.
ENGINE SELECTION: PERFORMANCE CYCLE ANALYSIS 155 The total pressure ratio of the mixer is the product of the friction loss (7rM max) and the mixing loss (rrM ideal) or.
20 Fig. 5.5
Component Matching
Jr or r vs rhv'CO/8) of every part in the engine is a "free" by-product of the calculations outside the design, even if the engine cycle is arbitrarily complex. Engine speed (N) is not included in the out-of-design equations and is only needed when the efficiency of the rotating components (fan, compressor, or turbine) varies significantly over the operating speed (N) of the engine. Part of the high altitude/low Mach number area is excluded from the operational range of many aircraft due to the low dynamic pressure and high lift coefficient (CL) required to sustain flight.
5.10, and the variation of the compressor pressure ratio at full throttle with changes in the flight condition, as in Fig.
Example Engine Selection: Performance Cycle Analysis
In the search here, the influence of each of the design parameters on engine performance at off-design critical flight conditions for the mission is determined. To find these limits for the baseline engine, a performance analysis is performed using the Engine Test function of the AEDsys program at full throttle. With the basic engine performance in hand, the engine design choices are now systematically varied to achieve the performance of other benchmark engines.
The fuel consumption of these candidate engines is compared to that of the base engine to find the engine with the lowest fuel consumption.
