Summary. Engine Sizing & Selection. Propulsion Integration Extremely Important. Airframe Integrator s Motto. Piston Engine.

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1 Engine Sizing & Selection Copyright 2006 by Don Edberg Summary Engine Sizing & Arrangement Introduction Performance Requirements Engine Geometric Characteristics & Placement Airframe Integrator s Motto Blame it on propulsion. Barnaby Wainfan, NGC El Segundo Propulsion Integration Extremely Important A small shortfall in performance can add up to millions of dollars in increased fuel costs Airframe supplier may have to pay penalties for shortfalls Engine Choices Piston Engine Types Piston engine with propeller Turbine engine with prop = Turboprop Turbojet Turbofan (low or high bypass ratio) Pulsejet Ramjet Rocket 1

2 Piston Engines Turbojet Engine Inexpensive Best fuel economy Relatively heavy Vibration issues with intermittent combustion process Performance decrease with altitude Solved with turbocharger or supercharger Inlet Diffuser Shaft Burner Compressor Turbine Nozzle Afterburners Turbojet With Afterburner AKA reheat Pour fuel into rear of engine and burn it Get more thrust Get empty tanks fast (higher SFC) Inlet Low-Pressure Compressor Burner Low-Pressure Turbine Afterburner Flameholders Nozzle High-Pressure Compressor High-Pressure Turbine Afterburner Afterburner Fuel Injectors Low Bypass Ratio Turbofan Low-Pressure Compressor Bypass Duct Nozzle Fan Burner Low-Pressure Turbine High Bypass Ratio Turbofan Fan Burner Low-Pressure Turbine High-Pressure Compressor High-Pressure Turbine Bypass ratio = , T SL /W eng = 6 10, TSFC Dry = , TSFC WET = Afterburner Nozzle Compressor High-Pressure Turbine Bypass ratio = , T SL /W eng = 4 6, TSFC =

3 Pulsejet Engine Rocket-Powered Aircraft Limitations Performance of all engines limited by thermodynamics Exhaust temperature must not damage engine Usually run lean using excess air for cooling Turbines use active blade cooling Thrust determined by mass flow, density drops with altitude (no issue for rocket) Thrust, T, lbs Thrust vs. Speed & Altitude (left: dry; right: afterburning) Sea Level 10,000 ft 20,000 ft 30,000 ft 40,000 ft 50,000 ft Thrust, T, lbs Sea Level 10,000 ft 20,000 ft 30,000 ft 40,000 ft 50,000 ft Mach Number, M Mach Number, M Figures of Merit HP per pound (higher is better) Specific fuel consumption SFC in terms of HP or thrust per weight of fuel Typically in terms of lb thrust/(lb fuel/h) watch units for range & endurance calcs Equivalent for propped engines (delivered power per fuel weight) Lower is better Power Available and Power Required, Power Available vs. Power Required, Prop Aircraft horsepower V for minimum Power Required Power Available Power Required True Airspeed, knots Vmax 3

4 Power Available vs. Power Required, Jet Aircraft Engine Selection Criteria: Cruise speed Cost Economy (fuel, maintenance, etc.) Redundancy etc. Engine data on Internet, AV Week Source issue, Janes, etc. Requirements Size Engines Constraint diagram provides required T/W Estimated weight W provides T = (T/W)W Item Takeoff Length Minimum Rate of Climb Sustained Turn Specific Excess Power Maximum Speed Civil Military () Engine Selection Rubber engine Use an engine deck for performance prediction (ref: AIAA competition history) High cost of engine development Existing engines Search information sources for off-the-shelf engines with sufficient performance No engine development costs Already in maintenance stores? Scaling An Existing Engine L = L actual (SF) 0.4 D = D actual (SF) 0.5 W = W actual (SF) 1.1 SF = scale factor (Raymer ) Engine Geometric Characteristics (Raymer) Non-afterburning and afterburning sizing data equations , Raymer Diameter, engine length,weight, SFC all are functions of takeoff thrust T and Mach no. M Other inlets and ducts as needed Boundary layer diverters Afterburners? Add to your aircraft drawing 4

5 Engine Nacelle Drawing Integration with Airframe Thrust or power level picks or scales engine Inlet air duct must be sized for airflow in ALL conditions Fuel lines Cooling Engine-driven accessories Installation and removal clearances, mounting structure Engine Placement Choices Under wing on pylon (traditional) Aft fuselage side-mounted engines (DC-9, 717) Center fuselage engines (DC-10, 727) Over wing (Honda jet) Other configurations (White Knight, etc.) Engine Placement Trades Locate nacelle(s) to be above or below wing wake Consider structural weight of pylons, etc. FOD ingestion, etc. Weight & balance considerations Wing location, fuselage upsweep, etc. Service & maintenance Local Flow Effects Angle nacelle for local flow direction (calculate upwash or downwash as needed) Example: B-717 engines at rear of fuse are angled upward Inlets Very important to engine performance Must provide enough air in all conditions Must diffuse (slow down) air to M = 0.4 ~ 0.5 Want as much pressure recovery as possible (best >90%) Geometry affects drag of aircraft Upwash Downwash 5

6 Inlet Types NACA duct (for aux air) Conical (SR-71) Normal shock or Pitot (airliners) 2-D Ramp (F-14, F-15) Inlet applicability summarized in Raymer Fig Normal Shock Inlet Geometry in Raymer Fig Lip radius very important No shock if subsonic Rotate front face or entire engine to account for up/downwash from wing Other Inlets May be used for subsonic or supersonic Often use variable geometry Adjust geometry so shock is swallowed or minimized Mechanism must be reliable Isentropic flow desired, but typically get some oblique shocks Raymer Figs to Location of Inlets/Nacelles Many choices (Raymer Fig ) Nose, chin, side, over/under wing, over/under fuselage, wing LE, etc. Want clean air to be ingested Minimizing length minimizes losses CG considerations OEI control ( one engine inoperative ) S-duct vs. Straight (L-1011 vs. MD-11) Internal separation in S-duct vs. structural weight issues with pylon mount Servicing buried engine must be more difficult Inlet Design Capture area estimated using mass flow Estimate area using Raymer Fig m If mass flow not known, rule of thumb is: mass flow = 26[D(ft)] 2 = 127[D(m)] 2 where D is front face diameter. Better to use isentropic compressible flow per Raymer equations 10.16, 10.17,

7 Boundary-Layer Air F-35 Has No BL Diverters Need to avoid BL air for better performance Use a diverter (Fig ) Diverter must be integrated with inlet location Diverter must work effectively at all angles of attack Space for BL air to bypass engine Nozzle Integration Nozzle must (or should) expand exhaust gases and accelerate them Depends on mass flow: often use variablearea nozzle Affects drag Lots of info Raymer pp Cooling also required, Fig Installed Jet Thrust Manufacturer data uses perfect inlet, exhaust, etc. Losses due to: actual inlet, air bleed, power extraction, actual exhaust nozzle, air temperature Aerodynamic losses: drag of inlets, nozzles, trim drag due to change in thrust Brandt suggests: installed T = 0.8T mfr, installed SFC = SFC mfr /0.8 May be offset by engine improvements Engines Mounted on Fuselage Propellers 7

8 One-Bladed Propeller Propeller Types Propellers Prop Blade Angles Prop s Helical speed = (V tip 2 +V 2 ) 1/2 = (ω 2 R 2 +V 2 ) 1/2 Inflow angle changes with velocity so variable pitch props used for maximum efficiency Propeller Blade Angles Variable Pitch Propeller 8

9 Prop Efficiency Efficiency typically depends on advance ratio J and power coefficient C p J = V/nD C p = P/ρn 3 D 5 Can get propeller maps and find sweet spot Corrections for fixed pitch Raymer Fig Power coefficient Propeller Efficiency Chart Propeller efficiency depends on: Power level RPM Blade pitch β Dimensionless numbers are advance ratio J and power coefficient C p Choose pitch and RPM for max efficiency η ( eta ) Advance Ratio J Prop Configurations Pusher allows shorter fuselage = less drag Pusher reduces efficiency because of disturbed airflow over prop (= noise) Longer landing gear required Other Propeller Notes Wing-mounted engines require larger tails for OEI control Rubber piston engine equations in Raymer Table 10.3, 10.4 Cooling vitally important Fuel System Fuel Considerations Tanks contain fuel Types = discrete, bladder, integral Volume depends on required fuel volume (approx. density is 7.5 gal/ft 3 ) Density varies with temperature (Raymer Table 10.5) Stow in wing or fuselage or tail or all Fuel CG must average near aircraft CG Calculate CG movement, show on CG plot (Raymer Fig ) Pumps needed in certain cases 9

10 CG Travel Diagram Valuable Info in Raymer App. E Contains curves from engine decks Based on Mattingly et al Aircraft Engine Design (good ref.) 10