Gary Hertzleryahoo com Terry Schubert Jschuberjuno com 1
Gary Hertzler@yahoo. com Terry Schubert Jschuber@juno. com 1
Efficient Pusher Engine Cooling and Drag Reduction Overview Basic cooling design subjects inlet pressure recovery (updraft, downdraft, NACA & ram) internal cowl airflow & pressure control cylinder baffles. Drag reduction subjects cowl shape winglets main gear fairing and wheel pants Making performance observation tools measurement Efficient Pusher Engine Cooling and Drag Reduction and 2
A/C operation effects cooling and temperatures Most demanding operations for cooling control: • Leaning (LOP ref. John Deakin) http: //avweb. com/news/colums 182146 -1. htm • • climb speeds climb power OAT let down Efficient Pusher Engine Cooling and Drag Reduction 3
Temperature measurement methods vary: probe style (bayonet or spark plug thermocouple), location (cold or hot side) Bayonet TC Spark plug TC Efficient Pusher Engine Cooling and Drag Reduction 4
Spark plug TC reading varies with location & plug design • TC on cold side of cylinder will read ~ 50 -75 F colder than if on the hot side • Spark plug TC will read ~ 60 F hotter than bayonet probe • ~ temps due to plug & cylinder design Efficient Pusher Engine Cooling and Drag Reduction • Unpowered TC system must be compensated for Cold Junction Temperature (CJT) • CJT is where TC wire ends • If CJT is higher than calibration point, then add difference to gage reading 5
Cooling air path overview inlet to outlet Delta P -(pressure difference) can be measured with an air speed indicator with diffusers on pressure & static probes ASI Efficient Pusher Engine Cooling and Drag Reduction 6
EXPANSION is the key to cooling Without it, air will not enter the cowl 11 degree max included angle for EXPANSION Large inlet opening scoops without double volume expansion do not cool efficiently Efficient Pusher Engine Cooling and Drag Reduction 7
Delta P - “difference in pressure” from one side of cylinder to the other • Pressure is measured in inches of water column. • 4” water column is minimal (O-320 needs 5. 5” wc @ 2500 cu ft, O-360 needs 6. 5” wc @ 2700 cu ft) • When measured with airspeed indicator -100 MPH ~ 4” wc - 110 MPH ~ 5. 5” wc - 117 MPH ~ 6. 5” wc Efficient Pusher Engine Cooling and Drag Reduction 8
Different cooling schemes all work when properly applied Efficient Pusher Engine Cooling and Drag Reduction 9
Up draft cooling, good & bad • Good - hot air rises, NACA inlets are at higher ambient pressure due to deck angle), self regulating at various speeds), induction system stays cooler- more dense charge, case in cool blast, oil cooler in easily expanded cool air alignment, less chance of vapor lock • Bad - more prone to carb ice since carb is in cooler location Efficient Pusher Engine Cooling and Drag Reduction 10
Down draft cooling, good & bad • Good - Engine manufacturers’ standard, cools top of case, hot air bathes induction system / reduces icing chance and increases fuel economy- better vaporization, located in ambient high pressure, no increase in cross section on tandem A/C. • Bad - hot air bathes all accessories, may increase vapor lock chance, less dense induction charge, harder to get cool expanded air to oil cooler, inlets tend to be in lower ambient pressure areas Efficient Pusher Engine Cooling and Drag Reduction 11
Inlet location and type There is not one universal “best kind” • Arm pit • Submerged - parallel wall (NACA) • Shoulder • Pitot / ram (P-51) • Submerged divergent wall (NACA) • Combination Efficient Pusher Engine Cooling and Drag Reduction 12
Arm pit inlet • Good - higher pressure inlet, close to cylinder alignment • Bad - slight increase in cross section, should align major dimension with wing/not fuselage side Efficient Pusher Engine Cooling and Drag Reduction 13
Shoulder inlet • Good - high dynamic pressure inlet at cylinder blister, good cylinder alignment, no cross section increase • Bad - lower ambient pressure above wing or fuselage, need oil filler neck duct modification Efficient Pusher Engine Cooling and Drag Reduction 14
Submerged divergent wall (NACA) • Good - potentially less drag, • if bottom mounted, better main gear interference drag condition & self regulating with deck angle changes • Bad - 80% pressure recovery, entry surface & alignment more critical, attached flow entry required, not good for oil coolers due to fin density issues, expansion duct required Efficient Pusher Engine Cooling and Drag Reduction 15
Submerged parallel wall • Good - potentially less drag, better pressure recovery than the divergent submerged inlet, self regulating with deck angle changes when on the bottom, • Bad - less pressure recovery than pitot/ram inlet Efficient Pusher Engine Cooling and Drag Reduction 16
Pitot / ram (P-51) • Good - higher pressure recovery, less sensitive to construction skill, used for oil coolers • Bad - best to be out of boundary layer, greater drag, needs flow diverter if above boundary layer to reduce intersection drag Efficient Pusher Engine Cooling and Drag Reduction 17
Combination Efficient Pusher Engine Cooling and Drag Reduction 18
General guide lines • Ducts must have expansion areas immediately aft of inlet • Expansion area outlet must be at least double the inlet area • Smooth duct transitions required • No SCAT hose or sharp bends in unexpanded areas Efficient Pusher Engine Cooling and Drag Reduction 19
NACA divergent inlet guidelines • Inlet throat MUST be smooth (NO bump) • No separation allowed • Square corners needed to trip surface flow • Expansion area aft of inlet is required and must at least double the inlet area • Expanded air moves around obstructions OK • Locate inlets at high dynamic pressure areas. Stagnation points are best • Locate outlets at low pressure areas Efficient Pusher Engine Cooling and Drag Reduction 20
NACA divergent inlet inefficiencies • NACA inlets do not work well on oil coolers due to low pressure recovery • Note: break in surface at inlet duct. It trips the flow and reduces amount of air drawn into duct Efficient Pusher Engine Cooling and Drag Reduction 21
Inlet expansion area converts low pressure/ high velocity air to high pressure/low velocity • rectangular expansion duct 11 degrees max included angle • Radius corners to keep by pass air attached • Inadequately expanded air will not pressurize cowl and force required air through fins • Excess angles cause turbulence and choke inlet area reducing flow Efficient Pusher Engine Cooling and Drag Reduction 22
Expanded air passes over unbaffled fins/ enters low pressure side of cylinder while moving through shell baffles • Air transfers heat from fins • Hot air exits shell baffles through curved openings • Shell baffle keeps air moving over all the fin area • Shell opening determined by fin channel volume (see Al Coha’s article CSA April 1995, page 11) Efficient Pusher Engine Cooling and Drag Reduction 23
Baffle must seal completely • Orient fabric so it expands against a hard point • Avoid non-reinforced baffle material in curved areas - it cracks easily Efficient Pusher Engine Cooling and Drag Reduction 24
Silicone & BID baffle seals • Si-BID for highly curved areas • Aids entry/exit with curled metal lips RTV attach to fins • No need for Hi. Temp Si, GE Silicon II works, less cost Efficient Pusher Engine Cooling and Drag Reduction 25
Cylinder head shell baffle Si-RTV and BID, barrel baffle of aluminum see technique articles Jan 1993 p 5 • Saran Wrap pattern, BID & Si rolled into matrix, Saran removed from bottom side, then Si wet out BID is pressed into place to cure & trim Efficient Pusher Engine Cooling and Drag Reduction 26
Plenums may cause more trouble than they fix • Require removal for engine service • extra sealing issues • an extra part to fabricate, maintain and carry around Efficient Pusher Engine Cooling and Drag Reduction 27
Heated air exits aft toward lowest pressure area • Do not allow air to exit perpendicularly through sides, top or bottom • rooster tail increases drag & reduces prop efficiency Efficient Pusher Engine Cooling and Drag Reduction 28
Small outlet recovers velocity & energy • Outlet size is the same to 20% larger than the inlet to allowing for heat expansion • Retreating cowl surface may cause separation if surface bends in too soon Efficient Pusher Engine Cooling and Drag Reduction 29
Reverse NACA ducts do not work well as outlets Efficient Pusher Engine Cooling and Drag Reduction 30
Exhaust augmentation aids cooling by reducing outlet pressure, thus increasing Delta P higher RPM also increases cooling flow pipes are recessed in cowl Efficient Pusher Engine Cooling and Drag Reduction 31
Exhaust pipes are recessed inside small cowl outlets to increase augmentation compress engine components to limit curve in lower cowl electronic fuel injection could eliminate the carburetor Efficient Pusher Engine Cooling and Drag Reduction 32
Prop heating prevented by proper “clocking” More than the engine needs to be cool • 2 -blade props must be positioned at 1 -7 o’clock when piston is at TDC • 3 -blade props have at least one blade in heat plume Efficient Pusher Engine Cooling and Drag Reduction 33
Trouble shooting - What if all CHTs are too high? • Probably insufficient delta P - measure delta P and photograph oil flows - add more expansion • Install diffuser aft of firewall 34
If only some cylinders run hot • Alter cooling flow with containment ramps • open duct may not be effective as closed duct Efficient Pusher Engine Cooling and Drag Reduction 35
Strippers & Trippers: special “last ditch” inlet devices, may fix original design flaws • Drag increasing vortex generators (VGs), fences, diverters, boundary layer strippers add energy to re-attach flow 36 Efficient Pusher Engine Cooling and Drag Reduction
more special “last ditch” inlet devices Efficient Pusher Engine Cooling and Drag Reduction 37
Even more special “last ditch” inlet devices Efficient Pusher Engine Cooling and Drag Reduction 38
Delta P measurement probes placed on high & low pressure sides of test article • probes must measure static, not dynamic, pressure Efficient Pusher Engine Cooling and Drag Reduction • aquarium air stones are effective probes. 39
- Slides: 39