1 Gary Hertzler - [email protected] Terry Schubert - [email protected]

1

Gary Hertzler [email protected]

Terry Schubert [email protected]

2

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 measurement and observation tools

Efficient Pusher Engine Cooling and Drag Reduction

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A/C operation effects cooling and temperatures

Most demanding operations for cooling control:• Leaning (LOP ref. John Deakin)

http://avweb.com/news/colums182146-1.htm

• climb speeds• climb power • OAT• let down


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Temperature measurement methods vary: probe style (bayonet or spark

plug thermocouple), location (cold or hot side)

Bayonet TC Spark plug TC


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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

• 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 Efficient Pusher Engine Cooling and Drag Reduction

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Cooling air path overviewinlet to outlet


Delta P -(pressure difference) can be measured with an air speed indicator with diffusers on pressure & static

probes

ASI

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EXPANSION is the key to coolingWithout 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

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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


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Different cooling schemes all work when properly applied


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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


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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


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Inlet location and type -There is not one universal “best kind”

• Arm pit

• Shoulder

• Submerged - divergent wall (NACA)

• Submerged - parallel wall (NACA)

• Pitot / ram (P-51)

• Combination


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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


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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


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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


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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


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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


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Combination


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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

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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


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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


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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


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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)


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Baffle must seal completely

• Orient fabric so it expands against a hard point

• Avoid non-reinforced baffle material in curved areas - it cracks easily


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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


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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


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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


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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


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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


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Reverse NACA ducts do not work well as outlets


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Exhaust augmentation aids cooling by reducing outlet pressure, thus

increasing Delta P higher RPM also increases cooling flow


pipes are recessed in cowl

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Exhaust pipes are recessed inside small cowl outlets to increase augmentation


compress engine components to limit curve in lower cowlelectronic fuel injection could eliminate the carburetor

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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


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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

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If only some cylinders run hot

• Alter cooling flow with containment ramps

• open duct may not be effective as closed duct


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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


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more special “last ditch” inlet devices


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Even more special “last ditch” inlet devices


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Delta P measurement probes placed on high & low pressure sides of test

article• probes must

measure static, not dynamic, pressure

• aquarium air stones are effective probes.


Documents

1 Gary Hertzler - [email protected] Terry Schubert - [email protected]