Air Cooled VS. Water Cooled

[1:1 Scale]

I think you're muddying the water (as usually seems to happen in these discussions) by introducing one particular type of engine that hasn't (AFAIK) been proven to make the advertised power, runs through a gearbox, and has (IMO) a far from optimum cooling design, and using that as your measuring stick. Personally, with the exception of the increased frontal area, I don't think you would loose much more speed if you took the Egg radiators and put them OUTSIDE of the cowling. Maybe I'm wrong, but I think at the very least, you're comparing an 85-90% optimized cooling system to a 40-50% optimized cooling system. Just like saying aircooled radials are cleaning up at reno and forgetting that they're double the displacement.

Let's try to keep on the subject of air vs. liquid cooled and not turn this into another Lyc. vs. Egg/Sube thread

 

[gmcjetpilot]

Water planes? 90 turns? Genius of air cooling?

How could there be a better comparison? Same airplane, same cowl, and right on topic.......air VS water cooling. What other comparison could be more valid?

ha-ha. They don't call them AIR planes for nothing. Otherwise they'd be WATER planes, , I'm kidding, LOL.

Seriously Todd like your radiator. That 90 degree turn will make the air mad, but its such a large core you should be OK. I think its cool (pun intended). To reduce cooling drag you need to try to accelerate the air again before it leaves the plane, but get that baby flying first!

The Egg folks and Tracy Crook prefer the small densely packed (4-5" thick) radiators placed in front of the engine, which restrict the air-flows out the back. These generally cool quite well at high speeds due to the high pressure of 200 mph air, but tend to be inefficient at slower (climb, during taxi) when the pressure is lower, and they present close-to-the-same drag profile as the air-cooled heads do.

A minor point some Egg and Mazda's over heat in climbs or high power. As far as pressure the problem is 200 mph has no pressure recovery. You need Delta Pressure to cool. The way to get pressure is slow the air down. That takes room in the form of a diffuser like the bell of a slide trombone, a nice curved shape going from small to larger area. Bernoulli again, slow air pressure increases. A lot of that 200mph that smashes into the up front radiators just builds up and spills out the front of the cowl like a bow wave of a ship. That bow wave is a form of drag.

The other school (where I'm personally leaning, btw) places a much larger, full-size aluminum automotive radiator that is less densely packed and thinner (maybe 1.5-2" thick) placed either parallel to the engine, or beneath it, or both.

Putting Heat Exchanger (HE) under the engine is fine, but a little crowded? Also the 90 degree turn issue. One I like is the belted air power guy's method (V6 chevy or buick, belt PSRU). They put the radiator up against the fire wall at right angles across the whole firewall. It uses the cowl as a pressure plume diffuser. I am not holding this up as an ideal, just another idea. The picture does not show the "shelf" baffle; I believe they divide the cowl into upper and lower hi/lo pressure plenums. Its easier to make a sharp turn after going though the radiator than before. I can see ways to improve this but it would take more duct work.

The issue is turning air a sharp 90 degrees (especially fast moving air not slowed down). It has terrible pressure recovery, but I do understand a necessary evil to make it fit. The way to do it takes room and proper ducts and diffusers. You really want to air to go straight in and straight out, no turns. You might say: "But LYCS have air do a 90 degree turn". Well kind of true, but there is room for a proper upper HIGH pressure plenum and air FLOW THROUGH the engine to a LOW pressure plenum. The genius of having the engine and "radiator" all-in-one as integral package, compact and light. Also you only need about 3 in-H2O to flow the wide optimally spaced fins. Of course the oil cooler is there as a liquid cooling supplement.

Power Sport may be had the best idea. The pic I assume does not show (complicated) baffling. The cowl has one chin scoop for all air, induction air for the engine and cooling air for the radiator. It looked slick. The old aerodynamic rule, "If it looks good it probably is good".

George,
I did some snipping here but I think you missed what Ross was talking about the 50 PSI comment. He was talking about pressurizing the SYSTEM to 50 PSI then the boiling point is significantly raised. They were also running a high level of EG mix which raises the boiling point even more.

Thanks Bill I got the 50 psi was to raise the boiling point. Also I believe it is to drive the pressure loss through a very very long system of hoses. The stock P-51 water pump is fine; I was talking about stock Mazda or Subaru water pumps driving a belly radiator at a lower elevation, many feet aft, verses a car radiator right in front of the engine, with foot long hoses. I agree about flow; that was the point I tried to make. I discussed this with the "Liquid Cool Jugs" guys. They use a "Formula One" water pump. Yes HP is used to pump water. That should be put in the Pro column for air-cooled engines. You say 5 HP? Interesting. Drag racers use electric fans and electric water pumps for their 1/4 mile blasts to reduce hp loss.

Speaking of P-51, here is a 30, December, 1942 war time report on

"Final Report On The Tactical Suitability Of The P-51 Type Airplane".

Radiator was mentioned 7 times! with comments like, "With the exception of the radiators, the airplane is completely satisfactory," showing even North American had a challenge with the radiators. So don't give up, it might take a few iterations.

http://www.wwiiaircraftperformance.org/mustang/p-51-tactical-trials.html

Great link on radiators and how Reno guys do it with radiator water spray bars (evaporate cooling) and ADI or I call AWI (alcohol water injection or "A-wee"). Alcohol and water injected into the induction to lower temps and increase detonation margins. They do this on turbines as well but for a different reason, but with the same result, more power. On a jet or turbine it lowers temps to allow more power with out exceeding thermal limits.

http://www.aafo.com/racing/news/98/intrepid.htm

 

[rtry9a]

GMJET...

All 90 degree turns are not equal- imho the worst scenario is the Lyc or Egg radiator layout where the high pressure fast airstream hits the flat plate head/radiator perpendicular to the airflow where it produces a lot of turbulence and drag. From there the hot air is left to go anywhere inside the cowl until it finds some space to exit. The better designs have ducts to direct the air out and should reduce some of that drag.

A proper water-cooled air duct will have a correctly sized and shaped scoop that minimizes drag and allows only the amount of air needed to cool adequately (size is often the result of numerous trials). The cool air then enters an expanding cone to slow the air flow, then after it has slowed down and reduced its presssure, it turns 90 degrees in a wedge diffuser and into the "trumpet" shaped radiator duct to disperse the cool air through the entire radiator's surface, all of which is designed to reduce turbulence and drag. The warm air then enters the exit area (either ideally into an expanding cone to speed up the hot air, or if space is limited, released out through louvers on the side or bottom of the cowling next to the radiator.

The big advantage to the P51 style belly scoop is that it provides space for the rear ducting and, if desired, an exit air flap (and it looks cool). The louvers also work well when they exit into a low pressure area- ideal with the side radiator arrangement. The downside to the belly scoop is that it also involves long coolant lines (adds an additional failure mode with slightly higher chance of breakage) and extra weight.

One nice option the rotary guys have (because of the engine's narrow shape profile) is that it is possible to offset one style of engine mount (not the engine) which allows side placement of either the oil cooler or the water radiator and a bottom placement for the other, rather than forcing both into the same space shared with exhaust and intake manifolds. The wide cowl cheeks common to the horizontally opposed Lycomming cowls are not really needed and can be replaced with something more aerodynamically shaped if desired.

 

[rv6ejguy]

It takes very little water flow to cool an engine and speculating that the OE water pumps are not up to the task is just that, speculation. I've got dozens of feet of tubing and hose on my installation, including a rad mounted aft of the baggage bay with 1/2 plumbing. As soon as I open the water valve to that HE, I see a drop in CLT of about 8C. Must be pumping coolant just fine through 18 feet of small tubing.

I've used OE pumps on every turbocharged racing engine I've ever built pushing specific outputs of 200hp/ liter- no problem.

Electric water pumps are now used in many forms of racing- not just drag racing and some OEMs are using them on production models now.

Water has 24 times the thermal conductivity of air and 816 times its mass per unit volume so it is possible to sink heat off surfaces at far higher rates than with air using very low flow rates. We can then sink heat from the water with a much more efficient device (the radiator) which divides the water into many thin sheets in the tubes and expose these to turbulent airflow via the fins which have very high surface area. The power densities of liquid cooled engines exceed air cooled engines by several orders of magnitude.

The pressure plenum can be a good idea maybe except that it suffers from the same issues as air cooled engines with regards to parasite drag of the air passing over bumpy obstacles and changing volumes and velocities as it progresses down towards the rad. Again, sharp turns near the firewall and no guide vanes give high drag and turbulence. Some of these work ok only because they have massive rads.

A dedicated duct which progressively slows the air and increases its pressure at the rad face while maintaining smooth flow is far superior from a cooling/drag standpoint- if you can fit the required length in the airframe. That is the hard part on many aircraft.

Any of you engineering types, feel free to correct me.

 

[Rotary10-RV]

Some yes some no

ha-ha. They don't call them AIR planes for nothing. Otherwise they'd be WATER planes, , I'm kidding, LOL.

Seriously Todd like your radiator. That 90 degree turn will make the air mad, but its such a large core you should be OK. I think its cool (pun intended). To reduce cooling drag you need to try to accelerate the air again before it leaves the plane, but get that baby flying first!

A minor point some Egg and Mazda's over heat in climbs or high power. As far as pressure the problem is 200 mph has no pressure recovery. You need Delta Pressure to cool. The way to get pressure is slow the air down. That takes room in the form of a diffuser like the bell of a slide trombone, a nice curved shape going from small to larger area. Bernoulli again, slow air pressure increases. A lot of that 200mph that smashes into the up front radiators just builds up and spills out the front of the cowl like a bow wave of a ship. That bow wave is a form of drag.

George, getting the air to accellerate again out of the cowling in a cowl dumped radiator system is EXACTLY the same as the air dumped through the plenum of a Lyc. Some of the racers have started streamlining the exit (Nemisis NXT) but most just leave the cowl slightly below the belly and generate a pressure difference that way. I'm sure you know this. Something else that should be mentioned the 90? turn comment is a misnomer in that K&W found that a wedge defuser works within 2-3% of a perfect bell defuser. The thing is that many people found out the hard way that the BACK of the radiator needs to be unrestricted. Many of the Mazda guys are getting it right BTW. There are several Lancair ES 20Bs flying and cooling just fine in climb thank you. The backward flying guys are also seeing success. The long EZ's cooling well too. No RV-10s so far but it will happen.

Putting Heat Exchanger (HE) under the engine is fine, but a little crowded? Also the 90 degree turn issue. One I like is the belted air power guy's method (V6 chevy or buick, belt PSRU). They put the radiator up against the fire wall at right angles across the whole firewall. It uses the cowl as a pressure plume diffuser. I am not holding this up as an ideal, just another idea. The picture does not show the "shelf" baffle; I believe they divide the cowl into upper and lower hi/lo pressure plenums. Its easier to make a sharp turn after going though the radiator than before. I can see ways to improve this but it would take more duct work.

It is interesting that the radiator setups that you have shown have been the least successful. The firewall mounted radiator has several tremendous drawbacks. First the downwind side is always restricted. Second the area in which the air can flow back to exit speed is very limited. The PowerSport aircraft worked well in most every area except 2. Their EFI was never optomised and the engine got poor mileage, and the firewall radiator system never cooled well. The belted air package worked OK, but the radiator was huge for the engine size and the V6 Jess was using was decidedly low-fi. Not a knock, rather a good choice on his part.
]

Interestingly there are tests that the NACA did where they ducted the air through a 180? turn and the difference in drag between that and a flat plate radiator was bearly measureable, IF the exit was in a low pressure area.

What has had to be RE-LEARNED is the layout of radiators and airflow for best effectiveness. The mention of the P-51 radiator system is interesting because for the D model and later they finally went to a truly bell curve defuser and exit, along with a better radiator and cooling after using that layout was never a problem. That change was late in the war, early 1944 I believe. Even in the later models where they went back to an Allison with 1700 HP cooled well. It was with that system that people were talking about the system actually generating thrust. That was never proved but drag was certainly lessened. We are learning about how to do cooling at 200 MPH, instead of 400 MPH so we have to deal with the lessened dynamic pressure. The latest issue of Contact! shows a Glassair with V8 they used a belly scoop and never had cooling trouble. I am sure they could improve the drag though. The issue of water FLOW pressure rather than system or cap pressure is something that has been being tested by WC guys. There are several cars with separate pumps and these have been tested in a closed loop system for both HP requirements and pump pressure. For reference the pressures being seen at the pump exit are maxing at about 24-28 PSI at the exit for the Mazda rotary with an obvious pressure drop through the block. that is at a maximum output of about 51 gpm, just short of cavitation. I'm not sure what the max would be for a Chevy. There are big arguements made about contact time vs. mass flow. Some places a huge arguement will start when someone talks about using an electric water pump. Anybody familiar with a power equation can easily understand that these pumps are not going to pump anywhere near the same amount of water as any decent mecanical pump, but people have had success with them. In fact both Ferrari and BMW have either experimented with them or actually have electric pumps in their roadcars. The only thing that is clear is that we still don't really know everything about how these systems work.
Bill Jepson

 

[Mike S]

Lots of good tech info in this site.

Cars, not airplanes-----yes, but hey, delta T dosnt know the diff.

http://www.stewartcomponents.com/Tech_Tips.htm

Most interesting tidbit for me was low pressure (old school) radiator caps opening at high engine RPM, specially when installed on radiator tank fed by pump output.

Good read.

 

[rv6ejguy]

Many V8s in the 400 hp range are cooled perfectly well with electric water pumps rated at about 35 GPM. The Subaru pumps only flow around 23 GPM at 4500 rpm. I tested a Toyota 2TC turbo (245hp) on the dyno and we saw no difference in water jacket temps running the water flow right down to 12 GPM. Most OEM pumps are total overkill for hp levels far above stock.

Higher water flow rates will reduce coolant temps theoretically by maintaining the tube exterior at a higher temperature near the end of the pass but in the real world, contact time of the air with the fins is far more significant to use air mass flow efficiently. This is again because we are exchanging energy between two mediums with very large differences in density. This is clear when we instrument the rad for inlet and outlet temps and also look at air inlet and outlet temps. The air picks up a lot of heat due to its low mass compared with water and the water loses very little temp passing through the rad- 10-15C is common.

Some of the electric pumps designs are considerably better than the mechanical ones so the input hp vs. flow is more favorable.

 

[SvingenB]

Even in the later models where they went back to an Allison with 1700 HP cooled well. It was with that system that people were talking about the system actually generating thrust. That was never proved but drag was certainly lessened.

A typical F1 car at 800 hp dissipates heat through the coolant radiators at 160 kW (214 hp) and 120 kW at the oil coolers (160 hp). So approximately 374 hp are just wasted through the rads. They do not produce any thrust, even though they are optimized for low drag. To produce thrust they would need to expand the air, and when the air only increases from 20 to 60-80? deg C typically, there really is not much expansion. With a perfect design, maybe it would be possible to create 0 drag?

I think BMW use electrical power for almost all secondary units. It is more or less a hybrid engine and use only the required power, and even shuts them down when they are not needed. This is done to reduce fuel consumption.

 

[Rotary10-RV]

And that's good!

A typical F1 car at 800 hp dissipates heat through the coolant radiators at 160 kW (214 hp) and 120 kW at the oil coolers (160 hp). So approximately 374 hp are just wasted through the rads. They do not produce any thrust, even though they are optimized for low drag. To produce thrust they would need to expand the air, and when the air only increases from 20 to 60-80? deg C typically, there really is not much expansion. With a perfect design, maybe it would be possible to create 0 drag?

I think BMW use electrical power for almost all secondary units. It is more or less a hybrid engine and use only the required power, and even shuts them down when they are not needed. This is done to reduce fuel consumption.

The other interesting fact is that of the available BTUs in the fuel 50% of the total energy available is going out the exhaust in pressure and heat! Turbo-compounding could recover a great deal of power too.
Bill Jepson

 

[gmcjetpilot]

Conservation of energy

Electric water pumps, don't they take AMPS or power to run? Right. That power comes from the alternator, which takes HP to turn. The more power required of the alternator, the more power it takes to turn it. Darn that no free lunch.

I wounder how much more (if any) efficent an elect pump is verses a mechanical. I am guessing not much, but it depends on the design and gearing of the mechanical pump. You don't see mechanical fans any more do you?

The mechanical pump runs all the time needed or not. The elect pump can be cycled on and off. Also for race guys they can cool between heats with the electric pump. I think Pro Comp drag cars run their pumps off battery only. Not practical for street or plane.

Interesting discussion. Great info!

Looking at summit racing and the many typical elect water pump, 35-37 gph at about 5.8 - 8 amps is about average. The big block / high hp application elec water pumps are 55 gph at 8-10 amps! Enough for a 600 hp oval track car they say. They go for about $350 to $500. Some say for competition only. It looks like some of the street mild elect pumps are just 15-20 gph.

 

[rodrv6]

I don't think there is any problem with using the stock, belt driven, water pump on an auto engine with remote radiators. One of my cars is a Pontiac Fiero which has the engine mounted at the rear wheels. The radiator and heater core are mounted in the front of the car and fed through tubes that run the length of the chassis and include a number of bends. The tube diameters are no different from the typical radiator and heater hose diameters found in front engine cars. The water pump on the engine is the same pump as the one used on the same engine in a front engine car and coolant flow is not a problem. My Eggenfellner Subaru, even with its three radiators and a heater core, does not have nearly as far for the coolant to travel in the cooling loop as my car. No need for an electric pump.

 

[rv6ejguy]

I don't think there is any problem with using the stock, belt driven, water pump on an auto engine with remote radiators. One of my cars is a Pontiac Fiero which has the engine mounted at the rear wheels. The radiator and heater core are mounted in the front of the car and fed through tubes that run the length of the chassis and include a number of bends. The tube diameters are no different from the typical radiator and heater hose diameters found in front engine cars. The water pump on the engine is the same pump as the one used on the same engine in a front engine car and coolant flow is not a problem. My Eggenfellner Subaru, even with its three radiators and a heater core, does not have nearly as far for the coolant to travel in the cooling loop as my car. No need for an electric pump.

Ditto the old mid engined Fiat X/19 which use the same pump as the front engined 128. No issues with cooling despite a good 16-18 feet of rad tubing.

I'm not saying that electric water pumps would be a good idea on an aircraft (maybe with twin alternators and batteries) but they do have continuous duty models now designed to run for years. The wattage required to pump say 20 GPM confirms that the parasitic losses on an engine driven pump are also very low. Some SCCA racers have switched to electric pumps now because overall losses are lower.

 

[elippse]

I think you fellows should do a lot of looking at back issues of liquid-cooling on the Contact! magazine web-site, www.contactmagazine.com. There was one on the P-51 that addressed the cooling drag/thrust issue and Charlie Airesman's article on the exhausted-augmented radiator on his EZ that would not overheat on the ground. Did any of you see the compact exhaust-augmented cooling duct I designed for Kevin's Relentless which he raced last year at Reno? Several people said they saw it at Oshkosh.

 

[Rotary10-RV]

Self made?

I think you fellows should do a lot of looking at back issues of liquid-cooling on the Contact! magazine web-site, www.contactmagazine.com. There was one on the P-51 that addressed the cooling drag/thrust issue and Charlie Airesman's article on the exhausted-augmented radiator on his EZ that would not overheat on the ground. Did any of you see the compact exhaust-augmented cooling duct I designed for Kevin's Relentless which he raced last year at Reno? Several people said they saw it at Oshkosh.

Paul,
Did you develop the sizes and shapes of the exhaust augmenter yourself, or did you use the NACA data to work from?
Bill Jepson

 

[SHIPCHIEF]

So....Back to radiator placement and cooling outlets

I'm wondering if I've got everything all gummed up.
I have the A/C condensers modified for 3/4" hose. Both are mounted on the right side. I couldn't use the traditional rotary mount behind the prop because the turbo takes up most of the room on the left side. Also radiated heat from it is a factor.
I plan to build a set of air inlet diffusers to pressurize the entire cowl, then control the heated air as it leaves the water and oil coolers.
I understand that liquid cooled engines need about 70% more cooling exit area than air cooled engines, so I plan to use the lower exit for engine exhaust and oil cooler discharge air. The water radiators on the right side will discharge heated air into the low pressure area behind the leading edge of the cowl cheek, and probably have exit scoops. I wonder what the plume drag will be like.
I didn't want any engine compartment air escaping down the left side of the fuse because the cabin air inlet is on the left side and I don't like to inhale exhaust fumes (in case of a leak).
My link takes you right to pics of the plane. Any constructive comments are welcome.
I haven't done alot of calculation to get here, I mostly read and observe, then try to build "TLAR" with an eye for safety and failure modes.
I'm trying to leave out anything that is not needed. You know "simplicate and add lightness" but it's not easy.

 

[rv6ejguy]

Looks pretty cool! A good heat shield around the turbine housing to protect the cowl should be used. The IR coming off the housing at full chat is extreme. The turbo should give you pretty awesome performance.

The GM evap cores work and are cheap but they have really high drag according to the flow bench tests. Might as well try what you have and see how this setup works. I'd guess that ground cooling will be poor.

You only need the large exit area on the ground and in the climb. In cruise you are just causing higher drag by flowing a ton of air through the heat exchangers you don't need- so your idea to control exit air is a good one.

 

[gmcjetpilot]

Yes I saw it, very cool

I think you fellows should do a lot of looking at back issues of liquid-cooling on the Contact! magazine web-site, www.contactmagazine.com. There was one on the P-51 that addressed the cooling drag/thrust issue and Charlie Airesman's article on the exhausted-augmented radiator on his EZ that would not overheat on the ground. Did any of you see the compact exhaust-augmented cooling duct I designed for Kevin's Relentless which he raced last year at Reno? Several people said they saw it at Oshkosh.

I read the EAA sport aviation article of the longEZ with the Subaru and the exhaust augmentor and a long long drive shaft and support. That was pretty cool (no pun intended). The level of workmanship and fabrication was on the high / difficult level. That leads me to my long held belief that pushers are better for water cooling than tractor just because of the room issue for ducts and radiators. You might get away with it on a tractor plane if you had a tunnel in the fuselage which would be quite a structural part of the airframe.

Exhaust augmenters have been used on air cooled piston planes for 70 years. My 1958 Piper Apache Twin had exhaust augmenters. The deal with them is they TAKE ROOM. There is no room on a Lancair or RV for such a device, in my opinion. However love the idea. NACA has stacks of reports going back to the 30's, 40's and 50's on the subject. Many of the WWII bombers and fighters had some form of exhaust "extraction" to promote air flow and cooling of the engine.

 

[rv6ejguy]

I've just finished reading Sighard Hoerner's famous book, Fluid-Dynamic Drag (1992 edition)

He worked for the German aircraft industry in WW2 and post war at Wright Field. He has compiled an amazing amount of info and presented it in a pratical, logical format.

I was especially interested in references to radiator and duct studies and he had some great real life analysis of various setups including those on the ME 109 which are not too flattering from a drag perspective.

He gives the ring rad setup used in the FW190D and TA152 top marks for low drag. He also makes comparisons of air and liquid cooled drag and efficiencies.

Radiators show a much lower pressure loss coefficient than air cooled engines in general but other general factors such as higher delta T, higher flow velocities through cooling fins compared to rad inlets and lower flow ratios tend to offset other disadvantages. In the end, subtle differences can make large drag differences and both air and liquid cooled installations, if done well can result in net thrust. The nod in this case going to air cooled engines.

It is clear that radial engined aircraft are at a disadvantage in frontal area, wetted area and external drag compared to inline liquid cooled installations. It would seem that opposed air cooled engines could be made to be very efficient with proper care although there are some extreme drag penalties associated with unfavorable inlet and diffuser geometry on air cooled engines due to low available inlet length.

It is clear that radiator setups must have a means to control mass flow through the core at cruise speeds or very high drag will be the result. The flap is very important to accelerate exit flow as well. A bad rad installation can have 10 to 15 times the amount of drag compared to an excellent one!

Oblique rad setups were not investigated but much of the text centers on momentum loss in the cooling airstream causing high drag so it would seem to confirm my observations on the flow bench with regards to these setups being poor from a drag standpoint.

Of additional interest was investigation into compressibility effects at high speed on cooling. Compression temperature rise at the face of radiators becomes quite a concern around Mach .7 as the delta T drops vs. coolant temp and heat rejection rates fall off. This explains the need on the Reno Unlimited racers for spray bar cooling to keep drag down at 500 mph. I'd heard stories that this was a major limiting factor in ultimate speed on both air and water cooled planes. We don't need to worry much about this in RVs.

There is a tremendous wealth of general aerodynamic drag info all all parts of an airplane and you come away with a better appreciation for small things and can see how some people have tweaked various airframes for a lot more speed over stock examples. I suspect they have read this book. Highly recommended if you can find a copy.

I'm even more interested in instrumenting the RV10 rad duct to gather information on the subject. It seems the good Dr. is not too fond of the belly rad setup due to extra frontal area but the guide vane I have would appear to be a good addition to avoid separation common in many submerged rad setups. It may work fine in the end.

 

[TSwezey]

I am curious to how much drag my set up creates and will probably work on it after everything else is running fine.

 

[N395V]

This thread is shaping up to generate a headache almost as bad as the torsional vibration thread

Why, oh why do I keep reading this stuff??

Pull over to the side of the road and have a beer with me.

 

[rv6ejguy]

I am curious to how much drag my set up creates and will probably work on it after everything else is running fine.

There were so many considerations and variables brought up in the book that it seemed only through flight testing are we likely to find out how good or bad our setups might be. At least with liquid cooling we have more options to improve things as far as inlet and diffuser losses go.

Todd, your submerged rads at least don't add the frontal area that my belly scoop does so even if the flow losses are higher, total drag may be very similar to my setup. Probably if it works well in hot weather, leave it alone. Something tells me you won't be short on speed.

 

[JDanno]

P-51s

Is the P-51 a direct drive system or does it have a gear box?

Wouldn't an electric auxillary fan eliminate the ground and climb cooling issues? Turn it on in climb or taxi and turn it off at speed.
I haven't heard much discussion about this ....so maybe it's a mooe point.
It seems like I heard the Geared Drive folks say they tried it but theirs cooled well enough that it wasn't needed. ???????

 

[Mike S]

PSRU, aka gearbox.

Other interesting things that most folks look at as being found on "Modern"
engines------I. E. Subie etc------dual overhead cams, four valves/cyl. liquid cooling.

 

[rv6ejguy]

Is the P-51 a direct drive system or does it have a gear box?

Wouldn't an electric auxillary fan eliminate the ground and climb cooling issues? Turn it on in climb or taxi and turn it off at speed.
I haven't heard much discussion about this ....so maybe it's a mooe point.
It seems like I heard the Geared Drive folks say they tried it but theirs cooled well enough that it wasn't needed. ???????

You need very powerful fans to move sufficient air through the typical 3-3.5 inch thick rad cores used in light aircraft. The fans create tremendous drag in flight and block a lot of air flow as well. Generally a bad idea.

Ground cooling works best with lots of area and thin rads but this is bad for high drag in cruise. A happy compromise is difficult unless you don't care about drag. I can ground run about 20-25 minutes from a cold start on a +25C day. Haven't had any problems to date.

One fellow who flies a Spitfire and P51 regularly up here says the Spit is pretty bad for ground cooling and you need to move right away and get it into the air. He says the P51 is a lot better and you can spend around twice as long on the ground. For formation work, the Spit always takes off first and he forms up on him.

 

[Jess Meyers]

Ross, I don't tune in for 3 days and you stir them up again, way to go.
Jess

 

[ergie63]

Ditto the old mid engined Fiat X/19 which use the same pump as the front engined 128. No issues with cooling despite a good 16-18 feet of rad tubing.

I'm not saying that electric water pumps would be a good idea on an aircraft (maybe with twin alternators and batteries) but they do have continuous duty models now designed to run for years. The wattage required to pump say 20 GPM confirms that the parasitic losses on an engine driven pump are also very low. Some SCCA racers have switched to electric pumps now because overall losses are lower.

The main advantage of electric pumps in automotive applications is that the coolant motor can be used intermittently during low load operations, saving fuel when not needed. Belt driven pumps consume power whether or not the thermostat is open or closed. This advantage is moot on continuous duty application such as an aircraft.

"The predicted results indicate that the cooling system with the electric pump can dramatically reduce the pump power consumption during the FTP 74 driving schedule and that the radiator can be downsized by more than 27% of the original size, under the grade load condition."
--http://me.engin.umich.edu/autolab/Publications/Adobe/P2004_09.PDF

Automotive belt driven pumps are intentionally designed with some level of slop (inefficient pumping action) so that the engine will run and seals not blown whether or not the thermostat is open. I suspect that SCCA racers (been there, done that) benefit from a more efficient impeller design afforded by a system that works on the coolant only when it needs to be moved. It also has the advantage being able to circulate coolant after the engine is shut down facilitating quicker engine maintenance during a busy race weekend. Also, they'd be looking for a system that provides all the cooling needed, but weighs less.

 

[Rotary10-RV]

ground up

Is the P-51 a direct drive system or does it have a gear box?

Wouldn't an electric auxillary fan eliminate the ground and climb cooling issues? Turn it on in climb or taxi and turn it off at speed.
I haven't heard much discussion about this ....so maybe it's a mooe point.
It seems like I heard the Geared Drive folks say they tried it but theirs cooled well enough that it wasn't needed. ???????

Danno, The P-51 does have a gearbox. The P-51 also has a "cowl" flap to open for ground handling. Typically called the "sugar scoop" it helped the ground handling a lot. The other advantage of the 51 scoop is that the propwash feeds the scoop better than a close fitted cowl opening.
The P-51 was a full ground up water-cooled aircraft so they could do the job right.

Bill Jepson

 

[Randy]

Ross and all,

I just read all through this thread and what a great one for me to find. One day I hope to get back to curing the cooling problems I am sure to have in summer weather. My Egg STI powered RV7A has not had any real cooling problems as a winter flyer but in hot temps I am sure to have issues. It just turns out that I have been doing my flying in cooler weather so far.

I am really tempted to try what you did with your 6 with the radiator in the rear fuse. This provides plenty of space for proper ducting to slow the air down and then speed it back up at the exit.

You had decent results with a NACA inlet but then added a scoop and saw improvement, perhaps it would be better to just use a properly sized scoop, or maybe they work well in combination with the NACA inlet?

I would love to simply get rid of the two small thick radiators at the front of the engine and replace them with a much more efficient radiator back in the fuse similar to what you did, probably a bit larger, and have it become the only radiator in the system. Mounting the radiator back there and the duct work would not be very difficult.

Getting the coolant back there is an issue, especially since my airframe is done and flying. I have been considering running aluminum tubing under the fuse, exposed. I guess there would be some concern about gravel getting kicked up by the front wheel and damage to the tubing but I don't see any dings in the skin down there.

This might even help cooling a little with the tubes getting some airflow around them. It would be nice to keep them down to 3/4" size. It seems that area behind the cowl air exit is pretty turbulent anyway so perhaps there is not much drag penalty to be had by hanging some tubing down there. Comments welcome on all of this...

I kind of thought a properly sized SPAL fan would be a good idea for ground cooling and not cause too much drag at speed. The blades are pretty thin, but I suspect they would be spinning even when not powered up in cruise flight. It would sure be nice to be able to turn on a fan to help allow plenty of taxi time in hot summer weather...

I am looking for good ideas for hanging the tubing to the fuse, something small and not too draggy...

Getting rid of the rads up front would make it easy to get some good air accross an oil cooler, intercooler, intake air etc.

CG would probably improve and if it did get too far aft I can move my 8 D cell ELT forward as a remedy.

Anyway I jsut had to post something since I have been thinking about how to improve my cooling while working on tubo stuff and getting ready to install an SDS system.

Randy C
RV7A Turbo STI

 

[rv6ejguy]

Welcome back Randy.

The rear mounted rad works well but can only be so big. First concern is exhaust and exit air from the cowling entering the rear scoop. My exhaust is offset to the right, scoop to the left. I instrumented the inlet for temps and no hot air is ingested on my setup. Second concern is exiting the rad air. I have two oval holes with aft facing wedge scoops under the stab in a low pressure zone to exhaust the air.

You can, with a lot of trouble make structural mods to the floor to be able to cut bigger holes but be very sure to keep the strength the same as stock.

The external lines would be fine and likely add no drag running under the belly. The flow is very disturbed already according to tuft tests in flight.

The NACA duct on the belly showed only average pressure recovery which was doubled with the boundary layer scoop added later. Generally NACA ducts are not that great unless they have at least a1-2 degree positive angle of attack. The fuselage is sloped the wrong way aft of the baggage bay so a forward facing scoop is probably required.

3/4 tubing with relatively straight runs appears to be fine up to 200nhp or so.

I had a SPAL fan on my 6A for a few months and finally did some timed ground running tests. It actually made no difference in ground cooling as it was too weak to pull any useful amount of air through a 2.5 inch thick core. I removed it after that. Others have had the fan disintegrate at high speeds in flight.

Jon Finley built a nice aft rad setup on a Dragonfly with Sube power complete with proper inlet diffuser and converging exit duct. He reports almost unlimited capability to run on the ground and he has a lot of flight hours on the plane.

Reg Clarke, also with a Dragonfly and Sube has done a really nice belly rad setup with very small inlet and rad. Cools well in all conditions.

 

[rtry9a]

IMHO, An air cooled engine will always have a higher cooling efficiency, if for no other reason than the delta H (difference between head temps and ambient air) is much higher than the radiator/ ambient air temp in a water-cooled engine.

The real question is whether the required high cylinder head temps are a good thing given other considerations like durability, lubrication (oil breakdown), weight differences, cowl shape, cooling capacity limitations, etc.

One interesting point regarding the P-51 ground cooling comment- I've been real interested in the Epps prop and the tiny cooling ports used, supposedly because it receives high pressure air from the center section of the propeller. Prop shape might be a big part of the cooling equation we have not considered when demands are highest and air speeds slow..

 

[rv6ejguy]

Hoerner's book outlines the differences in air and water cooled installations and as has been previously discussed, there are many factors involved in the entire process right from the heat sink process. What was not obvious to me was that the velocity of air past the typical air cooled engine fin was twice as high as on a typical rad setup and the heat transfer coefficient was substantially less per unit fin area so heat dissipation rates were little different despite the higher delta T. Radiators however show very high drag at high speeds unless flow is throttled. Air cooled cylinders show less drag penalty at high speed by comparison and were clearly superior with both in a fixed geometry outlet configuration.

His analysis stated that both air and liquid cooled installations could theoretically produce negative cooling drag (net thrust) if properly designed. The opposed engine offers lower frontal area than a radial but the cooling air path essentially turns through 180 degrees compared to straight through on radial and liquid cooled installations. Any time you turn air at high velocities, you have a pressure or momentum loss. If the cooling air exit velocity is less than the free stream velocity, you have net drag.

Hoerner explained in detail on how important throttling the cooling air (cowl flaps) was to reduce drag at high speed on both air and liquid cooling installations and showed that is was impossible to realize net thrust on either setup without them if cooling was to be adequate in climb also.

It's clear from many flying liquid cooled aircraft that ground cooling is worse with rad inlets close to the prop shank and best when the rads are wetted from air coming from the outer half of the prop disc. In flight, most forward facing components see about the same pressure so inlets close to the spinner work well in that respect.

Interestingly with improperly designed inlets and exits, inlet spillage at high speeds was quite common. This could be compared to an overflowing funnel where filling rate exceeds draining rate. This creates substantial drag.

Several examples of different rad placements and duct shapes show very large variations in drag despite only subtle differences.

 

[ergie63]

rv6ejguy:

I was wondering whether or not the fins in your radiator are louvered. In automotive apps they are used to promote turbulence to enhance heat transfer. Is this also true in aircraft apps? Turbulence, it seems, is anathema to aircraft design.

 

[rv6ejguy]

Yes, the fins are louvered on my heat exchangers. The flow within all radiator systems is clearly within the turbulent range anyway so this is not a factor in itself but flow bench studies show that louvered fins do create about 20% more pressure drop than unlouvered fins.

It should be noted that the rads on WW2 aircraft where much of the data originates were generally not louvered and were of thicker copper construction in most cases. While copper has higher thermal conductivity than aluminum, the solder joints interfered considerably with heat transfer. The modern Visteon type furnace brazed aluminum cores with louvered fins are at least 25% more efficient per unit face area than WW2 era copper rads. This would result in less overall drag and potentially higher Delta T on the inlet vs. outlet air compared to 60 year old designs.

It is interesting to compare percentages of glycol and water vs. heat transfer coefficient, boiling points and Delta T. While high percentages of glycol result in higher boiling points, glycol has a much lower rate of heat transfer than water. A low percentage of glycol to prevent freezing and minimize corrosion and higher system pressures actually gives the best of all worlds- best cooling, high boiling point and lowest drag. As I mentioned previously, the P51 pressurizes the system to 50psi and uses a 50/50 mix. Pre-war, many systems used 100% glycol and lower system pressures.

As hp development increased on the British Merlins, glycol percentages were reduced to 30% as the existing radiator designs were not capable of rejecting the increased heat without redesign. With the Merlin 61 and Griffon engines, larger rads had to be designed eventually despite this as hp was doubled finally in the 130 series Merlins and Griffons in 1944-1945.

 

[robpar]

As I mentioned previously, the P51 pressurizes the system to 50psi and uses a 50/50 mix. Pre-war, many systems used 100% glycol and lower system pressures.

Ross:

I have had dealings with the "Boilers Branch" if you even think of running high pressure cooling systems (over 15 psi) do not let them know about it or the plane will be so heavy you couldn't get it off the ground with a 1000 HP. Just kidding but there was an boiler inspector here that had bug in his bonnet about Cappuccino machines, if you can believe it. He wanted them certified! Calmer heads prevailed.

Higher boiling points would be very helpful. I have heard that the F1/IRL people are looking into additives that raise the boiling point but do not lower the Cp value of water. Raising the engine temperature would reduce the area of radiator required for a given amount of heat to reject, assuming constant Cp.

During a course on heat transfer I took many years ago the material the heat exchanger was made of did not effect the amount of heat that was transferred as long as the material was thin. There was a Length/Thinckness ratio that specified what was too thick. But in our designs we were using guage material and it made very little difference whether we used copper, aluminum or SS. The maximum difference if I remember was about 3%. Scaling on the surfaces of the heat exchanger made a much bigger difference than the material, so materials were chosen for compatibility with the fluid.

An interesting thread. The choice of air or liquid cooling is more complicated than it would at first appear.

Bob Parry

 

[rv6ejguy]

Yes, thinner is better from a thermal gradient point of view and that is one more reason why modern aluminum cores are more efficient than thicker brass or copper cores.

Some of the heat exchangers used for water and oil radiators on British and American engines were of the round honeycomb design and rather deep. These had relatively low pressure loss through the core but were not as efficient thermally as a traditional tube and fin type core. The very innovative Westland Whirlwind used two round honeycomb water radiators and one round oil cooler buried in each wing root. The inlets were located in the wing leading edges and air flowed straight though the spar which was a trusswork of streamlined tubing. Air exited out the rear spar through a long hinged door on the top wing surface just ahead of the flap nose.

An air cooled engine fin is very thick in comparison with a radiator tube and fin structure. What it loses in efficiency, it may make up with somewhat higher Delta T.

If you look at almost any WW2 design both air and liquid cooled, you will see cowl or rad flaps. The designers clearly understood the drag reduction possible in high speed flight using these. I submit that most of the liquid cooled experimental aircraft flying today suffer a much higher drag penalty than need be due to less than careful rad layout, poor inlet and exit ducting and a lack of rad flaps to recover much of the lost momentum lost to the rad core. Certainly my 6A is really bad at present.

 

[Rotary10-RV]

PROPER DUCT SHAPE

Ross,
I'm sure you understand the duct shape and length are very crucial to good water-cooled applications. The P-51 was one of the first aircraft that had proper diverging converging ducts. And that was only on the later versions. The use of a proper diverging duct slows the flow with less drag losses, which provides a proper speed of flow through the heat exchanger and the converging (exit) duct also speeds the flow with minimal loss. With proper duct work drag loss can be minimised.
The biggest problem with the Egg Subaru is the minimal distance allowed by the cowl to the front of the radiator face. There is little if any divergence in the duct unless the owner makes one on their own. The second sin radiator wise is the rads themselves are backed up close to the engine cylinder heads. A good system provides clear area for the radiators to exhaust into. A good rule of thumb is 3-4" of clear space before tapering the exit duct work. Obviously the Subaru FWF from Egg and others have ignored the exit side clearance rule due to the convenience of mounting the radiators near the cowl inlet holes. K&W (Kuchemann and Weber) found the exit to be equally important to the entrance to efficient cooling and low drag. Most of the inside cowl radiator mountings ignore this.
Bill Jepson

 

[rv6ejguy]

Absolutely and this is why I mentioned a few of the people like Finley and Clarke who have actually done things correctly and have had excellent results and even good ground cooling to boot.

This is why I suggest that people don't copy what I've done on my RV6A as there are no exit ducts. What I have works well for cooling but also must be creating high drag.

The data has existed for a very long time on how to do it right but this research has been largely ignored in recent times for ease of installation.