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Who Wants Zero Cooling Drag?

Rotary10-RV said:
You make your choice based on available information and then build it and test. No matter how good it looks on paper, it may or may not work. If you succeed, and I for one hope you do document what you have done and why. You can then help the rest of us "improve the breed" which is the truest spirit of experimental aircraft.
Bill Jepson

Absolutely agree. Sometimes we think we are pretty smart, build it and find out it doesn't work well at all. We learn just as much from the failures as we do from the successes.
 
On the surface conduction front, I'd recheck the calcs on the leading edge only idea. My calcs showed something like 120 square feet of aluminum would be required on a 200hp engine on a 100F day with 200F coolant. Naturally there are many unknowns to solve for so who knows until a test section could be actually tested. This is a crazy idea for a GA aircraft and nearly impossible to implement on an RV IMO.

Electric water pumps are widely available with current draws in the 4.5 to 8 amp range and flow rates up to 35GPM. These are not heavy and are way more than adequate to cool a 200hp engine. That would be the easy part.

Yes, 120 square feet sounds very reasonable. A typical radiator will have 10 fins per inch, so a rough calc shows a radiator that is 24" x 36" and 2" thick will have an internal surface area of 120 square feet. Radiators this size are typically seen in ~200HP aero conversions so the number works.

Could you really find 120 square feet of leading edge on an aircraft with a 23 foot wing span. Not likely. Sure it might be possible to deice a wing with coolant, however there will still need to be a radiator or supplemental cooling somewhere in addition.
 
Let's see if anyone gets it... :D
 

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Yes, no radiator scoop. But not using surface cooling either. It is ablative. Engine coolant circulated through a tank which boils and vents overboard. When the tank is empty, you are done.
 
One thought about diesel aircraft engines.

Spoke with the Cessna engineer developing the 8 cylinder engine for the 182.

Curious about the choice of 8 cylinders.

He explained that the firing impulse with 4 cylinder engines of sufficient displacement was too much for the (then) existing propeller technologies.

That may have been resolved as this was about 10 years ago, but it's a factor to keep in mind.

FWIW
 
Even in theory, you can not get a zero drag heat exchange from a skin radiator. Adding heat to the boundary layer will increase skin friction significantly. I think Hoerner covered this somewhat in his book, Drag. The gods are against us.

Someone mentioned ebullient cooling. As I recall reading, the 1930's ME-109 racer used it for record speed runs - no radiator, no cooling drag. Do it right and use the steam jets for a little extra push?
 
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One thought about diesel aircraft engines.

Spoke with the Cessna engineer developing the 8 cylinder engine for the 182.

Curious about the choice of 8 cylinders.

He explained that the firing impulse with 4 cylinder engines of sufficient displacement was too much for the (then) existing propeller technologies.

That may have been resolved as this was about 10 years ago, but it's a factor to keep in mind.

FWIW

This is why my good friend Bill Brogdon designed the Continental 4 cylinder diesel as a 2 stroke. Also, it is not just the prop, the reaction to compression torque at idle is a huge oscillating moment on the airframe. Smaller displacement engines (diesel) with high boost yield the highest hp/lb. Until direct injected gasoline is used as a stratified charge combustion to mitigate detonation, 100LL severely limits high BMEP with low BSFC.

A new product, gasoline or diesel, is limited by ROI not technology.
 
Even in theory, you can not get a zero drag heat exchange from a skin radiator. Adding heat to the boundary layer will increase skin friction significantly. I think Hoerner covered this somewhat in his book, Drag. The gods are against us.

Someone mentioned ebullient cooling. As I recall reading, the 1930's ME-109 racer used it for record speed runs - no radiator, no cooling drag. Do it right and use the steam jets for a little extra push?

I know that adding heat to the boundary layer will cause transition if it is laminar (in a gas. In a liquid, it has the opposite effect.) Once you have a turbulent boundary layer, IIRC heating lowers the skin friction coefficient. We did a flight test on a Lear Jet years ago where we heated the fuselage to lower the cruise drag. It was a modest benefit, not huge. The test results agreed with the pre-test predictions. There may be a trend with Mach no. where the opposite result might be true at very low Mach no., I don't remember. Certainly in supersonic boundary layers, the skin friction reduction from heating is very significant.

I think it is correct that the speed record runs with the ME 209 were indeed done without radiators, but I don't have a reference to verify that right now.
 
This is why my good friend Bill Brogdon designed the Continental 4 cylinder diesel as a 2 stroke. Also, it is not just the prop, the reaction to compression torque at idle is a huge oscillating moment on the airframe. Smaller displacement engines (diesel) with high boost yield the highest hp/lb. Until direct injected gasoline is used as a stratified charge combustion to mitigate detonation, 100LL severely limits high BMEP with low BSFC.

A new product, gasoline or diesel, is limited by ROI not technology.

I do a detailed comparison on various aero engines, including the new Adept 320T SI engine here: https://www.youtube.com/watch?v=GJy93vrbvw0&t=4s

From dyno data, it competes favorably on BMEP and BSFC fronts with the new Conti CD-300 diesel, using "old school" port injection.

On the cooling drag question some of you may have missed my 2015 Kitplanes article here: https://www.kitplanes.com/the-meredith-effect-fact-or-fiction/

I am currently working on another video on liquid cooling, looking at other successful flying examples from some of my customers and friends flying auto conversions.
 
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I know that adding heat to the boundary layer will cause transition if it is laminar (in a gas. In a liquid, it has the opposite effect.) Once you have a turbulent boundary layer, IIRC heating lowers the skin friction coefficient. We did a flight test on a Lear Jet years ago where we heated the fuselage to lower the cruise drag. It was a modest benefit, not huge. The test results agreed with the pre-test predictions. There may be a trend with Mach no. where the opposite result might be true at very low Mach no., I don't remember. Certainly in supersonic boundary layers, the skin friction reduction from heating is very significant.

I think it is correct that the speed record runs with the ME 209 were indeed done without radiators, but I don't have a reference to verify that right now.

Steve, you are correct. My first thought in reading the thread regarding putting the heat on the leading edge surface was that it would trip the boundary layer to turbulent. Maybe a skin radiator should be aft somewhat to preserve whatever laminar flow that might be possible.

I dug down and found my Hoerners. You are right. Adding heat to a turbulent BL will reduce the turb BL fiction somewhat. However, he goes on to say that the heated wing, being mostly in accelerated flow may produce a negative thrust component due to the added heat. Somewhat like a negative ramjet. Effects could be neutral.

OK, how about this:
We use ebullient cooling but dump the wet steam between the spars of somewhat conventional wings and recover the condensed coolant. Presto. The wings are radiators.:)

Ron
 
The folks who know most about this topic retained a traditional rad in a very modified ventral scoop sans scupper, to set the piston world speed record a few years back. Freeze frame at 1:00 to have a look at that: https://www.youtube.com/watch?v=jha18gdhI6Y

They are also spraying water through the HX of course. Best pass, 545 mph. Unfortunately the engine went south before 4 good passes could be completed.

Bottom line is nobody would consider surface conduction cooling on a GA aircraft. Completely impractical for numerous reasons.

Few liquid cooled GA aircraft apparently care much about low cooling drag as they continue to use dreadful radiator layouts. The new Diamond DA50RG, despite turbocharging and retractable gear, is no faster than a good 300hp RV-10. They must have massive cooling drag given the poor duct layout for the rads and intercoolers.
 
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