Successful Auto Conversion Trials, Tribulations and Tips

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Russell

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The subject of this thread is huge, I will just nibble at a tiny corner of it.

In the early 90’s I read a NACA report on how propellers effect air flow within three blade widths of the prop. The report said that air reverses direction for just a moment as each blade rotates by.

I mostly forgot the report for three or four years, then came face to face with this phenomena while my engine was on the test stand. A large radiator was mounted about five inches behind the propeller. I could not get the engine water temp higher than the thermostat setting. Wanting to get an idea of how much radiator might be needed in the plane, I decided to slide a piece of cardboard between the prop and rad. Once the temp started to rise I would have my rad size.

To my surprise, as the cardboard was slid between rad and prop the cardboard started to jump off the radiator and toward the prop then slam back against the radiator in rapid succession. The further the cardboard was slid in the higher it would jump off the radiator, as much as two inches!

Think about the tortured path that air must travel to cool our Lycomings. The inlets are too close to the prop and have reversing air. Air must then make sharp 90 degree turns, twisting, tumbling and finally exiting in a high pressure area. For water cooled engines, placing the radiator in the cowl where the normal air inlets were gives reversing air plus there is no room for a correct plenum that slows and smooths the air flow.

I have never seen a water cooled aircraft with the radiators mounted within the cowl where cooling was both effective and efficient. This doesn’t mean that it has never been done or can’t be done … just that I have not seen any. These past two paragraphs are just my opinion and are subject to error so accept it with caution.

The above is what drove me to look at the work done by designers of WWII aircraft. I have a copy the P-51 scoop and radiator.
 

Toobuilder

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This reverse flow theory is contrary to other studies and my own testing. On the Hiperbipe I made a pitot style induction inlet for the servo. The section of 3 inch tube that held the inlet nose ring could slide in and out of the airbox with the turn of a hose clamp. Back to back flight test (within minutes of one another) confirmed the highest MP the closer the inlet was to the blade. Final config was breathtakingly close to the prop and worked very well.

Lesson learned and applied to the Rocket. Induction inlet is way out on the prop arc and as close to the TE as possible. Cooling inlets are also well forward, and being fixed to the engine are even closer to the prop TE (about .125 inch at full pitch). Hard to quantify the mods because of so many other changes, but preliminary testing shows better MP and better cooling than before, despite a smaller inlet area.

I'm not suggesting that there is no reversion effect, but the net result might be an actual improvement. There are a lot of F1 guys that cool with minimal inlets and tractor propellers. There must be more to the story.
 

Russell

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This is one of the reasons that I like experimentals … we can experiment! My findings were close to those in the NACA report. However, Toobuilder came up with a totally different result that improved the plane. I wonder if anyone has similar or even a third finding?
 

Toobuilder

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Should be a very easy experiment: plastic pitot tube, altimeter, slide it fore and aft behind a turning prop and see where highest pressure is found
 

Russell

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Should be a very easy experiment: plastic pitot tube, altimeter, slide it fore and aft behind a turning prop and see where highest pressure is found
Good idea. However, a two bladed prop turning 1000 RPM, equates to about 33 blade passes every second. Being a mechanical device, I doubt that an altimeter would do anything but average the pressure at each location. To detect pulses, a fast pressure transducer may be required for the job. Detecting reverse flow may require a tool that can sense quick direction changes … I have no idea what that would be.
 

Toobuilder

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Which goes back to my statement of a net increase. Don't care about a momentary reversion if the remainder of the event is positive.
 

Russell

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My original post was aimed at finding aircraft with water radiators mounted near the prop that are both effective and efficient. With effective cooling the plane should be able to taxi, run up and fly without cooling problems. Efficient cooling has taxi temps, climb rates and air speeds that match or better the same model plane as yours but has a Lyc or Cont engine.
 

rv6ejguy

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Russell's ventral radiator success prompted me to finally get off my butt and ditch my "collection" of rads scattered all over my RV and replace them with a single ventral rad in a proper duct 6 years ago. Cooling was improved and drag and weight reduced at the same time. I was able to measure net thrust under some conditions.
 

Vigilant1

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My original post was aimed at finding aircraft with water radiators mounted near the prop that are both effective and efficient. With effective cooling the plane should be able to taxi, run up and fly without cooling problems. Efficient cooling has taxi temps, climb rates and air speeds that match or better the same model plane as yours but has a Lyc or Cont engine.
Tracy Crook's original cooling setup for his carburetted Mazda 13B had radiators under the cowl of his RV-4, and I don't recall him having effectiveness issues after he solved a bit of trouble with the turbulence around the inlet lip. He was keeping up with O-320s. That's not to say his installation was as efficient as it could be, but it did seem to work.
 

Toobuilder

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There's no doubt that a long divergent duct is easier to make work than a short one. This is one of the reasons you see 12 inch prop extensions on F1 racers, but it also appears that blade passage can have a net positive effect on a nearby inlet. Look at the blade cuffs installed on later P-51 models for an example.
 

mm4440

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Hi Ross, any update on this Rover installation:


Hi Ross, any update on this Rover installation?
Good luck at Reno.
Murry
 

rv6ejguy

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This one gathered about 1700 hours under the original builder. Sold to a younger guy who didn't like how the builders did some of the engine stuff. He pulled the engine out and re-did a bunch of things and had some serious issues with it then. Last I heard a few years ago, it was still grounded. Not sure what people are thinking in modifying something with that many trouble free flight hours on it... Leave it alone.
 

tailwind

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Look at the Canadian Northstar cooling package. It has the radiator under the spinner up front.
 

RSD

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Russell's ventral radiator success prompted me to finally get off my butt and ditch my "collection" of rads scattered all over my RV and replace them with a single ventral rad in a proper duct 6 years ago. Cooling was improved and drag and weight reduced at the same time. I was able to measure net thrust under some conditions.
Got any pictures or info of yours or Russell's setup?
 

rv6ejguy

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Look at the Canadian Northstar cooling package. It has the radiator under the spinner up front.
Power "egg" installations were common post war on liquid cooled aircraft like the Northstar and Shackelton. This meant for easier and quicker engine changes with everything attached to the engine. Not much to do with low drag. You need a long diffuser to slow the air and recover pressure for lowest drag and max efficiency. Generally hard to do with the rad mounted way forward.
 
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