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Turboprop style cowling for water cooled auto conversion

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rv7charlie

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Yep. Tracy Crook ran them on his Mazda 20B powered RV8 for years. He made them the same way that landing gear intersection fairings are made; they screwed to the spinner after spreading the trailing edges to go around the blade root. Took them off after one came loose at one of our rotary gatherings; not sure if he ever re-made them and reinstalled. IIRC, he said that they made a significant difference in both cooling and prop performance.

Charlie
 

Riggerrob

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Reno racers (biplane and Formula 1) will tell you to locate your intakes as close as possible to the propeller. This adds a bit more ram air. If your inlets are close to the hub, blade cuffs might help. However - as an earlier poster pointed out - then might become a maintenance head-ache.
OTOH If you locate your inlet(s) farther out/down, they will receive ram-air from a wider part of the prop blades.
 

rv6ejguy

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Reno racers (biplane and Formula 1) will tell you to locate your intakes as close as possible to the propeller. This adds a bit more ram air. If your inlets are close to the hub, blade cuffs might help. However - as an earlier poster pointed out - then might become a maintenance head-ache.
OTOH If you locate your inlet(s) farther out/down, they will receive ram-air from a wider part of the prop blades.
This isn't so important on liquid cooled engines. My intake is 3.5 feet from the prop, Russell Sherwood's is around 6. Ground cooling is just fine in both cases.
 
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Riggerrob

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Yes dear rv6jguy,
I was referring to how mounting carburetor intakes close to props increases manifold pressure by a few inches of mercury. This becomes a poor man's super-charger.
 

Chris Matheny

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Do you think the belly scoop is better/more efficient then a side cowling exit above the wing line like the Pipistrel Panthera uses? The side exit seems in theory to be useful as in climb you have more negative pressure area over the wing so it would cool more in climb and less in cruise. I guess if you need weight aft CG moving the radiator could help in a belly scoop. Just got me thinking as I have been going back and fourth on this same thinking for my KR2S I just started.
 

rv6ejguy

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The Panthera has no cowl flaps so duct design is a compromise between adequate cooling in climb and low drag in cruise. It's also air cooled so typically has higher momentum loss than a radiator in a properly designed ventral scoop. The low velocity at the duct exit almost certainly disturbs airflow aft of the exit, down the side of the fuselage. Cowling mounted radiators always have higher momentum loss than those housed in long diffusion/ convergent ducts but makes for easier packaging in some cases. There are compromises to every design decision in aircraft.
 
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BJC

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Do you think the belly scoop is better/more efficient then a side cowling exit above the wing line like the Pipistrel Panthera uses? The side exit seems in theory to be useful as in climb you have more negative pressure area over the wing so it would cool more in climb and less in cruise. I guess if you need weight aft CG moving the radiator could help in a belly scoop. Just got me thinking as I have been going back and fourth on this same thinking for my KR2S I just started.
Would a belly scoop be prudent on the low-slung fuselage of a KR?

BJC
 

rv6ejguy

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You could submerge the rad into the fuselage aft of the seat, only need a couple of inches projecting below the belly skin. Russell Sherwood did this on his Glasair with a very tall rad.
 
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rv7charlie

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Chris,

The Thorp T18 uses the cowl side exit, for that reason. The counter argument in some aero documents is that it messes with airflow over the top of the wing, where you want flow as perfect as possible.

One if the reasons given in some docs about the choice in the P51 was that it moved the inlet far enough aft to eliminate the prop pulses from entering the duct. The reason for *that* is that the pulses disturb flow in the duct; some of the energy hits the core and reverses, trying to push air back out of the duct. Note that circumstances are different in a cooler duct vs an induction inlet. When LoPresti did his 'Holy Cowl' work, the induction inlet almost touched the trailing edge of the prop blades, BUT, the prop was clocked so that each pulse raised pressure at the intake valves just as one was opening. At least that's what his advertising claimed.

Submerging ducts inside structure requires structural analysis; relatively easy on tube/rag because the fabric isn't structural, but can be hazardous on semi-monocoque fuselages like RVs, Thorps, etc if load paths aren't analyzed carefully. Probably easier on fiberglass, but still deserves attention.
 

Chris Matheny

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Would a belly scoop be prudent on the low-slung fuselage of a KR?

BJC
Mine will be set on tri gear taller than the original setup so not a big issue. As of now I am leaning towards the side cowling exits but have been tossing up both ways.
 

wsimpso1

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Water cooled auto conversion? I would not have to think long on that configuration. They are usually a little heavy, driving CG forward. Moving the radiator from forward to just aft of the CG will help with maintaining CG AND is the lowest drag radiator set up out there. The only debate in my mind is whether you also stick the oil cooler back there, and I think the answer is yes too. Then the sexy PT6 style inlet only has to be big enough for induction air, intercooler air, and incidental air over the engine. Have at it.

If the engine is actually lighter and moving HX's aft would get you in CG trouble, there is always a prop extension or a slightly longer engine mount or both. Way sexy still.

Letting air out through the side of the cowling is attractive: It lets you use a shallow splitter on the belly scoop, but it is dumping an extra load of air over the top of the wing. This is the same as having the fuselage wider through the front of the wing - it messes up flow a little bit. As long as the fuselage is straight walled and vertical through the wing, this looks like it should be OK. I always get concerned about the cabin getting exhaust gases this way.

The alternative is Ross' solution - a deep splitter for the scoop to keep the hot air from under the cowling out of the belly scoop. I know that it works for keeping exhaust gases out of the cabin and hot air out of the scoop, and only adds a couple inches if the exhausted air is small because the rad and oil cooler are elsewhere.

Billski
 
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