Subaru for RV-7 > Is It Real ?

[DanH]

Jan,
No numbers, eh?

<<And the efficiency does not have to be any worse than any other engine.>>

True. Given the higher efficiency of a water-to-air exchanger, an optimized system should cool with less mass flow than the air-cooled equivelent. The
requirement for a very large exit area is....curious.

 

[nucleus]

Jan,
No numbers, eh?

<<And the efficiency does not have to be any worse than any other engine.>>

True. Given the higher efficiency of a water-to-air exchanger, an optimized system should cool with less mass flow than the air-cooled equivelent. The
requirement for a very large exit area is....curious.

Not sure where you get the higher efficiency from... But consider that your delta T is a lot less with liquid. Your typical coolant temperature is around 200F with only about a 20 degree drop between the engine outlet and inlet. On a 90 degree day you have a delta T of about 100F, aircooled is more like 300F.


Hans

 

[David-aviator]

It does come down to the fin design of the radiators. It is clear that the more open fin design on an air-cooled engine can provide faster airflow. A radiator system, using a larger cooler, more open fin spacing, etc. might be more efficient but would require more coolant and ducting to relocate the radiator. The front mounted smaller coolers are a compromise, but an acceptable one to provide a compact, lightweight installation that will work in different airframes. More suction is required at lower speeds to pull air through the coolers, less is required as airspeed build and pressure increase at the front. Hence the need for a cowl flap. The cowl flap, pulled up to almost closed during cruise, will regain the 5 kts of speed lost over an air-cooled engine. The latest ECU will reduce the fuel flows and the benefit of modern engines again become evident. Just gave a ride to an instructor this morning and he could not believe the nice feeling of flying behind this engine. Just like Van himself now chose the Rotax for the 12 and like the smooth operation of it, we have the same technology engine for the larger airplanes. And the efficiency does not have to be any worse than any other engine. In fact, at cruising speeds the auto engine is more efficient. The latest reports from Europe, by Hans that fly the engine in a classic French Jodel, have better speeds and efficiency than the original certificated Lycoming airplane. This is a testimony to the latest ECU and higher reduction drive gearing.

Jan Eggenfellner

It is more than fin area, Jan.

If one researches the technology that made liquid cooling work on WWII fighters, it was smooth air flow into and out of the radiators. Someone figured out that a 6 degree diffusion slowed the air down so it had enough time to remove heat as it passed through the radiator and then gradually sped up again as it left the radiators into the exit area. In a car, radiators are skinny because air through them is relatively slow, in an airplane they need to be thicker to accommodate faster moving air. The EGG system does have adequate radiator thickness, but there is no provision for smooth air flow aft of the units. It is total air flow chaos as the rads are within inches of the engine block with no flow transition area whatever. That explains why leveling off at any altitude to cool the engine required setting 1700 rpm to force air through the rads, if one left the rpm at 2500, coolant temps would never come down even with much increased air speed. I believe it was all due to very disturbed air flow, especially aft of the radiators.

As you say, having the radiators up front is a compromise. Plumbing is simplified and they do work somewhat but there is a drag penalty. The exit cowl flap aids flow aft as it provides more escape for incoming air but I never believed it was the final answer.

I actually bought two linear actuators with the intention of using them but decided to test an enlarged exit area in a fixed mode first because I was concerned about adding more weight. What I found was that the increased exit area did help cooling, I could take off on a 90F day and climb to 8500' with just one of two level offs for cooling at 1700 rpm. But what made me not want to install the cowl flaps was coolant temperature at 8500 running at say 2400 rpm WOT. The coolant temp was well within limits at 190-210 but there was no room to close down the exit area, temps would have gone up for sure. I was still doing about 140 KTAS at 8 gph, down from 143 before I increased the exit area (which was 125 square inches).

The small radiators in the H6 were (are) marginal. If I attempted a second take off with a heat soaked engine on a 90F day, coolant temp hit 230 before reaching 1500 MSL and it was only the 1700 rpm procedure that brought it down. Leaving the rpm at 2500 and accelerating to 150 knots did nothing for cooling but logically it should have with more air hitting the radiators. When I say it must be the chaotic air flow situation aft of the rads that makes for that situation, I am just guessing because I have no other explanation. At one point I thought maybe the prop in fine pitch was actually blocking incoming air to the rads but I don't think that is true. Air has to go by the prop to create thrust which it is doing at WOT.

Anyway, I think overall you've done a good job getting the Subaru to work in an airplane, no one else has come close except Ross. But in retrospect, maybe there would have been a few less headaches if from the very beginning an expert in air flow and cooling had designed the cooling system. It is not late to consider such a move.

 

[N62XS]

Dead Horse!

There were never any cooling issues. Just a reluctance of some builders to listen to factory testing, showing that one difference, using a liquid cooled engine, was the requirement of a movable cowl flap. Jan Eggenfellner

Not true or factual. "Factory" cooling recommendations were changed as each builder completed R & D on "RTF FWF" packages they purchased.

Again, for me, this has everything to do with the realization that the past is in fact that, the past. The future is about the continued availability of alternative engines. I feel that way, you might not, but it is the only thing that matters for us, along with the support of earlier engines. Hence the new drive units. Dwelling on how the past could have been done differently, is not productive to the future. Jan Eggenfellner

Many have paid for that past, in many ways. Don't you mean "Dwelling on how the past could have been done differently, is not" profitable "to the future", your future.

Harping on low power and gearbox replacement is fine, if you choose to do so. You will then live in the past and refuse to see that the Subaru engine has again advanced to much higher power output and that a gearbox exist that can handle this power. The reality surrounding even the earlier drive unit is that it served many people well for thousands of accumulated hours for over a decade of flying. Only 4 drives had various issues and not a single airplane was lost due to any partial failure. That we want this record to stay good is a testimony to how serious we are about safety and is not in any way a negative, in our opinion.Jan Eggenfellner

Again, not true or factual. The install record does not support your past recollections or current statements. Use numbers and facts, not opinion. Many of us have no issue with alternative engines used in experimental aviation. The issue resides with individuals and companies that continually tout their products as superior alternatives to the traditionally used light aircraft powerplant. The facts clearly demonstrate that alternate engines are simply that alternate choices that are allowed by the rules that govern experimental aviation. Arguing that this alternate choice is a superior selection based on weight, cost, power, etc. is simply arrogant.

To go any farther would invoke the deletion fairy and as I grow older, frankly I tire of this useless exercise. In this economy, how people choose to waste their money is becoming a new spectator sport.

 

[Sam Buchanan]

Not true or factual. "Factory" cooling recommendations were changed as each builder completed R & D on "RTF FWF" packages they purchased.

<snip>

To go any farther would invoke the deletion fairy and as I grow older, frankly I tire of this useless exercise. In this economy, how people choose to waste their money is becoming a new spectator sport.

Yep, this is covering subject matter that has been thrashed plenty of times before, just use the Search engine if more info is needed.

Thread now closed.