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Old 08-22-2013, 05:49 PM
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Default Water Cooled vs. Air Cooled- Real Numbers

I've always been interested in the "Meredith Effect" as theorized back in 1936 by F.W. Meredith. He surmised that with a well shaped duct enclosing the radiator, it was possible to recapture the cooling air momentum lost passing through the rad after adding the heat and constricting the exit. These thoughts were applied in the famous rad setup in the P51.

I've been flight testing my 6A with the new ventral radiator scoop for a couple months. It's been fitted with 2 Magnehelic pressure gauges, one for rad inlet pressure and one for outlet pressure, an exit air temperature probe and a pitot tube to measure exit velocity.

It has always been the position of the air cooled advocates that those installations would always suffer less cooling drag simply because they were working with heads at about 400F vs. the 200F we have available in liquid cooled aircraft engines. In thermodynamics, Delta T or the difference in temperature between the object and the cooling medium means a great deal. All things being equal (which they are not in this case) you can sink off more heat or energy when the Delta is higher.

I always found this logic simplistic and flawed. While the the total energy we need to dissipate should be about the same to cool a 200hp Lycoming and a 200hp Subaru, the point of heat exchange and the duct geometry are vastly different.

In a Lycoming setup, we have a very short distance to slow down the air from the cowl inlet to the cooling fins and the air must turn first 90 degrees to go down through the head and cylinder fins, then must turn another 90 degrees to head towards the exit. We also have the steel barrels which are not nearly so efficient as aluminum at dissipating heat. Then we have inefficient changes in volume as the air passes through the system- large at the inlet, small through the fins, very large exiting into the cowling and then hitting many obstructions including the flat firewall on its way to the actual smaller outlet.

On the ventral rad setup, we have several times the length to slow down the air efficiently and gradually and recover the maximum pressure at the rad face. We don't change the air path 180 degrees like the Lycoming. Whenever you turn air suddenly, you have a loss of velocity unless it is done very gradually. We call this momentum loss. Whenever we take air on board the aircraft internally, unless we can return it to the same velocity, we create drag. By slowing down the air at the rad face, we reduce drag through the core. Then we reduce the cross sectional area of the duct gradually to accelerate the air again before it leaves the exit. The radiator core has many times the surface to volume ratio of an air cooled engines coarse fins and divides each hot part (rad tubes) into very thin sheets. This means that it is very efficient at transferring heat to the atmosphere.

I've been following Dan Horton's threads and test results on his IO-390 RV8 cooling experiments with great interest so I'll draw on some of the numbers he posted on VAF. This is not meant to degrade the fine work and testing Dan has done, as we can all appreciate that. I am just comparing the science of the two setups and I want to say, that these are just 2 points, who is to say either one is fully optimized?

So what do the numbers show?

Pressure recovery. The ventral rad duct appears more efficient. We recover 84% of the theoretical pressure available compared to 77% for the Lycoming, both with exit doors closed.

CHT / coolant to exit air Delta. These are hard to compare apples to apples as in the Lycoming setup, cooling air from the inlets also goes through the oil cooler and over the exhaust system before exiting. The exhaust likely contributes somewhat to the temperature rise of the cooling air since it is around 1400F and there is quite a bit of surface area there. If we compare at face value, we see the Lycoming setup is in the low to mid 50% range, depending on speed and CHT (I guessed a bit here). The radiator setup peaked at 92%. So despite the much lower Delta of coolant vs. cylinder heads, the radiator is far more efficient per unit temperature at transferring energy to the atmosphere.

Momentum recovery. I was not able to compare Dan's values here (maybe he can fill this part in). In the rad setup, I measured a 97% momentum recovery at the exit with the door mostly closed with a coolant temperature of 176F. This suggests that by raising the coolant temp and the Delta by 30F, we could probably reach velocity momentum parity inlet vs. outlet and possibly even add velocity (net thrust).

Now the remaining question is how much drag, internal and external does the ventral scoop present vs. the clean contours of the air cooled setup. (airplanes are all full of compromises). I can't answer at this point. I did my best to place the scoop in the frontal "shadow" of the existing lower cowl exit so the frontal area gain is minimal but I've also added some more wetted area.

Finally we have a couple more points to compare, more as food for thought:

My setup has excess inlet area still for the oil and intercoolers but my friend, Russell Sherwood has a well optimized (SARL winning) Glasair with a 230hp Subaru six. He has about 48 in2 of total inlet area (induction, oil, gearbox and rad vs. 56in2 for Dan's clean RV8. This suggests that if the same exit sizes are used, the liquid cooled setup is taking less mass flow on board to do the job with roughly the same hp levels. Russell's exit area in normal cruise with oil and coolant combined is around 27 to 30in2, Dan said his was around 30in2 if I got that correct.

Dan, please step in here to correct anything I posted incorrectly and your observations as well.

I welcome any other views on this fascinating topic.

Here are a couple videos doing some tuft testing: http://www.youtube.com/watch?v=QT8njoirTkU

http://www.youtube.com/watch?v=gPpWHvr1kJw
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 424.4 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm



Last edited by rv6ejguy : 08-22-2013 at 07:24 PM.
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  #2  
Old 08-22-2013, 06:22 PM
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So, how is it flying??

How does it compare to the previous radiator setup? Faster, same, slower?
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  #3  
Old 08-22-2013, 07:16 PM
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Quote:
Originally Posted by Mike S View Post
So, how is it flying??

How does it compare to the previous radiator setup? Faster, same, slower?
I've changed so many things, I am still over the airport for safety reasons (chicken test pilot) until I get 10 hours on it maybe so I have not been able to unleash it yet. I still am fighting an intermittent comm issue as well. My gut feeling is it is 5-10 knots faster than before as I keep having to pull back the throttle, below 22 inches and coarsen the prop to keep it below 110 IAS.

I have moved the velocity and temp probes internally now, deciding to leave them in place to gather more data at higher speeds when I can get outside the control zone.

Flight characteristics seem completely unchanged.

Cooling margin is WAY better. Cockpit management is reduced compared to the various water valves and shutters I had before.

Flights have mostly been short with the vid cams simply because the files become too large to deal with on the PC later- about 75Mb/ min. Plus, I like to take the cowling off and check things frequently. This is like starting over in some respects. But I love experimenting and learning.
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Old 08-22-2013, 07:59 PM
BillL BillL is offline
 
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Russ, Great work! This is so much better than stuffing the heat exchangers in the cowling. Good data to go with it. The laminar flow through the HX is so much more efficient than the high delta-P environment of the air-cooled flow paths. Lots of opportunity in both it seems. Please keep us posted as the final results of the aircraft performance modifications.

I am not an aerodynamicist, but at Conti when they were trying to offer liquid cooled engines, I did a series of analyses of duct flow for various inlet/outlet. radiator delta-p and heat rejection ranges and could never get actual thrust, but neutral was clear doable. It is great to see the liquid cooled potential being realized. Maybe in the airframe that thrust will truly be seen.
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Last edited by BillL : 08-22-2013 at 08:11 PM. Reason: added stuff
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  #5  
Old 08-22-2013, 08:35 PM
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Quote:
Originally Posted by BillL View Post
Russ, Great work! This is so much better than stuffing the heat exchangers in the cowling. Good data to go with it. The laminar flow through the HX is so much more efficient than the high delta-P environment of the air-cooled flow paths. Lots of opportunity in both it seems. Please keep us posted as the final results of the aircraft performance modifications.

I am not an aerodynamicist, but at Conti when they were trying to offer liquid cooled engines, I did a series of analyses of duct flow for various inlet/outlet. radiator delta-p and heat rejection ranges and could never get actual thrust, but neutral was clear doable. It is great to see the liquid cooled potential being realized. Maybe in the airframe that thrust will truly be seen.
You had an interesting stint at Continental I think!

I now have moved the exit pitot tube out of the boundary layer (I hope). I was going to remove all the test gauges after the basic tests but was having too much fun, so I left the pitot and temp sensor in place so I can check Delta V under more flight conditions. I am temptingly close to parity already at only 176F. The exit to face area sweet spot seems to be right around 17% with my duct design- (LAR "engineering").

One fun thing is playing with the exit door control on the runup and watching the exit ASI. Open and you have about 35-40 knots, closed about 60-65. Since I have a direct, push/pull cable actuation on the exit door, I can feel the internal pressure on the door, you need no force to open it, even fully, which is way into the slipstream but you really need to pull to close it.
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 424.4 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm



Last edited by rv6ejguy : 08-22-2013 at 08:49 PM.
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Old 08-22-2013, 10:03 PM
SHIPCHIEF SHIPCHIEF is offline
 
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Ross;
Very nice engineering, but I'm wondering why you mounted the athodyd radiator right along the fatest cord of the airframe?
The way I see it, you would get the most certain inlet pressure & velocity, which would be a positive factor in establishing that it works.
But the form drag of the airframe must also be increased, which would negate the speed increase we would hope for. I think you added more than wetted area, you violated the Area Rule, and increased the cross section of the fattest part of the whole plane?
My RV-8 Rotary 13b is only now starting taxi testing, so don't think I'm experienced or criticizing your work, I'll be reading and following. I put my cooling directly under the engine, like a P-40 to keep the 'cooling bulge' ahead of the CG and fat spot. P-51's put it out back.
That's my only question so far, but if you will keep posting, I'm sure to keep learning.
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Old 08-22-2013, 10:46 PM
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Quote:
Originally Posted by SHIPCHIEF View Post
Ross;
Very nice engineering, but I'm wondering why you mounted the athodyd radiator right along the fatest cord of the airframe?
The way I see it, you would get the most certain inlet pressure & velocity, which would be a positive factor in establishing that it works.
But the form drag of the airframe must also be increased, which would negate the speed increase we would hope for. I think you added more than wetted area, you violated the Area Rule, and increased the cross section of the fattest part of the whole plane?
My RV-8 Rotary 13b is only now starting taxi testing, so don't think I'm experienced or criticizing your work, I'll be reading and following. I put my cooling directly under the engine, like a P-40 to keep the 'cooling bulge' ahead of the CG and fat spot. P-51's put it out back.
That's my only question so far, but if you will keep posting, I'm sure to keep learning.
As I mentioned in the first post, airplanes are about compromises so the design considerations were numerous:

1. Short routing of the coolant pipes with minimal drag, weight, losses and had to be external to the cockpit.

2. The CFD plots didn't show that this position was any worse than anywhere else. I didn't want to compromise the structural integrity by chopping big holes to submerge the radiator and this would disrupt the efficiency of the scoop as well. For least momentum loss, we want the cooling air path as straight as possible. The exit is in an area of relatively low pressure according to the CFD.

3. Previous in-flight video and the CFD plots showed very serious turbulence just aft of the stock Van's cowling exit. By sticking the boundary layer "Vee" of the rad duct into this zone, we might be able to actually clean this up. The original cowl exit frontal area "hides" about 60% of the ventral scoop frontal area. I actually have less frontal area than before with various inlet and exit scoops.

4. Existing internal structure in the belly area was conducive to attaching the rad scoop mount structure to. Weight and ease of access to that area for riveting was also a concern.

5. Ground cooling at idle was a big consideration. The inlet placement would give the best possible velocity from the prop. It drops off rapidly further aft.

6. It was essential not to ingest hot air from the stock cowling exit (oil cooler, intercooler, clearance air) and exhaust into the radiator inlet. Mounting the rad further aft would give us no way to do this without huge mods to the exhaust system, cowling and systems under the cowling.

7. I considered a P40 type setup for a long time but the duct would be a big compromise and I was worried about the possible destabilizing effect the extra keel area out front might have with the small RV6 fin. The inlet geometry for ground cooling was not going to be as good also.

I considered these things for many months and decided on the best overall compromise for the 6A. I was out to improve cooling margin, reduce weight and complexity and try to reduce drag simultaneously.

I believe many people underestimate the drag momentum losses can cause. The cooling system passes thousands of CFM. My mission was not to "just" cool. That is easy. I wanted to cool with low drag. I see many really bad rad installations out there with huge radiators and marginal cooling. I'm only using a rad with 119 in2 of face area and the cooling margin is amazing.
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 424.4 hrs. on the Hobbs,
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http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm



Last edited by rv6ejguy : 08-23-2013 at 10:54 AM.
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Old 08-23-2013, 07:18 AM
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Although data from the liquid cooled install is interesting, I find it hard to get excited about a comparison using absolute values ("Real Numbers").

Quote:
Pressure recovery. The ventral rad duct appears more efficient. We recover 84% of the theoretical pressure available compared to 77% for the Lycoming, both with exit doors closed.
Good example. The 77% figure comes from upper deck piccolo tube pressure less aircraft static system pressure, divided by a calculated max Q for that altitude and TAS (i.e. a standard day value). The whole data set is here:

http://www.vansairforce.com/communit...&postcount=175

The intention was to look at the effect of different exit door positions. It compares figures within the same test setup taken moments apart.

Ok, the upper deck piccolo tubes are a paired set, 12" long each, positioned fore and aft in a specific position above each cylinder bank and connected together. The result is believed to be a reasonable average of upper deck pressure. Moving them forward toward the inlet would raise the indicated pressure. Positioning them perpendicular to flow or in another location would likely change the indicated pressure.

In addition, different aircraft tend to have slightly different static system pressures, a function of static port location and shape. That's why we calibrate the ASI in Phase 1. Yours and mine may or may not be identical.

There is an ongoing project (currently stalled) gathering data from a group of air-cooled installs. They all use the exact same piccolo tubes positioned in the same locations, plumbed the same and connected to the same brand/part# digital manometer. The intent is to make the data as comparable as possible, airplane to airplane, but even so we have some suspicion about accuracy.

Point is, your setup is different, and even identical setups are subject to errors. Doesn't mean your data is wrong, or not useful. It just means treating it as global data may not be reasonable.

OK, set aside instrumentation variables. For a moment let's assume the 84% and 77% figures are directly comparable. Does it prove anything about liquid vs air cooling? No. Either figure can be raised or lowered by varying the exit area. That was the point of the data set where you got the 77% figure.

Quote:
In a Lycoming setup, we have a very short distance to slow down the air from the cowl inlet to the cooling fins...
The distance issue applies to systems which rely primarily on internal diffusion. Examples would be your duct setup or something like Chris Zavatson's Lancair, with a prop extension to allow good internal diverging shape behind a small inlet. The distance issue is moot given primarily external diffusion. Again, it's not a divide between liquid and air cooling.

Quote:
Then we have inefficient changes in volume as the air passes through the system- large at the inlet, small through the fins, very large exiting into the cowling and then hitting many obstructions including the flat firewall on its way to the actual smaller outlet.
They're both modeled as inlet, plenum, exchanger, plenum, exit. That said, yes, the liquid system will have almost nothing in the post-exchanger flow, an advantage.

Quote:
Momentum recovery. I was not able to compare Dan's values here...
Dan hasn't published any velocity numbers because there is some doubt about accuracy. It may require an array of pitot/static probes spanning the exit area to compensate for local variation. Right now I'm using a single probe.

BTW, I noticed your exit velocity measurement was a pitot tube, with no local static?

Quote:
Now the remaining question is how much drag, internal and external does the ventral scoop present vs. the clean contours of the air cooled setup.
Best you're gonna do is find a similar 6A with comparable Lyc HP and run 'em. I think you're gonna get smoked trying to drag around that big belly box

Quote:
...my friend, Russell Sherwood has a well optimized (SARL winning) Glasair with a 230hp Subaru six. He has about 48 in2 of total inlet area (induction, oil, gearbox and rad vs. 56in2 for Dan's clean RV8. This suggests that if the same exit sizes are used, the liquid cooled setup is taking less mass flow on board to do the job with roughly the same hp levels. Russell's exit area in normal cruise with oil and coolant combined is around 27 to 30in2, Dan said his was around 30in2 if I got that correct.
Moderate inlet area differences should be moot given similar exit area. The exit is the throttle. Also remember that mine is optimized for excellent low speed cooling as well as reasonable top speed. Put another way, when I open the additional exit area I can indeed put a lot of air through the system.

Hey, don't get me wrong. You can put me in the cheering section that says a well done liquid cooled setup should have lower cooling drag, and may not have any more form drag if incorporated into the airframe from the start of design. Unfortunately, an RV isn't that airframe.
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Last edited by DanH : 08-23-2013 at 07:26 AM.
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  #9  
Old 08-23-2013, 08:24 AM
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Quote:
Originally Posted by DanH View Post
Although data from the liquid cooled install is interesting, I find it hard to get excited about a comparison using absolute values ("Real Numbers").



Good example. The 77% figure comes from upper deck piccolo tube pressure less aircraft static system pressure, divided by a calculated max Q for that altitude and TAS (i.e. a standard day value). The whole data set is here:

http://www.vansairforce.com/communit...&postcount=175

The intention was to look at the effect of different exit door positions. It compares figures within the same test setup taken moments apart.

Ok, the upper deck piccolo tubes are a paired set, 12" long each, positioned fore and aft in a specific position above each cylinder bank and connected together. The result is believed to be a reasonable average of upper deck pressure. Moving them forward toward the inlet would raise the indicated pressure. Positioning them perpendicular to flow or in another location would likely change the indicated pressure.

In addition, different aircraft tend to have slightly different static system pressures, a function of static port location and shape. That's why we calibrate the ASI in Phase 1. Yours and mine may or may not be identical.

There is an ongoing project (currently stalled) gathering data from a group of air-cooled installs. They all use the exact same piccolo tubes positioned in the same locations, plumbed the same and connected to the same brand/part# digital manometer. The intent is to make the data as comparable as possible, airplane to airplane, but even so we have some suspicion about accuracy.

Point is, your setup is different, and even identical setups are subject to errors. Doesn't mean your data is wrong, or not useful. It just means treating it as global data may not be reasonable.

OK, set aside instrumentation variables. For a moment let's assume the 84% and 77% figures are directly comparable. Does it prove anything about liquid vs air cooling? No. Either figure can be raised or lowered by varying the exit area. That was the point of the data set where you got the 77% figure.



The distance issue applies to systems which rely primarily on internal diffusion. Examples would be your duct setup or something like Chris Zavatson's Lancair, with a prop extension to allow good internal diverging shape behind a small inlet. The distance issue is moot given primarily external diffusion. Again, it's not a divide between liquid and air cooling.



They're both modeled as inlet, plenum, exchanger, plenum, exit. That said, yes, the liquid system will have almost nothing in the post-exchanger flow, an advantage.



Dan hasn't published any velocity numbers because there is some doubt about accuracy. It may require an array of pitot/static probes spanning the exit area to compensate for local variation. Right now I'm using a single probe.

BTW, I noticed your exit velocity measurement was a pitot tube, with no local static?



Best you're gonna do is find a similar 6A with comparable Lyc HP and run 'em. I think you're gonna get smoked trying to drag around that big belly box



Moderate inlet area differences should be moot given similar exit area. The exit is the throttle. Also remember that mine is optimized for excellent low speed cooling as well as reasonable top speed. Put another way, when I open the additional exit area I can indeed put a lot of air through the system.

Hey, don't get me wrong. You can put me in the cheering section that says a well done liquid cooled setup should have lower cooling drag, and may not have any more form drag if incorporated into the airframe from the start of design. Unfortunately, an RV isn't that airframe.
As always, appreciate your response Dan.

My main point of this post was more to show that the old, simplistic Delta T arguments are inaccurate and that the two systems can be very close in performance. I too want to gather more data especially since I modified the pitot tube to bring it out of the boundary layer.

Yes, my ASI probe is tied into the main static vent and I checked my exit ASI against the one in the panel. At 110 KIAS, they are within a knot so I chose this speed for most of the testing. The pressure probes were tried both to the static source and cockpit. At 110 KIAS, I could not see a difference more than .25 in with the vents open (I have big leaks at the back of the canopy). I have never done tests for CAS so from a strictly scientific and test flying perspective, my data at other airspeeds is suspect.

My piccolo tube for the inlet was placed right at the rad face since I am not concerned with pressure along the diffuser at this time. The exit one was placed about 4 inches forward of the end of the scoop. I did a lot of testing with piccolo tubes years ago and never found orientation made much difference in readings which is the point of a well designed tube- to get an accurate average pressure. I don't consider variations of less than .25 in H2O and 2 KIAS very significant and within experimental error here. I also corrected for observed OAT rather than standard in doing the recovery calcs. I was more interested in that data point than a day to day comparison. The inlet recovery is less interesting to me at this point than the velocity data.

The temp probe was located about 2 inches aft of the exit. The limitation is it will only measure to 70C and we often see temps exceeding this in the climb, so I can't accurately calculate TAS at the exit when it is hotter than this.

I agree, the data is not strictly comparable and as I said in the first post, these are just 2 data points.

On my setup, continuing to close the door past a certain point did not raise inlet pressure any more or velocity, that is why I wanted to investigate inlet spillage with tufts but there was no evidence of this with the door fully closed (exit area about 58% of inlet).

I believe we had the same design goals- excellent climb cooling and excellent cruise cooling with minimal drag. The last time I left Reno (Carson City) in 2008, we were loaded to gross, it was warm and then at 500 feet AGL, we hit an inversion and it was hot. I hated to lower the nose to keep things cool in the climb so I wanted something better.

I tried to grab your best figures for comparison of inlet recovery. Were you able to achieve something better than 77% and still keep the heads cool?

You could well be right, I could get smoked by an air cooled 6A. I have a prop which is not well optimized for top speed and likely a ton of drag from the existing cowl inlets/ exits which feed the oil and intercooler. Unless I can generate quite a bit of net thrust (unlikely in my view) to offset the drag of the scoop, I would have to go slower, all things being equal. However we don't know how much velocity the Lycoming setups have at the exit (you do ), which could be a big offset.

The part about the Glasair was only to illustrate that a good liquid cooled installation can get by on the same or less inlet and exit area for cooling. If we have the same speed, same Delta P, smaller inlet and exit area probably means less mass flow. This was simply to counter the often sprouted conjecture that the lower Delta T would mean the liquid cooled installation would require more mass flow to cool the same hp.

I agree, the RV is not the right airframe to start with to fully optimize a liquid cooled installation. It is just a testbed for ideas and has always been so for me. It wears the patches to prove it- with many failed experiments over the years but I have learned a lot from the failures as well as the successes.
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 424.4 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm



Last edited by rv6ejguy : 08-27-2013 at 06:49 PM. Reason: Spelling mistake
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Old 08-23-2013, 08:53 AM
David-aviator David-aviator is offline
 
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Interesting and complicated stuff going on here. I like Dan's analogy of the exit acting as the throttle. It sure does for controlling air flow through the engine compartment.

My objective has been adequate cooling, not winning a race. The stock Vans inlet to exit ratio is a compromise as are all things in airplanes.

I have a Dwyer magnehelic pressure gage but never got around to hooking it up to measure inlet and exit chamber pressures. When adequate cooling was achieved I felt the task was over. (by that I mean CHT's below 400 on a hot day climb)

I do know the RV-7A with a Lycoming engine matched or exceeded Van's speeds. Cooling was made adequate by opening the exit area, in this case with louvers. A moveable cowl flap works very well also but at the speeds we fly its complexity may not be warranted. I believe the Bonanza style louvers, which have been used for many years to cool oil, are most agreeable in terms of efficiency and simplicity.

With regard to radiator location with liquid cooling, there is a guy in Europe running a Subaru with heat exchangers in inboard wing leading edge. I have not heard from him for some time but he had reported excellent results. I believe it was in a Robin but my memory may not be recalling that correctly.

Liquid cooling works very well if everything is balanced as it is in millions of autos. The tough challenge Ross is taking on is getting all the elements working together in a small aircraft. The efficiency of it in aircraft such as the P-51 and the P-38 is well known but it has not been generally achieved in small aircraft.
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