What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Water Cooled vs. Air Cooled- Real Numbers

rv6ejguy

Well Known Member
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
 
Last edited:
So, how is it flying??

How does it compare to the previous radiator setup? Faster, same, slower?
 
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. :)
 
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.
 
Last edited:
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.
 
Last edited:
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.
 
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.
 
Last edited:
Although data from the liquid cooled install is interesting, I find it hard to get excited about a comparison using absolute values ("Real Numbers").

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/community/showpost.php?p=781564&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.

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.

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.

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?

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 ;)

...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.
 
Last edited:
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/community/showpost.php?p=781564&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.
 
Last edited:
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.
 
Proof of concept maybe??

Ross, it sounds like you are proving that all the work you put into the 10s cooling system is on the right track-------

rv10scoop78.jpg
 
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.

For exit velocity you need a static port in the flow at the point you wish to measure. In the case of a coaxial pitot-static I'm reliably advised that the exact location of the pitot tip isn't a sensitive detail. The static location sets the point of measurement. Put another way, the tip of the pitot might be 1/2" forward of the static ports...no big deal.

Piccolo pressure is fine with delta to the aircraft static.

My piccolo tube for the inlet was placed right at the rad face....

In the case of a diverging duct with a vertical exchanger that position would indeed result in the highest pressure indication. It's a little different with a horizontal exchanger (the air cooled cylinder set). See CR3405; pressure generally drops a bit as you move rearward from the inlet.

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.

Please accept my compliments for it.

THE MAN IN THE ARENA - Theodore Roosevelt
Excerpt from the speech "Citizenship In A Republic" delivered at the Sorbonne, in Paris, France on 23 April, 1910

It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat.
 
Last edited:
Ross, it sounds like you are proving that all the work you put into the 10s cooling system is on the right track-------

rv10scoop78.jpg

The RV10 scoop was built many years ago as at that point I'd already seen the light that cowling mounted rads were a poor compromise. This scoop is not as optimally shaped as the RV6 one but I like to believe it will do the job. I also built a test rig to quantify HX thermal performance a few years back and there were some rather larger variations in temp vs. pressure drop. The Rad in the RV10 is not as efficient thermally as the Rv6 one but has slightly lower pressure drop.

I had been scared ever since I built the RV10 one that the rad was too small for the hp but assurances from my friend Russell and now my flight testing seems to indicate this is no concern. (hopefully)

We also have a friend in Oz who has built many successful under wing rad installations and his face areas and rad volumes are way less than ours per unit hp. He may not recover the exit velocity we do, but they cool well in a hot climate. They are cooling a 400hp V8 with less total rad area than me and volume is only 1.63 X mine. I may have gone too big on mine but did not want to build a second duct if I was too small. Russell also said my inlet was still way too big but I had to have unlimited ground cooling, figuring I could close the exit and be almost as efficient in cruise.
 
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.

This may have been Hans in Holland with a Jodel? He had good cooling but the rad areas and volumes were huge for the hp. Easy to get good cooling, harder to do it with low drag which is what I am trying to learn about here. Actually there was NO modern data I could find anywhere and little historical data on real numbers. I wanted something actual to plug into the math rather than wild assumptions.
 
For exit velocity you need a static port in the flow at the point you wish to measure. In the case of a coaxial pitot-static I'm reliably advised that the exact location of the pitot tip isn't a sensitive detail. The static location sets the point of measurement. Put another way, the tip of the pitot might be 1/2" forward of the static ports...no big deal.

Piccolo pressure is fine with delta to the aircraft static.



In the case of a diverging duct with a vertical exchanger that position would indeed result in the highest pressure indication. It's a little different with a horizontal exchanger (the air cooled cylinder set). See CR3405; pressure generally drops a bit as you move rearward from the inlet.



Please accept my compliments for it.

THE MAN IN THE ARENA - Theodore Roosevelt
Excerpt from the speech "Citizenship In A Republic" delivered at the Sorbonne, in Paris, France on 23 April, 1910

It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat.

Good point on the static source for the exit pitot. I am going to move that today and I'll be able to switch sources in flight to see the difference.

Great quote from Teddy here and I salute you for your testing too- most interesting to have some real data with your attention to detail.
 
I did a bit more testing with the revised probe setup. It was cooler at altitude this time so rad exit temp was down to 52C. Despite this and the door in the neutral position, V recovery at the exit was 87% at 100 knots IAS and 70C coolant temp. I want to try correlate exit temp with V recovery at a fixed speed to reduce variables. The problem I have is to get the coolant temp up. I'll have to block off some of the inlet for the cabin HX which is doubling as a small extra rad in the summer, just dumping overboard. Either that or I need some dive brakes so I can work the engine harder...

A number of data points so far suggest this scoop works well at recovering momentum at the exit compared to even the throttled ducts others have tested and published the results for here on VAF a while back with air cooled engines. I'm seeing 59% with the exit door fully open and as high as 97% (old probe setup) with the door closed vs. 51% for a stock RV8 outlet and 64% for the throttled exit at similar TAS as my tests.

These are preliminary and my testing continues but from the data so far, there is nothing to support the old notion that air cooled engines will always have less cooling drag than an equally optimized liquid cooled setup.
 
Ross;
How is the cooling drag reduction project using the 'Meredith effect' doing?
(Thanks for the help on turbochargers)
 
Ross;
How is the cooling drag reduction project using the 'Meredith effect' doing?
(Thanks for the help on turbochargers)

I've been fighting a vexing intermittent comm problem for months now. I believe (dearly hope) I have finally isolated it down to a bad intercom channel. For the time being, I've crossed rigged the pilot PTT and headset jacks to pass through the copilot channel and wiring. A few more hops and I'll know if this is really the cause, then I can get outside the pattern and gather the rest of the numbers.

Everything except the radios have been performing perfectly. My new door actuation system works wizard and cooling is amazing even with the door closed.
 
Nice setup!

I had always been under the impression that a properly configured, fan forced cooling system resulted in markedly less drag. And the possibility of thrust. I have to wonder if the HP required to drive such a system would be put to good use.
 
Nice setup!

I had always been under the impression that a properly configured, fan forced cooling system resulted in markedly less drag. And the possibility of thrust. I have to wonder if the HP required to drive such a system would be put to good use.

I have a fan on the back side of my radiator, but I have my VP-200 shut it off during cruise - I guess I could turn it on & off manually and get a few data points....

John
 
Nice setup!

I had always been under the impression that a properly configured, fan forced cooling system resulted in markedly less drag. And the possibility of thrust. I have to wonder if the HP required to drive such a system would be put to good use.

Just having the fan there probably creates a lot of drag depending on how good/bad your radiator/ duct setup is. Probably does not matter if it's on or off. The blocking effect of the fan disc would be substantial at higher speeds.

There is no possibility of creating thrust with a standard electric fan on your rad and few if any rad setups would be able to create thrust unless they use a well designed divergent/ convergent duct with a variable geometry exit door.
 
I have a fan on the back side of my radiator, but I have my VP-200 shut it off during cruise - I guess I could turn it on & off manually and get a few data points....

John

I was thinking about a completely different type of setup. Not simply an automotive rad and fan assy. But rather, a compact, high density, multi pass, heat exchanger with a high pressure rise fan for rapid and accurate airflow through the heat exchanger. Pickup would be somewhere useful without frontal area. Discharge would be in an appropriately sized nozzle. Exhaust heat could be used here too.

Variations on this theme were used in industrial applications for years and are still used on certain jet aircraft, such as the Gulfstream family of jets (using fan air to cool bleed air) . Instead of using a large heat exchanger and modest airflow rates. Engineers would downsize the heat exchanger and force feed it air. Then recover the heated air for other uses. In our case, possibly thrust.

The G550 uses a thrust recovery outflow valve. It helps the aircraft achieve about 250 nautical miles more range than the GV. Clearly, that airflow is put to good use.
 
I was thinking about a completely different type of setup. Not simply an automotive rad and fan assy. But rather, a compact, high density, multi pass, heat exchanger with a high pressure rise fan for rapid and accurate airflow through the heat exchanger. Pickup would be somewhere useful without frontal area. Discharge would be in an appropriately sized nozzle. Exhaust heat could be used here too.

Variations on this theme were used in industrial applications for years and are still used on certain jet aircraft, such as the Gulfstream family of jets (using fan air to cool bleed air) . Instead of using a large heat exchanger and modest airflow rates. Engineers would downsize the heat exchanger and force feed it air. Then recover the heated air for other uses. In our case, possibly thrust.

The G550 uses a thrust recovery outflow valve. It helps the aircraft achieve about 250 nautical miles more range than the GV. Clearly, that airflow is put to good use.

The mass flow and pressure required to cool a 200hp engine in the climb are pretty immense. You'd need quite a few hp driving a fan or blower to do the job. You are talking about mass flow of around 160-200 lbs./min and delta P of around 5 inches H2O with a traditional radiator. Double this for a 6 to 8 inch deep HX. Now you are into centrifugal blowers in the 4-5hp range and you are stuck with finding some good way to drive this. On a 14V system- talking about over 200 amps here roughly...
 
Maybe this is an ignorant reply?.but what about drag-less watercooling for cruise flight, using wing D-section surface for example?
 
A while ago I was reading about a race plane which uses ADI fluid sprayed on a radiator to cool the engine in races.
Since cooling drag is a major factor, why not optimize the cooling system for cruise and use a sprayer for additional cooling in the climb?

Probably less complex then cowl doors, easier to optimize....

Tim
 
A while ago I was reading about a race plane which uses ADI fluid sprayed on a radiator to cool the engine in races.
Since cooling drag is a major factor, why not optimize the cooling system for cruise and use a sprayer for additional cooling in the climb?

Probably less complex then cowl doors, easier to optimize....

Tim

Water is heavy, you have to refill it. I've shown in testing so far that we can offset almost all the cooling drag with a proper scoop design. Spray bars don't make much sense on general use aircraft but make a lot of sense on race aircraft. Most of the Sport and Unlimited Reno races use both spray bars and ADI.
 
Water is heavy, you have to refill it. I've shown in testing so far that we can offset almost all the cooling drag with a proper scoop design. Spray bars don't make much sense on general use aircraft but make a lot of sense on race aircraft. Most of the Sport and Unlimited Reno races use both spray bars and ADI.


I get the feeling when I get the chance to go experimental, I will end up spending a lot of time on this issue.

Tim
 
I get the feeling when I get the chance to go experimental, I will end up spending a lot of time on this issue.

Tim

#1 priority is to have it cool in the climb on a hot day and also cool on the ground on a hot day. It's pretty well understood now how to do this with low drag. I've documented my design and testing extensively here and on my RV6A Page on our website. Really, we are just building on the work done in WW2. The problems and solutions haven't changed too much since then. We just have more efficient radiator designs these days. No need to re-invent the wheel.
 
There are some who have found spray bars useful on the ground, at events like OSH or SNF where they might get stuck idling for long periods.

Charlie
 
A properly designed setup should cool indefinitely on the ground without boil over on a hot day and without the aid of fans or spraying water on the rads. Several people have achieved this. You have a big fan out front after all...
 
A properly designed setup should cool indefinitely on the ground without boil over on a hot day and without the aid of fans or spraying water on the rads. Several people have achieved this. You have a big fan out front after all...

I have found that my radiator fan and spraybars help a lot during extended taxi and running on the ground in hot weather. Also, in climb out. Other than that, it does cool indefinitely until a hot day - about 80?F, or so.
I think the Lycoming guys have temperature issues, as well....

John
 
A properly designed setup should cool indefinitely on the ground without boil over on a hot day and without the aid of fans or spraying water on the rads. Several people have achieved this. You have a big fan out front after all...

Ross, the penalty may not be worth it. The liquid cooled engines in WWII over heated on the ground if not launched soon. Designers probably did not solve the problem because the cost was too high.

I really liked the turbine like operation of the Sub H6 but even if the PSRU had been more reliable and internally integrated into the engine as with the Merlin of WWII, I wonder if the engine could have with stood hours and hours of high HP output.

My Honda Pilot engine us now approaching 200,000 miles without a hitch and a service tech guy said recently after doing some routine maintenance, it should go another 100,000 miles. He knew of one engine at over 400,000 miles. But its entire life has been at 20-30% power. I do not believe this engine or a Subby will last that long running at 75% power, it simply was not designed to do so. And to fly around at 20-30% power is not acceptable to most pilots.
 
I have found that my radiator fan and spraybars help a lot during extended taxi and running on the ground in hot weather. Also, in climb out. Other than that, it does cool indefinitely until a hot day - about 80?F, or so.
I think the Lycoming guys have temperature issues, as well....

John

Yes, a Lycoming isn't usually too happy idling for 30+ minutes on the ground either if the OAT is over 85F in my experience.
 
Ross, the penalty may not be worth it. The liquid cooled engines in WWII over heated on the ground if not launched soon. Designers probably did not solve the problem because the cost was too high.

I really liked the turbine like operation of the Sub H6 but even if the PSRU had been more reliable and internally integrated into the engine as with the Merlin of WWII, I wonder if the engine could have with stood hours and hours of high HP output.

My Honda Pilot engine us now approaching 200,000 miles without a hitch and a service tech guy said recently after doing some routine maintenance, it should go another 100,000 miles. He knew of one engine at over 400,000 miles. But its entire life has been at 20-30% power. I do not believe this engine or a Subby will last that long running at 75% power, it simply was not designed to do so. And to fly around at 20-30% power is not acceptable to most pilots.

WW2 fighters had a different mission and used very deep rad cores and small face area for low drag at high speeds. This will cause poor cooling on the ground but was an acceptable compromise. Despite this, many types of liquid cooled fighters operated successfully at +40 to +50C OAT.

There are now at least 5 Egg Subes with over 700 hours on them without ever being touched plus hundreds of RAF 2000 gyros with 500+ hours on them, also without being touched, another training gyro in Australia which had 3800 hours on it, ditto. Also some Ford and Rover V8s with over 1000 hours on them, so we know the old thoughts about duty cycle don't always ring true. Didn't your teardown show no problems or wear inside too?

We also know that all auto engines these days are validated for a minimum 200 hours at WOT and some as much as 1600 hours at WOT, so I disagree that they are not designed for this. Even though their typical duty cycle in North American driving is low, they must be designed to be reliable if someone pounds on them. I've seen plenty of Showroom Stock type racers go many hundreds of hours being abused at much higher rpms and hp levels than they would normally see in aircraft use where we derate rpms down around 4000-5000 and are at higher altitudes where power drops off substantially from SL.

For reliability to be intact at high power, people are finding that they must observe the factory temperature limits and keep AFRs and timing within the same specs as the OEM does.
 
Last edited:
Most people have no idea that auto engines live their lives at 15-20 HP. Even a big SUV at highway speed will only need about 20-25HP.
 
Most people have no idea that auto engines live their lives at 15-20 HP. Even a big SUV at highway speed will only need about 20-25HP.

I think everyone knows this so I am not sure what your point is...

This thread is about cooling drag differences between air and liquid cooled engine installations. Let's keep it on topic if we can.
 
Humbly suggest that you do some research on the actual testing of typical automotive engines. It's highly unlikely that an a/c engine would last more than 10 hours into a typical auto test cycle that lasts 500+ hours.
 
But not for high HP continuous operation. Auto engines are tested extensively for efficiency and emission compliance at 15-20 HP. This does not make a 150 HP continuous duty aircraft engine. Want to see Extensive testing in reality? Look at how 2nd and 3rd year engines need to be fixed for problems in the first year run. In my case the Toyota 270 HP V6 used in RAV4's for example. Suffered failed ignition coils after 1 year requiring replacement. Hardly an example of "rigorous" testing.

I still once in a while buy auto mags. You need to get updated some. The testing on the engines used for the LS1/7.... Or BMW, or the new Ford 3.6L....
Now for the low end basic cars, the Chevy Cobalt, Dodge Neon.... yah, these cars generally have much less in testing performed. They also do not have as efficient engines, production costs are more of a focus.

Tim
 
Humbly suggest that you do some research on the actual testing of typical automotive engines. It's highly unlikely that an a/c engine would last more than 10 hours into a typical auto test cycle that lasts 500+ hours.



Agreed 100%.......

The testing protocol for aircraft engines is crude and substandard compared to auto engines....


And.. Water cooled motors don't suffer from shock cooling...
And... no chance of CO poisoning from the cabin heater....
And... Water cooled motors get better BSFC too...

Other then that, they are about as good as 60 year old technology.:rolleyes:
 
Agreed 100%.......

The testing protocol for aircraft engines is crude and substandard compared to auto engines....


And.. Water cooled motors don't suffer from shock cooling...
And... no chance of CO poisoning from the cabin heater....
And... Water cooled motors get better BSFC too...

Other then that, they are about as good as 60 year old technology.:rolleyes:

Aircraft engines don't suffer from shock cooling either. It's a myth disproved by RAM a long time ago.
 
Aircraft engines don't suffer from shock cooling either. It's a myth disproved by RAM a long time ago.

That is debatable.. But for the sake of keeping the conversation going.. What about the other two points I brought up ?
 
Our old air cooled engines actually have very good fuel consumption. A Continental (or tuned Lycoming) requires about .38 lbs of fuel to produce one horsepower for one hour. ie: BSFC = 0.38 lbs/hp-hr.

The engine in the 2010- Toyota Prius has a best case BSFC of .36-.38, depending on which data you look at.

The all-mechanical air cooled engine fares pretty well against the most efficient computer controlled car engines of today.

David
 
Last edited:
If people want to debate their preferences for traditional vs. automotive engines, it's already been done on this forum ad nauseam so you can pick up an old thread and beat it some more... pointless as it may be.

As I've said so many times before, fly whatever engine you want and whatever makes sense for your mission. Please respect the choices other people make. Trying to ram your opinion down other people's throats serves no useful purpose, especially on this thread which isn't about engine preferences.

'nuff said I hope...
 
Have you got that plane over 110 KIAS yet? What do you think will happen to pressure recovery (or not) when you're at 170 KIAS? Do you see a massive increase in airspeed not affecting pressure recovery? Will the Meredith Affect still apply when 3 times the mass flow is going into your radiator/scoop? I would think that your numbers arrived at 110 KIAS will be irrelevant at cruise speed. Probably even be a huge drag on the whole concept. Perhaps a smaller scoop/radiator combined with a secondary radiator for TO and climb operations that can be completely closed off at higher airspeeds.
 
Last edited:
have you looked at the rotary engine guys' stuff? the rotary creates even more heat than your suby. take offs on hot days push many limits. sprayer tubes have been tried. I always wondered why the jet effect of the exhaust wasn't pursued more. this has been used on some certified a/c. kevin [13B and 20B projects at one time]
 
have you looked at the rotary engine guys' stuff? the rotary creates even more heat than your suby. take offs on hot days push many limits. sprayer tubes have been tried. I always wondered why the jet effect of the exhaust wasn't pursued more. this has been used on some certified a/c. kevin [13B and 20B projects at one time]

It was used on older Cessna 310's. Not sure how well it worked. The 310's I seen had pretty ratty looking exit tubes. Maybe not worth the hazard of an exhaust leak vs. improved cooling air flow thru the cowling. In a water-cooled set-up you'd have to get the air/exhaust jet behind the radiator. Would be an odd arrangement with a P51 belly scoop. Most likely have to mount radiator in front of cowling and use exhaust venturi to extract air from the cowling behind the radiator.
 
Last edited:
I'm looking into this myself for a potential water-cooled aero-diesel install. It would solve two things at once. Help to pull air thru radiator (much better than a Meredith effect) and direct all the exhaust/heated air down a controlled path away from the camera hole I'm putting in the belly of an RV7.
 
Have you got that plane over 110 KIAS yet? What do you think will happen to pressure recovery (or not) when you're at 170 KIAS?

Static pressure will increase in both plenums.

Do you see a massive increase in airspeed not affecting pressure recovery?

Your example 60 knot increase is not massive. Given any reasonable inlet, static pressure before the exchanger will rise about 55%.

Will the Meredith Affect still apply when 3 times the mass flow is going into your radiator/scoop?

Yes, the effect will still apply....but it won't be three times the mass flow. Given a fixed exit, it will be about double, less if Ross closes the variable exit. Which he will; the transition from climb to cruise will reduce cooling requirement, so (1) there is no point in maintaining the excess mass flow, and (2) it will increase exit velocity, reducing drag.

I would think that your numbers arrived at 110 KIAS will be irrelevant at cruise speed. Probably even be a huge drag on the whole concept.

Wrong as a soup sandwich.
 
Back
Top