View Full Version : Radiator position
06-24-2005, 06:29 PM
This is for all you heating/cooling experts. I am using a VW Rabbit radiator, mounted vertically under the EA81 on my Bensen type. The radiator is actually mounted at an angle under the engine. Since the cooling fins are not aligned perfectly with the forward airflow, would it be better if the radiator was mounted horizontally, with the cooling fins alinged with the airflow? With the radiator mounted vertically, with both hose connections now at the top, how efficient does the coolant circulate, versus if the radiator was mounted with one hose connection at the top, and the other at the bottom, as in a cars set up?
06-25-2005, 03:42 AM
Note the radiator position on this gyro
If I am not wrong, the rabbit radiator has very small openings for air passage.
The pressure differential on a gyro is to small to use this radiator with enough efficienty.
Choose a radiator with bigger openings for air passage.
Choose a radiator with only 9mm distance between the pipes.
Radiators have different cores, the more pipes the better.
The size and the position of the pipes are not important as long as there is no air in the radiator.
06-26-2005, 02:47 AM
The rabbit radiators work fine if the engine is not on full power for long periods. The top tank has a divider in it so the water goes down one side, across the bottom and up the other side. I have seen plenty work OK mounted vertically like yours.
06-27-2005, 10:22 PM
Rabbit radiators are standard on all 582 SportCopters. Mine never gets hot. The highest I have ever seen was 160 degrees when I was flying wide open and it was 110 outside. Normally it stays right at 140 (thermostat opening temp). You can see by the picture it is mounted at a angle behind the seat and under the engine and gets plenty of airflow. I understand a Suburu is a bigger engine, but since it burns less gas per hour than a Rotax, it may generate less heat?
Scott Heger, Laguna Niguel, Ca N86SH
06-28-2005, 02:52 AM
There is so many things that can be wrong with a subaru, that can cause high temps, Besides the radiator.
Air inside the engine.... Wrong thermostat.... Bad head gasket.... etc....
06-28-2005, 04:22 AM
Since I wasnt the one that originally built my gyro, I am now looking at ways to make some corrections if needed. Looking at my radiator position, it is mounted under the engine, tilted at a 45 degree angle forward. The lack of airflow through it is probably causing most of the problems. On an 80 deg day, the water temp is 180, but the other day I flew, it was 90F, and the water temp was 200f.
06-28-2005, 04:32 AM
I don't think the angle is the problem Paul. And these engines will run that hot, they are working alot harder to push you and the gyro through the sky than they worked pushing the car they were designed to push down the road.
You need to check the thermostat, and make sure there is no air still trapped in the engine block or manifold.
Alot of airplanes use this engine and radiator and they mount the radiator flat up under the plane and use a small duct to allow air to pass through the radiator. They seem to work fine, once the rest of the system is sorted out.
06-28-2005, 05:10 AM
Ron...So running the EA81 at 200f on the water temp is not a problem? I have a new 180f thermostat in it. If thats what other EA81s are doing in this temp, then I will leave it alone.
06-28-2005, 05:12 AM
.......Alot of airplanes use this engine and radiator and they mount the radiator flat up under the plane and use a small duct to allow air to pass through the radiator. They seem to work fine, once the rest of the system is sorted out.Ron, while your comments are valid up to a point, the fact remains that for a radiator to work properly it has to have sufficient air flow thru it.
The owners of ones you see used in FW aircraft that are working properly have jumped thru many hoops to get to the point where they work.
There has been many an article published about the need to get the air into a properly baffled cooling plenum on a FW and then out and it isn't always easy. It comes close to being a science. So, we can't really use the under the cowl radiator of a FW to compare with the gyro's open installation. Both installations have unique problems and most of us aren't knowledgable enough to deal with them. Consequently we continue to have questions like this.
Everyone needs to remember that the radiator requires a continuous flow of positive air pressure on the inlet side so that there is air pressure differential across the radiator. No differential, no air flow thru the radiator.
Gyro builders typically don't provide an air duct to the radiator inlet so a pusher's prop is probably supplying a low enough pressure area on the outlet to make it work in most cases.
I've often wondered if the fin density couldn't offer enough resistance to air flow that a major part of the air flow would pass around rather than thru the radiator. High fin density certainly offers more heat transfer surface but there may be a case where more is not better!
06-28-2005, 06:00 AM
Paul I am not the subaru expert, but as long as the temps didn't get much past 200, I wouldn't be too worried about it. What you don't want is for the coolant to boil and I don't believe it will boil at 200.
Dean, Have you seen Carls gyrocycle with the same radiator Paul uses, on a 100 horsepower Hirth, and he has it laying flat under the engine with a small scoupe to try to get some air over the radiator?
Why not try a simple experiment..... Tie off some short pieces of Yarn string in front and behind the radiator and tie the gyro off on the ground and run the engine and watch the yarn and see if much air is being pulled through the radiator from the prop. You might be surprised. Not to mention that in flight you have even more air going through it.
I have a pic of two of a few other gyros with tilted radiators that had no problems keeping cool. Here you go. Three Subaru EA-81 powered machines and one Rotax 912S 100 horsepower machine. 3 of them have the same radiator as Paul has and all are tilted. I have personally flown all of them except the bright yellow Soma. All of them have no problems with cooling.
What about RAFs and Sparrowhawks? they have that huge cabin in front of the engine and radiator. Shouldn't they also have problems then according to your caculations?
06-28-2005, 06:23 AM
When Mike Morgan bought his EA81 powered Bensen, we had a hard time keeping the temperatures in check.
After trying everything else we could think of, we mounted a VW Rabbit radiator on it that I had sitting in the hangar.
The temps immediately dropped, and rarely get above 180º.
The old radiator was square and was mounted vertically in front of the prop.
The Rabbit radiator wouldn't fit in the same place, so we mounted it sideways on the diagonal engine supports.
No scoops or fins, and it works fine.
Also attached is a picture of Matt Steinke's subaru powered machine at the Waxahatchie fly-in in 2002. Note the slight angle that his radiator is mounted. He said hat it worked fine in that setup.
There was also a South African fella at BD this year with a Dominator-style machine with a 912S, and his radiator was mounted in the same fashion. I asked him if it cooled the engine well enough in that postion, and he reported no problems.
06-28-2005, 06:57 AM
Thanks for the info everyone. Up here near Dayton, years past, the summers havent been too bad, usually low to mid 80s. I try to fly a few times each week, but if I waited for it to cool off, I wouldnt be flying. As long as the water temp doesnt get too high, I will leave it.
06-28-2005, 11:42 AM
Yes, you guys have shown a lot of examples of radiators that work and that is good. I wasn't taking issue with any particular radiator or installation. What I was pointing out is that there are cooling system design parameters we should be using for maximum efficiency. The way we install cooling systems is to do what has worked for someone else without a thorough understanding why. Most of the time it works so there isn't a problem.
What I would like to see is someone that understands heat transfer systems try and optimize a cooling system(s). This may be an area where some weight can be saved because a large radiator with the attendant large coolant capacity may not be needed. I suspect this can be done as a paper exercise if someone has the BTU removal rate for any particular combo of radiator and coolant flow We know how many BTUs are being turned out from the amount of fuel used per hour. What we don't know is how much of that is lost thru the exhaust or from radiation. I'm saying we when it is me that doesn't know! But I bet some of our experts have the capability to get us a ball park number.
06-28-2005, 12:55 PM
You're right Dean.
The installations I showed above are probably not the most efficient.
But Paul, in his first post, wanted to know if a VW Rabbit radiator would work.
And it does.
If you figger out the BTU thing, don't forget to clue us in! :)
06-28-2005, 06:30 PM
I did not write this just found it and posted here.
The heat load to the cooling system is related to the flow through the radiator and the temperature drop through the radiator by the following expression:
Q = M * cp *dT
Where Q is the heat load BTU/min., M is the mass flow rate of the coolant in BTU per pound per degree F, dT is the temperature drop through the radiator in degrees F, and * indicates multiplication. Since a gallon of coolant weighs about 8.3 pounds, we can replace M in the expression by 8.3 times the coolant flow in gallons per minute, or GPM. The resulting expression is as follows:
Q = 8.3 * GPM * cp * dT
Since the specific heat of the coolant is essentially constant and the coolant flow rate is constant at rated engine speed, the expression tells us something that surprises most people.
That is, for a given heat load and coolant flow rate, the coolant temperature drop through the radiator will be constant, and nothing anyone can do to the design of the radiator can change that. Adding rows or fins or face area or whatever will not change the temperature drop through the radiator. As a general rule, cooling systems are designed to operate with a coolant temperature of about 190 degrees F at the radiator inlet and have about a 10 degree F temperature drop through the radiator at rated power and rated coolant flow. This will result in a bottom tank temperature of 180 degrees F. Note that the coolant temperature drop through the radiator must be specified in degrees F or degrees C, not percent. Taking a percentage of the radiator inlet temperature will yield different results depending on whether the inlet temperature is given in degrees F or degrees C.
Effects of Radiator Design on the Cooling System
A cooling system whose heat load and coolant flow rate results in a 10 degree F coolant temperature drop through the radiator will have that same coolant temperature drop whether the radiator has a very small face area and flat fins or a very large face area and louvered fins. The difference is that the large louvered fin radiator will be more effective than the small radiator at transferring heat to the cooling air, meaning that it can do it with a much lower difference in temperature between the core and cooling air. The small radiator may require such a high difference in temperature between the core and the cooling air and the core that the coolant may reach boiling temperature before the core is able to transfer all of the heat load to the cooling air. While both radiators would have the same coolant temperature drop through the radiator, we would say that the larger radiator had better heat transfer performance if its top tank temperature (Inlet coolant temperature) stabilized at, say, 180 degrees F while the smaller radiator stabilized at 220 degrees F.
06-28-2005, 06:46 PM
Water has a higher specific heat than an ethylene glycol or propylene glycol coolant mix. Therefore, it provides the best heat transfer performance in a cooling system. If a cooling system is marginal, that is, it only overheats on the hottest of days, then running with water as a coolant in the summer and an ethylene glycol or propylene glycol coolant solution during the rest of the year will probably solve the problem.
Most commercial coolant solutions provide cooling, anti-freeze protection, corrosion inhibitors to protect the metals in the cooling system, and a lubricant for the water pump. When running water as a coolant for maximum heat transfer, a product that provides a corrosion inhibitor and water pump lubricant should be added to the water.
In terms of the relative heat transfer performance of ethylene glycol versus propylene glycol coolant bases, they are pretty much equal when mixes according to the manufacturers’ recommendations, usually a 50/50 water to glycol mix. Ethylene glycol coolant solutions provide slightly higher heat transfer performance over propylene glycol solutions at low coolant flow rates.
We have been using a product called water wetter added to distiled water. It is a surfactant that allows more of the water to contact the metal thus increasing the heat transfer. We are also adding a lubricant / corrosion inhibitor to the mix. While this mixture provides better heat transfer it does not provide for freeze protection.
06-29-2005, 07:50 AM
.......We have been using a product called water wetter added to distiled water. It is a surfactant that allows more of the water to contact the metal thus increasing the heat transfer. We are also adding a lubricant / corrosion inhibitor to the mix. While this mixture provides better heat transfer it does not provide for freeze protection.Thanks for posting all this info, Michael! I think it provides a strong hint of what takes place and what the effect is of different configurations. I have only commented because gyro engine cooling seems to be an area that could stand more attention and understanding since it is not unusual for it to become a topic of discussion here.
I'm familiar with the terms and general principals since I spent 40 years in a work environment that sometimes required designing and incorporating heat exchange devices. I do not have the education and knowledge required to do the number crunching but fortunately I had access to the professionals on our team that could do that for me. And/or vendors were also willing to do that.
Heat transfer in general is pretty simple. It is strongly dependent on the difference in temp (commonly referred to as delta T) between the cooling media and the material to be cooled. The other factors involved are the specific heat values of the materials, heat transfer area and the flow rates. A change in any one of these will affect heat transfer; sometimes dramatically.
Oh yeah, while it is not really important if it works, I'm not sure Water Wetter is a surfactant. A surfactant is something that detergent manufacturers use while I suspect that Water Wetter is more of an agent to reduce water surface tension. A fact mentioned that some of us miss, is that straight water is the best coolant! But water can cause specific problems and therefore the need for anti-freeze with corrosion inhibitors and other agents. But we can't get carried away with their use because heat removal will suffer.
PR Gyro Pilot
06-29-2005, 08:16 AM
I had a bensen gyro with Subaru EA-81 engine and RFI reduction drive, and VW rabbit radiator, the temperature on the engine during climb was 210-220 deg. and around 185 during normal flight.
Im located in Puerto Rico where the temp is always between around 87deg-95deg
never had any problems with the EA-81. it flew nice always.
06-29-2005, 10:43 AM
Some of the guys who fly 582s with the Rabbit radiator in this part of the country (Oregon) find it necessary to block part of the radiator surface when flying in cool weather. I'm told the Rotax thermostat allows erratic swings in temperature if relied upon to regulate coolant temp. In the summer, the engine apparently needs all or much of the radiator surface to be exposed.
Does anyone else find this to be the case?
If blocking part of the radiator's surface is a common tactic, I would think that finding something other than a flat piece of cardboard could have a significant effect on drag. An adjustable, streamlined "nose" which could be positioned to block varying portions of the radiator's surface would at least get back some aerodynamic efficiency when part of the radiator had to be covered.
Or, is a radiator blocking 10 per cent of the prop disc area not really a problem? I don't know if anyone has actually measured.
A guy at El Mirage last year was running a Subaru EA-81, and using two heater cores instead of a radiator. He was grounded when it got above 90 degrees, but reportedly got away with it most of the time. The aerodynamic savings looked significant.
Has anyone tried Rotax's actual factory dual-radiator setup on a 582?
06-29-2005, 06:12 PM
A link to the redline info page for water wetter.
06-29-2005, 08:07 PM
From looking at the pics of other gyros, I can see were my problem might be. I have a direct drive EA81, with a 52in prop. Since the engine sits lower than one with a larger prop/redrive, my radiator is tilted at a 45 deg angle to the incoming airflow, which cannot be efficient, plus the top part is also blocked by the back of the seat. It would not cost me anything to turn the radiator sideways, plus mounted vertically, so it gets direct airflow to the front. This will be my next excuse to spend the day at the airport, and I will post my findings as to temp difference.
06-30-2005, 05:07 AM
Go ahead and try it and let us know how it turns out. I am willing to bet there is little to no difference...
Several of those gyros in the pics posted are direct drive subarus, and the radiator is mounted just like yours.
06-30-2005, 06:26 AM
Another thing to throw into the bit bucket is that if your flying a direct drive Soob your not making as much power (heat) as a belt drive machine.
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