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RE: To Bench Test or Not?
ORIGINAL: the Wasp ""a low pressure area created by the prop near the hub"" well I am sure this is true "near the hub", but as long as the cylinder stays cool,,,er, it "the cylinder" will pull the heat out of the crack case, and the fresh oil/fuel/air mixture interring the engine will also help cool the lower end of a 2 stroke,, ""this part about thinking the propwash will provide a cooling air flow is incorrect"",, I cannot agree,, I think a more correct statement would be that the average flyer would be surprised to see how little airflow an engine needs to keep it at a good running temperature, many times I have had my hands behind running engines/props, you can not tell me a prop alone will not cool an engine, a prop creates much more airflow than a flywheel-fan on a weedeater, ,, I have often put my hand behind my running weedeater and noticed "how little airflow there was yet there is so much heat being removed,, I will agree our engines do get more cooling in flight, , but fact is, most engine damage to our engines comes in flight from tuning too lean,, I can say I have broken-in my MVVS and my 180 4stroker on the bench and for some reason they remand cool with only a prop to create airflow over the engine,, and, how many pilots have we all seen starting and run their engines on the ground "in" their planes for quite a long time and these engines do not over heat and become damaged,, and, I am quite sure our well made plane engines have a tighter cylinder tolerances than our old air cooled dirtbike engines of the 70.. |
RE: To Bench Test or Not?
as I said,, ""I will agree our engines do get more cooling in flight"",,,
but,,,,,,, (as for idling on the ground) there is a big diff between a big air-cooled 370 CID oil circulating opposed 6 cylinder fullsize 4 st engine with 2 tiny holes in the front of it's cowl,, and our well fined single cylinder 2 st that idle/turn 3 times as fast "on the bench",, ""Unless you have access to cylinder head temps like the full scales to monitor"" after being around 2 strokes as long as I have been I certainly don't need a temp gage to know if my 2st is too hot,, now a big 6 cylinder 4 st under a tight cowl I would want a temp gage.. Jim |
RE: To Bench Test or Not?
""3W suggests a method of break-in on a test stand, but you are running the engine at part throttle under a very light load to prevent cooking it""
yes, and 3W is trying to protects themselves from people that don't know what they are dong,, fact is many many people accost the world brake-in their engines on the bench without baffles and without hurting their engines,, remember that 2st oil test 2 years ago on RCU whereby the guy ran his engine for hours and hours and hours on the bench "and I believe it was a 3W",, he did not hurt that engine,, no baffles there.. Jim |
RE: To Bench Test or Not?
Eagle Tree FDR for mine. Also have a Venom temp recorder. The Venom is about a half inch thick by three quarters wide by about a inch and a half long. It only records the highest temp for a flight or bench run whereas the FDR records and plays back readings taken a number of times per second for the duration of the flight or run. Can't tell you much about what goes on inside of a cowl because none of mine are inside a cowl but I have data on what happens when they aren't cowled. This kind of information made me feel good about the 16 bucks I spent on the Venom but I ain't so sure about the FDR. It gives me some good info for my race planes so I guess it was money well spent too.
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RE: To Bench Test or Not?
ORIGINAL: Zeeb Well there is a bit of confusion here. The reasoning is flawed about where the cooling air comes from since there is actually a low pressure area created by the prop near the hub. While your statement that there is less load in the air is generally correct, this part about thinking the propwash will provide a cooling air flow is incorrect. It may seem counter intuitive, but I promise you if you start and run a full scale on the ground with the cowl nose bowls still in place but not secured by the top section or restrained by some safety wire or something, it will pull the nose bowls into the prop. If you run a spinner on a model prop, it reduces much of this because the non-airfoiled shape at the hub is covered up. Note that the cylinder head on a typical gasser is out at least 35-40% of the tip radius and you can see that there will be significant flow over the cylinder head due to the prop. The reversion only takes place very close in to the hub, once the prop blades start looking like airfoils, they start pushing air much better. If you think about it, the prop wash ALWAYS has to be higher velocity than the free stream air or else the plane will fall out of the sky, so I am not sure how in-flight airflow can be any different than on the bench. Mark |
RE: To Bench Test or Not?
well air does have "Mass",, therefore when flying at speed our planes do have more Air Mass hitting the front of our planes, so more Air Mass more Airflow more cooling,,
my only argument is about keeping a engine cool on the bench,, I will agree an engine will work harder on the ground or on the bench because it does not have the extra Air Mass crossing the prop, but I have run engines on the bench and they have stayed cool enough to the point they were not damaged.. Jim |
RE: To Bench Test or Not?
The engines that are most airflow critical also have the least cooling fin area. I have abused brand new Sachs and Zenoah engines by over propping and over leaning on a stand, couldnt hurt either one.
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RE: To Bench Test or Not?
ORIGINAL: the Wasp ""3W suggests a method of break-in on a test stand, but you are running the engine at part throttle under a very light load to prevent cooking it"" yes, and 3W is trying to protects themselves from people that don't know what they are dong,, fact is many many people accost the world brake-in their engines on the bench without baffles and without hurting their engines,, remember that 2st oil test 2 years ago on RCU whereby the guy ran his engine for hours and hours and hours on the bench "and I believe it was a 3W",, he did not hurt that engine,, no baffles there.. Jim |
RE: To Bench Test or Not?
""I have abused brand new Sachs and Zenoah engines by over propping and over leaning on a stand, couldnt hurt either one""
I have a MVVS 35 I broke it in on the bench with 2 gallons, tuned 500 RPMs below the peak RPM I could hold my fingers on the head for a good 5 count.. Jim |
RE: To Bench Test or Not?
Ask 3w about all the "pink" cylinders they get back because people believed the prop was all it took to force air through the cylinders. Then use a true cht gauge and telemetry system to compare that temp gun reading to the actual cylinder temps. Two common misconceptions will be debunked when you get the answers.
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RE: To Bench Test or Not?
ORIGINAL: Pat Roy Ask 3w about all the "pink" cylinders they get back because people believed the prop was all it took to force air through the cylinders. Then use a true cht gauge and telemetry system to compare that temp gun reading to the actual cylinder temps. Two common misconceptions will be debunked when you get the answers. Mark |
RE: To Bench Test or Not?
Me, I just run my engines on the plane until I feel they idle, transition and run at full-throttle consistently and reliably - usually with the cowl in place (because I have it setup so I can access the needle valves from outside), then go fly. It usually takes just a few minutes to achieve this condition with a few tweaks of the needles. Of course, I run the engine slightly rich for the first few flights as well.
I guess the question is then why don't I run engines on a test stand? My answer would be there is no need to as I have always had good results just flying them slightly rich for a few flights and gradually lean them until they are still slightly on the rich side of achieving full power. But I also have about 35 years of modeling experience and know how to set up a fuel tank and fuel lines, properly mount an engine, etc. If, however, you are not as knowledgeable or confident in setting up a model, maybe it would be a better idea for you to go ahead and run your engine on a test stand until you learn all about it. I don't think there is one, single, correct answer. It just depends on what works for you. |
RE: To Bench Test or Not?
""It just depends on what works for you"" ,, there you go,,
was it Ralph ?? that said he brakes-in his Zenoah GT 80 on the bench (for Racing), at 4000 RPMs for a couple hours ?? Jim |
RE: To Bench Test or Not?
It seems like the more you spend the less you get when it comes to cooling fin area and mounting options.
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RE: To Bench Test or Not?
It does not matter if you break your engine in on a test stand or on a airplane. What matters is oil ratio, how lean the engine hi-speed needle is set and its always good to vary the throttle when breaking in a engine. Heat cycles gets them broke in quicker and maybe better! Capt,n[)]
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RE: To Bench Test or Not?
mmattockx,
I saying that both of your conditions change once the engine is airborn. An engine on a stand running on the ground has profoundly different airflow conditions than when moving through the air. With or without a cowl has different airflow issues. Just drive your car in a light snow fall with the lights on and watch the way the snow flows over and around your hood. A large percentage of the stuff never makes contact with the car at all. It separates and deflects around the vehicle before it gets there. The snow is not making the decision to deflect, the air carrying the stuff is. You'll recognize very quickly how different shapes have different effects. On the test stand you can't possibly run full throttle for any length of time without additional directed airflow. I've done this too many times using telemetry feedback and temps to know that without additional air, temps become dangerous quickly. What do you learn about an engine that can't be run at max performance levels except for extremely short time periods? And yes, baffling can be everything in lowering temps, just as ducting air from the side of a cylinder on a test stand will. No baffling on a cowled engine can be multiple times worse than flying without a cowl at all. Once a cowl is installed around an engine very little air makes it too the engine. It was deflected by the prop disc and the shape of the cowl. What does get there has to be properly directed to make the best use of it. If not directed it will simply "bend" and largely miss that big, obstructive cylinder head altogether due to the increased pressure of the small amount of air that actually does make it to the engine. Temp guns are absolutely terrible for accurately depicting an engine temp. What makes them bad is that people believe the number they see to be true and accurate. They're not, and can easily be 100 degrees lower than what is actual. People fly hot and hard, reduce power for an approach, and then leisurely taxi back for a temp reading. By the time the engine gets to them it's already cooled 100 plus degrees from the descent, approach, and taxi. And that's only if they read the right place....If the engine is fuctioning correctly, mixtures near correct, and enough oil is being used, an engine will begin a rapid cooling process immediately after reducing the throttle in a no or low load situation such as a descent. Essentially only three items are "breaking in". The ring is getting matched to the cylinder, and the alloy components are reaching their maximum strength and molecular bond through heat cycling. The correct oil ratio for this process is largely determined by the components used in an engine. An engine with bearings in all the right places requires less oil that one with bushings. Generally, a bearing engine can be broken in anywhere from 32 to 50-1. A bushed engine can require ratios between 16 and 32-1, with normal running ratios limited to 32-1. Check with the manufacturer to find out what the engine requires. Mineral or synthetic oils shouldn't matter. |
RE: To Bench Test or Not?
Well said Pat. IR guns are useless for trying to see what is happening in the air and not too much better for seeing what is happening on the ground. You learn a lot with recorders like the eagle tree and even more if it is equiped with a radio for realtime telemetry on the ground.
I appreciate you sharing your wealth of knowledge. I didn't know some of this stuff until I got the FDR, the best I had before that was an IR gun and now I know that it didn't tell me a single thing useful about what the temps looked like in the air. |
RE: To Bench Test or Not?
ORIGINAL: Pat Roy On the test stand you can't possibly run full throttle for any length of time without additional directed airflow. I've done this too many times using telemetry feedback and temps to know that without additional air, temps become dangerous quickly. Can you run that same engine at full throttle (actually at the same power setting, not throttle position) in the air without the same result of overheating? With no changes to the cowling or baffling from the test stand runs, that is. I know you have enough experience with instrumented engines to probably have tried this, which most of us never get a shot at. Have you seen significant differences between ground runs and in-flight runs on the same aircraft? My contention is that airflow around the engine is dominated by the prop disk effects on the air and that doesn't change a lot between a test stand and flying from what I can see. If a person runs an uncowled and unbaffled engine on the stand and it overheats, then runs the same engine baffled and cowled in the air and it is fine, that is not a valid comparison because the operating conditions changed. I say that the same power output and same cowling/baffling runs the same temps whether on the ground or in the air. Do you have experience that says different? Mark |
RE: To Bench Test or Not?
The time differential between when an open engine over heats on the ground and in the air can be substantial. The ground runs typically heat up much, much faster. Thus the reasoning for additional directed airflow when performing ground runs at greater than mid power levels. For extended run periods, additional airflow at anything greater than low power levels are beneficial. It's a hard balance to strike. How cool or how warm do you need or want the engine to run? Too cool and the alloys fail to meld. Too hot and you damage rings and bearings. Catch 22 sometimes.
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RE: To Bench Test or Not?
ORIGINAL: Pat Roy The time differential between when an open engine over heats on the ground and in the air can be substantial. I'm not trying to be difficult, I really would like to know if there is a difference and where it comes from. Mark |
RE: To Bench Test or Not?
Dead air in front of a prop vs. air at speed relative to the airframe. Volume and pressure. Everything is relative to the same power settings.
Example. Before the advent of aux electric cooling fans and fan clutches on automobiles (prolly before a lot reading this were born) engine water temps would often rise dangerously when the car was stopped with the engine running for long periods of time. The fan was still working since there was no fan clutch. You could temporarily lower the water temps by revving the engine, but only for limited periods of time before they would increase again. Once you got the car rolling again the temps would fall. The rpm could and would be the same stopped or moving but the air flow relative to the forward motion of the car increased as the car increased speed, increasing the amount of cooling air through the radiator. Cylinder fins are the radiator when it comes to two strokes, just missing the additional water heat transfer medium, making airflow that much more critical. Add a cowl to a stationary and a moving engine and the airflow issues change drastically. The more blunt the cowl or surface area of an engine the more the air flow departs from a straight path, missing the engine altogether. Pressure develops at the face of the object changing the directional flow of incoming air behind the pressure front. The low pressure area created at a propeller is at the wrong side of the propeller to assist cooling. The "low" is at the front of the propeller... Better example, Buy a telemetry system or data logger and do the comparisons personally. Saves the keystrokes of trying to convince those that don't want to believe. Not being a smart a**, just telling it like it is. |
RE: To Bench Test or Not?
ORIGINAL: Pat Roy Buy a telemetry system or data logger and do the comparisons personally. Saves the keystrokes of trying to convince those that don't want to believe. Not being a smart a**, just telling it like it is. Thanks, Mark |
RE: To Bench Test or Not?
Although the Eagle Tree is pretty expensive it provides quite a few user functions that others don't, and they are small. You don't need a live telemetry dashboard unless you do a lot of ground testing. You can't watch the dashboard when flying... The data logger they have (base unit for all the systems) is extremely useful. Buying the optional USB serial port for routing the data link to the outside of the fuselage is a very nice feature to have unless you like removing the hatch to download data. All of this presumes you have a laptop to facilitate gathering data. Obtain a couple extra thermocouples for different engines. That also allows room to screw one up experimenting with different mounting locations.
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RE: To Bench Test or Not?
ORIGINAL: mmattockx ORIGINAL: Pat Roy The time differential between when an open engine over heats on the ground and in the air can be substantial. I'm not trying to be difficult, I really would like to know if there is a difference and where it comes from. Mark |
RE: To Bench Test or Not?
ORIGINAL: Pat Roy Although the Eagle Tree is pretty expensive it provides quite a few user functions that others don't, and they are small. You don't need a live telemetry dashboard unless you do a lot of ground testing. You can't watch the dashboard when flying... The data logger they have (base unit for all the systems) is extremely useful. Buying the optional USB serial port for routing the data link to the outside of the fuselage is a very nice feature to have unless you like removing the hatch to download data. All of this presumes you have a laptop to facilitate gathering data. Obtain a couple extra thermocouples for different engines. That also allows room to screw one up experimenting with different mounting locations. I will take a look at it. The laptop is no problem. What type of thermocouple are they using? Do they clip or bolt to a cooling fin (I seem to recall hearing that mentioned elsewhere in this forum)? The kart racers have nice ones that replace the washer under the spark plug, so they get a reading right from dead center of the head in an area of minimal cooling. It has been a while since I looked at them, I will have to do some research. Mark |
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