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#61
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On Tue, 15 Jul 2003 11:54:37 -0600, "Bill Daniels"
wrote: Another area where I would like to see some experimental data is the "Radiator Ramjet" (just to pick a controversial term) where the radiator is in a tube and the heated air exits the rear of the tube at a higher velocity than the cool air entering the front of the tube, theoretically producing a small amount of thrust that offsets the drag of the radiator. Bill Daniels "Jay" wrote in message . com... Hi Bill, You don't need experiental data for this Bill, you just described the P-51 Mustang cooling system. However, even with three heat exchangers putting out heat into the exhaust air and a 1400 horsepower engine producing the heat, the Mustang never actually managed to get a net thrust out of the system. In addition, the point where the cooling system was ***ALMOST*** equalling drag was a very specific speed and altitude. I forget the exact height but it was above 20,000 feet and the speed was over 300 mph. Only under those circumstances did the power being generated and the speed being flown produce the necessary heat to accelerate the exhaust air flow to nearly cancel out cooling drag. By the way, most of the cooling systems did this to some fashion, but the Mustang was the first to actually design the cooling system to really benefit from it. This concept was researched and written up by a British aerodynamicist by the name of Meridith, and the produced thrust became known as the "Meridith Effect". North American designed the Mustang's system using the best aerodynamicists available at the time and with virtually unlimited resources to manufacture the kind of heat exchangers that would work in this environment. By the end of WWII, almost all research into liquid cooled systems came to a halt as jet powered aircraft became the future for military aircraft. I'm not an aerodynamics engineer, just a home builder. But my impression is that most relatively slow homebuilt or GA airplanes do not produce the heat needed to really accelerate the exhaust flow to make much out of the Meridith Effect. After all, we're always leaning out and cruising at reduced power settings. We have big wings, for the most part, and a lot of drag. Something really slippery like a Long EZ or Vari EZ or Glassair or Lancair might be fast enough to benefit, but getting the cooling system designed and fitted within the tiny wetted area of the fuselage might be nearly impossible. From my personal point of view, it's far more important to make sure the cooling system does the job all day and every day and on the ground too than to agonize over a few mph, real or imaginary. Corky Scott |
#62
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"Corky Scott" wrote in message ... On Tue, 15 Jul 2003 11:54:37 -0600, "Bill Daniels" wrote: Another area where I would like to see some experimental data is the "Radiator Ramjet" (just to pick a controversial term) where the radiator is in a tube and the heated air exits the rear of the tube at a higher velocity than the cool air entering the front of the tube, theoretically producing a small amount of thrust that offsets the drag of the radiator. Bill Daniels "Jay" wrote in message . com... Hi Bill, You don't need experiental data for this Bill, you just described the P-51 Mustang cooling system. However, even with three heat exchangers putting out heat into the exhaust air and a 1400 horsepower engine producing the heat, the Mustang never actually managed to get a net thrust out of the system. In addition, the point where the cooling system was ***ALMOST*** equalling drag was a very specific speed and altitude. I forget the exact height but it was above 20,000 feet and the speed was over 300 mph. Only under those circumstances did the power being generated and the speed being flown produce the necessary heat to accelerate the exhaust air flow to nearly cancel out cooling drag. By the way, most of the cooling systems did this to some fashion, but the Mustang was the first to actually design the cooling system to really benefit from it. This concept was researched and written up by a British aerodynamicist by the name of Meridith, and the produced thrust became known as the "Meridith Effect". North American designed the Mustang's system using the best aerodynamicists available at the time and with virtually unlimited resources to manufacture the kind of heat exchangers that would work in this environment. Yeah, North American did well with the Mustang given that it was just old "slide rule" engineers one generation ahead of me working on it. I imagine the kids these days using Computational Fluid Dynamics programs and modern materials could improve on the Meridith Effect - maybe a lot. I don't want to get too far form the original posters idea on skin radiators. That idea is worth some experiments too. Bill Daniels |
#63
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The Mustang's cooling system (an external combustion ramjet) is probably
about as good as it is going to get utilizing radiators for heat exchangers, because the thrust produced by a ramjet is very dependent on internal efficiencies (drag). A radiator is a very high drag ramjet heat source (combustor) because of its large surface area, and relatively poor aerodynamics. The net thrust of a ramjet type cooling system could be increased if a more efficient (lower drag) method is found to transfer the heat to the internal airflow. I agree with Corky's statement that effective cooling is more important (the Mustang's was inadequate for prolonged ground operation) than a few miles per hour in cruise for slower aircraft. However, for aircraft cruising above 150-175 MPH, I believe cooling drag is certainly high enough to be of interest to any designer. RJ "Corky Scott" wrote in message ... On Tue, 15 Jul 2003 11:54:37 -0600, "Bill Daniels" wrote: Another area where I would like to see some experimental data is the "Radiator Ramjet" (just to pick a controversial term) where the radiator is in a tube and the heated air exits the rear of the tube at a higher velocity than the cool air entering the front of the tube, theoretically producing a small amount of thrust that offsets the drag of the radiator. Bill Daniels "Jay" wrote in message . com... Hi Bill, You don't need experiental data for this Bill, you just described the P-51 Mustang cooling system. However, even with three heat exchangers putting out heat into the exhaust air and a 1400 horsepower engine producing the heat, the Mustang never actually managed to get a net thrust out of the system. In addition, the point where the cooling system was ***ALMOST*** equalling drag was a very specific speed and altitude. I forget the exact height but it was above 20,000 feet and the speed was over 300 mph. Only under those circumstances did the power being generated and the speed being flown produce the necessary heat to accelerate the exhaust air flow to nearly cancel out cooling drag. By the way, most of the cooling systems did this to some fashion, but the Mustang was the first to actually design the cooling system to really benefit from it. This concept was researched and written up by a British aerodynamicist by the name of Meridith, and the produced thrust became known as the "Meridith Effect". North American designed the Mustang's system using the best aerodynamicists available at the time and with virtually unlimited resources to manufacture the kind of heat exchangers that would work in this environment. By the end of WWII, almost all research into liquid cooled systems came to a halt as jet powered aircraft became the future for military aircraft. I'm not an aerodynamics engineer, just a home builder. But my impression is that most relatively slow homebuilt or GA airplanes do not produce the heat needed to really accelerate the exhaust flow to make much out of the Meridith Effect. After all, we're always leaning out and cruising at reduced power settings. We have big wings, for the most part, and a lot of drag. Something really slippery like a Long EZ or Vari EZ or Glassair or Lancair might be fast enough to benefit, but getting the cooling system designed and fitted within the tiny wetted area of the fuselage might be nearly impossible. From my personal point of view, it's far more important to make sure the cooling system does the job all day and every day and on the ground too than to agonize over a few mph, real or imaginary. Corky Scott |
#64
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#65
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Excellent answer A, in cruise flight, there is little difference between a
propeller's slipstream and freestream velocity. Today's props are very efficient (near 90% max) and create little turbulence, with the exception of the tip's vortex . I have opened the window on a c-150 and put my arm out to feel the propwash, and other than a slight pulsing the only substantial disturbance is where my hand penetrates the prop's tip vortex ring. Try it some time. This is not to say the flow across a fuselage behind a prop is laminar, I'm nearly certain it isn't as laminar boundary layers needs very little disturbance to become turbulent, but total chaos does not exist within the slipstream behind a prop. The stagnation point can also be defined as the point on the leading edge of a surface/component that the velocity normal (perpendicular) to the skin is 0. The Meredith effect, and ramjets in general, create stream flow by maintaining a negative pressure gradient (high to low pressure) streamwise within the duct/engine. This gradient is created by area differentials, fixed or variable, between the inlet and exhaust. The inlet area is smaller than the exhaust area, and thrust is a product of the duct's massflow times the inlet's and exhaust's differential velocities. In the case of a cooling system, if the internal aero losses are to high, due to friction across radiators, etc., then the net thrust may be negative. RJ wrote in message ... You'd be surprised how *little* a prop accellerates the air. The thrust is a product of how much air is accellerated and the change in velocity. A prop going 150 mph is working on a lot of cubic feet of air per second - a cyl of air the diameter of the prop and the distance the airplane goes in a second. Changing the velocity of that amount of air just 10 mph gives oodles of thrust. The back prop on a 337 is seeing air that's moving backward at about 15 mph. The slower the airplane is going, the more it has to accellerate the air to get the same thrust, because it's accellerating less air. The stagnation point is the point on the leading edge of the wing where the air is splitting - some goes over the wing, some goes under the wing. Above the stagnation point the air goes over, below, under. AT the stagnation point (which is very, very small) it piles up then falls off one way or the other. In article , says... I'm not clear on what the stagnation point is. Maybe you can expand you explanation more? My line of thinking was basically this: Airplane flies in clean air, propeller bites into clean air at high sub sonic speeds, creates column of high velocity air, but at the same time, wastes bunches of energy also churning it up. As air moves back past fuselage and wing roots, churning energy disipates (decaying exponentially with distance from prop). Now I'm sure that the airframe itself introduces some turbulence of its own, but this is distributed all over and is less concentrated at its source(s) I think a lot of people are thinking about airplanes minus the propeller or thinking of pusher configurations. This may be one of the reasons why pusher power plants seem often have over-heating issues. Taking the 337 for example: On the surface you'd say, the back of the airplane must be traveling at the same speed as the front of the aiplane so what's the deal? But going to the second order effects, the front engine lives in an air flow with speeds much higher than IAS and the turbulence of the cooling stream for the front engine is MUCH higher than it is the intake scoop for the rear engine because of the aformentioned exponential decay of the turbulence in the prop wash. On the Meredith Effect. Intuitively I don't see how its supposed to work. You take air in the front of the scoop, you heat it up, and it expands (ideal gas law). How does the air know thats only supposed to go out the rear? Regards "RJ Cook" wrote in message ... Jay, The turbulence within a boundary layer over an aerodynamic surface INCREASES with distance DOWNSTREAM of the stagnation point. The cowl of an aircraft would have less turbulence within the boundary layer than nearly anywhere else on the fuselage. RJ |
#66
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Thanks for contributing to the fray...
I wasn't really talking about cruise so much as I was talking about climb-out: high engine output, low vehicle speed, high AOA. Arm test- If you can feel pulsing 5 feet back from the prop, then you know that there is air of varying velocities washing over your arm. And since you also know that the distubance decays very quickly at first and then more gradually as time (and airplane) goes by, the disturbance is much higher a one foot behind the propeller. I'm sure its not total chaos, and it isn't laminar either, something between the two. Question is, does the amount of turbulence produced, boost the heat transfer capability of that air enough for a surface radiator on the front/under surface of the cowl to work for a 100-200HP engine? And yes, the first step is to make sure that you've got reliable cooling. Okay, check, you've done that. Now what can be done to eliminate one of the largest sources of drag? If this actually worked, it would be a huge incentive for more water cooled (AKA auto conversion) power plants in aircraft. "RJ Cook" wrote in message ... Excellent answer A, in cruise flight, there is little difference between a propeller's slipstream and freestream velocity. Today's props are very efficient (near 90% max) and create little turbulence, with the exception of the tip's vortex . I have opened the window on a c-150 and put my arm out to feel the propwash, and other than a slight pulsing the only substantial disturbance is where my hand penetrates the prop's tip vortex ring. Try it some time. This is not to say the flow across a fuselage behind a prop is laminar, I'm nearly certain it isn't as laminar boundary layers needs very little disturbance to become turbulent, but total chaos does not exist within the slipstream behind a prop. The stagnation point can also be defined as the point on the leading edge of a surface/component that the velocity normal (perpendicular) to the skin is 0. The Meredith effect, and ramjets in general, create stream flow by maintaining a negative pressure gradient (high to low pressure) streamwise within the duct/engine. This gradient is created by area differentials, fixed or variable, between the inlet and exhaust. The inlet area is smaller than the exhaust area, and thrust is a product of the duct's massflow times the inlet's and exhaust's differential velocities. In the case of a cooling system, if the internal aero losses are to high, due to friction across radiators, etc., then the net thrust may be negative. RJ |
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