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#161
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Electrically Powered Ultralight Aircraft
Dan Luke wrote:
"Charles Vincent" wrote: Just because it is not noticeable, or measurable by the lack of sensitivity with the instrument you are currently not using, does not mean that it does not exist. More weight on the bearings will cause more rolling resistance. That is fact, not open to dispute. If you say it is, I want to buy the rights to the bearings you are using, so I can patent them and make a fortune. If a bird craps on your windshield, it is more likely to noticeably influence your aerodynamic drag than rolling resistance.....I took Jim's "can't be found" to mean lost in the noise. According to SAE studies, aerodynamic drag accounts for 60% of the resistance that must be overcome for highway cruise, with tires being 25% and driveline friction making up the last 15%. Pardon the intrusion on this interesting discussion, but just how *does* added weight in a car impose extra load on the powerplant besides via bearing friction and tire deformation? Added weight means the powerplant is doing more work to maintain the same speed; there's no way around it, the laws of physics demand it. So where's the extra power going? Heating the brakes. :-) Matt |
#162
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Electrically Powered Ultralight Aircraft
In rec.aviation.piloting Dan Luke wrote:
"Charles Vincent" wrote: Just because it is not noticeable, or measurable by the lack of sensitivity with the instrument you are currently not using, does not mean that it does not exist. More weight on the bearings will cause more rolling resistance. That is fact, not open to dispute. If you say it is, I want to buy the rights to the bearings you are using, so I can patent them and make a fortune. If a bird craps on your windshield, it is more likely to noticeably influence your aerodynamic drag than rolling resistance.....I took Jim's "can't be found" to mean lost in the noise. According to SAE studies, aerodynamic drag accounts for 60% of the resistance that must be overcome for highway cruise, with tires being 25% and driveline friction making up the last 15%. Pardon the intrusion on this interesting discussion, but just how *does* added weight in a car impose extra load on the powerplant besides via bearing friction and tire deformation? It takes more power to accelerate the car to cruise speed in a given time. F=ma Added weight means the powerplant is doing more work to maintain the same speed; there's no way around it, the laws of physics demand it. So where's the extra power going? Ummm, no, quite the opposite. The laws of physics say once an object is in motion it takes no energy to maintain the velocity UNLESS there is some other force at work that would cause the velocity to decrease. Since at a constant speed, the a in F=ma is zero, the force is zero no matter the mass. Once at speed in a car (or airplane or rocket ship) the only energy needed to maintain speed is that equal to any drag forces that would otherwise slow the car down. Have you looked at the current crop of high mileage cars? They all have very aerodynamic profiles to get the air drag down. -- Jim Pennino Remove .spam.sux to reply. |
#163
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Electrically Powered Ultralight Aircraft
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#164
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Electrically Powered Ultralight Aircraft
In rec.aviation.piloting Ernest Christley wrote:
wrote: The laws of physics say once an object is in motion it takes no energy to maintain the velocity UNLESS there is some other force at work that would cause the velocity to decrease. Since at a constant speed, the a in F=ma is zero, the force is zero no matter the mass. Once at speed in a car (or airplane or rocket ship) the only energy needed to maintain speed is that equal to any drag forces that would otherwise slow the car down. Have you looked at the current crop of high mileage cars? They all have very aerodynamic profiles to get the air drag down. They also have very narrow, hard tires. Unfortunately, the DOT has laws against solid rubber tires or they could be made even harder. Your analysis would be mostly correct if we were talking about trains. My analysis of what? The biggest source of drag on a car is air followed by tires. Of course the makers are going to put hard tires on as well as streamline the vehicle to get mileage up. The less drag, the less gas the vehicle uses. What's your point? -- Jim Pennino Remove .spam.sux to reply. |
#165
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Electrically Powered Ultralight Aircraft
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#166
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Electrically Powered Ultralight Aircraft
In rec.aviation.piloting Charles Vincent wrote:
wrote: The biggest source of drag on a car is air followed by tires. Of course the makers are going to put hard tires on as well as streamline the vehicle to get mileage up. The less drag, the less gas the vehicle uses. What's your point? That is only true in cruise on the highway. In stop and go city driving driveline friction is the majority, followed by inertia. Air and tire is a small percentage combined. Inertia is not drag. Inertia is F=ma. In stop and go driving, F=ma dominates. If it didn't, hybrids converting the F in deceleration into energy in the batteries instead of heat in the brakes wouldn't get their high mileage numbers. -- Jim Pennino Remove .spam.sux to reply. |
#167
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Electrically Powered Ultralight Aircraft
"Phil" wrote I know you're only half serious, Yep, only half, until you really start to think about it. but yes, that would have to be considered. That's a risk in hybrid autos as well. EMTs and firefighters are taking special training to handle the wrecks of these cars. And the gasoline we use for our current airplanes poses the risk of incinerating the occupants in a crash. I am not sure that an electric plane would actually pose more risk. I think there is a higher risk, perhaps by many times. Ever seen a LiPo Battery have a catestrophic failure? One of the primary ways a LiPo can be caused to fail in that way is physical damage. Ask the electric RC guys. Most of them would never think of putting even a slightly physically damaged LiPo back into service, unless it was a really cheap plane that they wanted to see destroyed. Now imagine a battery many thousands (or even a few hundred) times larger, and larger capacity to match. I'll take my chances with the gasoline fire, thanks, ANY day. That speaks nothing of the chance of electrocution, or chemical burns or injury due to the cell's chemestry. I would think that the increased reliability of the propulsion system would decrease the risk overall. How many people are killed every year in crashes caused by engine failures? How much more reliable is an electric of that size ( to run a decent sized airplane with decent performance) and power going to be, especially if it is designed with lightness as a major design consideration? That remains yet to be seen. OK, even if we give the electric a given reliability superiority, that is not going to save all that many lives. Most power failures in I.C. powered airplanes are not that big of deal, and many times never even reported. Far more die due to stupid pilot tricks (a broad spectrum category to lump a bunch of other things together) than loss of power. Nope, lots of problems to consider before we start considering an electric aircraft. Lots more than we can maybe even consider, at the moment, even if we were to figure out a way to make a practical airplane electric powered, don't you think? -- Jim in NC |
#169
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Electrically Powered Ultralight Aircraft
wrote:
In rec.aviation.piloting Ernest Christley wrote: wrote: The laws of physics say once an object is in motion it takes no energy to maintain the velocity UNLESS there is some other force at work that would cause the velocity to decrease. Since at a constant speed, the a in F=ma is zero, the force is zero no matter the mass. Once at speed in a car (or airplane or rocket ship) the only energy needed to maintain speed is that equal to any drag forces that would otherwise slow the car down. Have you looked at the current crop of high mileage cars? They all have very aerodynamic profiles to get the air drag down. They also have very narrow, hard tires. Unfortunately, the DOT has laws against solid rubber tires or they could be made even harder. Your analysis would be mostly correct if we were talking about trains. My analysis of what? The biggest source of drag on a car is air followed by tires. Of course the makers are going to put hard tires on as well as streamline the vehicle to get mileage up. The less drag, the less gas the vehicle uses. What's your point? The point is that weight matters...even in land-locked vehicles. |
#170
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Electrically Powered Ultralight Aircraft
In rec.aviation.piloting Ernest Christley wrote:
wrote: In rec.aviation.piloting Ernest Christley wrote: wrote: The laws of physics say once an object is in motion it takes no energy to maintain the velocity UNLESS there is some other force at work that would cause the velocity to decrease. Since at a constant speed, the a in F=ma is zero, the force is zero no matter the mass. Once at speed in a car (or airplane or rocket ship) the only energy needed to maintain speed is that equal to any drag forces that would otherwise slow the car down. Have you looked at the current crop of high mileage cars? They all have very aerodynamic profiles to get the air drag down. They also have very narrow, hard tires. Unfortunately, the DOT has laws against solid rubber tires or they could be made even harder. Your analysis would be mostly correct if we were talking about trains. My analysis of what? The biggest source of drag on a car is air followed by tires. Of course the makers are going to put hard tires on as well as streamline the vehicle to get mileage up. The less drag, the less gas the vehicle uses. What's your point? The point is that weight matters...even in land-locked vehicles. In cars, weight matters most in acceleration and doesn't matter in any significant amount with modern tires in cruise. -- Jim Pennino Remove .spam.sux to reply. |
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