Post by flydiver on Dec 12, 2009 18:40:14 GMT 1
Props come in a LOT of sizes and configurations. It's pretty confusing. How do you sort it out?
Here's an analogy that may be useful.
You are the motor/propulsion, in this case you are rowing a canoe. The prop is the paddle. You get to paddle the canoe with an assortment of paddles-we have a bunch of sizes, and lengths available.
First, lets try one about the size and strength of a wooden yardstick, skinny and flexible. You would have no trouble pushing it through the water, the canoe would go no where. You might even break it.
So, I hand you a sheet of 4x8 plywood nailed to a 2x4. I can just hear you, "Keerist, you expect me to paddle that!" Not even possible.
Then we have a series of paddles graded in between the 2 extremes. One of them will work best for YOU. If we put a 6 year old kid (small motor) they would work best with a smaller and skinnier paddle. If you put an Olympic weight lifter in they would likely choose a much longer and fatter paddle. Of course the kid couldn't do well with a full size canoe and the weight lifter could really move the same canoe out. It works best if you match the power output to the need.
People, like electric motors have an 'optimal' work level. Go way under it and you don't do much. Go much over the optimum and they just can't do the work.
The analogy breaks down with KV because people are kind of limited in that regard, electric motors are not. But they are still limited in work load.
If you take 2 identically sized motors and wind them to change the KV (RPM/volt) the higher KV motor will handle a few more amps but to do that the prop size (either the length or pitch, or both) must come down. If you spin a smaller prop faster (like lots faster) it has the capability of higher speed but lacks thrust. As an extreme example: like a tractor moves slowly and powerfully (thrust), and a motorcycle can move fast but sure couldn't pull even a tiny plow.
Gearing is the ability to take a high KV motor, make the effective KV come down so it can swing a larger prop. The stock Cub is a 3:1 gear ratio (I think - don't have one anymore).
Lets's assume the motor is 3000KV (too high most likely).
3:1 > an effective KV of 1000 so it can swing the larger 10x8 prop.
If you put the same prop on it direct drive it would burn up in seconds. You would need to drop to something like a 6x4 to drive directly.
Gearing is like your car or for the personal analogy think of you being the power on a 10-speed bike. A low gear will allow you to climb a hill that you couldn't even attempt in top gear. OTOH the low gear will just spin kind of worthlessly on the flat and the high gear will get you some speed but it'll take you a bit to get up there.
(BTW -this IS done. The Multiplex Minimag is about Cub size. It's a very good aileron step up from the Cub. It comes with a 400 brushed direct drive motor and a tiny 5x5 prop. It's a stupid setup, done ONLY because it's dirt cheap, The company can claim it's giving you a motor. It is mostly worthless. It will fly the plane -barely if you give it enough time to get up to speed.)
Fat bladed slow fly props generally have small hubs. They are DESIGNED to handle the loads of a slower rotation. They are like the larger fatter paddle. If you try to drive them too fast they CAN break (the handle). They are most likely to do this at HIGH RPM so you have a pointy/skinny/sharp blade going for your eyes or those of your fly buddies. GWS slow fly props are VERY prone to this. You can hear the prop distort if driven too fast. They are barely adequate on a 7-cell Cub and I consider them dangerous on lipo.
Higher RPM direct drive props don't have as much surface area so get their thrust from moving smaller amounts of air but moving it more often. You can often compensate for the reduced blade surface area by going a bit larger.
The prop IS THE WORK LOAD. To sustain that work load the electrics have to be up to the job, ALL the electrics - motor, ESC, battery. On any given power setup one is the weakest link and that weak link will fail if over driven.
The ORIGINAL Cub was designed for a 480 geared motor, driven by 7-cells NiMh. The ESC/RX was designed to handle THAT load and a bit extra for 'buffer'. When it went LP the voltage went from 9.6v (7-cell fresh charge) to 12.6v 3S lipo, a 30% jump with a corresponding increase in motor RPM.
It's like you've taken your OLD VW bug and instead of driving it at 60mph, you are now asking it do do a steady 90mph instead. Sometime, somewhere, something is likely to break if you insist on doing that. It wasn't made for that.
To compensate for the increase in power the LP props down from 10x8 > 9x6. This keeps the work on the motor and electrics more in line with the design specs.
But the power hungry Cub crowd wants more and then wants even MORE. The site even SAYS you can use the 10x8 for float work. Why won't it work? It may, kind of like the VW can be used for passing up to 90 but I doubt it will sustain a long trip at that load.
So, how many of you are using a STOCK 10x8 (or similar) on a stock geared motor with a 3S lipo?
How many of you have burned up something with this setup?
How long did it take?
Here's an analogy that may be useful.
You are the motor/propulsion, in this case you are rowing a canoe. The prop is the paddle. You get to paddle the canoe with an assortment of paddles-we have a bunch of sizes, and lengths available.
First, lets try one about the size and strength of a wooden yardstick, skinny and flexible. You would have no trouble pushing it through the water, the canoe would go no where. You might even break it.
So, I hand you a sheet of 4x8 plywood nailed to a 2x4. I can just hear you, "Keerist, you expect me to paddle that!" Not even possible.
Then we have a series of paddles graded in between the 2 extremes. One of them will work best for YOU. If we put a 6 year old kid (small motor) they would work best with a smaller and skinnier paddle. If you put an Olympic weight lifter in they would likely choose a much longer and fatter paddle. Of course the kid couldn't do well with a full size canoe and the weight lifter could really move the same canoe out. It works best if you match the power output to the need.
People, like electric motors have an 'optimal' work level. Go way under it and you don't do much. Go much over the optimum and they just can't do the work.
The analogy breaks down with KV because people are kind of limited in that regard, electric motors are not. But they are still limited in work load.
If you take 2 identically sized motors and wind them to change the KV (RPM/volt) the higher KV motor will handle a few more amps but to do that the prop size (either the length or pitch, or both) must come down. If you spin a smaller prop faster (like lots faster) it has the capability of higher speed but lacks thrust. As an extreme example: like a tractor moves slowly and powerfully (thrust), and a motorcycle can move fast but sure couldn't pull even a tiny plow.
Gearing is the ability to take a high KV motor, make the effective KV come down so it can swing a larger prop. The stock Cub is a 3:1 gear ratio (I think - don't have one anymore).
Lets's assume the motor is 3000KV (too high most likely).
3:1 > an effective KV of 1000 so it can swing the larger 10x8 prop.
If you put the same prop on it direct drive it would burn up in seconds. You would need to drop to something like a 6x4 to drive directly.
Gearing is like your car or for the personal analogy think of you being the power on a 10-speed bike. A low gear will allow you to climb a hill that you couldn't even attempt in top gear. OTOH the low gear will just spin kind of worthlessly on the flat and the high gear will get you some speed but it'll take you a bit to get up there.
(BTW -this IS done. The Multiplex Minimag is about Cub size. It's a very good aileron step up from the Cub. It comes with a 400 brushed direct drive motor and a tiny 5x5 prop. It's a stupid setup, done ONLY because it's dirt cheap, The company can claim it's giving you a motor. It is mostly worthless. It will fly the plane -barely if you give it enough time to get up to speed.)
Fat bladed slow fly props generally have small hubs. They are DESIGNED to handle the loads of a slower rotation. They are like the larger fatter paddle. If you try to drive them too fast they CAN break (the handle). They are most likely to do this at HIGH RPM so you have a pointy/skinny/sharp blade going for your eyes or those of your fly buddies. GWS slow fly props are VERY prone to this. You can hear the prop distort if driven too fast. They are barely adequate on a 7-cell Cub and I consider them dangerous on lipo.
Higher RPM direct drive props don't have as much surface area so get their thrust from moving smaller amounts of air but moving it more often. You can often compensate for the reduced blade surface area by going a bit larger.
The prop IS THE WORK LOAD. To sustain that work load the electrics have to be up to the job, ALL the electrics - motor, ESC, battery. On any given power setup one is the weakest link and that weak link will fail if over driven.
The ORIGINAL Cub was designed for a 480 geared motor, driven by 7-cells NiMh. The ESC/RX was designed to handle THAT load and a bit extra for 'buffer'. When it went LP the voltage went from 9.6v (7-cell fresh charge) to 12.6v 3S lipo, a 30% jump with a corresponding increase in motor RPM.
It's like you've taken your OLD VW bug and instead of driving it at 60mph, you are now asking it do do a steady 90mph instead. Sometime, somewhere, something is likely to break if you insist on doing that. It wasn't made for that.
To compensate for the increase in power the LP props down from 10x8 > 9x6. This keeps the work on the motor and electrics more in line with the design specs.
But the power hungry Cub crowd wants more and then wants even MORE. The site even SAYS you can use the 10x8 for float work. Why won't it work? It may, kind of like the VW can be used for passing up to 90 but I doubt it will sustain a long trip at that load.
So, how many of you are using a STOCK 10x8 (or similar) on a stock geared motor with a 3S lipo?
How many of you have burned up something with this setup?
How long did it take?
