Contra Rotating Propeller Drive for f3a 2m Pattern Planes
02-24-2015, 07:23 AM
#1401
Hi Brenner,
Yes I'm sure that testing is the way to prove the new setup.
On another matter, have you done any experimentation on prop separation and noise? If an increase in separation produces a significant reduction would the new design be amenable to allowing this?
Malcolm
Yes I'm sure that testing is the way to prove the new setup.
On another matter, have you done any experimentation on prop separation and noise? If an increase in separation produces a significant reduction would the new design be amenable to allowing this?
Malcolm
02-24-2015, 09:45 AM
#1402
Thread Starter
Hey Paul,
Thanks for the tip. I'll take a look at it. Generally though, if a mount is soft enough to isolate vibration, then it should have enough compliance to mitigate impact loads. If it's stiff enough to cause vibration, then the stiffness will need to be reduced for this reason.
The fundamental problem here is that stiffening a flexible motor mount will raise the natural resonant frequencies of the motor/Drive/airframe system, and in order for the system to be robust, these resonant frequencies need to be either low enough so that you go through them at low rpm, where the energy content is low, or high enough so that you don't get close to them at max rpm.
In my day job I design washing machines, and with washing machines we need to keep the resonant frequencies in the structure at least 5hz above the maximum spin speed of the machine. This means for a washer that spins at 1000rpm, the lowest resonant frequency in the structure has to be: 1000 + 5*60 = 1300rpm.
This is 30% higher than the washer spin speed, so for a model airplane that spins at 5500rpm the lowest resonant frequencies in the structure would need to be higher than 5500*1.3 = 7150 rpm. Typically, this is difficult to achieve because natural frequencies are notoriously difficult to move around. In fact, I think that in many installations, even with a solidly mounted system, the resonant frequencies might be be lower than this, especially with some front mounted outrunner installations.
Anyway, I think that the Inspire mounting system probably doesn't take the "stiff approach, and it is more likely that it is still compliant enough to keep the natural frequencies of the system enough below the max rpm of the Drive to avoid vibration issues.
If this is not the case, then the plane will become very sensitive to unbalance in the props, and the fuselage will vibrate very aggressively when the Drive is at max rpm. If it does have sufficient compliance the reverse will be true. The system will be robust to unbalanced props, and there will be very little vibration transmitted to the fuse.
Brenner ...
Thanks for the tip. I'll take a look at it. Generally though, if a mount is soft enough to isolate vibration, then it should have enough compliance to mitigate impact loads. If it's stiff enough to cause vibration, then the stiffness will need to be reduced for this reason.
The fundamental problem here is that stiffening a flexible motor mount will raise the natural resonant frequencies of the motor/Drive/airframe system, and in order for the system to be robust, these resonant frequencies need to be either low enough so that you go through them at low rpm, where the energy content is low, or high enough so that you don't get close to them at max rpm.
In my day job I design washing machines, and with washing machines we need to keep the resonant frequencies in the structure at least 5hz above the maximum spin speed of the machine. This means for a washer that spins at 1000rpm, the lowest resonant frequency in the structure has to be: 1000 + 5*60 = 1300rpm.
This is 30% higher than the washer spin speed, so for a model airplane that spins at 5500rpm the lowest resonant frequencies in the structure would need to be higher than 5500*1.3 = 7150 rpm. Typically, this is difficult to achieve because natural frequencies are notoriously difficult to move around. In fact, I think that in many installations, even with a solidly mounted system, the resonant frequencies might be be lower than this, especially with some front mounted outrunner installations.
Anyway, I think that the Inspire mounting system probably doesn't take the "stiff approach, and it is more likely that it is still compliant enough to keep the natural frequencies of the system enough below the max rpm of the Drive to avoid vibration issues.
If this is not the case, then the plane will become very sensitive to unbalance in the props, and the fuselage will vibrate very aggressively when the Drive is at max rpm. If it does have sufficient compliance the reverse will be true. The system will be robust to unbalanced props, and there will be very little vibration transmitted to the fuse.
Brenner ...
Last edited by Brenner; 02-24-2015 at 04:01 PM.
02-24-2015, 09:53 AM
#1403
Thread Starter
Hey Malcolm,
We did this kind of testing several years ago when we designed the V3 Drive. What we found is that the noise increases dramatically if the prop spacing is too small, but it drops off quickly as the prop spacing increases. The current spacing is considerably quieter than the spacing we started out with, even though we only increased it by 6mm or so.
As far as increasing the spacing even further, we are somewhat restricted by the 2m length limit in f3a, and the space available for spinners in current designs. We could make the spinner longer and reduce noise a little more, but we'd have a lot guys who can no longer meet the 2m requirement.
Brenner ...
We did this kind of testing several years ago when we designed the V3 Drive. What we found is that the noise increases dramatically if the prop spacing is too small, but it drops off quickly as the prop spacing increases. The current spacing is considerably quieter than the spacing we started out with, even though we only increased it by 6mm or so.
As far as increasing the spacing even further, we are somewhat restricted by the 2m length limit in f3a, and the space available for spinners in current designs. We could make the spinner longer and reduce noise a little more, but we'd have a lot guys who can no longer meet the 2m requirement.
Brenner ...
02-24-2015, 10:19 AM
#1404
Hi Brenner,
Yes I thought that might be the case, although you could look at a spinner shaped front prop nut and do away with the existing front spinner cone. This might give you a few more millimetres to work with.
Malcolm
Yes I thought that might be the case, although you could look at a spinner shaped front prop nut and do away with the existing front spinner cone. This might give you a few more millimetres to work with.
Malcolm
02-24-2015, 12:11 PM
#1406
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If weight is the concern with motor shaft diameter, why not stick with the common 8mm but make it hollow? You get the strength of the bigger diameter and the lightness of a hollow shaft. Kind of like going from a 25mm wing tube to 30mm. Plettenberg do this with their Advance motor although the shaft is significantly bigger than 8mm.
Cheers,
Jason.
02-24-2015, 12:51 PM
#1407
Thread Starter
Hey Jason,
Yes, that would definitely be the way to go. I think the Hacker Q80 does this too, This is also how we do it on the Drive side as well.
We'd have to get the motor manufacturer to do it for us though, and that might be a tough sell.
Brenner ...
Yes, that would definitely be the way to go. I think the Hacker Q80 does this too, This is also how we do it on the Drive side as well.
We'd have to get the motor manufacturer to do it for us though, and that might be a tough sell.
Brenner ...
02-24-2015, 01:27 PM
#1408
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I not 100% sure but the shafts may only be case hardened in which case hollowing out may be possible on a lathe. Have done this before on other parts to lighten them up. It would however void any warranty so best if the factory does it really.
Cheers,
Jason.
02-24-2015, 03:53 PM
#1409
Thread Starter
Hey Jason,
Yes, Neu wants to sell motors, but I don't think selling motors to f3a pilots is a tremendously big part of their business. Like a lot of manufacturers, I think they make motors for f3a as more of a courtesy than anything else.
Brenner ...
Yes, Neu wants to sell motors, but I don't think selling motors to f3a pilots is a tremendously big part of their business. Like a lot of manufacturers, I think they make motors for f3a as more of a courtesy than anything else.
Brenner ...
02-25-2015, 03:26 PM
#1410
Hey Paul,
Thanks for the tip. I'll take a look at it. Generally though, if a mount is soft enough to isolate vibration, then it should have enough compliance to mitigate impact loads. If it's stiff enough to cause vibration, then the stiffness will need to be reduced for this reason.
The fundamental problem here is that stiffening a flexible motor mount will raise the natural resonant frequencies of the motor/Drive/airframe system, and in order for the system to be robust, these resonant frequencies need to be either low enough so that you go through them at low rpm, where the energy content is low, or high enough so that you don't get close to them at max rpm.
In my day job I design washing machines, and with washing machines we need to keep the resonant frequencies in the structure at least 5hz above the maximum spin speed of the machine. This means for a washer that spins at 1000rpm, the lowest resonant frequency in the structure has to be: 1000 + 5*60 = 1300rpm.
This is 30% higher than the washer spin speed, so for a model airplane that spins at 5500rpm the lowest resonant frequencies in the structure would need to be higher than 5500*1.3 = 7150 rpm. Typically, this is difficult to achieve because natural frequencies are notoriously difficult to move around. In fact, I think that in many installations, even with a solidly mounted system, the resonant frequencies might be be lower than this, especially with some front mounted outrunner installations.
Anyway, I think that the Inspire mounting system probably doesn't take the "stiff approach, and it is more likely that it is still compliant enough to keep the natural frequencies of the system enough below the max rpm of the Drive to avoid vibration issues.
If this is not the case, then the plane will become very sensitive to unbalance in the props, and the fuselage will vibrate very aggressively when the Drive is at max rpm. If it does have sufficient compliance the reverse will be true. The system will be robust to unbalanced props, and there will be very little vibration transmitted to the fuse.
Brenner ...
Thanks for the tip. I'll take a look at it. Generally though, if a mount is soft enough to isolate vibration, then it should have enough compliance to mitigate impact loads. If it's stiff enough to cause vibration, then the stiffness will need to be reduced for this reason.
The fundamental problem here is that stiffening a flexible motor mount will raise the natural resonant frequencies of the motor/Drive/airframe system, and in order for the system to be robust, these resonant frequencies need to be either low enough so that you go through them at low rpm, where the energy content is low, or high enough so that you don't get close to them at max rpm.
In my day job I design washing machines, and with washing machines we need to keep the resonant frequencies in the structure at least 5hz above the maximum spin speed of the machine. This means for a washer that spins at 1000rpm, the lowest resonant frequency in the structure has to be: 1000 + 5*60 = 1300rpm.
This is 30% higher than the washer spin speed, so for a model airplane that spins at 5500rpm the lowest resonant frequencies in the structure would need to be higher than 5500*1.3 = 7150 rpm. Typically, this is difficult to achieve because natural frequencies are notoriously difficult to move around. In fact, I think that in many installations, even with a solidly mounted system, the resonant frequencies might be be lower than this, especially with some front mounted outrunner installations.
Anyway, I think that the Inspire mounting system probably doesn't take the "stiff approach, and it is more likely that it is still compliant enough to keep the natural frequencies of the system enough below the max rpm of the Drive to avoid vibration issues.
If this is not the case, then the plane will become very sensitive to unbalance in the props, and the fuselage will vibrate very aggressively when the Drive is at max rpm. If it does have sufficient compliance the reverse will be true. The system will be robust to unbalanced props, and there will be very little vibration transmitted to the fuse.
Brenner ...
02-25-2015, 05:24 PM
#1411
Thread Starter
Hey Joe,
The 5hz number isn't actually a requirement. It's just that when all the work is done, that's usually what it turns out to be. It's also very washer specific, and it's with respect to one particular mode that is aggressively excited by any imbalances in the system.
If you're designing a system with vibrational modes that don't really have a large forcing function, you can obviously get closer, but in our case what limits the max spin speed in a washer is a vibrational mode where the wash tub assembly flexes towards the spinning basket, potentially resulting in tub to basket contact during spin, and this mode is directly excited by imbalances in the wash load, so we have to stay away from it.
The actual design criteria is basket to tub gap closure at max spin, under worse case load conditions. We also don't like to design systems with amplification factors much above 2, because our analysis methods start becoming unreliable.
As far as model airplanes are concerned, it's very likely that you can design the system so that max rpm is closer to first resonant frequency, but if you do this you are going to see increased transmission of vibration, and the system is going to become more sensitive to prop balance. Also, in some systems there is still the possibility of catastrophic failure. (front mounted outrunners for instance..)
Brenner ...
The 5hz number isn't actually a requirement. It's just that when all the work is done, that's usually what it turns out to be. It's also very washer specific, and it's with respect to one particular mode that is aggressively excited by any imbalances in the system.
If you're designing a system with vibrational modes that don't really have a large forcing function, you can obviously get closer, but in our case what limits the max spin speed in a washer is a vibrational mode where the wash tub assembly flexes towards the spinning basket, potentially resulting in tub to basket contact during spin, and this mode is directly excited by imbalances in the wash load, so we have to stay away from it.
The actual design criteria is basket to tub gap closure at max spin, under worse case load conditions. We also don't like to design systems with amplification factors much above 2, because our analysis methods start becoming unreliable.
As far as model airplanes are concerned, it's very likely that you can design the system so that max rpm is closer to first resonant frequency, but if you do this you are going to see increased transmission of vibration, and the system is going to become more sensitive to prop balance. Also, in some systems there is still the possibility of catastrophic failure. (front mounted outrunners for instance..)
Brenner ...
02-28-2015, 08:26 AM
#1413
Thread Starter
Hey Tony,
What would be the intent? If it's to isolate vibration it would certainly work, but we would need some way to support the spinning gearcase.
Brenner ...
What would be the intent? If it's to isolate vibration it would certainly work, but we would need some way to support the spinning gearcase.
Brenner ...
02-28-2015, 09:15 AM
#1414
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Got it. I thought it would get rid of any bending moment on the motor shaft. But having not fully thought it through you'd then have to support both ends of the motor and the drive.
04-05-2015, 08:24 AM
#1417
Thread Starter
A quick update on our Contra V4 development.
We have been testing various motors, and various gear ratios to try and find optimum combinations, and the setup that seems to be the best so far uses a Kontronik Pyro 650 motor with a 1030kv winding, and an 8.4:1 gear ratio. We also have 780kv, and 830kv motors on order that we haven't tested, but the basic architecture of all these motors is the same.
This setup draws 94A, and puts out 3375W. The heat rise with this motor after five minutes or so at full throttle is 91F (33C) with a 54F (12C) ambient temperature. (see pictures..) This means that on a hot 100F (38C) day in Arizona, the maximum motor case temperature shouldn't exceed 140F (60C). However, in practice the actual heat rise will probably be noticeably less, because no one runs at full throttle for five minutes.
All this is good, but the best part is that this motor only weighs 295g all up. This results in a total all up V4 Contra Drive weight of 640g or so, (Minus plates and mounts..) which is very competitive with single prop setups.
See attached for some pictures of our test setup.
Brenner ...
We have been testing various motors, and various gear ratios to try and find optimum combinations, and the setup that seems to be the best so far uses a Kontronik Pyro 650 motor with a 1030kv winding, and an 8.4:1 gear ratio. We also have 780kv, and 830kv motors on order that we haven't tested, but the basic architecture of all these motors is the same.
This setup draws 94A, and puts out 3375W. The heat rise with this motor after five minutes or so at full throttle is 91F (33C) with a 54F (12C) ambient temperature. (see pictures..) This means that on a hot 100F (38C) day in Arizona, the maximum motor case temperature shouldn't exceed 140F (60C). However, in practice the actual heat rise will probably be noticeably less, because no one runs at full throttle for five minutes.
All this is good, but the best part is that this motor only weighs 295g all up. This results in a total all up V4 Contra Drive weight of 640g or so, (Minus plates and mounts..) which is very competitive with single prop setups.
See attached for some pictures of our test setup.
Brenner ...
Last edited by Brenner; 04-05-2015 at 08:29 AM.
04-05-2015, 09:22 AM
#1418
Hi Brenner,
Very interesting.
An outrunner !!? Those are the motors of choice for a lot of heli guys !
What props were you testing with.
Look forward to more results.
Brian
Very interesting.
An outrunner !!? Those are the motors of choice for a lot of heli guys !
What props were you testing with.
Look forward to more results.
Brian
04-05-2015, 10:35 AM
#1419
Thread Starter
Hey Brian,
I'm running a 22x20 rear and a 22x20 front.
This is a heli motor, and if it continues to work, maybe we can piggyback on all of the motor development that has gone into helicopters in recent years.
It's also reasonably inexpensive as well. I think we paid about 300 Euros for this one.
We actually have done two things to this motor to make it more suitable for use in an f3a application. We swapped out the driveshaft for an extended version so that we could use a bearing at the back to provide rear support, and we added a second front bearing, and a brass spacer tube, to support the axial loads from the Drive.
According to my calculations this was needed in order to push the expected bearing life up to 1500 flights.
We are also using soft Budd mounts on the rear support as well as the front motor plate, which gives the mounting a very solid, but still compliant feel. This was needed in order to fully isolate the fuselage structure from vibration, and it seems to work well.
Brenner ...
I'm running a 22x20 rear and a 22x20 front.
This is a heli motor, and if it continues to work, maybe we can piggyback on all of the motor development that has gone into helicopters in recent years.
It's also reasonably inexpensive as well. I think we paid about 300 Euros for this one.
We actually have done two things to this motor to make it more suitable for use in an f3a application. We swapped out the driveshaft for an extended version so that we could use a bearing at the back to provide rear support, and we added a second front bearing, and a brass spacer tube, to support the axial loads from the Drive.
According to my calculations this was needed in order to push the expected bearing life up to 1500 flights.
We are also using soft Budd mounts on the rear support as well as the front motor plate, which gives the mounting a very solid, but still compliant feel. This was needed in order to fully isolate the fuselage structure from vibration, and it seems to work well.
Brenner ...
04-05-2015, 11:16 AM
#1420
Brenner,
Kontronik do a std,, and a comp version of each of those motors.
I'm not sue what the difference between them is, maybe coil resistance and or bearings (though the max RPM is the same in both spec's,, )
Yes the heli,, guys really are max'ing out this stuff.
A very interesting development by you guys.
Brian
Kontronik do a std,, and a comp version of each of those motors.
I'm not sue what the difference between them is, maybe coil resistance and or bearings (though the max RPM is the same in both spec's,, )
Yes the heli,, guys really are max'ing out this stuff.
A very interesting development by you guys.
Brian
Last edited by serious power; 04-05-2015 at 11:18 AM.
04-05-2015, 01:42 PM
#1421
Thread Starter
Hey Brian,
We're using the standard versions. I didn't see any reason why we needed to spend the extra money for the competition version if the standard version worked so well.
The competition Pyro 650 motor is rated for 3.5kW versus 3kW for the standard motor, and based on what I'm seeing so far, I think the standard version could handle more than 4kW intermittently, which is more than we can use with existing battery technology.
Brenner...
We're using the standard versions. I didn't see any reason why we needed to spend the extra money for the competition version if the standard version worked so well.
The competition Pyro 650 motor is rated for 3.5kW versus 3kW for the standard motor, and based on what I'm seeing so far, I think the standard version could handle more than 4kW intermittently, which is more than we can use with existing battery technology.
Brenner...
04-06-2015, 06:36 AM
#1424
Hey Brian,
We're using the standard versions. I didn't see any reason why we needed to spend the extra money for the competition version if the standard version worked so well.
The competition Pyro 650 motor is rated for 3.5kW versus 3kW for the standard motor, and based on what I'm seeing so far, I think the standard version could handle more than 4kW intermittently, which is more than we can use with existing battery technology.
Brenner...
We're using the standard versions. I didn't see any reason why we needed to spend the extra money for the competition version if the standard version worked so well.
The competition Pyro 650 motor is rated for 3.5kW versus 3kW for the standard motor, and based on what I'm seeing so far, I think the standard version could handle more than 4kW intermittently, which is more than we can use with existing battery technology.
Brenner...
Their website does not say a lot - other than resistance is lower - not specified for these motors though, but is for a couple in the pyro range.
I think that the comp versions have or may have different wind options - 'D' or 'YY' . Thus it may be possible to get different characteristics via the different motors.
I, for one, would like to retain the low end torque the current Neu offers - they are 2Y wound.
If not there are re-winders out there that can do this anyway - The 3D Heli world is a world apart from the rest of us.
As you said we could piggyback on all that development - if desired.
Might be worth having a conversation with Kontronik !!??
Brian
04-06-2015, 07:23 AM
#1425
Thread Starter
Hey Brian,
I certainly agree with you that the Neu motor works well. It runs cool, has lots of power, and is the lightest of all current options.
I'll ask Kontronik some questions. Although I think there are language barriers. (I don't speak German..)
Brenner ...
I certainly agree with you that the Neu motor works well. It runs cool, has lots of power, and is the lightest of all current options.
I'll ask Kontronik some questions. Although I think there are language barriers. (I don't speak German..)
Brenner ...
