I did a test on RC prop system. Put dual props on a test stand at various separations (something like -50% to +10%). Results showed not a huge impact on efficiency until overlapping quite a bit. But there is a significant noise difference. If you want maximum quiet, try 1". If you want compactness, 0.1" is also OK but a bit noisier. Most of my frames so far have been for world records so have used minimum spacing of 0.1" to remove weight.
carbon balsa also in switzerland. I try to build your x2 built in your custom octa, what diameter for the motor mast did you use? mast width 0.86, what does this mean?
Do you have any videos of your x2 built? Whats your experience in the meantime with it?
the octa X2 was my first ship and the first to set the world record for flight duration. then i went to a quad but will soon complete the octa X2. but getting there has taken a lot of steps.
- Figure out the gimbal (that was completed about 6 months ago)
- Learn FPV before i fly a valuable octa X2.
so now i'm focusing on a durable quad X spider to learn FPV. once i learn FPV, then I'll build another much larger quad to set the next record and thenl get back to an octa X2 when my payload costs go up.
i remember once losing an engine on a octa X2 and not even knowing it (really not paying attention). Started it up, took off, hovered for a bit, started flying and my brother says, "Hey, i think one of your motors is dead." The ESC wire had come loose. So really stable platforms.
The .86 was inches. The key on the mast is stiffness. You can do the calculations pretty easy.
- Let us say that a 15 mm carbon tube with 0.6 mm wall = 1 on a stiffness scale.
- A stiffness of 1 is more than adequate for a 17" prop quad
- A stiffness of 5.5 is adequate for a 17" prop octa X2 (a 24 mm dia. carbon tube with 0.8 mm wall)
On the Swiss site for tubing, use the following equation.
Stiffness Factor = (WallThickness/0.6) x (OD/15)^3
Use that equation on the 24 mm tube with 0.8 mm walls to make sure you get 5.5
Order that the tube you want and then to verify, give it the "Twang" test.
- Figure out how much of the tube will be unsupported
... a quad with a 17" prop this is about 9" to 12" on a spider
... on an octa X2 with 17" props this is about 26" to 29"
... the rest of the mast might be supported or stiffened by the Electronics Platform.
- Lay the full length tube to be used for the mast (most come in 1 to 2 meters long) on a table.
- Extend one end of the tube over the end of the table the unsupported distance calculated above.
- Press the rest of the tube tightly to the top of the table
- Twang the end of the tube that is hanging over the table
- Excellent is that there is no harmonic.
- Good is that the harmonic only lasts for one or at most two cycles and stops.
- Bad is that you hear a twang (any sustained note).
So make your best guess. Buy the tube. Then test it
It is also OK to:
- buy tubes that fit within tubes. The second ship that broke the world record used that method. Big OD on the mid section with normal OD on the final section.
- use two smaller tubes that are bonded on top of each other for z stiffness and then use the side spars between the masts to create x/y stiffness.
I tested the same craft, MT3515-400KV motors, same all up weight, same APM vibration dampening with two sets of propellers:
-APC 15x4 electric : i used to like the APC slow fly I had used in 10x4.7, 11x4.7 and 12x3.8 formats. Thay gave me previously good results with the MN3110-700Kv motors. Now with these 15x4 I measure vibrations at minimum +3/-3 which is awful. I balanced them as good as I could, it did not change a bit the level of vibrations.
weight of these APC props : 38g
-XOAR wood props 14x5, weight 27g (10g lighter!). Reduced the vibrations in half : +1,5/-1,5
I love these props, not only are they efficient but also they are nice looking!
You have inspired me to setup a small testbed to measure vibrations generated by different propellers.
I am thinking of measuring like so:
-I will use a well balanced reserve MT3515 motor I have on stock
-it will be fixed on a aluminum arm. It will be long enough to position a smartphone on the other end so that the propeller does not blow on it.
-I will run a seismic application on the smartphone that gives a frequency spectrum analysis on three axis.
-the measurements will be influenced by the supporting base of the arm. I might try to use Velcro beneath the arm to hold the arm without dampening the vibrations to measure.
-I will take different throttle measurement points for each propeller. Each point will be identified with a power meter.
-result: a main identified vibration frequency for each type of propeller, at different RPM
Does it look to be a correct measurement procedure to you?
This is a great test bench. I wish I could steal a room in my house to setup something like that (with a fire extinguisher of course).
Were you able to test a XOAR propeller ? These are apparently excellent wood props, prebalanced out of factory and as light as CF props!
I must replace the current APC 15x4 props I installed on the newly assembled X8 octo with MT3515-400KV motors because I get awful vibrations, even after balancing the props. I get twice as much vibrations on the IMU of APM (in the range of +3/-3 while I was at +1.5/-1.5 with the previous MN3110-700KV and 12inch props). Using the same APM vibraiton isolation (Kyosho zeal equivalent from 3M).
I'm pretty sure this is caused by these APM props. I see with naked eye the non perfectly flat surface of the moulded plastic. These props have also a very very thick hub ring that I can't seem to balance. I will just throw them away.
Obviously it is also not abnormal to get higer vibrations with bigger props, but I did not think the increase from 12inch to 15 inch would double my vibrations on APM !
This the the ship test stand in its current form. Shown are two ship and the two flight locations.
o It can be wood 2x4s (also not as hard as aluminum).
o It doubles as the structure to support the motor test stand on the right
o Place above for flight tests.
o Place below for initial tests.
o Structure line with pool noodles so props don't break
o Ship loosely held in place with thin bungee cords that can be affixed to multiple ship points for initial tests or at dead center for PID tuning (so it flies well enough for auto-tune and for fine tuning after auto-tune).
o Try to keep the upper platform easily to modify if you plan to use/build different shape ships.
o Think where the props will go as the ship rolls, pitches, yaws and climbs.
o Make it oversized
Most important on use. Always ramp up slowly. The copter can get out of control quickly. When held by bungees the spring affect can make out-of-control worse. At a minimum:
o Always wear protective eye wear.
o Wear thick leather gloves when in range of possible copter paths.
o Wear thick protective clothing.
o Wear full coverage shoes
o Always have your hand on the throttle
o Always have a dead-man switch in reach as throttle does not always remove power after a crash.
The test procedure is as important as the test equipment.
When taking energy (watts or watts, amps, and voltage) and thrust data:
o To compare the results of different rotors (motor + prop), calculate net thrust per watt. For copters, the rotor has to lift itself first. Net thrust (I have an excel worksheet that does this) is thrust read on the tared scale less:
- motor weight
- prop weight
- prop mount & screws
- motor mount & screws
- structure
o To compare the results of different props diameters, structure needs to be taken into account. I use a structure penalty of 2 grams per inch over 10 inches (so an 8" prop would receive a -4g structure weight and a 17" prop would receive a 14g structure weight)
o Record the voltage and keep it constant between tests (voltage matters and can bias results by more than 3%)
o Measure energy at various thrust levels as rotor efficiency varies (there is a curved peak in the middle of the thrust efficiency curve; it does not peak at maximum rpm as thought by many)
- estimated hover thrust per motor at AUW
- 80% hover
- 1.2x of hover
- 1.5x hover
- 2x hover
- max thrust
o balance props axial and in trace before beginning and check the result with a vibration meter before starting
o tare the scale, ramp up to full thrust for a few seconds and return to zero. Check tare as the wires will settle.
o when testing a new prop, record noise level at hover and elsewhere as needed.
o record temperature
o keep you eye on the thermometer (I burned up a Tiger Motor that belonged to Hugues by not paying close enough attention)
o when comparing different motors, always use the same ESC. If you change ESC settings, note those changes.
o label and record a prop number. CW & CCW props will perform typically around 2 - 7% different. Different props off of the same mold will perform typically up to 4% different or worse.
o Oh ... per Hugues recommendation, I added a fire extinguisher near by.
This is the current form of the test stand used for motor tests. The components are:
o a frame for holding the thrust post (in this case extruded aluminum machine frame)
o a thrust post that translates rotor thrust to the scale and is at least 2x prop diameter long to avoid ground effect bias (something light so the scale can tare with room for thrust)
o a soft connection device for lightly holding the thrust post to the frame (once the motor starts to spin, the rotor and thrust post will hold itself so there is a 6mm gap between the post and structure loosely held by an elastomeric speaker connector with a 4mm thick sorbothane washer between).
o a platform at the top to bolt down the motor mount (at least three places)
o a universal motor mount so fit to the base on any motor and made of highly thermal conductive material (in this case, aluminum)
o a thermocouple mounted to the motor mount to measure the temperature of the motor.
o a thermocouple display to show the temperature
o a vibration sensor connected to the top of the thrust post to ensure that the props are adequately balanced for the test.
o a vibration meter (in this case an Extech SDL800)
o velcro at the bottom of the thrust post connected to the top of the scale to counter rotor torque
o a gram scale accurate to 1 grams that can be tared (in this case a Polder 5kg x 1g KSC-350-90)
o a hard base for the scale (on carpet so needed a nylon slab)
o scale position markings so the thrust post stays perpendicular
o a sound level meter that calculates dba or dbc always placed 1m from the rotor (in this case a Radio Shack hand held)
o a hand held laser thermometer for sensing the temperatures inside the motor or on the housing
o a watt meter afixed close to the scale reading for near simultaneous visual reading (in this case a Hobby King HK-010)
o power cord adapter going from the watt meter to the motor
o a DC power supply to precisely control voltage (motor efficiency varies significantly with voltage)
o a power cord going from the power supply to the watt meter
o a switch on the power cord capable of handling the amperage
o a transmitter
o a receiver & power supply for connecting the ESC to throttle (this varies and is a topic in itself)
o a computer for recording the data
o a plexiglass plate separating the the computer from the test area
o a soft cover for the edge of the plexiglass plate (in this case foam pipe insulation)
o safety glasses hanging next to the computer for ease of access
o a first aid kit with anticeptic, large gauze and tape nearby
o a phone within reach for calling for help
o tools nearby for changing motors and props
What is lacking the most in this setup? A computer interface that reads in real time, the thrust, watts, dB, and temperature into Excel (I'd settle for the first two). When testing a lot, this would save so much time and be a bit more accurate.
Replies
I did a test on RC prop system. Put dual props on a test stand at various separations (something like -50% to +10%). Results showed not a huge impact on efficiency until overlapping quite a bit. But there is a significant noise difference. If you want maximum quiet, try 1". If you want compactness, 0.1" is also OK but a bit noisier. Most of my frames so far have been for world records so have used minimum spacing of 0.1" to remove weight.
hi Forrest, in the meantime i read most of your points in this forum, just fantastic!
I got 2216 scotch in switzerland. The carbon tubes i will get from here:
http://www.carbontubes.eu/
carbon balsa also in switzerland. I try to build your x2 built in your custom octa, what diameter for the motor mast did you use? mast width 0.86, what does this mean?
Do you have any videos of your x2 built? Whats your experience in the meantime with it?
the octa X2 was my first ship and the first to set the world record for flight duration. then i went to a quad but will soon complete the octa X2. but getting there has taken a lot of steps.
- Figure out the gimbal (that was completed about 6 months ago)
- Learn FPV before i fly a valuable octa X2.
so now i'm focusing on a durable quad X spider to learn FPV. once i learn FPV, then I'll build another much larger quad to set the next record and thenl get back to an octa X2 when my payload costs go up.
i remember once losing an engine on a octa X2 and not even knowing it (really not paying attention). Started it up, took off, hovered for a bit, started flying and my brother says, "Hey, i think one of your motors is dead." The ESC wire had come loose. So really stable platforms.
The .86 was inches. The key on the mast is stiffness. You can do the calculations pretty easy.
- Let us say that a 15 mm carbon tube with 0.6 mm wall = 1 on a stiffness scale.
- A stiffness of 1 is more than adequate for a 17" prop quad
- A stiffness of 5.5 is adequate for a 17" prop octa X2 (a 24 mm dia. carbon tube with 0.8 mm wall)
On the Swiss site for tubing, use the following equation.
Stiffness Factor = (WallThickness/0.6) x (OD/15)^3
Use that equation on the 24 mm tube with 0.8 mm walls to make sure you get 5.5
Order that the tube you want and then to verify, give it the "Twang" test.
- Figure out how much of the tube will be unsupported
... a quad with a 17" prop this is about 9" to 12" on a spider
... on an octa X2 with 17" props this is about 26" to 29"
... the rest of the mast might be supported or stiffened by the Electronics Platform.
- Lay the full length tube to be used for the mast (most come in 1 to 2 meters long) on a table.
- Extend one end of the tube over the end of the table the unsupported distance calculated above.
- Press the rest of the tube tightly to the top of the table
- Twang the end of the tube that is hanging over the table
- Excellent is that there is no harmonic.
- Good is that the harmonic only lasts for one or at most two cycles and stops.
- Bad is that you hear a twang (any sustained note).
So make your best guess. Buy the tube. Then test it
It is also OK to:
- buy tubes that fit within tubes. The second ship that broke the world record used that method. Big OD on the mid section with normal OD on the final section.
- use two smaller tubes that are bonded on top of each other for z stiffness and then use the side spars between the masts to create x/y stiffness.
Some test results about our prop discussion:
I tested the same craft, MT3515-400KV motors, same all up weight, same APM vibration dampening with two sets of propellers:
-APC 15x4 electric : i used to like the APC slow fly I had used in 10x4.7, 11x4.7 and 12x3.8 formats. Thay gave me previously good results with the MN3110-700Kv motors. Now with these 15x4 I measure vibrations at minimum +3/-3 which is awful. I balanced them as good as I could, it did not change a bit the level of vibrations.
weight of these APC props : 38g
-XOAR wood props 14x5, weight 27g (10g lighter!). Reduced the vibrations in half : +1,5/-1,5
I love these props, not only are they efficient but also they are nice looking!
You have inspired me to setup a small testbed to measure vibrations generated by different propellers.
I am thinking of measuring like so:
-I will use a well balanced reserve MT3515 motor I have on stock
-it will be fixed on a aluminum arm. It will be long enough to position a smartphone on the other end so that the propeller does not blow on it.
-I will run a seismic application on the smartphone that gives a frequency spectrum analysis on three axis.
-the measurements will be influenced by the supporting base of the arm. I might try to use Velcro beneath the arm to hold the arm without dampening the vibrations to measure.
-I will take different throttle measurement points for each propeller. Each point will be identified with a power meter.
-result: a main identified vibration frequency for each type of propeller, at different RPM
Does it look to be a correct measurement procedure to you?
This is a great test bench. I wish I could steal a room in my house to setup something like that (with a fire extinguisher of course).
Were you able to test a XOAR propeller ? These are apparently excellent wood props, prebalanced out of factory and as light as CF props!
I must replace the current APC 15x4 props I installed on the newly assembled X8 octo with MT3515-400KV motors because I get awful vibrations, even after balancing the props. I get twice as much vibrations on the IMU of APM (in the range of +3/-3 while I was at +1.5/-1.5 with the previous MN3110-700KV and 12inch props). Using the same APM vibraiton isolation (Kyosho zeal equivalent from 3M).
I'm pretty sure this is caused by these APM props. I see with naked eye the non perfectly flat surface of the moulded plastic. These props have also a very very thick hub ring that I can't seem to balance. I will just throw them away.
Obviously it is also not abnormal to get higer vibrations with bigger props, but I did not think the increase from 12inch to 15 inch would double my vibrations on APM !
This the the ship test stand in its current form. Shown are two ship and the two flight locations.
o It can be wood 2x4s (also not as hard as aluminum).
o It doubles as the structure to support the motor test stand on the right
o Place above for flight tests.
o Place below for initial tests.
o Structure line with pool noodles so props don't break
o Ship loosely held in place with thin bungee cords that can be affixed to multiple ship points for initial tests or at dead center for PID tuning (so it flies well enough for auto-tune and for fine tuning after auto-tune).
o Try to keep the upper platform easily to modify if you plan to use/build different shape ships.
o Think where the props will go as the ship rolls, pitches, yaws and climbs.
o Make it oversized
Most important on use. Always ramp up slowly. The copter can get out of control quickly. When held by bungees the spring affect can make out-of-control worse. At a minimum:
o Always wear protective eye wear.
o Wear thick leather gloves when in range of possible copter paths.
o Wear thick protective clothing.
o Wear full coverage shoes
o Always have your hand on the throttle
o Always have a dead-man switch in reach as throttle does not always remove power after a crash.
o Never get close to an armed copter.
The test procedure is as important as the test equipment.
When taking energy (watts or watts, amps, and voltage) and thrust data:
o To compare the results of different rotors (motor + prop), calculate net thrust per watt. For copters, the rotor has to lift itself first. Net thrust (I have an excel worksheet that does this) is thrust read on the tared scale less:
- motor weight
- prop weight
- prop mount & screws
- motor mount & screws
- structure
o To compare the results of different props diameters, structure needs to be taken into account. I use a structure penalty of 2 grams per inch over 10 inches (so an 8" prop would receive a -4g structure weight and a 17" prop would receive a 14g structure weight)
o Record the voltage and keep it constant between tests (voltage matters and can bias results by more than 3%)
o Measure energy at various thrust levels as rotor efficiency varies (there is a curved peak in the middle of the thrust efficiency curve; it does not peak at maximum rpm as thought by many)
- estimated hover thrust per motor at AUW
- 80% hover
- 1.2x of hover
- 1.5x hover
- 2x hover
- max thrust
o balance props axial and in trace before beginning and check the result with a vibration meter before starting
o tare the scale, ramp up to full thrust for a few seconds and return to zero. Check tare as the wires will settle.
o when testing a new prop, record noise level at hover and elsewhere as needed.
o record temperature
o keep you eye on the thermometer (I burned up a Tiger Motor that belonged to Hugues by not paying close enough attention)
o when comparing different motors, always use the same ESC. If you change ESC settings, note those changes.
o label and record a prop number. CW & CCW props will perform typically around 2 - 7% different. Different props off of the same mold will perform typically up to 4% different or worse.
o Oh ... per Hugues recommendation, I added a fire extinguisher near by.
This is the current form of the test stand used for motor tests. The components are:
o a frame for holding the thrust post (in this case extruded aluminum machine frame)
o a thrust post that translates rotor thrust to the scale and is at least 2x prop diameter long to avoid ground effect bias (something light so the scale can tare with room for thrust)
o a soft connection device for lightly holding the thrust post to the frame (once the motor starts to spin, the rotor and thrust post will hold itself so there is a 6mm gap between the post and structure loosely held by an elastomeric speaker connector with a 4mm thick sorbothane washer between).
o a platform at the top to bolt down the motor mount (at least three places)
o a universal motor mount so fit to the base on any motor and made of highly thermal conductive material (in this case, aluminum)
o a thermocouple mounted to the motor mount to measure the temperature of the motor.
o a thermocouple display to show the temperature
o a vibration sensor connected to the top of the thrust post to ensure that the props are adequately balanced for the test.
o a vibration meter (in this case an Extech SDL800)
o velcro at the bottom of the thrust post connected to the top of the scale to counter rotor torque
o a gram scale accurate to 1 grams that can be tared (in this case a Polder 5kg x 1g KSC-350-90)
o a hard base for the scale (on carpet so needed a nylon slab)
o scale position markings so the thrust post stays perpendicular
o a sound level meter that calculates dba or dbc always placed 1m from the rotor (in this case a Radio Shack hand held)
o a hand held laser thermometer for sensing the temperatures inside the motor or on the housing
o a watt meter afixed close to the scale reading for near simultaneous visual reading (in this case a Hobby King HK-010)
o power cord adapter going from the watt meter to the motor
o a DC power supply to precisely control voltage (motor efficiency varies significantly with voltage)
o a power cord going from the power supply to the watt meter
o a switch on the power cord capable of handling the amperage
o a transmitter
o a receiver & power supply for connecting the ESC to throttle (this varies and is a topic in itself)
o a computer for recording the data
o a plexiglass plate separating the the computer from the test area
o a soft cover for the edge of the plexiglass plate (in this case foam pipe insulation)
o safety glasses hanging next to the computer for ease of access
o a first aid kit with anticeptic, large gauze and tape nearby
o a phone within reach for calling for help
o tools nearby for changing motors and props
What is lacking the most in this setup? A computer interface that reads in real time, the thrust, watts, dB, and temperature into Excel (I'd settle for the first two). When testing a lot, this would save so much time and be a bit more accurate.
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