Need Help for Design/Build/Fly Contest
11-30-2009, 05:58 PM
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Need Help for Design/Build/Fly Contest
Hi all,
First off, a brief explanation: this Design/Build/Fly contest is sponsored by AIAA and college students from all over the country create a plane that must compete against the other teams in various "missions" for a score with the top three teams receiving several thousand dollars in scholarships. This is our school's (New Mexico State University) first time entering the competition, and only one other person on the team has previous experience in the RC hobby. I am the pilot of the plane to be, but I have practically no experience with electric motors so I come to you RCU on my team, my college's behalf. What would be the best brushless motor for a 8' plane weighing about 11 pounds dry and up to 18 with payload. There are several rule requirements that the electric system must meet, I've pasted the section of the rules that pertains to the propulsion of the plane
Any help is greatly appreciated, if anyone could tell me what the best motor and ECU to use for this situation it would help us greatly
Thanks so much, Bryndan
First off, a brief explanation: this Design/Build/Fly contest is sponsored by AIAA and college students from all over the country create a plane that must compete against the other teams in various "missions" for a score with the top three teams receiving several thousand dollars in scholarships. This is our school's (New Mexico State University) first time entering the competition, and only one other person on the team has previous experience in the RC hobby. I am the pilot of the plane to be, but I have practically no experience with electric motors so I come to you RCU on my team, my college's behalf. What would be the best brushless motor for a 8' plane weighing about 11 pounds dry and up to 18 with payload. There are several rule requirements that the electric system must meet, I've pasted the section of the rules that pertains to the propulsion of the plane
• Must be propeller driven and electric powered with an unmodified over-the-counter model electric motor. May use multiple motors and/or propellers. May be direct drive or with gear or belt reduction.
• Motors may be any commercial brush or brushless electric motor.
• For safety, each aircraft will use a commercially produced propeller/blades. Must use a commercially available propeller hub/pitch mechanism. Teams may modify the propeller diameter by clipping the tip, and may paint the blades to balance the propeller. No other modifications to the propeller are allowed. Commercial ducted fan units are allowed.
• Motors and batteries will be limited to a maximum of 40 Amp current draw by means of a 40 Amp fuse (per motor or battery pack) in the line from the positive battery terminal to the motor controller. Only ATO or blade style plastic fuses may be used. (e.g. "Maxi" size Slow Blow, 1.15"x0.85". Available online www.Mcmaster.com part #7460K51)
• Must use over the counter NiCad or NiMH batteries. For safety, battery packs must have shrink-wrap or other protection over all electrical contact points. The individual cells must be commercially available, and the manufacturers label must be readable/documented (i.e. clear shrink wrap preferred). All battery disconnects must be "fully insulated" style connectors.
• Maximum propulsion battery pack weight is defined in the mission rules section. This battery pack must power propulsion systems only. Radio Rx and servos MUST be on a separate battery pack. Batteries may not be changed or charged between sorties during a flight period.
• Battery packs must weigh less than 4 lbs
• Motors may be any commercial brush or brushless electric motor.
• For safety, each aircraft will use a commercially produced propeller/blades. Must use a commercially available propeller hub/pitch mechanism. Teams may modify the propeller diameter by clipping the tip, and may paint the blades to balance the propeller. No other modifications to the propeller are allowed. Commercial ducted fan units are allowed.
• Motors and batteries will be limited to a maximum of 40 Amp current draw by means of a 40 Amp fuse (per motor or battery pack) in the line from the positive battery terminal to the motor controller. Only ATO or blade style plastic fuses may be used. (e.g. "Maxi" size Slow Blow, 1.15"x0.85". Available online www.Mcmaster.com part #7460K51)
• Must use over the counter NiCad or NiMH batteries. For safety, battery packs must have shrink-wrap or other protection over all electrical contact points. The individual cells must be commercially available, and the manufacturers label must be readable/documented (i.e. clear shrink wrap preferred). All battery disconnects must be "fully insulated" style connectors.
• Maximum propulsion battery pack weight is defined in the mission rules section. This battery pack must power propulsion systems only. Radio Rx and servos MUST be on a separate battery pack. Batteries may not be changed or charged between sorties during a flight period.
• Battery packs must weigh less than 4 lbs
Any help is greatly appreciated, if anyone could tell me what the best motor and ECU to use for this situation it would help us greatly
Thanks so much, Bryndan
11-30-2009, 06:36 PM
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RE: Need Help for Design/Build/Fly Contest
I don't know of any volt limit anywhere in the rules, just weight and amps. As for the lipo's i think the reason they don't allow them is that it would prob make it far less of a challenge overall, just a guess...
11-30-2009, 06:45 PM
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RE: Need Help for Design/Build/Fly Contest
Here is one power system you can try: 18 or 20 inch prop., Axi 5330/18, Castle HV-30, 4 x 6 Cell NiMH 4600mAh (15.7 oz each).
11-30-2009, 10:56 PM
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RE: Need Help for Design/Build/Fly Contest
You did not say what the missions are, speed?, payload?, endurance?, etc. Each would have a different requirement. Also I assume 48 volts would be the maximum as that is considered the highest safe voltage, however if not there appears to be no limit on power.
Can the motor be rewound to a new configuration?
It does not appear that there are any limits on the ESC. Model airplane ESC's are OK but there are much better designs. One big problem is at part throttle where the flywheel or inductive current flows through the body diodes of the FETs which have a large voltage drop (.7 to over 2 volts). This is lost power, similar crappy switching power supplies. The highest efficiency units do not use diodes, instead use FETs.
I think sinusoidal drive is more efficient than trapezoidal drive. You could look at Field Orientated Control.
Can the motor be rewound to a new configuration?
It does not appear that there are any limits on the ESC. Model airplane ESC's are OK but there are much better designs. One big problem is at part throttle where the flywheel or inductive current flows through the body diodes of the FETs which have a large voltage drop (.7 to over 2 volts). This is lost power, similar crappy switching power supplies. The highest efficiency units do not use diodes, instead use FETs.
I think sinusoidal drive is more efficient than trapezoidal drive. You could look at Field Orientated Control.
11-30-2009, 11:56 PM
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RE: Need Help for Design/Build/Fly Contest
The missions are: 1. a 2 lap timed trying to get the fastest time 2. Payload, 3 laps with a internal cargo of up to ten softballs( dice rolled to determine how many we have to carry 1- 10) 3. Payload, 3 laps with 4 wooden dowels hung from under the wings.
So as you can see there isn't one optimal configuration that would work, rather an average compromise to create an overall effective plane. Now wee can change out the propeller in-between missions. so there's a little bit of fine tuning that can be done...
So as you can see there isn't one optimal configuration that would work, rather an average compromise to create an overall effective plane. Now wee can change out the propeller in-between missions. so there's a little bit of fine tuning that can be done...
12-01-2009, 12:00 AM
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RE: Need Help for Design/Build/Fly Contest
If anyone is interested more info on the competition or the rules, here ya go: http://www.aiaadbf.org/2010_files/2010_rules.htm
12-01-2009, 12:00 AM
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RE: Need Help for Design/Build/Fly Contest
Do any of the currently available hobby ESC's drive the motor with AC? I have industrial ESC's that are AC, but the extra weight would cancel out the increased efficiency. My hobby ESC's are all DC, 1st wire plus, 2nd minus, 3rd wire used for timing.
12-01-2009, 12:12 AM
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RE: Need Help for Design/Build/Fly Contest
The setup I suggested will work, but you would want to change the prop from an 18x14 for speed to a 22x6 for thrust, or something like that. Otherwise you will need to do some testing and select one prop that is a compromise. The CC HV-30 when set for soft cut-off and current sensing will limit the current to 30 Amps RMS or about 800 watts peak. Something like a big trainer, Telemaster ($130 kit) or piper cub would fly just fine on 30 to 50 watts per pound.
12-01-2009, 10:29 PM
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RE: Need Help for Design/Build/Fly Contest
It looks like everything is wide open throttle which helps. By just changing the prop it may be enough to cover all the missions, you could change the battery pack voltage by changing the serial / parallel combinations with just one battery pack.
BLDC motors have inductance so the voltage and current do not have to be in phase. If fact it is quite hard to get them to be in phase. Out of phase current does nothing for power but does waste energy as heat. It is possible to change the advance while the motor is running to control the out of phase current, however I do not know of any ESC that does it.
BLDC motors have inductance so the voltage and current do not have to be in phase. If fact it is quite hard to get them to be in phase. Out of phase current does nothing for power but does waste energy as heat. It is possible to change the advance while the motor is running to control the out of phase current, however I do not know of any ESC that does it.
12-03-2009, 05:35 PM
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RE: Need Help for Design/Build/Fly Contest
and would there be any benefit in getting the hv- 45 instead to get closer to 40 amps? because the other one only goes to 30.
Thanks
Thanks
12-03-2009, 05:55 PM
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RE: Need Help for Design/Build/Fly Contest
The NiMH are more energy dense than NiCd.
The HV-45, even on sensitive current sensing, will blow your 40 amp fuse. It sits at 40 Amps RMS. Setting the acceleration slower on a 40 amp fuse might work. A 50 amp fast acting fuse only went once when I ran into something.
Besides, with 30 volts or so and a Kv of 250 you won't get more than 15 to 20 amps in flight anyway.
The HV-45, even on sensitive current sensing, will blow your 40 amp fuse. It sits at 40 Amps RMS. Setting the acceleration slower on a 40 amp fuse might work. A 50 amp fast acting fuse only went once when I ran into something.
Besides, with 30 volts or so and a Kv of 250 you won't get more than 15 to 20 amps in flight anyway.
12-03-2009, 06:11 PM
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RE: Need Help for Design/Build/Fly Contest
for our battery we're looking at a , 24 cell (5000Mah per) pack, does that sound doable with that motor and one of those two ECUs?
