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Thread: Drak 4S motor options

  1. #1
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    Drak 4S motor options

    When the Drak prototype was first unveiled for Kickstarter almost a year ago it was running a 4S motor. Later testing and tweaking by Ritewing changed their preferred power source to a 5S system, and that's the voltage of the custom motor Ritewing will be making available for the Drak.

    For me, I'm really heavy into 4S systems and don't want to start collecting slightly oddball 5S batteries. Had I known 5S was going to end up as the preferred power source, I'd probably had serious second thoughts about ordering a Drak. As much as I'd like to get one of the custom Ritewing Drak motors, as far as I know they won't be making any available in 4S.

    So I went off searching for what limited info is out there for powering Draks with a 4S motor. Since the data was scattered over a number of places I thought I'd combine it into a single post for anyone else looking to do a 4S system. The following is a collection of assorted data I've been able to find on the web. The first group are posts on the FPVLab in the Ritewing forum in the main Drak thread.

    3-12-2015: By Ritewing, The Drak prototype used a 1200 KV 4S motor, probably the equivalent of a 2820 motor. It used an 11x7 prop, but Ritewing said 10x6 would be OK too.

    4-3-2015: By Ritewing, For 4S the KV should be about 1200.

    11-13&15-2015: By Ritewing, paraphrasing, 6S is good, 4S OK but 5S best in terms of power/efficiency. For 6S use about 740 KV, 5S about 950 KV and 4S 1100-1200 KV implied. Still seemed to use an 11 x 7 prop. A lot of good info in these two posts.

    Other data sources:

    2-14-2015: In the "aaron_gx Builds a RITEWING DRAK 60" thread in the FPVLab "Pilot's Lab" forum is a build of a 4S Drak. Aaron seems to use a Ritewing 1300 KV 4S motor with an 11 x 7 prop. He described it as being a 2820 size and "..same size as a 3542 Turnigy motor, maybe slightly larger".

    Stoneblue Airlines has flown and sells the Drak, and they seem to recommend a 4S setup. Their two recommended motors are either a Emax GT Series Motor 1090KV, or a Cobra Motor 1130KV. For a prop they recommend either a 10 x 7 or 11 x 7.

    If anyone else has more info on using a 4S motor on a Drak, please add to this thread.

  2. #2
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    I'll be sporting a Cobra 3515/14 950Kv with a APC 11x5.5 on 4S to start. Drak arrives tomorrow. I'll report back on performance after 1st flight.
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  3. #3
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    Gonna be doing a different drummer sort of thing with my motor, 780 KV for me

    Although Ritewing has recommended using 1100 to 1200 KV motors if using 4S, I'm instead going with a 780KV motor turning an APC 11x7 prop. Here's why...

    Since there haven't been any official figures published yet as to total wing area, I measured my own, including the eleverons and about 2.5 inches of the body closest to the wing where I assumed it would produce lift. I brought my dimensions into a CAD program and it calculated a total useful wing area of 744 square inches.

    I'm planning on using my Drak as a slower flying Pixhawk mapping bird, maybe running under a Section 333 exemption, so a modest flying profile. Looking around at the few prototype Draks out there and what weights have been posted, I'm assuming an AUW for mine will be in the ballpark of 6.5 pounds (2950 grams).

    This gives me a projected wing loading of around 0.14 oz/in^2 or 20.1 oz/ft^2

    For gentle flying (no 3D!) I set my desired static thrust to 0.75 x AUW or 4.9 pounds (2222 grams)

    All this stuff I put into Motorcalc and started doing sims. Initially I was using 1100 to 1200 KV motors, but the solutions had problems and cruise amps seemed high. So I let Motorcalc loose and it was coming back with desirable KVs in the mid-800 range with an 11 x 7 prop. Well, that was interesting.

    I happened to have on hand a very nice, barely used Scorpion II 3020 780 KV motor from an earlier project. Would that work?

    The motor is rated at 800 watts with 40 amps continuous. I plugged it into Motorcalc and it spit out a very nice solution using an 11x7 prop. It predicted a maximum current draw of 37 amps at static full throttle with 2360 grams of thrust. Top speed is predicted to be 50 mph (plenty fast for my purposes) with a most efficient cruise speed of 29 mph drawing only 4.9 amps (55% throttle). That all sounds pretty good, but how accurate is Motorcalc?

    To find out I put the Scorpion motor on a test stand I have and ran it with an APC 11x7 prop on a fully charged 4S battery. At full throttle I measured a thrust of 2500 grams at 39 amps and 575 watts. That's pretty close.

    Cutting the throttle back, at 5 amps I was still getting 586 grams of thrust statically. Motorcalc tells me I need to be generating 260 grams to keep the Drak flying at its calculated most efficient speed of 29 mph. What that static 586 grams will drop to when it's moving at 29 mph I don't know for sure. Motorcalc predicts the thrust at 29 mph to be 39% of the static thrust, and if so that would make my 29 mph 5 amp thrust about 230 grams. Again, in the ballpark.

    Since I have the Scorpion motor already on hand, as well as a very fine Castle ICE 50 ESC, it makes sense to give them a try. The numbers look good.

    I have run the numbers on an 11x8 prop and it also looks good, but the current increases a bit at full throttle and cruise, but top speed goes up to 55 mph. I may pick up an Aeronaut 11x8 folder and run that on my test stand.

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    Nice blog. I too was having this struggle. I was looking at cobra motors and could not find one that would give me 2700g thrust on 4s that would be efficient. In the end i opted to go for the ss motor and going to run 5s. I did find some cheeper 5s lipos at 10,000mah for 120ish. Granted its probably not a good lipo but i too am just looking to cruse so im not planning on pushing that battery either.

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    I worry more about altitude flying and launchin it on 4s with such a low kv.
    Are you in colorado?
    If so you mite have a bear of a time launching her with low kv on 4s.

    The one nice thing about the 5s setup is it will fly out of your hand with ease like a rocket on a mission, then it can also cruise a sipping rate of 7 to 10 amps.
    It also wont pull more than 60amps with the 11/7 on here will pulling strait up ver climbs.

    Let us know what you find out, these are my concearns.
    Quote Originally Posted by OtherHand View Post
    Although Ritewing has recommended using 1100 to 1200 KV motors if using 4S, I'm instead going with a 780KV motor turning an APC 11x7 prop. Here's why...

    Since there haven't been any official figures published yet as to total wing area, I measured my own, including the eleverons and about 2.5 inches of the body closest to the wing where I assumed it would produce lift. I brought my dimensions into a CAD program and it calculated a total useful wing area of 744 square inches.

    I'm planning on using my Drak as a slower flying Pixhawk mapping bird, maybe running under a Section 333 exemption, so a modest flying profile. Looking around at the few prototype Draks out there and what weights have been posted, I'm assuming an AUW for mine will be in the ballpark of 6.5 pounds (2950 grams).

    This gives me a projected wing loading of around 0.14 oz/in^2 or 20.1 oz/ft^2

    For gentle flying (no 3D!) I set my desired static thrust to 0.75 x AUW or 4.9 pounds (2222 grams)

    All this stuff I put into Motorcalc and started doing sims. Initially I was using 1100 to 1200 KV motors, but the solutions had problems and cruise amps seemed high. So I let Motorcalc loose and it was coming back with desirable KVs in the mid-800 range with an 11 x 7 prop. Well, that was interesting.

    I happened to have on hand a very nice, barely used Scorpion II 3020 780 KV motor from an earlier project. Would that work?

    The motor is rated at 800 watts with 40 amps continuous. I plugged it into Motorcalc and it spit out a very nice solution using an 11x7 prop. It predicted a maximum current draw of 37 amps at static full throttle with 2360 grams of thrust. Top speed is predicted to be 50 mph (plenty fast for my purposes) with a most efficient cruise speed of 29 mph drawing only 4.9 amps (55% throttle). That all sounds pretty good, but how accurate is Motorcalc?

    To find out I put the Scorpion motor on a test stand I have and ran it with an APC 11x7 prop on a fully charged 4S battery. At full throttle I measured a thrust of 2500 grams at 39 amps and 575 watts. That's pretty close.

    Cutting the throttle back, at 5 amps I was still getting 586 grams of thrust statically. Motorcalc tells me I need to be generating 260 grams to keep the Drak flying at its calculated most efficient speed of 29 mph. What that static 586 grams will drop to when it's moving at 29 mph I don't know for sure. Motorcalc predicts the thrust at 29 mph to be 39% of the static thrust, and if so that would make my 29 mph 5 amp thrust about 230 grams. Again, in the ballpark.

    Since I have the Scorpion motor already on hand, as well as a very fine Castle ICE 50 ESC, it makes sense to give them a try. The numbers look good.

    I have run the numbers on an 11x8 prop and it also looks good, but the current increases a bit at full throttle and cruise, but top speed goes up to 55 mph. I may pick up an Aeronaut 11x8 folder and run that on my test stand.

  6. #6
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    Quote Originally Posted by ritewing View Post
    I worry more about altitude flying and launchin it on 4s with such a low kv.
    Are you in colorado?
    If so you mite have a bear of a time launching her with low kv on 4s.

    The one nice thing about the 5s setup is it will fly out of your hand with ease like a rocket on a mission, then it can also cruise a sipping rate of 7 to 10 amps.
    It also wont pull more than 60amps with the 11/7 on here will pulling strait up ver climbs.

    Let us know what you find out, these are my concearns.
    Thanks for the input Chris. I'm mostly around sea level or flying in the SoCal deserts, so probably no density altitude issues. I'll see how easy it is to launch and if it's a bear I might just switch to a bungee setup. 7 to 10 amps at cruise isn't bad, but if it really can get down to 5 amps, per Motorcalc, that would be really great. With a 10,000 MAH battery a 90 minute mapping mission should be possible.

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    Looks like I've settled on a good prop combo for my 4S Scorpion II 3020 780 KV motor. I'll provide a brief summary of the data in case anyone else is thinking about using a motor around 800 KV with 4S. As long as the motor's wattage rating is above 700, it should be similar.

    After doing a lot of work in MotorCalc I found six props that looked viable, three APCs and three Aeronaut folding props (I'm told the Aeronauts are very similar to Graupners and are nice stuff). I then tested these six on my static test stand with the Scorpion motor, running on a fresh 4S battery for each run.

    The following is a condensed version of my data for each prop. All values are at wide open throttle. The first number is thrust in grams and the second number is the Amp draw.

    APC 11 x 7E: 2,450 gms / 36 Amps

    APC 11 x 11.5E: 2,330 gms / 30 Amps

    APC 12 x 4.5 MRP: 2,900 gms / 40 Amps

    Aeronaut CAM 11 x 8: 2,170 gms / 39 Amps

    Aeronaut CAM 12.5 x 6: 2,850 gms / 41 Amps

    Aeronaut CAM 13 x 6.5: 3,150 gms / 48 Amps

    Since I'd slightly prefer a folder, I'm going with the Aeronaut 12.5 x 6 as it appear to provide essentially the same performance as the APC 12 x 4.5 MRP. Both are hitting about 40 Amps, which is my motor's max continuous rating. I am tempted to use the 13 x 6.5 folder but won't. Its top Amp draw exceeds my maximum of 40, but for quick bursts that wouldn't worry me much. Of greater concern is the ESC I have on hand and am planning to use, a Castle ICE 50, and I'm just too close to its 50 Amp limit.

    So it's going to be an Aeronaut 12.5 x 6 folder. At full throttle it should be sufficient as my projected Drak AUW is 6.5 pounds (2,950 grams) so I'll have just a little short of 1:1 thrust to weight ratio. At 5 Amps I'm seeing it provide 670 grams of static thrust which is around what MotorCalc is telling me I'd need to push the Drak in most efficient cruise. So it appears to be on course for a 5 Amp or less cruise.

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    I'm going to give the SunnySky Angel 2820/1100kv a try with an 11x7.
    "Truly superior pilots are those who use their superior judgment to avoid those situations where they might have to use their superior skills"- Author unknown

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    Be careful buddy from what I have seen it will most likely It will smoke that motor if you push it hard. 28 mm diam just doesn't have the tork for props over 10 inches, so amps go way up. Let us know how it goes .

  10. #10
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    Quote Originally Posted by Scotttu View Post
    I'm going to give the SunnySky Angel 2820/1100kv a try with an 11x7.
    Per the prop data for that motor, aren't you going to be well over both the Amp and Watt limits?

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