Author Topic: Cobra 3520/14 Weight Limit (Read 2393 times)

Ken Culbertson · « **on:** January 31, 2019, 11:48:30 AM »

I am about to start the finish on my PA ship for this season. It is my 1st "60" size ship so I have to ask all of the dumb questions. It is about 670sq and will be powered by a Cobra3520/14 5s 3300. What I want to know is how heavy can I go with the finish before I start to get porky for that engine. I have read all I can from prior posts but all I can gather is that 69oz is not too heavy. 69 - Good Heavens, that is 30 over what my PA ship was in 1980! Brave new world. As it sits right now unfinished it is at 54 with motor, battery, esc, prop, wheels. The heaviest ship I have owned is 52.

Humongous battery compartment. This is exciting, I am actually going to get to use .018 lines!

Ken

Dean Pappas · « **Reply #1 on:** February 01, 2019, 01:36:34 PM »

Hello Ken,
As it turns out, the power handling capability of one of these motors is largely tied to its weight!
The Cobra 3520 should handle a plane in the mid or even high 60's ounces weight without a problem.
The battery should have just enough energy to fly high 60's ounce weights as well.
Of course, if you can juuust manage to keep things under 64 oz, you will be a whole lot happier with the thinner lines.
I look forward to the post-finish pictures.

Regards,
Dean Pappas

Ken Culbertson · « **Reply #2 on:** February 01, 2019, 04:32:36 PM »

Quote from: Dean Pappas on February 01, 2019, 01:36:34 PM

Hello Ken,
As it turns out, the power handling capability of one of these motors is largely tied to its weight!
The Cobra 3520 should handle a plane in the mid or even high 60's ounces weight without a problem.
The battery should have just enough energy to fly high 60's ounce weights as well.
Of course, if you can juuust manage to keep things under 64 oz, you will be a whole lot happier with the thinner lines.
I look forward to the post-finish pictures.

Regards,
Dean Pappas

I am really going to try. Is a 10oz finish realistic? It is the weight of that damn 5s battery that is killing me. I wonder if I could get by with a 2800? Nearly 50% of the weight of the plane right now is in the power plant. I am going to probably need tail weight. If I do I will add finish instead. At least with electric I can fly it first. I have ordered a set of .018's. Hope I don't need them!

I can't wait to see it in the air too. It is a rebuild of my 1965 Nats plane. Everybody thought I was nuts back then to have a long tail movement, huge stab, skinny flaps, 660 sq high aspect wing and a thick airfoil. They were right about being nuts but it was not over the design!

Ken

Tim Wescott · « **Reply #3 on:** February 01, 2019, 05:18:36 PM »

I run 018 lines on a 64 ounce plane and I'm quite happy. If you stick to the lower limit then you may experience some springiness in the lines that wouldn't be there if you go up a size.

Martin Quartim · « **Reply #4 on:** March 10, 2019, 06:23:00 PM »

Just for the record, my friend flew an 82oz 750sq model over 1000 flights with a Cobra 3520. Never had a problem with the motor.
He uses a KR Timer, Emax 60A ESC, 3300 5s with a 14x7 prop. He managed to make over 300 flights in one battery pack, it is a very robust propulsion system.

Ken Culbertson · « **Reply #5 on:** March 11, 2019, 08:39:14 AM »

Quote from: Martin Quartim on March 10, 2019, 06:23:00 PM

Just for the record, my friend flew an 82oz 750sq model over 1000 flights with a Cobra 3520. Never had a problem with the motor.
He uses a KR Timer, Emax 60A ESC, 3300 5s with a 14x7 prop. He managed to make over 300 flights in one battery pack, it is a very robust propulsion system.

Thanks, I am relieved. I should come in under 70oz at 710sq. I am running the same configuration with a Huben and Castle 75. Can't fit that large a prop so I will be using a 12 x 7 three blade. I can't fit a 6s so it has to work on the 5s 3300. I did a pre-finish trim flight at 9400 rpm on a 4s and was able do all of the maneuvers. I am looking forward to the added power of the 5s.

Ken

Ken Culbertson · « **Reply #6 on:** March 18, 2019, 12:51:47 PM »

Quote from: Ken Culbertson on March 11, 2019, 08:39:14 AM

Thanks, I am relieved. I should come in under 70oz at 710sq. I am running the same configuration with a Huben and Castle 75. Can't fit that large a prop so I will be using a 12 x 7 three blade. I can't fit a 6s so it has to work on the 5s 3300. I did a pre-finish trim flight at 9400 rpm on a 4s and was able do all of the maneuvers. I am looking forward to the added power of the 5s.

Ken

Just a follow up. At 66oz the plane has plenty of power at 9600 rpm on a 11.5 x 6. Lands with about 30% on the battery and everything is cool. Max esc temp recorded was 100. Sad part is that the controls locked on takeoff and it put a 1" deep hole in the asphalt most of which was filled with a pulverized plastic spinner.

Ken

Joe Yau · « **Reply #7 on:** March 18, 2019, 11:28:01 PM »

I use a Cobra 3520-12 on a 68.5oz (with batt) TF Score, it flys very well with authority. The 3520-12 has 820Kv , so it is a 5 cell motor for thin/light 11x5.5, 11.5x5.5 or 12x5N 3-blades props. It flys at around 5.2- 5.28 lap speed with .018" 64' eye to eye line length.

Ken Culbertson · « **Reply #8 on:** March 19, 2019, 07:40:57 AM »

The 3520/14 is a much larger motor than I am used to. First flights were amazing but a bench test after my recent encounter with a large flat surface gave me the impression that this motor has a lot of torque and dual (front and rear) mounts may be in order or at least a shaft retainer.

Ken

Ken Culbertson · « **Reply #9 on:** March 23, 2019, 11:01:11 AM »

After rebuilding the nose I am a bit nose heavy. Right now I am running on a 3000ma 5s 30-40c battery. At 5.5 laps and a 5 1/2 min run I use about 65%. I can save 2 1/2 oz by dropping to a 3000 5s 20-40c battery. From what I read the higher the "c" value the better within reason and our rule of thumb is 25c+. Also from what I gather the only penalty would be a bit more work for the ESC and it may run hotter but it s a Castlle75 and it has never gotten over 100 on the 30c so it should be fine.

What am I missing?

Ken

Fred Underwood · « **Reply #10 on:** March 23, 2019, 05:03:32 PM »

If the use is about 2000 ma in 5 minutes, that averages about 24 amps/hr. If I have a 2700 mah battery rated at 25c discharge, that would allow a rate of 25 x 2.7 or 67.5 amp/hr. We use about 8 – 10C discharge rate on 2700 mah batteries. Higher discharge rate batteries are generally heavier. You can look at Zippy compact 2700 mah batteries at 25C and at 35C discharge rates and get an idea. Higher C batteries may have larger gauge and heavier wiring. OTOH, they have less internal resistance and may not heat as much. For our use, 20/25C batteries seem to work fine.

Ken Culbertson · « **Reply #11 on:** April 22, 2019, 12:26:39 AM »

Just a final note. Got the nose rebuilt and in doing so it now has a 2700 6s 35C battery. My original question was if the 3520/14 had enough power for a 65 oz ship. It has more than enough. At 70 oz on 65' lines it zipped through it's first pattern with ease on it's second flight. This plane has been snake bit up to this point. Locked controls, bubbling paint, fly away motor. I should have given up on it ages ago but it survived and finally, after 18 months back in the hobby I have a competitive PA ship that qualifies for Classic. All it needs now is a competitive pilot! I have to admit that it flies better with the Cobra than it did when I designed it back in '63 and flew it with a Fox 35!

Thanks to all for the help - Ken

Ken Culbertson · « **Reply #12 on:** May 23, 2019, 01:33:19 PM »

Quote from: Dean Pappas on February 01, 2019, 01:36:34 PM

Hello Ken,
As it turns out, the power handling capability of one of these motors is largely tied to its weight!
The Cobra 3520 should handle a plane in the mid or even high 60's ounces weight without a problem.
The battery should have just enough energy to fly high 60's ounce weights as well.
Of course, if you can juuust manage to keep things under 64 oz, you will be a whole lot happier with the thinner lines.
I look forward to the post-finish pictures.

Regards,
Dean Pappas

Since you were right about the power being sufficient I thought I would ask the same question about the 2826. The prop charts show little power difference. I am starting on the 2020 PA ship which will be an electric and slightly modified Top Hat (the late Tom Neibuhr's design). He built rediculously light. I plan to be simply not heavy. Shooting for under 64. Will a 2826 pull it? Everything associated is lighter if I can use that motor and, even more important, I already have one!

The picture is the finished version of the plane this thread is all about. 73 w/6s battery. I owe you that.

Ken

Crist Rigotti · « **Reply #13 on:** May 23, 2019, 08:14:29 PM »

Skip the Cobra 2826 and go with a Cobra 3515. The 3515 runs a lot cooler that the 2826. I know these things trust me.

Ken Culbertson · « **Reply #14 on:** May 23, 2019, 10:48:29 PM »

Quote from: Crist Rigotti on May 23, 2019, 08:14:29 PM

Skip the Cobra 2826 and go with a Cobra 3515. The 3515 runs a lot cooler that the 2826. I know these things trust me.

Only problem is that I already have a 2826. Can I just tell it to act like a 3515?

Ken

Crist Rigotti · « **Reply #15 on:** May 24, 2019, 08:08:32 AM »

Quote from: Ken Culbertson on May 23, 2019, 10:48:29 PM

Only problem is that I already have a 2826. Can I just tell it to act like a 3515?

Ken

It'll run too hot in Texas! Beware, you've been warned.

TDM · « **Reply #16 on:** May 24, 2019, 11:45:16 AM »

2826 is going in the wrong direction. If you went in the opposite direction with a 3520-14 motor you will save a lot of battery usage get the CG forward like you wanted to get rind of tail weight and perhaps get away with a 5S on top of it. The are less energy losses too because the amp draw is lower so everything runs better so the battery usage will drop more than what you expect. Heat is lost energy, if the motor is running cooler, the ESC is cooler the battery is not loaded up you have all these items not generating heat and thus more energy left in the battery. Also try to up the pitch as much as you can get away with to save more energy.

Ken Culbertson · « **Reply #17 on:** May 24, 2019, 01:51:39 PM »

Quote from: TDM on May 24, 2019, 11:45:16 AM

2826 is going in the wrong direction. If you went in the opposite direction with a 3520-14 motor you will save a lot of battery usage get the CG forward like you wanted to get rind of tail weight and perhaps get away with a 5S on top of it. The are less energy losses too because the amp draw is lower so everything runs better so the battery usage will drop more than what you expect. Heat is lost energy, if the motor is running cooler, the ESC is cooler the battery is not loaded up you have all these items not generating heat and thus more energy left in the battery. Also try to up the pitch as much as you can get away with to save more energy.

OK, sometimes it is best to defer to superior knowledge and experience! I will get another 3520. It is just that the 2826 has been overpowering my ARF Nobler and it keeps asking me when it gets to fly a real airplane. It will be disappointed.

Ken

Author Topic: Cobra 3520/14 Weight Limit (Read 2393 times)

Ken Culbertson

Cobra 3520/14 Weight Limit

Dean Pappas

Re: Cobra 3520/14 Weight Limit

Ken Culbertson

Re: Cobra 3520/14 Weight Limit

Tim Wescott

Re: Cobra 3520/14 Weight Limit

Martin Quartim

Re: Cobra 3520/14 Weight Limit

Ken Culbertson

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Ken Culbertson

Re: Cobra 3520/14 Weight Limit

Joe Yau

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Ken Culbertson

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Ken Culbertson

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Fred Underwood

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Ken Culbertson

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Ken Culbertson

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Crist Rigotti

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Ken Culbertson

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Crist Rigotti

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TDM

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Ken Culbertson

Re: Cobra 3520/14 Weight Limit