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A bit unbalanced now...

Discussion in 'General Discussion' started by JaysDMC, Mar 9, 2014.

  1. JaysDMC

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    Hello everyone.

    I recently added a 2 axis gimbal to my Phantom 1 and it works great! However, I've noticed that the front of my Phantom is significantly heavier now with the gimbal and GoPro mounted to it. Before I mounted the gimbal my Phantom would simply rise as I took off. Now, as I take off it lifts and pushes forward a few feet until it gains enough momentum to correct itself.

    Here's an example of what I'm talking about. Note how it lurches forward as it initially rises:

    [youtube]http://www.youtube.com/watch?v=6xfdOVtW6Kw[/youtube]

    I was thinking about getting something like this:

    http://www.rc-drones.com/rcd-phantomount-acc.php

    to relocate the battery and relocate the weight.

    My question is, does anyone think relocating the battery under the drone and towards the rear will have an adverse effect on flight?
     
  2. joeflyer

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    I think that's typical. Adding a gimbal makes it nose heavy. Once it's off the ground the NAZA corrects for it but my Phantom is not nearly as stable as it was before adding the gimbal. You would think the guys selling these would come up with mount adapters that place the gimbal a little further rearward for better balance.

    To answer your question getting the CG back where it belongs is the right fix. That plate looks good, although a little pricey. I'm thinking about powering my gimbal and TX off of a small auxillary 11.1v battery and mounting it behind the gimbal for balance and possibly longer flight times.
     
  3. ElGuano

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    A gimbal mounted further back on a Phantom is problematic because you'd have the legs and front arms in view all the time, making for unusable video (other than FPV/quad-POV shots). The only "real" solutions are: 1) use retractable landing gear and keep the camera pointed a few degrees down, or 2) spin the Phantom around so the LED is in the front and the battery door is in the back.

    I did the latter with mine, and DJI did the same when the P2/P2V came out. The added benefit to this is you can add successively larger batteries and push the gimbal even further out front to keep the view clear even with extreme maneuvers, and still keep the COG balanced.
     
  4. OI Photography

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    While it may help with centering the COG a bit, the added weight of even a small extra battery vs. the tiny amount of drain from the vTx means you won't gain a net increase in flight time, and possibly even the opposite.
     
  5. ElGuano

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    Definitely even the opposite I'd say. There's no way to make the math work out on an extra battery with the same energy density. If you want to balance COG, that'll help. But the Phantom does a really good job balancing in the air so if it's just the forward lurch, I'd 1) take off a bit faster, and 2) give it a bit of back elevator to compensate on takeoff. I'd also consider dropping the prop guards, pushing more weight to the outer edges is probably not helping if you're in a tipping situation.
     
  6. joeflyer

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    Not necessarily, that depends on how much you move it. With my Tarot gimbal the camera lens is 3/8" in front of the legs, so there's obviously room for movement before the legs come into view. It would be easy to check to see how much it can be moved without seeing the stock legs. If you want to mount it right on the CG and the legs are in view then a set of these would resolve that.

    http://www.ebay.com/itm/DJI-Phantom-1-2 ... 417d4ce55a
     
  7. amrflyingdude

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    Quads (unlike fixed wing aircraft) have very little issues with off set CG's because they can overcome balance problems without increasing airspeed and/or affecting stall speed but rather by purely increasing motor RPM.

    If you want to balance it out you will cut your flight time but relief some stress out of the 2 front motors, so it is a trade.

    One of the reasons why I added FPV using a carbon fiber mount is to compensate for my gimbal making my bird nose heavy. My quad is very balanced but it is heavy overall so my flight times are nos as good as yours, so if you are happy with your setup leave the way it is, just pull back a bit on the right stick take off and you will be fine
     
  8. joeflyer

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    I'm more concerned about the drain of the gimbal. Whether it's worth the trip or not depends on a lot of factors like overall weight, what battery, what props, the amount of drain from both the gimbal and vTx, etc.

    I have a 11.1v/1300mah battery that weighs 100 gm. that I plan on trying. If I see an improvement in flight time with that powering both the gimbal and vTx then I'll shop for the smallest/lightest battery that will get me through several flights. I expect that about 800 mah would be enough and I can find one that weighs about 70 gm.
     
  9. OI Photography

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    Even the 70g batt will take up to a minute off your flight time without saving you anything useful in terms of juice on the main flight batt. I have a 450mAh 3S batt at 42g and even that one isn't worth it. I've checked the current draw while running a vTx, gimbal and some extra LED's all at once in addition to the NAZA/Rx, and it's still an order of magnitude lower than what the motors suck down at idle, and made no noticible difference in flight times when I tested without them. That's something you could even test easily on the bench (power on with no props) to demonstrate.
     
  10. ElGuano

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    You can see the current draw of the tarot in real time. With my motor power settings, that's ~150mA. It's minuscule compared to the added weight of a secondary battery.
     
  11. joeflyer

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    I'm still new to quadcopters so I wanted to see how the two configurations compared. I did two test flights just hovering the Phantom 6 ft. off the ground in GPS mode; Temp 34 F. The Phantom was working pretty hard due to 10-20 mph gusting wind and I had to use the sticks a lot to maintain position:

    1) Phantom 1.1.1 with P2 props, Tarot gimbal, Gopro 3, TS832 Tx, with stock 2200mah battery powering the gimbal and Tx.
    Overall wt. = 1155 gm.
    Time to 1st warning = 5m.55sec. Time to auto landing = 7m 25sec.

    2) Recharged the battery and repeated the above with added 100 gm. battery powering the gimbal and Tx. Overall wt. =
    1255 gm.
    Time to 1st warning = 5m.35sec. Time to auto landing = 6m 55sec.

    As you can see flight times are 20-30 sec. better with just the main battery. Now if I apply the 1 second/gram rule of thumb it would be about a wash if I were using a 70 gm. auxiliary battery, and if I were using a 40 gm. battery I could pick up about 30 sec. flight time. A 40 gm. 11.1v Lipo would be 400-450 mah so it would probably have to be recharged or changed after two flights.

    I'm not sure it's worth the trip. If I were also powering my Gopro off the gimbal definitely not. If I get hard up for flight times I may just get a 40 gm. battery and redo the experiment.
     
  12. OI Photography

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    Testing while flying in GPS mode in heavy wind makes it very difficult to get reliable comparison data for voltage consumption. If you use a more controlled testing environment, I think you'll find even flying with the 40gm batt powering the vTx and Gimbal won't gain you any net increase in flight time over flying with just the 2200mAh flight battery powering everything.

    You can do a test on your bench to demonstrate how little current the gimbal and vTx draw...just use a freshly charged battery to power on the Phantom/gimbal/vTx but don't start the motors. See how long it takes to drain the battery (get comfortable, it'll be a while). Now run the same experiment but also let the motors run at idle (no props).