"Diven is correct, but more important is the following,
As amperage increases under load the voltage drops.  Called voltage lag.  This drop in voltage increases as your actual amperage approaches the C rating for your battery.  When the voltage lag…"

"You have a large number of variables to consider, and each choice you make will effect the other issues, and your final results.  You cant make an useful decision on any of these issues with a table provided with a motor, and understand the effect…"

"jbabio, I  have a heavy Y6 built a few years ago and still in the air.  In my case I have three parallel 5s, 5000mah batteries.  It works for me and my Y6, but I would not recommend three parallel batteries as this creates battery issues you can not…"

"When choosing components for your project you must first use a program like ecalc.ch to estimate the performance of the system and provide predictions of the flight time, current, throttle at hover, and much more.  It sounds like you currently just…"

"Im happy to help.  Lots of good advice and support on this site and the others.  I have learned everything from the wikis and the blogs.  
Good luck moving forward!"

"Your first flight was totally in "position hold" mode?  You need to start with stabilize mode, then move to alt hold mode, then to loiter.  Only in this order can you really judge performance.
Until you succeed with completing the autotune routine…"

"See my comments on this post , previous page.
Voltage Lag is a potential cause of your problem.  A good review of your logs can confirm or deny this.  Compare the battery voltage and current over time.  As current go's up voltage go's down.  When…"

"Hard to tell if the values are to high (creating oscillation) or to low (inadequate reaction from the controller).
When I was getting off the ground I adjusted stabilized roll and pitch no more than 1.0 for each attempt.  Try upping the values to 8…"

"I have a heavy Y6, 7.5KG.  I had major problems taking off for my first flight.  The tendency to flip or do some other unwanted action was potentially very dangerous.  I needed to raise stabilize P just to get off the ground.  Once I completed…"

"Have you managed to complete Autotune?  This is a common problem with the first flight.

If you are unable to achieve stable flight in Althold, required for Autotune, then try increasing the "stabilize P"  value by a small bit, or better configure…"

"Even, I just came upon a video in the wiki that may help address your issue.  In the wiki, search for " ESC sync issues"  If this is actually your issue, this will confirm it."

"A few items.  from your diagram it looks like you have two batteries, not in parallel, but independently supplying power to two motor / ESC 's each.  This could be causing trouble as the batteries are not equal.  I have a parallel battery…"

"Well a bad power module is better than voltage lag creating your problem.
Have you measured your current during a flight from an independent source?
Why are you using 70A esc's when you hover at 10A?"

"This is normal.
The minimum voltage is under load.  As you raise your throttle the current or Amperage increases, this causes the voltage to lag, or go lower.  How much lower?  That's individual.  I'm accustomed to seeing .5v to 1.5v lower when…"

"Martin, the "jump start" seems to work for most, however, I have found that with a heavy lift beast anything fast during the maiden flight is terrifying.  some manual tuning is needed prior to being airborne.
Evan, has 70A esc's so I figure he has a…"