Over or Under which is best

[Avenger]

I have an F550 and currently site the battery centrally on top, however I've purchased a 10,000 mAh battery which seems to make the craft top heavy. So my question is should I resite the batter underneath the plates and will that offer me greater balance and stability

 

[Sandgroper]

Think about a boat.....if all the weight is topside, the boat is very unstable and will roll in the slightest sea, in fact it will probably turn turtle without much prompting. This factor describes one of the most important calculations in stability...the Metacentric height. The bigger the MH, the less stable. Same goes with aircraft...look at the stability of a high-winged Cessna compared to a low wing speed machine. The Cessna (with a Low MH) is positively stable...ie when left alone it will tend to right itself and resume normal flying, whilst any craft having a high MS number will be divergent...it will tend to become unstable. So for my money, the low battery (putting weight BELOW the CofG is always a better route for stability. Mind you...there is always a price !!! as MH moves further below CofG then the sensitivity decreases, leading to more sluggish response to control inputs

Yer pays yer money and takes yer choice :upset:

 

[DucktileMedia]

It depends a lot on the weight below as well. Are you adding a gimbal/camera? That weight could be balanced by the lipo up top. All in all you just need to get it somewhat close. Unlike a boat these are completely computer controlled so although it might be less work for the FC to stabilize it, it will. Having said that, if you dont have any payload i would put it down low and add landing gear.

 

[JoeBob]

There is a tendency to think that weight below makes an MR more stable - not true. If all the weight is below the thrust line, that makes each individual rotor work the most to adjust in flight. A perfectly balanced MR would have equal weight above and below the thrust line so that the rotors had to overcome as little leverage as possible to adjust the craft.

Important: your thrust line is NOT the top deck or the COG of your craft, it's the propellers. So if you're running standard 2214 DJI motors on your F550, your battery is probably already below the lift line on the top deck (2214s raise the props 1.25 inches above the deck).

 

[jdennings]

A perfectly balanced MR would have equal weight above and below the thrust line so that the rotors had to overcome as little leverage as possible to adjust the craft.

+1. The ideal (theoretical) set-up is one where the MR's CG, FC and propeller plane are all at the same point on the Z axis. With the CG, FC (accels and gyros) and combined propeller center of thrust at the center point in the XY plane.

 

[Avenger]

It depends a lot on the weight below as well. Are you adding a gimbal/camera? That weight could be balanced by the lipo up top. All in all you just need to get it somewhat close. Unlike a boat these are completely computer controlled so although it might be less work for the FC to stabilize it, it will. Having said that, if you dont have any payload i would put it down low and add landing gear.

Yeah I have a gimbal and gopro3 slung underneath, nothing on top other than the GPS tower and the battery.

thanks for the replies guys

 

[Avenger]

There is a tendency to think that weight below makes an MR more stable - not true. If all the weight is below the thrust line, that makes each individual rotor work the most to adjust in flight. A perfectly balanced MR would have equal weight above and below the thrust line so that the rotors had to overcome as little leverage as possible to adjust the craft.

Important: your thrust line is NOT the top deck or the COG of your craft, it's the propellers. So if you're running standard 2214 DJI motors on your F550, your battery is probably already below the lift line on the top deck (2214s raise the props 1.25 inches above the deck).

Very ry interesting and a good point, thanks