A Question on Drones and Battery Placement.

[JohnSSM]

I am a drone hobbyist and there is an argument in that community about where to place one's battery packs. They can either be placed on the top deck, strapped and hanging from the bottom deck, or ideally, they can be near the middle or center of gravity...Many folks cannot put the battery in the middle, or near the ideal center of gravity, so they must choose strapped to the top, or strapped to the bottom.

I talking about a four engine craft that tilts in the direction that it moves...

Is it ever advantageous to have the majority of the mass on top of any vehicle? Some folks claim that it can flip and roll better with the battery on top, but does this make any sense in physics?

I think we all agree that having one's mass at center is best. But if you have to chose top or bottom, does it really make any difference for the agility and moveablity for the craft other than how far it is off center? On some drones, the top of them is close to the center, on other drones, the bottom would put them closer to center.

Any thoughts and or proof to offer these other fellas on this subject?

[swansont]

If it's above the geometric center, it will tip over more easily. There will be a gravitational torque on the system once it's rotated. The further it is from the center, the smaller the angle has to be to where it will want to invert. Similarly, if the mass is below the center, you have a harder time getting it to flip.

[JohnSSM]

Yes...good point...

It will be easier to begin the flip when the mass is above the CG, but how about stopping the flip? Does it also become easier? That is what these pilots claim. That top loaded batteries make it easier to flip and roll...

The top load seems to start any flip easier, but ending it would take more power...and on bottom, starting the flip is harder, but ending it takes less power...less power equates to a more accruate control of power...so it seems that top loaded drones will flip and roll easier...

But how about turns? When a drone makes a turn, and does not flip or roll, how does a top or bottom CG effect that? It seems the bottom loaded drone may also drift more in turns, since it banks in the same direction that it turns and seems to "throw" the bottom balance out, creating a slide...what do you think?

[MigL]

Take a good look at an aircraft turning.

It does not turn in an upright attitude with a deflection of its rudder.

Rather, it banks in the direction of the turn and uses its 'pitch' to execute the turn

( pitch is easily available as all stable aircraft have their center of lift ahead of the center of gravity ).

 

Most high performance jets have a high roll rate for just this reason.

[Acme]

Take a good look at an aircraft turning.

It does not turn in an upright attitude with a deflection of its rudder.

Rather, it banks in the direction of the turn and uses its 'pitch' to execute the turn

( pitch is easily available as all stable aircraft have their center of lift ahead of the center of gravity ).

 

Most high performance jets have a high roll rate for just this reason.

I don't know the answers to the drone battery placement question, but it seems to me comparing drones to airplanes (which I presume you meant by 'aircraft') and jets is not a legitimate analog. At best you might make a comparison with a helicopter, but even then the drone is fundamentally different with it's multiple rotors. (At least 3 rotors on a drone; yes/no?)

 

As to battery placement, well, do some experiments. Put it on top and execute a set of maneuvers and take note of performance. Then put the battery on the bottom and do the same set and take notes and then compare the performances. Depending on the results and what you want to do during flights you pick the arrangement that works best.

[MigL]

Sorry, I had thought he meant fixed wing drones as opposed to rotating airfoil types.

 

For a rotating airfoil type ( helicopter ), the center of mass, for stability reasons, obviously has to be below the lifting surfaces

( or can you think of a helicopter that puts the rotor under the cabin ? )

[Acme]

Sorry, I had thought he meant fixed wing drones as opposed to rotating airfoil types.

 

For a rotating airfoil type ( helicopter ), the center of mass, for stability reasons, obviously has to be below the lifting surfaces

( or can you think of a helicopter that puts the rotor under the cabin ? )

Well, JohnSSM said "I [am] talking about a four engine craft that tilts in the direction that it moves.". Clearly the craft flies with the battery on top or bottom, but I don't think that moves the center of mass above the rotors in either case. Maybe it does, but such multi-rotor craft are unique and only remotely operate as do helicopters so perhaps the center of mass can be above the rotors.

 

I don't know if the rotor shafts tip or are fixed, or if they have variable pitch, but I had the impression drone rotors are fixed shaft and fixed pitch blades and that maneuvering is achieved by varying the relative speeds of the rotors. Perhaps this all varies by model. JohnSSM, can you edumacate me/us?

 

Another thought on the battery. Use 2 batteries, one top and one bottom. This could be either 2 batteries like the standard which would add weight but extend flight time, or two smaller batteries that together equal one of the usual type.

Addendum:

According to this Wiki article, I was correct. Still digging on possible tilt and variable pitch rotor models.

Quadcopter @Wiki

 

...Unlike most helicopters, quadcopters use two sets of identical fixed pitched propellers; two clockwise (CW) and two counter-clockwise (CCW). These use variation of RPM to control lift and torque. Control of vehicle motion is achieved by altering the rotation rate of one or more rotor discs, thereby changing its torque load and thrust/lift characteristics.[2][3] ...

Addendum addendum:

So yes, some have variable pitch rotors. From the same Wiki page:

 

 

Mechanical structure

 

The main mechanical components needed for construction are the frame, propellers (either fixed-pitch or variable-pitch), and the electric motors. For best performance and simplest control algorithms, the motors and propellers should be placed equidistant. ...

Still nothing on tilting rotor shafts, but reading that page on how maneuvering is accomplished I don't see such a mechanism as being any advantage.

Addendum addendum addendum

 

So yes, the center of mass can be above the rotors.

 

There's a sweet photo here, but the image is not linkable as this is a PDF. It's a dandy too! All kinds of equations on stability and such stuff as makes for interesting reading.

Modelling and Control of a Quad-Rotor Robot

Paul Pounds, Robert Mahony, Peter Corke

Australian National University, Canberra, Australia

CSIRO ICT Centre, Brisbane, Australia

Abstract

To date, most quad-rotor aerial robots have

been based on flying toys. Although such

systems can be used as prototypes, they are

not sufficiently robust to serve as experimental

robotics platforms. We have developed the X-4

Flyer, a quad-rotor robot using custom-built

chassis and avionics with off-the-shelf motors

and batteries, to be a highly reliable experimental

platform. The vehicle uses tuned plant

dynamics with an onboard embedded attitude

controller to stabilise flight. A linear SISO controller

was designed to regulate flyer attitude.

 

Mmmm...I should not have relied on the photo to determine the center of mass, or in their terms center of gravity.

What's the diff? Checking...

Center of gravity @Wiki

In the study of the dynamics of aircraft, vehicles and vessels, forces and moments need to be resolved relative to the mass center. That is true independent of whether gravity itself is a consideration. Referring to the mass-center as the center-of-gravity is something of a colloquialism, but it is in common usage and when gravity gradient effects are negligible, center-of-gravity and mass-center are the same and are used interchangeably.

Ok; kewl. On to the paper's clarification on the test quad-chopter.

4 Model Parameterisation and Stability

Designing a controller based on this model requires parameters

of the physical system to be specified. Most

of these values are dictated by the flight performance of

the system; some, most importantly h, can be chosen

freely. The error associated with each parameter defines

the envelope of the plant model’s dynamic response. We

analyse the system behaviour within this envelope to determine

the best value of h, the height of the rotor plane

above the CoG.

Had to add the Ozzian spellings of parameterisation, analyse, and behaviour to my dictionary. G'don those mates.

 

Nonetheless, the rotors can be below the center of gravity. >>

Of the composing terms, only h can change signs. For

a conventional helicopter, where h < 0, the craft has

an unstable pole pair. If the rotors are inverted (above

the CoG), the craft will diverge without oscillation. If

the rotors and CoG are coplanar, the craft is marginally

stable. This behaviour was demonstrated in a full 3D

simulation previously [Pounds et al, 2004].

...

The unforced stability analysis demonstrated that h

is also important in determining the behaviour of the

dynamic system. The root locus for h shows that the

structure of the open-loop poles changes significantly as

h changes sign (cf. Fig. 5). Analogous to the unforced

case, the system exhibits an unstable oscillation when

the CoG is below the rotor, pure divergence when it is

above the rotor, and neutral stability when coincident

with the rotor.

Prouty suggests that helicopters can benefit from an

inverted rotor configuration, as pure divergence is easier

for a human pilot to correct for than unstable oscillation

[Prouty, 2002, pp 603].

Edited September 4, 2015 by Acme

[swansont]

For a rotating airfoil type ( helicopter ), the center of mass, for stability reasons, obviously has to be below the lifting surfaces

( or can you think of a helicopter that puts the rotor under the cabin ? )

 

I can think of reasons why that would be a bad idea apart from possible stability issues, so that's probably not conclusive reasoning.

[Acme]

For a rotating airfoil type ( helicopter ), the center of mass, for stability reasons, obviously has to be below the lifting surfaces.

( or can you think of a helicopter that puts the rotor under the cabin ? )

 

I can think of reasons why that would be a bad idea apart from possible stability issues, so that's probably not conclusive reasoning.

Me too. Chop chop! And as to stability, that last bit I cited actually suggests it would be a good idea for stability.

Prouty suggests that helicopters can benefit from an

inverted rotor configuration, as pure divergence is easier

for a human pilot to correct for than unstable oscillation

[Prouty, 2002, pp 603].

Maybe it won't be too long before we start seeing quad-copters on the scale of current helicopters. ?

 

I was also thinking about battery life issues with the model quadcopters and thought perhaps they could use a small gas engine driving a generator. Oui/no?

[imatfaal]

Me too. Chop chop! And as to stability, that last bit I cited actually suggests it would be a good idea for stability.

 

Maybe it won't be too long before we start seeing quad-copters on the scale of current helicopters. ?

 

I was also thinking about battery life issues with the model quadcopters and thought perhaps they could use a small gas engine driving a generator. Oui/no?

 

Power from hydrocarbons distributed electrically is a lovely system Some of the largest ships in the world use it - some use gas turbines as the initial stage and others standard diesel motors, the power is then sent to small and very efficient electric motors in fully azimuthing (horrible word I am afraid) pods hanging underneath the hull. You get forward drive, navigation and stabilisation all at the same time through one single system. And because the motors/turbines are run at max efficiency for a greater percentage of the time (you use multiple small units which you attempt to utilise at max efficiency or turn off) and you get less adverse hydrodynamical effects you get an overall efficiency saving as well. But getting it to work and maintenance is very difficult compared to a single enormous diesel engine.