How to make a more stable UAV plane platform.

Ever heard of "Moment of Inertia"? Neither had I, but it's quite simple really.Object with the most of it's weight at the center of it's gravity is easier to rotate than one with the weight on the outer edges. So why put all the electronics and batteries in the center under the wing. By distributing the weight to the wings, should create a platform that has steadier flight characteristics.More on this and a few pictures in my personal UAV blog.
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  • I understand that the wings of passenger jets are so strong they would bend about 45 degrees before failing. It is true of course, that passenger jets load the wings with fuel.

    I'm not sure the lessons of a passenger jet apply to small airplanes, there are huge differences in aerodynamic realities. Let me just say that I found flying a glider without ailerons to be rather more difficult than my much more maneuverable GP MicroWizz. Reducing one's control capabilities increases the lag time between joystick and plane reaction. To each there own, and perhaps I'd get used to that Glider if I flew it more often.

    That said, in the context of ardupilot - a slow rate of change may be helpful... I'm not sure.

    I do agree that spreading the weight is a good strategy for overloading a plane.
  • It's true that solid core an light weight wings are beneficial in some situations, but having all the expensive electronics and cameras etc. in the solid core might not be the best bet if you accidentally hit a tree and the wings separate and the fuselage ends up in a rock :)

    Also in a traditional top wing model design having the camera and control electronics in the wings (not in the wing tips), might save them if the plane hits land nose first and the wings are designed to separate from the fuselage.

    But all in all I think there's no right or wrong design. I personally almost totally destroyed my plane when trying to do a loop with too much speed and the wings snapped. The reason was that I had more weight (motor + batteries) on the fuselage than the model was designed for. If the weight had been distributed on the wings the wings could have survived. If modern passenger jets had the fuel on the core, treir wings would snap under the load too.

    I think this is something that all UAV conversions should think about. You can make almost any heavy UAV camera plane conversion fly just by using a bigger brushless motor, but will the fuselage and wings survive the load when you try to manually recover from a coding bug induced steep dive...
  • Pekka,

    Airplanes that pick themselves off the ground and fly after a bad landing tend to have a solid core and lightweight extremities. Increasing the moment of inertia will also increase the stress of a wingtip landing for example as the energy absorbed will be much higher. There are structural advantages of having the heavy things close together and compact. Increasing the size of the heavy and therefore structural members, increases weight, or decreases structural integrity.

    I'm not saying yours is a poor design, just thinking through the consequences - with an evolutionary bias - ie there are reasons airplanes tend to have light wings and heavy centers.

    Also note that gliders are easy to control in big air, but trickiest to control as they slow down for landing. That's when you're most likely to need the control.
  • bGatti, thank you for your tip on positioning the camera with weight - I makes alot of sense. I've been thinking all kinds of camera stabilisation solutions, but they usually add more weight.

    While it's true that bulky objects should be placed behine one another on a streamlined fuselage rather than making the hole craft bulky, there's usually an option to use for example 4 smaller batteries in parallel than a one large pack that wouldn't fit in the wings.

    Also many readily available control electronics (see my last post) tend to be square and flat, so their ideal place could be in the wings. My current development craft is a modified glider with big wings and a tight narrow body.

    I also think that the servos won't be an issue in many situations. The craft I'm converting has always been a bit too agile (even before adding a motor), and I've had to restrict the control surface movements to counter my lack of skills :)
  • another benefit of moving weight to the wings, is that it puts less stress on the ... wings.

    The ultimate example was the UAV Helios slated to fly over Hawaii (ended badly), but none the less, it was designed in sections, with each wing section carrying its own battery, solar and motor.

    Sadly, it was so unstable that it crashed. Bear in mind that increasing the moment of inertia / increases/ the force necessary to stabilize the plane by the same degree. Bearing in mind that control force is roughly equal to control surface * motor thrust, one must increase control surface significantly if one increases the moment of inertia. Some flying surfaces are intentionally placed in the prop wash. The amount of prop-related control available is limited - increasing the moment of inertia - decreases the amount of control.

    This will also require larger servos, and thus add to the total weight.

    I'm sure you can trade maneuverability for slower response, but I'm not convinced it is necessarily a good idea. (see Helios).

    The other reason bulky things end up stacked behind each other is aerodynamics. Trains are hundreds of times more efficient than semi-trucks because they break the airflow only once for a hundred traincars. Stacking bulky objects sideways is inefficient.

    Lastly, the imaging problem as I've seen it, appears to be vibration related - rather than axis stability.

    Try a thought experiment in which vibration sources are moved far from the camera, while static weight is clustered nearest the camera. For my money, that is the better image.
  • 3D Robotics
    Great tip! Very sensible suggestion. Note that another way to increase the pitch moment of inertia is to distribute the equipment weight fore and aft.
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