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Feb 01, 2017, 07:46 AM
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Build Log

vtol time lapse camera platform


This project is intended to create a stable '25g mobius-type' camera platform which can hold position for at least an hour at a time. A quick 1 minute battery change should be ok at that kind of interval.
And complete at least 6 hours continuous total flying time in a single day.
It must not attract undue attention from the ground. It should resemble a bird at casual glance from below.
It must take off and land within a very small agreed area.
Must be easily transportable.

Within a one-off project without a commercial budget, there are not many other options than a VTOL.
Helium blimp.
Hydrogen blimp.
Hot air balloon.
Multirotorcopter.
Ultrastable ram kite with invisible strings.


I have no idea if the physics are correct or not - that a wing-based multirotor can reduce the battery drain.
I seemed to have stirred up some debate on this and other forum. There is no tether vector, like a kite has a string.
I found and lost the link to a really detailed study of the Albatross aerodynamics. Which looked at many other birds and the lift vectors involved when they appear to be 'stationary'. There must be an airflow to create lift. If the airflow is truly horizontal, the bird will descend at the same vertical rate as it would if gliding at maximum L/D.
However, it is not possible to determine if a bird is generating lift while adjusting wing tip feathers/angles. Think of a hovering Kestrel and what you see with binoculars. A Kestrel also has a relatively high wing-loading, 20oz ft2. To keep the weight and construction complexity/cost down, I had to decide this model would be flown in a maximum average breeze of 5mph. I can choose the day for the photography run.
The autolevel software during hover would be pointing the rotors forward (on average) when holding position against a gentle breeze.
This is effectively the same as a 'half' transition. So the average thrust needed just balances the drag of the wing. Which at say 500g AUW and L/D of 5 is 100g. At 5g/W that's only 20 W. If the breeze speed matches the max L/D speed. Just a starting point assumption.
In fact the first prototype observed L/D was better at 6 to 6.5, and AUW 450g. Rotor efficiency looked closer to 10g/W.
So I thought I'd put something together and see what happened.

I designed a prototype after researching as many previous examples and theory as I could find.
The rotors are in the wing so they are supported between two pivot points. Saves a lot of weight and vibration issues.
There is the possibility that the duct increases prop efficiency.
The centre of thrust is over the centre of gravity.
The planform has to jut out forward where the fans are located and then back again so most of the wing area is behind the COG and yet they are also close to the centre of wing lift.
The battery is mounted as low as possible.
Anhedral/dihedral puts the fans at the highest point.
A 3mm depron skin over accurate reflex wing profile ribs with built up spars was tried out and seemed stiff and strong enough.

Now for some images.
When it stops raining I'll take the model outside and do some close-ups of the mechanics etc.
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Feb 01, 2017, 08:46 AM
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photos


Close up of rotor tilt mechanism.
A 9g servo is linked directly on one side and a ptfe/nylon bush stuck in the depron at the other.
I ran the motors/props and turned the mechanism by hand, thinking there would be some gyrational forces developed which is why everyone used great big ball bearings and metal geared servos. There was hardly anything, what little there was went along the axis of the pivot mechanism.
'Aero' knitting needles(good alloy and free from charity shops) of course and aluminium litho plate stiffeners.
Lots of big prangs later and only problem is motor tubes coming loose and sliding within glued housing.

Wingspan 1200mm. Should be able to scale other dimensions from this.


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