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Camel

Fri Dec 26 2008, 11:45AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Hi,

I've been researching ionocraft recently. One of the main goals of hobbyists is to produce a craft that could lift its own power supply.

With the exception of helicopters and a few military fighters, aircraft generally weigh more than the thrust they produce. Lift is generated by the wings, not the engines. So I reckon instead of making a VTOL ionocraft, why not give it wings and use the Biefeldâ€“Brown effect purely for horizontal thrust?

The main difficulty is, obviously, building a power supply capable of delivering about 30kV and about 5mA and weighs less than a few hundred grams.
I'm guessing a flyback transformer would be the lightest and easiest option. However the flyback transformer which I scavenged out of an old monitor weighs 300 grams in itself. Batterys are also heavy, 8 AA lithium cells would weigh about 120 grams, but they have the best power to weight ratio.
Another problem is the size of the actual ion thruster. For a decent amount of thrust there needs to be a large length of capacitor. I'm thinking a bi-plane type design, where the top and bottom wings produce lift and the ion thrusters sit in the middle, functioning also as braces.

Comments & suggestions?

Dr. Dark Current

Fri Dec 26 2008, 12:00PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, the diodesplit transformer is indeed the lightest way to produce such voltage and current. Maybe you could have some luck finding less weighty transformers. I've just weighted one of mine at 230 grams. I'm pretty sure there are even lighter ones, but they might get pretty hot with 150 watts.

Another concept is a little ferrite transformer and a CW multiplier, but a lot of the "power" is lost in tha capacitors' reactance so the "little" xfmr would need to supply considerably more power (which would be only reactive, ie. NOT drain from the battery).

I've also heard of piezoelectric transformers but I'm not familiar with the technology.

Camel

Fri Dec 26 2008, 12:12PM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Mmm, I imagine the number of caps and HV diodes required to a CW multiplier would easily equal the weight of a flyback transformer.
or am I thinking of something else.

Carbon_Rod

Fri Dec 26 2008, 05:26PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Laminated PT (piezoelectric transformer) are a mature technology:

Few stacks run above 90% efficiency.

The mass of your propellant and the effective exhaust velocity of the ions would not be sufficient to overcome the frictional losses.

IIRC most ion thrusters can only achieve thrust:power ratios of about 10 to 70 mN/kW

Cheers,

Camel

Sat Dec 27 2008, 12:29AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Carbon_rod, those piezoelectric transformers look quite interesting . . . do you know where I might be able to score one? My usual electronics suppliers don't have anything like that.

Also, I'm not making an ion thruster. I'm thinking of making a model aircraft that uses this for propulsion.

rp181

Sat Dec 27 2008, 12:55AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

ion lifters are electrohydrodynamic thrusters. The type you point to uses tremendous amounts of energy. A small desk top sized piece consumed over 1 million watts (or was it joules?). The NASA wants to power them with nuclear reactors.

Camel

Sat Dec 27 2008, 01:05AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

err . . .
Lifter 3 on this page has a 24V 19W power supply. Its your standard run of the mill science fair lifter.

mikeselectricstuff

Sat Dec 27 2008, 01:41AM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

The most common use of piezo transformers is in CCFL backlight inverters, but IME they only tend to be used where smallest size (especially thickness) is important.
Digikey used to sell them but I think they discontinued - Complete BL inverters would be the most likely source for them in sensible sub-zillion quantities. I don't know what the limiting factor is power-wise but being designed for CCFLs the breakdown voltage is likely to only be a few kv. A CW multiplier on the output may be needed to get enough voltage, or possibly a series-connected arrangement with suitably isolated seperate battery supplies for each section.

The problem with conventional transformers is that for low core weight you want high frequency, but capacitance losses are an increasing problem as frequency goes up.

I wonder if another approach might be a construction of a very large number of very very small battery structures - this sort of thing was used for driving image intensifiers in WW2. With clever application of modern materials this could actually give the best power/weight ratio as there would be no conversion losses, but may need some serious development on the materials and structure of the battery - DIY Li-Po batteries are possibly a bit ambiitous..!
Or perhaps a hybrid of this and a CW multiplier - e.g. a few 1kv battery stacks and some HV MOSFETS driving a CW or marx type multiplier.

However something else that may be worth thinking about - coupling power from an external source - although not completely autonomous, it would be pretty impressive. Solar cells would probably b e too heavy, but how about inductively coupling the supply? A massive excitation coil with a ton of power going through it, to overcome the inefficient coupling to an onboard pickup coil. Maybe something like a Tesla coil with a very mobile secondary, and enough added secondary capacitance to minimise detuning from movement.

I wonder if the magnetic field would interfere with the ion stream?

A magnetron coupling power to a small onboard antenna may also be feasable, if somewhat dangerous...

Sulaiman

Sat Dec 27 2008, 08:35AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

sorry for the interruption,
I'm curious what the benefit is compared to
solar-powered electric motor with a propeller ?

Marko

Sat Dec 27 2008, 12:58PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Sulaiman wrote ...

sorry for the interruption,
I'm curious what the benefit is compared to
solar-powered electric motor with a propeller ?

As far as I know - absolutely none. A small motor with a propeller would beat it in efficiency and weight many times.

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