# The Naked Scientists Forum

### Author Topic: What is wrong with my ionocraft calculations?  (Read 7180 times)

#### Supercryptid

• Hero Member
• Posts: 606
##### What is wrong with my ionocraft calculations?
« on: 10/03/2009 03:53:06 »
Alright, so have you guys heard of a lifter/ionocraft? Here's some information if you need it: http://en.wikipedia.org/wiki/Ionocraft

Well anyway, there is an equation given on the page that tells you how to calculate the thrust produced by such a device:

F = (Id)/k

Where "F" is the force of thrust measured in newtons, "I" is the current measured in amperes, "d" is the air gap distance measured in meters, and "k" is the ion mobility coefficient of air measured in m2(V s).

I wanted to calculate how much thrust such a device could generate if it were powered by a AA battery. Wikipedia says that AA batteries can have 400 - 1700+ millamp-hours (0.4 - 1.7+ amp-hours), which I figure means that they can supply a current of 0.4 - 1.7+ amps for a period of 1 hour, right?

So here are the figures for a device with a 1 cm air gap:

I = 1.7 A
d = 0.01 m (1 cm)
k = 0.0002 m2 (V s) (given by Wikipedia)

F = (Id)/k
F = (1.7 x 0.01)/0.0002
F = 0.017/0.0002
F = 85 newtons

Now, 85 newtons is about equal to 19.1 pounds of force. This seems to be way too high. Ionocraft of this size are supposed to have a thrust-to-weight ratio too small to allow for an onboard power source to be carried. Yet AA batteries have a mass between 15 and 31 grams. The lifter's construction could also be made much under 19.1 pounds.

I must be doing something wrong.

What's the problem?
« Last Edit: 10/03/2009 03:55:42 by Supercryptid »

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### What is wrong with my ionocraft calculations?
« Reply #1 on: 10/03/2009 04:34:04 »
Looking at the links at the bottom, it seems as if the current, I, is the corona current, i.e. the current of ionized air.  You'll need massive voltage to convince the air to conduct 1.7 A of current.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11999
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### What is wrong with my ionocraft calculations?
« Reply #2 on: 10/03/2009 09:53:18 »
Yes Supercryptid. We definitely need to build those. As I have some defense contacts I've been able to procure this instructive film footage for your viewing pleasure.
We need this..
feature=channel_page

#### lyner

• Guest
##### What is wrong with my ionocraft calculations?
« Reply #3 on: 10/03/2009 10:18:23 »
3MV/m is needed to produce ionisation of air. For a 1cm gap, this implies that you would need something like 30kv. With a sharply pointed electrode, this could be less but it would have to be about 15kV.
Your 1.5V cell can provide, say, 1A  - which is 1.5W. The current for 1.5W at 15kV would be only 0.1mA. That's the sort of current which your 1.5 cell could provide at high voltage and is what you should use in your calculations. (I haven't included the inefficiency of the voltage converter circuit here)
I think that makes the force about 6mN.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11999
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### What is wrong with my ionocraft calculations?
« Reply #4 on: 10/03/2009 12:31:45 »

http://blazelabs.com/l-intro.asp#peek
And this one is good too.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040171929_2004178266.pdf

« Last Edit: 10/03/2009 12:58:33 by yor_on »

#### Supercryptid

• Hero Member
• Posts: 606
##### What is wrong with my ionocraft calculations?
« Reply #5 on: 12/03/2009 08:49:25 »
3MV/m is needed to produce ionisation of air. For a 1cm gap, this implies that you would need something like 30kv. With a sharply pointed electrode, this could be less but it would have to be about 15kV.
Your 1.5V cell can provide, say, 1A  - which is 1.5W. The current for 1.5W at 15kV would be only 0.1mA. That's the sort of current which your 1.5 cell could provide at high voltage and is what you should use in your calculations. (I haven't included the inefficiency of the voltage converter circuit here)
I think that makes the force about 6mN.
3MV/m? Can you point out a source for that?

#### lyner

• Guest
##### What is wrong with my ionocraft calculations?
« Reply #6 on: 12/03/2009 10:24:36 »
Yes - you can find it on the link yor_on has given you!

#### The Naked Scientists Forum

##### What is wrong with my ionocraft calculations?
« Reply #6 on: 12/03/2009 10:24:36 »