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Science Olympiad coach needs info on building a hovercraft

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Old 11-26-2016, 03:38 PM
  #1
drcubbin
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Default Science Olympiad coach needs info on building a hovercraft

Hovercraft is a new event for us this year, and we are having a bit of a rough start (even though we g go to our first Invitational 12/7/16). Here's the skinny of our allowed specs: max craft weight = 2kg (about 4lbs - a lot considering motor restrictions) it can be 200 g, but the closer we come to the 2kg limit, the more points: 2 motors - MUST BE BRUSHED, no brushless allowed; Must be powered by 9v - expected voltage (according to labels) cannot travel > 9v across any wires; I am leaning toward 2-blade props after reading the pros and cons of adding more blades. So what I am looking for is a powerful (very, very high rpm) 9v brushed motor. I am ok with the build, but finding the right motor... not ok. Any suggestions? Much appreciated in advance! Mike
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Old 12-01-2016, 08:13 AM
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Jennifer Curtis
 
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Mostly motors don't have voltage limits. (technically
they do, but it is really high, like when the insulation
breaks down, which is well above 9 volts.)
So you don't choose your motor based on voltage.

The real limiting factor is how much current the
wires will handle. In addition to this, is the motors
ability to dissipate heat.

Motors have a speed that they will run at any
particular voltage. They will try to run at that
speed no matter how big the propeller is. If
the propeller is really big, the motor will draw
more current than the wire thickness will allow,
and the windings will burn out.

A smaller propeller may draw less current,
than the wire size limit, but the motor will
still get too hot if it can't get rid of the heat
quick enough. Heavier motors can hold
and dissipate more heat.

You didn't say what the motor restrictions
were, but if there is a restriction on motor
weight you probably want to get a motor
at that weight.

Now for a hovercraft, you want a fan that
develops pressure rather than lift. The
total pressure will be equal to the weight
of the craft. (If you use a propeller that
creates LIFT equal to the craft's weight,
it won't hover, it will lift off and fly away.)

The skirt around the craft holds the
pressure and directs it downward. If the
craft gets too high the pressure is released
and the craft settles back down. You can
calculate the pressure required to hover
based on the weight of the craft and
calculate the power required to generate
that pressure.

You should figure that the motor weight
in grams should be about three times
the number of watts of power needed.
That will be a motor that will be able to
dissipate the heat it produces. (you still
need to make sure it has good airflow
to keep it cool)

The motor's top speed is a function
of its windings. This is the motor's Kv.
You can't change this without re-winding it.
You can use a speed controller to limit the
motor speed Also if the load is great, the
motor won't reach its maximum speed anyway,
but it could burn out trying. So use a speed
control. You should use a speed control
anyway to actually control the craft.

So you get a motor, control, etc and build
a craft and try it out. If it doesn't hover,
even at full speed, you add more blades
to increase the pressure. You use a
wattmeter to make sure you are not
overloading the motor. Don't worry
too much about how many blades
it has. You want it to generate pressure
not lift. Generally fans used in airplanes
(as in turbofan - turbine engine driving
a fan) have more than two blades.

You may need to try different pitch
propellers to get the pressure you want.
You can also mix different pitch propellers
on the same shaft, but there should not
be too much difference, such as a 9x4
plus a 9x5, not a 9x4 and 9x6. Also
put the higher pitch one closer to the
ground.

So if you can't get a good hover at this
point, you might need to try another
motor, or re-winding it for a higher or
lower Kv.

You didn't mention how long it needs
to hover. Of course the longer it needs
to run, the larger the battery will be,
meaning more weight, more power,
etc.

You also didn't mention if there are battery
restrictions. If there are not, a 7.4 volt
(two cell) lithium polymer battery is probably
your best bet. Figure the power(roughly)
and the duration you need to bet the mAh
size battery, then with the weight of the battery
re-calculate the power needed to carry the
weight of the craft with that battery to be sure.

Finally be sure the shroud around the fan is
very close to the blades to keep pressure
from leaking up around the tips.. Streamline
the supports that hold the motor over the
hole, and the wires to the motor.

Jenny

ps there was an article in the mid to late 1970's I
n a magazine (probably Popular Mechanics) about
how to build a small full size hovercraft using
a lawn mower engine.

pps I have seen a hovercraft made from a cd and
a CPU fan. It was totally uncontrollable and only
able to work on an absolutely smooth surface, but
it was technically a hovercraft.

Last edited by Jennifer Curtis; 12-01-2016 at 08:19 AM.
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Old 12-01-2016, 09:49 AM
  #3
drcubbin
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Thanks SO much, Jenny! Lots of information I did not know.

To clarify so information I didn't add at first:

1. There is no limit on the size of the motor, only that the power source cannot be > 9v across any lines. Since the maximum weight we are looking to lift is a little over 4 lbs (2kg), we really need the motor with highest rpm that can be run for a very short distance on that 9v.
2. No restrictions on blades either, but your advice (pressure vs. lift) is excellent.
3. It need to hover no > about 1 minute, so we are not too worried about overheating or power loss.
4. We will close our shroud in as well.'

Finally, are there any motors you can think of that might fit this bill?

Thank you so much for your great insight!

Best,
Mike
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Old 12-01-2016, 12:14 PM
  #4
Jennifer Curtis
 
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If you search for "brushed motor" on tower hobbies,
you will see quite a few available. Most don't give
any real specifications. Two that caught my eye
were:

http://www3.towerhobbies.com/cgi-bin...?&I=LXEXSG&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXEXSF&P=7

These are 27 and 20 turn motors to turn pretty high rpm. They
will produce 35 and 55 watts respectively. The best part is
they only cost about $10, so if you burn one out experimenting
it is not a great loss, and you can try to re-wire it or just toss it.

I would try these propellers:


http://www3.towerhobbies.com/cgi-bin...?&I=LXFYAL&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXFTMF&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXFYAM&P=7

They are 5 inch APC bullnose props 4.3 4.5 and 4.6 pitch

I can't say for sure that it would hover at 4 pounds with one
motor. I'd feel confident at 1 pound.

You could try multiple motors for heavier lifting.
Multiple brushed motors should work fine off a single
speed controller, unlike brushless motors.

As an alternative to a speed controller you
could use a length of coat hanger and an
alligator clip. Hang the alligator clip on
the wire at different points to act as a
variable resistor. It is highly inefficient,
but also very affordable. The wire will
get hot so be careful. How long the wire
is will be a matter of experimentation.

If you do this, you will need to keep an
eye on your battery voltage, since it won't
cut off automatically. Lipo batteries can't
be allowed to get too low or they will be
permanently damaged. I use 3.6 volts
per cell under load as my minimum voltage,
but the "official voltage" is 3.3 volts.
Double that for a 2 cell battery.

Jenny
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Old 12-01-2016, 12:23 PM
  #5
Jennifer Curtis
 
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When I mentioned using multiple motors (above)
I meant with each one driving its own propeller.

Jenny
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Old 12-01-2016, 04:49 PM
  #6
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As said above, you don't need a prop. Props make lift because they are little wings. What you need is a fan, because fans are designed to move air.

You are also barking up the wrong tree looking for the fastest motor. More speed=less torque, so you could easily wind up with a motor that can't spin the fan you pick.

I'd think a small desk fan would do the job, possibly with a belt drive and gear reduction to get the speed right. Another option would be a squirrel cage blower set up with a direct drive.
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Old 12-02-2016, 05:39 AM
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Jennifer Curtis
 
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For a fan, speed is actually good, so you are on the right track
there. (Jester: Note that the electric jets we fly have very high
revving motors driving their fans. Reduction drives are complicated
troublesome devices that sometimes improve propellers, but not
fans, enough to outweigh their drawbacks.)

On the track of using propellers for a fan, you could also
use a larger propeller with the tips cut shorter to fit a
smaller opening. This would change the airflow
characteristics, possibly giving better performance.
Be sure to carefully balance the props, cut or not.

There are also ducted fans available that we use
for our model jets. They can get pricey and don't
allow for much experimentation like propellers
do.

Since the Science Olympiad is mostly and
educational exercise, the more experimental
approach is probably a better solution.

Jenny
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Old 11-05-2017, 06:31 PM
  #8
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Default Battery for hovercraft

I have purchased a motor and blades from Tower Hobbies as suggested by Jenny in an earlier post . I tested a model with a 12 v. auto battery and get good levitation. Now I need to get a battery that can be mounted on the hovercraft and not be too heavy. As a reminder, the Science Olympiad limits the voltage to 9v.and no lithium. I'm probably looking for a NiMH battery pack with as high a mah possible. Can anyone offer me some suggestions?
Jim
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Old 11-07-2017, 08:09 PM
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Yep. Get an 11.8v NiMh that is big enough to do the job. If you'll check the amp draw on your hovercraft and multiply that by the amount of run time you need in hours, that will tell you how many amp hours you need. Multiply that number times 1000 to get Mah, which is how RC batteries are generally measured.
Of course, you'll draw more amps with the battery attached since the hovercraft will weigh more. That will require more battery, which will then require more amps. So the fan you tested may or may not work. Designing things is hard. I teach engineering.
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Old 11-09-2017, 08:28 AM
  #10
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Thanks, Jester s1, for the response. I'm sorry that I failed to mention that the Science Olympiad limits voltage to 9 volts so I would be limited to an 8.4 volt NiMh battery. There are a few available online, more and less costly at 7.2 volts. Most max out at 3000 to 4000 Mah. Two batteries may be needed, one for propulsion and the other for levitation. The competition period may be as long as 8 min. I don't know how low I can go with the batteries since I was not able to get levitation until I tried the auto battery. I suspect that was because I needed a power source that could provide the current needed to extract max power from the motor. The rules allow the boys to add as many as 16 rolls of pennies to the completed hover craft so it must be possible to create a lot of pressure, although battery weight will reduce this weight capacity. Although I'm a retired electrical engineer, its been many years since I worked on designs like this. You folks seem to be on top of the current generation of parts. Thanks again for any suggestions you can give me..
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Old 11-10-2017, 07:15 AM
  #11
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NiCads will give more current than NiMH.
You can get AA or sub-C nicads from tower.
Seven in series and as many in parallel as
you need for capacity.

You can use the same battery for both lift
and propulsion.

Jenny
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Old 12-03-2017, 06:41 PM
  #12
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Does anyone have ideas for controlling the thrust of the propulsion fan? If a rheostat would work, could someone suggest one and where to get it. Unfortunately Radio Shack no longer exists.
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Old 12-04-2017, 04:32 AM
  #13
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You need an electronic speed control. All hobby supplies have them. If you bought a brushed motor (2 wires coming out) you'll need a brushed ESC.
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