# Hovercraft

I did math myself starting with your equations and got the same answer, so you’re probably correct.

Now, how big would the swept area on this hypothetical fan be? You’ve got a column of air 18.5 meters long (for my calcs) long passing through through every second, that contains 2.7kg of air, which is 2.11 cubic meters.

V = pi * r * r * h
2.11 = pi * r * r * 18.5
sqrt(2.11 / (18.5 * pi)) = r
0.19 = r = 19cm radius fan, or 7.5 inches. That’s a pretty big fan, but not unusably big. This could be do-able, but the speed out the back and the CFM rating seems pretty insane. I searched for 4000CFM fans, and came up with stuff sold as “whole house fans”, and I doubt that those would put out the kind of velocities needed.

Is this a correct calculation?

Don’t want to get into all the math, I don’t really remember it, but from fooling around with a human-carrying hovercraft a few years ago I have a couple of things for you to consider:

First: You have to be careful with the center of mass when using a hovercraft- any manipulators carrying over the curtains will tip it. Tipping makes the air curtain hit the ground (a rule violation) and will make your 'craft drift because of the uneven airflow.

And second: Yes a hovercraft will float the 120 lbs of robot. It will do so for awhile too. I remember using two car fans and the same 12 volt batteries we use now to carry myself as well as the weight of the actual craft and batteries (probably a little over 120 back in the day)

O.k. Being less far-fetched, from this I’ve gotten that using fan(s) to boost a drive trains output is possible but to cut out the drive completely is just stupid (or not prudent).

The hovercraft would have been sweet though. Oh well

I did the math a little differently and got 7.2 inches for the radius. Probably a round-off difference between you and me. If anyone was seriously considering this, it would probably be better to make the fan bigger, thus it could run slower.

I was thinking about this some more, and I realized the original spec of 50N is a bit ridiculous. Sure it would make your robot fast, but once your fan spooled up, you wouldn’t be able to stop (without cutting and reversing the fan, which would take a long time). 5-10N as a booster would be more reasonable. [strike]I’d bet that 50N is more in the range of what an FRC robot gets on carpet, as it implies acceleration of nearly 1G for a max-weight robot. You’d have a 120lb rocket. [/strike](Edit: this is wrong. You’d have acceleration of 1m/s^2, which is nearly 10 times less than 1G. I need to go to bed).

I made an excel sheet to play with these numbers:
If you had a fan with a radius just wide enough to fit on the short length of your robot (35cm), you’d only need an output velocity of 3.1m/s for a 5 newton boost. It would only require 7.8 watts. For a 10N boost with the same fan, you need a 4.5m/s output. However, since a lot of that boost would get caught on the trailer (thus slowing you down), you’d need more.

Edit: But it occurs to me that a giant fan would pose an entanglement risk, and so probably isn’t practical, if QA allows it at all.

Just for arguments sake, remember robots actually weigh about 150 pounds. The 120 base that passes inspection, plus a 13 poundish battery, plus 15 pounds of battery.

Also, can you provide some documentation about motor outputs? I though CIMs output about 300 watts.

I’m going by what we’re allowed circuit-breaker wise. We have a 12V system with 40amp breakers. Thus, the most power we’re allowed using for a single motor is 480 watts. Due to conduction losses, the most useful power we’re allowed is a bit less than that, but it is in that region of power.

I don’t know the specs for the individual motors, it is possible that 480 watts through one of the kit motors is impossible.

This might sound stupid but where would you go looking for fans / propellers? And what kind of gearing would get a CIM to spin the most efficient.

EDIT: Talking about using this to give a drive a little boost not to push an entire bot around.

This is an R/C fan that generates 4N, but spins at a pretty crazy RPM to do so: http://www.modelflight.com.au/rc_model_accessories/wemotec_micro_fan.htm.

I think it would be worth your time putting a standard house fan on a physics cart and use a spring scale to see how much force it generates. Then you could maybe replace the house fan motor with a CIM and speed it up a bit.

Again, I note that you probably don’t want to invest too time or money in this idea unless QA says it is legal. Their “only traction must come from rover wheels” rule could potentially be interpreted (FIRST has reached pretty far in their rule interpretations in the past) that you can’t get thrust from fans either.

You’re right, max output of a CIM is somewhere in the 300 W range. For these back of the envelope calcs, I was assuming 100% efficiency in the CIM motor (and in the gearbox, and in the fan). 12V * 40A = 480W is the maximum input to the motor. It’s not 100% efficient. But, you could always use two of them put together if you wanted to…

This question has been vexing me as well. My good friend (http://en.wikipedia.org/wiki/Propeller) tells me that propellers max out around 80% efficiency. I’m afraid that we won’t get anywhere near an optimum prop, so let’s assume 10% efficiency with 300W of CIM, yielding 30 W. When your robot is moving at 2 m/s, 30 W should translate to 15 N, about 3 lbs. If you power your fan or fans with 2 CIMS, you get 6 lbs of thrust, which would make a nice adder to the 7.5 lbs you get from traction.

Now, if someone who knew what they were doing made the prop a mere 30% efficient, then they would have an additional 18lbs of thrust; enough to push anyone around.

Unfortunately, I have no idea where you would purchase or how you would design this propeller. There are some big RC planes out there, but I can’t find much information on thrust produced or efficiency. Anybody have any leads?

This one is for EXARKUN666… You posted somewhere that you were unsure of the numbers that were used in the “LUNACY: initial game layout” in chapter 7. I realized that the <=7 on the carts and the 13-20 mean
the 13-20 is starting count for the moon rocks for that station 20 is max and if u load moon rocks on your robot before start, which max is 7, then that explains where u get the 13 moon rocks at start from, because you would have taken 7 from 20 to put on robot. And the <=4 is indicating that you will have 4 or less of the empty cells at start. I believe that you can load moon rocks and/or empty cells on your robot before start, but i don’t see why you would put empty cells on your robot before start.

this is out of context from this thread but i could not find the thread that u originally asked about this so i quoted u here and posted here. Not to get off topic.

I think everybody’s best bet as of now would just be to check out some hobby shops to get a look at propellers used for RC planes, copters, airboats or anything similar.

lol, i think it will be more like a game of air hockey

hey you guys, your thinking to hard…
You could do the opposite and get more grip… Like that model car that can go on the ceiling! I don’t know if that would work… What do you guys think?

is the coeficient of friction independent of surface area under the present conditions

i think that someone already discussed earlier in this thread using a fan to generate downforce, effectively increasing the normal force on the wheels.

my question is, how would you calculate the amount of suction force that a fan can generate against a solid surface like the playing field? using drive motors to power a fan is really only worth it if there is a significant traction increase.

also, is it legal to generate downforce with a fan?

it is illigal to genarate downforce with a fan

I think you’re not thinking hard enough, since you asked.

Yes, as coefficient of friction is a property of the interaction of two surfaces. But I think you want to know about pushing power, which theoretically remains the same because a larger area means a smaller force per unit area, but in reality a larger surface contact area means a very slightly increased pushing power.

Can you quote a rule please? Or is this speculation?

I’ve built hovercrafts before and let me tell you that by time your robot is all said and done, you won’t be able to hover. I mean, it could be possible, but that would require so much power that it’s just not plausible. I just seriously doubt that you could build a hover craft and efficiently complete the mission.