Hovercraft

[smartkid]

I know that this has crossed other teams minds and so I'm just going to throw it out there. It comes down to who can build and control it better anyway...

Rule R-06 states that we must use the wheels provided to "provide traction between the ROBOT and the ARENA." However if we throw caution to the wind and think about the idea of a hovercraft the shirt used to hover doesn't provide traction, it reduces traction. Then fans provide thrust.

This would normally make you more easily pushed but this year everyone is going to be getting pushed around (I've seem the math, with wheels alone you simply cannot get more traction). This could give you more maneuverability and better control of your bot (and with four CIM's available and a compressor I'm sure it's doable).

Is it allowed?
Would it work?
Am I crazy?

[NickJames]

Try it out and tell us how it goes. [=

[Bongle]

I think a pure hovercraft would be a silly idea. Throwing away the traction that you ARE allowed from your wheels and opting instead for an even harder to implement and control drive system doesn't seem like the right idea.

However, using a fan to augment your acceleration and deceleration might work. The wheels can only give you a maximum of about 26.7N of acceleration¹ force. If you build a fan that's good for a few newtons of thrust², then that can be a substantial increase in your robots straight-line handling, allowing you to outrun someone intent on dropping rocks in your trailer (and maybe even causing a bit of aerodynamic drag against them).

¹: Maximum mass of robot: 54.54kg. Weight = 54.54 * 9.81 = 535N. Maximum traction: ff = uFn -> ff = 0.05 * 535N = 26.7N

²I know nothing about fans. This may very well be outside of the possibilities for a 12V fan, but it would be worth running numbers on.

[Bongle]

Originally Posted by Bongle

²I know nothing about fans. This may very well be outside of the possibilities for a 12V fan, but it would be worth running numbers on.

To continue this train of thought and get an idea of what would be possible. Note that neither of these would be legal:

A $2600USD model aircraft jet engine puts out 59N: http://www.trimair.com.au/index2.html. However, it is petrol powered and obviously illegal.

A $740 set of ducted fan components can put out 13-15lbs of thrust (57.9N), but uses a minimum of 3200 watts, which is 6.6 times how much power a FIRST robot can use.

[drkiraco]

to keep going:
Thrust = mass flow * velocity
Power = (1/2) mass flow * velocity^2

For a given thrust, we could pick any combination of mass flow and velocity, but to minimize energy used, we want greater mass flow and less velocity.

max power for a single FIRST motor is 480 watts (12V * 40A). call it 500W for round numbers. So for 50 N you could do, for example, 2.5 kg/s air at 20 m/s. Now, to put this in perspective, 20 m/s = 44 mph. Seems a little high to me, but maybe not out of the question. Probably you wouldn't get anything faster than this. Now put 2.5 kg/s of air in perspective. That's 5.5 lb/s. Since a cubic foot of air has a mass of ~0.075 lb at standard temp and pressure, we need 73 ft^3/s. That's 4406 ft^3/min (scfm). That's a rather large number. Maybe not totally out of the question though.

So, unless I messed up the math, I'd guess that 50N of thrust could be achievable without violating the laws of physics or any FIRST rules, but I don't think it would be cheap or easy...

Originally Posted by Bongle

To continue this train of thought and get an idea of what would be possible. Note that neither of these would be legal:

A $2600USD model aircraft jet engine puts out 59N: http://www.trimair.com.au/index2.html. However, it is petrol powered and obviously illegal.

A $740 set of ducted fan components can put out 13-15lbs of thrust (57.9N), but uses a minimum of 3200 watts, which is 6.6 times how much power a FIRST robot can use.

[Bongle]

Originally Posted by drkiraco

to keep going:
Thrust = mass flow * velocity
Power = (1/2) mass flow * velocity^2

For a given thrust, we could pick any combination of mass flow and velocity, but to minimize energy used, we want greater mass flow and less velocity.

max power for a single FIRST motor is 480 watts (12V * 40A). call it 500W for round numbers. So for 50 N you could do, for example, 2.5 kg/s air at 20 m/s. Now, to put this in perspective, 20 m/s = 44 mph. Seems a little high to me, but maybe not out of the question. Probably you wouldn't get anything faster than this. Now put 2.5 kg/s of air in perspective. That's 5.5 lb/s. Since a cubic foot of air has a mass of ~0.075 lb at standard temp and pressure, we need 73 ft^3/s. That's 4406 ft^3/min (scfm). That's a rather large number. Maybe not totally out of the question though.

So, unless I messed up the math, I'd guess that 50N of thrust could be achievable without violating the laws of physics or any FIRST rules, but I don't think it would be cheap or easy...

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?

[mray1031]

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)

[smartkid]

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

[drkiraco]

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.

Originally Posted by Bongle

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?

[Bongle]

Originally Posted by drkiraco

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. 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. (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.

[Ian Curtis]

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.

[Bongle]

Originally Posted by iCurtis

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.

[smartkid]

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.

[drkiraco]

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...

Originally Posted by iCurtis

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.

[minisimon]

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?