Crazy robot idea for this year 2

[Jon Stratis]

Originally Posted by EricH

It gets better. You also have to have something to hold the helium. However, you could possibly get better lift by pressurizing the helium, allowing more liters to be added... which opens up its own set of problems, including the fact that helium escapes from containers very easily if there are any holes at all.

Helium balloons float because helium weighs less than air. Shoving more helium molecules into the same volume won't increase lift, it'll actually decrease it! Imagine you have two 1-liter bottles of soda. They each weigh a certain amount. Now I hand you a 1-litter bottle that's been insanely pressurized and actually contains 2 liters of soda - which is heavier? The pressurized bottle weighs the same (discounting the container weight) as the two unpressurized bottles!

[EricH]

Of course, I didn't specify how much extra you pressurize it. The kind of pressures I'm used to dealing with for air/helium, you can't measure them in PSI unless you really like small decimals.

I'd also note that at least one maker of the size of helium balloons that can possibly lift an FRC robot does specify that they be pressurized, and goes to great lengths to have the balloons maintain that pressure. This is partially because said balloons need to maintain a given shape, granted--but you have to wonder if it doesn't give a tad bit of extra lift.

(Then again, these helium balloons can weigh several hundred pounds empty.)

[JamesCH95]

A perfect battery can support around 60A draw for the whole match, at around 11.5 volts. Lets be generous and say you only want a few moments of flying and the battery can support 150A at around 10.5V. This is 1575 Watts, which should be able to lift around 16-17lbs.

I do not believe that it is possible to lift all of the required hardware (battery (13lbs), cRIO (1.4-2lbs), cRIO modules, power board (1.6lbs), digital sidecar (0.25lbs), main breaker, bumpers, motors, radio (0.5lbs) etc) with the battery's available power. This is even without accounting for losses such as resistance in wiring and motor inefficiencies. In fact, that 1575W of theoretically available electrical power is probably around 1200W if you use RS775-18s at peak efficiency (78%). Voltage at the motors will be lower than 10.5V, so ding some more power...

I think it's safe to say that the battery cannot output enough power to lift the absolute minimum FRC legal robot.

[remulasce]

I'd also note that at least one maker of the size of helium balloons that can possibly lift an FRC robot does specify that they be pressurized, and goes to great lengths to have the balloons maintain that pressure. This is partially because said balloons need to maintain a given shape, granted--but you have to wonder if it doesn't give a tad bit of extra lift.

This is still not how buoyancy works. The idea is to get the least weight to take up the most volume. The volume determines how much lift is produced. The lifting element is there to maintain volume with as little weight as possible. Because pressure of a gas is not related to the molecular weight of that gas, elements with very low molecular weights are used to match the outside atmospheric pressure, so the volume of the balloon is maintained. Your manufacturers suggest inflating beyond that pressure to give additional rigidity and in case of leaks, but once the balloon volume is full, all you're doing is putting more weight into the balloon.

[pfreivald]

A leaping robot that jumps forward over the fender and then clambers up the baskets -- blocking all four of them with a shape change -- is both technically feasible and within the rules.

We thought of it day one, then said, 'naah.'

[maxweberh]

You could have always put a fan, on the robot, went over the bump/bridge. Sat in your alley, and aim at the top 3 hoops. I think that is a smart plan for defense.

But once again, I would love a flying robot.

[Akash Rastogi]

Originally Posted by maxweberh

You could have always put a fan, on the robot, went over the bump/bridge. Sat in your alley, and aim at the top 3 hoops. I think that is a smart plan for defense.

But once again, I would love a flying robot.

Don't encourage him Max

[PAR_WIG1350]

Originally Posted by remulasce

I stand corrected. The required battery + crio, combined with the limited set of motors, are more than enough to make this impossible in my view, but if you can do it, I think the "safety" rule is the only thing standing in your way. Which would also be near-impossible to satisfy, but you may try as you like.

???
This years set of motors has one of the highest power densities I have seen in FIRST. If you ran all of the 550s (all 8) into a single gearbox (low reduction, high speed), and put a large prop on it, you could get some substantial lift.

Motor power breakdown:
-AndyMark motors (AM- 0912)> 180.83W * 2 = 361.66W (current is below limit)
-Banebots RS-550-120-------> 253.52W * 4 = 1014.08W **(slightly less due to current limits, at this power level, they draw 43.2 amps each)
-Fisher Price 00801-0673-----> 291.59W * 2 = 583.18W **(prob. not quite this high, this would require 54.76 amps per motor)
----> this comes out to 1958.92 Watts, theoretically. So, How much would that be able to lift? (more specifically, how much would that be able to lift using a propeller?)

[EricH]

Originally Posted by PAR_WIG1350

So, How much would that be able to lift? (more specifically, how much would that be able to lift using a propeller?)

It depends on the prop, and the air density. Quite possibly even the specific prop. The one overarching question which won't be answered by motor power is prop thrust--motor power is good for getting that thrust, but can't be precisely correlated without some testing. A 27x13 is quite a hefty prop, even for a hefty motor like an FP.

And I've seen a case where the same prop tested at two different thrust levels in two different locations. Not fun trying to figure out why...I suspect it had something to do with either the air density or a mounting change, or both.

[PAR_WIG1350]

Originally Posted by EricH

It depends on the prop, and the air density. Quite possibly even the specific prop. The one overarching question which won't be answered by motor power is prop thrust--motor power is good for getting that thrust, but can't be precisely correlated without some testing. A 27x13 is quite a hefty prop, even for a hefty motor like an FP.

And I've seen a case where the same prop tested at two different thrust levels in two different locations. Not fun trying to figure out why...I suspect it had something to do with either the air density or a mounting change, or both.

Well, air density at FIRST venues, which are often climate controlled and at fixed altitudes, shouldn't be too hard to calculate, plus the mounting won't change as long as it is on the robot. Additionally, the same website also sells 26, 24, 22, 21, 20.5, and 20 inch props in varying pitches.

Does anyone have performance data from 2009 for similarly sized propellers?

[JamesCH95]

Don't get caught up in the details with props and their inefficiencies, sizes, etc. Do the broadest, most basic calculation possible to check and see if it is within the realm of feasibility.

If we take a minimum robot mass (25lbs) and use the entire cross sectional area of the robot envelop, 28inX38in we find that we will need to generate approximately 0.0235psi (162Pa) of dynamic pressure just to 'hover' in free air, ignoring ground effects.

Pdynamic = ( fluid density * fluid velocity^2)/2

fluid density = 1.18kg/m^2 (~STP)
Pdynamic = 162Pa

Fluid Velocity => 16.9m/s

Total air flow rate is 28in*38in*16.9m/s = 11.6m^3/s

In a basic sense we need enough power to accelerate 11.6m^3 of air to 16.9m/s in 1s. This can be simplified to a basic energy problem:

Air mass = 11.6m^3*1.18kg/m^3 = 13.7kg

air velocity = 16.9m/s

Air's kinetic energy = 1/2*mass*velocity^2 = 1950Joules

Time available to apply this energy = 1s

Total power required by this method = 1950J/1s = 1950W to maintain hover, including no inefficiencies.

This is about what PAR_WIG1350 figured that eight motors, drawing well over 280A, could produce.

It is technically unfeasible with our legal battery. It is technically unfeasible with our required main breaker. Even if these barriers were removed, the motors would not last long at peak power.

[PAR_WIG1350]

Originally Posted by JamesCH95

Don't get caught up in the details with props and their inefficiencies, sizes, etc. Do the broadest, most basic calculation possible to check and see if it is within the realm of feasibility.

If we take a minimum robot mass (25lbs) and use the entire cross sectional area of the robot envelop, 28inX38in we find that we will need to generate approximately 0.0235psi (162Pa) of dynamic pressure just to 'hover' in free air, ignoring ground effects.

Pdynamic = ( fluid density * fluid velocity^2)/2

fluid density = 1.18kg/m^2 (~STP)
Pdynamic = 162Pa

Fluid Velocity => 16.9m/s

Total air flow rate is 28in*38in*16.9m/s = 11.6m^3/s

In a basic sense we need enough power to accelerate 11.6m^3 of air to 16.9m/s in 1s. This can be simplified to a basic energy problem:

Air mass = 11.6m^3*1.18kg/m^3 = 13.7kg

air velocity = 16.9m/s

Air's kinetic energy = 1/2*mass*velocity^2 = 1950Joules

Time available to apply this energy = 1s

Total power required by this method = 1950J/1s = 1950W to maintain hover, including no inefficiencies.

This is about what PAR_WIG1350 figured that eight motors, drawing well over 280A, could produce.

It is technically unfeasible with our legal battery. It is technically unfeasible with our required main breaker. Even if these barriers were removed, the motors would not last long at peak power.

With the exception of the battery, the limits are all purely thermal, Through strategic placement of the breakers and motors the propwash might keep them cool enough to avoid anything failing mid-match.

Also, I find it strange that you bring up ground effects while referencing a post made by someone with a ground effect related username.

[JamesCH95]

Originally Posted by PAR_WIG1350

With the exception of the battery, the limits are all purely thermal, Through strategic placement of the breakers and motors the propwash might keep them cool enough to avoid anything failing mid-match.

Also, I find it strange that you bring up ground effects while referencing a post made by someone with a ground effect related username.

Hah, I did not know that PAR WIG was a reference to ground effect aircraft until I looked it up just now.

The 25lbs I figured for the minimum required hardware did not include eight motors, controllers, and wiring. Add another 6-8lbs for those motors, and it's too heavy to take off again. One would probably need 10-14 motors of similar power to get airborn.

The RS550, FP, and AM motors are not especially efficient, especially at max power. They are also quite heavy for their power output. For example, this brushless motor (just something I found on a google search) brags a higher peak power rating than any FRC motor, save the CIM, at 2.8oz of weight, almost a 1/3 the weight of an RS550. FRC equipment is just a bad way to go about trying to make a flying robot.

[pandamonium]

what if during auto mode a robot placed a ball on the opposing alliances bridge then had a manipulator push on that ball to lower the bridge. This action alone would actually be quite helpful and if another robot on you alliance got the balls of the coopertion bridge there could be 6 balls on their side and 12 on yours. The balls stolen from their bridge could even be collected and scored which would be quite awesome!

[fox46]

You guys need to read up on flight theory, and EricH needs to read up about buoyancy.

In order to make a craft fly you don't necessarily need to generate more thrust than weight. Furthermore, a helicopter isn't exactly a giant propeller on top of a body. It is a rotating wing. A quadrotor relies on conservation of momentum to stay aloft - push air under it to force it upwards. A wing however relies on bernoulli's principle where fast moving air over the top of a wing results in a lower pressure which "sucks" the craft upwards.

The maximum takeoff weight of a 747 is ~980000lbs however its engines only generate ~67000lbs times 4 = 268000lbs How does it fly?

By this ratio, a robot weighing 120lbs.. if shaped like a 747, would only need 33lbs of thrust to fly which is quite possible!

Even though it is possible, to achieve it within the confines of FIRST rules makes it extremely difficult.