UpForce

[Guyute]

what do you think the up force would be needed to lift a 130lb robot in the air 12' using a 4.5' propeller. i have an idea that it might be 130lbs of force, but wouldn't the size of propellers matter. if there are any AE's out there could you give me a bit more info...

[Frank(Aflak)]

Originally Posted by Guyute

what do you think the up force would be needed to lift a 130lb robot in the air 12' using a 4.5' propeller. i have an idea that it might be 130lbs of force, but wouldn't the size of propellers matter. if there are any AE's out there could you give me a bit more info...

most engines out there aren't capable of lifting even themselves straight up. in order to build a flying robot, you would have to use ever shred of power to lift it, leaving none availble for things like, oh, drivetrains, ball grabbers, etc.

I think the idea of watching a robot fly up and grab the bar is cool, but I doubt that it is possible with the motors and power supplies we are allowed to use.

If you were going to try that, I would suggest using a larger propeller diameter.

[CrazyCarl461]

John, don't give away our robot design!

[Brant Bowen]

I'm not very good at physics, but the force would have to be greater than 130 lbs. i think, maybe something like 130 times 9.8 to counteract gravity???

Anyways I still don't think you can get a prop spinning fast enough to take off. Also take into consideration how much it would take to control where that thing goes.

Sound like a safety hazard to me.

[Frank(Aflak)]

Originally Posted by Brant Bowen

I'm not very good at physics, but the force would have to be greater than 130 lbs. i think, maybe something like 130 times 9.8 to counteract gravity???

Anyways I still don't think you can get a prop spinning fast enough to take off. Also take into consideration how much it would take to control where that thing goes.

Sound like a safety hazard to me.

no, a 130 pound force acting directly up on your robot will cancel gravity completely. to actually go up instead of hovering, you need more.

But you aren't going to get 130lbs of force out of a propellor powered by the kit motors.


HEY!! What if you used a wind up propellor using all that elastic tubing . . . .

hmmmmmmmm.

Ach, I will never be able to sell that to my team, but hey, its an idea.

The bad part is this: if you miss the bar and your prop winds down 15 feet in the air, you no longer have a robot!

Heh, imagine it flying out of control into the crowd, with rapidly spinning blades of DEATH!!

[Mercutio]

D'oh! Frank(Aflak) posted what i was going to say as i was typing! That happens to me WAY too much...

Actually, if your prop wound down in midair the bot might not crash. You'll start sinking slowly at first as the force from the prop got below 130 lbf, and then you'd accelerate faster and faster. It all depends on how fast the prop winds down.

Now if your prop got jammed and stopped in midair...

[DaBruteForceGuy]

Who said the robot needed to be 130lbs??

[Frank(Aflak)]

Originally Posted by Mercutio

D'oh! Frank(Aflak) posted what i was going to say as i was typing! That happens to me WAY too much...

Actually, if your prop wound down in midair the bot might not crash. You'll start sinking slowly at first as the force from the prop got below 130 lbf, and then you'd accelerate faster and faster. It all depends on how fast the prop winds down.

Now if your prop got jammed and stopped in midair...

No, it would still lead to a vicious crash . . . you see, when the prop winds down from giving more than you weight to giving a force equal to your weight . . . your robot is still going upwards, so that yes, your prop will slow your descent . . . but you will still hit pretty hard. Hard enough to bend thin driveshafts, break bearing blocks, tweak frames, etc.

It would be cool to see.

and the robot doesn't need ot be 130 lbs, but when he says 'how to get an upforce of 130lbs" we are assuming he is talking about moving a 130lbs platform.

Everything we said still applies, just change the numbers.

[mtrawls]

In the classical system, as I understand it, a pound of force is the force gravity makes on an object whose mass is one pound of mass.

If you have to talk about force in the classical system, I suggest you use the unit of force called the slug.

Whoa! Physics is confusing enough without confusing the terms "mass" and "weight." Let's get things straight. The POUND is the unit of FORCE in the Imperial (British) system. Weight is defined to be W = mg, which is a force vector. Weight is then a force, and the POUND is also a unit of WEIGHT. The SLUG is the unit of MASS in the Imperial system. Here on Earth, where g=32ft/s/s, assuming that the robot is short enough that g does not change drastically (probably a safe assumption assuming the height limit!), then we can convert between weight and mass relatively easy (W=mg) -- but beware not to confuse the two in a physics equation, or in your understanding. If all that sounded a bit confusing (too many wierd names ... afterall, who's heard of a slug?) ... that's why we use the metric system (well, I guess not many more have heard of a Newton, but that's inconsequential).

[IronSlayTallica]

maybe if you use 26000 muffin fans.

Do you know the muffin fan...

[Frank(Aflak)]

Originally Posted by IronSlayTallica

maybe if you use 26000 muffin fans.

Do you know the muffin fan...

that would be less airworthy than our robot last year (which is saying lots).

Because a muffin fan cannot lifta a muffin fan. 26000 muffin fans cannot lift 26000 muffin fans.

[Guyute]

most engines out there aren't capable of lifting even themselves straight up. in order to build a flying robot, you would have to use ever shred of power to lift it, leaving none availble for things like, oh, drivetrains, ball grabbers, etc.

- Frank(Aflak)



who said that we were going to have a drive train or ball grabbers...our pure intention to this would be to control the bar 100% i'm not going to go into depth on how we will do that specifically, but its going to be done.

[Mercutio]

First of all, please ignore me if you already know this and correct me if I'm wrong... I'm just trying to be of use.

Originally Posted by Guyute

what do you think the up force would be needed to lift a 130lb robot in the air 12' using a 4.5' propeller

You need the same amount of force to lift your robot no matter what you lift it with. A 5-foot propellor, a 15-foot propellor, a rope, your arms... it doesn't make any difference.

Originally Posted by Brant Bowen

I'm not very good at physics, but the force would have to be greater than 130 lbs.

Yes!

Originally Posted by Brant Bowen

i think, maybe something like 130 times 9.8 to counteract gravity???

No.

Talking about mass and force in the classical system can be misleading, because the units for pounds of mass and pounds of force act really weird. Let's do it in metric first.

Imagine dropping your robot out of an airplane. Neglecting air resistance, it will accelerate at a rate of around -9.8 m/s/s (notice the acceleration is negative, because the robot is gaining downward speed!). If the mass of your robot is 60 kg (about 130 pounds), the total force on it will be

F = ma
F = 60 * -9.8
F = -588

-588 Newtons. The force is negative because it's pulling your robot down.

Now, add a propellor to your robot that provides 588 Newtons of up force (lift). The total (net) force is

F_total = F_gravity + F_propellor
F_total = -588 + 588 =
F_total = 0

zero! So is your robot flying upwards?

F = ma
a = F/m
a = 0/60
a = 0

No! The total acceleration is zero, so your robot's speed will not change. If it started out falling, it will keep falling. If it started out rising, it will keep rising. If it started out stationary, it will stay still.

So bring your robot back down to earth. If you put it on the ground and start the propellor, the net force on it will again be zero, as will its acceleration. Since it's stationary to start with, it won't rise off the ground. If you put it on a scale, however, the scale will read zero, because the force of the propellor is exactly balancing the weight of the robot.

So if you made the propellor put out even a tiny bit more lift, the robot would accelerate upwards!

In the classical system, as I understand it, a pound of force is the force gravity makes on an object whose mass is one pound of mass. So putting 130 pound of force on a 130-pound-mass robot would just make its acceleration zero -- it would not lift off. Any more than 130 pounds of force, and the robot *would* lift off! So (9.8 * 130) pounds of force like Brant suggests would definitely be enough, but less force would work as well, and (9.8 * 130) pounds of force would probably be overkill.

If you have to talk about force in the classical system, I suggest you use the unit of force called the slug.

Hope that was helpful!

~Aaron

[gc02]

Originally Posted by Guyute

what do you think the up force would be needed to lift a 130lb robot in the air 12' using a 4.5' propeller. i have an idea that it might be 130lbs of force, but wouldn't the size of propellers matter. if there are any AE's out there could you give me a bit more info...

There's a rule of thumb that says you need 1 hp to produce 2.5 lb of thrust, so you'd need at least 52 hp to lift 130 lb.

[Frank(Aflak)]

Originally Posted by gc02

There's a rule of thumb that says you need 1 hp to produce 2.5 lb of thrust, so you'd need at least 52 hp to lift 130 lb.

You might get that out of an extremely strecthed elastic tubing for a few seconds. Long enough to get to the bar.

And you might get added boost from the ground effect, also.