# Correct me if I'm wrong

I’ve been thinking about the robots that are using fans/props for propulsion.
Aren’t they inadvertently supplying traction to the front wheels? The force of the the fan/prop pivots on the rear wheels and applies a force to the front wheels. Yes or no?

Reference this Q&A
Q) Can we increase the normal force with a fan or other component, so long as it does not harm the ARENA?

A) No. Any vacuum/suction/fan system that alters the traction characteristics of the ROBOT would be considered a violation of Rule <R06>.

As long as the force vector from the fan is parallel to the ground, or has an upward/lifting component, then it’s not increasing the normal force. This is from a simple sum of forces in the vertical direction, and the assumption that the robot center of mass isn’t accelerating vertically. F=ma, and all that, so if a = 0, then F=0. If the fan isn’t contributing any force in the vertical direction, then there’s no NET increase in the normal force.

Now, it’s definitely the case that the fan is shifting the balance of the normal forces on the wheels. The force it’s applying isn’t going to be entirely balanced by other horizontal forces, so the sum of the moments on the robot isn’t going to balance out without the front wheels applying more normal force than the back wheels. But this doesn’t increase the total normal force, it just changes where it’s being divvied up.

You’ll have the same shifting of normal forces from your robot simply accelerating forwards using its wheels. This is why the front end of cars pop up and the rear end sinks when you floor the gas from a stop. But the car isn’t getting pushed into the ground any more or less than it was before, unless it actually lifts the front wheels off the ground.

This distinction confused me as well.

Lets say that you have a fan that can create 100 N thrust. (I’m pulling a number because it makes the math easier.

If you choose to use it to add horizontal thrust, you get the full 100N benefit.

If you choose to use it to add vertical thrust, you get 4 or 5N benefit, because it has to be transferred through the wheels.

The only advantage of a fan that pushes you into the floor (that I can see) is that the force can be applied in any horizontal direction that you need (if you have crab drive OR you are being pushed.

A horizontal fan on a gimbal knocks the socks off of a vertical fan. Can anyone give me a good reason to use a vertical fan instead of a horizontal + gimbal?

Therefore, I’m unsure why the lesser solution is illegal while the better solution is legal.

I’ve made a few gross assumptions here, but I’m very interested in hearing other thoughts.

EDIT: I forgot about suction devices. This is a special case, and I understand its illegality. My above thoughts only apply to fans pushing, rather than ground force suction fans.

I assume it’s simpler to outlaw all downforce increasing systems than just one specific one, thus making them all illegal. Of course, it creates its own set of problems since there are a large number of transient situations where downforce is increased temporarily, like landing after popping a wheelie, etc. Most of these are usually offset by decreased downforce at other times, of course, but that doesn’t seem to be taken into consideration in any of the rulings.

There was a robot @ DC that utilized fan motion with drive wheels, I guess FIRST didn’t remember the ruling.

It depends. If the fan applies force directly through the center of mass of the robot (vertically speaking), then no torque would be applied to the robot and no traction characteristics would impact the wheels. If the force is applied beneath the center or gravity, it would create a torque that would apply additional force to the rear wheels and remove force from the front. If applied above the CG, the opposite would occur.

Ultimately, it’s not different that acceleration of your robot via a standard drive (which, since the force is applied beneath your CG creates a torque favoring your rear wheels). The additional force on one set of wheels should be identical to the force removed from the other set, so the normal force of the robot as a whole won’t change (just of individual wheels).

Just make sure I have my camera ready when you inspect the Kats bot on Thursday at Boiler and make that ruling. I want to get a good shot of Schnabel’s face.

There were multiple robots at DC with fans, including ours. Which are you referring to?

GDC

*
…Regarding the third part of your question - there are no rules prohibiting the use of forced air for propulsion or thrust, provided it is not a safety hazard and does not cause damage to the field.*

I’ve only seen two robots with fans and both were pointed horizontal, so they add thrust but not traction. If I remember right, one of the updates had a clause outlawing “mass manipulation” to gain a traction advantage. I guess it is up to the inspector to judge whether a fan is mass manipulation.

On a separate note, why the heck to they have this rule in the first place? The possibility of increasing traction got our entire team talking physics forced everyone to come up with creative designs. Why limit creativity??

There’s more than 2.

The mass manipulation was in the Q&A; a team asked if they could, say, move the orbit balls for the sole purpose of traction. Fans are NOT mass manipulation; they simply move air. As they are explicitly allowed by the Q&A (given a horizontal direction so as not to violate <R06>), then any team can use them.

Why the rule? Why which rule? <R06>? It’s part of the game challenge. However, you can’t add traction by altering the reaction with the ground under <R06>. There only remains the chance of adding a reaction with air to increase speed, acceleration, and/or pushing power. Fans provide that, and are legal.

The point he was trying to raise is that if the fan isn’t transmitting force directly through the center of mass of the robot then it will create a torque (rotational force) on the robot as a whole. This torque will apply additional force to one set of wheels (the front if the fan is above the center of mass) and lessen the force on the other set. He was asking if this change in force distribution would be considered to be changing the traction properties of the robot.

Think of a car. When it accelerates the weight of the car shifts towards the rear wheels. When it brakes it shifts towards the front. Same reason motorcyclists do wheelies while they’re accelerating and decelerate to bring the front wheel back down.

Now, I don’t think this violates the rule, because the total normal force does not change. It is just shifted. Beyond that, if this violates that rule, so do any drivetrains where they don’t apply force through the directly through the center of mass, which is likely every single team in FIRST

.

The real problem is that FIRST has been equating normal force or frictional force with traction. They’re not the same, but are obviously related: traction is a motive force derived from friction between surfaces and normal forces on the scale of interlocking surface features, while the overall normal force and frictional force are related to any downward components of force (e.g. from weight, downward thrust, momentum over time, etc.).

This has led to Q&A posts that do not frame the response in terms of the stated objectives and context of <R06>—to restrict devices other than rover wheels that would increase traction by interacting with the arena.

Only the craziest interpretation of <R06> leads to the conclusion that downward-thrusting fans that interact with the air are traction devices. It’s crazy, because if you’re going that route, the rules don’t give any leeway to ignore the traction-increasing effects of weight and momentum. As far as the rover wheels are concerned, all downward force increases traction, no matter whether it is a result of gravity, or a fan pushing air upward.

And of course, because this information was released in Q&As, a team might quite reasonably object on the grounds that it is a non-binding statement, rather than an enforceable rule.

Note, also, that the statements about vacuum cups and similar devices stem from <G29> and <G30> issues (i.e. field damage). Even then, there is no prohibition; just a stern warning and the threat of being restricted from using the robot.