Re: For those who are skeptical about propellers - Team 2526
 

I have to ask: How do you draw that conclusion? Sorry, but I just don't see it. They just say, this is one example of a safe design, and we expect comparable ones. They don't say what it's in respect to, just that it has to be comparable. The design given is for a general propeller, that's it.

Re: For those who are skeptical about propellers - Team 2526
 

Prop size isn't the issue, nor is RPM. It's people not understanding the forces that they are attempting to control.

Unfortunately, there is no simple formula that makes a prop of a specific size safe at a specific RPM. There are many factors that go into max prop RPM. While diameter is one of the factors there are others such as composition of the prop (wood? Nylon? etc ...) as well as pitch, and design (Master airscrew, APC, etc). What is safe for one type is dangerous for another.

All props should have a specification sheet that will tell Max RPM for that prop. I would expect teams to have a safety margin built in so that the prop cannot ever exceed that speed (In fact I would stay well below the manufacturers recomended max RPM).

Again, teams will need to prove that their design is safe and that the protection they have put in will work in the event of a catastrophic propeller failure. Anything less should not be allowed on the field.

Re: For those who are skeptical about propellers - Team 2526
 

Yeah, I don't either now. I drew the conclusion through misreading. Sorry.

Note to zrop: Good luck meeting all the updates' standards.

Re: For those who are skeptical about propellers - Team 2526
 

To be picky on this item, it's the current. That is why GFI devices are calibrated at current only. Voltage causes current to flow so that is why it is often labeled "Dangerous". A person with highly conductive skin and tissue will cause a greater current to flow than a person with poor conductivity for the same applied voltage.

Re: For those who are skeptical about propellers - Team 2526
 

As to Team Update 11, there is three significant specifications there. The prop must be protected by a 1/4" mesh. The shroud needs to be at least 16 guage aluminum and it's length should be 1/2 of it's diameter. Those are at least easily determined by your inspector.

Re: For those who are skeptical about propellers - Team 2526
 

Zrop & others, any updates? Do you guys have any automation built-in, i.e. automated yaw functions to keep you going straight, etc?

I can see that you guys are putting much effort into the design of the props, and are experimenting to get the maximum bang out of the props. This is the process that FIRST loves to see. I remember seeing a team from Florida doing a prop design -- anyone remember who they are? Assuming good safety practices, I definitely look forward to seeing this in person! (Even if bumper impact forces wreak havoc on a blade who's tip is spinning over 200mph :ahh: :cool:)

Re: For those who are skeptical about propellers - Team 2526
 

Al -
Not to put too fine of a point on it, but Team Update #11 provides an example of "what will be considered safe," not specific regulations on what must be done. The example shows one way to cover the three things that I would consider important:

- protection from ejected debris in the event of prop failure
- protection from ejected debris in the event of FOD ingestion
- protection from insertion of body parts (fingers, toes, noses, etc)

The update leaves enough room for teams to come up with alternate designs for their protection methods. As long as that alternate method covered the three items listed above with equivalent or better levels of protection, then it would appear there is still some design flexibility here.

-dave


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Re: For those who are skeptical about propellers - Team 2526
 

Agreed!

Re: For those who are skeptical about propellers - Team 2526
 

Currently we are working on replacing our props with a similar 3-prop version of the one shown. The good news is that mostly everything is mounted and working to some extent (just needs a little tweeking). The bad news is that something happened with our computers causing a lot of our code to get erased or damaged (i don't know exactly what happened). I was actually talking with some of the people are Superior Tools who are machining some stuff for us and one of the guy is a model plane enthusiast, and i started talking with him. He actually recommended a 3-prop blade for low speed high torque. He also mentioned that those props we were using are so strong that its actually illegal to make props bigger than 16 inches (somewhere around here) out of that material, and he was saying how the only thing that would break them would be foreign objects coming in direct contact. He said that sudden changes in force due to collisions would have no effect on them or cause them to break and same with vibrations. He also mentioned that little objects like nuts and bolts and washers probably won't have to much effect on the prop its self, but its best not to find out (it may just turn that nut into a bullet).

As for the safety part, we are planning on having 2 cages, the first one as shown on the first page, will be used to keep large objects (mostly orbit balls) away from the props, then inside of that cage will be the aluminum housing and mesh.

I am not sure if this has been said yet, but in addition to being prop driven, we have the option of a wheel drive. We are using a modified bike gear where it will freely spin if the speed surpases the speed its being driven at, we hope this combined with the props will allow for faster acceleration (what we were lacking only prop powered).

Well its time to pull the all nighters to get this thing working, tomarrow i think we will be ready for the triblade + driven axle test to see what kinda performance we get out of it :).

Best of luck to all teams in this final week

Re: For those who are skeptical about propellers - Team 2526
 

I've seen props made of carbon fiber break. That's pretty strong stuff, right? Especially if it's done right, and the company that makes them does it right. And they still break. There is, for safety consideration at any rate, no such thing as an unbreakable prop. Same goes for wood. Those plastic ones that you show? I've seen plastic ones break too.

Also, if you have the aluminum housing and mesh, you probably won't need the outer cage. It's a very good idea, though, as it adds a second line of defense against various things getting into the prop.

Re: For those who are skeptical about propellers - Team 2526
 

I know they are likely to break, but it just makes me feel better that the chances they will break are very low compared to what i first thought.

My biggest concern about the aluminum housing for the props is that if a orbit ball hits it at a high speed (don't know how fast the bots can throw them) it will potentially warp or displace the housing, and if we have it within a 1/4 inch from the props we could potentially hit the housing with the prop. Using the outer cage will prevent and bigger object (and more likely to cause damage) from getting to our props.

Re: For those who are skeptical about propellers - Team 2526
 

Be aware, one of our mentors this build season was helping with the chains when the driver asked for "Is everything clear?" and a guy said out loud "CLEAR!" He was clear himself but not the mentors. Of course, guy behind the joystick was smart enough not to move the chains, but had the driver guy just looked down to check the voltage and move the chains at the same time, who knows if we'll still be in build season.

The guy who yelled our clear is mentally impaired and is in the special ed program. We can't quite "exclude" people so he's part of us, but it's hard dealing with having a safe season and excluding people who can not be safe. We have those "liability" forms for the school but they won't save anyone's fingers or hands.

Re: For those who are skeptical about propellers - Team 2526
 

Oh what I love even more is how "No!" sounds very close to "Go!". We've already experienced a problem with that.

I think the formal testing terms are now "Red light", "green light". ;D

Re: For those who are skeptical about propellers - Team 2526
 

I don't mean to derail, but I didn't think this was worthy of its own topic: Has anyone done the math on a verticle propellor pushing the robot down harder?

Re: For those who are skeptical about propellers - Team 2526
 

Yes. The GDC has. Their math says that you have to divide by zero, then integrate with respect to X, then again with respect to Y...

Or, to put it more simply, you can't. I've brought this up here before, and in another thread that went on a similar tangent: http://forums.usfirst.org/showthread.php?t=11025