For those who are skeptical about propellers - Team 2526

[Molten]

Originally Posted by GaryVoshol

Check out the protection requirements in Team Update 11.

Only problem is that the update 11 doesn't have any specific requirements. It gives an example of a safe setup. It does not state any need for this particular setup. I personally won't feel that this is adequately covered until they specify max blade size, max rpm, and mandatory specific protective equipment. Any less and it will be too ambiguous.

[EricH]

Originally Posted by Molten

Only problem is that the update 11 doesn't have any specific requirements. It gives an example of a safe setup. It does not state any need for this particular setup. I personally won't feel that this is adequately covered until they specify max blade size, max rpm, and mandatory specific protective equipment. Any less and it will be too ambiguous.

They "expect that propellers...will be protected in a comparable way." That's not exactly just an example...

I'm not sure that they want to specify those items. They've already spec'd the bumpers to death, so it's either "Do we REALLY want to go farther?" or "Why stop there?". Judging by team reactions to the bumpers, I'd guess the former. I could be wrong.

If they do specify one thing, I would say they should do specific minimum protective equipment, possibly with relation to the size of the prop and the speed.

[Molten]

Originally Posted by EricH

They "expect that propellers...will be protected in a comparable way." That's not exactly just an example...

When it says comparable. It says that it is in respect to the danger of the blades. In reality, it really isn't saying much.

Also, I know it is the blade size that makes it dangerous. I also know it isn't the rpm that makes it dangerous. Lets compare to electricity. The current alone won't kill you and voltage alone won't kill you. It has to be together. However, there are standards for both. If nothing else, they could set up a simple formula that would allow for various rpm/diameter possibilities while still restricting the real danger.

[EricH]

Originally Posted by Molten

When it says comparable. It says that it is in respect to the danger of the blades. In reality, it really isn't saying much.

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.

[Molten]

Originally Posted by EricH

I have to ask: How do you draw that conclusion? Sorry, but I just don't see it.

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

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

[Daniel_LaFleur]

Originally Posted by Molten

When it says comparable. It says that it is in respect to the danger of the blades. In reality, it really isn't saying much.

Also, I know it is the blade size that makes it dangerous. I also know it isn't the rpm that makes it dangerous. Lets compare to electricity. The current alone won't kill you and voltage alone won't kill you. It has to be together. However, there are standards for both. If nothing else, they could set up a simple formula that would allow for various rpm/diameter possibilities while still restricting the real danger.

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.

[Al Skierkiewicz]

Originally Posted by Molten

Lets compare to electricity. The current alone won't kill you and voltage alone won't kill you.

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.

[Al Skierkiewicz]

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.

[JesseK]

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 )

[Koosley]

Originally Posted by JesseK

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 )

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

[EricH]

Originally Posted by Koosley

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'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.

[Koosley]

Originally Posted by EricH

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.

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.

[dlavery]

Originally Posted by Al Skierkiewicz

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.

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


.

[Al Skierkiewicz]

Originally Posted by dlavery

Al -
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


.

Agreed!

[Daniel_LaFleur]

Originally Posted by Molten

Only problem is that the update 11 doesn't have any specific requirements. It gives an example of a safe setup. It does not state any need for this particular setup. I personally won't feel that this is adequately covered until they specify max blade size, max rpm, and mandatory specific protective equipment. Any less and it will be too ambiguous.

First off, Max blade size has little to do with the safety of a prop. A 32" prop can be made safe while a 10" prop could be very unsafe.

Max RPM is a function of prop size and construction. some props are perfectly happy at 20,000 RPM whil others are dangerous at 5,000 RPM. To set an arbitrary 'MAX RPM' may actually encourage poor design and unsafe robots.

As far as mandatory specific protective equipment goes, what may hold a 32" 5,000 RPM prop may not hold a 11" 20,000 RPM prop. Instead of blanket protective equipment (which may or may not work in all cases), require the teams to prove that the setup is safe with proper engineering numbers. Show that the cage will completely contain a catastrophic failure of the prop.