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

A couple of years ago at the Peachtree regional, one team added ballast to their robot using a container filled with bb's. We only discovered this when the container ruptured during a match and spilled this former ballast across the floor. Despite a serious vacuum session immediately and at every break thereafter, we were still finding bb's when we packed up the field.

I've been a regional and a championship ref and every year we pick up a small box full of parts dropped off robots during competition. Please do not underestimate this hazard that others have already described to you.

Your design has to take into account safety in conditions which are out of your control. This is why we have negative reactions to your insistence that some of these failure scenarios won't or can't happen. What might seem safe while working in your own controlled environment is possibly highly unsafe when surrounded by hundreds of people completely unaware of your design and dozens of robots that might not be as well built as yours.

I would not be surprised at all if a lead inspector deems your design as it stands too unsafe for competition. The head ref at each regional also has the prerogative to prevent an unsafe robot from competing and might not take an engineering inspired view. It might be more of an emotional view like the previous poster who would not drive a Smart car no matter what the safety engineers say.

Personally I hope to see your robot on the field, even if just to see the backwash clear the scoring table of all its paper.

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

Next year's game involves apples. What could it be? Let's all head to our local orchards to do research!:P

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

Dave's Mac is autographed by "Woz".

I don't think he's very shy about being an Apple supporter...

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

Don't ask Dave about apples, he'll start talking about penguins,

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

my team has one and it only creates two punds of thrust

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

I know others have said it, but I'm concerned about the safety here.

If those blades break, which could happen, they will be sent flying at extremely high speeds, with lots of energy.

I'm usually the guy telling people worried about safety to shut up, it's fine. But this is one case where it seriously scares me. You could send shrapnel at extremely high speeds into the audience, which could kill somebody if hit right. It's unlikely, but too high a risk for my liking.

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

With two equal weight robots in a pushing contest on this regolith surface, a pound or two of thrust will make a big difference. Done properly (a good safety cage), a propeller in addition to all-wheel drive makes a lot of sense. The robot that pushes another robot into a hostile corner can decide the game.

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

I know they will work, I'm just a little frightened by them on a robot =P

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

I totally agree with that.

I've seen similar sized objects with similar energy and similar speeds break up; it will take a serious, serious object to hold it in. A cage won't suffice, plates of metal or polycarb will be needed, and not thin stuff at that.

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

The propeller blades are pretty low mass. Put a curved Lexan duct around the fan and call it good. The nice thing is that you know the blades are going to fly straight out -- it's not like they're made of C4. Wrapping the perimeter with a 6-inch wide Lexan duct should do the trick. Even if the blade hits the Lexan and bounces back to penetrate the finger-grid, there certainly won't be enough energy left to kill someone standing around the field.

How often do RC plane propellers spontaneously explode? This is a really mature technology -- I would be surprised if they weren't overbuilt for safety. How about replacing those 2-blade props with 3- or 4-blade units? For the same power you can have shorter blades with much lower tip velocities. At least, that's how it works on real airplanes.

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

RC airplanes aren't hit by other things very often.

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

A properly mounted propeller on an FRC bot shouldn't ever get hit at all. If they are mounted out in the open like the prototype in the first item in this thread -- that's unsafe and negligent. I was assuming (I know, I know) that it would be properly protected from ball-strikes and other structural indignities.

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

The R/C airplanes don't get hit either, at all (other than maybe on a hard landing or a midair collision). It's not the props getting hit that we're worried about, it's the robot getting hit and jarring around.

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

I'm pretty sure that if the props can resist any vibrations that they and the motors generate, they should hold in a robot collision. I mean yeah, it will be a significant change in acceleration, but that's still no reason for the props to spontaneously explode.

Oh and update: We've decided to add power to the back 2 wheels powered by a CIM motor. So basically, that'll add some extra acceleration that we lack, and the props should handle well with the turning. ;D

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

Thats two completely different forms of impact. First is vibration. That usually slowly tears something up. The second is turbulence and blunt force. Basically your comparison is as reasonable as stating, 'This doesn't break when I shake it so it shouldn't when I take a baseball bat to it.' Not really a valid statement at all.

A personal request: Once you build your robot so it can drive, purposefully run it into a couple of walls at full speed. See what happens. If it doesn't do any damage, you might be ok in competition, but you still might have trouble.

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

Why is it that I shudder every time I hear "I'm pretty sure" when they are talking about engineering?

I'm not too concerned about the vibrations of the motors. I'm far more concerned with the shock of impact from another robot.

**Gets up on soapbox**
Why not be "sure" rather than "pretty sure" by running the bending calculations. I am "pretty sure" that your robot will see a better than 10G impact with another robot or the wall (this will be transmitted throughout the robot and anything cantilevered will feel the full force across its beam.). Calculate that force across the beam of your prop and use the weight of your prop divided by the number of blades. Personally I'd use 20G as a safety margin. If the material does not bend further than where it will permanantly deform then I'd call it fairly safe for the forces it will see ... then you need to protect it from outside forces like other robots and orbit balls.

Props can be very effective. Lets just make sure they're safe as well.
**Gets off soapbox**

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

It's called a propeller explosion because that what it looks like when contained.

A 12 inch prop rotating at 9000 RPM has a wing-tip velocity over 470 RPM. A 22 inch at that RPM is traveling at over 3/4 the speed of sound.

Propellers do not spontaneous explode when standing still, nor undamaged and moving within their design limits in the applications for which they were designed, tested, and proven. They explode when forces beyond their material strength limits essentially rip them apart. When that occurs two things happen; the failed piece (maybe an inch of tip) no longer has any centripetal force and begins moving in a straight line (usual outward and a bit forward) while aerodynamic forces still act on it, often causing it to tumble; At the same time, the remains are now horribly unbalanced and the remaining rotational energy goes into ripping off the other tip and maybe some other parts - hence the term explosion.

Back to the tip - it will usually bounce off something but since it will be at an angle its direction will change. This happens maybe 200 microseconds after tip separation. If the tip is wood it may splinter into dust. If its nylon it may elastically deform and retain a good deal of its energy after the bounce. Murphy then takes over and the path of the tip may cause it to pass through any protective screening if the opening are wide enough. Or it may bounce inside, hitting the rotating remains. You are told to stay out of the propeller arc because in the open air setting of an R/C field, that is where the prop is likely to go (except if the prop nut loosen off with a tractor prop and the prop travels forward until it hits the face looking at the front of the propeller (i've found props 100 feet away).

On a FIRST field, the whole chain of events could be triggered by the robot hitting the wall, inducing abnormal loads. The point is that if Murphy has his way the collision may be on the side of the field where people like referees, MC's, and cameramen are standing. Tragedy does not require that someone be killed. A nylon tip is plenty sharp, especially when its travelling at 200 fps.

Yes, R/C propellers are a mature technology for the conditions and best practices in which it has been used. That is not what we have here, especially as configurations stray from the conventional. Exactly how are you going to predict the vibrations and resonances of a pimped up belt drive for the prop?

When you move outside the envelope, the technology is no longer mature. And prop failures in R/C do occur surprisingly often where the envelope is being pushed.

I have no doubt that some thruster designs will be deemed insufficient to be operated within a reasonable margin of safety for the conditions present on the field (5 other 150 lb robots sliding under marginal control all over the field.) I'm hoping that some teams will have confronted and surmounted the challenges presented and be allowed to demonstrate their engineering on the field.

I do like this added dimension to the competition. But to paraphrase a movie quote "The lack of humility before nature that's being displayed here, uh ... staggers me."

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

I think the following comments on propeller safety from a manufacturer are worth the lengthy quote- From APC.

APC Propeller Safety Concerns

All propellers are inherently dangerous. Model airplane propellers are especially dangerous. Model airplane propellers used in high performance racing are extremely dangerous. Model airplane engines designed and modified to achieve maximum operating capabilities create unpredictable and potentially severe loads, leading to various forms of potential propeller failure. Ignoring reasonable safeguards may likely be catastrophic. This concern is the motivation for the following discussion.

Warnings included with propellers are intended to protect consumers. They also protect manufactures against claims resulting from misuse of the product. Most products with potential for causing injury contain ample warnings about misuse. Some advertisements for products now contain warnings, even before the product is sold! There is a strong proliferation of warnings in most products having potential for creating injury or damage. This inundation of warnings may cause consumers to become inured to product warnings.

The warnings about propeller use must be taken seriously, especially for racing applications. It is very risky to assume that a racing propeller blade will not fail, especially when used with state-of-the-art racing engines. Yet, nevertheless, occasionally model aircraft operators are observed standing in the plane of propeller rotation of high performance racing engines running at full power. This is very frightening. The following information reinforces the assertion that dangers of misuse are very real.

Ideally, a product can be designed with credible knowledge of the environment (loads acting on the product) and capabilities of the product to withstand that environment (not fail). There is nothing ideal about designing a model airplane propeller because some major components of propeller loads are very uncertain. The principle load components acting on a propeller are:

Centrifugal (from circular motion causing radial load)
Thrust/drag (from lift and drag acting on blade sections)
Torsional acceleration ( from engine combustion and/or pre-ignition)
Vibration (from resonant frequencies or forced excitation)
Another potential source of loading is aero elastic tip flutter. This may be caused by self exciting aerodynamic loads at a resonant frequency.

These loads are discussed next in order.

Centrifugal loads are very predictable, given rotational speed and mass density distribution of a blade. Their contribution to total stress is relatively small.

Thrust/drag loads are somewhat uncertain due to complexities of aerodynamic environments. The relative axial speed at the prop (at any radial station) is aircraft speed plus the amount the air in front of the blade is accelerated by the mechanics creating thrust. The latter may be approximated using first order classical theory. Much empirical lift/drag data (from wind tunnel tests) exists to quantify lift/drag loads, once relative velocity and angle of attack distributions are established.

Torsional acceleration loads are generally not known. Analytical estimating technique used by Landing Products to quantify torsional acceleration loads suggests that they can become dominant when pre-ignition or detonation occurs. These analytical observations are supported by test experience with very high performance engines running at elevated temperatures. The latter causes a high torsional load (about the engine shaft) which creates high bending stresses, adding to those from centrifugal force and lift/drag effects. These torsional acceleration loads depend on unique conditions for specific engines. Engines "hopped up" for racing appear to be especially prone to create high torsional loads when lean mixtures lead to high cylinder temperatures and pre-ignition/detonation.

Vibration causes additional loads from cyclic motions. These motions occur when resonant frequencies are excited or when cyclic load variations exist on the blade. The magnitude of these variations depends on how close the driving frequency is to the resonant frequency and the level of damping in the propeller material. Engine combustion frequency is an obvious excitation. Obstructions in front of or behind the blade can cause cyclic variations in thrust load. Once a blade starts to flutter, those motions alter the flow, causing variations in loading. High performance engines have caused propeller tips to break, presumably due to fatigue failure from vibration.

Aero-elastic flutter is speculated to be a dominant mechanism causing rapid fatigue failure near a tip when insufficient or destabilizing tip stiffness exists. The interaction between variable loading and deflection induces a high frequency vibration with unpredictable magnitude.

Efficient propeller design practice utilizes analytical/computational models to predict propeller performance and stresses. However, the uncertainty in impressed and inertial loading from complex phenomena requires testing to assure safe performance. Unfortunately, it is not possible to assure testing that convincingly replicates worst case conditions. The large combinations of engines, fuels, temperature, humidity, propeller selection, aircraft performance and pilot practices creates an endless variety of conditions. If the origins of severe loads were well understood, quantified, and measurable, structured testing might be feasible that focuses on worst case stack up of adverse conditions. However, since the origins of severe loads are really not well understood, it is essential to provide sufficient margins in material properties and design to assure safe performance. Propellers that are used in fairly routine and widespread applications (sport and pattern) lend themselves reasonably well to test procedures that provide reasonable confidence. In time, a sufficient data base develops that can be used to empirically quantify performance and "anchor" or "tune" assumptions used in analytical models.

However, propellers that are used for increasingly extreme performance applications do not benefit from the large empirical data base sport and pattern propellers enjoy. Assumptions and design practices developed for current generations of engines may not be valid for emerging engines whose technologies continue to push engine performance to greater extremes. Consequently, propellers that are used in applications where performance is already relatively high (and expanding) must be used with great caution.

An adverse cascading effect occurs when propellers are permitted to absorb moisture in high humidity environments. Composite strength, stiffness and fatigue endurance all reduce with increased moisture content. Reduction in stiffness typically causes resonant frequencies to move toward the driving frequency (increasing torsional loads) and, the reduction in strength reduces fatigue endurance. Composite propellers should be kept dry.

In summary, please abide by the safety practices recommended by propeller manufactures. This is especially important for high performance propellers. Assume that propellers can fail at any time, especially during full power adjustments on the ground. Never stand in or expose others to the plane of the propeller arc.

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

In FIRST, safety has always been #1, and its imperitive that teams take it seriously. As several others have mentioned, and linked to in this thread, accidents DO happen, and CAN cause serious injury. For the 2008 season, 1075 built a robotic forklift cart, driven by an IFI system from 2004. When we built it, we knew for sure that the safety advisors would be all over us about it, since driving a vehicle through the crowded pits has the potential to be quite dangerous, never mind that the vehicle itself has its own concerns. It was built with a very long list of safety features designed to prevent accidental operation, as well as to reduce possibility of serious injury as a result of its use. It was also built to reduce RSI's in our own team members, associated with lifting the robot, and working on it at the wrong height.

Its features are as follows:

on the old white joysticks:
trigger: enables the lift to be operated with aux1 and aux2
trigger + thumb button: enables the X and Y axes of the joystick to drive the cart

other lockouts:

seat is spring loaded, with a switch to prevent operation of the cart without a driver IN the drivers seat

front bumper on the end of the deck has a switch in it as well to stop the cart from continuing to drive into something (people, walls, whatever)

the lift has holes crossdrilled in the legs for safety pins, to ensure the deck doesnt fall.

the motor driving the deck up and down has a shroud over the chain, to protect any stray fingers, etc

it has a rotating light off our 2003 robot, to warn the area of its presence and ability to move unexpectedly.

for 2009 I believe we are planning to add an automotive horn, for an audible warning.

AND, its ALWAYS operated with a crew of flagpeople to clear the path and raise awareness to its presence.

As for benefits it provides to us, it was designed to be able to reach over the edge of the field, however, none of the events we've taken it to so far have allowed us to do so. This was to reduce back strain associated with lifting it on and off the field.

It allows us to raise up the robot to a proper height to work on in the pits without back strain and similar issues.

It also has a number of features to aid in testing and debugging the robot at competition.

This is the kind of attention to detail required in FIRST when it comes to safety. I hate to use my own team as an example, because it sounds like i'm bragging or gloating, but when there's a high risk for danger (ie. props breaking up at thousands of rpm, with the potential for sending shrapnel flying at high velocity), safety should be paramount.

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

Not just breaking and having shrapnel in the field, but if something flies into the stands it's going to be trouble.

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

Just a reminder of how important safety is:
http://chiefdelphi.com/forums/showth...540#post815540

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

To any teams that will be using propellers on their robot, be prepared for serious scrutiny from your robot inspectors. I'm still developing the inspector training materials for this particular design element and I'm leaning towards placing a lot of the burden of safety verification on the teams.

Be prepared to defend your design. I would strongly encourage you to bring a "standard finger" to demonstrate general protection.

I would strongly encourage you to calculate the amount of energy stored in your propeller(s) at max speed and determine whether that energy is sufficient to punch through your enclosure. For example, how much energy is required to "rip" a steel wire of diameter X? Maybe you can find such destructive limits on online materials databases.

Bring plenty of documentation to support your design.

Good Luck and BE SAFE!
Russ Beavis
Chief Inspector

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

Russ,

Thank you very much for addressing the safety issue of propellers here on CD as a warning. I have two questions though:

1) Will the "inspector training materials for this particular design element" be officially released on the FIRST website so that teams will know exactly what they need to be prepared for?

2) Will the "standard finger" be defined? Something like a 3/4"D x 3"L wooden dowel or something? If left to the teams to define I would expect as many different definitions of a "standard finger" as there will be propeller designs. Actually it seems like this should be something that the inspectors provide at inspection rather than the teams if it is to become a "standard" similar to the sizing box or scale.

I ask because the safety considerations for propellers should be taken seriously but the only defined requirements that teams have access to at the current time is the Inspection Checklist. Maybe a section could be added to the checklist covering minimum propeller safety requirements?

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

Chuck,
The standard finger that Russ speaks of is a test fixture used by UL to test fan guards. As I understand it, the device takes into account the distance from the guard to the blade and the size of the openings in the guard as it would pertain to a finger intrusion. As this is a new concept in robot design for First, and has potential safety issues, I am sure that several different disciplines are involved in setting a standard for inspections.
Finger intrusion is only one of those concerns. As have been discussed above, noise, flying debris, robot damge, etc. are all concerns as well.

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

Dude. Those props are still awesome.

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

I look forward to seeing all documentation and substantiation that will support the safety guidelines that Russ will be issuing to the inspectors.

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

For those who are curious about how serious fan blade containment is the aerospace industry, check out this certification test performed by Rolls-Royce.

Think of all of the analysis that went into the design of that engine to ensure survival from such a catastrophic failure. The FAA and other aerospace agencies require proof that the design is robust, hence the analysis is backed by this very dramatic (and expensive!) test.

I'd be interested in seeing what came out of the the exhaust of that engine. I bet it wasn't pretty...

BTW, I didn't post this to discourage propeller advocates for robots. I just want them to see what kinds of challenges engineers sometimes encounter.

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

You're right. Years of effort goes into these fan designs in the hope that you only have to run this test once.

Some of the coolest testing we do is this test and the bird strike test, which basically involves shooting ducks at an engine running at full power. As was recently proven in NY, it's a good thing we do it. A friend of mine in the test department designed the equipment to do our most recent bird strike test.

As an MC and announcer...I hope the safety restrictions are very tight on prop-driven robots. Otherwise, I'll be announcing from outside.

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

As a scorekeeper about three feet from the edge of the arena, who has the added disadvantage of sitting down most of the time and is therefore less mobile, I couldn't agree more.

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

I agree that that needs some serious protection. Don't underestimate the energy in large diameter spinning objects.

Perhaps you could make a ducted design that turns with the blades. Then you would get more performance, while reducing the chance of pieces coming out of the fan space.

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

Check out the protection requirements in Team Update 11.

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

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.

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

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.

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

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.

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

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.

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


.

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

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

Sorry, I didn't see that thread, I'm actually a FTC team member so I haven't looked the FRC Q+A

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

Except the ground...

The (plastic) prop on my Electric Firebird XL stood up to everything. Flew into trees, fences, all that, handled it just fine... But, your hand grazing a spinning prop HURTS. Almost cut a few fingers off once...

I like the idea, but safety first man. Better safe than sorry. The lexan sheet wrapped around should be fine, me thinks. The Blades aren't super heavy and shouldn't penetrate too far. Plus with the lexan you can add the little lip-thing they do on automotive ITB's/Carbs to help clean up the airflow coming in. And it looks fairly cool.