Quote:
Originally Posted by Rick TYler
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.
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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."