Aluminium Strength: Building Hovercraft Blades for Safety

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    Aluminium Strength
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Discussion Overview

The discussion centers around the design and safety considerations for building hovercraft blades, specifically focusing on the thickness of an aluminum shroud to contain potential blade failures. Participants explore various engineering approaches, safety measures, and empirical data related to hovercraft design and blade containment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Colin De Bruyn inquires about the appropriate thickness for an aluminum shroud to prevent injury from a spinning blade weighing 200 grams at 3000 rpm.
  • One participant suggests adapting existing designs from commercial fans, implying that empirical examples could guide the shroud's design.
  • Another participant argues that precision calculations may not be necessary and recommends a robust, industrial approach to the shroud's construction.
  • A different viewpoint emphasizes the complexity of impact calculations and suggests conducting tests to determine the necessary energy absorption capabilities of the material.
  • Concerns are raised about the fan's performance and whether the specified 3000 rpm is achievable with the current setup, indicating that more power may be required.
  • Colin provides additional context about the fan's specifications and the speeds at which the blades would operate, seeking to understand the energy involved in a potential failure.
  • One participant recommends ensuring that the protective measures are at least as strong as existing hovercraft designs.
  • A suggestion is made to reference a specific engineering paper that discusses blade containment design and provides a formula for calculating the energy a containment shell can withstand.
  • The importance of incorporating a significant safety factor for man-rated vehicles like hovercraft is highlighted.

Areas of Agreement / Disagreement

Participants express a range of views on the necessity and methodology of calculating the shroud's thickness, with some advocating for empirical adaptation and others emphasizing the need for rigorous testing and calculations. No consensus is reached on the best approach to ensure safety in the design.

Contextual Notes

Participants note the complexity of impact dynamics and the need for empirical data, indicating that assumptions about material properties and design specifications may vary. The discussion reflects a range of engineering perspectives without resolving the technical challenges presented.

Colin De Bruy
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Hi Guys
This is my first post. I hope someone can help. I'm building a hovercraft, the fan that lifts the hovercraft is made of pine timber and the blades are carved from the pine. The blades weigh 200grams each and are spinning at 3000rpm. Question, if a blade broke of wilst spinning at 3000rpm, how thick should I make a duct or shrould (from alumimium) to keep the blade from breaking though and hurting anyone? Many thanks Colin De Bruyn
 
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i do not have an empiriacl answer..but in my fuzzy way of think
i always look at things that have already been developed that i can adapt
i would look at a floor fan that has a similar weight fan blade.chances are it is made of plastic.
i would replicate the fan shroud..better yet..use a shroud from one of these commercally available fans.
this is commonly called hill billy engineering in the colonies!
 
Yeah this isn't the kind of thing that's worth calculating to be honest. Figuring out show it likely to strike the shroud and all that is all a bit redundant.

This doesn't need to be precision engineering, just go industrial on it and make it uber chunky so that nothing would ever get through.
 
Coming from someone who has had to contain "egressing" blades, I can tell you that this is not a trivial calculation. You can take a basic approach and look at it in terms of energy, but even then, you need some tribal knowledge on energy absorption capabilities of the material, which most people don't have. Your best bet is to try to rig up some tests to actually measure what you need.
 
I agree with Ranger Mike - you won't be able to construct yourself a very good fan with that method, so you are better off spending a small amount of money buying one.

Also, where did the 3,000 rpm come from? Have you calculated the necessary fan performance based on the weight of the vehicle and its performance requirements? You may find that you need a whole lot more fan energy than you realize.
 
Thanks to all who posted. The fan was built by Universal Hovercraft it's purpose built for a small hovercraft designed for kids to ride on. The fan is driven by a 5hp petrol motor which has a top rpm of 3000 rpm. I doubt the engine will get to this rpm with the fan attached but at 3000rpm the tip of the fan is doing 339km per hour or 210 mph and the inner root of the blade is doing 169km or 105mph. So does anyone know how much energy a 200gram pine blade has at this speed? Many thanks Colin
 
Impact problems are fairly complicated. I'd just make the protection at least as strong as the original hovercraft or other existing hovercrafts.
 
If you can find the paper, check out "Missing Firing Tests at Stationary Targets in Support of Blade Containment Design", by J.I. Goatham and R.M. Steward in the Journal of Engineering of Power. April 1976, vol. 159. It's something that I've used to calculate containment properties. The paper describes what is referred to as "Just-Constrained Energy", which is the theoretical energy that the shell can withstand before failure. It is defined as (knowing apparently that laxtex is down):
[tex]E_c = 16 A t S_{ut} e[/tex]
Where A is the area of impact (going to be hard to quantify); t is the thickness of the shell; [tex]S_{ut}[/tex] is the ultimate strength of the material, and e is the elongation at failure.

This can get you in the area, but if I were you and really wanted to design this from scratch, know that for a man-rated vehicle such as a hovercraft, you want to put a HUGE factor of safety on it.

You can also look at containment rings for gas turbine engines.
 

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