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What are the requirements for a racing hovercraft thrust system?

Monday, April 1st, 2013

Performance:

1. Produce loads of thrust, especially at lower speeds ie 0-40mph

2. Stable performance at high duct angles of attack- ie when the craft is travelling sideways, or even backwards!

3.The thrust system should be designed to cancel out most of the swirl with stators or similar: to cancel any torque acting on the craft itself

4.Good performance in dirty enviroments- ie water spray, mud and sand are all unavoidable in racing. No poncy laminar flow sections.

(If you think the ride looks pretty smooth from the second video you are wrong! Fast forward to 2:52 and you can see Marcus racing in spray and bumping around in mid-field)

Practicality:

1.Guarding: should not be able to accidently cut arms off on blades. Duct must contain fan blade failure in a crash. (See racing hovercraft regulations)

The baseline: multiwing/hascon 5Z

Sunday, March 31st, 2013

To compare the baseline design directly with the amazing new blade, you need to be able to compare it on level ground. Because I dont yet have my own rapid prototyping and testing facility I will be making a computer model of the multiwing blade and testing it against the new design in the same computer program. The thrust numbers you get out of the program can’t properly be compared to thrust coming out of the real hovercraft yet because it does not take into account things like blockage from the fan guard, driver and other real life things. In time i will write a bit about the computer model, DFDC to explain this better.

To make a model of the blade you need to find out aerodynamic characteristics at various sections along the blade, and also measure the chord and twist along the blade. To get the properties you need to cut the blade up to get sections:

To do this the chord distribution and twist of the blades is measured. the 5Z bladesand using a combination of 2D airfoil programs, xfoil/javafoil, we find the following information about it:

(specifically this is the Hascon blade but im told this is a direct rip-off of the multiwing aerodynamic shape)

The thick blunt trailing edge is neccessary for mass production reasons but gives a very high minimum drag coefficient. Ideally you want a thin knife-edge at the trailing edge, this is definitely an area that could be improved upon. To find out how hard the blade is working along its length, the geometry and aerodynamic details are put into DFDC for a typical formula 50 fan. 6 blades, at 2450 RPM with the pitch set to absorb 50Hp. From DFDC you can see that the blade is working fairly hard.

DFDC pic showing aerodynamic Cl along blade.

For all of the variations in number of blades and various duct diameters i have tried running the multiwing blade, it never appears to operate with constant Cl along the blade. Theoretically the most efficent blade has uniform lift coefficent along the whole blade span.

A great benefit of the DFDC software (apart from being free) is that you can very quickly change inputs such as blade pitch, # of blades, duct diameter etc in seconds (vs hours and $$$ needed to run CFD). The main focus for my blade design will be for the higher power categories towards 100-200HP fan systems, so I carried out a few runs with the model for a GSXR750 powered thrust fan (88 fanHp), and F1 809 powered craft (175 fanHp)

Table showing powers/ thrusts for higher power systems

Fan fun!

Sunday, March 31st, 2013

I have always thought there must be something better than the plastic air-conditioning fans we use to lift and propell our racing machines.  One aeronautical engineering degree later, and I am just about getting a decent enough understanding to think about doing it!

Its also taken a long time digging up decent information on ducted fans for propulsion (let alone getting it into my head), so i plan to share what im doing. So if you’re a fan design guru reading this and can see a better way please put me back on the right track! My main sources are shown below.

Before we try to start a new design from a clean sheet, we need to have a better idea of our requirements (MORE THRUST isn’t really helpfull…).

What are the requirements for a racing hovercraft thrustfan?

Next we need to find the baseline using the blade which most people are using- the Hascon/Multiwing 5Z blade. Whats wrong with it and how could it be improved?

The baseline design: Multiwing 5Z fan

Now its time to design a new blade in aerodynamics land, and find out if the possible improvements are big enough to go to the effort of actually making it, bearing in mind how you will actually make it, and if it can be made strong enough.

-Prototype fan design aero

Finally its time to think of the best way to make the shape of the blade and connect it to as many horsepowers as possible without it bananaing uncontrollably.

Structural design

Making the moulds

Also to race it you need to test it!

My original threshold to get to the last few steps was 10% improvement in thrust for the same engine power and fan diameter, however on paper 6% is looking more likely. Im gonna do it anyway, because I love to make things and I see this as an essential step in my goal to becoming the F1 world hovercraft champion!
Not to mention the hours and $$$ I have invested to date (£27k student loan…)

Sources:

R.A. Wallis: Practical fan design

Ducted fan design book by F. Marc de Piolenc & George E. Wright Jr.

Ducted fan design code by Marc Drela and Hal Y. as well as the ES2 version by Philip Carter

Theory of wing sections by Abbott and Doenhoff

Javafoil and Javaprop by Martin Hepperle

Multwing Optimiser fan selection program

-Many NACA and NASA papers

Thanks for help from:

Rupert Baker, Tom Brufato, Alister Simpson, Marc Piolenc, Bob Parks, Philip Carter, Keith Oakley and many others i have pinched ideas from