The new fan design (aero)


(For structural design click here)

The following is an article i wrote for the famous hovercraft club mag, ‘Light Hovercraft’:

Fans, what’s wrong with what we have and what performance gains could you get with the hoverclub’s own blade design?

The short answer is not much. The best improvement in thrust for a given power I’ve found so far is around 7%. These £15 lumps of plastic are actually pretty perfect for our needs!

I set myself a threshold of 10% improvement to get me in the workshop making the moulds, but since that’s not looking likely I will share my design with anyone interested in making them. This is still a work in progress and there is (or will be) much more information on my website: www.camracing.co.uk. (…one day i will post a walkthough of my whole process from excel spreadsheet to DFDC to CAD to handcalcs and FEA)

How did I come up with this?

To arrive at this number I have chopped up multi-wing blades, made computer models of them, and run them through the fan design program which I talked about in an earlier article (Ducted Fan Design Code by Marc Drela, free online). This is the baseline fan, which produces X thrust for a given duct diameter, power and fan rpm.

I then design a fan specific to a hovercraft’s needs, which produces Y thrust at the same duct diameter, power and fan rpm. Then I compare its results to the baseline in the same program, to find out the relative improvement (Y/X-1)%. I’m not writing any thrust values here because the model I am using does not take into account duct blockages from engines/ guards/the hull. It’s a waste of time comparing values from a real thrust test with numbers from a computer program which does not take important things like this into account. The important thing is the difference between two fan designs when compared on a level playing field.

A few fans are shown in a Pintail duct below:

Pros Cons
5Z (baseline design) Available now at good retailers! Designed to cool your office
Prototype (5%) 5% more thrust and could be more resistant to fan stall. Fairly easy to make without CNC moulds Not quite pushing ‘rip your face off’ performance
Optimum (7%) 7% more thrust, looks cool Difficult to make and may not work well off design point

What are the changes from the multi-wing blade?

To get the 7% improvement, there have been two main changes to get from the multi-wing to the new blade:

  1. The airfoil section of the blade is changed to a much less draggy one, giving about 5% improvement. Because the multi-wing is an injection moulded part, it has to have a fairly thick trailing edge which gives it pretty crap performance. Fortunately for multiwing, it turns out that the airfoil you use has surprisingly little effect on overall fan performance (1/2 of the airfoil drag does not equal double fan performance).

2.   The twist and chord of the blade is altered so that every station along the blade is working at its optimum angle of attack. This gives the extra 2%, and you only get the full benefit of this effect at one fan RPM and fan loading. If you start changing the pitch, diameter and number of blades to suit a different engine/ duct, you may lose some of this performance.

Shape distributions of the three fan blades shown above

This is where some sort of practical experience is required; there are many combinations of twist and chord distribution that can give similar performance, but may give a fan with a stupid amount of twist or a foot long chord at the root. I have been reverse engineering as many different fans and propellers as I can lay my hands on, to understand how to make something that won’t just work on paper! For a few of the captivating nuances of the various approaches i have tried see this page. For a shed builder it may be necessary to sacrifice a bit of performance to get to a blade without complex changing shapes like the optimum fan shown here:

F2 thrustfan model

Most of the models I have been running are for the thrust fan on my new F2 GSXR 750 using the following design space: 2400 rpm, 229Nm torque, duct diameter 1100mm (Pintail section), 6 blades. I have also tried fans in the formula 3/50 range and found similar results in terms of thrust improvement (but you only get all the benefits if you design for a specific set-up). Flow straightener angle is tailored for each case, but does not affect performance that much.


What about fancy pictures from CFD?

The methods I have been using so far do not take into account details like sweep, dihedral and wavy surfaces like you may have seen on some ‘revolutionary’ fan designs. For this you need to use CFD (Computational Fluid Dynamics), which apart from giving you pretty pictures, is only really chasing the last 1 or 2 percent of performance. For a fan or propeller, simpler models which only require  excel and patience capture the basic physics very well, CFD is really only the icing on the cake. I have some experience using CFD for a rotating fan and my opinion is that you can only go into this sort of detail if you have a few months spare and some very experienced friends to help you (not to mention a powerful computer with the software installed).

Looks good, but what does it all mean?

Upshots:

-My 7% result may be pessimistic due to the fact that the multi-wing blades deflect slightly in use. The model assumes the blades do not twist or bend like they do on the hovercraft. If the new blades were in a stiffer material and didn’t twist under load, they might perform better. The only way to know if this fo sho, is to make it and test it!

-Increased thrust per horsepower may not be the only benefit to a re-designed blade; it may be possible to delay fan stall by operating at lower lift coefficients. Proof is in the pudding!

-Because the airfoil shape does not have a massive effect on the overall fan performance, it may be possible to get good performance from a flat plate type fan, which would be much easier to make.

-Noise: a more efficient fan may be quieter; again a case of actually measuring it to see if this is true.

-Like any ‘system’ the performance depends on how everything works together. No matter if you have the best fan in the world, you won’t get much thrust if you obscure the inlet with a massive engine (why did you buy a GIXXER then!?!). Everything needs to be right including the duct and cover, hull shape, straighteners, centre cone.

I’ve spend the last 5 years getting to this stage so I probably will have to go ahead and make some prototypes to get some closure; does anyone fancy helping out?

Next time: how to design prototype blades and make them without proper moulds.

Posted in: Fan design, General on April 1st by admin


3 Comments

  • Pingback by Fan fun! - Hovercraft Racing — April 1, 2013 @ 2:25 pm

    […] -My best design so far […]

  • Pingback by Prototype fan design - Hovercraft Racing — September 15, 2013 @ 11:47 am

    […] propellor with very wide chord at the root and not much at the tip. In a previous post the twisted optimum fan shown is basically […]

  • Comment by William B. Stovall — December 11, 2015 @ 3:34 pm

    The Raspet AV-14 ASME paper on the USA army Marvel-Marvelette ducted fan STOL project states they achieved substantial thrust improvement by increasing the propeller wist angle in the outer blade radius portion by 6-7 degrees. This was to account for the actual versus theoretical inflow air distribution across the duct.

    They achieved 600 lb thrust from a 5′-6″ shrouded prop with a 90 hp engine. 10 hp of this was consumed by a BLC system. I see little or no reference to this accomplishment by manufacturers of hovercraft (like Turnstone), airboat people, fan or prop makers etc. They seem to achieve only 2.5 to 4 lbs/hp static thrust. even the airbus publicity E-1.0 numbers I have seen suggest primarily a PR stunt.

    Does the oft mentioned Drela design program take the inflow distribution into account? Have you any comments on the above?

    I have looked through a number of theses, r&d papers and tech notes but found nothing that indicates (i.e. without a government budget) possibilities for ducted/shrouded fan/prop real world applications. Matters of interest to me include passive or simple (ha) powered BLC, possible benefits from VG’s.

    Any comments, info sources, contacts etc. will be much appreciated.

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