Prototype fan design


It became apparent that the best fan design would be a very tricky shape to realistically make, even if you do have a fancy CNC machine to make the moulds for you. The balance i am trying to find is:

New design must be at least X% better than the baseline = Cool looking sexy shape

> What I want  is somewhere in between>

New design must be realistic to manufacture = Saw it from a 2×4

To get the best theoretical performance you need to design the blade to work under ‘free-vortex flow’. This essentially means that the air travelling through the fan is all travelling at the same speed from hub to tip, and that you have a constant pressure rise at all points on the swept area. If you follow this by the book, you end up with a fan looking like a human powered aircraft 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 this.

The problem with this is actually laminating this structure in as simple a way as possible. With the elegantly sculpted optimum design, I couldn’t think of a way to reliably mold a blade without making it out of at least 2 mouldings and bonding them together. Gluing mouldings together can be done but it is an extra process and one that needs careful control. It is also better practice to have fibres running around the trailing edges of the blades anyway, as is standard for most commercial propellors/ fans out there.

My aim is to infuse the blade in a single piece, which requires the use of a core rigid enough to wrap the glass and carbon around as you put it into the mould. Each blade must be the same weight therefore each core must be identical- they cant just be hand sanded out of a block of foam like ive done in the past. Therefore I plan to use CNC feather-cut blue polystyrene, as model aircraft builders typically use. For this technique the core can only be lofted from a section at either end- the complex optimum fan is out of the question for this then!

So back to the aerodynamics to make the best fan you can that is tapered from two sections. While this sounds pretty crap, this is what happens in real life on aircraft wings. Most airliners have straight tapered wings even though you may be able to get 0.5% less induced drag using an elegant Spitfire elliptical plan form (which is actually not the reason the spitfire wing is shaped like that…). With fans you lose a bit more performance by making this decision, i think its good for 1-2% efficiency loss. But if i can find a straight tapered design that is still at least 5% better than the baseline, im prepared to go for it.

Which brings me to the frozen prototype fan design! Which is the result of many iterations and much frustration and swearing. Is it really worth it for 5% more thrust? Well the more Hp you have the more it is worth i suppose. (I do this because in a twisted way i do enjoy the learning process but also because i want to beat the Tony Gs and Dan Ts of the hovercraft racing world.)

End view on blade showing all the sections if the blade as it will be made

What I have come up with is a blade that will look something like the larger wing-fan blades. The two main design points are:

(in an 1100mm duct spinning at 2510RPM (145m/s tipspeed), using a 12 blade hub)

  1. Around 100Hp with 3 blades
  2. True 200Hp with 6 blades                   (with room for more)

Compared to a multiwing 5Z blade, the new blades should produce 5% more thrust for a given Hp. As well as this they are larger, so require fewer blades to get the same power through, saving weight and $$$.

So details about the new blade are shown below:

This plot shows some general performance figures for the fan operating with 3 blades taking around 100Hp

From the plot above you can see that the lift coefficient of the blade is not linear, due to the nature of a straight tapered fan. The Cl at this point is 1.44, which is near the limit of this airfoil’s performance, its may be necessary to go upto 4 blades, which is easy with the 12 blade hub.

To check if this is alright, you need to do a quick analysis of the blended airfoil section to the radial station where Cl=1.44. At this point (about 50% blade span) the section will be a mix between the sections at the root and tip (the parent airfoils).

Plot showing the parent and blended airfoil sections as they lie on the blade

When you create a section by blending two parents, which is what esLOFT does for you, you don’t necessarily get good children (funny joke about your brother etc…) so you need to check using airfoil analysis programs that the section does what you expect.

A visual check to see if the blended section leading edges have come out OK

So anyway, we take station 5 coordinates and input them into JavaFoil, which will give you a polar of the airfoil, as long as the relevant Mach number and Reynolds numbers are input for that section at that operating point.

But will it be ok running at Cl=1.44, which is pretty high? Im not sure…


Below are some pictures of the blade after it has been lofted in CAD from the sections taken from DFDC and esLOFT (and many spreadsheets, 2D airfoil tools hundreds of text files etc.). Ater you’ve done it a few times it only takes a few hours to get a design from a set of requirements to a CAD model which is pretty good. All of these tools are freely available too (apart from the CAD) all you need to do is give a few hundred hours to learn how they work (and how to run linux).

Javafoil polar showing section 5 performance- looks similar to original airfoils which is good.

Now you can have a nice solid model, but the hard bit is still to come… Making it from real materials

In this picture you can see the prototype in green and the 5Z in blue

As for actually making a blade you can bolt into the hasconwing hubs, tune in next time!

Posted in: Fan design, Featured, General on September 15th by admin


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