Structural design and how to build it

For some reason people in engineering call this ‘design for manufacture’ which apparently is another skill you have to learn. Why would you be designing things that are un-manufacturable in the first place? Mongloid business twats dont understand that the whole design and engineering process is fully interlinked from concept to production!

Anyway… I’ll just put a few pictures up until I have a few spare minutes to add proper words.

There are pages and pages of scribbles and doodles and concepts dating back years from when I started thinking about making my own fan. I didn’t finish the aerodynamics one night and start the structural design the next, there are many iterations between all stages. (the design shown here is iteration U, and i only started using letters a few months ago).

After you get to a decent aerodynamic design, get the geometry into your generic CAD package and get the loads into excel.

That bending moment breaks one of the rules I made when starting this project: the new blade loads cannot be more than the original blades. The Hascon root bending moment was in the region of 100Nm, and in terms of loads acting on the hub, is not as bad as it looks I hope. In hindsight this wasnt a great rule, its pretty obvious that to create more thrust you need more force going through the blade, D’oh!

Anyway next you need to estimate the mass from surface areas, estimated thicknesses, materials. From which you can do basic calcs, and start creating inputs for a slightly better model. (Know that these screenshots are the tip of the excel iceberg).

By the way excel, paper, calculators, books and daydreams are probably where most of the work is done, but they dont really make for interesting pictures. (Also if i could recover from my deep hole of a youtube addiction I could have finished this years ago)

But anyway FE does give you lots of pretty pictures, and they are only that if you don’t know how to use it properly, just like CFD. Thanks to work for letting me use it in my spare time.

The general aim of all models and calcs is to be conservative everywhere, so you have as much safety factors built in when you realise you havn’t taken something into account for example:

-Worst case aero loads are used to get highest root bending moment (1200mm diameter fan assumed)

-Heaviest blade that is likely is modelled at max tipspeed to get highest centrifugal loads

-For the hub model there is a loaded blade in each of the 12 hub sockets which is double what would be seen in reality: 6 blades can absorb 200hp

Above you can see how its necessary to align elements with the direction of the fibres, and to try and make them fairly square and uniform if you are modelling carbon fibre. All of the carbon fibres point along the length of the blade to resist the bending from aerodynamic forces, and the the tension from centrifugal loading (21000xgravity). There are glass fibre braids holding it all together and resisting twisting. The skin is laid on top of all that- the aim is to assemble and infuse the whole thing in a single shot- no tricky trimming and gluing bits together, hopefully it will even be painted in the mould.

Above is how the blade deflects when you assume that the hub is 100% rigid- 9mm seems pretty good. In reality its die cast aluminium, and as i said above my root bending moment is much higher than the original blades so its a good idea to do some checks to see if it will be ok. (Obviously the real test is the first time you blip the throttle, but these tools allow you to at least quantify the problem).

So when a model is built including the hub, the deflections are getting towards double compared with assuming a rigid hub. The model above shows 12 blades, but actually for the real fan i plan to have only 2-6 blades. I am taking the worst case, and also this was the easiest to model (the actual model is only one blade and 1/12th section of the hub with cyclic symmetry. I have no measurements of hascon/ multiwing deflections for high power levels in a 1200mm duct, but i would be interested to hear if you have.

Bear in mind these numbers are all for the worst case- in reality the fan I plan to make and use will be 1100mm diameter which has a 26% lower root bending moment. I have optimised the design for the 1100mm duct, but I thought it makes sense to extend the mould slightly so its big enough for 1200mm blades for the future.

Stress in the hub could be a problem, and is definitely an area I will carry on looking into- the yield strength of most grades of die-cast-aluminium is 320MPa and as you can see there are higher stress regions here. However this doesnt mean it will fail in real life, because this model has simplifications, like not including the little stiffening ribs and fillets that exist on the real part. I can do that later when I am sure this is the final design and i have time to run slower, more complex models. I have also put imaginary bolts on the holes and a tolerance on the blade root so the hub halves squeeze onto the root, both of which may not be quite right.

(By the way if anyone knows what grade the hub material is i would be interested, but without knowing I design for the lowest grade its likely to be).

The next thing is to see how the stress in the blade itself is. My current design has a tubular spar running all the way along the blade, which has a 1.5mm thick skin of UD carbon innit. This is the main structural member which runs through the centre of pressure and centre of gravity of the blade as closely as possible, while also running through the thickest part of the blade for max bending resistance. By my reckoning the stresses are manageable even for a lot of fatigue cycles (or hours of blipping the throttle on the grid).

In the pretty picture below you can see the stress along the fibres, positive numbers and bright colours are tension (pulling), negative numbers and cold colours are compression (squeezing). The tensile stress is highest because the blade is spinning around at 2500RPM which makes the weight of the blade 21000 times as much as if it was sitting stationary in your hand.

In another plot below the same result is shown for the model which includes the hub, you can get a good idea why compressive stresses exist: the blade (wing) is trying to bend up due to thrust  (lift). The action of bending means the top of the spar must get shorter (compression) and the bottom must get longer (tension).

Just to highlight why this is a good place to use long fibre composites, which have a lot of strength only in the direction of the fibres, the stress at right angles to the fibres is only about 1/10th of the stresses above.

After you’ve spent a while farting around making changes so many times you cant remember which model was the best, you get organised and write down stresses, and check how they stack up against the strength of the materials:

And we are done (with this iteration)… There are still things that need looking into further (red fail boxes above) and possible tweaks that need to be done (yes I know it needs some sort of blend at the root).

The thing is that i’ll never win any races if i continue iterating fan design and trying to live around my youtube addiction. So at some point you have to call it and get your amazing girlfriend to lasercut sections for the plug, courtesy of UoN Architecture dept.

Posted in: Fan design, General on December 8th by admin


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