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I am in the process of constructing a linear compressor for the propulsion system of a small UAV. To manufacture the compressor blades and give them their required airfoil shape, I intend to first milling pieces of mild steel (1/8 x 3/4 x 2 inches) into a semi-airfoil shape. Then heat them to the proper temperature required for deformation of the steel before placing them into a press die to be compressed into blades with a true airfoil shape, with the necessary twist and bend added during the compression stage.

To my understanding, an industrial method of creating compressor blades is to heat small tungsten cylinders to 900°C and then compressed them at 2000 metric tons to give them their required shape. However, I do not think that the process I outlined above will require a similar pressure and temperature. Hence, my question is what range of temperature and pressure would be necessary to perform this process? And, maybe I should first ask if this process is even possible utilizing a 50 ton press and a hobby-sized ferrier furnace capable of reaching 1100°C?

I should add that there will be no combustor used in this propulsion system, therefore most of the heat entering the system will come from compression of the air. Normally, inlet and outlet air temperatures of a compressor can reach a difference of 300°C to 600°C (to my knowledge); however, I do not expect such high temperature differences in the compressor I am building. Absent accurate mathematical calculations as to expected temperatures, I would, at present, speculate a temperature increase of ~100° between inlet and outlet. This is based upon simplistic calculations I have performed comparing the rotational speed of the rotor and volume of air flow of the compressor I am building with those of normal-sized compressors used in jet engines.

Moreover, given the enhanced air flow produced by the airfoil shape, I would be reluctant to abandon it for the sake of a simpler design and easier method of blade manufacture.

I would appreciate any all information you could provide to me on this subject and thank you for your time and assistance.

HEY, GUYS! LET ME CORRECT SOME INACCURACIES.

The metals used for compressor blades include stainless steel and titanium--not tungsten. My error (I knew it started with a "t":)

Second, the possibility of the blades cracking or whatnot because of brittleness of the metal is offset by the blades having an amount of play after they are set in the rotor. This excess movement helps to dissipate the stress exerted upon the blades.

Third, the blades are heated twice during the forming process (the first time as a ceramic coated cylinder), both times for 15 minutes at 980°C. Subsequent to the first heating, they are then placed into dies and are subjected to 1000 metric tons of pressure: after their second, 1600 metric tons. They are both times cooled in water to temper them which gives them greater resilience.

Lastly, it appears that the airfoil design is not an essential attribute to the blades as some have it and some do not: its inclusion is dependent upon the design of the compressor. However, a slight twist in the blade (the degree of which is also determined by the design) appears consistently in the videos, books, and web articles I have on compressor blades. So I maybe able to forgo the intricate process of incorporating an airfoil design in the blade and just give it a good twist in a pair of dies in my hydraulic press after a little heat treatment.

Sorry, and again, thanks.

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  • $\begingroup$ Sounds like you have the bare minimum to process steel. There's a reason compressor and turbine blades are made of higher temperature materials though- what temperature will your combustor reach, and what stages of your compressor and turbine will see enough heat to anneal steel? $\endgroup$
    – Abel
    Commented Feb 11 at 12:25
  • $\begingroup$ compressor blades have it much easier than the turbine blades. for a junky UAV you might even be able to keep uniform cross section (ie not even a proper airfoil), cold-form them, and then heat treat. $\endgroup$
    – Pete W
    Commented Feb 11 at 17:41
  • $\begingroup$ Re: "junky" - sorry I was not intending to demean your project, was more referring to the now many homebrew sheetmetal turbojet contraptions you can find on youtube. Point being, the designs were more optimized for ability to construct in a well equipped garage, rather than ideal performance. To your edited question: With +100K at compressor outlet, it sounds like the compression ratio, and thus the efficiency, isn't being pushed to its max, so efficiency from ideal blade shape (vs just a curved uniform thickness) would not be quite as critical, but it's been a long time since I studied this. $\endgroup$
    – Pete W
    Commented Feb 11 at 20:43

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Plain ("mild") steel is likely to fly to pieces via centrifugal stresses. You must use a higher grade of steel. I recommend you use drill rod, which is a high-strength steel which can be machined to shape and then baked in an oven, which converts it into something so hard and strong that it can no longer be easily machined, cut, or pulled apart.

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