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Post by racket on Aug 13, 2016 5:36:12 GMT -5
LOL at the turbine shaft , gear it down by 10 times and you'll have 30 ft lbs at 4500 rpm
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Post by captinobeans on Oct 27, 2016 12:32:46 GMT -5
hello just getting into this jet designing head mash i have got two books the first is "gas turbine engines for model aircraft by kurt shreckling" and "jet engines by thomas kamps"
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Post by captinobeans on Oct 27, 2016 13:13:27 GMT -5
hello just getting into this jet designing head mash i have got two books the first is "gas turbine engines for model aircraft by kurt shreckling" and "jet engines by thomas kamps"
My question as i am trying to do an Excel spread sheet for all the equations is one of the first in the book by Shreckling where the axial speed is desided the one for nMax = Umax/diamiter * pi... he uses 250 for Umax this is 250meters/second for Ni-Cr steel but where did this number get genarated from??? i know this is all about the blade pulling its self appart at speed so when i look up tensile strenght i keep getting results in MPa ? how do i get the meters per second...this is going to change with each diferent material used
sorry to all that use feet and inches thats 820.2 feet /sec
i want to get the spread sheet as acurate as posible so everyone can tap in the material the diameter, and blade angles, amount of stages, spools, number of blades and tempratures etc to get all values worked out without going threw the pain of tapping multiple calculations out for every change made so all you need to worry about is the building of your perfect jet and knowing your result is close to expected.. i know there are programs for some bits but they dont seem acurate or give every posible bit of information needed for everyone.
less tears money and hard work by working it out before starting building
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Post by racket on Oct 27, 2016 16:14:39 GMT -5
Hi Kurt would have done the calcs taking into account all of the variable of his wheel design and running conditions .............LOL, the information required in creating a first rate turbine wheel fills volumes . I hope you have several years to spare for creating your spreadsheet to the accuracy you desire ...........and as for creating the perfect jet first time ..........Heh heh , even the multinational aero engine manufacturers can't do that after spending billions . The guys over at the GTBA www.gtba.co.uk/ have much more experience with both Kurt and Thomas' engines , it might pay to ask there as well . Cheers John
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Post by finiteparts on Oct 30, 2016 14:24:42 GMT -5
Hi, If you look at Kamps book, he works through this also. He approximated the rotor as a thin disk with a bored hole and has a very basic equation shown. I would suspect that M. Schreckling did something similar. I made a Excel sheet a while ago that calculates the stress of a rotating disk based on the equations provided in Shigley's "Mechanical Engineering Design". If you plot the radial and tangential (hoop) stress for a constant thickness disk, you see: This was generated for a disk of similar dimensions to the KJ-66 turbine wheel rotating to 95,000 rpm (tip speed ~ 830 ft/s). As you can see, the peak stress occur in the bore. This makes sense because the bore has to "carry" all the material at the further radial locations and the centrifugal loads that are placed on all that material. This is a very approximate approach because the thermal differences between the tip and the bore act to modify the stress field in the disk and these thermal forces can be very large. So to get the tip speed into stress, you must calculate the rotor rotational speed and then solve for the stresses. The bore stress will be the highest, so you could use that as the limit based on material properties. The equations can be found here: homepages.engineering.auckland.ac.nz/~pkel015/SolidMechanicsBooks/Part_II/04_ElasticityPolar/ElasticityPolars_04_BodyForcesRotatingDiscs.pdfAs for material properties, you want to use the 0.2% yield strength. If the bore starts to yield, you may loose the centering of the turbine wheel or have very low fatigue life. I am not sure which tensile strength you were looking at, but just in case you were not looking at the 0.2% YS, I thought that I would mention that. As a point of interest, you can see from the peak hoop stress at the bore the reason that manufacturers take such great pains to keep the inner bores as smooth as possible. For example, if you look in most compressor wheels, the bore is almost a mirror polish in order to keep any stress concentrations due to grooves or nicks, from reducing the impellers fatigue life. Good luck! Chris
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