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Post by Br. Marius on Feb 3, 2015 18:38:36 GMT
Question 1 - reprinted here
1. A simplified Mach-scale rotor model of Black-Hawk was built to test in the Glenn L. Martin wind tunnel. The rotor characteristics are given as:
Rotor Number of blades = 4 Rotor radius = 2.77 ft Blade Chord = 3.15 in (uniform) Blade Twist = 0 degree Blade Airfoil = NACA 0012 Rotor RPM = 2300
Assume induced velocity correction factor kappa_h of 1.15, and airfoil characteristics as cl = 5.7alpha for alpha < 12 deg cl = 1.2 for alpha > 12 deg cd = 0.01
Calculate hover characteristics at sea level with collective variation up to 15 deg (a) Rotor thrust T in lbs and (b) Shaft HP and (c) Show angle of attach distribution radially
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Post by MZ on Feb 4, 2015 3:30:10 GMT
These are the graphs I got. I am having a hard time accepting the results but it will have to do for now. Max thrust about 394 pounds and max SHP about 30 HP. ENAE633HW1.pdf (147.78 KB)
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Post by Br. Marius on Feb 4, 2015 16:31:56 GMT
For my own sanity check, what don't you like about your results? I'd agree with you, but only from the standpoint of having run the numbers myself and yielding different answers that agree with my intuition about radial AoA variation. My code and plots are attached.Attachments:hw1hoveringrotor.pdf (147.02 KB)
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Post by Deleted on Feb 4, 2015 16:43:42 GMT
Well, my plots are similar (to Mario's) but the y-axis values (Thrust, Power, etc...) are slightly off. This depends on how I used the induced velocity correction factor, Kh. If I don't apply it when calculating inflow for C_T (like Leishman 3.23), then I get the same plots. But Chopra is now saying in his text and notes that inflow for C_T should have Kh applied to it. However, I also remember him saying last class to assume uniform inflow... so should we just ignore Kh completely?
And for angle of attack with radial distribution, will we have a surface plot? Not only does alpha vary with radial distribution but also with the collective setting. I'm assuming we still have to show this as a function of collective. Or maybe just multiple curves of constant collective on one plot?
** Oh wait, maybe we assume constant inflow just for calculating phi to calculate angle of attack?
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Post by Deleted on Feb 4, 2015 17:31:21 GMT
The AOA plot is what I did not like much since it did not make sense. It should be greatest at the root and lower at the tip sort of like you got Br. Marius.
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Post by Br. Marius on Feb 4, 2015 18:15:50 GMT
I agree Mario. How did you do your computations? I noted the equation numbers from Leishman in my code if you still have that text (I need to re-reference them to his book) and would like to reference that.
Justin--I remember what he said about uniform inflow from last class, but, to be honest, I just implemented BEMT because it seemed easier and he said it was OK to do something like that. The kappa value shouldn't create nonuniform inflow, only correct the value to something that will yield a CT more in line with actual values, so I'd think that you would want to correct the inflow values before utilizing them to compute AOA. That's my thought anyhow....I didn't actually utilize kappa myself because BEMT doesn't assume uniform inflow.
Also, thanks for the tip. I've redone my plots so that my radial AOA variations in the collective pitch sweep are all on 1 plot.Attachments:hw1hoveringrotor.pdf (120.31 KB)
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Post by Deleted on Feb 4, 2015 18:47:05 GMT
So how come you did not divide the lambda by radial position before you subtracted it from theta collective pitch
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Post by Br. Marius on Feb 4, 2015 19:25:25 GMT
Because lambda is the inflow velocity ratio, it is equal to the inflow angle for small angles. Since it's the inflow angle, I should be able to subtract it directly from the blade pitch angle and get the local AOA. I don't see why it should need to be divided by radial position...
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