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Cannot properly increase propeller pitch and engine throttle  Topic is solved

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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 2:16 pm

okay
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Mon Nov 29, 2021 9:28 pm

No. of blades = 3
rpm = 2600
Diameter = 2.032
Spinner dia = 0.3
Velocity = 105
Power = 212500

shrouded = no
square tip = no
open hub = no

Press 'Design it'

Go to the airfoils tab and change

Airfoil 0.0 = Clark Y, Re=25,000
Airfoil 0.0 = Clark Y, Re=100,000
Airfoil 0.0 = Clark Y, Re=500,000
Airfoil 0.0 = Clark Y, Re=500,000

Go back to design tab and press 'design it'

The prop pitch in the POH says it can be set between 12.4 to 28.5 degs at 0.75r. so we need to find out what the table values for Ct and Cp would be at those 2 data points.

Go into the geometry tab and look at the beta angle at 0.75, it's 30.8, so we need to rotate the prop by 2.3degs

Go into the modify tab and enter -2.3 in the 'change blade angle by'.... press modify it once only
Go back into the geometry tab and look at the beta angle at 0.75, it's now 28.5.

Go into the multi analyse tab and press 'Analyse'
Press 'copy text' and paste this into a spreadsheet

28.5-12.4 = 16.1
Go into the modify tab and enter -16.1 in the 'change blade angle by'.... press modify it once only
Go back into the geometry tab and look at the beta angle at 0.75, it's now 12.4

Go into the multi analyse tab and press 'Analyse'
Press 'copy text' and paste this into a spreadsheet

you now have all the data needed to create two tables for ct and cp

remove excess rows down to
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2

btw your plane wont go faster than 1.2.... no table will ever go up to 6

Now you notice the 12.4deg table only goes as far as 0.7... so some manual extrapolation is needed

Create 2 charts in your spreadsheet and observe the curves.
For the 12.4 degs chart we need to create values to cover the missing ones and you should end up with something that looks like these

Code: Select all
   ct   
   12.4   28.5
0   0.070471   0.059169
0.1   0.075647   0.057872
0.2   0.075084   0.053236
0.3   0.073785   0.046486
0.4   0.071892   0.03723
0.5   0.070132   0.025331
0.6   0.069065   0.011572
0.7   0.068736   -0.00298
0.8   0.067296   -0.018
0.9   0.063196   -0.035
1   0.054459   -0.055
1.1   0.041856   -0.075
1.2   0.028984   -0.095


Code: Select all
   cp   
   12.4   28.5
0   0.070429   0.023747
0.1   0.077167   0.023881
0.2   0.075751   0.023797
0.3   0.073886   0.022897
0.4   0.071894   0.02079
0.5   0.069845   0.016573
0.6   0.069028   0.009758
0.7   0.068737   0.000482
0.8   0.06909   -0.01
0.9   0.068348   -0.022
1   0.063237   -0.035
1.1   0.052714   -0.048
1.2   0.039982   -0.062
Last edited by S&J on Mon Nov 29, 2021 9:45 pm, edited 2 times in total.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 9:31 pm

No, the propeller's maximum RPM will be 2700, not 2600 !
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Mon Nov 29, 2021 9:36 pm

yes but that data we have is specific for 2600rpm... so we use this.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 9:38 pm

Okay …
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 9:46 pm

Weird … I initially got a beta angle of 27.8 with the data you posted, and when I Modify with a blade angle of 0.7, the beta doesn't change ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 9:53 pm

Sorry about the non-changing blade angle - it did change, but I looked in the Design tab instead of the geometry tab ! :oops:
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 10:28 pm

Weird - I get much higher values than you got … I probably messed something up, so I will just try your tables.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 10:59 pm

Hmm … that was not a good idea … your tables have much lower power required coefficients … that made my cessna go like a rocket - 240 kts at > 4000 RPM, 370 HP and 250 pph.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Mon Nov 29, 2021 11:23 pm

I think my tables are better … the climb performance is a bit low though. And it still goes like a rocket in level flight - up to 250 kts ! I think this is due to that I have only data up to an advance ratio of 0.7 - can you please do the extrapolation for me ? I've no idea how I should do this. But HP, PPH and RPM were good this time. I deduce from all that that thrust for low advance ratio is too low, but too high (well, it's using the low AV values) for high advance ratio - is that correct ?

Here are the tables - I've commented out everything below AV=0.7:

Code: Select all
  <table name="C_THRUST" type="internal">
      <tableData>
                          12.4              28.5
                    0    0.114996    0.133809
            0.100    0.103658     0.137833
            0.200    0.093862     0.137363
            0.300    0.080296     0.135799
            0.400    0.062841     0.134164
            0.500    0.062841     0.133284
            0.600    0.019471     0.134623
            0.700    -0.004907    0.132284
            <!--0.800     0.125542
            0.900     0.113687
            1.000     0.093263
            1.100     0.071824
            1.200     0.049625-->
      </tableData>
  </table>

 <!-- power coefficient as a function of advance ratio and blade angle -->
  <table name="C_POWER" type="internal">
     <tableData>
                          12.4                28.5
            0            0.046561      0.133809
            0.100    0.047045      0.137833
            0.200    0.046274      0.137363
            0.300    0.043624      0.135799
            0.400    0.038364      0.134164
            0.500    0.029759      0.133284
            0.600    0.017741      0.134623
            0.700    0.001950      0.132284
            <!--0.800        0.125542
            0.900      0.113687
            1.000      0.093263
            1.100      0.071824
            1.200      0.049625-->
     </tableData>
  </table>
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Tue Nov 30, 2021 8:53 am

Can I ask why you're doing doing 4000rpm when the top rpm for the engine is 2600 ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 10:58 am

I was not doing 4000 RPM - the engine did it in a quick level flight test when I used your tables ! With my tables RPM stays within limits.
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Tue Nov 30, 2021 1:29 pm

oops my mistake I had the 12.4 and 28.5 columns the wrong way round

Code: Select all
   ct   
   12.4   28.5
0   0.059169   0.070471
0.1   0.057872   0.075647
0.2   0.053236   0.075084
0.3   0.046486   0.073785
0.4   0.03723   0.071892
0.5   0.025331   0.070132
0.6   0.011572   0.069065
0.7   -0.00298   0.068736
0.8   -0.018   0.067296
0.9   -0.035   0.063196
1   -0.055   0.054459
1.1   -0.075   0.041856
1.2   -0.095   0.028984


Code: Select all
   cp   
   12.4   28.5
0   0.023747   0.070429
0.1   0.023881   0.077167
0.2   0.023797   0.075751
0.3   0.022897   0.073886
0.4   0.02079   0.071894
0.5   0.016573   0.069845
0.6   0.009758   0.069028
0.7   0.000482   0.068737
0.8   -0.01   0.06909
0.9   -0.022   0.068348
1   -0.035   0.063237
1.1   -0.048   0.052714
1.2   -0.062   0.039982
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 3:20 pm

:mrgreen: :lol: :mrgreen: Yep, the new tables are better than the old ones.

I went to 3500 feet (there our optimum AFR is reached the first time) with full throttle - climb performance is perfect, I think - just above 1000 FPM. Then at 3500 ft, level flight with the AP. Throttle adjusted to maintain 83 % HP, RPM 2600. I waited a few minutes, and found the cruise speed at these settings to be … 210 kts. That's a lot better than before, but still 50 kts too much. Also, I got an overshoot of the HP already when not moving - 287 instead of 285. This became worse as soon as I released the brakes - at 100 kts climbing, I got over 310 HP. So, the Cp tables is too low overall, I would say. How do we fix that ? Edit the table manually until it is about right ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Tue Nov 30, 2021 3:43 pm

Well ya could or you could accept the engine and prop are now matched and adjust the planes drag.
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