New Aircraft - Promecc_Freccia

[TheEagle]

For the airfoils, use the Clark Y Re=500000 for all stations - it fits the best.

[Ysop]

Maintenance Manual says:

2.19. PROPELLER
Manufacturer: DUC HELICES
Model: 3-blade Inconel SWIRL clockwise
Diameter: 1730 mm
http://www.duc-helices.com/produit.php?id_produit=1060&id_rubrique=89

You can of course code the possibility to change the pitch on the ground.
The actual pitch angle is of lesser importance as there are a lot of unknowns of the geometry as well.
Suggest to optimize to an economical (!) cruise condition.

[SDeAstis]

This is my first attempt to use JAvaProp and javaprop2jsbcpct.

Code: Select all

PS P:\Programmi\Python\Python310\Lib\fgtools\aircraft> py .\javaprop2jsbcpct.py -i F:\FlightGearDev\1.ProMecc-Freccia_developement\Src\Propeller\test -b 0 -m 3
['F:\\FlightGearDev\\1.ProMecc-Freccia_developement\\Src\\Propeller\\test']
        <table name="C_THRUST" type="internal">
                <tableData>
                                0.0
                        0       0.096595
                        0.05    0.095693
                        0.1     0.094791
                        0.15    0.093889
                        0.2     0.092987
                        0.25    0.092085
                        0.3     0.091183
                        0.35    0.089949
                        0.4     0.088715
                        0.45    0.087716
                        0.5     0.086717
                        0.55    0.086187
                        0.6     0.085657
                        0.65    0.085211
                        0.7     0.084765
                        0.75    0.083458
                        0.8     0.082151
                        0.85    0.079904
                        0.9     0.077656
                        0.95    0.071794
                        1.0     0.065932
                        1.05    0.058352
                        1.1     0.050771
                        1.15    0.042991
                        1.2     0.035210
                        1.25    0.027237
                        1.3     0.019265
                        1.35    0.011152
                        1.4     0.002904
                        1.45    -0.005422
                        1.5     -0.013739
                        1.55    -0.022056
                        1.6     -0.030374
                        1.65    -0.038691
                        1.7     -0.047009
                        1.75    -0.055326
                        1.8     -0.063644
                        1.85    -0.071961
                        1.9     -0.080279
                        1.95    -0.088596
                        2.0     -0.096914
                        2.05    -0.105231
                        2.1     -0.113549
                        2.15    -0.121866
                        2.2     -0.130184
                        2.25    -0.138501
                        2.3     -0.146819
                        2.35    -0.155136
                        2.4     -0.163454
                        2.45    -0.171771
                        2.5     -0.180089
                        2.55    -0.188406
                        2.6     -0.196724
                        2.65    -0.205041
                        2.7     -0.213359
                        2.75    -0.221676
                        2.8     -0.229994
                        2.85    -0.238311
                        2.9     -0.246629
                        2.95    -0.254946
                        3.0     -0.263264

                </tableData>
        </table>

        <table name="C_POWER" type="internal">
                <tableData>
                                0.0
                        0       0.096280
                        0.05    0.095251
                        0.1     0.094222
                        0.15    0.093193
                        0.2     0.092164
                        0.25    0.091135
                        0.3     0.090106
                        0.35    0.089128
                        0.4     0.088150
                        0.45    0.086946
                        0.5     0.085742
                        0.55    0.085348
                        0.6     0.084954
                        0.65    0.085190
                        0.7     0.085425
                        0.75    0.085615
                        0.8     0.085805
                        0.85    0.085432
                        0.9     0.085058
                        0.95    0.081177
                        1.0     0.077296
                        1.05    0.070693
                        1.1     0.064090
                        1.15    0.056173
                        1.2     0.048256
                        1.25    0.038975
                        1.3     0.029695
                        1.35    0.019348
                        1.4     0.008245
                        1.45    -0.003558
                        1.5     -0.015448
                        1.55    -0.027338
                        1.6     -0.039228
                        1.65    -0.051118
                        1.7     -0.063008
                        1.75    -0.074898
                        1.8     -0.086788
                        1.85    -0.098678
                        1.9     -0.110568
                        1.95    -0.122458
                        2.0     -0.134348
                        2.05    -0.146238
                        2.1     -0.158128
                        2.15    -0.170018
                        2.2     -0.181908
                        2.25    -0.193798
                        2.3     -0.205688
                        2.35    -0.217578
                        2.4     -0.229468
                        2.45    -0.241358
                        2.5     -0.253248
                        2.55    -0.265138
                        2.6     -0.277028
                        2.65    -0.288918
                        2.7     -0.300808
                        2.75    -0.312698
                        2.8     -0.324588
                        2.85    -0.336478
                        2.9     -0.348368
                        2.95    -0.360258
                        3.0     -0.372148

                </tableData>
        </table>
 

Could you have a look and tell me whereas I'm doing mistakes?

[TheEagle]

Great for the first attempt ! You made two small mistakes: first, you missed the following part of my instructions:

So, after pressing Design, go to the Geometry tab and find the station whose r [mm] is closest to (<propeller radius in mm> - 250), then read off the blade angle at that station. Now, go to the Modify tab, and enter (<desired pitch angle (try 25)> - <the just read-off angle>) into "Change blade angle by", then press Modify. Now go back to the geometry tab and again read off the pitch angle - it should be the desired one now.

Second, you need to give the correct blade angles with the -b argument - but this is only important if you have a variable pitch propeller with multiple input files.
I also made a small mistake in my code (formatting the table wrong when there's only one blade angle) - so please make sure to update your local copy of the FGTools (either git pull or redownload through a ZIP snapshot, depending on your installation method (git clone or ZIP)), then do the whole thing again, this time with the correct blade angle in JavaProp !

[S&J]

Good to see Javaprop being used, the more developers the better.

One thing

So, after pressing Design, go to the Geometry tab and find the station whose r [mm] is closest to (<propeller radius in mm> - 250), then read off the blade angle at that station.

This is not how I do it.
Blade angle is conventionally read off at 0.75 radius.

[TheEagle]

Blade angle is conventionally read off at 0.75 radius.

Yup, i know, but the propeller's specification does not conform to that - specifying the blade angle at 25 cm from the tip instead - so we have to go along to get correct results - at least that is my understanding.

[S&J]

Ah page 15.... well spotted

Then the next issue I have is the top speed and max power.
I believe these are not at the same time. Max power is for climb out of 5 minutes and you're not doing top speed.

So I'd suggest top speed at continuous power.

Can I ask what values have been used to calculate and the value of the prop inertia ?

[TheEagle]

Can I ask what values have been used to calculate and the value of the prop inertia ?

Huh ? No one said anything about calculating it … but good point, I'd like to now how to do it !

[S&J]

I'm assuming this is the up-to-date rep.

https://github.com/sdeastis/ProMecc-Freccia

Is there a more up-to-date ?

[SDeAstis]

Is indeed the latest!

[S&J]

We know that a slender rod of uniform thickness rotated around one end can have it's mass inertia expressed as 1/3 m r2.

What we know about this rod is that it's COG is 1/2r, yet it's inertia calcs uses 1/3. mmmmm interesting.

What we know about the prop being used is that the CoG is 1/3r (page 23), which we'd expect as a prop is not uniform along it's length.

So if an object with it's COG at 1/2r has it's interia at 1/3r, where does the inertia exist for an object who's COG is 1/3r.... answer 0.222r

Resulting in an equation

ixx = 0.222 m r2

0.222 * 4.2 * 0.865 * 0.865

ixx = 0.6976kgm2
ixx = 0.51slugft2

anyone wishing to check the logic, please do

[dany93]

We know that a slender rod of uniform thickness rotated around one end can have it's mass inertia expressed as 1/3 m r2.

What we know about this rod is that it's COG is 1/2r, yet it's inertia calcs uses 1/3. mmmmm interesting.

Hi, S&J,

Your first idea seems to be that the mass of the blade is concentrated at a single point (at 0.5R).
Moreover, even for a point mass at 0.5R, its moment of inertia would be M * (0.5R)^2 = M * 0.25R^2 = 0.25 *M * R^2.
But the inertia calculation is the result of an integral along the rod, between 0 and R.
sum(mi * ri^2) for mass elements at ri
In this sum, because of the r^2, the mass elements far from the center have more "weight" (influence) than those close to the center.

See Wikipedia Moment of inertia
"The moment of inertia of a thin rod with constant cross-section...." (calculation for a rod of length L, where we would have L = 2R)

Please check me too, easy to make a mistake...

[S&J]

As much as I'd like to integrate calculations of inertia for every particle of mass at r. I'm not sure it's practical.

We can only work with the data we have plus allow for a reasonable level of extrapolation.

We have a prop weight of 4.2kg
We have a mass distribution of the prop blade that puts it's cog at 1/3 r
And we have a prop dia of 1730m

I've always used 1/3mr^2 as a 'standard' across the range of my props.

https://www.engineersedge.com/calculato ... nertia.htm

1/3 clearly results in too great a number. So the number I'm looking for is to describe a prop shape not a rod.

[dany93]

Hi, S&J,

My response was not to say that we should take 1/3 m r^2 for the propellers, but to justify this apparent contradiction:

What we know about this rod is that it's COG is 1/2r, yet it's inertia calcs uses 1/3. mmmmm interesting.

This said, I agree that a propeller doesn't have the same weight distribution as a rod.
This needs some estimated correction. And the hub weight is not easy to estimate, either....

Fortunately, the accuracy of the moment of inertia for the propeller is not that critical. It has some effect on the its spin acceleration (uneasy to observe) and on the gyroscopic effect (uneasy to estimated at flying, often weak in FG). Although the gyroscopic effect could be more interesting.

Maybe that this can help Measures of Propellers Moments of Inertia

[S&J]

Using my existing data of props and the data from the table in the above link.
The prop shape factor works out at 0.114

Ixx = 0.114 * m * r^2

Ixx = 0.26slugft

A big reduction in inertia of my props