Talking propellors

[bomber]

Javaprop

http://www.mh-aerotools.de/airfoils/javaprop.htm

but because of security issues you may have a problem with the copy box being greyed out... here's the solution.

http://forum.outerra.com/index.php?topic=2851.0

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First off on the design tab.. enter these values.

number of blades = 2
revs per min = 2750
Diameter = 1.778
Spinner Diameter = 0.15
Velocity = 57
Power = 75000

and press the design it button.

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on the airfoils tab change the airfoils to these..

MH126
MH114
MH116
MH116

these are just my preference of airfoils you can play around and pick from the selection if you so wish

Having done that go back to the design tab and re-press design it..

Don't do it the other way round, to miss a step as it doesn't produce the correct values.

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on the geometry tab you can have a look at your prop.
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on the mulit-analysis tab press analise

and have a good look at your initial prop....

it's not finshed by the way, not that you couldn't use it but we know something about our prop and thats the 18.74 degs pitch angle... and this prop doesn't have this it has a 20.9 degree pitch angle.

how do I know ?

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on the geometry tab you can have a look at your prop.

left hand side of the table find 0.75... then across to the pitch angle which is the 3rd collumn.... 20.9
or even easier look at Beta at 75%R on the design tab

[bomber]

oh I missed this question..

One thing. should I make a branch on the puppy with any modified engine? that way we can just tell pilots test version A and version B and report...

The thing is I'm doing this for sanhozay.. he's back Friday and I'm kinda thinking it's a question to be asking of him..

Also bare in mind that with prop planes the ability to select different engines and prop from a drop down menu, similiar to fuel and loadout would be an idea.

If someone would look at it..

[bomber]

ok from here on in Javaprop there's more than one single solution..

But the aim is to get a prop that's close such that you could get a planes aiframe in the air and see how it performs..

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what you could so is in the design tab adjust the speed unit the pitch angle is 18.7........48.7m/s (press design it)
or you might not and instead go into the modify tab and change the blade angle until the pitch at 75% is 18.7 (don't press design it, on the design tab)

don't forget to go to the multi analysis tab and analyise it each time

but even if you do one of those two or a combination of both adjustments in the multi analysis tab you'll note that the rpm at 0 velocity with 75kW of power being produced by the engine is still too high.... so

in the modify tab make sure all the values were as before... or in this case put the change angle back to zero and instead scale cord by 1.1.... ie 10% each time..

after a few of these adjustments you should bring your rpm's at zero velocity to under 2400rpm

So you have a prop's Cp and Ct tables now but without an airframe you've no idea what speed the plane will actually fly at with this prop and engine.
but we do know some facts.... 57m/s top speed... now at that speed the prop generates (from the table) approx 1.075kN of thrust... at top speed we know that thrust equals drag.. so 1.075kN = 242lb of drag force. (if like me you're doing your planes in imperial)

So.... did we all uinderstand that... and it's taken me years to get this far so don't be upset if you need to read it again and experiment..

oh yeh the tables...

Code: Select all

   Ct
   [-]
0   0.112115
0.05   0.104577
0.1   0.1131
0.15   0.1115
0.2   0.109289
0.25   0.106339
0.3   0.10259
0.35   0.097825
0.4   0.092387
0.45   0.086231
0.5   0.07918
0.55   0.071791
0.6   0.064189
0.65   0.056473
0.7   0.048533
0.75   0.040399
0.8   0.032089
0.85   0.023613
0.9   0.014982
0.95   0.006205
1   -0.00102
1.05   -0.009797
1.1   -0.018574
1.15   -0.027205
1.2   -0.035681
1.25   -0.043991
1.3   -0.052125
1.35   -0.060065
1.4   -0.067781
1.45   -0.07517
1.5   -0.082221
1.55   -0.088377
1.6   -0.093142
1.65   -0.096891
1.7   -0.099841


Code: Select all

   Cp
   [-]
0   0.054598
0.05   0.05778
0.1   0.054248
0.15   0.055037
0.2   0.055782
0.25   0.056638
0.3   0.057253
0.35   0.057347
0.4   0.056991
0.45   0.056055
0.5   0.054261
0.55   0.051865
0.6   0.048889
0.65   0.045346
0.7   0.041154
0.75   0.036288
0.8   0.030725
0.85   0.024439
0.9   0.01741
0.95   0.009617
1   0.002719
1.05   -0.005074
1.1   -0.012867
1.15   -0.019896
1.2   -0.026182
1.25   -0.031745
1.3   -0.036611
1.35   -0.040803
1.4   -0.044346
1.45   -0.046742
1.5   -0.048536
1.55   -0.049472
1.6   -0.049378
1.65   -0.048763
1.7   -0.047907


[jwocky]

Hi Bomber,

problem #1
The exchange of the engine and propeller for example by dropdown menu will be a hard one. I think, there are some properties involved that can't be written at runtime and it would anyway call for a complete FDM-reset, right? So this is more like reaching the limits of the concept.

Problem #2
I haven't checked on all details, but it makes sense the way you explain it. The problem is however, the rpm are still too high. Now, you can over-rpm any motor if you for example drove a car too fast down a long hill, I wonder why we have a similar effect here. Alas, I have currently no time, I promised a BBJ2 and my serial killer work also just called again, but right now, I feel, I may should go the whole way and do the math of engine and propeller from the scratch. There is something, we miss.

J.

[bomber]

Well 1) was just wishfull thinking...

I don't however see how the rpm is too high... remember the engine can only grunt a max of 75kW so in the following table this is a fixed value.. the rpm varies depending upon the load on the prop, the faster the plane goes the less load....kinda like peddling a bike in high gear, hard to start but once up to speed the load on the legs drops off... Also remember the plane cannot go any faster than 57m/s so any values above this are applicable.

Code: Select all

v      rpm      Power
[m/s]   [1/min]   [kW]
0.000   2391      75
3.480   2347      75
7.100   2397      75
10.60   2385      75
14.07   2374      75
17.50   2362      75
20.93   2354      75
24.40   2353      75
27.94   2357      75
31.61   2371      75
35.51   2396      75
39.65   2433      75
44.11   2481      75
49.00   2544      75
-----------------------------
54.51   2628      75
60.90   2740      75
----------------------------
68.67   2897      75
78.74   3126      75
93.36   3500      75
120.1   4266      75
190.7   6500      75


So even when at the top speed of the plane this particular prop will be spinning less that 2750rpm.

[jwocky]

Well too high compared to the entry in the bible which says between 2300-2400 rpm at maximum throttle.

[bomber]

Code: Select all

v      rpm      Power
[m/s]   [1/min]   [kW]
0.000   2391      75

at zero m/s ie static the rpm is 2391...

[SHM]

Maybe I should learn FDM development

[IAHM-COL]

YEAH!
I Know
Now this is the thing:
JWocky promised a JSBsim tutorial, per parts... But, it's just he is soo busy with a handful of projects.
But FDM dev sounds like a cool skill.

I did some tweaking in the Hondajet, but that was just scratching the surface.

[bomber]

Well we've not even started on the juicy bits...

Understanding engine and prop design is a start, and getting a process that people can follow and reproduce with results that stand up to rigorous challenge by our fellow flight sim enthusiasts is a goal that's worth achieving.

And it might encourage changes of the jsbsim code to fill in any gaps we expose.