A password will be e-mailed to you.

Welcome Guest 

Show/Hide Header

Welcome Guest, posting in this forum requires registration.
Pages: [1] 2
Author Topic: Constant speed prop
WarpD
Advanced
Posts: 81
Send Message
Post Constant speed prop
on: June 4, 2012, 22:26

How is Prepar3D calculating the correct blade angle to maintain a given prop RPM?

Ed Wilson
MikeSchroe-
ter
Lockheed Martin
Posts: 96
Send Message
Post Re: Constant speed prop
on: June 5, 2012, 12:49

It's utilizes a basic 1st-order closed-loop feedback controller where the input is a reference RPM derived from the prop lever and the output is the rate of change of the angle. The time constant is "prop_tc" from the sim.cfg. The beta limits are also specified in the [propeller] section.

Hope this helps,
Mike
WarpD
Advanced
Posts: 81
Send Message
Post Re: Constant speed prop
on: June 7, 2012, 11:57

Ouch... can't they do anything simple?? (just kidding!!)

Essentially a rather simplistic PID controller.
MikeSchroe-
ter
Lockheed Martin
Posts: 96
Send Message
Post Re: Constant speed prop
on: June 7, 2012, 12:58

Yep, basically with just the "P".
WarpD
Advanced
Posts: 81
Send Message
Post Re: Constant speed prop
on: June 18, 2012, 20:05

Ok... followup question... how does the sim derive the reference RPM from the prop levers? Since we only specify a maximum RPM... is it a strictly linear extrapolation for what RPM target value is desired?
MikeSchroe-
ter
Lockheed Martin
Posts: 96
Send Message
Post Re: Constant speed prop
on: June 18, 2012, 20:32

Min RPM is also specified. e.g:
[propeller]
min_gov_rpm= 1100

Linear interpolate between min and max based on prop lever position..
WarpD
Advanced
Posts: 81
Send Message
Post Re: Constant speed prop
on: June 19, 2012, 01:01

Ok... that makes sense... but in the case of a turboprop... is the min_gov_rpm still strictly prop RPM or is it the the value of N2 that's being governed? I've seen turboprops with values of 25520 for min_gov_rpm.

And let me say thanks for answering these questions.

Ed Wilson
MikeSchroe-
ter
Lockheed Martin
Posts: 96
Send Message
Post Re: Constant speed prop
on: June 20, 2012, 01:29

It's based on the engine RPM. In this case N2. This was originally implemented this way when modeling big WWII gear engines and the engine RPM seemed more relevant. I'll admit it seems a little odd with the turboprops.

Mike
WarpD
Advanced
Posts: 81
Send Message
Post Re: Constant speed prop
on: June 20, 2012, 10:25

Thank you Mike for the quality answers.

Ed Wilson
WarpD
Advanced
Posts: 81
Send Message
Post Re: Constant speed prop
on: June 20, 2012, 16:46

Being the 'genius' I am... I'm certain I'm missing something.
I'm trying to set the proper torque/prop RPM/Ng for a 350 and am running into some odd results. I can set the appropriate torque tables to values that match the actual aircraft's graphs. However, when I do this... the prop RPMs are never correct.

Torque needs to be running at around 6% with 70% N1 and the props should be at 1050. I can get 6% with 70% N1... but the props are at 870??

What have I missed?

Ed Wilson
MikeSchroe-
ter
Lockheed Martin
Posts: 96
Send Message
Post Re: Constant speed prop
on: June 21, 2012, 13:26

Heh, I find props to be the hardest thing to tune. Remeber there are two sides to the equation, torque in from the engine, but also torque absorbed by the prop turning through the air. You probably need to look at that table in the .air file.

Mike
WarpD
Advanced
Posts: 81
Send Message
Post Re: Constant speed prop
on: June 21, 2012, 18:01

I'm using two tables... #509 and #1508... one is Engine/Turboprop Shaft friction and the other is CN1 to Shaft Torque.

It is the combination of those two tables (sorry I don't recal their defined names) that I assume controls the torque. #1548 sets Atmospheric Density on shaft torque... and at sea level is set to 1... which means torque would be 100% of whatever the value is calculated by the other two tables... correct?

So... how is this affecting the actual prop RPMs? That's where my mind's drawing a big blank.

Ed
MikeSchroe-
ter
Lockheed Martin
Posts: 96
Send Message
Post Re: Constant speed prop
on: June 21, 2012, 18:18

Prop RPM results from the difference of the engine applying torque (as you desribed above) and the resistance of the blades through the air. The RPM stabilizes when those two are equal. Apply more engine torque and the rpm increases, increase the blade angle and the extra drag reduces the rpm. As with most things in physics, equilibrium occurs when the sum of the forces are zero, right?

The table I was referring to earlier is AIR_61S_PROP_PWR_CF, or #512. It takes two inputs: beta and advance ratio. It outputs the power coefficient required to turn the prop given those inputs (power and torque are proportional for a given RPM). A description of this power coefficient can be found in any textbook discussion on propeller dynamics. The key is to get the inputs and the outputs tuned in coordination for your aircraft.

Hope this helps,
Mike
WarpD
Advanced
Posts: 81
Send Message
Post Re: Constant speed prop
on: June 22, 2012, 09:14

Ok... so in the #512 array the lowest beta value I'm seeing is 10... does that mean that if the actual blade angle goes below 10 it still only uses the values for the beta 10 array?

As for books... I have the one you recommended in the other thread... the math hurts. :)

Ed
MikeSchroe-
ter
Lockheed Martin
Posts: 96
Send Message
Post Re: Constant speed prop
on: June 22, 2012, 11:27

Yes, that's correct. You can change that from 10 to something lower though.
Pages: [1] 2
Mingle Forum by cartpauj
Version: 1.0.31 ; Page loaded in: 0.102 seconds.

Products

Community

Learn

Support

Policies
Prepar3D Overview Latest News Image Gallery FAQ Privacy Policy
Model Converter Forum Videos and Tutorials Refund Policy End-User License Agreement
Model Placer Prepar3D SDK Case Studies Acceptable Use Policy
Developer Network Program Add-On Installation Contact Us
Exporting
© 2012 Lockheed Martin Corporation. All Rights Reserved. Forum RSS News RSS