Cessna 210 "Centurion" family development topic

[dany93]

The disagreement may be due to improper propeller curves (other blade section? efficiency?) or to aircraft drag at cruise velocity.

I think I've found an a explanation: the Aeromatic propeller efficiency is almost equal to 1 (97%) in cruise conditions. Differently, the efficiency curves I've seen (NACA 640 measurements) are at max about 86%. The curves are close to each other for C_Thrust, but C_Power is about 15% lower for Aeromatic at useful advance ratio (J=1). Thus, the needed torque is lower for the Aeromatic one in the same conditions. Which gives that, for the same torque, either the propeller can spin at a given blade angle and higher RPM, or for a given RPM (here, 2500), it can spin with a greater blade angle. Hence, a higher airspeed. (not easy to explain, I hope not writing mistakes)

To summarize, the Aeromatic curves are slightly optimistic, above realistic usual ones IMO.

More recent propeller might have a better efficiency than those from NACA 640, but not 97%. Unfortunately (on purpose?), these values are not given by manufacturers.

[TheEagle]

Does anyone of you have an idea why /engines/engine[0]/thruster/torque is showing negative values while the aircraft is cruising happily at 180 kts ?
Also, how can I lower the idel rpm of the engine ? I've set <minrpm> to 100, but the engine is running 500 RPM with the throttle at 0. I've also included <min-throttle>0</min-throttle>.

[D-ECHO]

Is it possible that your propeller is turning counter-clockwise (CCW)? In this case, it'd be expected that the torque is negative during all positive-power operations.

For the idle RPM, set <idlerpm>

See also https://wiki.flightgear.org/JSBSim_Engines#FGPiston

According to the above wiki entry, minthrottle is deprecated/unused and minrpm isn't even an existing tag.

[S&J]

100 rpm idle ?

An ocean going liner yeh maybe, an aeroengine, not likely.

600-800rpm

[dany93]

Code: Select all

/engines/engine[0]/thruster/torque

is minus the JSBSim propeller torque, this is the value explained below.
JSBSim can be a "stand alone" FDM, independent of FG. Here, it sends its values to FG.
You can read the torque exerted by the propeller to the aircraft in

Code: Select all

fdm/jsbsim/moments/l-prop-lbsft

But, as written above, this is a torque exerted by the propeller to the aircraft. As the propeller is clockwise, this moment is negative (in the Body frame), but its value is the right one.
It is this one which is sent to

Code: Select all

/engines/engine[0]/thruster/torque

. Confusing, arguable, don't ask me why.... But the value is here.

[dany93]

Also, how can I lower the idel rpm of the engine ? I've set <minrpm> to 100, but the engine is running 500 RPM with the throttle at 0. I've also included <min-throttle>0</min-throttle>.

How much is the idle RPM in the POH?
If I remember well, the idle RPM is set by <minmp unit="INHG"> 10.0 </minmp>
In the wiki, you can read for idlerpm:

this value affects the throttle fall off and the engine stops running if it is slowed below 80% of this value. The engine starts running when it reaches 80% of this value.

It does not set the usual (real) idle RPM.

[TheEagle]

minrpm isn't even an existing tag.

I mean't <idelrpm>, of course

@S&J: I've set the <idelrpm> so low because I had problems wiith the starter. I'll dig into the POH and see if I can find the real idelrpm.

[S&J]

Use the particular engine hand book.

[dany93]

I've set the <idelrpm> so low because I had problems wiith the starter

Another solution would be increasing the starter torque or /and starter-rpm (I've not tried). Better if it works.

Code: Select all

  <starter-torque> {number} </starter-torque>
  <starter-rpm> {number} </starter-rpm>

starter-torque: A value specifying the zero RPM torque in lb*ft the starter motor provides. Current default value is 40% of the maximum horsepower value.
starter-rpm: A value specifying the maximum RPM the unloaded starter motor can achieve. Loads placed on the engine by the propeller and throttle will further limit RPM achieved in practice. The actual RPM needs to be more than idlerpm * 0.8 for the engine to start running.

However the engines starts with its current settings, even if slightly long.
The current effective idle RPM (about 900 if I remember well?) seems correct, usual.

Or, if lowering the <idlerpm> 700.0 </idlerpm> value in the file, not too much....

[EDIT]
I've observed.
The starter RPM is close to 900.
The engine starts slightly below 700 RPM, in less than 2 seconds.
The effective idle RPM is close to 1000.
I don't understand what you are blaming.
[/EDIT]

[TheEagle]

Okay … Do you think it's normal that the plane starts rolling on the rwy with the thrust at ~100 lbs, or should I increase the rolling friction ?

[dany93]

I've no measurements, but by guessing I would say that's normal. I think it is roughly the force exerted by two people who pull the aircraft on the ground.
If someone else has a better based opinion...

IRL, one important security rule is to run the starter with the brakes ON.

[TheEagle]

IRL, one important security rule is to run the starter with the brakes ON.

Well, that rule is there for a reason ! I will just test with a few other Cessna's how much thrust is needed to get them rolling, that will give us a rough idea.

[dany93]

Anyway, you have

Code: Select all

<rolling_friction> 0.02 </rolling_friction>

which is the usual value on a hard runway.

[S&J]

Isn't it the static friction value that's more relevant

[dany93]

No. The static friction is at rest, with the brakes "ON".