C421C advanced physics: the real stuff for OEI training

[AndiKunzi 62]

C421C advanced physics

 

1. Why I developed the advanced physics version

I was asked via this forum, via PMs and also by e-mails to provide a modified version of the Alabeo C421C which can be used for serious multi-engine training. It should have accurate type specific performance for both AEO (all engine operative) and OEI (one engine inoperative), the latter for either a windmilling prop at various speeds and for a feathered engine. Feathering should work as realistic as possible.

Thus, I developed an advanced physics version of the Alabeo C421C. It shows realistic physics for negative thrust (prop drives engine). This is necessary for OEI training and more realistic idle power simulation.

 

2. Where to download

The modifications to the “c421.air” and “aircraft.cfg” files,
aircraft data, equipment list, checklist and other documents for N100L,
as well as some images of C421C, registration N100L, can be found here:

https://www.dropbox.com/sh/9fbvh4w0lpkv3zk/AACFtH_FhttbtkGkuZB8iTB-a?dl=0

Unfortunately, due to copyrights, I can’t upload the full files and can’t provide the AFM and other copyright protected documents.

 

3. How to install

To update “c421.air”:
- Copy the original Alabeo C421 to another aircraft folder, e.g. “C421C advanced physics”.
- Use AirEd (freeware) and open the original “c421.air”.
   Open AirEd a second time and open “modifications advanced physics c421.air”.
   Copy the modified tables 511 and 512 to the original .air file
   (right click in modification file -> copy -> right click on original file -> replace from clipboard)

To update “aircraft.cfg”:
Just copy the modifications to the corresponding position in the original file.

 

4. Legend of modifications

Modifications in aircraft.cfg:
- Basic Empty Mass (actual mass for N100L 5,154 lbs inserted; most C421C are more heavy, ours is on diet)
- correct basic data for engines + props
- engine friction included (necessary for all negative thrust scenarios)
- flaps lift + drag scalar adjusted (split flaps)
- 2 vacuum pumps
- VMO = 151 KIAS; VNE= 240 KIAS
- cabin: max. differential pressure: 5.0 psi

Modifications in c421.air:
- implemented negative thrust scenarios in prop thrust and efficiency curves (tables 511 + 512); adjusted thrust curves for high TAS at high ALT

 

5. Side effects of modifications:

- more realistic engine starting:
starting the engine needs the throttle to be 2 cm above idle (see checklist) for the first 15 seconds

- if you start the engine and don’t retard the prop to medium RPMs immediately, you will overstress engine and prop. So be careful when doing an air start, you might lose the prop and the aircraft. (In the real aircraft, these effects are less severe, but you have to avoid high RPMs until the engine is warmed up and have to avoid rapid change of RPM.)

 

6. Additional Notes:

- Basic aircraft data as published by the manufacturer, like range and service ceilings, especially OEI, are based on MTOM.

- The use of high payloads close to MTOM and higher ambient temperatures are making for a more challenging OEI training.

- In the sim, climb power is too low compared to full power (about 65 % instead of 75 %). I have to find a way to adjust that. 

- I have buttons for engine failure under the mixture (see images)

- Seat Position is, like in many Aircraft, higher than as per Default by the sim. For the C421C and me being 5 ft 7 in, that's about 7 clicks arrow up.

 

Stay sharp!
Enjoy the training!

Best regards,

Andi

Edited June 7, 2018 by AndiKunzi

[Les Parson 99]

Andi, 

A thousand thanks for the mods! Will get to work on these straight away. 

 

[rightseat 277]

Wow! Thanks, Andi! 

We really appreciate the effort! 🙂

[Ted Striker 338]

Thanks for sharing your work Andi. You’ve just convinced me to make my first Carenado purchase......If I have any funds left after I get back from the FSExpo in Vegas.

Ted

[gregmorin 199]

Thanks Andi!

Greg

 

[RobertBernardK 7]

A huge congrats, and big thanks, for make this plane to some basic "Alabeo"  to a closer tool for training 421C, 

Without doubt best plane of Alabeo at the moment with your help. 

Redgards 

Robert Bernard 

[AndiKunzi 62]

If you wonder about the brutal effect when an engine goes inop: try it in the real plane. The asymmetric thrust is an eye opener until you feather the dead one. The advanced physics C421C is not exaggerating here. That's why EFATO is leading to VMC rolls (speed below red line -> aircraft uncontrollable) by far too often. If pilots train for that using a sim, they will be able to handle this kind of emergency. 

Edited June 7, 2018 by AndiKunzi

[qqwertz 770]

Hi Andi,

how could you possibly do that? I was firmly set on NOT buying this plane because I already have their PC12, which fulfills a similar role. But your mod is making me really curious. 🙂

My question was actually sort of serious: I really would like to know how you created this mod, in principle. Are you using the laws of physics, or are you trying to reproduce the RW numbers by a lot of trial and error?

Best,

Peter

 

[AndiKunzi 62]

3 hours ago, qqwertzde said:

My question was actually sort of serious: I really would like to know how you created this mod, in principle. Are you using the laws of physics, or are you trying to reproduce the RW numbers by a lot of trial and error?

First, I must admit that I was rarely flying sims since the early 1990s. Before that, I learned quite a lot when training instrument procedures on my home sims (LAS86 / LAS90, instrument trainer). That started in 1985 - I was 15 by then and addicted to flying.

September last year, I decided to go into sim flying again for training emergencies. I found out that many profesional sims for General Aviation aircraft are still based on FS2004 or similar. I evaluated FSX, P3D and XP11. XP11 hat strong points, but could not offer me a cessna 340, 40X, 41X or 421 twin with a GTN750. Also, I hoped on LMs power for future development (and still do).

No piston twin on neither platform I tested showed realistic behaviour for windmilling props. XP11 was by far best, however, their Baron G58 climbs at 250 ft/min with a windmilling prop at blue line and the prop turns only 600 RPM then. Min prop pitch might be the case, or few resistance of the engine. When you cut the mixture at blue line speed in the real plane and the prop is forward, it will continue to be windmilling at rather high speed, at least 1500 RPM, causing a lot of drag. Feathering on XP11 was nice, though - contrary to FSX and P3D.

I opted for P3D. I asked for help - also here at AVSIM - to get a better simulation of windmilling and feathering, but found no solutions. I only found the answer once that there is no solution. I read about AirEd here and found tables 511 and 512. I tried to understand what they are doing. I could find an old documentation which helped me understanding. I found out that the settings for the prop being driven by the wind (negative angle of attack) were completely off. A simple approach would be to have kind of symmetric values for thrust vs. positive and negative AOA. I calculated some values using prop geometry and my experiences about prop RPM at idle engine vs. airspeed, and filled the table by interpolation and extrapolation.

On the other side of the thrust equation, the reason for the low (basically missing) negative thrust of the prop was obvious: the engine did not have substantial resistance, especially at higher RPMs. I found settings for that in the aircraft.cfg.

(Due to the similarity of those tables and settings to the ones I found on default single engine piston aircraft, where simulating windmilling and feathering is not that important, I believe that for many aircraft, those settings had been copied over decades and just got some alterations.)

After entering the supposed settings in tables 511 and 512 and guessing that engine resistance was off by a factor of 2, the first flight test was more than I expected: the prop was windmilling at a more realistic medium speed, cost me 300 ft/min compared to feathered and feathering worked. 

The following 3 days I slept for about 12 hours cumulated, did quite a bit of fine tuning and simulated about 200 EFATO at places where I would never even simulate that with the throttle lever in a real plane: out of St. Moritz, out of Aspen, intersection take-off at my homebase Stuttgart over the Weidacher Hills and so on...

Now I want an Aspen EFD1000 in the C421 and realistic leaning. I gave up on both.

Edited June 8, 2018 by AndiKunzi

[qqwertz 770]

Very interesting. Thanks for sharing, Andi.

Peter

[rightseat 277]

I am really enjoying the mod, Andi. Thanks for making it. I had fun simulating engine out procedures,  and you're right, it IS a handful near near VMC.

I know you did touch it, but can I ask if you feel the yaw damper is actually doing its job in the sim? I can't tell it's doing much because I still do a lot of pedal dancing to stay coordinated with two engines running.

Thanks,

Scott

[AndiKunzi 62]

Hi Scott,

Glad you like the mod. Thank you!

I must admit that I didn‘t focus on the yaw damper so far. The shorter C340 and C303 need one, the C421 is stable without and thus, I don‘t have one in N100L.

For OEI training on the C303 25 years ago, we switched of the yaw damper. Thus, I don‘t know how it would have helped (assumption: not much, since it had limited rudder travel). I know it helped on the C303 in turbulences. The C421 is rock stable even in severe turbulences. Beside turbulences, the YD won‘t help staying coordinated. It is just dampening yaw oscillations. Twins are a bit sensitive to them.

I‘ll try the YD within the next days and let you know if I think it is working as supposed.

Edited June 8, 2018 by AndiKunzi

[rightseat 277]

Don’t put yourself out, Andi, but if you get some free time, I’d like to get your opinion for sure.

 

many thanks.

[akmatov 4]

Many Thx, awesome stuff.  You have a great instrument suite!  Wish Alabeo had used your c421 as it's base model.

[AndiKunzi 62]

I contacted them about half a year ago and offered my help (for free)...

If someone should know someone there, I would be happy if I‘d be allowed to provide the full files for download.

Edited June 9, 2018 by AndiKunzi