Can someone help me get to grips with stab trim?

[ezfrag]

Hi all. I’m hoping to get some help as I’m struggling with the “feel” of the challenger in the air. 
 

Firstly, is there a way to calculate the correct pitch trim based upon weight and balance? I can’t find a page in the FMC that will tell me what to set as stab trim before takeoff. I’ve seen and read people saying to shove it at 5 for takeoff so I tried this and it was way too light-nosed and started oscillating as I fought the nose wanting to rise during takeoff. I reset to the runway and dropped the stab trim to 3.6 and it was way better and felt much more stable during the climb out. So how do I determine this?

During landing I experienced the exact same thing with the nose feeling too light. Is it standard procedure to disconnect the AP and then reset the stab trim? I felt like I had to fight the aircraft onto the runway. Is there a pitch sync button or something that I’m missing? I found an AP pitch sync button on the yoke but it can’t be this because AP is disconnected during landing.

Can someone talk me through how they work with trim in the aircraft? My experience is with Airbus A320 and A350 where the aircraft  feels really stable when AP is disconnected. Also with the Airbus I’m used to making small inputs with the side stick with the aircraft holding attitude in between so you don’t have to hold the bank input or pitch input because the aircraft holds on its own. Clearly the Challenger doesn’t do this and naturally I know I need to fly it differently but it feels like the stability is all over the place as soon as I disconnect the AP.

Appreciate any help as I try to learn to fly this aircraft. Cheers!

Edited August 16, 2023 by ezfrag

[bpcw0001]

Yes, it seems to be a bit of an issue. Trim too much nose down, and the aircraft may climb nicely, but you can't get it off the ground. Trim to get her easily off the ground, and she'll shoot up in the air at a high pitch angle.

We're essentially missing a W&B tool that matches the HS650.

Also see 

 

From watching some Challenger takeoff videos, it appears that the pilots always trim the nose a little down soon after liftoff, which indicates a compromise in the TO trim setting between getting the aircraft off the ground, but not wanting to climb too steeply.

Maybe some of the real-world Challenger drivers can comment on that.

 

Meanwhile, I've resorted to flying with a fixed payload (4pax + 1FA 200lbs each, and 250lbs baggage).
My usual trips take around 6.000-8.000lbs of fuel, which I can handle with a 29% trim input into the FMC, yielding a trim value of about 4.6

More fuel will require more nose-down trim.

It's not perfect, you'll have to kind of pull her off the ground, but the ensuing climb is not as brutal and only requires little nose-down trim after liftoff to ease her into a reasonable climb rate.
 

 

Edited August 17, 2023 by bpcw0001

[VirtualGAaviator]

Trim is generally a function of Weight and Balance. The only simming solution I've found, thanks to FoxTrot Alpha Aviation, is the Pan-Aero weight and balance tool. The tool is specific to the C600/C650. Unfortunately, an Apple product is required to run the tool (i.e. Ipad, Iphone, etc.).

[bpcw0001]

17 minutes ago, VirtualGAaviator said:

Trim is generally a function of Weight and Balance. The only simming solution I've found, thanks to FoxTrot Alpha Aviation, is the Pan-Aero weight and balance tool. The tool is specific to the C600/C650. Unfortunately, an Apple product is required to run the tool (i.e. Ipad, Iphone, etc.).

I use that, too.

What's your experience? Have you found this tool to yield values that work well with the Hot Start 650 in all cases?

[VirtualGAaviator]

 

18 minutes ago, bpcw0001 said:

What's your experience? Have you found this tool to yield values that work well with the Hot Start 650 in all cases?

TBH, I rarely use the tool. I tend to set trim to some value I can't recall at the moment or just accept the default value. Now that Pan-Aero is on my radar again, I may use it more often. Maybe then I can talk about my experiences by updating this thread.

[bpcw0001]

11 minutes ago, VirtualGAaviator said:

 

TBH, I rarely use the tool. I tend to set trim to some value I can't recall at the moment or just accept the default value. Now that Pan-Aero is on my radar again, I may use it more often. Maybe then I can talk about my experiences by updating this thread.

Same with me. As written before, I usually fly with a fixed config that is accommodated by one trim setting.

However, I found the tool useful to figure out where to move the trim with extreme payloads or fuel loads.

[ezfrag]

@bpcw0001 & @VirtualGAaviator thanks for the insight. I have downloaded that tool and had a look and also have mapped the pitch trim to the hat switch on top of my joystick instead of on my throttle quadrant where it is slightly more difficult to adjust on the fly. I will then try multiple take-offs to see if I can find a workflow whereby I can rotate enough to get a positive rate and then trim down a certain number of clicks until I get a less aggressive nose-up intention from the aircraft. Appreciate the help.

[bpcw0001]

One of the issues is the very strong attitude change when retracting/extending flaps 20.

Typically, when flaps 20 are retracted on takeoff, you'll need a good deal of nose-down trim applied immediately.
From real-world takeoff videos, you can see that pilots do apply this node-down trim after flaps 20 retraction as well, but it doesn't look as pronounced and dramatic of a change as it is in the HS Challenger.

Same goes for flap 20 extension, which makes the aircraft "balloon" quite a bit. 
Not sure if the real thing does that as well to this extent.

[VirtualGAaviator]

 

On 10/10/2023 at 4:58 AM, bpcw0001 said:

One of the issues is the very strong attitude change when retracting/extending flaps 20.

Typically, when flaps 20 are retracted on takeoff, you'll need a good deal of nose-down trim applied immediately.
From real-world takeoff videos, you can see that pilots do apply this node-down trim after flaps 20 retraction as well, but it doesn't look as pronounced and dramatic of a change as it is in the HS Challenger.

Same goes for flap 20 extension, which makes the aircraft "balloon" quite a bit. 
Not sure if the real thing does that as well to this extent.

Rob Hammer, a real world Challenger 650 pilot, and an advisor to HS on flight modeling says the flight dynamics are pretty realistic.

[bpcw0001]

15 hours ago, VirtualGAaviator said:

 

Rob Hammer, a real world Challenger 650 pilot, and an advisor to HS on flight modeling says the flight dynamics are pretty realistic.

Well, good to hear.

We should all get a ride in a CL650 some day and see what it is really like.

[bpcw0001]

My point being:  a lot of simulations tend to exaggerate certain characteristics of an airframe in order to make a point.

I'm not claiming this here in our case, but still, the massive trim changes required to control flaps 20 retraction/extension are hard to believe. I'd definitely like to hear more real-world Challenger pilot input on this matter.

 

Edited October 16, 2023 by bpcw0001