FSX-MS Question regarding aircraft trim settings

[Hammer308 4]

Am flying primarily addons for the default aircraft in FSX Gold, along with the Honeywell FMC Mod. But am having an issue with having too high of a pitch angle during cruise. I've tried setting various settings during pre-flight, but its pretty hit & miss. The FMC mod doesn't give any info for this.

Is there a general way to determine this in order to set it properly?

Any info is appreciated.

 

 

[lzamm 143]

You haven't provided much info as to what you're trying to do or which aircraft you're using.

In general, lift (a) depends on air density (b),  speed (c) and angle of attack (equivalent to pitch angle in stable, level flight).  So if your pitch angle is too high, then you're either (a) too heavy, (b) too high or (c) too slow. In any combination, of course.

The Honeywell FMC won't help with that as it's a generic add-on that is not specific to any aircraft.

 

[FDEdev 955]

Furthermore trim has nothing to do with the pitch attitude and what is 'too high' in your opinion? E.g. 2°-3° is quite normal for most jets.

[Hammer308 4]

I'm usually flying addons for the default A320/21, B738, CRJ-700, B744, also flying Posky & Camsim addon a/c, plus many others incl. bizjets, etc.

I know what affects trim, but I normally have to guess what stab trim to set during pre-flight & no matter what I try, I usually have an in-flight nose-up attitude of somewhere around +8 to 10deg. even when I have the stab trim set in the green area during pre-flight. This happens even if I have the weight balance generally neutral, and doesn't matter what a/c I'm flying in level flight.

I'm looking for a relatively easy way to determine what stab trim to set during pre-flight so I don't get so much nose-up attitude during flight.

[FDEdev 955]

1 hour ago, Hammer308 said:

I usually have an in-flight nose-up attitude of somewhere around +8 to 10deg. even when I have the stab trim set in the green area during pre-flight.

I'm looking for a relatively easy way to determine what stab trim to set during pre-flight so I don't get so much nose-up attitude during flight.

8-10deg is way too high. Looks like your airspeed indicator shows true airspeed instead of indicated airspeed.  

Again, stab trim has absolutely nothing to do with the attitude during flight. The stab trim setting is only important to be able to rotate at the correct VR on takeoff. 

There is no easy way to determine the stab trim setting since it usually varies a lot with weight, CG and flap position.

 

 

Edited February 17, 2020 by FDEdev

[lzamm 143]

11 hours ago, Hammer308 said:

know what affects trim, but I normally have to guess what stab trim to set during pre-flight & no matter what I try, I usually have an in-flight nose-up attitude of somewhere around +8 to 10deg. even when I have the stab trim set in the green area during pre-flight.

Take-off trim has nothing to do with cruise trim - the speeds are very different, also flaps have an effect.  You are trimming in flight, aren't you?

If you're leaving the trim in the takeoff setting it's quite likely the plane will settle in a low-speed, nose-up attitude in cruise, hands off.

Edited February 18, 2020 by lzamm

[Angels40 17]

14 hours ago, Hammer308 said:

I'm usually flying addons for the default A320/21, B738, CRJ-700, B744, also flying Posky & Camsim addon a/c, plus many others incl. bizjets, etc.

I know what affects trim, but I normally have to guess what stab trim to set during pre-flight & no matter what I try, I usually have an in-flight nose-up attitude of somewhere around +8 to 10deg. even when I have the stab trim set in the green area during pre-flight. This happens even if I have the weight balance generally neutral, and doesn't matter what a/c I'm flying in level flight.

I'm looking for a relatively easy way to determine what stab trim to set during pre-flight so I don't get so much nose-up attitude during flight.

 

I use the default B744 and its default settings (as per its configuration file) will not resemble anything like the real aircraft.   I have had to make some changes to bring it somewhat closer to its real aerodynamic values.   With that in mind, you will need to use trim settings, etc. that are different from the real aircraft.   

For example, the real B744 might use 3 to 6 units of trim, depending on aircraft weight, flap setting, runway, wind data.  But in FSX, you will have to be satisfied with using 7.5 to 9 units of trim.   So, to give you an example of a flight:   you have a take-off weight of 800,000 lbs, on a 12,500 ft runway, with a moderate headwind and at 50 ft above sea level.   So without having the necessary data from the FMC and elsewhere, you will probably need 8.5 units of trim, 20 deg. of flaps, and 94% thrust (may be derated).   At a V1 of 150 kts and Vr at 160 kts, you will climb easily with minimum of yoke pull back.   However, as soon as you reach V2 and a positive climb rate, you will need to pull back on your trim (to about 8 units) or else either you will climb at too high a rate or you will be pushing your yoke forward to prevent a stall.   After you complete your turn to your first waypoint/SID and you engage the autopilot, then the autopilot will control the trim.   Your next responsibility (as per your problem), will be to raise flaps incrementally at correct speeds, stay under 250 kts below FL100 and maintain altitude as per ATC instructions.   Your initial altitude may be around FL300 to FL320;  if you climb further without first reducing weight (i.e. burning off fuel), then you will have the problem you're seeing:  high angle of attack.   The B744 typically has a 3 deg aoa at cruise, but that figure varies slightly depending on other factors.

So, as has been suggested, take off trim is only for taking off (to lift the aircraft off the runway with a small movement of the yoke).   Once in the air, and if you're hand flying, the trim should be brought back to 7.5 to 8 units.   If you would like a copy of my take off settings for the B744, just PM me.

Chris

[FDEdev 955]

5 hours ago, Angels40 said:

At a V1 of 150 kts and Vr at 160 kts, you will climb easily with minimum of yoke pull back.   However, as soon as you reach V2 and a positive climb rate, you will need to pull back on your trim (to about 8 units) or else either you will climb at too high a rate or you will be pushing your yoke forward to prevent a stall....

 After you complete your turn to your first waypoint/SID and you engage the autopilot, then the autopilot will control the trim.   Your next responsibility (as per your problem), will be to raise flaps incrementally at correct speeds, stay under 250 kts below FL100

A few notes; You don't want to start the rotation with a minimum pull on the yoke because this greatly increases the risk of a tail strike and you need to re-trim as soon as you are airborne. 

Trim is usually set so that a significant pull is required to start the rotation and you can maintain at least V2+20 without the need to re-trim. If you need to re-trim during the initial climb it's much safer if you have to apply nose up trim than if the aircraft continues to pitch up on its own.

Most pilots perform the flap retraction before engaging the AP. One reason is that the acceleration for flap retraction usually involves a considerable decrease in pitch attitude which is much faster when performed manually.

Btw, At 800000lbs you will have to increase the speed above 250kts during the clean up, even below FL100/10000ft 😉    

[Angels40 17]

8 hours ago, FDEdev said:

A few notes; You don't want to start the rotation with a minimum pull on the yoke because this greatly increases the risk of a tail strike and you need to re-trim as soon as you are airborne. 

Thanks for your comments, but I offer the following:   I would have thought that a greater pull on the yoke at rotation would make a much higher probability of a tail strike.   Even when observing this aspect in in-flight cockpit videos, one can observe only a slight pull back of the column by the PIC.   Your second paragraph would be correct if, as you say, a significant pull back of the yoke is required (hence lower trim units required).  My understanding is that the reason that a small movement of the column is required at rotation is because this allows the PIC to have the extra degree of available movement if the need arises.   Of course, the a re-trim will be required for climb out.

 

8 hours ago, FDEdev said:

Most pilots perform the flap retraction before engaging the AP.

This of course is the pilot's decision.   I've seen the AP engaged soon after the initial turn and en route to the SID.

 

8 hours ago, FDEdev said:

Btw, At 800000lbs you will have to increase the speed above 250kts during the clean up, even below FL100/10000ft

In the U.S., as you know, the 250 kts limit is maintained below FL100, (FAR 91.117 (a)).   So, if an aircraft can't safely maintain a speed of less that 250 kts, then the required amount of flaps would need to be set;  i.e. in some cases, an aircraft cannot fly in clean configuration till above FL100.

Just my thoughts.

Chris

[FDEdev 955]

2 hours ago, Angels40 said:

1. I would have thought that a greater pull on the yoke at rotation would make a much higher probability of a tail strike.   Even when observing this aspect in in-flight cockpit videos, one can observe only a slight pull back of the column by the PIC. My understanding is that the reason that a small movement of the column is required at rotation is because this allows the PIC to have the extra degree of available movement if the need arises.   Of course, the a re-trim will be required for climb out.

2. This of course is the pilot's decision.   I've seen the AP engaged soon after the initial turn and en route to the SID.

3. In the U.S., as you know, the 250 kts limit is maintained below FL100, (FAR 91.117 (a)).   So, if an aircraft can't safely maintain a speed of less that 250 kts, then the required amount of flaps would need to be set;  i.e. in some cases, an aircraft cannot fly in clean configuration till above FL100.

1. No, since you can feel the aircraft start to rotate with enough aft movement of the yoke and you have to consider that a real yoke requires considerable force to be moved, so even a 'slight' pull actually requires quite a bit of force.

Don't forget that the loadsheet isn't always precise. If the aircraft is more tail heavy than on the loadsheet, your aircraft might even start to rotate on its own before VR. Trim is not set to cater for additional pitch authority, but for speed/pitch stability after takeoff. 

2. Flap retraction is always based on altitude and has nothing to do with the initial turn or SID. Most pilots like to FLY, hence they usually engage the AP as late as possible. Especially in the sim where people fly much less, I highly encourage hand flying.

3. That's not how it is done IRL. You always fly at least the minimum clean speed after clean up at low altitude. In case of a heavy DC-10 this means >290kts!

Edited February 19, 2020 by FDEdev

[Angels40 17]

Thanks again for the time you've taken to respond.   Instead of adding comments to all what you've written, the only points that I would have trouble accepting are:

1.    Flap retraction is always based on altitude....    Since I was referring to the B744 in my response to the OP, I understand that flap retraction is undertaken based on speed and not altitude.   Altitude may play a part in the pilot's decision, but he adheres to the flap retraction markers that come up on his speed tape as his speed increases.   I agree with the rest of your point #2.

2.     You always fly at least the minimum clean speed after clean up at low altitude. In case of a heavy DC-10 this means >290kts!   Well, if ATC has permitted >290 kts below FL100, then so be it.   But for adherence to noise abatement regulations, that shouldn't be the norm.

At least I hope that the OP (Hammer308) has gained some benefit and insight from these responses.

Chris

 

[FDEdev 955]

37 minutes ago, Angels40 said:

1.    Flap retraction is always based on altitude....    Since I was referring to the B744 in my response to the OP, I understand that flap retraction is undertaken based on speed and not altitude.   Altitude may play a part in the pilot's decision, but he adheres to the flap retraction markers that come up on his speed tape as his speed increases.  

2.     You always fly at least the minimum clean speed after clean up at low altitude. In case of a heavy DC-10 this means >290kts!   Well, if ATC has permitted >290 kts below FL100, then so be it.   But for adherence to noise abatement regulations, that shouldn't be the norm.

1. Ok, bad formulated on my part. Flap retraction is based on speed but starts at a fixed altitude. Once you are at the correct altitude you usually reduce the pitch attitude and start increasing speed for the flap retraction according to the speed schedule. So the correct 'sequence' for flap retraction is altitude first, then speed.

2. You don't need an ATC permit to exceed the 250kts and it is the norm for heavy Heavies. ATC knows the approximate minimum climb speeds for the aircraft they are working with. Noise abatement is only applicable during the initial climb at low altitude. That's one of the reasons why flap retraction is based on altitude. 

Edited February 19, 2020 by FDEdev

[Angels40 17]

Thanks for your comments.   And noted your amendment.

Chris

Edited February 19, 2020 by Angels40