Trim and pitch balance - How does it really work?

Hey folks,

I'm trying to understand how to properly trim the L-39C so that it keeps level flight without much interaction required by the pilot. I'm aware that the trim system is not an auto pilot and trimming doesn't necessarily mean that you can go AFK for some minutes and find the plane still in level flight and on course.

The main reason for my question is that I find the L-39C to wildly oscillate in pitch which makes the plane very hard to control. This is enormously annoying when flying, e.g., mission 2 of the Kursant campaign, where you have your hands full with referencing visuals with your kneeboard, calculating ETA etc. I find it nearly impossible to have enough time for those things while trying to keep the aircraft level and not having the instructor yell "Altitude 200" at me all the time. And due to me yanking on the stick all the time, I easily get off course and speed as well. And due to the low level flight at 200m, the pitch oscillation quickly brings me down to uncomfortable levels which is why I HAVE to constantly watch the VVI.

I'm very confused now because I have all those different concepts in my mind. AFAIK you trim for a certain angle of attack (translating to a certain speed). I read the awesome book "Stick and Rudder" a while ago and I had many "AHA!" moments. Armed with all that (half-absorbed) knowledge, I tried to apply it to DCS... and failed miserably. According to the book (IF I understood it correctly), if left to its own devices, i.e. no control input, the plane will always try to fly at a given AoA (~ speed) determined by trim. If it's too slow, it will pitch down until it reaches its designated speed, if it's too fast, it will pitch up. Also, the book goes a long way to hammer into your head that the stick is an AoA/speed control and not a height control and the throttle vice versa. Still, the L-39C throttle has certain markings, e.g. Cruise Speed which suggests to just leave it there and do everything else with the stick and trim. Where's the missing link between the book and practical application in DCS? I highly doubt that the principles in that book have become obsolete since they are so fundamental.

I tried to find out if the plane really stabilizes on a certain speed when left to its own with the Free Flight instant action. Setting throttle to Cruise Speed, neutral elevator trim and leaving the stick alone (except for an occasional small aileron correction to keep the plane level), the plane oscillates wildly. It climbs and dives with huge spikes until it finally somewhat settles after around 10 minutes. It still oscillates but the frequency is so low that each cycle takes a very long time now. Also the amplitude decreases with every cycle. During all that time, on average, the plane climbs. After 10 minutes it has climbed from ~2000m to 5700m. Speed at this point oscillates somewhere around 460km/h. So I guess it does indeed try to attain a certain speed but without any help from the pilot it just takes a very long (maybe even asymptotic?) time to actually reach it?

Incidentally, in the flight manual, it says that the plane is balanced at 380km/h with neutral elevator trim and no stick input. How does this relate?

How do those observations help me in keeping the plane straight and level without constant attention? Or better asked:

Given a certain speed and altitude, what are the specific steps to take to attain those parameters and trim the plane to keep it straigt and level with only an occasional small correction?

Thank you! :)

amatuer point of view:

if it pitches up, it looses speed > slowly pitches down > gains speed > pitches up again.

so you have to use trim (which is a bit crude, no idea if its that crude irl) and throttle to maintain a neutral pitch.

and just accept small changes in pitch up down and correct them yourself.

just be happy its not a warbird with prop-torc :)

my guess is that in no wind conditions the plane is slated to fly at published speed hands off. If there is windage then that speed will vary.

In regards to procedures in flight of calculating TSD and navigation, and since mission 2 is VFR flying, I would suggest you attempt to open the map and take screenshots of your route and print them. That should keep your eye on the road as per VFR scenic flying while keeping one eye on your instruments.

There is no sweet spot where you actually could fly it hands off, IMHO. What I do to adhere to the strict parameters imposed is to trim nose down so that during level flight I am slightly pulling back on stick. VVI becomes neutral but speed changed so I manipulate throttle alone, while using stick to control my VVI since its vey responsive. So when speed is reached I keep throttle always moving and not static. Allow me to refly it with a video.

Hey folks,

I'm trying to understand how to properly trim the L-39C so that it keeps level flight without much interaction required by the pilot. I'm aware that the trim system is not an auto pilot and trimming doesn't necessarily mean that you can go AFK for some minutes and find the plane still in level flight and on course.

Trim is primary to eliminate stick forces (not so important for simming) to be able to make small corrections around the center with good feedback from the stick. IRL you will notice if the plane wants to pitch up/down because the stick will push your hand around.

The main reason for my question is that I find the L-39C to wildly oscillate in pitch which makes the plane very hard to control. This is enormously annoying when flying, e.g., mission 2 of the Kursant campaign, where you have your hands full with referencing visuals with your kneeboard, calculating ETA etc. I find it nearly impossible to have enough time for those things while trying to keep the aircraft level and not having the instructor yell "Altitude 200" at me all the time. And due to me yanking on the stick all the time, I easily get off course and speed as well. And due to the low level flight at 200m, the pitch oscillation quickly brings me down to uncomfortable levels which is why I HAVE to constantly watch the VVI.

At 200m you should check the cockpit only for important information on your instruments IRL, you can't read briefing material. In SP pause the game and take a look.

I'm very confused now because I have all those different concepts in my mind. AFAIK you trim for a certain angle of attack (translating to a certain speed). I read the awesome book "Stick and Rudder" a while ago and I had many "AHA!" moments. Armed with all that (half-absorbed) knowledge, I tried to apply it to DCS... and failed miserably. According to the book (IF I understood it correctly), if left to its own devices, i.e. no control input, the plane will always try to fly at a given AoA (~ speed) determined by trim. If it's too slow, it will pitch down until it reaches its designated speed, if it's too fast, it will pitch up. Also, the book goes a long way to hammer into your head that the stick is an AoA/speed control and not a height control and the throttle vice versa. Still, the L-39C throttle has certain markings, e.g. Cruise Speed which suggests to just leave it there and do everything else with the stick and trim. Where's the missing link between the book and practical application in DCS? I highly doubt that the principles in that book have become obsolete since they are so fundamental.

Like Nero said: Set your desired attitude (with the stick) with your desired speed (throttle). By

that you will force a certain AOA for that attitude and speed. Now trim neutral (no force on stick). If you change your attitude or speed (again stick/trim) you need to trim again.

I tried to find out if the plane really stabilizes on a certain speed when left to its own with the Free Flight instant action. Setting throttle to Cruise Speed, neutral elevator trim and leaving the stick alone (except for an occasional small aileron correction to keep the plane level), the plane oscillates wildly. It climbs and dives with huge spikes until it finally somewhat settles after around 10 minutes. It still oscillates but the frequency is so low that each cycle takes a very long time now. Also the amplitude decreases with every cycle. During all that time, on average, the plane climbs. After 10 minutes it has climbed from ~2000m to 5700m. Speed at this point oscillates somewhere around 460km/h. So I guess it does indeed try to attain a certain speed but without any help from the pilot it just takes a very long (maybe even asymptotic?) time to actually reach it?

yes, it's a long time. How long depends on how stable the the A/C was designed. The A/C will overshoot the AOA you set with trim. If you trim for 8°AOA and are at 4° and you let the stick go (and no throttle change) your plane will pitch up and pass 8°. Then it will get too slow/AOA to big and will pitch down again

Incidentally, in the flight manual, it says that the plane is balanced at 380km/h with neutral elevator trim and no stick input. How does this relate?

It also gives you a total weight. For that speed and weight, at 0 pitch trim there are no stick forces. The manuals tries to explain there how and why you trim and not how to fly the plane.

How do those observations help me in keeping the plane straight and level without constant attention? Or better asked:

The planes needs your constant attention but the trim helps you to keep the attention low.

Given a certain speed and altitude, what are the specific steps to take to attain those parameters and trim the plane to keep it straigt and level with only an occasional small correction?

 

Thank you! :)

See above.

The L-39 is one of the best modules and the best one to learn this stuff. Have fun! :pilotfly:

I generally find that I can hit a balance between power and trim to where I can 'nearly' fly hands-off. I like to get it to a point where corrections to course are done with the trimmer switch.

my guess is that in no wind conditions the plane is slated to fly at published speed hands off. If there is windage then that speed will vary.

This is one of the most common misconceptions among non pilots.

(steady) wind doesn't have any influence on aircraft handling and performance in IRL (or in any sim), never, (except in e.g. the DCS F-5).

Thank you all for your comments. After a lot of fiddling with my input curves and giving myself exercises (e.g. increase speed to X or change altitude to Y while keeping the other parameters constant) I can stabilize my VVI more or less around 0 with a reasonable amount of required pull on the stick.

It's more difficult when flying low because there's not much room for error and level turns are still quite difficult but I think I'll get there eventually. :)

This is one of the most common misconceptions among non pilots.

 

(steady) wind doesn't have any influence on aircraft handling and performance in IRL (or in any sim), never, (except in e.g. the DCS F-5).

True, that's also a point handled in that book. I find the analogy very fitting. Imagine you're in a train and cannot look outside. If the train moves at a constant speed you cannot know in which direction the train is moving (~ A/C movement WITH the wind). You can only feel your own movement inside the train (~ A/C movement THROUGH the wind).

(Ofc you can feel the bumps in the road but they still don't tell you direction (~ sudden gusts of wind))

In regards to the level turns with the plane, this is a load factor issue EG:

in the campaign, often times you are required to make a 45 degree bank turn... In order to hold level altitude and not increase it (too much back-pressure on stick & reduction in airspeed) or decrease it (not enough back-pressure on stick & increase in airspeed)

What is needed is to "pull" a certain amount of "G" in the turn, to maintain level flight... Not too much and not too little... Its kinda a Goldilocks zone type thing.

check out this here it explains it well:

http://www.boldmethod.com/learn-to-fly/aerodynamics/why-does-stall-speed-increase-with-bank-angle/

Then after reading that check out this accelerated wing stall IRL (Which if you fly the P-51 in DCS you know darn well how easy it is to over-control and get into)

http://www.boldmethod.com/blog/video/2015/03/pilot-skims-the-water-in-air-race/

Also in a turn your stall speed is higher, which means you will stall sooner in a banked turn (accelerated wing stall)

So you need to add power in turns as well (to keep from stalling the aircraft wing)