Trying to Master Trim

Keystoner

Keystoner

Pre-Flight
Joined
Oct 10, 2021
Messages
73
Display Name
Display name:
Keystoner
First post here. To preface, I am a student pilot with about 25 hours experience. I’m training in a Cessna 150.

I’m writing to request assistance mastering trim. I understand the basics that trim is used to relieve yoke pressure. I can trim for cruise pretty well. I want to be able to utilize trim better, especially during landing. The idea of setting the trim to establish my desired approach landing speed, and then not having to worry about it, is very appealing to me.

I’ve found the “Handling Notes” for the Cessna 150 presented here to be very useful: http://www.pilotfriend.com/training/flight_training/fxd_wing/cessna150.htm

Here is an excerpt regarding trim:

From cruise to 1500 rpm four fingertip trims down and then back to 2000 rpm gives no flap minimum controllable. The same operation with only three trims down gives no flap slow flight at 60 knots. The same operation again but leaving the power at 1500 gives a 60 kt descent. The transition from pattern slow flight to descent requires only a reduction of power, easy.

Every 10 degrees of flap has a one-to-one relationship to a full fingertip turn of trim from the above configuration. From the 60-kt 1500-RPM power glide an application of 10 degrees flap lowers the speed to 50 kts. A full fingertip trim movement up returns the speed to 60 kts. 20 degrees does the same, as will 30/40 degrees.​

The author says there is a 1:1 relationship between one full fingertip turn of trim to 10 degrees of flaps. Add 10 degrees of flaps, the airspeed reduces 10 kts. After one full fingertip trim nose down, the airspeed regains the 10 kts. This repeats for 20 degrees and 30 degrees. So, after 30 degrees of flap insertion and 3 full nose down trims, I’m back to the same speed before any flaps? Why am I putting flaps in then? I thought to slow the plane down. This fellow uses this technique during landing. In his article (the link above), he suggests: from cruise, reduce power, add 3 trims nose up, and take them all out with each successive 10 degrees of flaps. Don’t I want to be a little slower on each leg of landing?
 
Trim is used to ease control forces. If you feel like you need it, use it. If not, don't. I never bothered with it in my Cessna 150, the aircraft was so light I never used it. I used trim in my Cherokee (ceiling mounted window crank!) though my partner never did, he's a bigger fellow and never felt the need. I couldn't land the Mooney without it, the stick forces are just too great.
 
Just set your pitch to achieve desired result, trim out pressure. Four fingertip trims is BS, too much thinking involved. Set the pitch you want, adjust trim to remove pressure, that's pretty much it. You are trimming the airplane to an airspeed, if you remove or add power the airplane will seek that airspeed, if you remove power the airplane will descend, if you add power the airplane will climb if you are not applying pressure. There are a lot of combinations of events and configurations that require trim changes, coming up with a separate recipe for each will slow you down. Just get the nose where you want, adjust the trim to hold it there. Keep in mind that going from climb to cruise means you lower the nose, the airplane will take a little while to accelerate, so it might require a couple adjustments to stabilize things.

I really don't even think about trim any more, it just happens. I check once in a while by letting go of the stick, if the plane doesn't move, I'm good, if not I adjust. It's really that simple.

You should become a master at setting trim and always try to fly in trim, it will make your flying more accurate and easier. It will also help when you go for your instrument rating.
 
PaulS said:
coming up with a separate recipe for each will slow you down.
Click to expand...
Thanks. This 'recipe' advice is good. I'm an engineer and I'm so inclined to try and make one for everything. I know it's interfering with my learning.
 
The C-150 (and other cessna's) have a very well designed trim/flap interactions. unlike many airplanes adding flaps reduces the trim speed. Generally 10 degrees of flaps will reduce the trim speed about 10kts. As I recall adding more flap doesn't change the trim speed much. (Definition Trim speed equals the speed that the plane will fly when hands off the controls).

With 150/172's you can pretty much set the trim for hands off at about 80kts on downwind, adding 10 degrees of flaps will slow you to 70kts for your approach speed. Add more flaps as desired. For most Pattern work I don't touch or need the trim in Cessnas and generally like to demonstrate flying the pattern with only one or two fingers on the yoke in smooth air for just about the whole pattern other than the round out, flare and touchdown.

In some cessna configurations I might have 1 to 3 turns (finger tip trims) of trim change between landing and takeoff to get the speeds I want.

Brian
CFIIG/ASEL
 
Keystoner said:
Thanks. This 'recipe' advice is good. I'm an engineer and I'm so inclined to try and make one for everything. I know it's interfering with my learning.
Click to expand...

Same here, there are a few instances a "recipe" may help, but I don't use them. Better to understand what is going on, then through repetition you automatically zero in on what it takes. Also remember things like fuel load, passenger and baggage load may make the airplane react differently, so don't get frustrated, just keep at it, set the trim, let things settle down, then readjust if necessary. After a while, you won't have to readjust as much.
 
Use trim as needed. I have never count fingertips, turns, or anything else. Trimming should become almost subconscious after a while, you feel a force on the yoke, you fix it. Calculating, counting, figuring, just takes a lot more mental power away from the task at hand.
 
PaulS said:
You are trimming the airplane to an airspeed, if you remove or add power the airplane will seek that airspeed, if you remove power the airplane will descend, if you add power the airplane will climb if you are not applying pressure.
Click to expand...
What airplane do you fly where trim sets a particular airspeed? It's not true of four types I've tried it in.
 
Keystoner said:
First post here. To preface, I am a student pilot with about 25 hours experience. I’m training in a Cessna 150.

I’m writing to request assistance mastering trim. I understand the basics that trim is used to relieve yoke pressure. I can trim for cruise pretty well. I want to be able to utilize trim better, especially during landing. The idea of setting the trim to establish my desired approach landing speed, and then not having to worry about it, is very appealing to me.

I’ve found the “Handling Notes” for the Cessna 150 presented here to be very useful: http://www.pilotfriend.com/training/flight_training/fxd_wing/cessna150.htm

Here is an excerpt regarding trim:

From cruise to 1500 rpm four fingertip trims down and then back to 2000 rpm gives no flap minimum controllable. The same operation with only three trims down gives no flap slow flight at 60 knots. The same operation again but leaving the power at 1500 gives a 60 kt descent. The transition from pattern slow flight to descent requires only a reduction of power, easy.

Every 10 degrees of flap has a one-to-one relationship to a full fingertip turn of trim from the above configuration. From the 60-kt 1500-RPM power glide an application of 10 degrees flap lowers the speed to 50 kts. A full fingertip trim movement up returns the speed to 60 kts. 20 degrees does the same, as will 30/40 degrees.​

The author says there is a 1:1 relationship between one full fingertip turn of trim to 10 degrees of flaps. Add 10 degrees of flaps, the airspeed reduces 10 kts. After one full fingertip trim nose down, the airspeed regains the 10 kts. This repeats for 20 degrees and 30 degrees. So, after 30 degrees of flap insertion and 3 full nose down trims, I’m back to the same speed before any flaps? Why am I putting flaps in then? I thought to slow the plane down. This fellow uses this technique during landing. In his article (the link above), he suggests: from cruise, reduce power, add 3 trims nose up, and take them all out with each successive 10 degrees of flaps. Don’t I want to be a little slower on each leg of landing?
Click to expand...

Mastering use of trim is an admirable goal, but I am not sure this is a good way to do it. I've never taught anyone to memorize trim amounts. If the airplane is in steady-state flight (constant airspeed and level or constant vertical speed) it should be in trim. You can check whether it's in trim by periodically letting go of the yoke and making sure there is no pitch change.

The second paragraph of your quote is not telling you how to trim, it is merely describing a relationship, and familiarity with that relationship can help you know how much trim to use and which direction.

Extending flaps in a C150 usually requires slight nose-down trim or the airspeed will be reduced excessively. Flaps not only add drag, which slows the airplane down, but in a high-wing low-tail airplane they increase the airflow and downwash on the tail and cause a nose-up pitching moment.
 
Slightly off topic but one handy rule-of-thumb for 172 trim is that full up trim will give you hands off best glide speed - 65kts- with no power. Just roll that wheel all the way down when your instructor/examiner pulls the power or of course should the engine fail for real. Otherwise just trim for the pitch/airspeed you want to maintain. Those handling notes to me only seemed to make a simple concept complicated. Have fun with your training.
 
MauleSkinner said:
What do your airplanes do when you add or reduce power without changing trim?
Click to expand...
Pitch up/down, and change speed accordingly. For example, I climb out and fly final in the J3 at ~60, but there's 9 cranks of nose-up trim between takeoff and landing.
 
There is an established trimming procedure in the Canadian flight training syllabus, and it seems to me that it's also in the FAA stuff. Trimming can be done the wrong way in too many ways, and it results in frustration and sloppy flying. And if you can't get it right in a light featherweight airplane like a 150, you won't find it any easier in a heavier one. Best to learn it right.

APT: Attitude, Power, Trim. You're in the climb and have the airplane trimmed so there's no pressure to hold the airspeed. Then you level off, and this is where the trouble starts. So many PPLs I flew with would level off and at the same time pull the throttle back and trim. The airplane would slowly accelerate, which made it raise its nose and start climbing, so they'd trim some more. And then some more. And then they'd get a little too much trim in and it would start descending. So much fooling around.

Leave the throttle in, push so that the airplane stops climbing, wait until you reach your cruise airspeed, and then set the throttle to hold that speed, THEN trim the pressure off. One and done. In a more powerful airplane you might need to take some of the pressure off while you wait for the airspeed, but leave some pressure in until the airspeed and throttle are set. Attitude first, then power, then trim. Be patient.

Starting descent? PAT. Power first, then attitude, then trim. Power back, hold the nose up until you get your descent airspeed, then lower the nose to hold that airspeed, and then trim. All done.

Trim is always last. Don't use it as a pitch control; it's for taking the pitch pressure out, not for flying the airplane.
 
Lindberg said:
What airplane do you fly where trim sets a particular airspeed? It's not true of four types I've tried it in.
Click to expand...

An airspeed, not a particular airspeed, once everything is in equilibrium the airplane will maintain an airspeed. If something changes, like less power, the airplane will seek the airspeed to which it was trimmed. For less power, the airplane will begin descending to maintain the trimmed airspeed.

If I want to set a particular airspeed, generally I'll use a power setting that I've pre determined, let the airplane settle down after the power change, trim to no pressure, and I'm at the airspeed I want. Really helps flying approaches.

When you get this stuff figured out, it takes a lot of the mystery trimming an airplane.
 
PaulS said:
An airspeed, not a particular airspeed, once everything is in equilibrium the airplane will maintain an airspeed. If something changes, like less power, the airplane will seek the airspeed to which it was trimmed. For less power, the airplane will begin descending to maintain the trimmed airspeed.
Click to expand...
In what airplane have you done this? I've tried it in a Cessna 170, a Citabria, various J3s and PA-18s, and an RV7. In none of them will the "trimmed airspeed" be the same after you change the power setting.

If it works in your airplane, I would love to see a video: Trim for an airspeed at a high power setting, reduce power, and show the plane still flying at the same airspeed.

This is an oft repeated truism, but I have never seen it work in practice.
 
Fellow engineer here. Just set the power, let things start settling out, hold back pressure as needed, then adjust the trim until there is no pressure and you’re done. Easy peasy. There is zero need to overthink this one.
 
Lindberg said:
In what airplane have you done this? I've tried it in a Cessna 170, a Citabria, various J3s and PA-18s, and an RV7. In none of them will the "trimmed airspeed" be the same after you change the power setting.

If it works in your airplane, I would love to see a video: Trim for an airspeed at a high power setting, reduce power, and show the plane still flying at the same airspeed.

This is an oft repeated truism, but I have never seen it work in practice.
Click to expand...

Yeah, I've never looked at it that closely because I don't care. For the OP, it doesn't matter as long as he understands that if he is trimmed out to maintain an altitude with a constant power, if he pulls power, the airplane will lose altitude, if he adds power, the airplane will climb. Will the ASI not move? I doubt it, but once everything stabilizes it should be close.

@Keystoner , don't get caught up in this minutia. For your purposes, you want to get to your desired altitude, set your pitch, set your power, trim out the stick force, remember that as you accelerate to cruise speed you may have a little more trimming to do. When you think you have it, take your hand off the stick for a few seconds, the airplane should stay level, if it doesn't level it out, and trim again.

Lindberg, next time I'm flying I'll give it a try and see what happens.
 
I agree with others..... seems to be overly complicated

Lance F said:
Slightly off topic but one handy rule-of-thumb for 172 trim is that full up trim will give you hands off best glide speed - 65kts- with no power. Just roll that wheel all the way down when your instructor/examiner pulls the power or of course should the engine fail for real. Otherwise just trim for the pitch/airspeed you want to maintain. Those handling notes to me only seemed to make a simple concept complicated. Have fun with your training.
Click to expand...

in all my cessna time I never noticed that one about best glide. seems useful.

About the only "recipe" like that I've heard and found useful is in a 172, when entering a steep turn, roll down two cranks of trim...gives a pretty close approximation for level flight through the turn.
otherwise, I was always in the trim as needed camp.
I will add this. I've flown some planes that were very hard to trim for me. just a touch down to far, a touch up too far the other way.
 
Dan Thomas said:
APT: Attitude, Power, Trim. You're in the climb and have the airplane trimmed so there's no pressure to hold the airspeed. Then you level off, and this is where the trouble starts. So many PPLs I flew with would level off and at the same time pull the throttle back and trim. The airplane would slowly accelerate, which made it raise its nose and start climbing, so they'd trim some more. And then some more. And then they'd get a little too much trim in and it would start descending. So much fooling around.

Leave the throttle in, push so that the airplane stops climbing, wait until you reach your cruise airspeed, and then set the throttle to hold that speed, THEN trim the pressure off. One and done. In a more powerful airplane you might need to take some of the pressure off while you wait for the airspeed, but leave some pressure in until the airspeed and throttle are set. Attitude first, then power, then trim. Be patient.
Click to expand...

Excellent! I was watching a video from Jason Miller (The Finer Points) that reiterates this same teaching ...
 
I have a friend learning in a 150. He says he doesn't need trim, control forces are light. But, he has a problem with varying airspeed on final. Trim is your friend, light forces or not, use the trim.
 
Trying to master trim, ain’t we all brother!
 
Lots of awesome responses.

There is one time I count trim “turns”, that’s when doing steep turns. In a 172, it’s something like 2 turns of the wheel. Any other time in flight it’s to take pressure off of the yoke.

Lance F said:
Slightly off topic but one handy rule-of-thumb for 172 trim is that full up trim will give you hands off best glide speed - 65kts- with no power. Just roll that wheel all the way down when your instructor/examiner pulls the power or of course should the engine fail for real. Otherwise just trim for the pitch/airspeed you want to maintain. Those handling notes to me only seemed to make a simple concept complicated. Have fun with your training.
Click to expand...
Thanks Lance! I just learned something!
 
Lindberg said:
Pitch up/down, and change speed accordingly. For example, I climb out and fly final in the J3 at ~60, but there's 9 cranks of nose-up trim between takeoff and landing.
Click to expand...
So what aerodynamic parameter does trim control?
 
Trim sets the pitch, ergo airspeed.

I take off at the same airspeed I land, so the trim setting is the same. I climb out at an airspeed higher than I take off, so a trim adjustment is required.

Fundamentally, you trim for an airspeed. Your power setting dictates whether you will then climb, remain level, or descend.

Flaps, as they change the location of the center of pressure, change the base attitude of the airplane. In everything I’ve flown, they give a flatter, more nose down attitude at the same airspeed. Therefore, a reason to use flaps other than slowing, is to increase your visibility over the nose as when trying to see the runway during low vis approaches.
 
Brad W said:
I agree with others..... seems to be overly complicated



in all my cessna time I never noticed that one about best glide. seems useful.

About the only "recipe" like that I've heard and found useful is in a 172, when entering a steep turn, roll down two cranks of trim...gives a pretty close approximation for level flight through the turn.
otherwise, I was always in the trim as needed camp.
I will add this. I've flown some planes that were very hard to trim for me. just a touch down to far, a touch up too far the other way.
Click to expand...
The "two cranks" thing works sufficiently well for me, too.

Lindberg said:
In what airplane have you done this? I've tried it in a Cessna 170, a Citabria, various J3s and PA-18s, and an RV7. In none of them will the "trimmed airspeed" be the same after you change the power setting.

If it works in your airplane, I would love to see a video: Trim for an airspeed at a high power setting, reduce power, and show the plane still flying at the same airspeed.

This is an oft repeated truism, but I have never seen it work in practice.
Click to expand...
I do it with a C-150 and C-172. The airspeed is sufficiently close to the same airspeed for my purposes. I don't remember how close it was in a Piper Cherokee, but I don't remember retrimming for airspeed in tht plane, either.
 
Tools said:
Trim sets the pitch, ergo airspeed…

Fundamentally, you trim for an airspeed...
Click to expand...

OK. I’ll bite.

That may be true, but I think it’s a terrible way to envision it.

The elevator sets the pitch, while the trim relieves the needed pressure to maintain any given pitch/power. I literally never think, “I need 10° nose up pitch, so let me adjust my trim”, or “I’m cruising at x airspeed, so here’s how much trim I’ll need”. I use the trim solely to relieve pressure - it’s that simple.

In fact “flying the trim” is a bad habit that leads to sloppy pitch control. It means adjusting trim to try to maintain a given pitch or altitude, rather than using the yoke/stick to do so. It can be difficult for an instructor to pick up, but if a deviation from desired altitude is met with trim first, it’s harder for a student to maintain a given altitude precisely. Put the nose where you want it, and only the trim out any pressure as needed.
 
FastEddieB said:
OK. I’ll bite.

That may be true, but I think it’s a terrible way to envision it.

The elevator sets the pitch, while the trim relieves the needed pressure to maintain any given pitch/power. I literally never think, “I need 10° nose up pitch, so let me adjust my trim”, or “I’m cruising at x airspeed, so here’s how much trim I’ll need”. I use the trim solely to relieve pressure - it’s that simple.

In fact “flying the trim” is a bad habit that leads to sloppy pitch control. It means adjusting trim to try to maintain a given pitch or altitude, rather than using the yoke/stick to do so. It can be difficult for an instructor to pick up, but if a deviation from desired altitude is met with trim first, it’s harder for a student to maintain a given altitude precisely. Put the nose where you want it, and only the trim out any pressure as needed.
Click to expand...
Pilots’ poor understanding of trim has allowed me to see some really cool stuff…among other things, I’ve been to .94 Mach. ;)
 
MauleSkinner said:
Anything but airspeed, apparently.
Click to expand...
Is airspeed an aerodynamic parameter? What are the others?

In some aircraft I fly, the trim adjusts a servo tab on the elevator. In some, a balance tab. In some, the horizontal stabilizer's angle of incidence. In some, the centering force on the stick itself. None of them will fly the same hands off speed at 100% and 50% power without adjusting the trim. But I don't have experience in most of the types people here fly, so I ask the question I did above when this comes up.
 
Last edited:
Lindberg said:
Is airspeed an aerodynamic parameter? What are the others?

In some aircraft I fly, the trim adjusts a servo tab on the elevator. In some, a balance tab. In some, the horizontal stabilizer's angle of incidence. In some, the centering force on the stick itself. None of them will fly the same hands off speed at 100% and 50% power without adjusting the trim. But I don't have experience in most of the types people here fly, so I ask the question I did above when this comes up.
Click to expand...
Regardless of type of trim system, the closest parameter that it targets is airspeed. Power changes make a difference for a few reasons, including thrust lines and the fact that prop wash is moving faster than the airplane. Basically you’re changing multiple parameters to get an airspeed change with the same trim setting. But if you try to tie trim to just stick force or pitch, you’re setting yourself up for misuse. It’s equivalent to saying an inclinometer ball is a yaw instrument.
 
Last edited:
Pitch and airspeed changes with power changes are going to be dependent on CG and horizontal stabilizer placement in the prop blast. If the CG is near the forward limit, that stab/elevator is trimmed to have more downforce to hold the nose up. If it's in the prop blast, that nose will drop a lot as power is reduced, and speed is likely to increase. At aft CG the stab is making much less downforce and the pitch change will be less, and airspeed might decrease. The thrust line in relation to the center of drag is another factor, and in some amphibs (flying boats) that's a big factor. You might get pitch-up on power reduction. In a 180 or 185, lightly loaded and with a lot of trim in on approach, the overshoot can become dangerous as that effective variable-incidence stabilizer trim make the nose rise enough to stall the thing, and you have to push real hard.

So there is no simple rule of thumb, and being taught such rules of thumb can handicap you. Use the trim to remove the pressure, and forget such stuff as full-up trim in a 172 on approach. The loading of the airplane will change that; the flap setting will change it; the rigging of the trim system will change it. I have found plenty of trim systems far out of rig. Mechanics and service manuals sometimes don't agree, and so we get misrigged airplanes.

So learn the theory from the books, not from private pilots. Then learn to use that knowledge by thinking a little. Misconceptions get spread farther and faster than the facts.
 
You must log in or register to reply here.
Back
Top