One thing I would like to know is: on a typical landing in a 172, flying final at 65 with full flaps, would you expect to land with the trim up or down relative to the take off position? I've avoided asking that before because I know everyone will say "Don't worry about that, just set the speed with the yoke and trim the pressure away, whichever way it's needed". But I think it would help me to know, because it might shed some light on what I'm doing wrong before that point.
No worries on the long post, I type fast.
I think a lot of this explanation stuff gets inadvertently avoided when instructors rush ground work. If the standard flight is show up, jump out of the car, and jump straight into the airplane, well, airplanes make for a lousy classroom. They’re noisy, the student is busy and overloaded/distracted, and it’s not a good place for an aerodynamics chat.
One positive compared to when I learned to fly is online forums like this one MIGHT fill in some gaps there, so we post and answer stuff. We can’t replace the instructor who SHOULD be doing more ground questioning to figure out which piece of the building blocks got missed or is being forgotten in the airplane when busy, but we can give your brain some stuff to mull over when you’re not trying to think and fly the plane at the same time.
Ground is often where the stuff you know from the books all of s sudden makes more sense when applied to “so what did we see on the last flight” which makes debriefs also important. Very important.
So anyway, your question. Where will the trim be?
I think it’s a reasonable question. And you framed it for a specific aircraft type, so you know this will change in other airplanes, eventually.
(Don’t worry about people who say a question is dumb or you think they will, just ask. They asked “dumb questions” once too.)
You’d probably be very nose up trimmed at that speed in a Skyhawk. Probably not full up but read on, you could be.
The slower you are the less air across the control surfaces the more deflection the surface needs to be moved to exert the same force it did at a faster speed. You get that part in your head already I’m sure. Apply it to the trim tab. Lots of trim for slow, less trim for fast. You’ll see this in cruise. It doesn’t take much of a trim tweak to make a big change in pitch at high airspeed. It takes a couple of half rolls of the wheel at low speed to do the same pitch change. Right?
One thing to be aware of when using a LOT of up trim on landing is that you always have to be ready for a go-around. When you push the throttle forward the air moving over the elevator and horizontal stabilizer is instantly greater because it’s coming from both the airplane accelerating AND the propellor.
The airplane is going to pitch up, and it can be a bit dramatic of a pitch up too. You have to be ready to hold the pitch angle you want with the elevator control and re-trim immediately because the trim will be, essentially, far too “deflected” for that new airflow it just got, and will force the nose up.
This is why that one CFI I mentioned, doesn’t teach students to trim much in the pattern. I don’t ascribe to that, instead I’d rather teach what’s going to happen when the power comes in quickly like that, and ask the student to put the cowl edge (or whatever their usual mark for a normal takeoff pitch angle is) on the horizon. Let the airplane pitch up to that angle but push to STOP the upward movement and now re-trim for that.
Some instructors might also MIX those methods. Maybe pre-solo and through solo, set a general pitch and speed with trim and fly it all with the yoke so the student learns the relationship between airspeed and control forces and effectiveness and bring in more demands for trimming it all after solo as the student develops a feel for it.
Anyway back to this glide path idea barely touched on above...
For most trainers, a “stabilized” (don’t get me started on that word... I’m just using it here because you’ve probably heard everyone harp on it) three degree glide path with full flaps is going to take some power. So there’s a trim that will hold that and the airplane will just fly down to the runway with that trim and power setting.
But let’s say you started closer to the runway and had not descended from pattern altitude at all? Throttle to IDLE and full flaps and again, there will be a trim setting that flies that steeper glidepath all the way to the runway.
In my very very slow 182 with a STOL kit on it, I can set up a steeper than standard glide path with flaps at 40 and very little power, if any, and full up trim.
The two trim settings likely won’t be the same because of a concept we were discussing earlier. Air flow from the prop over the tail.
(Ah-ha! Lightbulb!)
In the idle scenario and super steep glide path there will not be as much airflow over the tail pushing on that trim tab. In the power on scenario there will be more.
Okay, kinda. I’m exaggerating it a bit to make the point.
If the airplane is flying 65 knots in both cases the airflow over the tail will be nearly identical. But the concept here is, the trim wheel won’t always be in the exact same location. Nor would you want to think about it that way.
But...! You’re question about where would it generally be? Up. Quite a bit up. Because you’re slower than cruise, and they designed it (at least on this airplane) to be fairly effective across the entire normal operating airspeed range of the airplane.
It wouldn’t be nice of them to design it to not have enough throw to be able to help you out as the airplane slowed down. Make sense?
So now you ask, “so why is there a takeoff trim location marked on the thing if the trim is that variable?” Excellent question, grasshopper!
It’s because, what power setting do we almost always use for takeoff? FULL power. Whatever we’ve got! Full smash, full rental power, stoke up the boiler and let’s get this thing into the air and off the ground!
They knew that, so they could mark a reasonable place for it. Cool huh? Nice of them.
Thus, you “trim away pressures” because pressures are created by different airspeeds. That’s the connection in the knowledge chain. If the airplane slows, it’ll need more trim deflection just like if you just pull a little more on the yoke as you slow. Same the other way, if it speeds up, too much trim deflection for the faster speed and you’d have to roll it back out.
And that leads to the somewhat misleading but accurate for a constant speed descent, “trim for airspeed”. You’re really trimming for a particular pitch angle (and really an angle of attack, but we’ll ignore diving into that for the moment) that results in an airspeed. But it’s accurate enough as a reminder on final. Trim the nose to where when no power changes or flap changes are happening, your approach airspeed is whatever you’re shooting for. Essentially the trim is “holding the yoke for you” so you don’t have to work muscles as hard.
Now introduce those “power for altitude” concepts. You’re a little high. You reduce the throttle. The airplane should just pitch down a little on its own and fly your “trimmed speed” and descend. It might need a trim tweak, but just a tweak, however because less airflow over the tail from the prop. Maybe the nose dropped a little too much and it’s accelerating.
That ending paragraph is where your “feel” for this particular airplane comes in. You’ll just know after a while that a single engine prop plane nose will drop a little more with a power reduction and it’ll try to speed up a little. Just a little. Very subtle when you start to notice these things.
So there ya go. More thoughts on trim that you ever wanted! Summary version is “trim away pressures” and that’s why you’re confused. If you talk about where the pressures are coming from you backtrack into what you already know about airspeed and control surfaces and how effective they are at different speeds. Cool, huh? All that silly “booklearnin’” actually does apply. Just nobody mentioned how!
Remember trim is a helper. Positive aircraft control is always done with the main flight controls. If the nose needs to come up or go down RIGHT NOW, do it. Trim should be kept close enough that it can wait. And that trim up can fight you pretty good on a full power go-around. Don’t forget that one. It’s a low altitude pitch up rapidly to a stall accident that can sneak up on you.
One student I know is VERY small. She’s short, not incredibly muscular, and she’s modified her go-around procedure in high horsepower airplanes to bring about 70% of go-around power in, then quickly re-trim to remove the large control forces trying to push her nose too high, then add remaining power and finish trimming. She HAS to use trim to her advantage. She’s not a fat old flight instructor with chiseled arms of steel, like some of us.
But that trim wheel makes her equally capable of putting the nose right where she wants it.
Hahaha. Right. Chiseled. I’m about as chiseled as a brick.
Here’s some fun for you. You know how tired you are of hearing the instructor say right rudder? My STOL airplane, the STC changes to the POH day this in the pre-takeoff checklist for short and soft field takeoffs.
Rudder trim: Half right.
Hahahaha. No kidding. And it works. And I STILL need to push a little more right rudder with my foot. Because when you rotate and fly off at the silly-low speeds of a STOL airplane, the rudder just isn’t fully effective at liftoff. It’s flapping way over there to the right trying to find some air to grab and most of it is coming from the prop!
Have fun thinking about all this. It’ll help in he airplane when you realize the control pressures are changing because airspeed is changing!
You’ll settle right in knowing how that trim really works!
