Why do people say this about trim?

Clip4 said:
It would have been better if you instructor had explained maneuvering speed and turbulent air penetration so you didn’t have to needlessly dink around.
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He didn’t have to. Matter of fact he was just relating a story. The point is this is mountainous terrain around here. I’m perfectly happy sticking between 90-100 on a descent into potential unstable air. I fly on the conservative side of everything though, and I don’t mind dinking around.
 
dmspilot said:
But it's true to enough extent to give a student pilot an idea of how trim and power affect the airplane.
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Or make an approach to an airport in a DC-10 with no other control than power.

Al Haynes explained they concept/correlation pretty well in his talks about Sioux City.
 
Tantalum said:
^same thing with descent. I don't understand when I fly with people and the first thing they do to descend is cut the power, why not just push the nose down and stay out of the yellow arc if the air is rough? It's a little bumpy out I'll reduce power to stay out of the yellow and ask for a 500 to 700 foot per minute to send
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Depends on what your flying and how you want your descent profile to be.

Some airplanes I pull power first. Others I push the nose and trim first.
 
On many but not all planes, power adjustments and the associated changes in prop wash over the elevator change the wing angle of attack at a given elevator trim setting. Assuming the plane remains at 1 G and depending on aircraft type, that may mean adding power reduces airspeed.

Elevator trim serves to move the stick/elevator position at which there is no fore or aft stick force. Assuming no change in prop wash over the tail, a given stick/elevator position will maintain a certain wing angle of attack. If you want a different wing angle of attack and/or to change power setting, you likely need a new stick/elevator position. Trim can then be adjusted to move the neutral force position fore or aft to coincide with the newly required stick/elevator position.
 
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Tantalum said:
...That's just an obvious relationship, physics and all that.. but nothing stops me from retrimming the airplane and flying at 100% power and say 160 knots, or 50% power and 140 knots..
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When you're told that power = altitude we're not talking about "retrimming" the airplane. Without touching the trim you're not going to fly faster by adding power, you're going to gain altitude. Same goes for reducing power, you're not going to slow down, you're going to descend. The same goes for trim - it's not what makes you go up in down, it's what controls speed.

This is being "over-simplified" for a reason. When the typical student first takes the controls of his aircraft he doesn't know this, he thinks pull back to go up, push forward to go down and step on the gas to go faster, just like a car. This is one of the first fundamental principles he is taught and being told it in this manner does not hinder his further development of skills. Yes, it's more complicated. He's gonna figure all that out in due time. But he has to grasp this idea first or he's basically gonna crash.
 
David Megginson said:
And people, seriously, stop with the "Pitch for X, throttle for Y" nonsense. That's not how airplanes work. Stick and Rudder was trying to stomp that out 75 years ago, but people are still repeating it.

The airplane does not have a single up/down control. There are circumstances where you might change your vertical path using only the throttle (e.g. tiny tweaks on an ILS glidepath) or only the elevator (e.g. pitching down to Vy after clearing an obstacle at Vx, while already at full power), but neither is the norm.

The art of flying is learning the complex interplay of power and angle of attack, not putting out oversimplified "rules" that can kill a naive newbie if applied in the wrong context.
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DM: you have nicely condensed the "art (and skill) of flying" by pointing out that one variable (e.g. AoA, airspeed, or rate of descent/climb) can be held as a perfect constant while holding all other control inputs constant is an impossibility (absent conditions not found in the real world). Your clarity does not go unnoticed, eh? (Me, too, by heritage: Cape Breton, NS)
 
Bobanna said:
(Me, too, by heritage: Cape Breton, NS)
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I flew my PA-28 out to Port Hawkesbury with my family once, over a decade ago, and we spent a week visiting Cape Breton. We even stopped by a local ceilidh at a town hall one evening—we were the only ones from away that evening, as far as we could tell—and Buddy MacMaster himself walked up on stage unannounced and played, because he was there to support one of his young fiddle protegées. You come from an amazing part of the world.
 
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Silvaire said:
Without touching the trim you're not going to fly faster by adding power, you're going to gain altitude. Same goes for reducing power, you're not going to slow down, you're going to descend.
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True, and I agreed with this in my post. Cruising along trimmed and level at 8K if I go from 75% to 100% power and do nothing else the plane will generally maintain the airspeed (same for some oscillations, etc.) and it will climb. Pull power and it will descend. But that doesn't mean power = altitude.. that just means that for the existing aircraft configuration that set power setting resulted in X speed and Y altitude.. as @David Megginson wrote there's a complex interplay happening. Power changes just that... POWER.. what you do with that power, whether it's going up or down, or faster or slower, or some combination of the two.. depends on how you fly the airplane

Silvaire said:
The same goes for trim - it's not what makes you go up in down, it's what controls speed
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"Controls airspeed" for a given plane config and given power.. also different airplanes react differently to different power settings and the same trim setting will not mean the same speed at all power settings. Trimming for 65 knots at 40% power, will, in most planes, not yield 65 knots at 100% power or 25% power. It's a mnemonic that sort of works to help people grasp concepts, but it's not an edict

I appreciate that flying is complex and people have to start somewhere.. but you climb up on these ladders and at some point have to throw them away.

Anyway.. the cow has been milked, and is now dead, along with the beaten horse next to it

Do whatever works for you.. if "pitch for airspeed and power for altitude" works for you.. fine then, happy flying! In 19 years of flying though it's part of my list of aviation things that has been past down and bug me, in addition to the following (let's see how many I can trigger with this :D ):

-find a rivet on the cowl for your steep turn
--there's a lot more to flying a smooth steep turn than that

-aim for the end of the runway
--why, I'm planning to land in the touchdown zone, why am I aiming for the end?

-all the mnemonics like "potato flames" "arrow" etc.,
--they're not always accurate, and learning these things and understanding why they're important is not hard. One thing to recite something, another to comprehend and understand it

-high wings
--wings belong on the bottom

-Piper's don't need carb heat
--dangerous to teach that a carb'd aircraft can't get carb ice

-You don't need carb heat if you're in the green arc
--also dangerous, I've had a rough running engine in the clouds at 2,400 RPM which carb heat resolved

-the plane won't break at or under maneuvering speed
--I keep hearing this, and it's not true

-if you have an engine failure you can extend your glide by nose diving to get within ground effect
--theoretically true, and there are cool vids of sailplanes doing this.. but telling someone to do this with zero practice when they're already freaking out to fly 20 feet off the ground is suicide

-overshooting your climb and desired altitude just to descend back down to it will make you go faster, since you're getting "on the step"
--so stupid to try this in anything outside of an XB-70 (or the like) it doesn't even warrant a response

-turning an engine backwards will break your vacuum pump
--no, the vacuum pump blades slide in and out of their grooves

-an engine can't start being turned backwards
--it can, and you can even get an engine to "run" backwards for a second or too if the start procedure was messy and you have fuel/air mixture in the exhaust manifold

-"my plane is fine running above 400* CHT"
--ok

-don't extend flaps while banking
--why not?

-max demonstrated crosswind component is a POH limitation
--no it's not

-the more you lean an engine the hotter it gets
--to a point this is true, but at some point the engine begins to cool again based on where you are in the curve, but a firmly held belief by many is the "lean = hot"

-the plane doesn't know it's over water
--yes it does, and the engine will run rough and will be seconds away from imminent and complete failure
 
Tantalum said:
Piper's don't need carb heat
--dangerous to teach that a carb'd aircraft can't get carb ice
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Good point, when it's phrased that way. It would be more accurate to say "It's not recommended to apply precautionary carb heat in a PA-28 automatically on every descent, like it is for Continental-powered carbureted Cessnas, but you should sure-as-hell turn it on if you have the slightest reason to suspect carb icing."

I am a rare PA-28 pilot who actually has experienced carb icing—right after takeoff, under conditions that normally wouldn't cause it—so I'm glad I reacted quickly, and didn't delay because I mistakenly believed my O-320 was totally immune.
 
David Megginson said:
Good point, when it's phrased that way. It would be more accurate to say "It's not recommended to apply precautionary carb heat in a PA-28 automatically on every descent, like it is for Continental-powered carbureted Cessnas, but you should sure-as-hell turn it on if you have the slightest reason to suspect carb icing."
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Very true. I've read all sorts of theories about the Piper carb heat.. ranging from "it was marketing in the 60s and 70s to compete with Cessna" to "it's located differently so it stays warmer and really shouldn't get it" to "there's no air filter so don't use it" .. I'm sure there's at least some (if not complete) truth in many of these

David Megginson said:
experienced carb icing—right after takeoff, under conditions that normally wouldn't cause it
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what was your indication? Reduced power, roughness, all of the above? When did you disable carb heat?
 
This discussion contains some fascinating views of physics. There should be no misunderstanding of the basic concept that to increase altitude will require the input of energy to overcome the force of gravity. Power, which is the expenditure of energy per unit time, is how we accomplish this feat. This is where the simplified version of power=altitude comes from. Trim will change the angle of attack, which, up to a point (maximum L/D), will increase the efficiency of converting power expenditure to potential altitude gain. If you have insufficient (zero) excess power, no amount of nose-up trim will make you go up.

At intermediate trim and power settings, one can use either trim or power to go up or down. Power, by adding more excess energy that can be converted to lift, and trim by increasing lift through increased angle of attack. Instrument training teaches how to utilize these techniques. Coarse adjustments (transitioning from cruise to climb or descent) are often accomplished by changing power without changing trim. (For climbs, we like to go up faster than a cruise AOA would allow, so pitching to a more efficient attitude is expedient.) Precision instrument approach tracking is typically done in reverse fashion, using small elevator control inputs to control descent rate while leaving power settings alone--unless a coarse adjustment is needed to relieve continuous elevator trim forces, when a power setting change is normally applied.
 
woodchucker said:
He didn’t have to. Matter of fact he was just relating a story. The point is this is mountainous terrain around here. I’m perfectly happy sticking between 90-100 on a descent into potential unstable air. I fly on the conservative side of everything though, and I don’t mind dinking around.
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Actually your instructor should have explained this to you better. The less weight in your Aircraft the slower you need to be descending.

As an example, the Vb (Maximum Turbulent Air Penetration Speed) in a Cessna 172 ranges from 80 to 98. So at loading less than gross (the way people fly at high elevation) you can still damage the plane at 90-100 in turbulent air.

In another make an model the values can be significantly higher. Know the plane you are flying and don’t be so quick to accept rules of thumb.

You can find Vb for you Aircraft in POH Section 4 under speed for normal operation or use the values for Va.
 
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^That's a peculiarity as well that's not immediately intuitive, the lighter the plane the slower you need to go in order to not break it in turbulence.. I've been told 1-2 knots for each 100 lbs below max gross as general rule of thumb for the crap we fly

But are that many people seriously encountering structurally threatening turbulence and mountain waves on descent?
 
Tantalum said:
^That's a peculiarity as well that's not immediately intuitive, the lighter the plane the slower you need to go in order to not break it in turbulence.. I've been told 1-2 knots for each 100 lbs below max gross as general rule of thumb for the crap we fly.
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Every "V" speed for your airplane varies by weight...at least it does for mine. Here's a spreadsheet I put together and that's kept on the backside of the clipboard that holds approach plates. Should be easy to modify for other aircraft.

(NOTE: the site won't let me upload the spreadsheet, only the .pdf of it. If anyone wants the .xls file, PM your email address to me)
 

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timwinters said:
very "V" speed for your airplane varies by weight...at least it does for mine. Here's a spreadsheet I put together and that's kept on the backside of the clipboard that holds approach plates. Should be easy to modify for other aircraft.
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Nice work on the spreadsheet.. I have a set I keep on my phone in Google Sheets for reference. It makes sense that the V speeds vary by weight, but for maneuvering speeds I think it's counter intuitive for most people that when heavy the maneuvering speed is higher than when light.. you're already scared the wings will break off, so asking them to carry more load and go faster is counter intuitive at face value.. but of course that ignores the more complex aerodynamics of flight
 
Peter Anderson said:
Nose up to go slower and nose down to go faster. This rule always applies no matter the power setting.
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Not if you're inverted at the top of your loop (related to site picture - you're statement is still true);)

zaitcev said:
When the air is smooth, I can actually affect the trim by about a knot or two just by leaning forward and back.
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I used to run my seat as far back as possible and luggage in rear on the Tiger on solo flights and see a 1-2 knot difference. In the RV, I can pick up 2-3 knots consistently by closing my air vents. Us long XC guys get bored after 3 hours and already calling in PiREPs coming up with various "tests" like above:p:p
 
atbroome said:
I thought maneuvering speed was only related to the load that control surfaces can but on the structure at full deflection - one control surface at a time.

This is what I recall from my training (link works for non-AOPA members):

https://www.aopa.org/news-and-media...ning-magazine/a-new-look-at-maneuvering-speed
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True...but look in your AFM. For many light airplanes, it’s listed as “Rough Air or Maneuvering Speed”, so Va and Vb are the same number.

In the Beechjet, on the other hand, Va varies between 210 and 246 depending upon altitude (in the Limitations section), but there’s an abnormal checklist for turbulent air penetration that says 200 knots/.58M.
 
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atbroome said:
I thought maneuvering speed was only related to the load that control surfaces can but on the structure at full deflection - one control surface at a time.
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It's confusing because there are actually two maneuvering speeds, design maneuvering speed (VA) and operating maneuvering speed (VO).

As the name implies, the first is a design criteria. The FAA says that the aircraft must be designed such that full control deflection doesn't break anything at or below VA. It also says that this speed can't be less VS √n (stall speed under the limit load factor). In that sense it's a minimum speed. The aircraft manufacturer could choose a higher VA, meaning they could choose to make the aircraft structure stronger than that.

Operating maneuvering speed is, also as the name implies, an operating limitation. It's a maximum airspeed. It must be equal to or less than VS √n. This speed protects the aircraft against exceeding the limit load factor and since it's equal to or less than VA, at the same time also protects against stuff breaking when full control deflection is used.

VA used to be used for both purposes, which obviously led to confusion, and, if the aircraft manufacturer chose a higher VA than the minimum required by the regulation, then being below VA did not assure the pilot of not exceeding the limit load factor. So the FAA came up with the concept and name of operating maneuvering speed and the symbol for it VO, in 1993 (NPRM).
 
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Tusayan said:
On many but not all planes, power adjustments and the associated changes in prop wash over the elevator change the wing angle of attack at a given elevator trim setting. Assuming the plane remains at 1 G and depending on aircraft type, that may mean adding power reduces airspeed.

Elevator trim serves to move the stick/elevator position at which there is no fore or aft stick force. Assuming no change in prop wash over the tail, a given stick/elevator position will maintain a certain wing angle of attack. If you want a different wing angle of attack and/or to change power setting, you likely need a new stick/elevator position. Trim can then be adjusted to move the neutral force position fore or aft to coincide with the newly required stick/elevator position.
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The details are necessary.

For a given airspeed and gross weight, the AoA will always be the same no matter whether the airplane is in level flight, in a steady climb, or in a steady descent, as long as there is no turning involved. It's basic physics: to support a given weight at a given speed takes a given AoA. Slow down and you'll need a larger AoA. Speed up and it will get smaller. If you slow down and don't increase the AoA, the airplane will start descending until the AoA it wants is achieved. And the reverse is true, too.
 
bflynn said:
I triggered on the word "passed".
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Oh damn, that triggers me too! I blame it on speech-to-text.. incidentally, most of my friends think I fly something called a Citrus not a Cirrus
 
Tantalum said:
What does an airplane do when it takes off? I might be crazy but I usually pull up at takeoff to climb

Pedantry strikes again!
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What do you pull up on? Most yokes move fore and aft. Same with the stick on my RV, and the side stick of the Cirrus.

When I want to takeoff and climb I pull back.

Does you know what Pedantry meanz?!?
 
Ravioli said:
What do you pull up on? Most yokes move fore and aft. Same with the stick on my RV, and the side stick of the Cirrus.

When I want to takeoff and climb I pull back.

Does you know what Pedantry meanz?!?
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Screenshot_20191210-140835.jpg
 
@Tantalum - I catch your drift, but the pedantic members love to pick the nits.

The saying is, ‘Pitch for Airspeed, Power for Altitude’ which is true, but some here don’t seem to realize that the two almost always coexist together.

*ahem @Clip4 *
 
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