Trim for speed? Pitch for speed? Power for altitude

[BrianL99]

Ok, it's the age old argument, but having now put about 50 hours on my LSA, it becomes more relevant than when flying a Cirrus ... a LSA takes more "flying" and energy management.

So ...

In cruise flight, does anyone really "trim for a specific speed"? Or do you trim for altitude and allow speed at a given power setting, to simply reach equilibrium?

I understand the aerodynamic realities, but the practical realities don't seem to coincide. "Speed" is an unnecessary parameter to control, but not altitude. If ATC assigns me 4500', they expect me to be there and don't really care how fast I'm flying. If I'm flying a course of 150 degrees over 3000', I'm expected to be at "odd + 500'" and no one cares what my speed is.

Curiously enough, as far as I know, the standards for a PP or even CP ticket, don't require you maintain air speed within a given tolerance, but you are required to maintain "altitude" within a +/- range.

[comperini]

Yes, there are speed tolerances in several of the maneuvers

[FastEddieB]

So ...

In cruise flight, does anyone really "trim for a specific speed"? Or do you trim for altitude and allow speed at a given power setting, to simply reach equilibrium?

I was taught that the sole function of the trim was to relieve control pressure.

And, since that's what I use it for, I don't really understand your question.

But certainly in instrument training, the ability to hold a certain desired airspeed is necessary, and tested for. And I would use power to achieve that airspeed.

[SportPilot]

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[Nomore767]

"In cruise flight, does anyone really "trim for a specific speed"? Or do you trim for altitude and allow speed at a given power setting, to simply reach equilibrium? "

In cruise once you establish your speed you trim the control pressure to ease the load required to maintain the attitude and altitude.

I was taught like Eddie to always trim the control pressure off.

Yes, trimming to keep altitude is important. However, you'll note that if you set power for max sped then the trim required for that will be different if you then decide to reduce power and fly at a slower speed. The airplane will 'try' to fly the trimmed speed.

In airplanes with auto-pilot ,and auto-trim, which I have ( I know it's only an LSA )…you'll see the plane trimming for the power/speed, altitude you want. In an LSA it can change constantly.

In training your CFI will have you practice slow-flight and then increase back to cruise speed all without losing/gaining altitude. This is an exercise in precision, power management, speed…and yes…trim control.

[MrMorden]

I trim to relieve pressures. That said, at a particular power setting and altitude, once the pressures are neutralized you are "trimmed" for that airspeed. Any disturbance to the aircraft will tend to return to the equilibrium state, which should roughly equate to the same speed.

If you put the plane in a shallow dive and let go, the airplane will speed up, then pitch up to try to return to the equilibrium state. It may take several oscillation cycles, but the airplane should eventually return to roughly the same speed.

Is this what you mean?

[BrianL99]

"In cruise flight, does anyone really "trim for a specific speed"? Or do you trim for altitude and allow speed at a given power setting, to simply reach equilibrium? "

In airplanes with auto-pilot ,and auto-trim, which I have ( I know it's only an LSA )…you'll see the plane trimming for the power/speed, altitude you want. In an LSA it can change constantly.

That's sort of where I'm getting to. My Cirrus had auto-trim of course, so it wasn't an issue. With an LSA, the airplane is much more sensitive to just about everything and flying a Rotax with a "reduction gear" seems to make it even more of an issue. As you say trim (or pitch) is constantly in a state of flux.

I guess the issue I'm trying to get my mind around, is how much pitch effects RPM in an LSA (or Rotax). A slight variation in pitch can result in a 300-800 rpm change, that's simply not the case with a more powerful engine. I suspect it has more to do with the reduction gear than anything else, but I'm not sure. An LSA is much harder to "fly by the numbers". If I set the throttle of my Cirrus at 1600 RPM's, plus or minus 20% or so of pitch would have no impact on the RPM's.

& a question for the more mechanically inclined: How is manifold pressure effect by pitch or is it entirely independent? In other words, could you fly a Rotax "by the numbers", by using MP instead of RPM ?

[sandpiper]

Don't over think it. Just trim to fly hands off. No matter what phase of flight you are in.

[SportPilot]

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[MrMorden]

Don't over think it. Just trim to fly hands off. No matter what phase of flight you are in.

Good answer.

Agreed, though as Brian said, that can be easier said than done in an LSA. My CTSW basically can't be made "hands off" in anything but smooth air. Every gust and bump upsets it, so I'm constantly having to manage it. I'm lucky in that I have an autopilot if I really get tired of fighting it on long trips, but it is a handful in rough air.

Just the price you pay for a really light airplane that has a low fuel burn, I suppose.

[3Dreaming]

"In cruise flight, does anyone really "trim for a specific speed"? Or do you trim for altitude and allow speed at a given power setting, to simply reach equilibrium? "

In airplanes with auto-pilot ,and auto-trim, which I have ( I know it's only an LSA )…you'll see the plane trimming for the power/speed, altitude you want. In an LSA it can change constantly.

That's sort of where I'm getting to. My Cirrus had auto-trim of course, so it wasn't an issue. With an LSA, the airplane is much more sensitive to just about everything and flying a Rotax with a "reduction gear" seems to make it even more of an issue. As you say trim (or pitch) is constantly in a state of flux.

I guess the issue I'm trying to get my mind around, is how much pitch effects RPM in an LSA (or Rotax). A slight variation in pitch can result in a 300-800 rpm change, that's simply not the case with a more powerful engine. I suspect it has more to do with the reduction gear than anything else, but I'm not sure. An LSA is much harder to "fly by the numbers". If I set the throttle of my Cirrus at 1600 RPM's, plus or minus 20% or so of pitch would have no impact on the RPM's.

& a question for the more mechanically inclined: How is manifold pressure effect by pitch or is it entirely independent? In other words, could you fly a Rotax "by the numbers", by using MP instead of RPM ?

In any airplane with a fixed pitch propeller will change RPM with pitch changes. The direct drive engines have a smaller change in RPM than the Rotax because of the gearbox. When you factor in the 2.43 to 1 gear reduction you will find the change in propeller RPM is about the same.
What I tell students is set the RPM and don't try and chase it. Just fly the airplane. I will pull the power back a little if the RPM gets into the yellow.

[HAPPYDAN]

All very interesting - and I thought all those fluctuations I have been experiencing were just the Skycatcher or, more likely, my inexperienced handling. It's somewhat comforting to know it's just a characteristic of LSAs in general.

[SportPilot]

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[MrMorden]

All very interesting - and I thought all those fluctuations I have been experiencing were just the Skycatcher or, more likely, my inexperienced handling. It's somewhat comforting to know it's just a characteristic of LSAs in general.

I was just going to say the same. The 3 SkyCatchers I fly do not have autopilot, so you have to keep your hand on the stick and eyes out the window at all times. They are also very pitch sensitive and hard to hold precise altitude. The least little bump up or down to the elevator trim makes a significance difference.

Even with the autopilot on, in rough air the airplane pitches and rolls quite a bit, but the autopilot keeps the heading and altitude quite well. But it's not above making some pretty strong corrections to do it.

[BrianL99]

All very interesting - and I thought all those fluctuations I have been experiencing were just the Skycatcher or, more likely, my inexperienced handling. It's somewhat comforting to know it's just a characteristic of LSAs in general.

That's another reason I brought up the subject. I've never really seen much discussion of the stability of an LSA and quite honestly, they're damn unstable.

It's sort of the "dirty little secret" of LSA's ... they can be a handful to fly.