Trim - Flying's Unsung Hero - Part Three - Trim, Speed, and Safety: What the Airplane Is Trying to Tell You

I recently ran a simple experiment in three simulators — Prepar3D, X-Plane 12, and Microsoft Flight Simulator 2024 — using a Cessna 172.

Full throttle. Trim set to neutral. No pitch input. Autopilot used only to hold heading.

Then I let the airplane climb.

Across all three platforms, the aircraft settled into a steady climb at a consistent indicated airspeed while the rate of climb steadily diminished with altitude. At first glance, it looked like the airplane had simply chosen its own speed — and held it with remarkable precision.

In a sense, it had.

A statically stable airplane will seek an equilibrium angle of attack where lift, drag, thrust, and pitching moments balance. That angle of attack determines indicated airspeed. Trim doesn’t create that balance — it simply allows the airplane to hold it without continuous control pressure.

But here’s where the practical lesson begins.

In normal flight, trim (pitch) selects airspeed, and power determines what the airplane does with that airspeed. Add power without changing trim, and the airplane climbs. Reduce power, and it descends — all while holding essentially the same speed. Change trim, and you change the speed the airplane will seek.

That’s not theory. That’s how the airplane is designed to work.

At the same time, it’s important to recognize what the simulator may be smoothing out.

In the real world, a normally aspirated engine loses roughly three percent of its available power per thousand feet of altitude. Climb performance depends on excess power — the difference between power available and power required. As altitude increases, excess power decreases, and rate of climb follows.

But that’s only part of the story.

Vy decreases with altitude. Propeller efficiency changes as air density drops. Slipstream effects over the tail diminish. Induced and parasite drag do not scale identically. And let's not forget CG.  All of these factors subtly shift the airplane’s equilibrium condition as it climbs.

For indicated airspeed to remain perfectly constant from sea level to service ceiling, those effects would need to remain in near-perfect balance.

That’s unlikely.

In a real airplane, I would expect small variations — not instability, but not the near “ruler-flat” IAS trace the simulators produced.

And that brings us to safety.

The equilibrium speeds a stable airplane seeks is not random. It exists well above the stall region. The airplane is, in effect, biased toward a safe operating condition.

Professional aviation takes this further. Pilots flying aircraft like the Boeing 737 don’t estimate safe speed — they are given it. Every phase of flight uses defined speeds; all intentionally set with margins above stall.

The takeaway for student pilots and simulator flyers is straightforward.

Trim is not about comfort. It is how you select and hold a safe airspeed.

If you are holding pressure on the controls, the airplane is telling you something.

Trim it. And let stability — not guesswork — do its job.  

Understanding effective trim makes flying by the numbers that much easier.

Your thoughts?

Kenneth (Ken) Butterly