Trim Speed Band: Static longitudinal stability flight-test technique

In October we introduced non-maneuvering static longitudinal stability. Through examples we explained how the amount of force you must apply to the control stick or yoke to fly an airspeed different from your trimmed airspeed can affect the ease or difficulty of flying your airplane. We also showed how different stability characteristics can be benign in cruise but insidiously dangerous during final approach.

With the importance of this stability characteristic established, let’s discuss how you can evaluate your airplane’s static longitudinal stability. Before you can assess stick forces and airspeeds, you must determine your airplane’s trim speed band. As mentioned last month, the trim speed band (sometimes called the “free return speed”) is a range of airspeeds in which your plane will fly hands-free without retrimming.

Friction in the longitudinal control system that prevents the elevator from returning to its pre-deflected position is the usual cause of this phenomenon. Held at its new deflection by friction, the elevator now commands a slightly different angle of attack-and a slightly different airspeed. Not all airplanes have a trim speed band, but you won’t know until you check for it.

Airplanes with no apparent control friction on the ground can have a trim speed band. Elevator floating could cause it, but it’s more likely that the slight bending and twisting occurring in flight creates friction that’s not there on the ground.

If you’ve never been frustrated while trying to trim your airplane for an exact airspeed, your plane may not have a trim speed band. If you’ve had difficulty nailing an exact speed, or found that your plane seemed happy to fly a few knots faster or slower than the speed you trimmed it for, it may have a trim speed band.

Test Procedure

Start by establishing a straight-and-level flight condition with the airplane trimmed for hands-off flight. You need not be absolutely level; a slight climb or descent is acceptable-but a steady airspeed is essential. Note your observed airspeed (what you read on your airspeed indicator). Let’s say this “trim speed” is 120 knots.

Now slow down a couple of knots by holding some back-stick. Do not re-trim or adjust the engine or propeller controls. Be patient and let the airplane stabilize at the slower speed. Let’s say it’s 117 knots. You’ll probably be climbing slightly, and that’s okay.

While watching the relationship between the airplane’s nose and the horizon, slowly release your pull force on the stick. If the nose drops when you release the back-stick, you’re outside the trim speed band. If the nose position doesn’t change, you’re still inside the trim speed band.

Do not rely on the artificial horizon, altimeter, and vertical speed or airspeed indicator for this test. The nose movement is a much finer gauge. If the nose moves down, it will happen as soon as you relax the back-stick, but it probably will not pitch nose-down quickly.

You have to watch closely to observe this sometimes subtle pitch change. When you release the back-stick, hold your head perfectly still because any head motion can give the appearance of the nose moving relative to the horizon. If your seat back is high enough, put your head against the rest to make sure it doesn’t move.

Don’t become so engrossed in looking for the pitch change that you ignore your pilot obligations-especially seeing and avoiding.

If the airplane’s nose dropped when you released the back-stick, you know that 117 knots is outside the trim speed band. Repeat the back-stick pull and stabilize at 118 or 119 knots. Let’s say you stabilized at 118 knots, and the nose dropped again when you relaxed your pull. At this point you can repeat the test at 119 knots or quit, knowing the slow end of the trim speed band is between 118 and 120 knots.

If the airplane’s nose did not drop when you relaxed your pull at 117 knots, slow down a few more knots and repeat the test. And keep repeating the test at progressively slower speeds until the nose drops. Then repeat the bracketing procedure to identify the slow end of the trim speed band.

Let’s say the nose didn’t drop at 117 knots, so you tested it at 112 knots, where it did drop. You now know that the slow end of the band is somewhere between 112 knots and 117 knots. You might try 115 knots next. If the nose drops, the slow end of the band is 116 knots. If it doesn’t drop, the end of the band is somewhere between 112 and 115 knots. Continue this bracketing technique until you find the slow end of the trim speed band.

Once you know the speed of the slow end of the trim speed band, repeat the test at airspeeds faster than the original 120-knot trim speed. Using only the control stick, push to a few knots faster than 120. Do not touch the engine/propeller controls.

After the airplane stabilizes at the faster airspeed, relax your push on the stick. If the nose rises, you’re outside the band. If the nose does not move relative to the horizon, you’re still inside the trim speed band, so accelerate a few more knots and try again. Use the same bracketing technique and careful observation to find the fast end of the trim speed band.

Let’s say the band’s fast end was 124 knots and the slow end was 114 knots. Under these flight conditions your airplane has a 10-knot trim speed band, and hands-free it will maintain any airspeed within that 10 knots without re-trimming. Knowing this can ease your frustration when you have difficulty trimming to an exact speed.

During normal flying one way to deal with a 10-knot trim speed band is to make those final airspeed adjustments by nudging the stick forward or aft a tiny bit, and let the control system friction hold the elevator at its post-nudge deflection. This is often easier than chasing an exact airspeed with trim. An even easier solution is to accept flying a couple of knots faster or slower than your target cruise speed.

In our example we identified the 10-knot trim speed band between 114 and 124 knots, but it will not always be 6 knots slow and 4 knots fast. Because it’s a band, it could lie anywhere around the 120-knot trim speed. The next time you establish a 120-knot cruise speed the band could be from 112 to 122 knots, or 119 to 129 knots, or even 120 to 130 knots.

You can never be sure where you are within the trim speed band unless you test it. Naturally you’re not going to perform this laborious test every time you level off, and it’s not necessary. When you’re in cruise flight around 120 knots, just know that your airplane has a 10-knot trim speed band.

You should repeat the entire test with your airplane configured for the landing pattern because the trim speed band for it could be different from the cruise configuration. Another important reason to perform the test is because we tend to fly more precise airspeeds in the pattern, particularly on final approach. A large trim speed band can mask any stick-force cues to airspeed deviations, making it easier to drift off your desired final approach speed as you direct more attention outside the cockpit and scan your airspeed indicator less often.

Knowing you have a small trim speed band lets you trim for the final approach airspeed and concentrate on flying the final approach. When you feel yourself holding back or forward stick, you’ll know you’re flying slower or faster than the trim speed, assuming you’ve not changed the airplane’s configuration, trim, or power setting.

Whether they realize it, all pilots use tactile cues like these. If your airplane has a large trim speed band, you’ll know that a diligent eye on the airspeed indicator is essential for tight airspeed control on final approach.

By the Numbers

1. Establish straight and level flight with the airplane trimmed for hands-off flight at a steady airspeed.

2. Using only the control stick, decelerate a few knots. Do not touch the trim or engine/propeller controls.

3. Stabilize at the new airspeed.

4. Note the relationship between the plane’s nose (or some fixed structural reference) and the outside horizon.

5. Relax your stick pull while observing the nose and the horizon.

6A. If the nose does not drop relative to the horizon, you’re inside the band. Repeat the process starting at Step 2.

6B. If the nose drops relative to the horizon, you’re outside the band. Using only the control stick, establish a new steady airspeed between this speed and the last speed at which the nose did not drop. Do not touch the trim or engine/propeller controls. Repeat the process starting at Step 3.

7. Continue this bracketing technique until you determine the slow end of the trim speed band.

8. Using only forward stick, establish a steady airspeed a few knots faster than the original trim speed from Step 1. Do not re-trim or adjust the engine/propeller controls. Repeat Steps 3 through 8 substituting the word “push” for “pull,” “rise” for “drop” and “fast” for “slow.”

9. After determining the slow and fast end speeds of the trim speed band, subtract the smaller number from the larger number-the difference is the trim speed band.

If you’ve never had a problem trimming your airplane or tightly controlling its airspeed, you don’t have to rush to test your airplane. On the other hand, if you’ve had these symptoms, maybe it’s not your fault. It’s worth a check during your next flight.

This month we showed how to determine your airplane’s trim speed band. And we discussed how the trim speed band can affect airspeed control, frustration, and safety on final approach. Now we’re ready to delve into static longitudinal stability testing, and that will be next month’s topic.