April 1999Features

A pilot's guide to airspeed

Trimming away troubles


Airspeed seems to be a straightforward bit of aeronautical terminology, but the word actually covers a variety of types. And the ability to use airspeed correctly is testimony to a pilot's skill.

What is airspeed? Well, it's lots of things. First there's indicated airspeed (IAS) that we read directly from the airspeed indicator. Then there's true airspeed (TAS), which is indicated airspeed corrected for non-standard temperature and pressure. We use TAS in flight planning and performance measurements. The third type is calibrated airspeed (CAS), or indicated airspeed corrected for the effect of unusual angles of attack on the pitot-static system, an effect known as "position error."

In flight we generally take a "what we see is what we get" approach to airspeed - that is, when flying the airplane according to manufacturers' recommended operating speeds, or when complying with airspeed margins on a practical test, we go with what we see on the airspeed indicator. But that's not to say we simply accept what we get when flying the airplane. A good pilot selects a specific, appropriate airspeed for every flight operation and knows how to achieve it. Limiting speeds such as maximum flap and gear extension speeds, never-exceed speed, and minimum controllable airspeed, must not be exceeded for structural reasons on the high side and to avoid stalls on the low side. What methods do you employ when observing the airspeed requirements for your airplane?

Knowing the "book" is a good place to start, but your technique must be up to the task. Fortunately, as pilots gain experience they learn that airplanes provide constant clues as to whether they are running in the proper range. Can you recognize by feel and sound the approximate airspeed at which your aircraft is flying? During your next training flight with an instructor on board, cover the airspeed indicator and take a few minutes to estimate airspeed. You may be surprised by how well you can act as a substitute for the airspeed indicator using sensory information and power and pitch settings as clues. This may prove valuable some day if you discover after launch that the instrument isn't working properly or if your senses warn you that you've strayed from the proper airspeed.

The mechanics of airspeed control require managing three dynamic elements: power, pitch, and trim inputs. We set the power as desired for takeoff, cruise, descent, or landing. Then we adjust pitch to achieve the desired airspeed. We follow up by trimming the airplane to maintain the resulting performance profile. Power, pitch, trim.

After sufficient practice it becomes automatic. Consider an example from each phase of flight. You are ready for takeoff in a Cessna 152 from an 1,800-foot grass strip with tall trees at the end. You are planning to execute a short-field procedure using VX, best angle-of-climb speed (54 knots indicated), until you've cleared the obstacles. Then, after retracting the flaps at 60 knots, you will accelerate to VY, best rate-of-climb speed (67 knots), to reach a safe altitude quickly. You apply takeoff power and hold the recommended tail-low pitch attitude. As the airplane lifts off, you adjust pitch to achieve 54 knots. In a few seconds you are clear of the obstruction. Now you lower the nose a bit and accelerate to 67 knots for the climb to altitude. At this point, no further pitch changes are required for a while, so it is a good time to trim off the yoke pressure you are holding. You can now maintain this power-pitch-trim combination until you are ready to level off in cruise. In essence, the airplane is trimmed to maintain the pitch attitude yielding 67 knots with the throttle at the full-power position.

Leveling off, you need to establish a new pitch-power combination, and then trim the airplane. In some low-powered trainers it is a good idea to lower the nose first and let the airplane accelerate to cruise speed before reducing power. If you don't, it can take several minutes for the airplane to accelerate, requiring more trimming. After the airplane accelerates to the expected speed for the power/pitch combination used, say 95 knots at 2,300 rpm in the 152, trim away remaining forward pressure. On a smooth day, it may now be possible to fly hands-off for quite some time. (don't relax too much; you still may need to apply occasional rudder-pedal pressure to keep the nose from yawing off the desired heading.)

When the time comes to descend, a power reduction will cause the trimmed airplane to nose down in a constant-airspeed descent. Recall that airspeed will not change if the trim is untouched; the power setting will determine whether the airplane descends, flies level, or climbs at the trimmed speed. There may be some oscillations as the airplane stabilizes, but it should return to the trimmed airspeed after a few pitch cycles. Yes, of course you can also start your descent with forward pressure on the yoke. But remember that this will increase your airspeed, requiring you to re-trim nose down to maintain the new descent speed (because the airplane is trying to pitch back up to its trimmed airspeed as you descend).

Trim is the aerial equivalent of a car's cruise control. Incorrect use of trim is evident in chronic overcontrolling of the airplane or trying to use the trim wheel as a lazy pilot's elevator control. Doing this causes a loss of control over airspeed and altitude. Change the trim and you signal the airplane that you want it to change speeds. To do this (without an accompanying power change) it must depart from level flight - the very opposite of what the pilot is trying to do when struggling to hold altitude in turbulence. So the airplane and the pilot find themselves locked in a silent, continuing argument, proving only that pilots are from Mars and airplanes are from Venus. This can fatigue the pilot and can make passengers uncomfortable. Remember, if you have the airplane trimmed to the desired speed and the airplane is not in level flight, it is the power setting that needs adjusting. Too much power for level flight at the trimmed airspeed and the airplane will climb; not enough power and it will descend. it's that simple.

In the traffic pattern, adding flaps creates drag, requiring a lower pitch attitude, and therefore a trim change, to maintain constant airspeed. A skillful pilot may use the drag of the flaps to slow the airplane a few knots in its current trimmed condition, adjusting the glide angle as needed with the throttle.

One common error is to try to maintain a constant pitch attitude in the glide after reducing power. This results in airspeed decay, and perhaps a too-high sink rate. (Failing to recognize this problem is the cause of many "too low, too slow" accidents in which the airplane is too low to be pitched down and accelerated, and too slow to be held off with further back pressure without stalling.) If you reduce the power, let the nose come down to maintain the trimmed speed, as it was designed to do.

Over the threshold, reduce power to idle. As you encounter ground effect, increase the pitch attitude to slow the airplane to a speed just above stall for the instant of touchdown (but not enough to cause ballooning). This final pitch change also reduces the sink rate to almost nil as the airplane nears the runway, guaranteeing a soft arrival.

Note that even during the ground roll, the elevator control is still held full back, harnessing drag induced by the high angle of attack to slow the airplane as quickly as possible. This too is airspeed control.

That's a lot to think about, but once the ideas are understood, the execution is very simple. Two exercises demonstrate the important points about the power-pitch-trim dynamic. The first is an armchair drill: Ask yourself how you would conduct a flight if you could fly at only one airspeed, say 70 knots in the Cessna 152, from takeoff until just before landing, when you would transition to a full-stall touchdown. (To keep from complicating the issue with configuration changes, Let's make it a no-flap landing.)

How many trim settings would this flight require? Just one! We take off and use pitch control to achieve 70 knots, then trim to maintain that speed. When reaching cruise altitude, we reduce the power (to about 1,700 rpm in our 152) until our 70-knot ball-of-fire was no longer climbing. Returning to the airport, we reduce power a little more and enter a 70-knot descent. We could vary that rate of descent with more power changes - but we would touch neither the trim nor the control yoke until it was time to flare and land.

Another exercise can be done in flight - a tried-and-true demonstration of the aircraft's built-in positive-stability characteristics. Experience this one and you will never overcontrol an airplane again! Most pilots know intuitively that when you add power, an airplane in flight will pitch up and climb, and that when you reduce power, it will lower its nose and descend. But what happens next?

Let's see: Reduce power when in trimmed level flight. Yes, the nose drops, and the airplane accelerates a few knots as it starts to descend. But note that after a few seconds, something interesting happens. As the descent continues, the down-pitching of the nose ceases, and in another few seconds, the nose begins to rise again, and airspeed decreases. A second cycle of down- and up-pitching follows, within a smaller range than before. Eventually the oscillations stop and the airplane stabilizes in a descent at the speed it was flying when straight and level. Adding power from level trimmed flight produces a similar cycle in the resulting climb. Yes, climbs and descents were always this fuss-free. it's what your airplane has been trying to tell you all along.

Book knowledge counts, but having the technique for producing results is vital to airmanship. If you ever notice a pilot who seems to do very little in the way of controlling an airplane but still gets big results, it may be because that pilot knows an airplane will do most of the work if it is allowed to fly the way it was designed to.


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