Flying Qualities

"That is one good flying airplane...." Every aeronautical engineer would like to hear every pilot say those words after landing the engineer's latest creation. Trouble is, one pilot's "good flying airplane" might be another pilot's slug. Characteristics that endear an airplane to a pilot who enjoys aerobatics might drive a pilot right up against the wall who is flying on instruments.

The Federal Aviation Administration's requirements on handling qualities are rather loose. You can read the pertinent pages and tell that the basic regulation on flight characteristics sums it all up neatly: "The airplane must meet the requirements...at the normally expected operating altitudes without exceptional piloting skill, alertness or strength." The latitude that is given means that we, as pilots, have to understand the differences in airplanes and make the necessary adjustments in technique. We also have to recognize that there will never be an airplane that meets everyone's standards of "one good flying airplane."

To begin, we can learn some things about how an airplane might fly before we start the engine. Does it have a long wheelbase? If it does, chances are the stick force required to rotate the airplane to the takeoff attitude will far exceed the stick force required to hold that attitude immediately after liftoff. If the airplane has light stick forces, the chances of a pilot- induced oscillation (PIO) in pitch will he high right after takeoff. All this will be even more pronounced if the airplane is "short coupled," that term related to the distance between the horizontal tail and the wing.

The best example of this I have seen was the Prescott Pusher, offered for a brief time as a kit. After looking at the airplane on the ground, I hesitated about flying it. It looked that bad. I did fly it, and it met my every expectation. I wrote in my log, "Finally met one I didn't like." Besides having absurdly poor performance, the airplane had quite marginal pitch stability, which was accentuated right after takeoff by the fact that the main landing gear was too far aft — which it had to be to keep the airplane from falling on its tail when at rest. It took a big pull to get it off the ground, and a substantial reduction in that pull was required right after liftoff — all the ingredients for a PIO in pitch.

A production airplane that has the main gear too far aft is the Cessna 303 Crusader. It has light pitch forces, and most pilots get a good bobble out of it right after liftoff.

Pitch stability is where we give an airplane a quick grade after takeoff. Because power tends to adversely affect pitch stability, we see an airplane at its second worst right after it leaves the ground. The only worse time is in a go-around where landing flaps are added to the equation. Anyone who saw the video of the F-22 accident on an attempted go-around has seen an example of a condition where an airplane became uncontrollable as the configuration was changed and full power, including the afterburners, was applied. It was a classic oscillation in pitch with a happy ending because the pilot walked away. If an airplane does not seem friendly right after takeoff, it might get better, but it also may never become entirely satisfactory to your touch.

You can tell a little about roll control qualities on a walkaround just by looking at the ailerons and considering the ratio between wingspan and aileron span. This is no absolute, but larger ailerons farther out on the wing tend to relate to higher roll rates — especially when compared with airplanes of like speed. On a Bonanza, for example, the aileron span is greater in relation to wingspan than on a Cessna 210. Guess which one has lighter roll forces?

The cowling is also an important place to look because power has a lot to do with pitch stability, especially in an airplane that is built in different versions. When Piper put turboprops on the Navajo and called it a Cheyenne, they had to add a stability augmentation system to the airplane to make it meet stability requirements with the additional power. Even then, it barely met the requirements. Because center of gravity also affects pitch stability (the farther aft, the less stable the airplane), a version of the Cheyenne was later built without stability augmentation — you guessed it, less power and an aft CG limit that was farther forward.

When you get in an airplane, wring the controls out to learn something about what is there. A lot of airplanes have downsprings or bobweights in the pitch control system, put there to buttress pitch stability or modify G loading with control inputs in one phase of flight or another. If moving the rudder also moves the wheel, there is an interconnect, most likely to meet a certification requirement that the airplane be able to pick up a wing with rudder alone in the go-around mode. Rudder-aileron interconnect does not necessarily mean that the rudder is ineffective in general, it just means that it won't influence roll a lot with full flaps and full power.

Also, before flying an airplane, consider its origin. If it is an original, meaning that it was designed to be exactly what sits before you, chances are it will fly better than if it is an airplane that had been re- powered, stretched, and subjected to other modifications to make it more palatable in the marketplace. Because I bad-mouthed the Prescott Pusher, I must go to another stillborn kit project to make the point in a different way. When I looked at the Swearingen SX300, I thought that the airplane looked right, like it held no secrets. Because the landing gear retracted into the fuselage, there was no compromise on gear placement. The surfaces all appeared in good harmony. I felt comfortable making a formation takeoff in the airplane and then flying it for a photo shoot before doing anything else with it.

Something can be learned about an airplane while taxiing out for takeoff. How is the ground steering? If it is awkward or oversensitive, that gives you a little peek into the designer's head. If he wasn't worried about making it easy for you to drive the airplane to the runway, how much did he worry about making it a nice airplane to fly? It all counts.

If that look under the cowling suggests that the airplane has a lot of power relative to its size, that tells you something about the flying qualities immediately after takeoff. Because the airplane will be accelerating rapidly, it will be above the trim speed for takeoff almost immediately. That brings a requirement for nose-down trim, and if it is a powered trim system, the speed at which the trim moves is one of the flying qualities of the airplane. Because of runaway trim requirements, many of the systems move more slowly than might be desirable. If you are old-fashioned like me, you far prefer manual trim systems where you determine how fast it moves, and there is no possibility of a runaway. It isn't possible to completely judge a trim system before you go, but in doing the checks on a powered system, you can see how fast it moves.

Once off the ground, there are two distinct types of airplanes when it comes to pitch control. Some require muscle, others don't. The requirement on the pilot is to use whatever force might be required to place the airplane in the desired attitude. If you hop from a muscle airplane (a 210 and Mooney are good examples) into one that requires a light touch, such as a Bonanza, there might be some overcontrolling to begin. If this happens, a pilot has only to lean on the stability requirements, which dictate that the airplane must return to within 10 percent (plus or minus) of the trim speed if a control force is slowly released. If the flying qualities of an airplane result in a PIO, taking pilot inputs out of the loop just naturally takes care of things.

The two places where we come in direct contact with flying qualities are in configuration changes and turbulence.

Because configuration changes are momentary events that only happen at certain times in flight, they are the least important. The pitching moment with flaps extension and retraction is the most notable event. I mentioned the 210 and the Mooney as being "muscle" airplanes. A 210 pitches strongly nose up with flaps extension. A Mooney pitches strongly nose down. That is the reaction that would be expected from a high wing and a low wing, though there are exceptions. If you put a Bonanza pilot in either airplane, he might well feel that too much force is required to compensate for flaps extension. Pilots of 210s and Mooneys hardly notice the forces after they fly the airplanes for a while.

More important than flaps extension is flaps retraction because, except for takeoff flaps, this is done infrequently in flight. In normal flying, there are few go-arounds, so if an airplane requires strong forces with flaps retraction, this is something that should be practiced on a regular basis. The airplane's reaction will be opposite that encountered with flaps extension.

Flying qualities in turbulence are much more important because we see at least some turbulence on most flights. The airplane's response to turbulence determines how hard the pilot has to work to maintain altitude and hold a heading. It also determines the barf frequency in the back seat, where turbulence always seems more pronounced.

An airplane's short-period response to a disturbance from turbulence must be damped. It must try to return to a given flight condition after it is disturbed. This is in relation to pitch because we know that without a roll- stability augmentation system, airplanes are not stable, or at least not as stable, in roll as they are in pitch.

The best way to judge an airplane's stability in relation to turbulence is to take it out on a bumpy day, and in correctly trimmed, level cruise flight, watch the airplane ride through the bumps without offering control inputs other than those necessary to keep the wings within 30 degrees of level in roll. We learn two things here. One is that many of the control inputs we use in turbulence are probably not necessary. Two is that an airplane with "truck-like" handling qualities probably exhibits better dynamic stability than an airplane that is delightful to fly in calmer conditions. Dynamic stability is the manner in which an airplane tries to return to its original flight condition after being disturbed.

A good example of how a change affected pilot reaction to dynamic stability came when the Cessna 310 was introduced. The airplane had a barrel of gas on each wing tip, the first such in general aviation. This had what might be called a "flywheel effect." Start it rolling, and the weight at the wing tips would make it want to roll even more. Pilots would overcompensate for this, and PIOs in roll were common on turbulent-day approaches. The fix was simply for the pilot to stop all roll inputs and let the airplane settle itself down.

Static stability describes what the pilot feels in the control system of the airplane as he flies. The requirement is simple. The pilot must have to push or pull progressively harder to move the airplane farther away from the trim speed. Another way to look at this is in the control force required to pull 1 G, or the stick force per G. If an airplane has a bobweight in the pitch control system, the effective weight increases as Gs increase, meaning that the pilot would have to pull even harder to add the second G. That is a way of protecting the structure of the airplane. A stability augmentation system, and downsprings, are a form of this device. These simply induce a force into the pitch system that keeps the stick force required per G progressively positive.

None of the basic stability and control requirements say anything about the handling qualities of an airplane. An example might be found in the elevator control requirements, one of which is that, at forward CG, a tricycle-gear airplane must have the elevator authority required to land on the main gear. On a Baron, it requires an understanding of the airplane and a little practice to get tail-low landings. The same is true on a Twin Comanche. On a 210 or Bonanza, you can almost scrape the tail; the elevator power is that good. Yet all the airplanes flew out of the same rule book. The difference is that the Baron and Twin Comanche were developed from single- engine airframes, and the forward CG had to be pushed to the allowable limit because of having two engines forward of the CG instead of one.

Control power is important in other areas, too. Rudder control is a critical item for crosswind landings. When I had a Cherokee Six and based it on a single-strip airport, there were frequent crosswinds the airplane couldn't handle. I would simply run out of rudder control and would have to go elsewhere. I got a little ice on the airplane one day and found out why rudder authority was limited. Only about the top half of the vertical fin had any ice on it, demonstrating that the flow over the vertical tail and rudder was none too good. The high-wing Cessnas have very effective rudder control and are excellent in crosswinds.

Another handling item that is important is related to the increase in drag as speed decreases. This is not a big factor in light airplanes, right down to the stall, but in some heavier airplanes if the speed gets low, the amount of power required to patch things up increases — to the point where full power won't cut the mustard. If an airplane gets into a situation like this, the only way out is an increase in airspeed to reduce the drag. That takes altitude, and depending on the phase of flight, there may not be much of that available. The moral to the story is to not get too slow in the first place.

As handling qualities go, there have been few problems with certified airplanes. The Aerostar had to have some handling quality modifications to assure adequate control at low airspeeds. Because of accidents, some airplanes have undergone a review of their certification. Some models of the Learjet, the MU-2, and the Malibu are examples, and in no case did the FAA find that the airplane had any shortcoming. The Cessna 303 had problems with certain ice formations, and that was cured.

On balance, though, the flying qualities of our airplanes have withstood the tests of time. They sure don't all fly alike, though, and that is good, not bad. In many cases, what might be considered a quirk is related to an expanded CG range that pays back by making the airplane more useful. Or it might be more power, and breathes there a pilot who won't work a little in return for the enjoyment of some extra horses? In the end, whether the engineers like it or not, the statement, "That is one good flying airplane," depends as much on the seat of the pants of the beholder as anything else.