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e x p l a i n e d h o w to assess your airplane's static longitudinal stability, and we provided a basic test that allows y o u t o d e t e r m i n e E Effects of static longitudinal stability w h e t h e r y o u r p l a n e has positive, neutral, or negaED KOLANO tive static stability. This qualitative approach also provides a feel for h o w the pull and "trim speed" y o u fly, the higher the push force on the stick varies at air- stick force. Being aware of this tactile speeds different from the airspeed stick-force cue tells you that your airfor w h i c h y o u ' v e t r i m m e d . T h e n speed has c h a n g e d . But, y o u say, we s h o w e d h o w to q u a n t i t a t i v e l y that's what the airspeed indicator is test y o u r p l a n e ' s static stability, for. You're correct, but if y o u drop qualifying this m e t h o d as probably the airspeed i n d i c a t o r from y o u r a more in-depth approach than scan for some reason, this alternate most of us need. airspeed information avenue comes This m o n t h we'll see h o w y o u r in handy. airplane's static stability can make p e r f o r m i n g r o u t i n e p i l o t i n g tasks Final Approach easier or harder. Some planes seem Let's say y o u ' r e f l y i n g an ILS apto handle better than others. Han- proach in actual instrument condidling qualities are important, and tions. There's nothing to see outside, your airplane's stability characteris- so you're completely focused on the tics play a major role in determining instruments. The closer y o u get to them. the decision height, the more sensiIn a positively stable airplane you tive the glideslope and localizer neemust maintain a pull or push force dles become. o n the stick to fly slower or faster D u r i n g t h i s p h a s e of t h e apthan the speed for which the plane p r o a c h p i l o t s s o m e t i m e s d e v o t e is t r i m m e d . The further from the more attention to the needles and

less a t t e n t i o n to other instruments. If y o u find yourself h o l d i n g back-stick to keep the glideslope needle centered, you're probably flying slower than your trimmed approach speed. You might n o t have noticed the airspeed indicator deviation because you're c o n c e n t r a t i n g o n the ILS needles, but the stick-force feedback cue is there. It's "free" information in that it's delivered to your brain through an entirely separate channel. All you have to do is notice it. We're certainly n o t a d v o c a t i n g that y o u replace your airspeed scan with stick-force cues. We're suggesting y o u be aware of these cues because t h e y provide useful, and potentially lifesaving, information about your airspeed. If your airplane has neutral static stability, it will m a i n t a i n a n y airspeed w i t h n o c h a n g e in the stick force. This can be an insidious hazard because every speed feels like the trim speed, and t h e airplane will maintain the n e w airspeed, handsfree, whenever you release the stick. If y o u drop the airspeed indicator from your scan or an iced-over pitot Sport Aviation

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tube renders your airspeed indicator useless, you'll never know it through stick-force cues. An airplane with negative static stability can provide stick-force cues that are opposite of an airplane with positive stability. If y o u ' r e h o l d i n g back-stick in an airplane with negative static stability, it probably means y o u ' r e flying faster t h a n y o u r trim speedmhardly intuitive, especially if y o u r o u t i n e l y fly p o s i t i v e l y stable airplanes. To correct this situation you'd have to pull the stick even furt h e r to d e c e l e r a t e b a c k to y o u r proper approach speed. An actual instrument approach is

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y o u away f r o m y o u r n o r m a l scan. The stick-force cues of an airspeed change help y o u r e m a i n situationally aware whenever anything compromises your instrument scan. W h e n troubleshooting problems, static longitudinal stability testing c a n be u s e f u l . R e c e n t l y I f l e w a mildly unstable airplane. Its instability didn't compromise the safety of this day VFR-only airplane, but it did affect m y piloting performance. On the first c o u p l e of circuits I f o u n d myself gaining 5 to 10 m p h on final approach. A little analysis revealed that as I t r a n s i t i o n e d from a n o r m a l instru-

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n o t t h e o n l y t i m e these stick cues c o m e i n t o play. As y o u p r o c e e d down the glidepath, visual approach devices like VASI and PAPI c o m m a n d progressively more attention outside the cockpit. The more time you spend looking through the window, the less time y o u have available for scanning your instruments. Noticing a stick p u l l or p u s h a f t e r y o u ' v e t r i m m e d for t h e p r o p e r f i n a l approach speed is a clue that your airspeed has probably changed. Making radio changes, looking for traffic, checking an i n s t r u m e n t approach chart, or tending to other responsibilities like checklists can take log

JANUARY 2002

m e n t scan to a more external observation, I relied on stick-force cues to keep m y airspeed c o n s t a n t . Maintaining the same stick force hid the airspeed migration from the cue that would have been present in a plane with positive stability. It's i m p o r t a n t to r e m e m b e r t h a t using stick-force cues for airspeed dev i a t i o n s d u r i n g l a n d i n g does n o t w o r k if y o u d o n o t r e t r i m a f t e r changing the power setting. Adjusting power changes the pitching moment caused by the engine/propeller and therefore changes the elevator trim r e q u i r e m e n t . If y o u d o n ' t retrim, the stick force needed for your

final approach airspeed will change e v e r y time y o u m o v e the t h r o t t l e . Retrimming after the power change s h o u l d restore the stick-force/airspeed relationship.

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Different Cruise Story Longitudinal static stability describes the relationship between stick force a n d airspeed. To take a d v a n t a g e of this relationship as a stick-force cue to a changed airspeed situation, y o u need a reason to m o v e the stick. On final a p p r o a c h y o u ' r e always tweaking the nose a hair up or down to follow the VASI or glideslope needles or to put the r u n w a y where it's s u p p o s e d to be in the windshield. Cruise flight is different. Once established o n the desired altitude and airspeed, you trim out the stick force. Because you're not making any control i n p u t s , y o u s h o u l d n ' t e x p e c t any stick-force cues, so rely on your airspeed indicator and altimeter to note deviations. Despite this limitation, static stability still plays an i m p o r t a n t troubleshooting role in cruise flight. You don't need to trim a neutrally stable airplane because it m a i n t a i n s a n y airspeed hands-free. The difficulty is e s t a b l i s h i n g t h e desired airspeed. You must select a stick position and wait to see w h e t h e r it's the correct one. Unlike the positively stable airplane, where y o u can finely adjust y o u r stick f o r c e to e s t a b l i s h t h e proper airspeed, in a neutrally stable plane the stick force disappears when y o u s t o p m o v i n g t h e stick. This means a series of trial and error stick placements to get it right. Even in a p o s i t i v e l y stable airplane, h o w the required stick force varies with airspeed can affect the ease or d i f f i c u l t y in t r i m m i n g for hands-free flight. Figure 1 shows the static stability curve for a homebuilt kit n o longer in production. Notice that it takes o n l y 2 p o u n d s of stick p u l l to fly m o r e t h a n 100 m p h slower than trim speed. That s a gra d i e n t of m o r e t h a n 50 m p h p e r

routine piloting tasks easier or

harder. p o u n d of stick force or about 3 m p h per ounce of stick pull. Okay, m a y b e this m a t h drama is unfair, but this airplane was difficult to trim because small airspeed adjustments required tiny stick forces. Instead of a trim tab this plane had a trim lever connected to the elevator c o n t r o l system b y a spring. W h e n

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you moved the lever it pulled on the control system to reposition the elevator (and the stick). Friction h e l d t h e lever in place, and it required a 3-pound effort lust to m o v e the lever. W h e n you're trying to change the trim b y an ounce to two, the 48 ounces of force needed to m o v e t h e l e v e r c a u s e d q u i t e a problem. If y o u h a v e difficulty t r i m m i n g y o u r airplane for cruise flight, it's n a t u r a l to b l a m e y o u r s e l f at first, and then the trim system, but maybe it's y o u r a i r p l a n e ' s static stability. P e r f o r m i n g the static stability tests should answer this question.

Other Implications Positive stability is a good thing, but if the stick-force-versus-airspeed gradient is too steep, it can cause problems. Perhaps the m o s t obvious one •

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Test Pilot is it will take a lot of effort to fly an off-trim-speed airspeed w i t h o u t ret r i m m i n g . Naturally, y o u ' d retrim f o l l o w i n g a n y l o n g - t e r m airspeed c h a n g e , b u t t h e r e are times w h e n you need to make a temporary speed change. If you fly the glideslope with the stick a n d adjust y o u r d e s c e n t rate with throttle, y o u r effort level will probably be higher in a plane with a steep gradient, but you're more likely t o . m a i n t a i n the desired airspeed w i t h o u t i n a d v e r t e n t excursions once you've trimmed for it. Flying wing in a formation is ano t h e r t i m e too m u c h stability can increase y o u r workload. W h e n flying close formation your eyes are essentially locked on the lead airplane. You trust the leader to fly the correct airspeeds, and y o u steal a glance at your instruments whenever you can. If the leader changes airspeed, ~r, ....

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102

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you'd know it by the requirement to h o l d back- or f o r w a r d - s t i c k to remain in position in a positively stable airplane. This might be a bit of a stretch if you're constantly m o v i n g the throttle, but you shouldn't have to if your lead is smooth. Describing the e x t e n t of an airplane's stability by h o w m u c h stick force it takes to fly off-trim airspeeds is a traditional metric, b u t it's n o t entirely correct. You could change !he stick forces b y a d d i n g springs into your airplane's longitudinal control system. This doesn't change the airplane's stability, b u t it does change your impression of it. That's w h y t h e stick-force/airspeed relationship is called " a p p a r e n t stability," and it is an absolutely valid way to assess h o w y o u r plane's stability affects your ability to accomplish piloting tasks. Before you install springs in your flight controls, understand that they will probably change other static and dynamic stability characteristics. Indiscriminate use of c o n t r o l system springs or other stick-force-modifying devices can make y o u r airplane b e h a v e in an u n s t a b l e m a n n e r . If you don't like your plane's static stability characteristics, talk with the plane's manufacturer or designer before making control system changes. This m o n t h we touched on a few s t a t i c s t a b i l i t y issues t h a t c o u l d make y o u r p i l o t i n g tasks easier or harder. We showed h o w you might use the test techniques presented in the last three "Test Pilots" to troubleshoot seemingly unrelated problems. Finally we c a u t i o n e d against solving apparent stability problems with home-grown flight control system modifications. Next month we'll address Jim McCulley's question about maneuvering speed. Send y o u r c o m m e n t s a n d suggestions to Test Pilot, EAA Publications, P.O. Box 3086, Oshkosh, WI 54903-3086 or to [email protected] with TEST PILOT as the subject of your e-mail.