Stick & Rudder

Test Pilot IN JANUARY WE EXAMINED A Aircraft designers can few flying scenarios influcombine all of these words enced by an airplane's lonand numbers into a single gitudinal static stability. picture they call a "V-n diaWe showed how various gram," because it's plotted in stability characteristics terms of calibrated airspeed Flying safely inside the V-n boundaries make your piloting tasks (V) and load factor (n), and easier or harder, and how Figure 1 is the V-n diagram ED KOLANO you might use the test for our airplane. You can techniques presented in think of load factor as g; in the October through December 2001 knots calibrated airspeed, and stalls, other words, the airplane has a load "Test Pilot" columns to troubleshoot wings-level, at 60 knots. It's a home- factor of 2 when it's "pulling" 2g, as it seemingly unrelated problems. Fi- built, but it meets the structural re- would in a level 60-degree-bank turn. nally, we cautioned against solving quirements for an airplane certifi- (Because of this, the diagram is someapparent stability problems with cated in the normal category, times called a "V-g" plot.) homegrown flight control system including a 3.8g limit load. Stall Line modifications. A while back EAA member Jim The curved line extending from the zero n, zero V point to the point laMcCulley wrote to "Test Pilot" and beled "A" is the stall line. You can't asked for clarification about maneufly to the left of this line because the vering speed (V^). Like the rest of us, wing will stall before the airplane Jim had read different (and often will fly that slowly. Another way to conflicting) explanations of maneuvering speed. Misunderstanding V'A look at it is that you can't fly above this line because the wing will stall could lead to in-flight structural damage to your airplane, so let's before you can achieve the higher Our airplane's limit load means load factor. This is an important feaclear up the confusion. Here are a few definitions of ma- the airframe can bear 3.8 times the ture of the V-n diagram. From Ig draw a horizontal l i n e neuvering speed you might have airplane's 1,000-pound weight without suffering deformation or dam- across to the stall line. Note that it inheard: VA is the maximum airspeed at age. This is another way of saying tersects at the Ig, or wings-level, stall which the airplane will stall before the airplane, at its 1,000 weight, is speed. You know from experience that designed to encounter 3.8g in flight the airplane will not sustain flight structural damage can occur. V A is the minimum speed at without damage. It has nothing to slower than its stall speed. Attempting which the limit load factor can be do with exceeding the airplane's to fly slower would be trying to operate to the left of the stall line. specified maximum gross weight. developed aerodynamically. Like a production airplane that You also know from experience V A is the maximum speed at which the controls may be fully de- complies with the Federal Aviation that pulling the stick back while flyflected without over stressing the Regulations, our airplane has a 50- ing just faster than the stall speed percent safety factor designed into its leads to a stall without generating airplane. Confused? You should be, because structure, giving it an ultimate load any more g. This would be trying to not all of these definitions are cor- factor of 5.7g. Exceeding 3.8g can operate above the stall line on the Vrect. Let's see where V^ comes from, cause structural damage like popped n diagram. Draw another horizontal line for and then you can select your own rivets, buckled skin, and compromised composite material, but noth- 2g flight. It intersects the stall line at definition. Our example a i r p l a n e weighs ing should fall off the a i r f r a m e . approximately 85 knots. This is our 1,000 pounds, has a 200-knot never When you exceed 5.7g, things can airplane's 2g stall speed, and if you try to pull 2g when you're flying slower exceed speed (V NF ), cruises at 150 start falling off—like wings.

Maneuvering Diagram

The more you know about how your airplane flies, the safer you can fly it.

Sport Aviation

95

Test Pilot than 85 knots, the plane will stall before it achieves 2g. Similarly, you can't pull more than 2g at 85 knots because

the plane will again stall. If you want to pull more than Ig, you must fly faster t h a n 60 knots, and to pull more than 2g, you must fly faster t h a n 85 knots. This may sound like a restriction, but it's actually a nice built-in protection against over stressing the airplane. At these slow airspeeds you can't reach the airplane's 3.8g limit load factor because the airplane will always stall first. Load Factor Limits The diagram's 3.8g horizontal line represents our airplane's limit load factor. You already know that exceeding this load factor can damage the airplane. It may not come apart, but it very well may bend permanently. Above the 3.8g limit load factor line is another horizontal line drawn at 5.7g. This is the ultimate load factor line. The 5.7g represents the 50percent safety factor required by the FARs (1.5 x 3.8 = 5.7) for certificated general aviation airplanes. Exceeding this line can cause the plane to break up in flight. Maneuvering Speed The stall line meets the limit load factor line at our airplane's maneuvering speed at its 1,000-weight, 117 knots. Flying our hypothetical airplane at 1 1 7 knots or less ensures that it w i l l stall before over stress damage occurs. When flying faster than V,\, it's up to you to ensure you don't over stress the plane. This V A corner of the maneuvering envelope is sometimes called the corner speed. Fighter pilots like to operate here because VA is the slowest speed where they can achieve the m a x i m u m safe g—and the slower the speed, the smaller the airplane's turn radius. Limit Airspeed

The vertical l i n e defining the right 96

FEBRUARY 2002

side of our airplane's maneuver diagram is the limit airspeed line. This is our plane's VX1,. Flying faster than VN1. at any g level may r e s u l t in structural damage or failure. Aileron reversal due to wing warping, des t r u c t i v e f l u t t e r , and other bad things for an airplane's structure can happen out here. Don't ever fly faster than VNl.-. Less Than Ig Notice that the bottom of our maneuvering diagram looks like a smaller and slightly distorted mirror image of the top. Maneuver l i m i t s

also apply in the negative sense. The regulations require certificated airplanes to have a negative limit load factor of at least 40 percent of their positive limit load factor. For most n o r m a l category a i r p l a n e s , this works out to be negative 1.52g (0.4 x 3.8 = 1.52). The ultimate negative limit load factor retains its 50 percent safety factor. It's negative 2.28g (1.5 x 1.52 = 2.28) for our example. Is It g or Weight That Matters? Discussing airplane maneuvering in terms of load factor or g is convenient because pilots can relate to it. POS Ultimate Load Factor

"O CD

O

Airspeed (knots)

Neg Ultimate Load Factor

Figure 1

Structural Failure POS Ultimate Load Factor

Structural Damage POS Limit Load Factor

1000-pound airplane _ "U ~

co

o

150

-3-

Figure 2

200

Airspeed (knots)

We know what pulling g feels like. We know that our bodies feel twice as heavy during a 2g maneuver. We also know that our passengers' bodies feel twice as heavy to them, even if they weigh less. Here's where you have to be careful when specifying load factor limits. Our airplane's wing spar can support a certain weight or force; anything more and it begins the journey

from bending to breaking. The spar doesn't care if the force comes from a 2,000-pound airplane flying straight and level or a 1,000-pound a i r p l a n e p u l l i n g 2g. The 2,000pound bending force on the wing is the same in either case. Pulling 2g feels the same to you (and reads the same on your plane's g meter) in the airplane loaded to either weight. But the force exerted on the wing is twice as much on the airplane with a 2,000 pound gross weight (2g x 2,000 = 4,000 pounds) than it is with the 1,000-pound loading (2g x 1,000 = 2,000 pounds). Let's use more realistic loadings to see how an airplane's weight affects its VA. We're flying our plane solo, so its gross weight is 750 pounds instead of the 1,000 pounds it weighs with our usual passenger. If Figure 1 were based on the 1,000-pound airplane, with the lighter gross weight, you m i g h t determine that you could safely pull more than 3.8g because the limit load factor line accounts for 3,800 pounds (3.8 x 1,000 = 3,800) of wing strength. Your math says with only 750 pounds, you can safely pull

(and I'm not guaranteeing it will), but it may not help you get home with a self-destructed engine compartment, bent and extended landing gear, and that big hole in the floor behind the seat. As long as you observe the a i r plane's V,\, you should be fine. But wait—the V,\ in Figure 1 is based on a 1,000-pound airplane. The V,\ for the same airplane loaded to 750 pounds is slower.

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5.07g (3,800 / 750 = 5.07) without

exceeding the wing's capability. This is not safe. The wing is designed to support 3,800 pounds based on 3.8g at a 1,000-pound weight, but other airplane components may be designed for 3.8g, period. Landing gear uplock strength may be based on a limit load factor of 3.8, and so might the engine mount, baggage compartment floor, and who knows what else. The wing may be okay at 5.07g

Figure 2 shows the stall line for both loadings. Experience tells you that an airplane stalls at a slower speed w h e n it's lighter—8 knots slower, in this case. This is true for all load factors, too. When you move the stall line for the lighter-weight loading to the left of the original stall line, notice that VA also moves left, or slower. V A for the lighterloaded plane is 101 knots, or 16 knots slower than the 1,000-pound

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Test Pilot airplane V.\. Flying the 750-pound airplane at the 1,000-pound airplane VA will not ensure that the airplane will stall before structural damage occurs.

felt the r e s u l t i n g momentary increase or decrease in load factor. An airplane wing doesn't discriminate between a load factor increase the pilot causes with back-stick and a load factor increase caused by a gust. These load factors combine as far as

When flying the 1,000-pound airplane's VA in the 750-pound airplane, the lighter airplane won't stall the wing is concerned. until it achieves 5.07g, dangerously Encountering a 30-foot-per-secclose to the ultimate load factor! ond vertical gust when you're alOur airplane may have i n s u f f i - ready pulling 3.8g will over stress cient elevator authority or too much your airplane. If you fly no faster induced drag increase to actually get than VA in turbulence, the vertical to 5g at 117 knots, but I don't sug- gust will cause a momentary stall gest betting your life on it. If you rather than increase the load factor know your airplane's V A for a spe- past the safe level. cific weight, you can calculate its VA VA is not the speed where the for any other weight using Formula green and yellow arcs meet on your 1, where V A1 is the original maneu- airspeed indicator. This is V N Q, or vering speed for the original W j the maximum structural cruising weight, and VA2 is the new maneu- speed, which is defined in part with vering speed for the new W2 weight. an assumption that in turbulent air Even with your weight-adjusted you'd be flying straight and level VA, it's always good practice to know and not maneuvering. where you are in your plane's maThe V-n diagram does not acneuvering envelope at all times. count for rolling maneuvers. The diDon't rely on the stall line's auto- a g r a m — a n d VA—assumes a pure matic over stress protection as a p i t c h i n g maneuver. If you roll as guarantee. you pitch, parts of the airplane exBesides weight's effects on the V-n perience more g than other parts. diagram, you should know about Let's say you're pulling 2g and several other factors. Each V-n diagram per- Formula 1: VA1 is the original maneuvering tains to a single airplane speed for the original W^ weight, and VA2 is the configuration. Changing new maneuvering speed for the new W2 weight. the configuration with VA2 = V. the position of the landing gear, flaps, and canopy can all affect the diagram— rolling right-wing down. To roll, the and your airplane's VA. left wing must produce more lift There will probably be additional than the right wing, and that means limitations for other configurations more g at the left wingtip than at the that create different boundaries on right wingtip. It also means more g the V-n diagram. For example, most at the left wingtip than you and your airplanes have speed limits on gear g meter sense in the cockpit. Your and flap use, and most have load airplane's limit load factor is lower when rolling, and its VA is slower. factor limits as well. Finally, let's dispel a dangerously popular myth that V A is the maxiBehind the Picture The V-n diagram as shown in Figures mum speed at which you can fully 1 and 2 does not account for turbu- displace the controls without over lence or wind gusts. We've all been stressing the airframe. Fully deflectbounced around the pretty blue sky- ing your plane's ailerons w h i l e filled with white puffy clouds and 98

FEBRUARY 2002

pulling near-limit g at V^ can damage your airplane. One might argue that the increased lift demanded of the wing with the downward-deflected aileron will cause it to stall before exceeding the limit g. It might, but here are a couple of reasons not to try. First, if you're correct, a sudden stall of one wing could make for a very impressive snap-roll-type maneuver begun at nearly 4g. Second, if you're a few knots faster than VA when you slam the stick to the side, over stressing the airplane is almost guaranteed. Third, startled by a single wing stalling unexpectedly during such a loaded maneuver, a pilot might make control inputs that could place the airplane in a dynamic environment where the load factor increases could come from non-aerodynamic forces. These "mystery" forces, such as gyroscopic precession, could turn your airplane in very unexpected directions. How many sideways g can your airplane handle before the vertical tail snaps off? We spent a lot of words this month talking about the V-n diagram. The more you know about how your airplane flies, the safer you can fly it. We can boil these few thousand words down to a couple of must-know items: Maneuvering speed is d i f f e r e n t for different weights and configurations, and if your maneuver involves rolling and yawing, don't do it near your airplane's limit load factor. Next month we'll jump back into performance testing. We'll take review performance testing basics and look at data obtained during performance test flights in the EAA's Young Eagles RV-6A. Thanks, Jim, for your question about maneuvering speed. Send your comments and suggestions to Test Pilot, EAA Publications, P.O. Box 3086, Oshkosh, WI 54903-3086 or to i>ditorial(a\>aa.org with TEST PIEOT as the subject of your e-mail. EAASport Aviation

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