## Test Pilot: Maneuvering Diagram

Maneuvering. Diagram. Flying safely inside the V-n boundaries. ED KOLANO knots calibrated airspeed, and stalls, wings-level, at 60 knots. It's a home- built, but ...
Stick & Rudder

Maneuvering Diagram

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

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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

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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)

Figure 1

Structural Failure POS Ultimate Load Factor

Structural Damage POS Limit Load Factor

1000-pound airplane _ "U ~

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o

150

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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

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