Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/113663
possible. This is our proof that the smallest, or shallowest, flight path angle occurs when L/D is maximized. Notice there's no W in the last equation. That's because your airplane's glide range does not depend on its weight. Okay, so far we know that the maximum glide range is achieved when your glide has the smallest flight path angle, and that occurs when your plane is flown at its maximum L/D. Now we'll show that the maximum L/D occurs at a single value of AOA. If we look only at the lift equation, the biggest value of CL would produce the biggest value of L, which might imply the greatest L/D, but this is not the case. Take a look at the drag equation. Notice the similarity to the lift equation. CD is the drag coefficient, and it accounts for induced and parasite drag. Figure 3 shows a generic relationship between the lift curve and the drag curve versus AOA. Notice how the increase in CD dramatically exceeds the increase in CL at higher values of AOA. The maximum L/D, which is the same as maximum CL/C D, occurs where the vertical distance between the two curves is greatest—well below the AOA for maximum CL. You can also see that there is only one AOA where this occurs. (For the purists, the lift and drag curves in Figure 3 would normally be vertically separated. They're shown as they are to make the illustration more clear. The maximum CL/CD AOA is the same.) You already know from last month that the lift curve in Figure 3 applies to any airplane weight at any altitude. The same is true for the drag curve. The bottom line here is there is only one AOA that will give you the farthest engine-out glide range, and this is why you can't stretch a glide. Your glide speed and descent rate will be faster when your airplane is heavier, but the range will remain the same. If you change the airplane's configuration, as in lowering the flaps or leaving the prop in flat pitch, the lift and drag curves in Figure 3 will change, but there will still be just one AOA (probably different from the clean configuration AOA) for each configuration that produces the farthest glide distance. There's a technical aviation term called the pucker factor. It's a variable whose intensity depends on the na- ture of the event that causes its occurrence. No one has yet derived a reliable equation for the pucker factor, but empirical data and qualitative evaluations allow us to conclude that a sudden loss of engine power yields an immense pucker factor. Sizable pucker factors inhibit a pilot's math skills and recollection acumen. What's my best glide speed? What airplane weight does it apply to? How much does my plane weigh right now? What's the speed adjustment I have to make to make sure I'm flying at the best glide speed? Wouldn't it be reassuring to know that little "tic" mark labeled "Max L/D" on your AOA gauge guarantees the best glide? Following an engine failure, most of us will probably transition to some memorized glide speed as we attend to the other restart and forced landing procedures. This procedure might be sufficient if your selected landing site is nearby or if your restart is successful or if the weight variation of your plane is small enough that a single glide speed guarantees 99 percent maximum range for any weight. For all other conditions, that AOA gauge could be a lifesaver. An AOA Bonus Beyond the value of AOA as a stall margin indicator and maximum glide range instrument, there's another benefit. Your airplane will cruise farthest when you fly at its maximum L/D. Your airplane's maximum L/D AOA is the same for maximum cruise range as it is for maximum glide distance, assuming the airplane configuration (landing gear, flaps, etc.) is the same. Let's take a less technical, more intuitive approach to this. Maximizing range is about efficiency. The more efficiently you fly your airplane, the farther it will travel given its available altitude or fuel. Maximum lift and minimum drag would certainly be ideal, but the laws of aerodynamics don't allow this. Maximum lift occurs just as the wing stalls. Even if you flew a couple of knots faster than stall speed, you wouldn't get very far because of the slow airspeed and high power (and fuel flow) requirements of slow flight. Minimum drag seems like a good idea, but what you gain in endurance because of the lower power requirement you lose in the resulting slow airspeed. Your fuel would last longer, but you wouldn't travel as far. EAA Experimenter 45