July 21, 2008
If a nonpilot asks you "How do you take off?" how would you answer? Line up with the runway, add power, accelerate to liftoff speed, raise the nose and go. But is it really as simple as that? Every year airplanes fail to get off wet or muddy runways, or to clear obstacles past the departure end. Every summer we hear of airplanes that can't get airborne or, if they make it into ground effect, don't have the power to climb any higher. The answer to "How do you take off?" is "I have to choose the technique to match the takeoff conditions." So how do you choose your takeoff?
Rolling Takeoff, Or Power Up On The Brakes?
Do you stand on the brakes and power up, or do you prefer to add power smoothly, letting the airplane roll as you advance thrust? A rolling takeoff is less stressful on the airframe (and occupants). Although some pilots have a personal preference one way or another, there is no "correct" answer; what you do depends on the requirements of that takeoff.
Try this experiment: On a calm-wind day, put an observer in the copilot seat. Line up with the runway centerline on a specific spot, such as the runway numbers. Add power for a "rolling" takeoff ... that is, let the airplane accelerate naturally as you advance the throttle. Accelerate until you lift off, having your observer count the number of runway stripes and spaces from power-up to liftoff. If the runway has standard markings, each stripe-and-space combination is 200 feet long. Regardless, fly the pattern and land, having recorded your estimated runway requirement for takeoff.
Now repeat the experiment, except from your identified starting point power up while holding the brakes. As soon as you achieve full power release brakes (you'll need more rudder initially than a rolling takeoff, because low air flow over the rudder reduces its effectiveness at countering torque). Accelerate to the same takeoff speed, with your observer counting the runway stripes-and-spaces.
Do this a few times each way to gauge the difference at a variety of weights and density-altitude conditions. What you may find in many airplanes is this: A rolling takeoff results in a longer takeoff roll. The heavier the airplane and/or higher the density altitude, the greater difference you're likely to see. So back to answering that nonpilot's question: If the objective is to get off a short runway, or to take off when you have a nearby obstacle, or when departing with a high-density altitude (you'll have to explain that to a nonpilot), you'll probably choose to make a "power up on the brakes" departure. Otherwise, you'll likely make a rolling takeoff to reduce stress on the airplane.
Flaps Or No Flaps?
Do you use flaps on takeoff, or not? The effect is going to vary widely by airplane type, so you'll need to do another experiment. (Getting to better know your airplane's flight characteristics ... what a good excuse for doing a little proficiency flying!) First, look for any limitations on flap use for takeoff in your airplane's flight manual or POH (not just Performance section recommendations -- I mean Section II, Limitations, which are imperatives).
Repeat the rolling/braking experiment, but this time using a consistent technique (i.e., rolling) but with any flap settings that is not prohibited by the manufacturer. On each takeoff, call out when you're 50 feet agl (or, if it's easier to read, 100 agl) and have your observer note where you are over the ground when you make that call.
Determine what flap position gets your airplane off the ground soonest and what gets you to the altitude goal (50 or 100 agl) in the shortest distance. Again, it will vary by type, weight and density altitude, but you might find that your airplane gets off the ground sooner with flaps, but takes a shorter distance to meet the altitude goal without them. If that's the way your airplane flies, your answer to that nonpilot might be, "If the runway is short but there's no obstacle, I'll use flaps, but if there's a power line or trees off the end of the runway, I'll take off with the flaps up."
"How fast is your airplane when you take off?" your nonpilot friend inquires. As you may have already figured, the answer is again, "It depends." Depending on the vintage and complexity of your airplane, the manufacturer may have specified a takeoff speed. It will even vary with airplane weight in heavier aircraft. For your experiment, pick a technique (rolling or braked power-up) and a configuration (flap setting) and have your observer note takeoff distance and the point you reach your altitude goal when lifting off at the "book" speed. Next, repeat the trial but let the airplane lift off "when it's ready" (the heavier the airplane, the less wont it has to do this). Finally, consistent with the airplane's capability, try a couple "soft-field" takeoffs, lifting off into ground effect at a slower airspeed.
See what effect different liftoff speeds have on runway distance and initial climb. Can you generalize your airplane's performance for the nonpilot, and your own choice of takeoff technique?
VX or VY?
Closely aligned with liftoff speed is the target speed for initial climb. If your goal is to climb steeply over an obstacle, use VX. It'll be listed in the POH, perhaps modified (and thinly disguised) as a recommended 50-foot target airspeed, adjusted for airplane weight, on the takeoff performance chart. Note that the airplane may have different published speeds for different takeoff flap positions. On some of your "experimental" takeoffs, find the attitude that results in VX speed passing over that mythical 50-ft obstacle and have your observer record the distance it takes to reach that height from rolling/braked, flaps/no flaps conditions. Do the same at VY speed passing 50 agl. What you'll likely tell your nonpilot inquisitor is, "I will begin my climb at VY unless I have to clear a close-in obstacle, in which case I'll climb at VX.
But wait: You may want to climb out even faster than that. If engine cooling is an issue, or in multiengine airplanes where speed is your best defense against loss of control in the event of an engine failure, you may want to climb out at an even shallower angle and higher airspeed if obstacles permit. Figure a few of these faster initial climbs into your exercise and see how much distance it takes to get to 100 agl. You may tell that nonpilot you'll normally climb out even faster than "book" unless conditions dictate otherwise. That's powerful information to know as you choose your takeoff technique on any given day.
Chances are your nonpilot friend won't think to ask you about fuel-mixture technique. But you need to make a conscious decision about it. Mixture technique for high density-altitudes is a common topic in aviation publications (see "Hot and High How-To" in the July 2008 issue of our sister publication Aviation Safety). Yet it's obvious from the summertime mishap reports that the lesson needs to be reviewed each time the weather begins to warm.
The trend in more powerful engines is to crank up the fuel flows to their maximum to provide additional cooling at high power settings. This is especially true with turbocharged engines. The high flow rates now in vogue serve their purpose, but they may inhibit takeoff performance by creating too rich a fuel-air mixture. In hot weather (with a long runway), experiment with full rich and leaned mixture, measuring their effect on takeoff roll and the distance required to get 50 or 100 feet into the air. You may find to clear an obstacle, especially at a high density-altitude, you need to sacrifice a little long-term engine cooling (full rich mixture) for short-term performance (leaned for maximum horsepower). Afterward, if your nonpilot friend does ask you about mixture technique for takeoff, you might reply, "I use full rich except as need for takeoff performance, and if so, then I enrichen the mixture as necessary for engine cooling after transitioning to en route climb."
Choosing Your Takeoff
Planning, flying and interpreting these experiments would make a great instructional session. Combined with the required ground instruction and with a CFI as your observer, it could be a very informative flight review (see last month's Leading Edge column). Use a long runway so maximum performance isn't required for safety; adhere to all airplane and engine limitations.
Airplane weight and density altitude will greatly affect performance, so be careful that you generalize only for a given set of conditions. Fly extremely conservatively when outside your area of experience. By thinking about how you'd answer that simple nonpilot's question, "How do you take off?" you'll ask yourself what technique you should use every time you choose your takeoff.
Fly safe, and have fun!
Thomas P. Turner's Leading Edge columns are collected here.