Let the Airplane Make the
Takeoff |
 |
by
Budd Davisson, appeared in Flight
Training Magazine
A take off should be a thing of beauty, not a brutal experience |
Here’s an absolute fact: an airplane knows how to fly better
than we do. In many situations, the takeoff being a perfect example,
if we just set up the right parameters, the airplane will execute the
maneuver perfectly with little or no help from us. In fact, the less
we mess with it, the better off we’ll be. That being the case,
let’s talk about a takeoff technique that is very nearly hands-off,
but at the same time produces a takeoff that is not only safer, but
borders on being artistic.
First, let’s decide what it is we’re trying to accomplish on a takeoff. Is it just getting it off the ground or is there more to it than that? The answer is “yes” and “yes:” we are trying to get off the ground but there is much more to it than that.
Some will argue that a takeoff is a takeoff and any way you can get off the ground is okay, but that’s simply not the case. The same factors that affect a landing (wind, weight, temperature, runway surface, etc) also figure in on a takeoff. Sometimes these factors are even more critical taking off than on landing. Still, it is seldom that a pilot gives much thought to those factors unless he is one of the few who is working short, high-density runways with trees at the end.
At the same time, it’s seldom we give much thought to the “art” of the takeoff. We don’t think of it as a maneuver that should be executed with grace and precision. After all, all we have to do is ram the throttle in, get the nose up and the airplane will take off.
The reality is that the art and technology of the takeoff are tightly intertwined and, if you strive for one, you wind up with the other, whether you want it or not.
The factors that affect the efficiency and safety of a takeoff are well known to one and all. They are, in no particular order, aircraft load, temperature/humidity/altitude, which combine to determine density altitude, runway surface, runway grade (slope), runway length and obstacles at the end. If we assume a “normal” runway in which length and obstacles don’t present an extreme situation, then we’re left with factors that affect us on every runway, chief among them being density altitude and load.
There are lots of rules of thumb about handling takeoff parameters, but the reality is that most of them are a little nebulous and difficult to quantify for the airplane we’re flying. The numbers below are factors compiled from various (mostly Cessna) POH’s that, while giving good starting points, don’t tell the entire story.
Factor Factor
Increase Increase
in Takoff Roll
Weight 10% 20%
Altitude 10% 10%
Temperature 10°C,
18°F 10%
Slope 2% 10%
Tailwind 10%
of takeoff speed 20%
Grass Dry 20-25%
Wet 25-30%
Soft 25%
It’s instructive to study the above factors because it takes only a few seconds before you’re asking yourself questions like, “Okay, so I’m 500 feet higher than my home airport, I’m heavier than I normally fly, it’s hot out, so how much does that effect my takeoff?” The answer isn’t easy because it’s hard to determine the exact effect of all the factors added up. All you know is that the airplane isn’t going to be developing either the power or the lift and you’re trying to lift more weight than usual so your takeoff roll is going to be longer. But, that’s not an answer. We knew that before we asked the question. We still don’t know exactly what takeoff speed is required to compensate for all of the above factors.
Airliners and other heavy, fast movers have charts and graphs (now contained in a calculator) that give them exact rotation speeds. We have POH’s that also give us rough guidelines, but they almost never include any of the above factors. So, what do we do?
The key to any takeoff is to assume the airplane knows more than we do. Every airplane has two distinct factors that, once the factors discussed above are taken into account, determine when it’s going to takeoff in any situation: speed and angle of attack. At a given angle of attack, there is a specific speed at which enough lift will be generated that it overcomes all of the factors. This takes into account the airplane’s weight, the temperature, the humidity, what you had for breakfast and every other aspect of the airplane that changes the takeoff. So, if a given angle of attack is selected and held, as the airplane accelerates, when it reaches the magic speed, the curves will cross and the airplane will fly off the ground.
So, what angle of attack do we hold and how do we measure it? The answers are, we don’t know what angle of attack is needed and we have no accurate way of measuring it. About the best we can do is make guestimates as to what angle is “a lot” and what is “not very much.” Oddly enough, that’s close enough.
The higher the nose and the more angle of attack that’s held, the closer that angle will be to the critical angle (stall) for that operating condition and the airplane will lift off close to stall speed. Conversely, the more shallow the angle, the larger the margin on takeoff between stall and lift off speed. So, we chose our nose angle based on what we’re trying to accomplish during the takeoff.
When you’re talking about speed and angle of attack, it’s important to remember that even though a given angle of attack at a given speed will usually generate about the same amount of lift (this is an approximation), that doesn’t necessarily mean the airplane is ready to fly. That amount of lift may not be enough to lift the airplane in those conditions, so it needs a little more speed at that angle. For that reason, even though it may look as if you’ve held the nose in the same place every time, that doesn’t mean it’s going to lift off at the same speed or in the same distance. Too many other factors come into play.
There is, however, one hard and fast rule of thumb we can adhere to: the higher we hold the nose on takeoff, the slower it will lift off, while the flatter we hold the nose the higher the lift off speed will be and the airplane will be more solidly in the air. A softfield takeoff is an example of a max angle takeoff, but the airplane is barely flying and doesn’t give us much energy to fight gusts or crosswinds, so it’s not an appropriate technique for normal flight operations.
So, what is an appropriate technique? Funny you should ask. In nearly all flight situations, if we establish a nose-gear-barely-off-the-ground attitude before reaching flying speed, the airplane will lift off at stall speed plus an unknown, but fairly sizeable. factor. What that means is the airplane leaves the ground with enough energy that you can handle anything the environment can throw at it.
In a tailwheel airplane this technique translates into picking the tail up and holding an attitude that is just a little tail down from level.
The procedure is fairly cut and dried but does entail just a few nuances calling on you to really see what’s happening in the windshield.
We’ll start the takeoff (we’re assuming a tri-gear airplane) as we normally would, but, as soon as the airplane is rolling fast enough that we sense some life in the elevators, we position the yoke aft of neutral and hold it there. What we’ve done is insert a nose-up command in the elevator before the airplane has enough speed to pick the nose up. Then, when the airplane accelerates through the speed required to unstall the tail and pick the nose up, we immediately release some backpressure. At the same time, we look over the nose and visually fixate on where the nose is in relation to the horizon and, from that point on, our entire focus is to hold the nose in that position.
Because the airplane is accelerating, if we don’t release back pressure and set a given nose attitude, the nose will continue coming up as the airflow over the tail increases with speed. What we’re trying to do is establish a slightly nose up attitude that positions the nosewheel something like six inches off the ground. Our goal is to hold that attitude firm, which will require us to gradually ease off the backpressure as the airplane accelerates. The first few times you do it, it’ll be a little counter intuitive because we’re actually moving the yoke forward slightly as the airplane takes off. The name of the game, however, is to use the nose like a rifle sight and absolutely nail it on a given position on the horizon and hold it there no matter what.
The net result of this little exercise is that the nose will come up, the airplane will run on the mains for a few seconds and will lift off when, and only when, the lift/speed curves cross and exactly match the environment in which the airplane is operating. It’ll compensate for every single thing having to do with the takeoff and won’t leave the ground until every factor is right. This is an instance where we can’t possibly know as much as the airplane does in terms of what is needed at that precise moment, in that precise location, with that specific airplane to safely leave the ground and be guaranteed of a positive rate of climb.
Not everyone ascribes to this type of takeoff and prefers, instead, to simply accelerate the airplane, nose on the ground, until they are sure they have a safe margin and then forcibly rotate the airplane into the air. To many, however, this displays both a certain arrogance, in assuming the pilot knows the airplane is ready to fly, and a general lack of feeling for the airplane, because it is a brutish, less-than-graceful maneuver.
For many pilots, the concept of letting the airplane run on the mains and gracefully lift off in its own time is another way of saying they consider piloting to be a form of bio-mechanical partnership. They are trying to unite with the airplane and, between the two of them, create something that is both beautiful and natural, rather than being mechanical and semi-crude.
The real pay off for letting the airplane make the takeoff decisions, however, is increased safety. Most western states pilots understand that it’s easy for a flatlander to get spooked by a really long takeoff roll on a hot day and force the airplane into the air. Folks who deal with density altitude all the time know it’s not a good idea to “force” an airplane to do anything. The willingness to force an airplane into the air in the wrong situation is why we have so many takeoff accidents in the west during the summer. In the west, we let the airplane do the deciding.
In reality, you can yank an airplane in the air and be okay, most of the time. But it’s not right. It’s also not being one with the airplane. Plus it’s down right ugly. It’s your choice: incorrect, mechanical and ugly or smooth and elegant. That’s really not much of a choice, is it? BD
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