Page 16 - Volume 11 Number 9

Ask the Expert Lighten Up!
by Tom Clements
No, I am not talking about your morose attitude. the chart proscribes, then takeoff distance will be
(Although converting that downer into an upper
will probably make you a much more pleasant cockpit occupant.) Instead, I am talking about lighten- ing up the weight on the nose tire during the takeoff roll.
Some airplanes exhibit a strong tendency to create nose-down force during the takeoff roll. The Beechcraft Model 60 Duke is infamous for this. As beautiful as the Duke’s profile is, there is no doubt that its nose resembles a door stop – flat on the bottom and sloping down on the top. That sloping top tends to force the relative wind upward and, in turn, a force develops that pushes the nose downward. Less obvious, the 200- and 300-series of King Airs also develop a nose-down force during the takeoff roll. It is not because of a door stop nose design, but instead is caused by the relatively high thrust-line of the power plants. To accommodate their larger diameter propellers, the centerline of the engines on the 200- and 300-series was raised four inches above the centerline used on the 90- and 100- series. The higher thrust line of the propellers creates a rotation force pushing the nose downward.
Unless this nose-down force is compensated for some way, then the faster the airspeed, and the more load exists on the nose tire. In itself, this extra nose tire weight adds drag by making the tire have more rotational resistance. More drag equates to a longer ground roll. Also, it becomes more difficult for the elevators to provide enough down force to lift the nose up to the proper rotation attitude at the correct time.
For the airplane to have performance identical to the airplane that the test pilots used to create the takeoff data charts is probably, in my opinion, difficult and very rare to achieve. Maybe I am just an old “doubting Thomas,” but my experience leads me to believe that all performance charts need to be viewed with a bit of suspicion since it is rare for each airplane and each pilot to perform identically.
One important data point for all takeoff calculations is rotation speed, VR. If it occurs earlier or later than
14 • KING AIR MAGAZINE
affected. So, if we don’t “lighten up,” it is impossible to use the proper VR. Instead, if we wait to add elevator back-pressure until at or near VR, then either (1) we actually rotate at a significantly higher speed due to the time it takes to overcome the downward nose force, or (2) we pull the wheel back so forcefully that the airplane “leaps” into the sky abruptly, giving the passengers quite a surprise.
Therefore, my recommendation is to pull back on the control wheel rather early during the takeoff roll ... lighten up the nose tire. This is a subtle technique that requires a bit of trial and error to get right. The goal is to make that nose rise exactly at the proper VR speed, with smoothness and precision. Can anyone do it perfectly all the time? Well, I sure can’t and I have observed very few pilots who do it right consistently. Although no one can guarantee perfection all the time, let’s at least stack the deck in our favor such that we have a realistic chance of using a proper VR.
That means that we must lighten up – lighten up the weight on the nose tire by making the airplane assume a neutral pitch attitude. To do this, we must pull the wheel back enough, and early enough, so that we find that neutral zone: no nose up nor nose down attitude. To decide if we are doing this correctly, here’s the test: Adding just a touch of additional back pressure will make the nose rise a little; releasing just a touch of back pressure will make the nose drop a little.
If we can add some back pressure and yet the nose stays where it is, however, it is telling us that we are not in that neutral zone. We are setting ourselves up for an abrupt and/or late rotation. It gives us a warm and fuzzy feeling when the nose starts up exactly at VR and liftoff occurs when it should.
Finding the proper neutral pitch attitude is a bit easier to accomplish on King Airs with conventional tails than those with T-tails. When the elevators are mounted down low, they are subjected to prop wash, the faster-moving stream of air behind the propellers.
SEPTEMBER 2017