The Take-Off Roll
By Mike Andresen
As a flight instructor, I get to peer into the procedures of people of various training backgrounds while giving Biennial Flight Reviews. Some were trained in local or distant flight schools, some have training from professional backgrounds, and some were trained by a free-lance CFI that they just happened to know.
Just before taking off on a BFR, I ask the pilot what our rotation (Vr) and climb speeds (Vy) will be. One particular pilot responded to me that he does not rotate. He said that he just lets the airplane fly when it "wants" to. My response was: if it doesn't "want" to fly and the airport fence starts looking really large, at what point do you become a pilot rather than a passenger?
The purpose of this article is to review a topic that is somewhat taken for granted - the take-off roll and how to minimize take-off distance. The physics involved is actually quite simple. Apply a force (thrust minus drag) to a mass (the airplane) and it accelerates. To accelerate to take-off speed in the shortest time possible we need to maximize thrust, minimize drag and minimize the mass accelerated.
Minimize Airplane Mass
Your rate of acceleration is directly proportional to the weight of your airplane. If I fly my airplane at one-third fuel (1,970 lbs) versus full fuel (2,210 lbs) my take-off roll will be 11% shorter. Carrying the right amount of fuel and fuel reserves will improve performance.
Maximize Engine Thrust
Full rich and wide open throttle are the norms. If you do lean for high altitude, monitor temperatures to be sure you don't overheat the cylinders.
Maximize Propeller Efficiency
Variable pitch propellers should be in the low pitch position which minimizes blade drag and requires the least amount of engine torque to turn the propeller. Propeller efficiency will be poor at the start of your take-off but will improve as airspeed increases.
Minimize Drag
This is where our BFR pilot was wrong. To minimize drag, the wing should be held at low angle of attack until reaching rotation speed. That will minimize a large drag contributor and minimize take-off distance. You can convince yourself of this by demonstrating the soft-field take-off which is performed with the wing held in high angle of attack and has a longer ground roll compared to the normal take-off.
Rolling resistance is another form of drag that is minimized by proper tire inflation.
So you've done all of the above and it's time to rotate. What does that mean? We can find the answer in Part 23.51 of the federal regulations for small single engine aircraft certification. For normal, utility, and acrobatic category airplanes, rotation speed, Vr, is the speed at which the pilot makes a control input, with the intention of lifting the airplane out of contact with the runway. By certification standards, Vr, must not be less than VS1.
Now for the best part: Vr is the speed that will result in a speed at 50 feet altitude that is shown to be safe under all reasonably expected conditions, including turbulence and complete engine failure or 1.20 VS1. So Vr is the speed that has been demonstrated to allow you to accelerate to a safe speed by the time you are at 50' altitude.
Next month we will talk about climb speeds.