Video #11: Takeoff


Script (with weird spellings and lots of comma’s to make the helpful American gentleman narrator speak properly!)

Hello! Welcome to the eleventh video in the prof pilot dot co dot ewe kay flight training video series. This video will be pee king at what’s going on with the aircraft when you take off, and things you might need to be aware of, in order to keep your own takeoff from being less than optimal.

Looking at a normal departure first of all, let’s assume you’ve done your runup checks, and your pre-takeoff checks. You’ve been given takeoff clearance, so you check the approach path to make sure the controller isn’t an idiot, and sent you to become the brand new undercarriage of a seven four seven. Turn on to the runway, and line up with the centreline. The exact method for takeoff will vary depending on the type of aircraft you fly, but at some point you are probably going to need to use a lot of engine power to gain airspeed. If you don’t do that, you’re going to get in the way.

This massive increase in engine power, and therefore thrust, will send the aircraft accelerating down the runway. While you are hurtling along, make sure you check that your airspeed indicator has started to work, the engine is developing the correct power, and all the engine instruments are in the green. Since this is a normal takeoff, we can assume everything is okay, and the aircraft is not trying to kill us on this occasion. At a certain point of the take off roll, known as rotation, the pilot pulls back on the stick, increasing the angle of attack of the wing. The combination of increased airspeed and angle of attack, cause the lift of the aircraft to increase, and if the lift from the wings is greater than the weight of the aircraft, then we can fly, and will climb away. This will please the people on the ground.

There are a number of factors that affect the distance in which you get airborne, which can be quite important to bear in mind on a short runway. The main protagonist in this thrilling story is the wind. Remember in the lift formula, that lift is increased with an increase in airspeed. Airspeed is not the same as groundspeed. For example, here is you, in a stationary aircraft. Let’s start a headwind up blowing towards you at 15 knots. Since you are not moving, your groundspeed is zero. However, air is moving over your aircraft at 15 knots, and this is your airspeed. Put this into takeoff, and you can see on the airspeed indicator that if you have a 15 knot headwind, then you already have a 15 knot bonus in airspeed over the aircraft taking off in no wind. Running the takeoff side by side, you can see that the takeoff distance in a headwind is shorter, than the no wind scenario. Likewise, a 15 knot tail wind means that you start off with a 15 knot disadvantage, and you need to have a groundspeed of 15 knots before your airspeed is zero. This increases the distance required to take off, when compared to the no wind aircraft.

Weight also affects takeoff distance. The heavier your aircraft, the longer it will take to accelerate, and the longer takeoff distance you will have to travel to reach takeoff speed. You will also climb away at a shallower angle. So leave as many Americans as you can behind. Hah hah hah, my sides are splitting. That was as droll as it was original.

If the runway slopes downwards, gravity helps you accelerate, and you will need less runway. Upwards sloping runways slow you down. If the runway is wet, uneven, made of grass, or a combination of these, that increases drag, decreasing acceleration, and increasing distance required.

A lower air density, which can occur if you are high up, or if it is hot, will cause the engine to produce less power. Air Density also features in the lift formula, and as you can see, if the air density decreases, so too will the lift generated, and we must make up for this by flying faster. Both of these effects cause our takeoff run to be increased.

You can also decrease your takeoff distance by extending your flaps a little bit. As we have already discovered, a small amount of flap increases lift a lot, by increasing the coefficient of lift. It also increases drag a little, but this is greatly overshadowed by the bonus lift.

Another thing to ponder, is what effect a large piece of rotating metal, known as a propeller, does when it starts twirling round at 2,700 revolutions per minute, as it does at takeoff. First of all, it chucks a lot of air backwards, and this air, which is rotating round the aircraft due to the propeller, hits the tail on one side. As we discussed in episode 7, this will cause the aircraft to yaw, commonly to the left. As well as this, the propeller rotating anti-clockwise, will want to turn the rest of the aircraft clockwise, as Newton describes in his third law of motion. This puts extra pressure on your left tyre, and this, along with the tail force, will turn the aircraft to the left during takeoff. Thankfully, we have a rudder to control this, so things on the left of the runway can breathe a sigh of relief. Unless you forget.

Next episode, we will be looking at some of those less that optimal takeoffs, and what you should do if you find yourself in one.

See you next time!

Tags: , , , , , , ,

Leave a Reply

Sponsors