2012. április 17., kedd

Soft field takeoff

I hear lots of times that taking off is easy, you just push the throttle all the way and let the airplane fly. Yeah baby! Or maybe not?

Let's see the case of the soft field takeoff. It can be applied not only if the surface is soft - like sand or wet mud - but in case it is rough, stony or the grass is long. In case of rough, stony runway with a normal takeoff we are stressing the tires, landing gear and the prop as well. If the runway is soft, wet or the grass is long then the problem is even bigger. The drag on the wheels increases the required takeoff distance. The drag can be so much that the airplane will not be able to reach the rotation speed! 
Now these stones don't really save a propeller
So we need to do something to get the weight of the airplane off the wheels as soon as possible. Thus reducing not only the drag of the soft surface but also the forces that stones and rough surface put on the wheels and propeller. We then get the airplane off the ground at a low speed and accelerate in ground effect, above the runway and free from the rolling drag of the wheels.
Propeller after normal operations
Start with 10 degrees flaps. Needless to say that 10 flyps will produce more lift at a low speed, thus taking some weight off the wheels early in the takeoff roll. Yoke should be held fully back, same as when you taxi, to keep the weight off the nosewheel and keep the propeller as high as possible to avoid chipping from small stones. As you add power the airflow from the prop makes the tail effective, and will create a tail-down force, it will take most of the weight off the nose. At this point, although the airplane will have a nose high attitude the nose wheel is still running on the ground, with the nose strut mostly extended.
Sand isn't your friend...
The nose-high attitude means that the wings are at an increased angle.  This is good, because it means that the wings will generate some lift even at a low speed. Not enough to fly, but it will decrease the load on the wheels. First of all if the field is wet/soft it gradually allows the tires to rise and start to surf on top of the mud. Secondly, on a rough terrain the wheels will not hit the bumps with big force. 
Soft field takeoff (via flickr/fireboat895)
Speed will start building up, the airflow over the tail will continue to increase and the nosewheel will finally come off the ground. If we would hold the yoke in the initial position, completely back, the nose would continue to rise, we wouldn’t be able to see where we’re going and we’d be dragging the tail on the ground. So relax a small amount of elevator backpressure so that the nose remains spmewhere around the climb attitude. If you keep the nose too high the wing creates too much drag. Just hold the nose high so that the nose wheel does not run on the ground anymore. As the airplane is accelerating further you will need to make small corrections to keep the nose in the proper attitude. 
With the increasing speed at one point the lift will be compensating for the airplanes weight and thus we become airborne. 
Soft field technique went wrong (read the article here)
Happy? Not yet... We're just a couple of centimetres from the ground, we have low speed and a high angle of attack, creating a huge amount of induced drag. At the moment we're just floating in the ground effect. But we've got rid of the nasty drag that was created by the rolling friction of the tyres. Lowering the nose slightly - but carefully, not to put it beck down - will help us get rid of more induced drag and gain a couple of more knots. As airspeed builds up just keep the airplane in the ground effect, let it accelerate to a safe climb speed. Upon gaining the desired airspeed simply start to climb out with Vx or Vy.

Here's a nice video on this topic:
Now, I need to point out that you shouldn't jump in the first airplane find a mudhole land there and then try to perform a takeoff. Always practice any new maneuver with an instructor on a paved runway until you are proficient.