Wednesday, September 19, 2012

Navigation, Speed and Wind Drift

Navigation - Which is Better -Slow Plane or Fast Plane 

Remember, a large plane and a slow both drift with the wind in straight and level flight. 

Speed does make a difference in navigation. Weight, size and horsepower do not make a difference.

The Setup: 

Make a 100 mile flight to a destination with a 20 mph wind blowing across your course to your destination. You take off and point the nose of your plane directly toward your destination without an allowance for drift in your 100 mph cruise in your slow plane. You correctly determine it should take one hour to complete your flight. After an hour you look at the ground and your destination is nowhere in sight. In one hour of flying through the air your plane has drifted with the air 20 miles to the downwind side of your course. If it is a hazy day with visibility of 7 miles or less you will not see your destination.

If you attempt this same trip in, a 300 mph plane, point it directly toward your destination without compensating for drift and, after an hour, it too will drift exactly 20 miles.  But, at 300 mph your faster plane will arrive at the planned destination in 20 minutes. If the wind is 20 mph, then in 20 minutes your plane will drift in the wind a little less than 7 miles. Now, when you look out of your plane after 20 minutes you will see your destination off to one site easily visible. Your navigation is better when you can see your destination when you are supposed to see your destination. In a fast plane, even though it is drifting in the wind just as helplessly as your flight in a small plane, the drift is proportionately less important to the faster plane. The drift angle is less in the faster plane and that is why a faster plane is much easier to navigate. 

Tuesday, September 18, 2012

Training - The Figure of Eight

Illustration of Figure of Eight in Flight Training - Wind

In pilot training an instructor had you practice figures of eights over a conspicuous object on the ground. Some times this was incorporated into a steep turn exercise. For an illustration of wind effect a 20 to 30 degree bank was sufficient.

When you pass over an object you can identify from a distance start a 30 degree bank. Hold this 30 degree bank for 360 degrees. It is better to pick a direction to start the instruction heading, for instance , straight West, as you pass over the object, to begin the 30 degree bank for 360 degrees.

In calm air conditions, when you have completed 360 degrees of 30 degree bank, and are just beginning to head out on a due West course you would expect a complete circle was flown and the object you picked out to begin the turn was again right under your plane. You are correct.

In wind conditions it changes the flight path. If you hold the same 30 degree bank for 360 degrees you describe an exact circle through the air. The air in which you fly moves on with your plane in the "soup." Your path on the ground is not a perfect circle. It is more like the figure "6" at the moment you are again heading exactly due West. But, after starting your 30 degree constantly held bank and flying the bank for 360 degrees your position is not exactly over the object where you started your turn. Your plane is several hundred feet downwind form the object.

If you understand the concept of a turn in the wind above you understand the whole problem of drift in turns.

The drifting of the plane during the maneuver is not accompanied by any forces from any direction. It is accompanied, though, by confusing sensations for you, the pilot.

If you did not watch the ground you would not notice any wind effects. But, you can not help seeing the ground, while turning at a constant 30 degree bank for 360  degrees and noticing how the ground speeds up, now it slides sideways, now it slows up and now it slides to the other side.

How you react depends on where your attention is centered. If you are centered on your path over the ground you would notice how your path is pulled out of shape. The proper response to the changing path over the ground is to do nothing. Let the drift take place and make no effort to resist the illusions of what drift does and causes you to try to keep the plane in a perfect circle. 

You don't want to involuntarily try to keep the shape of the turn you started perfect. You may involuntarily try shallowing or steeping your turn or the danger of "holding rudder against the drift" and slipping or skidding during the parts of the turn  when you have the "wind" from your side.

If your attention is on flying a nice turn the slipping and skidding tendencies will be opposite. As you notice your planes apparent sideways sliding you will tend to skid as you turn from upwind to downwind and you will tend to slip while you turn downwind to upwind.

You have to learn to fly the plane by attitude and the feel in the seat of your pants. You have to disregard the strong impressions of sideways and improper motion the ground transmits to your eyes.

Monday, September 17, 2012

Downwind Landings - Wind Effects

Downwind Landings

Straight downwind landings by mistake. Pilot fatigue leads to flight errors. Misread wind indicators or tower instructions.
 (more serious)

Landing downwind, when you think you're landing upwind, leads to confusion. 

You approach the runway through the air at 75 mph and, at the same time drifting with the air at 20 mph. Since the two motions are both in the same direction the plane's speed, relative to the ground, is 95 mph.

You sense the altitude above the runway, when landing, from visual clues. The speed of the runway surface and/or the speed of runway lights in his field of vision provide that information. You think you're close to the runway from your experience when landing upwind. You don't think you're in any danger from stalling and begin to slow the plane up  preparing to touch down momentarily.

You just fell into a dangerous visual illusion. It is true the plane has plenty of speed and speed is what keeps a plane from stalling. What you forgot, because of the illusion, it is the speed of your plane through the air that is important. Thats the speed your plane's wing is pushing through the air. That speed is 75 mph. 

Of your 95 mph that you think you are going only 75 mph is actually going through the air.

The apparent relative speed of your plane to the ground, 95 mph, gives you illusion that your plane is closer to the runway. In fact, your plane is higher than the visual clues that you mistakenly used to determine the altitude of your plane.

Since you are tired and you think you are landing upwind, you may instead be close to a stall as you slow your plane down for a landing while still high above the runway.

If you discover your mistake in time you may, depending on actual altitude above the runway, lower your plane's nose, give immediate power application, recover from the stall and think things through before you attempt another approach.

Too many pilots don't get a second chance.