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PRECISION MANOEUVRES

Elementary precision manoeuvres include steep turns and wind drift correction.

STEEP TURNS

The only difference between the steep turn and the normal turn is the steepness of bank. Many instructors have made this statement, and many stu­dents have nodded agreement but mentally noted that they didn't believe a word of it. The fact is that the techniques are the same with one or two minor additions.

A steep turn is one with a bank of over 30°. In Chapter 8 and "The Turn" in Chapter 9, you noted that in medium turns (15°-30° bank) you were able to take care of the need for increased lift by increasing back pressure. As the turn gets progressively steeper, you will find that the lift must be increased so much that you run out of angle of attack. In other words, the airplane stalls before you get enough lift to maintain altitude.

In the steep turn you may need increased power to maintain altitude because two things are happen­ing: (1) The required added back pressure means a higher angle of attack and greater induced drag (see Chapter 2) and (2) the added load factor in the turn causes an increase in the stall speed. The added power serves to compensate for the added drag (which might otherwise finally result in an altitude loss), and it also helps lower the stall speed. You'll note in doing stalls that the stall speed is lower when power is used (Chapter 12).

The 720° Power Turn

The 720° power turn is a confidence - building pre­cision manoeuvre that consists of 720° of turn with a coordinated roll-in and roll-out and a bank of 45°-60°. You will use added power and vary your bank as needed to maintain altitude (Fig. 101).

Pick a point on the horizon or a road below as a reference point. The turns are done at an altitude of at least 1500 feet above the surface. Look around for other traffic before starting the turn.

As you roll into the turn, open the throttle smoothly so that as the desired angle of bank is reached the power setting is approximately that of climbing power. Because of the great amount of back pressure needed after the turn is established, the angle of attack will be such that the airspeed will drop appreciably. You have a high power setting and low airspeed, so you will have to correct for torque effect.

Once the desired amount of bank is reached, the ailerons are neutralized, sufficient right rudder to cor­rect for torque is held, and back pressure is held as needed to keep the nose at its proper place on the horizon. This nose position may be slightly higher than that in medium turns because of the required increase in angle of attack (added back pressure). A good rule of thumb is to have your bank and power established before you have made over 45° of turn.

You will find that the back pressure required is somewhat greater than you had anticipated.

If you see that the plane is losing altitude, the best method of bringing it back is to shallow the bank slightly. This will increase the vertical component of lift and the nose will return to the correct position. Then resume your steeper bank. Or if the plane is climbing, steepen up slightly and/or relax back pres­sure slightly.

The reason for not making the back pressure take care of all of the altitude variations is this: If you are in a 60° bank, the stall speed is increased 1.414 times that in level flight. If you have to climb or increase the lift at this bank, obviously you will run out of angle of attack. At 60° it takes 2 pounds of lift to get 1 pound of the vertical component. This is a losing proposition, and with steeper banks these odds jump sharply (see Figs. 94 and 95).

Check your wings, nose, and altimeter as you turn. Watch for the checkpoint. Some students make three or four complete turns instead of two because they aren't watching for the point. After all, this is a precision manoeuvre.

Pick your reference point

Smoothly apply power as you roll in

Start the rollout about 45° before 
completion of the second turn

The rollout is complete.
The plane is at cruise power and attitude.

Fig. 1. Elements of a 720° power turn.


After you've completed the first 360° of turn, if you held altitude, the plane may wallow or bump slightly as it flies through its wake turbulence. The more slipstream you hit on the second 360° the better your turn (though this could be argued). It's possible, however, to hit a bump even if the altitude ran wild.

The idea is not to start out, say at 3000 feet, lollygag around from 2800 to 3200, and then feel proud because you happen to be back at 3000 feet when you roll out.

While your instructor won't be too critical about your performance at this stage of the game, it's a good idea to learn the technique now. Then you won't have to sweat 720s later when you should be practicing advanced stalls or other manoeuvres.


The second 360° is a continuation of the first; make corrections as needed. Watch for the checkpoint and start rolling out and throttling back about 45° before you are lined up with it. The roll-out should be smooth and timed so that the plane is in a straight and level attitude and at cruising power when the checkpoint is reached.

The hardest part about the roll-out is keeping the nose from coming up. It takes a lot of back pressure to hold it up in the turn, and it will take concentration in easing off that back pressure. During the first few times, it will seem that you don't relax back pressure - you press forward to get the nose down where it belongs. To review the 720° turn:

The 720° power turn uses the same fundamentals as the shallow or medium turn. A steep turn requires only a longer period of deflection of the ailerons and rudder in order to obtain the steeper bank.
The reason right rudder may be needed is that the torque
effects are caused by added power and slower airspeed, not the steepness of bank.
Power is used because (1) the angle of bank is such that the stall speed is increased, and (2) the required added angle of attack is increasing drag to the point where altitude may no longer be
efficiently maintained by back pressure alone.

Some students get the idea that the nose can be held up by top rudder. The infinitesimal amount of the upward thrust angle doesn't compare with the great loss of efficiency suffered by slipping the airplane and slowing it down.

PROBABLE ERRORS

  1. Back pressure added too soon at the beginning of the turn the nose rises and the plane climbs.

  2. Applying full power or opening the throttle too soon airspeed has not dropped yet and the engine overspeeds.

  3. When the nose drops, trying to pull it up by back pressure alone, not shallowing bank results in high stresses on the plane.

  4. Failure to watch for the checkpoint. (Make a mental note as it goes by the first time. That way you won't make the instructor sick as you go around and around.)

  5. Not releasing all of the back pressure on roll-out the plane climbs.

  6. Forgetting to throttle back as the plane is rolled out also causes the airplane to climb.

  7. The usual coordination problems of slipping and skidding.

 

WIND DRIFT CORRECTION MANOEUVRES

The instructor will usually introduce you to the idea of wind drift correction by choosing a road or railroad that has a crosswind component and having you fly directly over it and then alongside it. You'll reverse course and fly in the opposite direction so that you can get practice in correcting for both a left and right crosswind. But first, let's take a look at the principle of wind drift correction.

You've seen lightplanes flying on windy days and noticed that they sometimes appeared to be flying sideways. In relation to the ground they were, but as far as the air was concerned the planes were flying straight as a string.

Suppose you wanted to cross a stream in a boat, paddling or motoring briskly from X to Y. The current is swift, so if you start out, as in Figure 2A, you can see what happens‑there's much sweat but little progress toward your destination.

By pointing the nose directly across the stream at Y, you wind up at Z. In Figure 2B you play it smart. You point the boat upstream, how far up depends on the speed of the current, and you keep experimenting until you get the correct angle.

The same idea applies to the airplane. The wind is the current in this case. If you want to fly from X to Y with the wind as shown, you must angle into the wind, or set up a "crab."

If you start out pointing the nose at Y, you end up as shown in Figure 3A. So you correct for the wind by making a balanced turn and rolling out at the correction angle as shown in Figure 3B.

Current

Current

A

B

Fig. 2. Boats and river currents.

Fig. 3. The principle applied to the airplane and air mass.


The Rectangular Course

The purpose of the rectangular course is to give you a chance to fly the airplane while your attention is directed outside. As you learned the Four Fundamentals, you concentrated on the airplane and only noted its reference to the ground as a whole. Now you'll begin to use the plane and make it follow a definite path over the ground. During the first few minutes you'll feel as frustrated as a one-legged man in a kicking contest.

This manoeuvre consists of flying a rectangular pattern around a large field or fields and is done for three reasons:

  1. To get into the habit of flying the plane while dividing your attention between the cockpit and objects outside.
  2. To learn to correct for wind drift in flying a straight course in preparation for cross-country flying.
  3. To get practice in low precision flying in preparation for traffic pattern flying.


The rectangular course is done at 600 feet because mistakes are easily seen at that altitude, and it builds confidence to fly at a lower altitude.
The instructor will try to pick a field lying so that the wind is blowing diagonally across it. This will give you practice in wind correction on all four legs (Fig. 4).

Fig. 4. The rectangular course is normally done at 600-1000 feet above the surface.

PROCEDURE. Enter the pattern at a 45° angle, flying downwind (point 1). It is easier to set up your first correction downwind because drift can generally be seen more easily. This is just a human foible, but it works out that way. The turns around the field can be made either to the left or right. Assume in this case that the turns are to the left (Fig. 10‑4).

Roll out of the turn at what you think is the proper correction angle. The plane's distance from the field should be far enough away for the boundary to be easily seen and yet not so far away that your mistakes can't be seen. About 600‑800 feet from the boundary would be a good distance for the average lightplane. Some students roll out parallel to the boundary and then make the correction, but it's advisable for you to get into the habit of making a correction as soon as possible.

Assume that at point 2 you undercorrected and the plane is drifting into the field. Make a balanced turn of a few degrees into the wind and see how things fare. If you are now holding your own and are not too close to the field to see the boundary without straining, fly the plane to the end of the field.

At point 3 you'll have to make a left turn. Since there is a quartering tailwind, it will tend to push you away from the field, particularly if a shallow turn is made (point 4). The best thing, then, is to make a fairly steep turn at this point. The steepness of bank will depend on the wind velocity‑the greater the wind velocity, the steeper the bank. Roll out of the turn with whatever correction you think necessary (point 5). Don't count on having the same angle of correction as you had on the other leg. The wind probably won't be crossing this leg at the same angle. As shown in the diagram, the wind is more from the side here, so more correction is needed. The vector of the wind pushing you into the field on the first leg was not as great as the vector you're fighting on this second leg.

In the second leg, the wind is still somewhat behind the plane, so when you get to point 6 you can figure on another fairly steep turn. This turn will not be as steep as the one at point 3 because the wind component on your tail is not as great as it was at point 3.

Your angle of bank (the steepness of the turn) is directly proportional to your speed over the ground.

Continue the rectangular course. At point 7 the bank must be shallow because you are flying into the wind (your groundspeed is low). At point 8 the first part of the turn is shallow, then it is steepened to keep the plane at the correct distance from the field.

PROBABLE ERRORS

  1. Poor wind drift correction‑not setting up correction soon enough.
  2. Not recognizing drift or not making a firm correction after recognizing it.
  3. Not maintaining altitude. Most students tend to climb in a rectangular course -there are a few rugged individualists who lose altitude.
  4. Coordination problems. Some students who make perfect turns at higher altitudes get so engrossed in watching the field that their turns are awesome spectacles indeed.

When the airspeed indicator was being discussed in Chapter 3, it was mentioned that the airplane was part of the air when it was flying. Let's go into that a little deeper.

Suppose that you are flying from point A toward point B, as shown in Figure 5. The wind is 20 K. You are to point the nose at B and make no correction for the wind. At the same time you leave A, a balloon is released there. At the end of an hour both you and the balloon are 20 nautical miles south of the A-B line. You covered a lot of ground but still ended up as far south of the line as the balloon. You were both carried by the air mass itself.

Fig. 5. The air mass will move the three aircraft equally in a given amount of time.

A jet or rocket plane, flying the same heading, would be 20 NM south of the A-B line at the end of an hour, even if it cruised at 1500 K.

Old-time pilots used to say, "I was flying along and all of a sudden the wind got under the wing and almost flipped the plane over." They hit turbulence, or disturbances within the air mass. The air mass itself didn't suddenly "blow" against just a part of the plane.

Another idea that died a hard death is that holding rudder corrects for wind drift. Stories are still told of pilots flying the mail in the 1930s and landing with one leg numb from holding rudder for "that blank crosswind." If you hold rudder, you defeat your purpose. When you skid a plane, it tends to continue in a straight line, and because of the added drag its airspeed will drop off. You'll end up in cross-controlled flight.

Skidding the plane defeats your purpose. The most comfortable and efficient means of compensating for drift is to maintain balanced f1igh1(no slip or skid). You will be shown later that there is one time, and one time only, when this does not apply.


S-Turns Across a Road

S-turns across a road, like the rectangular course, are good manoeuvres for getting you used to dividing your attention between the airplane and the ground. The primary purpose, however, is to show you how to correct for wind in a turn. There was a brief introduction to this idea in the rectangular course, but S-turns give you a chance to acquire finesse.

S-turns are a series of 180° turns of about a quarter mile radius using a road as nearly perpendicular to the wind as possible. (This is for planes of 90-95 K cruise. Faster planes will use a greater radius.)

The reasons for S-turns are:

  1. To fly the airplane while dividing your attention between the cockpit and the ground.
  2. To learn to correct for wind drift by varying the bank. This will come in handy later in circling and remaining near the same spot in a strong wind.
  3. To get practice in precision flying in preparation for advanced manoeuvres later.

S-turns are also done at 600 feet for the same reasons as those given for the rectangular course.

The object is to fly a series of semicircles of the same size, making smooth balanced turns and correcting for wind drift by varying the steepness of the bank. The plane should cross the road in a level attitude with the wings parallel to the road.

PROCEDURE. A road that runs as nearly 90° to the wind as possible is picked. It is best to enter the S-turns downwind, because drift is more easily detected and corrected by the student if correction requires a steepening of the bank rather than shallowing it out. The first turn can be made in either direction.

As shown in Figure 10‑6, the initial bank must be steep; otherwise, the wind would "push" the plane too far from the road before the turn is completed (point 1).

Assuming that you have set the correct steepness of bank, when point 2 is reached the bank must be shallowed or the plane would be turning at the same rate of turn in degrees per minute, but as it began to head into the wind, the groundspeed would drop. It would appear to "pivot" and would not follow the smooth curve of the semicircle but would end up at point 3.

Fig. 6. S-turns across the road.

The shallowing of the turn should be such that the wings are level as the plane crosses the road, still at 600 feet (point 4).
After you cross the road, the bank should be a shallow one in the opposite direction. If a steep bank is used, a path such as at point 5 would result.
When point 6 is reached, the bank must be steepened in order to have the turn completed as the plane crosses the road again. If the bank was not steepened at point 6, a path like that of point 7 would occur. The plane would not cross the road at the correct place nor would the wings be level or parallel to the road.
Continue the series until you get tired or run out of road. Remember: The angle of bank is directly proportional to the groundspeed. (The greater the groundspeed, the greater the angle of bank.)
The angle of bank in S-turns must be constantly changing. The hardest part of trying to keep the manoeuvre smooth is at point 8, where you have to roll from a steep bank one way to a steep bank in the opposite direction.
If the wind is strong, many students do not have a shallow enough bank on the upwind side of the road. Strangely enough they are able to correct on the downwind side without much trouble. Many times in this situation the plane's path over the ground looks like Figure 7.
One problem with these manoeuvres is that you may spend all of your time looking at the ground references. You should keep an eye out for other traffic as well.

PROBABLE ERRORS

  1. Failure to properly correct for drift.
  2. Rolling out of the turn too soon or too late resulting in crossing the road with wings not parallel to it.
  3. Gaining or losing altitude.
  4. Coordination problems in the turns (jerky or slipping and skidding).

In the S-turn you are interested in a definite path over the ground and therefore must correct for the movement of the air mass of which you are a part.

Fig. 7. A common error in S-turns across the road.

 
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 Wednesday, 07 August, 2002