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RC University: Airplanes



 


Advanced Aerobatics
Written by: Russ Petersen
 

In our discussion presented below we treat all of aerobatics as a combination of some fairly basic maneuvers. Most aerobatic maneuvers consist of some combination of a loop, a simple axial roll and inverted flight. So, we will begin the discussion with a more careful look at these basic building blocks of aerobatic flight.

The Basic Inside Loop.

The Inside Loop is entered at acrobatic cruise throttle setting, straight and level and into the wind as the aircraft passes in front of you. Done properly the aircraft will describe a circle and will re-enter level flight by returning to the point of entry, on speed and on heading. The maneuver is entered by smoothly adding full throttle and pulling back stick to describe a symmetrical raising of the nose to vertical flight while maintaining a wings-level constant heading. The maneuver is continued by holding back stick until the airplane is nearing the top of the loop in inverted flight. At this point most airplanes will be decelerating and you will notice a tendency for the airplane nose to move to the left caused by the torque of the engine having a yawing effect as the airplane slows.

To accomplish an on-heading loop you will need to correct for this yaw as the airplane goes over the top of the loop with a small amount of right rudder. Also, in order to describe a symmetrical maneuver over the top you will need to relax back stick when you are upside down. This will enable to aircraft to fly a smoother, loop at reduced airspeed while inverted. At this point the nose should be starting toward the ground and the speed should be building up, so it’s a good idea to begin to retract the throttle as you go down the backside of the loop to the vertical position. Remember also that if you added right rudder over the top of the loop it would be returned to neutral as the speed builds up. As you continue through the last ¼ of the maneuver, you will need to add back stick and smoothly add throttle to return to acrobatic cruise power setting. Remember that this maneuver is to be flown as much as possible at a constant airspeed, so good throttle management will help greatly in this regard.

Inverted Flight.

We will find that there are a number of good ways to enter inverted flight, but since we have just discussed the loop, lets plan to use what we have learned to enter level inverted flight. So, let’s use the first ½ of the loop to get the airplane on a heading that is a reciprocal of the loop entry heading, upside down at the top of the ½ loop. Entry then, would be by flying the airplane level, in a hands-off trim condition at constant speed, into the wind at acrobatic cruise power setting. Then, consistent with our discussion above you will smoothly and simultaneously pull back stick and add full throttle. The aircraft nose will rise and you will have to add some right rudder as it slows and enters the top of the loop in inverted flight. At this point you will need to smoothly return the throttle to acrobatic cruise and add some forward (down) elevator to keep the nose from coming down and continuing though the loop maneuver. Done correctly, you should now be in inverted flight, wings level on a heading that is the exact reciprocal of your entry heading. You should begin by holding this flight attitude for at least 5 seconds. After five seconds has passed you should release the forward stick and let the nose drop in order to complete the last ½ of the loop. Don’t forget to retract the throttle as you enter the vertical part of the second ½ of the loop.

So, you now have a maneuver that looks more like a racetrack on its side, with the straight legs on the top and the bottom added to the simple loop. As you gain experience and wish to lengthen the inverted portion of the maneuver you may wish to enter the ½ loop somewhat further upwind so that you can have a longer portion of inverted flight. While you are in inverted flight you should notice that the ailerons function in the same direction (with respect to aircraft response) as they did with the aircraft in an upright condition, but that the elevator and rudder are reversed! So, to cause the airplane to climb you must add down elevator and to cause it to yaw to the right, you would want to add left rudder.

The Axial Roll.

The axial roll maneuver is a maneuver in which the aircraft is rolled either left or right from an upright, level flight condition completely over until it returns to the same upright flight condition on the same heading. The maneuver should take about 4 seconds to complete, so you do not want to hurry to do this properly. Entry, as always, is in a hands-off neutral trim condition, into the wind at acrobatic cruise power in level flight. (For some intermediate trainers with limited power and/or a symmetrical airfoil, it might be useful to begin the maneuver with a slight displacement of back stick in order to get the nose started up prior to beginning the roll.) The goal here is to roll the airplane while keeping the nose of the airplane as near to the center of rotation as possible. The maneuver should be executed in a manner such that the airplane does not gain or lose altitude at the completion of the maneuver. In order to understand the necessary control displacements to execute the axial roll we should examine what is happening to the airplane in a continuous roll condition. Remember that when an airplane is placed in roll, the rising wing has an aileron that is displaced downward, which generates more lift and drag on that side of the wing. Similarly, the falling wing has an upwardly displaced aileron, which is generating less lift and less drag on the opposite ½ of the wing. Now if you view the airplane from the top, this drag differential has a tendency to pull the nose of the aircraft toward the rising side of the aircraft (because there is a differential in drag caused by the rising/falling differential drag/lift conditions).

Most writers call this adverse aileron drag and it is always present in any conventional airplane during a rolling maneuver. If this condition is not corrected for, the nose will describe a circle as the aircraft rolls to inverted and back to level flight. This results in a strange slipping maneuver (any maneuver in which the tail slides toward the direction of turn is a slip) when viewed from the ground, and is almost always what is observed during most RC sport aerobatics. But, the addition of some rudder in the direction of the roll will correct for this condition, so you should plan not only to displace aileron in the direction of the roll, but also to add some rudder in the direction of the roll as well. Another factor exists in the axial roll that must always be accounted for and that is, with the ailerons displaced and a rudder correction entered, the aircraft will tend to slow down during the roll resulting in slightly reduced control effectiveness. In addition, as the aircraft rolls through the inverted, the nose will tend to drop, so you should plan to add a little down elevator to hold the nose up as the aircraft passes through the inverted position. This elevator correction (and the rudder correction as well) needs to be eliminated as the aircraft finishes the roll and is returned to upright flight, at acrobatic cruise power setting on its entry heading. Properly executed, (not too fast) it is one of the prettiest maneuvers in flying and is a basic aerobatic building block maneuver. Combined with inverted flight and a loop, one can execute a lot of interesting and challenging aerobatic flight maneuvers.

The Slow Roll.

You may want to skip reading about the slow roll until you have completed the other sections of this discussion or you may find it convenient to rely on your reading of the “Axial Roll” above to enhance your understanding of the Slow Roll. Since we have just discussed the simple axial roll and the factors that are at work during its execution it may be useful to consider what should be expected to occur if the rate of roll (4 seconds for an axial) is slowed such that the roll takes 6 or more seconds to execute. All of the control factors discussed above are still at work. That is, adverse aileron drag will work to pull the nose of the aircraft outside the center of rotation, and that needs to be corrected by adding some rudder in the direction of the turn. But, unlike the axial roll, the rate is now much slower so the aircraft now has a tendency to drop its nose as you enter the first ¼ of the roll and are passing through a knife-edge type of flight condition. Therefore, some rudder must also be added to correct for the tendency of the nose to drop in this attitude. For example, if you are executing a slow roll to the left you will need to add some right rudder as the aircraft passes through the first ¼ of the roll. As it reaches half way through the roll and is in an inverted flight condition, the rudder correction should be returned smoothly to neutral, while remembering to hold the nose up with some up elevator. The amount of up elevator needed here will probably be more than was needed in the axial roll, since the aircraft will pass through inverted flight much more slowly than should be the case in the axial. As the aircraft continues in its left roll to about ¾ of the roll, left rudder will be needed to hold the hose up and elevator will probably be returned to neutral. As the aircraft completes the roll and enters level upright flight the left rudder displacement necessary to hold the nose up at the ¾ roll point should be returned to neutral.

The Immelmann Turn.

As was discussed in our earlier presentation on aerobatics, the Immelmann grew out of a WWI fighter pilot maneuver in which the airplane needed to both reverse heading and to climb. So, an Immelmann is just the first ½ of a simple loop followed by a ½ axial role from inverted flight to normal upright flight. The maneuver is entered at level flight, and acrobatic cruise power setting into the wind. The entry to inverted flight discussed above is executed except that as the aircraft is entering level inverted flight on top of the loop, it is rolled either right or left through 180 degrees (1/2 roll) to normal upright flight. You may choose to roll to the left in the beginning because the engine torque will be pulling you that way anyway. The power will be added to full at the entry to the bottom of the loop, and returned to acrobatic cruise at the top. Similarly, most airplanes will decelerate during the climb in the ½ loop and that will result in a slight yaw to the left unless it is corrected for with right rudder. As the aircraft enters inverted level flight at the top, the right rudder should be relaxed to neutral. So now, we have combined a ½ loop, inverted flight and a ½ axial roll to make the first complex aerobatic maneuver for you to practice!

An Alternate Entry To Inverted Flight.

You could take advantage of some of the practice described above to develop an alternative entry to inverted flight. It would also work to make a standard maneuver entry (into the wind, level, neutral trim at acrobatic cruise power) and as the airplane passes in front of you just execute a ½ axial roll. At the completion of the roll you will need to add some down elevator to hold the nose up and prevent the airplane from beginning to lose altitude. Remember to make some rudder correction in the direction of the ½ roll to correct for adverse aileron yaw and also remember to take the rudder correction out as you enter inverted flight. You should take from two to three seconds to execute the ½ roll and you should hold the inverted fight for at least 5 seconds at the beginning. When you have reached the end of the maneuver you can complete the maneuver by executing the second ½ of the axial roll and return to upright level flight.

The Split-S.

A simple maneuver that is essentially the Immelmann in reverse is called a Split-S. To enter this maneuver you should plan to climb to about the altitude that you obtain at the end of the Immelmann. The maneuver is entered into the wind at acrobatic cruise in hands-off neutral flight. To begin, a ½ axial roll is executed in accordance with our discussion above to accomplish simple inverted flight. After holding the inverted flight for 2 to 3 seconds, the throttle is retracted to idle, and the forward stick is relaxed to allow the nose to drop. At this point you are executing the second ½ of a simple loop, and the procedures to accomplish that are described above. That is, you will want to keep the throttle at idle as you pass through the vertical to avoid to much speed build up and if you were holding some rudder at the entry to the ½ loop, that should be returned to neutral. As you hold back stick to transition to level flight at the bottom you should smoothly add power to acrobatic cruise. This will place you on a reciprocal heading to that which you were holding on entry, straight and level with the wings level with the horizon. So, we have used the alternate entry to inverted flight and the second ½ of a simple loop to add another maneuver to your acrobatic portfolio.

The Upright Spin.

A pleasant and gentle maneuver that is an extension of the stall that you have practiced in an earlier stage of this program is called a spin. Essentially a spin is a maneuver in which a stalled or nearly stalled airplane somehow generates more lift on one wing than the other, causing the airplane to begin to rotate around its vertical axis while in a stalled condition. In order to enter this maneuver a normal simple power-off stall is entered by retracting the throttle to high idle, maintaining wings level and gradually adding elevator to maintain a slightly nose up attitude. As the airplane slows, it will gradually reach a flight condition that does not generate enough lift to support the weight of the airplane and it stalls! In the past we have taught you to simultaneously add power, relax back stick and fly back to a normal level flight condition with a minimum of lost altitude. In the present case however, after entering the stall, a sharp displacement of full rudder in the direction of the desired spin is added while continuing to hold back stick. You may also have to add aileron in the direction of the rudder displacement as well to force the airplane into a spin. These control inputs will have the effect of forcing a rotation around the vertical axis of the airplane, resulting in the outboard wing (with respect to rotation) generating a bit more lift than the inboard wing.

In this condition, an airplane will begin a rotation in the direction that you displaced the rudder (and aileron if applicable). The nose usually remains in a level or slightly down attitude and the aircraft begins a gentle rotation around its vertical axis. With most airplanes, that are designed to be aerodynamically stable, the recovery from the spin is accomplished by relaxing the rudder and elevator to neutral (and the aileron if you are holding aileron). Most airplanes will terminate the rotation in about a ½ turn and enter fully controllable diving flight. Recovery is then just a matter of adding some back stick and returning the throttle to acrobatic cruise. If your airplane has a slightly rearward center of gravity it may rotate a full turn or more before it recovers. If you would like recovery to occur sooner, add opposite rudder at the recover point and that will accelerate a return to straight-ahead flight.

The Stall Turn.

The Stall Turn is a maneuver in which an airplane is flown on an upward vertical line and after establishing the upward line, the throttle is placed in high idle. As the airplane slows and just prior to a stall, rudder is added such that the airplane rotates around its vertical axis and enters a downward vertical line exactly coincident with the upward line established at the entry to the maneuver. The rotation should occur prior to the stall and with enough forward motion to still have rudder effectiveness. The aircraft should not “flop” over on its back or toward the belly in this maneuver but should rotate smoothly around its vertical axis from a straight nose up attitude to a straight nose down attitude. In order to enter this maneuver, the aircraft is flown straight and level, into the wind at aerobatic cruise. Similar to the entry to a loop, entry should be accomplished as the aircraft passes in front of you by pulling back stick, maintaining wings level and adding throttle to full. The aircraft is placed in the vertical up line however, and unlike the loop this means that you must relax the back elevator while maintaining wings level such that the aircraft is going straight up, in front of you with full throttle. At that point, the throttle is reduced to high idle and the aircraft is maintained in the vertical up line, wings level as it slows. As is mentioned above, as the airplane approaches a stall, full rudder is added in the desired direction of the turn to cause the airplane to rotate around its vertical axis and enter a straight down line, still maintaining wings level.

Recovery (or completion) of the maneuver is accomplished by smoothly adding back stick while returning the throttle to aerobatic cruise. You should exit at the same altitude that you entered the maneuver on heading that is opposite to the one you selected for entry. This would mean that you would exit in level upright flight downwind. Some practice will be needed in order to get the correct timing of the rudder input. If you input the rudder too soon you will have a kind of “flat turn” at the top, and if you wait too long the aircraft will stall and fall out of the maneuver. If this happens, let the nose fall to a safe, nose down position, and recover. With a little practice this is a fairly easy maneuver to execute and can be useful as a building block for other aerobatic maneuvers.

The ½ Cuban Eight.

An interesting extension of the Immelmann turn discussed above is called a ½ Cuban Eight. You will recall that in the Immelmann, a normal loop entry was accomplished until the airplane was at the top of the loop, in inverted level flight. At that point a normal axial roll is executed to place the airplane in upright level flight on a heading opposite to that which was used to enter the maneuver. In a ½ Cuban eight, instead of entering level inverted flight at the top of the loop, the loop maneuver is continued until the airplane is in approximately a 30 to 45 degree nose down attitude. At this point a normal axial roll is executed to return the airplane to upright flight. At this point the airplane will be in about a 45-degree nose down condition. The throttle should be reduced from full throttle to aerobatic cruise and the aircraft returned to normal upright flight by adding back elevator. The exit altitude should be the same as was used to enter the maneuver and the exit heading would be the opposite of the one used to enter the maneuver.

The Full Cuban Eight.

If you have mastered the ½ Cuban Eight described above you may enjoy extending it such that you connect two ½ Cuban Eight’s together (one right and one left) to produce a smooth maneuver which is great practice and done properly, very pretty to watch. In order to couple the two ½ Cuban Eights together, execute the normal ½ Cuban Eight maneuver as described above to the point of having completed the axial roll. That will place you on a reciprocal to entry heading, in a nose down condition (about 45 degrees), wings level at aerobatic cruise. To enter a second ½ Cuban Eight from this position, smoothly transition through level flight to enter the second ½ Cuban Eight while adding throttle to full. Again, after going over the top of the loop and down the other side, execute a ½ axial roll to upright flight at about 30 to 45 degrees nose down. This will place your airplane on the entry heading. Complete the maneuver by reducing the throttle to aerobatic cruise while adding back elevator stick and return to level flight at entry altitude on the same heading that was used to enter the first ½ Cuban Eight When you gain some experience you will find that you can continue doing “connected” ½ Cuban Eights, but the normal maneuver is accomplished with two partial loops and is complete when you are in level upright flight on the entry heading with wings level.

More on advanced aerobatics can be found in our full text. Click the link below.

 
This article was furnished by Russ Petersen. If you are interested in the full text of this publication, please click here.
 



 



 
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