This well recognized aerobatic maneuver was invented in 1915 during WW 1 by German ace fighter pilot, Manx Immelmann. Like the split S you have already learned, it is a vertical maneuver.
The Immelmann consists of an upward half loop, rolling out of the half loop at the top into level flight, which means the plane is now traveling back the way it came, but at a higher altitude. It is the mirror image of the form of split S that is performed without an initial pull up, as we discussed in lesson 2.
Unlike the three inside loops, the Immelmann is rather quick and simple. Pull up, roll out, complete. In fact this is really a better first maneuver than the three loops if the the plane has ailerons. Without ailerons, however, the roll out can be very sluggish on rudder only. In a sluggish roll the plane tends to lose some altitude, giving the transition from half loop to level flight an awkward looping dip. If you have both ailerons and rudder, applying full aileron and partial rudder can give you a brisk, snappy looking roll.
Allowing for the delay in your reaction time, and the response of the controls, start that roll out just before top dead center of the half loop. Another nice feature of the Immelmann is that it sends you back the way you came, do when you turn around, you are set to practice the maneuver again. With loops you'll have to put in a dead pass or team it with a downwind maneuver in order to reposition your model for another try at the loops. Thus the Immelmann can be diligently practiced in an unbroken series of Immelmann turns and split S turns.
One drawback to the Immelmann is that you are returning at twice the altitude you entered. If you then perform your usual pull up style split S, you'll be coming out of the split S too high.
Two ways to correct for this are: (1) Don't pull up very much on your split S. Use the full scale technique of rolling inverted in level flight, and then going into a downward half loop. (2) If you would rather pull up some then make your downward loop larger, adjusting its exit to conform with your target flight corridor altitude. We want always to use that same flight corridor we used in lesson 2. A very large half loop is almost like a power dive. If you panic a bit and pull out rather sharply, the g forces are enormous. A wing rubber banded on may lift off the saddle somewhat. To guard against this, try throttling back in your power dive.
Like the loops, the Immelmann is designated an upwind maneuver, though it can be done credibly well downwind in lighter winds. Also, like the loops, the g forces will generated in the pull up will cause the heavy wing to roll outwards.
At the top of the half loop, you will be faced with a plane showing one wing hanging low. If you have good reactions and can roll out toward the low wing, you'll have a snappy axial roll to upright flight that will look pretty good. But if you roll the other way, the extra time it takes to bring the low wing back to level and then all the way around over the top will make the roll sluggish, plaguing you with a dip in altitude.
The answer is to go back to the inside loop part of this lesson, and concentrate on correcting for the heavy wing effect by applying aileron or rudder on the way up the loop. That way you come out level on top, and can roll either way.