The following was received from Richard Goode and is reproduced here with his permission. Richard is a world-class competition and display pilot. He knows of what he speaks.
After his letter, a personal friend and YAK expert scribbled someopinions on the subject which may be far less authoritative than Richard's but you may find them useful.
The Hammerhead, more than most maneuvers requires very specific and careful training. The words that follow are intended to be used as a reference during that training and NOT to teach you how to perform the maneuver so you can try it yourself.
Hammerhead, or stall turn.
These are my thoughts on the hammerhead, based on some years of performing them and teaching them. Please let me know if you disagree or have a different opinion. I am here to learn too. This is written with the YAK 52 in mind and assumes an engine that rotates in that direction.
Although the HH looks simple to do, it actually requires a good sense of timing and a well developed sense of orientation. I make it one of the last figures taught in a course of aerobatic instruction. In my opinion, the most important part of the figure is to know when it does not look right and abandon it safely, don't hang in there hoping it will "sort itself out". - It won't. The consequences of a badly flown HH can be as described by Richard above, or a tail slide which can be damaging to an aircraft, particularly if fitted with flaps.
This is my procedure:
In preparation for the HH proper, we will fly the "Climb & Dive" exercise: Set aerobatic power (80cm + 82%). Let the aircraft accelerate to a steady speed and ensure that the slip ball is centered (no slip or skid). Now, look straight ahead and pull back on the stick until you have achieved a pitch up of 30 or 40 degrees. Look at one wingtip and then the other and check that they look the same with respect to the horizon. (adjust with coordinated movement of aileron and rudder if not). Keeping you feet absolutely still and not allowing the rudder to move at all, return the stick to the elevator-neutral position. The ball should still be about centered.
As you sit there, speed reducing, look at the airspeed and the slip ball. As speed reduces, the ball will move to the left, further and further. At a speed just above the spin entry speed (say 120 kph) push forward to level flight while still looking at the slip ball. Now it is hard over against the left edge of the tube right? Why? - Because as the speed reduced, the fin & rudder became less effective at counteracting the torque, 'P' factor and gyroscopic precession of the engine & prop. The ‘P’ factor and torque are the dominant forces as you maintain constant pitch and the aircraft slows in the climb. The precession is dominant during the change of pitch as you push to level flight.
Now let's do that again, this time though, use the rudder to keep the ball in the center. You will find that at the point of minimum speed, as you push to level flight, you have close to full left rudder applied to keep the aircraft straight!
So in a hammerhead, would we not expect the same phenomenon to occur? Yes and no. There is little or no 'P' factor because the angle of attack should be zero during the climb but there is still plenty of torque from that big geared engine. The bottom line is that if you spend the vertical up part of the figure with your feet still, you will be skidding wildly by the time you need to pivot. Skidding is a principle ingredient of the spin remember.
So now let's look at the hammerhead for real.
We set aerobatic power and dive, if necessary, to achieve the required entry speed. I use 280~300 kph in the YAK52. Return to level flight, check wings level and ball centered. Look straight ahead and pull. Pull hard enough to minimize the loss of energy, at least 3G, 3 1/2 to 4 is better. (In a YAK52, I temporarily use both hands to help avoid unwanted aileron input) Now as you pitch up, two things will happen: You will loose sight of the horizon over the nose and the aircraft will yaw to the right just like it did in the practice exercise. So look at a wingtip now and anticipate the need for some left rudder to keep the ball centered. Glance at the ball as you pitch. Look at both wingtips, one after the other and check that they are in the same place, relative to the horizon. Correct with small coordinated movement of stick and rudder. When you hit the vertical, check forward with the stick and climb. The slip ball won't work now, it is laid on its face, so instead, we will use the wingtips to judge. Look at one, the other and back again over and over so that you can be sure they are the same distance above the horizon still. Just like in the exercise, you will need to feed in left rudder, a little at first, then more and more as the speed reduces.
At this point comes a vital check. Are you perfectly vertical and straight? If not, immediately abandon the figure and try again. DON'T NOT CONTINUE with an imperfect beginning. Remember that the top surface of the wing is curved so allow for that in judging the vertical but don't overdo it. Not quite vertical (too little pitch) is much better than past the vertical (slightly on your back) - The Russians often use the Attitude Indicator to judge the vertical. In the West, that is unusual.
Assuming all is well, the next item on the agenda is the pivot, or rotation which is the heart of the figure. You are going to pivot to the right of course. In time, you will come to feel the point at which rotation should be initiated but for now, use the airspeed indicator. A figure of 100 kph is a good starting point.
A little more theory: - All the way up the vertical line, you have been feeding in left rudder so now, at the top of the climb, you most likely have at least half rudder applied. When you stomp on the right rudder pedal, the rudder will move from half left to fully right. It will generate 50% more side thrust than had it moved from the neutral position to full right. Adding that rudder to keep straight on the way up will pay dividends at the top!
Back to the HH! - When you reach the pivoting speed, let's say 100 kph, look at the right wingtip and note EXACTLY where it lines up with the horizon. With your left hand on the throttle, smartly apply FULL right rudder (press hard). Keep your eyes on that same point on the horizon. After the aircraft has rotated through 90 degrees, the nose will be pointing at your reference spot. Keep looking at it! You may use SMALL aileron inputs now to guide the left wingtip so that it will pass exactly thought you reference point.
· Many people teach that as the nose passes through the reference point, you should look at the right wingtip (which is now pointing at the ground) and use this to find a new reference point to use on the down line. I don't do this because, if you use the reference point on the horizon and try to make the left wing pass through the same place the right wing did, you will be sure that you rotated in a perfectly flat plane, with zero angle of attack. Zero AOA means it is impossible to stall and hence to spin.
· If the aircraft begins to roll on you or fall over onto its back, DO NOT push the stick forward and left to counteract! Those are the control positions for an inverted spin! It is rolling because you screwed up the entry. Your airspeed was incorrect or you were not straight at the point of rotation. Close the throttle and center the controls. Let the aircraft get itself pointing downwards safely and recover. If the controls are centered, you cannot stall.
Now, when the left wing is just above your reference point on the horizon, quickly look over the nose at the ground and swiftly apply left rudder. When the rotation stops, reduce the rudder to a position just slightly left of neutral. Why? Because you are at zero airspeed with a big torquey engine trying to pull you to the right again. As the aircraft gathers steam on the down line, move the rudder to the neutral position and then, even, a little to the right as the aircraft begins to slip. (Check both wingtips again to judge the rudder required). Recover from the dive with a good straight pull. Use the same acceleration (3 1/2 G or so) that you used on the pull up.
Many people will reduce the power setting on the way down. They argue that it prevents the aircraft from building up too much speed and allows for a more comfortable pull out. Others prefer to keep the power constant so that they don't have to consider the sudden change in torque. I prefer to reduce power but you may not. it's up to you.
That is all I know about the hammerhead. I have no doubt that Segei Boriak would apply his trademark "Bull-shiiit" to the above but it works for me. As always, I remind you that there is no substitute for competent instruction and that you should not try this without first availing yourself of some. There are plenty of good aerobatic instructors around these days. To learn more on the subject, I suggest the excellent book "Flight Unlimited" by Eric Muller and Annette Carson.
A YAK 52 Aerobatic Instructor