This is a short note on some of the ways a hammerhead turn might become unstuck. The context was the question whether an experienced aerobatic pilot might use the turn in an emergency to exit a box canyon. There are no bad questions, and this question is worth exploring in more detail.
The hammerhead turn is an aerobatic manoeuvre and should only be attempted in an aircraft approved for aerobatics, with properly qualified flight crew. Good VMC and a good horizon are part of good airmanship during aerobatics. Mountainous terrain does not offer a good horizon.
The stall turn is known as a hammerhead turn in North America, and this term is a better description. In a correctly executed hammerhead turn no stall actually occurs, or is intended to occur. The British perhaps should call it more precisely, the no-stall turn.
The manoeuvre is a combination of various manoeuvres:
entry from level flight and on axis;
a quarter loop into
an upward zero lift alpha vertical line, with wings level and aircraft remaining on axis;
a 180 degree turn with no pitching, or rolling, and staying within one wingspan;
a downward vertical line into
a quarter loop to level.
It has a k factor of 17 on the Aresti scale which makes it an important figure in the Sportsman category. The k factor for the loop is 10, for example.
https://www.iac.org/legacy/aerobatic-figures
Alan Cassidy’s Better Aerobatics has a chapter dedicated to the manoeuvre, and is an excellent analysis of the correct technique for the manoeuvre.
The aircraft is pivoted using rudder through 180 degrees when the aircraft is approaching zero airspeed, and on a zero lift upward vertical line. In effect approximately zero G. The combination of zero lift angle of attack/alpha, and zero G make it a no-stall turn. At zero G there is no lift, also at ZLA, there is no lift – no lift, no critical alpha, no turbulent layer, no stall.
However, there are threats during the various components of the manoeuvre.
In the initial quarter loop the aircraft will experience around 4g+, which is above standard category envelope. It also means the aircraft stall speed will be double the 1g stall speed (4^1/2) if entry speed is not comfortably above this speed, the aircraft will enter an accelerated stall. A typical GA four seat tourer (eg 172), at a high density altitude may barely have an IAS in cruise that is twice 1g stall speed.
If a ZLA upward vertical line is not established, the aircraft potentially is being set up for either an inverted or upright spin, when you apply full rudder to turn. The combination of zero G and ZLA at the point of pivot is important to control this threat. Your average 172 does not have an aerobatic sight line on the leading edge, and I don’t believe Cessna offered it, so judging the vertical will take lots of practice. There are many STCs out there, but possibly not for this.
When the aircraft is pivoted using rudder lots of interesting forces take over. The aircraft has slowed to nearly zero IAS, so aerodynamic flight controls have lost a lot of their effectiveness. Propeller slipstream, engine/propeller torque, and gyroscopic effects (both from the gyro effect of the propeller and from the airframe as it pivots) play a much bigger role. If these forces are not controlled the aircraft will roll and pitch as it turns, and therefore not accomplishing a 180 degree turn as intended. Again if a wing is generating lift, the pivot will induce roll as one wing is advancing and generating more light than the other wing, the gyro effect will induce pitch, we are now not where we wanted to be, but in a high alpha situation with lots of gyro and yaw – spin entry territory.
The other threat is that you initiate the turn too late and you set the aircraft up for a tail slide. Many aerobatic aircraft are not allowed to attempt a tail slide. As airflow reverses the flying controls are moved against their stops, can be damaged and/or control restrictions occur. If a tail slide is encountered there are specific techniques for recovery.
Assuming we have managed to accomplish something approaching a 180 degree turn (highly unlikely, but nevertheless) we now have to establish a ZLA vertical down line. If we do not apply opposite rudder and then centralise the rudder, the aircraft is descending in a skid. The ground is filling the windscreen and the aircraft is zooming towards it. If we pull up early and we are not in a clean vertical line, an accelerated stall/flick roll can be expected – the flick roll potentially occurring above any theoretical safe flick roll speed for a 172. Even in a clean vertical line if you pull g early before re establishing a safe speed an accelerated stall will occur.
The last manoeuvre is the quarter loop back to level. Here there is a potential for over stressing the aircraft.
I believe an experienced aerobatic pilot (say a 1,000 hours of aerobatics), is almost certainly never likely to attempt this as an emergency manoeuvre. She has probably seen too many hammerhead turns get unstuck to even think of doing one as an emergency turn.
…I should have added you typically lose several hundred feet in executing the manoeuvre. This is explicit in the Aresti notation.
And there’s probably turbulence in the canyon. And you don’t have a horizon outside.
Speaking of canyons, I suspect the original suggestion actually meant a wingover rather than a hammerhead turn.
Ultranomad wrote:
Speaking of canyons, I suspect the original suggestion actually meant a wingover rather than a hammerhead turn.
that, and the fact there was no space to turn.
Not sure how much an aerobatic guy needs to be experieneced to pull such manoeuvre in emergency?
Probably, someone who can fly Red Bull air race in constrained airspace/runway like Ascot, though they fly Extra 300 not a C172 and they do hit the pilon once or twice…
Looking at the Extra POH, recommended minimum entry speed for hammerhead is 100kts. Same as for a loop.
Would rather do an Immelman. Simpler, not so technical.
loco wrote:
Looking at the Extra POH, recommended minimum entry speed for hammerhead is 100kts. Same as for a loop.
Would rather do an Immelman. Simpler, not so technical.
The whole point is taken out of context. The idea (as I remember) is to turn around the plane 180 degree using as little lateral space as possible (closed, narrow canyon with no chance of out-climbing the terrain). The idea was never to execute a perfect competition aerobatics hammerhead (stall turn) maneuver
If a “lazy 8 kind of turn” will do (30-40 deg pitch, turn using rudder when reaching close to stall speed), there is no need to pull 4 Gs, going vertical etc. If it’s so narrow that a hammerhead, or other pure vertical maneuvers are the only solutions, then I would think you have continued way beyond what anyone would ever do in any real situation, and you are out of luck.
But, given that a pure vertical maneuver is not the only solution, and you have a real plane (not a C-172, Cirrus and such), then you do what suits you best. You wouldn’t improvise a new maneuver, but rather do one you knew and have trained for. Immelmann probably or something else (lazy 8 kind) in a Cub.
A wingover is a less demanding figure, but still requires energy to start and the radius of turn is not optimal (probably around one mile). There is no prescribed bank angle at the apex, although a 180 degree wingover would typically have a minimum 90 degree bank at the apex. The turn is a nicely coordinated turn with speed decreasing to 1g stall speed, but as you should be around 1/2 g at the apex with the nose falling, and turning to point at the target on the ground, you should be safely above stalling, if you have practised. Energy management and coordination is required to execute with finesse – hence being a featured manoeuvre in gliding aerobatics.
Doing the manoeuvre in a gentler manner, say peak bank of 60 degrees (technically the limit for non aerobatics), is unlikely to result in a 180 degree turn without loss of height.
Setting up the manoeuvre with too little energy will also result in a potential stall while in an unusual attitude.
It would be interesting to know has a. anybody actually used this technique in mountainous terrain b. being taught this in an approved mountain rating course? or an experienced mountain pilot suggested it?
I have zero knowledge of aerobatics but was the one who suggested the hammerhead turn as a means to get out of the narrow canyon in the C172 accident in Greece thread.
Thanks everyone and Robert especially for the explanations. I always thought that a hammerhead turn was the ultimate get out of a box canyon trap manoeuvre and was executable in almost any plane, I now know better.
