The Avalance

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A LOOP with a flick roll in the top was always called a ‘Porteous Loop’ (named after pilot Ranald Porteous) when I was a University Air Squadron student at Cambridge in the late 1960s. However, in her excellent book Flight Fantastic, Annette Carson corrects this ‘mistake’ by recording how the first person actually to demonstrate such a thing was American Earl Daugherty in the early 1920s, flying his “faithful Jenny to the unheard-of speed of 120mph”.

It was Auster test pilot Ranald Porteous who flew much the same thing in his 145hp Aiglet at Farnborough in 1949 and coined the term ‘Avalanche’ by which this figure is now most commonly known. To quote again from Flight Fantastic: “He took the Aiglet up one day at Rearsby in view of his managing director, Frank Bates. Porteous landed and approached Bates for a first-hand reaction. Bates regarded him quizzically for a moment, then replied: ‘Are you trying to tell me that was intentional?’”

ROLLING IN A LOOP
Last month, I wrote about the orientation challenge presented by making a ballistic aileron roll in the top quarter of an otherwise normal loop. Early aircraft would have found such an aileron roll impossible to achieve with consistency and control, simply because their ailerons lacked the authority to command a full roll in the time available.

This would certainly have been true of both the Jenny and the Aiglet, and is pretty much still the case with some more modern aircraft, including the Cessna Aerobat and the American Champion Decathlon.

The good news for pilots of all such aircraft, however, is that while the ailerons lack authority at low speed, the rudder does not. Remember, in vintage aerobatic terms, the rudder roll was invented before the aileron roll, which is why the term ‘Rudder Roll’ is not in common parlance.

So, in this month’s article, I will describe how to perform a positive flick roll at the top of a positive loop. If your aircraft flight manual permits flick or snap rolls, and if you are conversant with recovery from incipient spins and unusual attitudes, it is something you might try.

A SPECIAL LOOP
To initiate a flick roll that can be sustained through 360 degrees of roll, it is necessary to have an airspeed perhaps 40% higher than the normal 1g level stall speed VS1. While it is quite a simple matter to calculate such a speed, it is less simple to achieve it at the 1 o’clock position of a loop (as seen anticlockwise in the illustration). So this is how I teach myself to fly this sort of loop in a new aircraft type.

I dive to achieve perhaps 20 knots faster than the normal ‘book figure’ for the loop entry speed. I then start looping, but I look primarily at the accelerometer (G meter) rather than the speed indicator. I start to pull initially at about 4g, but this naturally decreases as the speed reduces during the climbing part of the loop.

As I near the apex of the loop I keep pulling enough to keep the G meter above 1.5g. If I can keep the acceleration above this figure, without pre-stall buffet, all round the top of the loop, then I reckon to have enough speed for the flick roll. At ‘VS1 + 40%’, the wing will stall if I pull 2g. But if I keep to just 1.5g the wing will keep flying, although it will be close to the critical angle of attack.

I fly this loop from a number of alternative entry speeds, until I find a starting point that gives me a consistent result that doesn’t stall but allows me to keep 1.5g.

Of course, this loop looks nothing like a circle from outside. It is a different animal altogether, having a very small radius at the top compared to that at the bottom. This will change, though, once I add the flick
roll as the next step.

RUDDER AND UNLOAD
Once I am sure I can fl y the modified loop just described, I start again at the bottom, but this time planning to execute the flick roll. I apply full rudder, as firmly as possible, just when I detect the aircraft has reached the attitude when it is about 30 degrees from the top of the loop.

To see this, I look out of the top of the canopy and judge the distance of the nose above the horizon.
At this point, I apply rudder but no more upelevator.

I already have a very high angle of attack, enough to stall one wing and start the flick. Pulling the control column back further at this point will risk stalling both wings, accompanied by a huge increase in drag and a much slower rotation. This must be a flick roll, not a horizontal spin.

As soon as the rudder bites the wing will drop, signalling the start of the flick. The wing that drops, though, on the same side as the rudder, will actually move skywards rather than towards the ground.
Such is the result of starting the flick while upside down at the top of the loop.

As soon as the wing drops, I start to move the control column smoothly to a position forward of neutral, perhaps adding a little aileron on the same side as the rudder.

The amount of this elevator ‘unloading’ and in-flick aileron, will depend on the type of aircraft fl own. Different types with different wing designs have different stalling characteristics. Some wings have wash-out, which lets the ailerons keep working a little as normal, even though the central portion of the wing is stalled. In this case, more aileron will probably help the flick rotate quicker.

Putting the elevator control forward will accelerate the rotation by the conservation of angular momentum, otherwise known as the ‘ice-skater effect’. You know, if you are spinning on skates and pull your arms close to your body, the speed of rotation increases. It also works in a swivel chair, of course, as bored office workers will know.

Just how far forward to unload is again dependent on type. It’s a matter of trial and error. Generally, the further forward I put the stick, while maintaining full rudder, the faster the aircraft rotates. In some machines, though, put it too far forward and the rotation will stop. If this happens, I double check to make sure it is the elevator that has stopped the flick, not my lazy rudder work.

STOPPING
An Avalanche has at least one of the characteristics of a packet of chocolate-chip cookies. It is difficult to know when to stop. Of course, you mustn’t stop until it is over. The Avalanche, that is.

This is another reason for unloading fully with the elevator. It is much easier to look ahead and monitor the extent of the progressing rotation if it is accelerated. The accelerated flick roll has much less ‘conic’ angle to it. The horizon appears to rotate much more closer to a ‘point’ than a great huge circle lost under the nose of the aeroplane.

I stop the Avalanche with full opposite rudder, which I apply hard and fast after probably two-thirds of the 360-degree rotation. Unlike the situation at the end of a level flick, the flight path of the Avalanche must continue in the looping sense during the rotation and after it has stopped. It is not really appropriate, therefore, to help to stop the flick with a lot of forward stick.

The rudder must be the principal stopping control, with the elevator starting to come back from the forward, unloaded, position to a place near or slightly aft of neutral. With the stick coming aft and the nose just below the horizon, I have to use peripheral vision to make any small roll correction to ensure the remaining loop stays on heading.

Picking up the roll situation quickly, from the limited visual cues available, and continuing in a smooth looping motion is another tricky problem, overcome only with lots of practice.

WHICH FOOT?
The flick roll can, of course, be done with either foot, just as when on the level. The flick roll in the top of the loop, however, is often initiated at an air speed quite a bit lower than a level one. This can have implications for the recovery, especially in shortcoupled aircraft with a large torque effect and an aft C of G position.
Put bluntly, the flick can be a bother to stop, and can easily carry on for another half-roll or so, driven by the engine, if you use left rudder in a Pitts Special or similar aeroplane.

In this situation, closing the throttle will reduce the torque and help with the recovery, but it may still be slightly delayed because of the reduced rudder effectiveness at reduced power.

An Avalanche in a rear-loaded Pitts is, therefore, much easier to control when initiated with right rudder, and this is the direction I teach first in these aircraft. In larger aircraft, it is usually fine to use either foot.

AVOID OVER-CONTROLLING
It is also important to avoid over-controlling at the point of recovery. Full opposite rudder is required, as mentioned above, but beware of the sudden, unplanned (and certainly not recommended) application of full forward stick. I have seen this done by mistake at relatively low level and the aircraft has transitioned seamlessly from a positive flick roll into an inverted spin.

Some quick thinking is then required on the part of the pilot to change feet again and release the grip on the control column. As always, precision is better than aggression.

ENERGY MATTERS
An Avalanche consumes energy. The energy destroyed during the auto-rotation is more than you can replace during the course of the remainder of the figure. This means that there are two possibilities at the beginning and end of the figure.
The most usual case: the loop finishes lower but almost as fast, because of the lost energy. Less usual, but actually preferable geometrically: the loop finishes at the same height but perhaps 15 or 20 knots slower than it started.
The consequences of this are simple. Be careful of what you intend to do immediately after the Avalanche is finished. You may be lower or slower
than you need!

CONCLUSION
The Avalanche is another great figure for developing and improving spatial awareness. Stress on the aircraft is low because of the relatively low speed for the flick roll. Stress on the pilot may, however, be higher, so be sure to try this with an experienced instructor on board or plenty of height below you.

Revise your procedures for recovering from loss of control in unusual attitudes before you try this exercise.
Be safe and enjoy your flying.
 

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