The sometimes surprising effects of the controls
So it’s the mid 1990s, I think – I’m not good with dates and times. I am flying from Jan Smuts to Port Elizabeth in one of SAA’s new Airbus A320s. Tupperware aeroplanes I’m told. Those were the days when SAA was still ranked amongst the world’s top airlines.
I worm my way up to the front – it’s sort of allowed because I know the driver. I’ll call him Fanie because if I tell you his real name he will have me cost-rated – I think that’s what he said.
The cockpit is scary. It’s as if you are about to swing a leg over your Harley when you spring back in horror. Some nut has replaced the round clocks with TV screens, turned all the switches upside-down and, worst of all, stolen the handlebars. There’s nothing to hang on to.
We are climbing out at an impressive angle into a crystal blue sky. I am much puzzled by many things in the cockpit, but the ones that have really grabbed my attention are the thrust levers, or throttles, or whatever you call them.
I ask Fanie about them and he tells me that I must rid myself of foolish old-fashioned thinking like that. They are not thrust levers at all – they are actually three-position switches. He explains that for instance nothing will happen if he pulls them back – like this…
He is quite wrong. Lots of things happen simultaneously and alarmingly. The aeroplane’s nose sags below the horizon as if we have gone over the crest of a wave on a fishing boat. We are light in our seats. The whole thing goes all soggy like a hot-air balloon that has been shredded by a thousand drones.
At the same time the first officer, an earnest youth with glasses who had been fiddling with the typewriter, looks up in horror.
“No, no, NOOOO, Captain, we are in Alternate Law Five with the push/pull interconnect setting programmed through the FMC to maintain airspeed, when the CADC overrides the aft toilet smoke detector.”
“Of course. Silly me!”
I told you this story for three reasons really:
- It’s critically important that when you are around aeroplanes never do anything in a hurry.
- The controls don’t always do what you expect.
- I thought Airbusses were meant to make things easier for pilots. I’m sure they do if the pilot remembers all the rules – which is not easy.
I have a few more examples of aeroplanes behaving badly, and not doing what you expect. Like when you are on the edge of a stall and she drops a wing, and you try to pick it up with opposite aileron. And the stupid wing drops further and will probably drag you into a spin..
Or when you are flying a C150 or a 172, and you have to keep adding power to maintain altitude. Suddenly your pilot brain springs into action. Of course – you need carb-heat. You yank out the knob expecting an increase in power, but what happens? You lose 600 revs and the whole bloody aircraft starts shaking. Of course, quick-as-a-flash you whack in the carb-heat and regain most of the power. But over the next couple of minutes you realise that you are in for a forced landing – the power just keeps fading. You lob down in a field, or on a farm road and congratulate yourself on doing a good job.
Actually not. You were wrong to expect carb-heat to give you more power. It will eventually, but first you must expect a drop in power for two reasons:
- The hot air going into the carb is much less dense than the normal air – so it’s like flying on a hot day, or at high altitude. You get less power.
- The ice in the venturi has been allowing less air through so the mixture is too rich. Suddenly you feed less-dense hot air into the system and the mixture becomes much too rich – so the engine wants to die,
Obviously the longer you let the ice accumulate the worse it becomes and the more severe the reaction when you do eventually use carb-heat.
Gee – that was a long story about controls not doing what you expect. And here’s another common one.
If you think you are Bob Hoover doing a wing over after a beat-up (see the accident report on page…) and you add rudder to get yourself round more positively, the secondary effect will invert you.
This is not wishy-washy theory – it’s pure aerodynamics and physics. In three consecutive annual airshows, in the late ’60s three pilots died in front of massive crowds while doing wing-overs. One was Henry Hunt with three pax in a 200HP Beech Musketeer, another was also a Musketeer – a 150HP Sport, and the last one was at Parys – I think in a Bonanza.
There are two reasons why a beat-up and wing-over is such a common way of plummeting to an early grave.
- It looks so easy that everyone and his butler can do it.
- The controls behave normally relative to the aircraft, but the aircraft is steeply banked and you have not thought it through.
Here’s what happens. You stuff the nose down, zoom along the runway, past your worshipers, looking back over your left shoulder to see their waves and happy smiling faces. Then you pull up sharply into a steep climbing left turn.
Okay let’s freeze-frame there for a moment. You are a couple of hundred feet up, almost at right angles to the runway with 70° of bank, but this turn is going to take you over the heads of your devotees instead of in front of them, so you need to tighten it up quite a lot.
You use left rudder and it does what it always does – it moves the nose towards the left wingtip. That means down. And the further effect of the left rudder as I will explain shortly is to bank (or roll) the aircraft to the left. This steepens the already steep bank – often to the extent of inverting you.
Of course you don’t like this, so you use right aileron to prevent the bank from increasing
You are looking over your shoulder again to see how you are doing with lining up in front of your admirers.
Now you are low, the nose is dropping and you are banked far more steeply than you have ever been before. Your airspeed has died away, the stall warning is shouting and you have lost half of your 200’.
Whether the aircraft flicks into a spin or not – the result is the same – you are going to smack into the countryside.
As my Nav instructor in the RAF used to say, “A dead pilot is a burden upon the Treasury and an eyesore upon the landscape.”
I did tell you that you will probably never need to think about the effects of the controls again unless you go in for aerobatics or instructing. So, as you are planning to instruct, follow me closely as we look at the “further” effects of the three main flying controls.
I’ll give you the guts of the pre-flight briefing and the patter (in blue again).
The Further effect of the Elevator
Remember I told you that if you ease the stick back like this (demo with your model in the briefing) the nose moves in the direction of the cabin roof, and if I ease it forward the nose moves in the direction of the undercarriage.
This always applies regardless of the aircraft’s attitude. But the elevator has a further (indirect) effect that applies if you start from straight and level – which is the majority of your flying. Here’s how it works.
Notice that we are now straight and level – and we are doing 105 knots and the rev-counter shows 2300 RPM. If I ease the stick forward, like this, the airspeed increases, (point at the ASI) because we are going downhill. You can also hear an increase in the revs, and in the slipstream noise. (point at the rev-counter). And if I ease the stick back, like this, the airspeed decreases (point at the ASI again) because we are going up hill. You can also hear a decrease in the revs and the slipstream noise (point at the rev-counter).
This is very important because, in light aircraft, we use the elevator to control the airspeed. This applies not only to straight and level but also to climbing and gliding. If you are going too slowly you ease the stick forward to increase airspeed, and if you are going too fast you ease the stick back and the airspeed will decrease.
This may seem counterintuitive – you probably expect to use the throttle to control airspeed, but we actually use the throttle to climb or descend. We will come back to this during your lessons on climbing and descending.
Notice that the airspeed does not respond immediately – it takes a little while to overcome the aircraft’s inertia.
In the light of the accident report on page … I would like to tell you more about the further effects of the ailerons and rudder that we started looking at last time.
Further effect of the Ailerons
Remember I told you that if you ease the stick to the right, like this (demo with your model in the briefing) the right aileron moves up, reducing the lift and drag, while the left one moves down, increasing the lift and drag. This causes the aircraft to roll to the right (as we saw previously). But the difference in drag actually pulls the nose in the direction of the left wingtip.
This undesirable yaw, in the opposite direction to the movement of the stick, is called “adverse aileron drag”. You will learn how to counteract this when we do the coordination exercise.
So, to summarise, when you move the stick to the right, the aircraft rolls to the right and yaws to the left.
Now the aircraft sideslips to the right, and this causes it to weathercock and yaw to the right. This means the nose moves in the direction of the lowered right wing. So we start losing height in a right turn. In other words, using right aileron eventually results in a spiral dive to the right. (Note: keep this demo gentle otherwise you will (a) frighten the pupe, and (b) run out of time to talk your way through it.)
Now the idiotic AIC tells you that you must talk the pupe through the spiral dive recovery. Forget it – he hasn’t even learned to fly straight and level yet, so he can hardly deal with spiral dive recoveries.
So that’s the further effects of the aileron.
Now let’s think about the rudder.
The further effects of rudder
Remember that the right rudder moves the nose in the direction of the right wingtip – so the nose yaws to the right. This means the right wing slows down and the left wing, on the outside of the turn, speeds up and gets more lift. The result is that as the aircraft yaws to the right, it also immediately banks to the right.
If you maintain right rudder the nose will continue moving in the direction of the right wing – in other words down, and to the right.
The result is a spiral dive to the right – the same as the secondary effect of aileron – but for a totally different reason.
Again demonstrate this very gently to avoid alarming the pupe and to allow yourself time to talk your way through it as it happens.
Here’s a quick summary of the further effects of the controls if you start from straight and level.
| CONTROL | EFFECT | FURTHER EFFECT |
| ELEVATOR | PITCH | AIRSPEED ALTITUDE |
| AILERON | ROLL | SPIRAL DIVE |
| RUDDER | YAW | SPIRAL DIVE |
Now we come to the interesting bit – how we use the controls for normal flight on a day-to-day basis.
The elevator
We use the elevator to control the airspeed whether you are flying straight and level or climbing or descending. Don’t expect an instant response – the rate at which the speed changes depends on the inertia (basically the mass, or weight, of the aircraft).
Heavy aircraft and big jets are so slow to respond that the pilots have to use thrust to change the airspeed – but that’s not for us in our puddle-jumpers.
We also use the elevator to prevent the aircraft from losing height when turning.
The ailerons
We use the ailerons to either deliberately bank the aircraft, usually for entering a turn, or to keep the wings level in turbulent conditions.
The rudder
This is the interesting one. Ask a dozen pilots why aeroplanes have rudders, and you will get a dozen blank looks, followed by vague mutters about you need a rudder to turn the aircraft, duh.
But actually you don’t. The aircraft will turn very happily without rudder. In fact next time you are in a turn, even a steep turn, take your feet off the rudder pedals and see what happens. Absolutely nothing. The ball will stay in the middle and she will keep turning happily.
In fact, the rudder is not there to turn the aircraft – it has three main purposes:
- To deliberately sideslip. We’ll deal with sideslips later.
- To counteract ‘P’ effect which is yawing caused mainly by the propeller’s slipstream.
- To counteract adverse aileron drag. As you move the stick to the right to start a right turn, remember that the right aileron moves up and the left one moves down – causing drag like a flap. So the nose yaws to the left. So the rudder is there to counteract that undesirable yaw in the wrong direction. When you are established in a turn the ailerons are pretty well neutral – so you don’t need rudder. But you do need it again as you use aileron to roll out of the turn.
So the rudder is there to counteract two design problems that affect us every time we fly – ‘P’ effect from the propeller slipstream, and adverse aileron drag
We will talk more about the ‘P’ effect during the climbing and descending exercises. And more about aileron drag during a coordination exercise which we will look at next time. Strangely this is not in the syllabus, but good instructors, like us, teach our pupes how to use the rudder properly.