Jim Davis
What’s the big deal about a steep turn? Hell it’s only 15° more than an ordinary 30° banked turn – and 15° is nothing – right?
Now I’m not good at sums so it took me a while to understand that it depends where you put that 15° of bank. If you start from straight and level it’s hardly noticeable.
Now imagine a pansy, 30° banked turn – the one you do every day. So if you increase that bank by a woosie 15° it can’t make much difference – or can it?
That would take you to a standard 45° steep turn. Later, we will see what yet another 15° of bank does when you take it up to 60°.
But, what’s the point of a steep turn? Well, it has three important functions, plus one optional one. You would use a steep turn if you:
• Suddenly find yourself on a collision course with another aircraft, or a bird, or a radio mast.
• Get into trouble in a valley.
• Want a brilliant coordination exercise. To find out how a pilot handles his aircraft simply ask him to do a steep turn to the left and then go straight over to do one to the right, without stopping in the middle. If you can do this a few times – going from side to side while keeping the ball near the middle and keeping the nose where it should be to hold altitude – then you can fly. It is not an easy exercise.
• You suddenly get it into your head that pylon racing is a good idea.
Now, before we get technical, I must tell you that I found out about steep turns the hard way. I did them at low level without any dual – and very nearly saw my tail-light.
You know those flickering black and white movies where everyone walked fast? Well that’s when I was building hours for my PPL. Whichever instructor happened to be around would ask what I did on my last flight, then usually tell me to go and do more of the same.
This meant you dragged a little yellow Cub out of the hangar, preflit it, found a prop-swinger and you were on your way to beat up boats on Bronkhorstspruit Dam, or entertain yourself with whatever cowboying took your fancy.
On the occasion of my self-taught steep turns I took myself to Garsfontein, which is about three nm east of Waterkloof. It’s all built up now, but at the time I was growing chickens there, and couldn’t even see my neighbour’s farmhouse.
Anyhow I headed there so that my young wife and baby son could admire me.
Our house was on a hillside, so to get low enough to impress my devotees, my turn had to be partly climbing and partly descending. Now, anyone who has flown a 65 HP Cub on the Reef will suck in their breath and confirm this was not a good plan.
No one had explained that turns cause more drag and a loss of airspeed. Or that the stall speed increases in a turn. So the aeroplane kept going all sloppy as I circled the house. Every now and then my left turn would sort of wobble into a bit of a right turn. Eventually I had had enough of this obviously defective aircraft, I took it back to the club and asked them to take it to Placo to get it repaired before it hurt someone.
I had no idea that the aeroplane was stalling and trying to spin. Had it been anything other than a Cub my promising career would have been hacked off at the roots.
So let me tell you why that little bit of extra bank makes so much difference.
[Pat can you shove the “steep turn diagram”, with the four aeries, here, please]
The diagram shows how increasing bank by even a small amount calls for a huge increase in the lift required. Going from a 30° to a 45° turn almost triples the amount of extra lift you need.
Two more 15° instalments will, respectively, take you to 100%, and then blow your socks off with a 300%, 4G manoeuvre. Look at the Lift required v. angle of bank graph.
(Pat can you shove the “Lift required v. angle of bank”, here, please)
So you’ve got the picture – a little increase in bank calls for a lot of pulling. And with that comes lots of drag, so you need a whole lot more power to stay in the sky.
While this has its disadvantages, I must tell you how instructors use this as a tool for knocking some of the bull out of gung-ho students, in the early stages. Instructor manuals tell you to give overconfident pupes a difficult task, and then criticise them when they don’t get it right. Sound cruel? Maybe it is, and maybe it saves their miserable lives. There is nothing an instructor hates more than a cocky pupe.
Ask them to do a medium turn. Quite often they steepen it on their own – just to show you how good they are. But if they don’t, then you simply mutter something like, “That’s a bit pansy, isn’t it?” They only need to steepen the turn very slightly before they start losing it. This is your chance to complain about increasing airspeed and loss of height. Pupe climbs back into box – mission accomplished.
To understand why going from 30° to 45° is such a big deal, we have to do a couple of sums. Now I am not good at sums, so I try to make things simple.
Have another look at the first diagram. If the wings are level your lift needs to exactly balance the weight. If your aeroplane weighs 1,000 kgs, then it needs exactly 1,000 kgs of lift to keep it flying level.
If you bank the aircraft, then its lift banks with it, and there’s not enough vertical lift to balance the weight.
That extra lift comes from easing back on the stick and increasing the angle of attack. But this causes two problems.
• If you get more lift (by any means) it costs you in drag. So the aircraft slows down,
• If you ease back on the stick, you increase your angle of attack, and that brings you closer to the stall.
So in any turn, the airspeed decreases and stall speed increases.
To get a sense of how much stall speed increases with bank have a look at that diagram again and imagine what happens when we do a 90° bank.
Correct – no vertical lift at all – it is all horizontal, this means that the aircraft will fall at the same speed as the battery for a diesel truck. So it’s impossible to maintain altitude in a 90° banked turn unless you have an aerobatic aeroplane with a fantastic power to weight ratio and a fuselage that generates lift on its side. Vertical lift is then supplied partly from the fuselage, and partly from the down-thrust from the propeller, or jet engine.
So I was saying that the need for extra lift goes from zero, with no bank, to infinity at 90° of bank. You can see this on the Lift Required v. Angle of Bank graph.
[Pat can you shove the “Stall speed v. Angle of bank graph”, here, please]
Okay, I have put off the dreaded formulae long enough – actually they are very easy.
The first one tells you how to find your G loading in a turn. It’s simply 1/ Cos of bank angle. So if you are banked at 60° you look up the cosine of 60, which is 0.5. And 1 divided by ½ = 2.
Now, to find your new stall speed, you take the square root of 2 and multiply that by the normal stall speed. So the square root of 2 is 1.4 and we multiply that by a normal stall speed of say 50 kts, and that gives our new stall speed of 71 kts.
I told you it was easy.
Okay, relax, we have finished with the sums. So let me tell you another seriously scary story about steep turns just to remind you why all this is so important for passing exams – and staying alive.
A million years ago I was a hangar-rat at Placo. My boss, Zingi Harrison, said, ‘Davis, get in the back of that aeroplane – we need some weight, and you can’t stuff that up.’
I was thrilled. Jack Jay, our new wonder-boy from England, was putting the finishing touches to his conversion to a brown 235 Apache.
Major Bomb-doors Pidsley was the instructor, Zingi and someone else sat in the middle row and I was in the single seat in the boot and had to scramble in through the hatch. No matter, I was excited to be in any aeroplane. The mission was to do three full-load landings.
The first two were great, the trouble started on the third downwind when Bomb-doors decided to simulate an engine failure on the left engine. Yes, the one with the hydraulics to pump out the gear and flaps. All went well while Bomb-doors and Jack busied themselves with the donkey’s-dong emergency pump the get the gear out and half flaps set.
The trouble is that they were so busy with their heads in the cockpit that they allowed the downwind leg to converge towards the field. Twin pilots are now starting to break out a sweat and squirm in their seats – they can see where this is going. We had almost no base leg and Jack did a classic trick – he overshat the centerline, forcing us to do a hammerhead. This became an ever steepening turn into the dead engine.
Although I didn’t know it, we were in an extremely precarious position. We could do the steep turn and risk spinning in. Heavy twins, turning into the dead engine, with everything hanging out, and the airspeed bleeding off, are prone to inverting themselves. Our only other option was to level the wings and go-around.
But up-country heavy twins, with the gear and flaps deployed, don’t like to go around on one engine. Even if Jack and the major pumped like hell and managed to get the gear and flaps up before we hit the ground there was still little chance of climbing away unless they managed to start the dead engine while struggling with the hydraulics – but there just wasn’t time for that.
We continued with the turn. The aircraft shuddered on the edge of a stall.
By looking between all the intervening shoulders I could just see the ASI with something like 85 mph showing, so I knew we couldn’t be stalling. I thought the shudder must be something to do with the feathered prop.
Later I asked Zingi about it. He said we were on the verge of stalling and spinning in. I asked how that could happen at 85 mph when the aircraft normally stalled at 65 mph. He just said, “Bugger off Davis and do some reading.”
So that was the second time the gentle Clark-Y wing saved my life.
As a third option Jack could have relaxed the back pressure and unloaded the wing. This would have meant we wouldn’t get back on finals properly. We would have landed half way down the field or on the grass – but so what? Definitely the right thing to do in a C310, a Baron or a Twin Comanche.
Which brings me pretty much to the crux of the matter. If you are in danger of stalling in a steep turn, or by pulling too hard to get out of a dive, all you have to do is unload the wing by relaxing the back pressure on the stick. Of course this will mean a loss of height – but that should be taken care of by reducing the bank, or levelling the wings.
Okay, we have looked at the technical side of steep turns, but I haven’t told you how to do them. First let’s look at the classic one in your flight test.
You start with doing the HASELL checks – particularly lookout, behind in the direction of the turn.
Enter as normal, with balanced aileron and rudder. As you go through 30°, smoothly increase power to help maintain airspeed. Many light aircraft need full power.
Allow the bank to increase to 45° (on the AH), and keep it there. You need to ease back to keep the nose up and supply the extra lift. You will have to use a lot more back pressure as the bank gets steeper.
Once you are settled in the turn, you have to divide your attention between the horizon, the A/H, the ball, and lookout for other traffic. Actually, with a steady angle of bank you can take your feet off the rudders and the ball will stay pretty much centred. The story about using top rudder to keep the nose up is rubbish. Don’t even think of it.
If the bank gets too steep, you will not be able to keep the nose up and the airspeed will start running away. If this happens, reduce the bank and ease the nose up. These corrections must be positive and immediate. If your inputs are too late or too gentle the airspeed increases very quickly.
If you can’t get the nose up, then throttle back and level the wings before the airspeed or revs become excessive.
If you get the nose too high and the airspeed decreases, possibly to the extent that you get a stall warning, reduce back pressure and level the wings.
Steep turns to the left don’t need as much rudder as right-hand ones. This is because the extra power of the slipstream tends to turn the aircraft left. Also, in side-by-side aircraft the nose position will appear different depending on which way you turn.
To recover, smoothly reduce to cruise power as the bank goes through 30°. As you roll out, use enough rudder to counteract aileron drag and the power reduction.
In a real traffic avoidance situation, you obviously skip the lookout and throw the aircraft into probably a right hand turn. And in a valley, the slower you are the less space you need, so use landing flap and reduced airspeed.
Don’t forget to practice the left-to-right exercise I described. Get it right and soon word will get around that you are a great stick and rudder pilot.
Postscript
On a SAAF instructors’ course at Potch we had to do 75° gliding turns in those magnificent C185s. These required a hell of a lot of pulling, and serious G to the extent that they eventually stopped us having fun because the Cessnas were popping rivets in the top of the wings.
So, using our neat little formula that says G loading = 1 divided by the cosine of 75° we find that we were pulling 3.86 G. If we did it smoothly and got it exactly right – which we mostly didn’t. It’s an extremely tough exercise.
Most light aircraft are certified for a positive G loading of 3.8Gs in ‘normal category’ ops. However, in ‘utility category this increased to 4.4Gs.