Stalling the Tiger Moth Part 2

After experiencing the stall last time, this time I get to do it for myself. But even making the aircraft fall out of the sky turns out to be less exciting than dodging other Tiger Moths, Spitfires, and even a Boeing 777.
Despite a weather forecast that threatened rain all day (which never came), Sunday was a high-traffic day at the airport. I was flying with an instructor I've not flown with before. The excitement started early when we were taxying out. Just after crossing the main runway, the tower came on the radio to ask if we could expedite backtracking the grass runway: the Group's other Tiger Moth was on its downwind leg to land. If we could hurry to the start of the runway and vacate it to do our pre-take-off checks, we'd be able to take off as soon as the other aircraft landed. Turning down the offer would mean we would have to wait until they'd landed before continuing our taxi.
As we didn't know how long we had, my instructor took over at this point to expedite crossing the runway. I was taught to taxi at a brisk walking pace, but he was nearly breaking into a jog. If we didn't vacate the grass runway soon enough, the other aircraft would have to go around: that is, abort the landing and climb away to do another circuit.
Once we were off the end of the runway, we could watch the other Tiger Moth land while we did our pre-take-off checks, after which I had the opportunity to perform the take-off myself. Lined up on the numbers, I kept an eye on the formation of trees I always use as a visual reference. Gradually I pushed the throttle lever forward, as well as my left foot, using the rudder to keep us straight. There was a slight crosswind from our left, so I pushed the stick somewhat to the left to prevent the aircraft being blown over. (Remember the Messerschmitt.) It got harder to keep straight the faster we got, and I had to continually adjust the rudder. Soon the speed had increased enough for the tail skid to lift off the ground, and we were rolling along on our main wheels. With the reduced drag, we picked up speed even more, and once we'd passed the magic number of 60 mph, I eased the stick back a little, and we were airborne.
I adopted the climb attitude, and focused on keeping our airspeed at 65 mph while also staying in line with those trees. But because of that crosswind, by the time we flew over the end of the runway we were closer to the concrete runway than the grass one.
We reached 500 ft and crossed the aerodrome boundary, which meant it was time to turn left. In a circuit this would be the crosswind leg, but since we were departing in that direction, instead of turning again onto downwind we just kept climbing out. It would be a long climb to get to the 3500 ft we needed to practise stalls, so a few clearing turns on the way ensured there was no traffic in the blind spot under the nose.

Eventually we got to the correct height, so we did our aerobatic checks and the instructor took control to demonstrate as I followed on the controls. After the previous demonstration, I was ready for the nose-high attitude and the feeling of dropping out of the sky, and I got to feel that again. But this time, once we'd recovered the small amount of height we lost, it was time for me to do it.
Causing the aircraft to stall was the hardest part. The first time, I didn't get the nose high enough, so as the wings started to produce less lift, the aircraft started to pitch down on its own even while I was pulling the stick further and further back. We recovered to a nose-down attitude without even stalling. (Quick note for physics-minded readers: you can see how this happens by looking at the lift-AoA chart. I got the AoA past the peak of the curve, but then the wings didn't have enough lift for me to increase the angle right to the end of the curve.) The instructor told me to treat the stick more decisively, and the second time I was ready to fight the nose-down tendency.
A chart of coefficient of lift against angle of attack. It increases until about 16°, then decreases again, but at 25° the curve stops completely.
As angle of attack increases (to the right), the amount of lift you get increases until the critical angle. It drops off slowly at first, but when the angle gets even steeper you get no lift at all. The curve is slightly different for different aerofoils, but the shape is typical. (Advanced readers: be careful not to confuse this curve with the lift-drag curve.) Image courtesy of Wikipedia user Botag.
With the throttle completely closed, and our nose sticking up as our airspeed decreased, I was pulling the stick progressively back to keep us like that, until eventually I felt the aircraft try to drop away under me, and we were stalled. I knew the next action to recover would be to bring the stick forward, but it was hard to demonstrate, as again, the aircraft was already trying to recover itself. I've had this described to me as, "The aircraft wants to fly," but really it's a deliberate feature of the aerodynamic design of the aircraft.
It wasn't all that simple, though. One thing I'd been warned about is to keep the stick central, and not use it to try to roll the aircraft if a wing starts to drop. As I explained last time, in the stall the wings are no longer acting to stabilise the aircraft, so even a tiny left-right imbalance will make one wing start to drop. Normally if one wing drops, you correct that roll by using the stick to roll back the other way. In the stall this will have the opposite effect!
Say I move my stick to the left. This makes the aileron go down on the right-hand wing, which has the effect of making that wing steeper (increasing its camber). Normally, that makes it produce more lift, and having more lift on that side makes the aircraft roll left. But when the wing is already stalled because its angle of attack is too high, making it steeper just makes the stall worse. Now that wing is producing less lift, so you roll further to the right. Eventually you go into a spin, which is a much worse situation.
Because of this, when you're stalled, you don't move the stick left or right at all: only forward along the centreline to recover. If there is some unwanted roll, you use the rudder to stop it getting worse.
This effect is pretty much the only reason stalls are dangerous. If you don't realise it's happened, and you keep flying the aircraft as usual, you get into a spin. Even if you do realise it's happened, it's like telling an experienced driver that suddenly as he's driving along, he has to keep his hands on the wheel but now the accelerator and brake pedals control the steering. It's so hard to not do something you've done so much it's automatic.
So of course, the first time I got a wing drop, my hand had twitched the stick over to that side even before I realised. As expected, the wing dropped further, and we were heading for a spin. Since spins are a topic for a future lesson, the instructor stepped in and recovered the aircraft, but he told me he was glad I'd had a chance to see how that really works rather than just having it explained to me.
The rest of the lesson was to consist of me practising that a bit more. Getting trimmed for straight and level, cutting power and pitching up for the stalling, recovering, and then climbing again to recover lost height. But this was about the time traffic started appearing.
Cessnas and Pipers, a common sight in the area, started going past below us. Below is the hardest direction to see from the cockpit, and it's also where you don't want traffic when stalling, because that's what direction you go. A few times we were all ready to do the stall again, but saw traffic and had to keep circling. (Also, am I the only one who thinks "do the stall" sounds like a dance?) On one occasion, my instructor had already closed the throttle to demonstrate when I called out, "Traffic, eleven o'clock low," and he had to return to cruise power to let it pass.
After a while, we had a warning on the radio of some quite unusual traffic: a Spitfire, practising its display for the Duxford air show the following weekend. We scanned the horizon, the clouds, and the fields below for the distinctive Spitfire shape, but couldn't see it anywhere. Even so, we knew it was out there somewhere. It was a modest honour doing aerobatics in the same skies as a national icon.
Spitfire planform (shown from below) showing the distinctive wing shape visible against a backdrop of cloud. The underside is painted grey-blue to match the cloud.
This Spitfire is quite easy to spot. But now imagine it's five miles away and the clouds are only a thousand feet above you. Picture courtesy of Wikipedia user Arpingstone.

After four or five times, the stall seemed a whole lot less exciting. Any sailors who remember drilling dry capsize recovery will recognise the pattern: it starts out thrilling and dangerous, but it becomes routine. That is, after all, the idea of drilling it. So once we were both satisfied that I could recognise the stall characteristics, I turned us back towards the aerodrome for a standard overhead join of the circuit. After concentrating on the stall for a long time, I wasn't that confident I'd remembered the pattern correctly. The idea of the overhead join is that it lets you join the circuit with good visibility, and keeps you well away from other traffic while you're manoeuvring, so even if I'd got it a little bit wrong it would have been alright. Even so, it's good to fly a neat circuit, and I think I'll learn more by doing it right than by doing it wrong, so I kept checking with the instructor at each stage to make sure I was descending and turning at the right times.
Just as I turned onto downwind, we heard the tower on the radio again, asking if we could make a tight circuit to land quickly. My instructor recognised the subtext of the message as meaning they were trying to fit us in ahead of other traffic, so he took control to get us on the ground as soon as (safely) possible. As we turned onto base leg, we heard a Boeing 777 call in, only to be told, "You are number two to a Tiger Moth on base." We'd managed to get in ahead of him, so he would have to continue his approach but wait until we had landed.
We turned again onto final approach, and my instructor radioed in,
"Golf India Zulu, final to land, 23 grass."
There was silence for a few seconds, so he tried again, with the same answer. By now we were less than 100 ft up, and still heading towards the runway. He had his hand on the throttle ready to go around if we still didn't get landing clearance. One last time, he tried, "Golf India Zulu, final to land, 23 grass." We were nearly at hangar-top height by the time the answer came back,
"Golf India Zulu, clear to land on 23 grass."
We breathed a sigh of relief and touched down on the grass, and I took control again to taxi us in. At the holding point (before crossing the main runway), we had the perfect view of the 777 as it landed on the main runway. As it turned out, this had been the longest lesson yet, a whole hour of flight time. If we hadn't managed to make that quick circuit to land ahead of the 777, we would have been circling for another five minutes to wait for the turbulence caused by its landing to clear. But my instructor's quick thinking meant I could sit and rest and enjoy the show.
B777 in AeroLogic livery, on the ground, photographed from about 20 m away.
It looked a little like this, but in the amber and red of DHL. It was nearly as close as this, too! Picture courtesy of Wikipedia user Molch-Entertainment, GNU Free Documentation License 1.2.
So although the main point of this lesson was to learn how to recover the stall, that wasn't the biggest thing for me. At the start and end of the flight I had two examples of how ATC tries to slot in aircraft around each other. Tidy flying (expediting a taxi, or keeping a neat and tight circuit) helps them to do that: it's not just good airmanship, but saves you having to wait around for other things to happen. Today the airspace was the busiest I've seen yet, so all through the lesson I got a lot of experience at looking out for other aircraft and knowing what to do when I do see them, something I've struggled at before.

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