The trick is not to have run out of PE and KE before deciding to turn, i.e. point nose downstream before climbing.
Unfortunately turbulence and density altitude usually mean that you are out of KE and with precious little PE.
I did some quick calculations using the rule of thumb for halving knots to get m/s, and 10 m/s2 for g.
Your standard bug smasher flapless Cub stalls at 42 mph. 60 mph IAS is a safe margin that takes into account the slight increase in stall speed for a 30 degree bank level turn. DA of 10,000’ gives you a TAS of 72 mph and a turning radius of 170 m, so the valley would need to be at least 340 m wide.
Alternatively a turn with a 75 degree bank which is approaching the utility category G corner for a Super Cub and a 90 mph IAS, gives you a 10,000’ DA turn radius of 67 m. But do you really want to do this outside the scenario of bringing down a Messerschmitt?
The wing over technique would allow you to unload g and steepen the bank as you followed the descending terrain.
Mountain rating courses did teach wing overs as a technique, but there are so many visual illusions in the mountains, that today I think the preference is for spotting early that you are in a box canyon and the visual clues telling you you won’t clear a ridge, and then proceeding with a safe turn early while your aircraft is still operating within a comfortable turn radius.
You can do a single “lazy eight turn”. That is pretty tight, as tight as it gets without doing a real stall turn, and is easy to do with little practice. There are several accidents in Norway where people have augered in tight valleys, but I think it has happened because they turn too late. A typical valley gets tighter and the ground starts to rise at the same time further into them. There is a point of no return, where you cannot make a turn anymore, and if the ground rises faster than the aircraft can climb, you have had it. Otherwise, the rule is just to fly at one side, so you are able to turn. To fly an aircraft that is able to do tight turns, and is maneuverable is also a good idea.
In VMC the standard energy-management turn (in which kinetic energy is traded for potential energy before banking and allowing the nose to fall back to the reciprocal track) is a safe, easy and effective alternative to the level turn taught for the PPL.
The trick is not to have run out of PE and KE before deciding to turn, i.e. point nose downstream before climbing at less than Vy and minimum height in a valley.
As for where to practice energy-management turns: at safe height between linear ground features – road, rail, river, field boundary etc. One can estimate the width between such features or measure it with the PC version of Google Earth.
Radius of turn is proportional to V squared, hence the lowly L-4 Cub, and L-5 Stinson were able to claim ME-109 and FW-190 kills in WW2 by leading the, presumably, inexperienced pilot into a box canyon.
If partial flaps lower the type’s stall speed, then I would use them for a slow safe speed turn using 30 degree bank. Probably at a nice 1.3 x Vs0.
The flapless L-4, or PA18 has a ridiculous small turning radius, say at 55 mph and 30 degree bank. Later will calculate this vs the G corner turn radius.
Regarding the use of flaps to facilitate the turn – flaps of course reduce the stall speed but the secondary effect is they reduce the Va (nothing to do with strength of flaps or supporting structure)
It almost looks like he is doing descending turns there, but I can't tell...
These Youtube videos all have a non-emergency turn in a valley, usually using half the valley. When going close to the side, it's important to be aware a downdraft could turn you into the wall.
At normal cruise speed, pull up until you get close to stall speed, then full rudder and aileron in the desired direction
Isn't that how to do a flick roll?
Perhaps uncautiously aggressive chandelle.....
Oh, and chandelles in a 182RG!!! 'Been there, done that, should not have... It flies them beautifully, but it sure speeds up fast coming out the other side.
Pilot DAR, what happened in the 182? I can't see how you would end up nose down at high speed exiting a chandelle? Or did you mean wingover or stall turn?
