Yaw String
en.wikipedia.org
en.wikipedia.org
A couple of other examples are an adjustable indication on the windscreen of where "straight in front of you" is, as well as an angle of attack indicator.
Angle of attack, pitch angle, and yaw commonly don't have their own indicators, as important as they are. Instead, pilots have to infer them by combining readings from multiple other sources.
Though imagine rewatching after years scenes from The Final Countdown, and noticing that the F-14 have your well known yaw strings installed in very familiar way (and finding out later that it was unauthorized, but very common, modification known to increase accuracy heavily).
The default speedometer doesn't actually tell you how fast you're going, and the default altimeter doesn't actually tell you how high you are.
More concerned with the technical details about keeping their precious machinery working than anything practical like am I going to get to Malaga in time for dinner.
joke
As to altitude there isn't a single always applicable definition of "altitude" hence pilots have to adjust it based on their current task, there simply isn't a way around that. You could try to always use "true altitude" in WGS-84 or something, but that's far more complex than a simple pressure altimeter. For example it generally cannot be done without external inputs, such as very accurate pressure maps or gps-like navigation aids. More complex = more failure opportunity.
But once you’ve been flying a while, you don’t really need it, you can feel it.
I'm not too well versed in the details, though.
Comment from a glider instructor of my acquaintance: Yeah, and that big fan in front of the yawstring doesn't help.
Might explain why yawstrings are more common in the twin engine case...
The slip indicator, which replaces a yaw string in modern aircraft, is part of the standard avionics 6 pack.
But that's no why that particular guy had two strings :). He just wanted to be double sure that his aircraft will perform at all times. I think he even had 3 certified flight recorders since misconfiguration or other failure of the certified flight recorder is the most common technical problem during a competition flight (you might not be aware that this little box is faulty until after you land). Actually quite a few "serious" glider pilots carry a second flight recorder.
Also, I'm not sure that it'd be even possible to do that much better. You need something light, flexible and visible. Almost sounds like the definition of a red piece of string. And then you need to attach it somewhere useful (where you can see it, ideally without looking at your instruments etc), which in most cases would be the canopy. And then you need to somehow affix it. The piece of tape holds well enough during flight, and anything more permanent could be annoying during maintenance etc or might attack the material of the canopy.
Changing up the design might also be annoying at the least, since you'd have to relearn a bit.
In the end, there's just not much wrong with the string. I works, it works well, and changing it is just not worth it.
and this, just a bit of yarn on a halyard. sometimes with a cone on the end!
The first major modification was the total energy variometer, which takes account of the fact that you can trade speed for altitude and vice-versa. It gives a figure (expressed as an equivalent rate of climb) that corresponds to whether the sum of your potential and kinetic energy is increasing or decreasing. This is particularly useful in rough air, where, with a simple variometer, it is difficult to center on the strongest updraft.
As the performance of gliders improved, and the speed for optimal glide angle diverged from that for minimum sink rate, it became useful to know the vertical speed of the airmass, as, when flying fast, your descent rate is quite high even if the air is rising. Netto / airmass variometers compensate for the still-air rate of descent you would have at the current airspeed. Relative netto variometers show the difference between this and the minimum sink rate.
All of this can be achieved by clever plumbing, though electrical instruments can have less lag, and potentially adjust for flap setting and ballast weight. One orthogonal development that requires electricity is the audio variometer, which emits a sound of varying pitch to indicate rate of climb or descent - an important safety issue, as you can monitor this datum while looking outside of the cockpit.
Another thing that variometers are usually equipped with is the speed-to-fly ring, invented by Paul McCready of 'Gossamer Albatross' fame. It gives a useful (even though imprecise, on account of the parameters that have to be estimated) indication of the optimal speed to be flying at for the current variometer reading.
As in the case of indicated vs. true airspeed, it is important to understand what you need the information for.
I have done IT consulting in the field, and have been genuinely happy to get a tour of some facilities and debugging software while sitting in a life-size simulator. It really tickles your engineering senses.