Friday, February 05, 2010

Simple is Good

Every once in a while I will describe a system, such as the oil pressure line running directly to a dashboard gauge, and someone will be aghast that "that's all it is?" Yes. And the less 'middle management' that an airplane designer can get away with, the better.

Some of the simplest aircraft systems are more direct than that. I've flown an airplane where the wing tanks drained to a nose tank and the level in the nose tank was indicated by a wire that poked out of the cowling. The wire was on a cork inside the nose tank, so when it started to sink, that meant the wing tanks were empty and only the nose fuel remained. Simple, direct. I suppose the cork could get stuck. That's what your watch is for. Some airplanes run a sight gauge from the wing tanks to the side of the cockpit so you literally see the fuel level. And most ultralights go one simpler and simply use a transparent or translucent fuel tank so the fuel level can be seen directly. You can't do that in an Airbus, but I'll bet Robert Piché would have liked to be able to see the fuel in his tanks rather than having to make decisions based on electronic diagnostic systems and lengthy checklists.

Another example: my engine combustion air intake is just the opening in the front of the cowling, and if that intake becomes blocked, the pressure drop inside will pull open alternate air doors. When closed, those doors are held by magnets. See the picture? Simple, directly reacting to the problem with no need for any intermediary, power source or interpretation. Those doors can also be opened by mechanical cockpit controls. The same system could have been designed with electric sensors, but why bother?

Some parts of my airplane are operated by electric motors. That includes the wing flaps, which change the shape of the wing airfoils for takeoff and landing, the cowl flaps, which control cooling air to the engines, the elevator trim, which allows me to hold an attitude without effort, and the starters, which turn over the engines until combustion in the cylinders is sufficient to sustain rotation as I start each engine. That's about it for electric motors. I've seen all those fail.

I can land or continue flight with inoperative flaps, although I need more runway for the landing and more fuel and time for the same journey. There are a lot of safeguards built into the flap controls to prevent asymmetric flap extension from flipping the airplane over. I would need a huge lever to operate those flaps without a motor. Manual flaps are great. You can move them at any speed you like and get tactile feedback of your airspeed.

I shouldn't be starting up if the cowl flaps are stuck closed, but I'm unlikely to notice if they are stuck open. I'll lose a few knots in cruise and see lower engine temperatures, but the indicator is also electric and may not indicate correctly if there is a fault. I've flown airplanes with manual cowl flaps. It's probably easier to route an electric wire than a set of push pull cables from the engine to the cockpit and I know I'd need bulky cowl flap levers rather than little toggle switches in order to open the cowl flaps against the pressure of the airflow. I think it's a shame that the indicators are electric, though.

Electric starters introduce more parts that can break, and weaker starting power in the cold, when you really need more. But I really appreciate having starter motors and not having to hand prop these engines.

The electric trim is conveneient, because the control is right on the yoke, but there is a back up manual elevator trim wheel on the centre console, just as there is on a B737, because that's an important control and designers don't pretend that switches and electric motors don't break. There are a lot more electrical components in my airplane, but I think I've covered the motors here.

Modern airplane design calls on the airplane to have a computer brain that knows and controls everything that is going on. This precludes some of the simplest systems, but also prevents a lot of stupid pilot errors that an airplane with no brain cannot object to. Ideally the pilot is smart enough and the airplane well enough designed that the two can work together to get the job done. Just don't knock simple.

This post brought to you by Gore to Wiarton again, this time with strong winds and weather at minima.

4 comments:

Kevin said...

Aviatrix

At least four more motors:

- Electric (backup) fuel pumps (1 per engine)
- Electric fan for the Janitrol
- Electric fuel pump for the Janitrol

And didn't you say you have a separate defrost fan? That's another motor. And the rear heater; that's another two...

Then there's the two servo motors on the autopilot (roll and pitch), and if you have a yaw dampener, then that's got another servomotor.

Does your plane use an electric or a hydraulic gear motor? Bet it's electric - there's another.. :-)

How about retractable landing lights? I can't tell if you're flying a big Cessna, but if you are, don't the landing lights retract and extend electrically? That's another motor per landing light...

I don't mean to be a pedant, but there are a lot of quite complex systems, even on our light twins... :-)

Kevin K

Aviatrix said...

Not pedantic.

Interesting in what it reveals of the way I think about my airplane. My gear is hydraulic; there are some electric triggers in the sequence, so the doors won't close if the master is off, but they aren't motors. My lights are on the nose gear leg, so they deploy automatically with the gear. For some reason pumps and fans didn't trigger my "motor" thinking. I was considering, for no formed reason, motors that moved things from place to place, not round and round. Which means the starters didn't belong there, except they're moving such giant things round and round.

And just a moment while I laugh hysterically at any suggestion that my autopilot be included in an inventory of anything. I think I shall DYMO-label it BALLAST.

I wasn't even trying to claim that my plane is particularly simple, just that simplicity is not a fault.

Jimmy said...

'DYMO-label it BALLAST' LOL. If it is the model I think it is, that is not unusual. I've only seen one properly working example out of a dozen I've tried.

If I didn't need mine for single pilot IFR it would be long gone. Problematic and expensive to fix. I also believe that real pilots hand fly the machine, A/P or not ;)

Darren said...

Some aircraft still have mounts for sextants. :)