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Comment by rsync | Hacker News Reader

Here's a question:

Tell me about single engine (what you appear to have) vs. multi engine (twin, I guess) ?

Should I be thinking about redundancy in plane engines the same way I think about redundancy in ... say ... kidneys ?

It seems to me that I would really, really want a twin engined plane...

edit: I found this [1] but I'm not sure I buy it. I am all about simplicity in systems, but redundancy in a critical component is a different thing. Further, Perrow speaks of fortuitous surpluses (or whatever) as an unalloyed good in [2]. This seems like just the kind of surplus I'd want...

[1] http://c2.com/cgi/wiki?AirplaneRule

[2] Normal Accidents (run, don't walk, to buy and read it)

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This is one of the oldest debates in the pilot world. Single vs Multi and there is no one answer. Ignoring the cost of extra fuel and maintenance for an extra engine. The usual argument against multi is like this - The asymmetrical thrust developed when one engine dies. Basically if one of your engines quits, especially with a high angle of attack (takeoff), you have a very small window to compensate and keep the plane flying. Otherwise you get what is called a Vmc rollover. That is probably the biggest argument against a twin.

But of course if you fly over water to the Bahamas every weekend or the Rocky mountains the pros outweigh the cons right?

My point is it's all relative. Being a relatively low time pilot in NJ I see no need for a twin. Assuming I could afford one in the first place.

A friend of mine owns an airplane, and his brother is a pilot in the USAF. I asked him this once and his opinion was that 2 engines makes the plane more complicated and increases the chance of pilot error. He had suffered 2 or 3 engine out incidents in his single engine plane, and landed all 3 with no issue. Obviously if your plane doesn't glide very well then the redundancy of a second engine is necessary (i.e a jet), but in your typical Cessna-esqe plane, gliding is a viable option.

I always assumed "gliding well" is a fundamental principle of an airplane.

I read somewhere (pre internet, so it must be true!!!1!) that the wing loading of a fighter is roughly equivalent of using a manhole cover as a frisbee.

In other words, anything can fly if you give it enough afterburner.

Many military aircraft are designed to not glide well, because gliding well means that a plane is less agile. Most modern fighter jets go almost immediately into a tumble if they lose power, since they require constant computerized corrections to remain stable.

Its probably a bit more important in a hobbyist or even a commercial plane.

Just about any can glide, but not necessarily well.

See: https://en.wikipedia.org/wiki/Gimli_Glider

I never knew about the glide ratio of large jets! And this story is amazing, mostly for the simultaneous levels of stupidity and competence involved.

Isn't one person's competence offsetting another person's stupidity what programmers and sysadmins do all day, every day anyway? ;)

It actually is - each plane as an ideal glide speed that you pitch the plane to match during an engine-out. 1 mile per 1k feed above ground level is a good rule of thumb for glide distance at this speed.

This comes down to 2 things.

1) How disciplined are you as a pilot, Are you going to insure your plane is ready to fly every single time to mitigate risk. Most fatalities in GA could of been avoided. You will have to make hard calls like. "I have a meeting tomorrow but the weather looks terrible, should I fly through it anyway to get there?" or "My oil pressure is low but I need to get home". You will get stranded and have to get a hotel room or car if you plan on using your aircraft for travel, I guarantee it.

2) Where are you flying? are you flying over water, mountains, cities? Are there roads where you could easily put down or nearby airports? One of the first things you learn as a pilot is what to do in an emergency. Step 1 when you lose engine power - Get the best glide speed and find a place to put down.

As an aside there are aircraft that have parachutes. Cirrus is the best known right now with the CAPS system. But, if you have a parachute are you going to try to fly through the storm to make it to that meeting above?

I think in the past, aircraft engines were relatively unreliable and would often quit midair for reasons other than fuel starvation so there'd be some chance of the other engine saving you. Now, however, almost all engine failures are the result of fuel starvation and two engines will die just as quick sans fuel as one so the added complexity isn't quite worth it, from a safety perspective for piston powered aircraft. The only real advantage now is that you get to go a faster (though not 2x) in a twin usually and get to earn lots of miles on your credit card since you'll be doubling your fuel bill.

The situation on turbine powered engines is completely different. The above only applies to piston engines.

Why do you say the turbine situation is completely different? Turbine engines are if anything more reliable than piston ones.

Sorry, I meant different in that I know very little about it. My language was imprecise.

With turbo{fan|jet}s I think there is an issue that its just difficult to construct aircraft with a single engine as opposed to wing or tail mounts.

I've been secretly lusting after one of these aerial SUVs for 10 years now:

http://en.wikipedia.org/wiki/Pilatus_PC-12

I think its because you can not go significantly faster by adding another turbine and if you got a jet plane, you probably don't care about fuel efficiency.

But as you say, you don't go significantly faster by adding another piston engine, either. Aerodynamic resistance at high speed goes as v^2, so adding twice the power increases your speed at most 70%. In practice, it will be less due to increased drag from engines in the wings, more weight, etc.

I pulled a comparison from Wikipedia:

Cessna 310 (twin): 2x240hp, max speed 220mph, range 1000mi.

Cessna 182 (single): 230hp, max speed 201mph, range 930mi.

I'm sure the twin has more useful load, but it sure doesn't go much faster or farther.

Aerodynamic drag (the force required) goes as the square of the velocity.

Power required is force times velocity, so power required goes as the cube of velocity. Twice the power adds only 26% to speed.

An engine failure in a twin can be difficult to handle during take-off; the plane can roll over, and there is not much time to react.

http://www.skybrary.aero/index.php/Engine_Failure_After_Take...

Think about it this way: have you ever had your car engine die on the road? For the vast majority of people, this just doesn't happen, even though people spend much, much more time driving than they would flying. Modern engines are remarkably reliable. And if it does quit, you can glide the plane to a forced landing.

Long flights over water, mountains, or in poor visibility is a different matter.

In addition, part of a private pilot license training is stall recovery, and engine loss scenarios.

I remember sitting in on a lesson while my brother was getting his private pilot's license. The instructor would randomly idle the engine in flight and have my brother practice the procedures for a engine loss in flight (put the plane into the ideal glide position, identify all possible landing spots, runway or field, etc...)

> have you ever had your car engine die on the road?

Yes, several times in the ten years I've been driving automobiles. Is that not common?

As far as I can recall, it hasn't ever happened to me in the 24 years I've been driving. Are you doing the required maintenance?

I suspect it is strongly correlated to how much you spend on cars (particularly, how old your cars are). I've had a car that liked to overheat, which took a couple of trips to the mechanic before they sorted it out. I also had a truck that had some sort of electrical problem (I don't remember what exactly, and I know nothing about automobiles).

Never happened to me, and I also asked my dad who was a professional driver for a while, and used to drive trucks in the old soviet union - all in all,he's got 30 years of driving experience - never happened to him, unless he ran out of fuel or the batteries failed.

Most common question I got when I flew: "What do you do if the engine stops?"

My answer: "Turn around go home and land"

It does help that I started out on gliders (sailplanes) first, of course... but losing the engine really isn't that big a deal in level flight in many small aircraft (or even big ones actually - a 747 has a surprisingly decent glide ratio (not actually surprising if you think about it, tbh)).

I've never flown twin engine, but as others have mentioned, losing one engine in a twin can be catastrophic - unless it's the Boomerang of course ;) http://en.wikipedia.org/wiki/Rutan_Boomerang

In summary: I would choose zero engines, one engine, or four engines - but not two.

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