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Are 2 smaller engines more efficient then 1 big one? (Read 128 times)

Sep 16^th, 2011 at 1:34pm

michaelb15 Offline
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I was wondering why a lot of mid sized aircraft have twin engines, instead of 1 large one? Would it not be more efficient to have 1 large one?

1 large engine would be lighter then 2 engines that produce the same amount of power as the large one, it would use less fuel, and would be cheaper/easier to maintain. And it would also have less surface area then 2 smaller engines, making it more aerodynamically efficient.

As for saftey, if you loose 1 of the 2 engines on the twin, the 1 engine would only get you to the crash scene Grin

Grin

I realize that the Cessna Caravan is a turboprop, but even so it uses a bit less fuel then the twin Beechcraft baron, and can carry even more of a load then the twin.

So whats the idea with twins?

I understand larger aircraft having 3 or more engines... but the twin... I dont know... It just seems inefficient.

But all being said, twins are a joy to fly, and have a very appealing look Tongue

Tongue

Tongue

What do you's think of twins?

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Reply #1 - Sep 16^th, 2011 at 3:24pm

Dave71k Ex Member

It's a interesting point, especially when it comes to small prop aircraft.
Obviously with ETOPS the way they are it makes sense for something like a 737 or bigger to have 2 engines for safety a 737 can fly quite happily on one engine.

But like you said the Cessna Caravan is a extremely successful aircraft and does it's job amazingly well.

I'd say twins are good in bad wind because you can balance the wind using differential power settings. However Caravans are used in the harshest conditions in Alaska and get on fine. So it's not that.

I guess one option would be to cancel torque if you want to pile on the power.

From what I've read this is one of the most debated topics in aviation and I don't actually think there's a reason. It's just preference and safety.

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Reply #2 - Sep 16^th, 2011 at 10:11pm

RaptorF22 Offline
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I think the main reason for GA aircraft is redundancy, especially when flying in the bush having a backup is really nice.

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Reply #3 - Sep 16^th, 2011 at 10:40pm

Dave71k Ex Member

RaptorF22 wrote on Sep 16^th, 2011 at 10:11pm:

I think the main reason for GA aircraft is redundancy, especially when flying in the bush having a backup is really nice.

Yet the most common bush planes are Cessna Caravans or 206s. It's certainly a big question.

I for one would much rather 2 engines if I was flying over somewhere big and uncharted or even large expanse of open water to be honest.

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Reply #4 - Sep 17^th, 2011 at 5:06am

expat Offline
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You have forgotten a couple of important things. Firstly, because I designed it, I can and secondly, and this plays a big part, because it looks good. Often looks are far more important then the reasons behind them, even in aviation.

Matt

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Reply #5 - Sep 17^th, 2011 at 5:50am

C Offline
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There a balance to be struck; two engines may offer a bit of redundancy, but with the type of operation and operator likely to be using the aircraft, cost is probably a big factor, both in terms of purchasing the aircraft, and, in terms of servicing - two engines cost twice as much to service.

It works in the airline world too, hence with the advent of ETOPS a lot more long haul aircraft are now twins, rather than 3 or 4 jets.

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Reply #6 - Sep 17^th, 2011 at 7:07am

NNNG Ex Member

From what I understand, the problem with a lot of light twins is if an engine is lost at low speed, the rudder isn't powerful enough to overcome the asymmetric thrust and thus the plane crashes. So often they are not hugely safer than a single engine aircraft but use more fuel and have more complexity.

I'm only guessing here, but I think there is a limit to how big propellers can get before the tips end up supersonic, which is loud and inefficient. And large propellers get less ground clearance so longer landing gear legs are required. More blades could be added, but then the propeller hub becomes more complex. And a single engine generates more torque and p-factor. So perhaps for light aircraft it is easier to simply add more engines rather than make one big engine.

Twin engined airliners however have no problems flying on one engine even if it fails right at V1, there are many cases where if a airliner had one engine then they would of crashed. There's a reason the 787, A350, 777 have two engines - generally less engines are better. More fuel efficient and easier to maintain. Four engined airliners are becoming a niche.

But on an airliner, if there was only one engine, where exactly would you put it? The tail with the vertical stab on top? The plane would then be extremely tall and tail heavy and would need to have the wing placed way aft to "balance" the aircraft so to speak. Mounting engines on the wings makes the wings lighter as they counteract the bending forces from the lift. So a single engine aircraft on an airliner would be less safe and probably less efficient.

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Reply #7 - Sep 17^th, 2011 at 7:33am

C Offline
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Quote:

From what I understand, the problem with a lot of light twins is if an engine is lost at low speed, the rudder isn't powerful enough to overcome the asymmetric thrust and thus the plane crashes. So often they are not hugely safer than a single engine aircraft but use more fuel and have more complexity.

In a lot of places where "bush" aircraft are used, you're probably not wrong. Performance would be so marginal on one engine that the only advantage of having the redundancy is getting to the crash site slightly later.

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Reply #8 - Sep 17^th, 2011 at 2:31pm

patchz Offline
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Being that, personally, I am only involved in aviation in the sim world, so I don't really care about fuel used, and efficiency only to a point.

As already stated, looks are paramount, but sound is also important. I love small to medium GA twins.

If God intended aircraft engines to have horizontally opposed engines, Pratt and Whitney would have made them that way.

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Reply #9 - Sep 17^th, 2011 at 5:36pm

expat Offline
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patchz wrote on Sep 17^th, 2011 at 2:31pm:

As already stated, looks are paramount, but sound is also important.

Yes, a nice pair of Bristol's covers both those points nicely Grin

Grin

Matt

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Reply #10 - Sep 17^th, 2011 at 7:08pm

hyperpep111 Offline
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expat wrote on Sep 17^th, 2011 at 5:36pm:

patchz wrote on Sep 17^th, 2011 at 2:31pm:

As already stated, looks are paramount, but sound is also important.

Yes, a nice pair of Bristol's covers both those points nicely Grin

Grin

[img]

Matt

What are those? Aren't they just engines? Undecided

Undecided

But on the other hand both single, double, triple and quad engines look great Wink

Wink

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Arguing with a pilot is like wrestling with a pig in the mud, after a while you begin to think the pig likes it.

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Reply #11 - Sep 18^th, 2011 at 1:00am

Jayhawk Jake Offline
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Wichita, KS

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Quote:

From what I understand, the problem with a lot of light twins is if an engine is lost at low speed, the rudder isn't powerful enough to overcome the asymmetric thrust and thus the plane crashes. So often they are not hugely safer than a single engine aircraft but use more fuel and have more complexity.

I'm only guessing here, but I think there is a limit to how big propellers can get before the tips end up supersonic, which is loud and inefficient. And large propellers get less ground clearance so longer landing gear legs are required. More blades could be added, but then the propeller hub becomes more complex. And a single engine generates more torque and p-factor. So perhaps for light aircraft it is easier to simply add more engines rather than make one big engine.

Twin engined airliners however have no problems flying on one engine even if it fails right at V1, there are many cases where if a airliner had one engine then they would of crashed. There's a reason the 787, A350, 777 have two engines - generally less engines are better. More fuel efficient and easier to maintain. Four engined airliners are becoming a niche.

But on an airliner, if there was only one engine, where exactly would you put it? The tail with the vertical stab on top? The plane would then be extremely tall and tail heavy and would need to have the wing placed way aft to "balance" the aircraft so to speak. Mounting engines on the wings makes the wings lighter as they counteract the bending forces from the lift. So a single engine aircraft on an airliner would be less safe and probably less efficient.

Vertical tails are designed for the engine out on takeoff condition. Takeoff power, engine dies, that's where you're going to have the most imbalance. Look at a 737 versus a Cessna Citation X. A 737 has a HUGE vertical tail, and it's engines are very far apart. Something like a Citation X has a much smaller vertical tail, but it's engines are much closer.

Efficiency is measured in specific fuel consumption, sfc for short. SFC has units of pounds of fuel/horsepower/hour (or for jets, pounds of fuel/pounds of thrust/hour). Because of this relationship, it doesnt matter how many engines you have, you have the same SFC for the same thrust in theory. Two engines doesn't double your fuel consumption per se, you just have more thrust and burn more fuel, so the efficiency balances out.

Efficiency depends much more heavily on the engine. Powering a prop with two engines that have an SFC of .35 would burn less fuel (in theory) than a single engine with an sfc of 0.5.

Turboprops are usually more efficient than turbofans, which are always more efficient than turbojets. Diesel reciprocating engines are usually more efficient than others.

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*The opinions expressed above are my own and are in no way representative of fact or opinion of any other person, corporation, or company.*

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Reply #12 - Sep 18^th, 2011 at 3:51am

expat Offline
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hyperpep111 wrote on Sep 17^th, 2011 at 7:08pm:

expat wrote on Sep 17^th, 2011 at 5:36pm:

patchz wrote on Sep 17^th, 2011 at 2:31pm:

As already stated, looks are paramount, but sound is also important.

Yes, a nice pair of Bristol's covers both those points nicely Grin

Grin

[img]

Matt

What are those? Aren't they just engines? Undecided

Undecided

Errr, no, not just engines Alex, Bristol's is an old expression to describe........."lady bumps"...... Grin

Grin

But that image was available on a t-shirt from the Blenheim aircraft restoration company. The engines on the Blenheim are Bristol Mercury's..............They sold out over night Grin

Grin

Grin

Matt

PETA People Eating Tasty Animals.

B1 Boeing 737-800 and Dash8 Q-400

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Reply #13 - Sep 18^th, 2011 at 3:01pm

patchz Offline
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I think Matt has hit the proverbial nail, squarely on the ~~nip~~ er head. Apparently, it is simply an aesthetic situation because the largest percentage of aviation enthusiasts are male.

So, when viewed from the front, twins remind us of something we all hold dear. Roll Eyes

Roll Eyes

Grin

If God intended aircraft engines to have horizontally opposed engines, Pratt and Whitney would have made them that way.

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Reply #14 - Sep 18^th, 2011 at 5:42pm

Steve M Offline
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Cambridge On.

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patchz wrote on Sep 18^th, 2011 at 3:01pm:

I think Matt has hit the proverbial nail, squarely on the ~~nip~~ er head. Apparently, it is simply an aesthetic situation because the largest percentage of aviation enthusiasts are male.

So, when viewed from the front, twins remind us of something we all hold dear. Roll Eyes

Roll Eyes

Grin

I've always maintained that twins are a lot of fun. Cool

Cool

Flying with twins is a lot of fun..

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Reply #15 - Sep 19^th, 2011 at 3:04pm

michaelb15 Offline
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"nobody ever takes the one big one" - I think thats off the simpsons

I am somwhere I don't know where I am!!!&&

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