Why do Dassaults have 3 engines?

[tuna hp]

Why does Dassault still design 3 engine planes when from everything I read about airplane design, 2 engine configurations are cheaper and more efficient?

From a safety standpoint, I have read a lot of arguments that 2 engines are just as safe. You're less likely to lose an engine to begin with, and at least with the old airliner trijets, you couldn't maintain altitude with 2 engines out anyway. Are Dassaults designed to maintain altitude with 2 engines out? With 2 engines out could they attempt a landing and climb out of it if they needed to?

The question about the choice of 2 or 3 engines has really captured me because everything I read about airliners choosing 2 vs 3 vs 4 engines says that 2 engine designs are more fuel efficient (lot of reasons: higher bypass ratio, less tip spacing loss relative to thrust, less internal drag compared to thrust, less nacelle drag) and lighter (2 engines will be lighter than 3 that make the same thrust, airframe design can be lighter), and better takeoff performance (because of higher max thrust).

And yet I look at the trijet Falcons and compare them to other planes in their weight and range classes and the Falcons always win in both fuel consumption and runway performance. Sometimes by large margins. How is that? Is it only because Falcons also typically have the smallest cabins in their class?

Or is it for some more interesting technical consideration in choosing between 2 and 3 engines? I completely understand that if you wanted X total thrust, it would be more efficient to get it out of 2 engines than 3. However, from what I have learned the dominant factor in figuring out the thrust that you need for an airplane is the regulation that mandates that a plane must be able to lose its most critical engine at its most critical point in takeoff and still be able to complete it. So say that this amount of thrust needed is A, then a bijet needs 2A total thrust while a trijet only needs 1.5A total thrust. A little less actually because the bijet will get more drag off the dead engine and the vector of its thrust will be farther from the planes center of gravity so plane has to waste more power trimming. Could this single consideration mean that when you take into account this regulation that trijets are actually more efficient than bijets?

And what about takeoff performance? Planes like the Global Express and G550 destroy the Falcon 7X in thrust to weight ratio, and yet the Falcon only needs 5,500' of runway compared to 6,000' or 6,300'. What could possibly be the reason for this? Only wing design? That seems doubtful to me, because a wing that so completely dominated others in low speed lift would also have to perform more poorly at high speeds and yet the 7X gets maximum efficiency at M.80, can fly somewhat efficiently at M.85, and an article in flying magazine claims that they took the plane up to M.93. Just as fast as those other planes. I don't know anything about turbine or airplane design but I know that for turbochargers in cars, smaller turbines spool up much faster. Could this mean that trijet designs have an inherent advantage in takeoff performance? Do they?

[wondering]

Another advantage: More direct routings when compared to ETOPS limited twin engine aircraft.

And dont they just look beautiful

[Arkwright]

The Falcon 900 EX has similar range and speed to a G4, but uses a LOT less fuel!

We practice single-engined go-arounds, (yes, thats TWO engines failed with 1000' AGL DA), on the recurrents.

The performance manual has data on two engines operating departures.

And yes, they do look beautiful!

[capt.sparrow]

yet the Falcon only needs 5,500' of runway compared to 6,000' or 6,300'. What could possibly be the reason for this?

Leading edge slats. Allows low speed arrivals/departures but high speed wing for cruise.

[Pugilistic Animus]

ETOPS =extended operations in the US applies to three/four engine jets, but that's only if they are operating more than 180 minutes from a suitable alternate

[tuna hp]

Originally Posted by wondering

Another advantage: More direct routings when compared to ETOPS limited twin engine aircraft.

And dont they just look beautiful

They are by far the best looking jet, but I'm pretty sure that ETOPS doesn't apply to privately operated planes (probably including fractional operators) and does it even apply to charter operators?

Originally Posted by Arkwright

We practice single-engined go-arounds, (yes, thats TWO engines failed with 1000' AGL DA), on the recurrents.

So Dassault does in fact take advantage of their 3 engine design to give a safety advantage. That is very interesting. I was reading an article comparing different number of engines and the probabilities of losing engines. If the probability of losing any single engine is P, then on a twinjet the probability of losing 2 engines is p^2 and on a trijet its 3p^2... 3 times as high. I also know that the central engine on the 7X has fewer redundant systems than the outside engines and what twinjet engines would have. In this equation that would make P higher in the trijet. But if the plane can operate safely on 1 engine then thats still probably a big safety advantage.

Originally Posted by capt.sparrow

Leading edge slats. Allows low speed arrivals/departures but high speed wing for cruise.

The Gulf doesn't have the leading edge slats but I believe that the Bombardier does and it still requires 6000'. So you don't think that there's anything inherent to the 3 engine design that would aid runway performance? Dassault trijets have always been known for their runway performance, but then if you look at the performance of their Falcon 2000 twinjet its nothing special at all. Does Dassault simply coincidentally engineer its trijets' wings for good runway performance or is there something inherent to having 3 engines? Thats what I'm trying to find out. I have a guess that more smaller turbines could produce more thrust early in the takeoff run but I could be way off.

[Pugilistic Animus]

those operating under 135 rules

[tuna hp]

Originally Posted by Pugilistic Animus

those operating under 135 rules

What? If they are operating under part 135 then they need to follow full ETOPS? Without special maintenance programs they have to stay within 60 minutes of a diversion, and with ETOPS certification they still have to stay a max of 180 minutes? Would a normal American bizjet charter operation have to operate part 135 / follow ETOPS? If so, is it something that everyone does and they all have the 180 minute clearance or is it something that is very rare?

[tuna hp]

Originally Posted by Arkwright

The Falcon 900 EX has similar range and speed to a G4, but uses a LOT less fuel!

I know and I'm desperately curious to find out how

[poss]

I don't know if the Dassault is an example of this but I read somewhere that some tri engined a/c use the 3rd engine, mostly, for take off... perhaps what I read was utter crap, as I say I cannot confirm the source as it was a while ago.

[Pugilistic Animus]

no private jets can do 91 if they want

but if you decide to use the benefits of etops flight then you will have to pay the associated costs; if you don't want to stop for refuelling in Kiribati with the canibals

[Red Goose]

Why do Dassault's have 3 engines?

Pax feel better with 3 engines when coasting out at FL370 overhead Seattle on the way to Maui (so do I!)

yet the Falcon only needs 5,500' of runway compared to 6,000' or 6,300'. What could possibly be the reason for this?

Two-engine aircraft are much more limited in the 2nd segment climb perf than three-engine aircraft.

The Falcon 900 EX has similar range and speed to a G4, but uses a LOT less fuel!

It's also about 10 tons lighter!

My simple thoughts about it,

Red

[tuna hp]

Originally Posted by Red Goose

Two-engine aircraft are much more limited in the 2nd segment climb perf than three-engine aircraft.

Why do three-engine aircraft with lower maximum thrust than two-engine aircraft getter better second second climb performance? Do you mean with one engine out? I don't know much about airplanes or flying: would that takeoff distance (5500') be calculated based on how fast the plane must get going before it leaves the ground in case it loses an engine?

It's also about 10 tons lighter!

Yeah how the hell does Dassault build its planes so lightly?

G450
Range: 4450nm
Cabin Volume: 1525 ft^3
Basic Operating Weight: 43,000 lbs
MTOW: 73,900 lbs

900EX
Range: 4500nm
Cabin Volume: 1264 ft^3
Basic Operating Weight: 24,700 lbs
MTOW: 48,300 lbs

Same range, roughly same speed and altitude performance, and the G450 has only 20% more cabin volume, why the hell does it weigh 75% more? And Dassault cabins are supposed to be the quietest even with the engine right behind it so I know that they aren't skimping on insulation...

[Red Goose]

Second segment climb (from gear up to 400ft above ground, 2.4% climb gradient for 2 engine aircraft, 2.7% for 3 engine aircraft), a certification requirement, is more limiting on 2 than 3 engine aircraft. This is simply due to the fact that with a 3-engine aircraft, with one engine out, you only lose 1/3 of your thrust (more drag, true, but you're still doing better!), and that counts for take-off performance.

how the hell does Dassault build its planes so lightly?

Probably, because they are used to building fighters, where every pound counts.
As an example, Cessna and Dassault developed the Citation X and the Falcon 2000 at the same time.
Cessna realized their aircraft was heavier than expected, so they had the power of their engines increased.
Dassault found out their aircraft was performing better than expected, so they had there engines de-rated.

[Arkwright]

Dassault trijets have always been known for their runway performance, but then if you look at the performance of their Falcon 2000 twinjet its nothing special at all.

Dassault Falcon 2000 (two engines) only has outboard leading edge slats. Falcon 7X and 900 (three engines) have both inboard and outboard slats.

[Red Goose]

Dassault Falcon 2000 (two engines) only has outboard leading edge slats. Falcon 7X and 900 (three engines) have both inboard and outboard slats.

And that increases the Vref, at similar weights, by 4 knots.
Take-off performance is not affected that much, though.

[Paradise Lost]

2 V 3

It's all been said now....G4 is 10 tons heavier= worse fuel burn.
Pax prefer 3 engines.
Second segment climb better with just 33% loss of power.
Short field performance due to a slatted wing.
......and they DO look good too........

[tuna hp]

Originally Posted by Red Goose

Second segment climb (from gear up to 400ft above ground, 2.4% climb gradient for 2 engine aircraft, 2.7% for 3 engine aircraft), a certification requirement, is more limiting on 2 than 3 engine aircraft. This is simply due to the fact that with a 3-engine aircraft, with one engine out, you only lose 1/3 of your thrust (more drag, true, but you're still doing better!), and that counts for take-off performance.

Yes but if you compare the Dassaults to their competitors they have engineered the planes to take advantage of this. They have significantly lower thrust/weight ratios to begin with. After losing an engine and 1/3 of their power their thrust/weight ratio is still within 10% of a gulfstream losing one of its engines since the gulfstream has so much more thrust to begin with. If Dassaults started with higher maximum thrust maybe that would change their efficiency and they wouldn't be as economical relative to the other planes.

My basic questions about 2 vs 3 have not really been answered.

-Suppose that you would engineer a trijet to have 80% of the total thrust you would have engineered a twinjet to have assuming you were making a plane with roughly the same characteristics. Is the plane heavier with 2 engines making X thrust or 3 engines making .8X thrust? Assuming an S-duct layout like the Dassaults

-Is there something about engines operating closer to their maximum rating being more efficient? If you are making a twinjet and a trijet with the same characteristics, is it going to get better cruise economy with 2 engines that produce Y cruise thrust out of X maximum thrust or the trijet that needs about the same Y thrust to cruise out of .8X maximum thrust?

-What are some of the performance tradeoffs between having 3 smaller turbines versus 2 larger ones? If you have a plane that needs 50 "thrust" for takeoff, then the twinjet needs 50 thrust per engine and the trijet needs 25. So the twinjet engines will be twice as powerful. I know for example that the twinjet engine will be more efficient in terms of energy used per lb of thrust produced. Will the smaller engines spool up to their higher thrust levels faster? Is it significant enough to effect takeoff performance? What are the other tradeoffs?

[DA50driver]

Why three negines on the 900? Because they didn't have room to fit a fourth:)

Assume you need 100 units of power to maintain level flight. If you have three engines that produce a total of 300 units you have 200 units available for climb. (Excess power available for climb). If you have two engines that produce a total of 300 units and one stops delivering power you are down to 150 units of power, in a three engine airplane you still have 200 units left. (50 units more, or 100%). This is simplified greatly, but it is the concept.

It is all about the percentage of power loss. This also affects runway performance numbers for take off. You get to see the end of a lot of runways in three and four engined airplanes, as opposed to 2 engine airplanes that rotate at a much earlier point due to the requirement to cross the end of the runway at 35'.

I spent a lot of time in the Falcon 900EX, I loved the airplane. It handles like a dream and it has redundancies that make the cold, dark Atlantic look a lot less intimidating I am currently flying a Gulfstream 450, which has as much power in one engine as the 900 had in 3. It is also a great airplane, very comfortable, spacious and built like a tank. Prior to the 450 I flew the Gulfstream 5, which has a different wing than the 450. The 5 had runway performance numbers close to that of the 900 for the same stage lengths. (4000' of runway is plenty for flights up to 6 hours).

The cruise power setting in a Gulfstream is approximately 82% power, giving a total fuel burn of approximately 2500lbs/hr. In the 900EX the power was in the low 90% range with a total fuel burn of 1900lbs/hr at mach .80. (you can cruise faster in both, but .80 seemed like a good trade off for speed vs. fuel burn).

They are all great airplanes, just different. The Gulfstream crowd refer to the engines on the 900 as hair-driers by the way.

[tuna hp]

Thanks for the details, DA50driver.

This also affects runway performance numbers for take off. You get to see the end of a lot of runways in three and four engined airplanes, as opposed to 2 engine airplanes that rotate at a much earlier point due to the requirement to cross the end of the runway at 35'.

Thats very interesting I didn't know that. That would seem to give 3 and 4 engine planes an inherent takeoff advantage when you include following regulations. Is that a significant difference in actual takeoffs?

The cruise power setting in a Gulfstream is approximately 82% power, giving a total fuel burn of approximately 2500lbs/hr. In the 900EX the power was in the low 90% range with a total fuel burn of 1900lbs/hr at mach .80. (you can cruise faster in both, but .80 seemed like a good trade off for speed vs. fuel burn).

Those percentages are what I was trying to articulate. Twinjets cruise at lower power percentage because they have higher maximum power/ weight ratio. Do turbofan engines operate significantly more efficiently at either 82% or 92%? From what I know intuitively about engines, the higher the pressure ratio of the combustion, generally the more efficiently it operates. If both a small turbofan and a big turbofan were built to the same level of technology and could operate at the same maximum pressure ratio, then the twinjet operating at 82% should be working at a lower pressure ratio than the trijet at ~92%. I wonder if that is significant to efficiency differences between twinjets and trijets.

Also thats relevent to your example of the twinjet and the trijet that both need 100 units of power to maintain flight. A twinjet would need to produce at least 200 but I trijet should be able to do it with 150. So if you do it with the smallest engines possible, then with 1 engine out both planes will have the same performance (100). I know that falcons are engineered to be able to fly on 1 engine, but isnt the real relevant aspect that they have to be able to complete a takeoff run on 2 engines? I don't know if the takeoff run requires more than twice as much power as maintain level flight... but my point is that with all engines, twinjets have higher thrust/weight ratios than Falcons. Yeah they beat Gulfstream thrust/weight ratios with one engine out, but not by 33% as in your example, I think its actually under 10%. If Falcons had the same all-engine thrust/weight ratio as Gulfstreams I imagine that they would have to be much less efficient.