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Topic: Question For The Aeronautical Engineers ...
Username: Sesd97
Posted 2006-07-15 01:03:42 and read 5184 times.

I've always wondered why it is that some smaller planes have 3 jet engines instead of just 2 ?  Confused

Great example being the Dassault line of bizjets ......


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Photo © Shaun Edelstein
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Photo © Shaun Edelstein



I can understand why Lockheed dropped 3 big holes into the MD-11 for instance...(Big plane, big weight, NEED MORE POWER !!)

But does the same equation work on a small bizjet? Surely it's very much more expensive to run 3 engines than 2 (with slightly more power) ?

That would be 1 extra of everything...cabling, fuel lines, avionics etc etc...not cheaper just to add more power to the existing 2 engines and run the plane with 2 ??

Looking forward to your insight ........thx, Shaun

Topic: RE: Question For The Aeronautical Engineers ...
Username: 474218
Posted 2006-07-15 01:36:16 and read 5171 times.

Quoting Sesd97 (Thread starter):
I can understand why Lockheed dropped 3 big holes into the MD-11 for instance...(Big plane, big weight, NEED MORE POWER !!)

Lockheed had nothing to do with the MD-11?

Quoting Sesd97 (Thread starter):
But does the same equation work on a small bizjet? Surely it's very much more expensive to run 3 engines than 2 (with slightly more power) ?

When the Falcon 900 and 50 were introduced it took three engines to get across the Atlantic.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Bri2k1
Posted 2006-07-15 01:43:38 and read 5168 times.

Quoting Sesd97 (Thread starter):
(with slightly more power)

It should be easy to see there's half again as much power with three identical engines compared to two of the same stock. This can also be a benefit in terms of redundancy, which is one reason some manufacturers and owners might want three instead of two larger powerplants.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Starlionblue
Posted 2006-07-15 02:51:07 and read 5147 times.

In the case of the Falcon, it's probably a case of long-haul overwater redundancy. Also, three engines allows for a lower total weight all things being equal since total thrust for all engines can be lower than with a twin. Same logic as the 340.

However for a new bizjet, with modern engines, the added complexity of an extra engine will not be worth it.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Darrenthe747
Posted 2006-07-15 07:41:13 and read 5114 times.

it's mainly to do with over-water range.

Topic: RE: Question For The Aeronautical Engineers ...
Username: DEVILFISH
Posted 2006-07-15 17:15:45 and read 5071 times.

Quoting 474218 (Reply 1):

When the Falcon 900 and 50 were introduced it took three engines to get across the Atlantic.



Quoting Starlionblue (Reply 3):
In the case of the Falcon, it's probably a case of long-haul overwater redundancy.



Quoting Starlionblue (Reply 3):
However for a new bizjet, with modern engines, the added complexity of an extra engine will not be worth it.

I think with the Falcon 7X, the Atlantic is no longer the aim. They've reportedly just increased the range via drag-reducing winglets and an additional fuel tank to 6,000 NM (11,100 KM) - albeit for the sole demonstrator of the company patriarch. Dassault said the first 15 production airframes would have to be retrofitted to have the same performance, adding that customers were 'happy' for the cause of the delay - apparently less anxious that they would be flying such a long distance on three instead of just two engines. http://www.flightglobal.com/Articles...ut+customers+remain+'happy'.html

Topic: RE: Question For The Aeronautical Engineers ...
Username: Kaddyuk
Posted 2006-07-16 20:45:50 and read 4963 times.

Quoting Starlionblue (Reply 3):
However for a new bizjet, with modern engines, the added complexity of an extra engine will not be worth it.

Really? You'd not have to worry about ETOPS and the extra costs that incurrs...

Topic: RE: Question For The Aeronautical Engineers ...
Username: Starlionblue
Posted 2006-07-16 21:04:14 and read 4961 times.

Quoting Kaddyuk (Reply 6):
Quoting Starlionblue (Reply 3):
However for a new bizjet, with modern engines, the added complexity of an extra engine will not be worth it.

Really? You'd not have to worry about ETOPS and the extra costs that incurrs...

You have a point but...
- Aren't private aircraft exempt from ETOPS?
- I still think the added expense of an additional engine is more than ETOPS or equivalent measures in today's environment. I may be wrong.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Kaddyuk
Posted 2006-07-17 00:16:51 and read 4945 times.

Quoting Starlionblue (Reply 7):
- Aren't private aircraft exempt from ETOPS?
- I still think the added expense of an additional engine is more than ETOPS or equivalent measures in today's environment. I may be wrong.

The only aircraft exempt from ETOPS are those 5000kgs (i think 5700kgs) and under... those aircraft under that weight are limited to 60 minutes from a diversion airport...

There are more costs incurred yes, but there are also severe limitations with ETOPS aircraft.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Bri2k1
Posted 2006-07-17 01:44:03 and read 4924 times.

Quoting Kaddyuk (Reply 8):

The only aircraft exempt from ETOPS are those 5000kgs (i think 5700kgs) and under... those aircraft under that weight are limited to 60 minutes from a diversion airport...

5000kgs? I don't think so. A Learjet 45 weighs more than that, completely empty. Even a Beechjet 400 can't take any fuel and weigh less than that.

ETOPS is a Part 121 issue. Private jets, being operated under a different Part of the FARs (Part 135 in most cases), are completely exempt. In general, any time limits have been a result of aircraft performance and capability limitations when operating on a single engine, and these are typically in the ballpark of 180 minutes or more. I believe, however, that under JAA rules, they are subject to a 120-min rule.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Starlionblue
Posted 2006-07-17 03:15:51 and read 4909 times.

As Bri2k1 clarifies, it seems to that private aircraft, at least in the US, have less restrictive twin engine regs.

Topic: RE: Question For The Aeronautical Engineers ...
Username: 411A
Posted 2006-07-17 05:40:13 and read 4890 times.

It has absolutely nothing to do with regulations folks, with regard to private operations.
Rather, it simply is the thrust required for the particular engine/airframe combination selected.
Sometimes, three is indeed better than two.

OR, in the case of the FIRST dedicated business jet, designed for the purpose, the Lockheed JetStar, 4 is better than two.

And, yes, I have flown this quite remarkable airplane, and a good one it is...for sure.

Just like all post-war Lockheed designs...the 1649 Constellation, Electra....and especially the TriStar...fantastic airplanes, all.
And yes, I have flown 'em all, as well, so I should know.

Lockheed, far ahead of its time.

Having said this, there is positively nothing wrong with twin engine designs...Gulfstream comes to mind.
A nice airplane this, especially fit and finish.
A great flying aircraft as well.

Topic: RE: Question For The Aeronautical Engineers ...
Username: 2H4
Posted 2006-07-17 07:02:10 and read 4875 times.




Quoting 411A (Reply 11):
And yes, I have flown 'em all, as well, so I should know.

We get it, 411A.....you remind us in virtually every one of your posts.  Wink




2H4


Topic: RE: Question For The Aeronautical Engineers ...
Username: Kaddyuk
Posted 2006-07-17 08:37:27 and read 4867 times.

Quoting Bri2k1 (Reply 9):
Private jets, being operated under a different Part of the FARs

I was talking about EASA rules...

Topic: RE: Question For The Aeronautical Engineers ...
Username: Mrocktor
Posted 2006-07-17 15:01:20 and read 4844 times.

Quoting 411A (Reply 11):
Rather, it simply is the thrust required for the particular engine/airframe combination selected.
Sometimes, three is indeed better than two.

Uh, no. Barring the case where there is no engine in production with the thrust rating you need for the twin or one that can be modified to be suitable (hint: this does not happen), the twin will be more efficient.

Reliability has ceased to be a problem for twins and business jet operations under part 91 or 135 are not subject to ETOPS.

Falcon continues to make 3 engine airplanes because its their trademark, and because the business jet owner (even corporate) is not cost driven.

mrocktor

Topic: RE: Question For The Aeronautical Engineers ...
Username: 411A
Posted 2006-07-17 16:56:16 and read 4829 times.

Yes, the twin will generally be better from an efficiency standpoint, Mrocktor, but in some circumstances, engines are simply not available that HAVE the required thrust, when the airframe is first designed.

Let us not forget as well, that the European aviation authorities are now investigating the possibility of mandating ETOPS requirements for charter operations with business jets of a twin engine design, and indeed have even proposed expanding this requirement to private operators that are registered within the European EASA countries.

This will not affect US operators of course, but on the other hand, sometime in the future it might, if the FAA decides to harmonize the FAA regulations with EASA, in some way.

It could well be that the three engine design will stay in favor for quite some time.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Lufthansi
Posted 2006-07-18 22:36:17 and read 4761 times.

Well if you have more engines providing more power you can go to shorter runways which means more locations to travel to. If you own your Falcon you want to got to your house or office as close as possible.

If you're living for example in Braunschweig (Brunswick in English/ and hell yeah -> definetly a place to be!) you will be happy to get there without any problems. You will start laughin when Volkswagen is pulling out their A319CJ for a hop to Hanover (20 miles!?) because they have to refuel there. If they fuel up their babybus at Brunswick (homebase) they'll end in the airport's fence. You and your Falcon will make it to anywhere! Maybe that's why Volkswagen also has Falcons...

And if you sent the Falcon to pick up you bunny you wanna make sure she is safe. You don't offer one engine... nor two engines... but three for a safe trip to your royal suite on the other side of the atlantic. So you can sleep much better while honey is coming home.

The old ETOPS things might have been important when developing this jet (mentioned above/best idea by far I think).

And don't care about fuel prices. If you have a Falcon, a bunny (or honey or whatever) and your royal suite that'll be the cheapest. (But what of your 3 hobbies might be the most expensivest?)

If you are now heading for to buy one, let me know please!  Smile

[Edited 2006-07-18 22:41:19]

Topic: RE: Question For The Aeronautical Engineers ...
Username: Mrocktor
Posted 2006-07-19 17:29:32 and read 4703 times.

Quoting Mrocktor (Reply 14):
Barring the case where there is no engine in production with the thrust rating you need for the twin or one that can be modified to be suitable



Quoting 411A (Reply 15):
in some circumstances, engines are simply not available that HAVE the required thrust

?

Quoting 411A (Reply 15):
Let us not forget as well, that the European aviation authorities are now investigating the possibility of mandating ETOPS requirements for charter operations with business jets of a twin engine design

And let us not forget that ETOPS twins are still more efficient than multi engine designs, despite the added costs, and that the FAA is considering extending ETOPS to multi engine aircraft  Wink

Quoting 411A (Reply 15):
It could well be that the three engine design will stay in favor for quite some time.

But it is not in favor, only Dassault uses it and for non technical reasons as I have explained.

Quoting Lufthansi (Reply 16):
Well if you have more engines providing more power you can go to shorter runways which means more locations to travel to. If you own your Falcon you want to got to your house or office as close as possible.

Incorrect. You size the engines to the desired performance you don't stick on an engine and see what takeoff field length you end up with. Yes the twin will have bigger engines, and still be more efficient.

Quoting Lufthansi (Reply 16):
And if you sent the Falcon to pick up you bunny you wanna make sure she is safe

Yup, and if you are ignorant of the fact that engine reliability is not the limiting factor in aircraft safety you might believe that the extra engine actually makes a significant difference.

Quoting Lufthansi (Reply 16):
And don't care about fuel prices



Quoting Mrocktor (Reply 14):
because the business jet owner (even corporate) is not cost driven.

?

mrocktor

Topic: RE: Question For The Aeronautical Engineers ...
Username: Jetstar
Posted 2006-07-19 17:42:40 and read 4698 times.

Quoting Bri2k1 (Reply 9):
ETOPS is a Part 121 issue. Private jets, being operated under a different Part of the FARs (Part 135 in most cases),

Privately owned corporate jets used strictly for corporate transport of their executives operate under FAR Part 91.

If the aircraft is used for compensation or hire, then it operates under Part 135 .

Topic: RE: Question For The Aeronautical Engineers ...
Username: Dw747400
Posted 2006-07-19 21:00:42 and read 4661 times.

Quoting Mrocktor (Reply 17):
But it is not in favor, only Dassault uses it and for non technical reasons as I have explained.

Engine out takeoff abilities from short strips with obstructions are improved with a three-engine design. Twins need to meet obstacle clearance too, but at some more difficult airports they must be payload restricted to do so--the other options would be to install a massive amount of thrust that was rarely ever used. Thus, the tri-jet is more flexible in a small number of cases.

Additionally, a tri-jet has a higher engine out ceiling than a twin, enabling the Falcon to take routings over terrain that may not be suitable for competing aircraft. Again, this is rarely a factor, but it is still a design benefit.

Nonetheless, twin-jets are dominant and will remain so.

Quoting Mrocktor (Reply 14):
Falcon continues to make 3 engine airplanes because its their trademark, and because the business jet owner (even corporate) is not cost driven.

You are right that the Dassault trademark of the tri-jet is probably the biggest driver behind the configuration, but you are DEAD WRONG that business jet owners are not cost driven. Part 135 companies want to maximize profits, and part 91 operations are still accountable to the shareholders. There are people who don't care about money--look at John Travolta's 707--but the majority do. The vast majority of biz jets operate for companies interested in making money--not wasting it.

Topic: RE: Question For The Aeronautical Engineers ...
Username: SlamClick
Posted 2006-07-19 23:52:38 and read 4633 times.

Quoting 411A (Reply 11):
OR, in the case of the FIRST dedicated business jet, designed for the purpose, the Lockheed JetStar, 4 is better than two.

I always thought it interesting that Lockheed - Burbank operated this two-engine version for decades.

View Large View Medium
Click here for bigger photo!

Photo © Royal S King


(or one just like it) I used to see it on their north ramp all the time.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Liedetectors
Posted 2006-07-20 00:40:45 and read 4626 times.

The ETOPS issue isnt really an issue anymore for the Falcons. Having three engines is kind of a distinctive look for the Falcon that sets them different from the rest. I understand that many operators insisted that Dasault leave the 3rd engine for both looks and reliability reasons.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Mrocktor
Posted 2006-07-20 14:57:03 and read 4574 times.

Quoting Dw747400 (Reply 19):
Twins need to meet obstacle clearance too, but at some more difficult airports they must be payload restricted to do so



Quoting Dw747400 (Reply 19):
Additionally, a tri-jet has a higher engine out ceiling than a twin, enabling the Falcon to take routings over terrain that may not be suitable for competing aircraft.

This is exactly what I was talking about. If you want field performance, you size your twin for field performance. If you want a high engine out ceiling, you size it for that.

The fact that most airframers don't is not a limitation of twin engine airplanes - it is a recognition that these "advantages" are not worth the cost (for any airplane).

Quoting Dw747400 (Reply 19):
The vast majority of biz jets operate for companies interested in making money--not wasting it.

Of course. But that does not stop a CEO/CFO/COO from spending "a little" more to have "the best" jet.

mrocktor

Topic: RE: Question For The Aeronautical Engineers ...
Username: Jetstar
Posted 2006-07-21 01:49:16 and read 4525 times.

Quoting SlamClick (Reply 20):
I always thought it interesting that Lockheed - Burbank operated this two-engine version for decades

This 2 engine JetStar is one of 2 prototypes built and it also differs from the production versions in that it has a single wheel on each main landing gear where the production versions have dual main wheels. The one is this picture stayed with its 2 engines and was used by Lockheed for company transport and then donated to a aviation school in Vancouver Canada. The other prototype was used as the flying test bed for the 4 engine installation, as far as I know it was never used for company transport. The airplane was disassembled and donated to an aviation school in the Atlanta area. I do not know if this airframe is still in existence

Over the years I have heard 2 different stories from Lockheed people, some say it was designed as a 2 engine airplane and the others say 4 engines.

The JetStar was designed by Kelly Johnson of the Lockheed skunk works fame in response to a proposal from the US Air Force for a 10 passenger utility transport.
I heard he actually designed it in a restaurant on a napkin.

The official story I was told when I went to the 2 week JetStar maintenance school in Marietta GA in the early 70’s was because the P&W JT12 engines were not available at that time because they were still in the development stage. Lockheed used 2 British Orpheus engines which were the only small engines available at the time so they could at least test fly the airframe and begin the certification process. Once Lockheed got the USAF contract, as in any contract there are dates listed and Lockheed would have to meet certain requirements by these dates or face penalties. By flying the 2 engine version first they were able to verify the aircraft flight characteristics. When the P&W’s were available from Pratt & Whitney then the second prototype was changed over to the 4 engines and was used in the fly off against its competitor from McDonnell.

The unofficial story is in the early flight testing of the 2 engine model, it could not make the second segment climb requirement in the Air Force contract so it was decided early on in flight testing to go over to the 4 engine version.

I go with the official story because the JetStar’s competitor for the USAF contract was McDonnell with its model 220. This was a 4 engine 10 passenger airplane with the engines mounted under the wings and it looked like a baby B-707. I believe the engines on this airplane were Westinghouse J-47’s. The Air Force plans also included using this small transport for presidential transport and at the time there was a requirement that the President only flies on 4 engine airplanes. So I feel that the JetStar was designed to be a 4 engine airplane from the beginning.

One of the JetStar’s main competitors in the biz jet corporate world was the twin engine Dassault Falcon 20. Both airplanes had similar range and could seat 9 to 10 passengers. The Falcon 20’s cabin was about the same diameter and height but was a little shorter in length. The JetStar had a storage closet aft of the lavatory and a fairly large baggage compartment opposite the entrance door so it offered more space when all the seats were full.

Dassault then took the Falcon 20. stretched the fuselage, and added a third engine on the tail like the B-727 and it became the Falcon 50 and this model is still in production today. Dassault then took the Falcon 50, added a wide body fuselage and called it the Falcon 900, which also is in production today. Dassault then eliminated the center engine and called this wide body the Falcon 2000. So from the original 1960’s Falcon 20, Dassault has spawned an entire family of airplanes that are still in production.

In the height of the Jetstar’s days, Lockheed had tech reps based around the country and one was based in White Plains and he was responsible for the Northeast. At one time Westchester Airport had 13 Jetstar’s on the airport alone and in the Northeast there were over 30 JetStar’s so the tech rep was kept busy. One of the tech reps showed me some drawing that Lockheed was working on for future JetStar’s. This drawing was for a 3 engine version using the newer TFE 731 fan engines and a redesigned much more efficient wing with all the fuel stored internally. It had the same profile as the Falcon 50 with the “S” duct for the center engine. Lockheed decided not to go this route but simply changed the 4 engines over to the fan engines and called it the JetStar 2. They had hoped to sell the JetStar 2 to the Air Force as a replacement for the older JetStar’s but the Air Force had dropped the requirement that the president only flies on 4 engine airplanes and decided to go with the Gulfstream 2 instead because it was much larger and had intercontinental range.

In Dallas on Love field there was a company called Executive Aircraft Service that was one of the best JetStar completion and maintenance centers in the country and was owned mostly by Dick and Ralph Emery. They had plans to buy up old JetStar’s, rewing the JetStar with Lockheed’s new wing design and also convert over to the new fan engines. The wing would have been built by Short Brothers in Ireland and installed in Dallas. I saw a proposal from them with a drawing with the new wing installed. It was contingent on getting a certain number of contracts signed with paid deposits. From what I heard no one signed a contract and they could not get financing themselves so this never got beyond the paper proposal.

About 10 years ago another company in California came out with a proposal for JetStar’s to be re engined with 2 CF34 GE engines, they actually had a prototype flying for demonstration purposes. Financing was to be through GE Capital, which would benefit GE itself because they would be able to sell more of their engines. GE Capital then ran the numbers through and found that there was not enough interest in this program because of the age of the airplanes and other newer airplanes coming on the market so they pulled out of the program., The modification company could not get any other financing and the program was dropped.

Lockheed, now Lockheed Martin being a military company was not interested in continuing the civilian JetStar program and dropped production after 40 JetStar 2’s were built.

Topic: RE: Question For The Aeronautical Engineers ...
Username: 2H4
Posted 2006-07-21 02:12:02 and read 4518 times.




Quoting Jetstar (Reply 23):
a proposal for JetStar’s to be re engined with 2 CF34 GE engines, they actually had a prototype flying for demonstration purposes.

Ooooooo.....I want a photo!




2H4


Topic: RE: Question For The Aeronautical Engineers ...
Username: Jetstar
Posted 2006-07-21 02:58:56 and read 4517 times.

Quoting 2H4 (Reply 24):
Ooooooo.....I want a photo!

Somewhere in my files I have an 8” x 10” glossy photo of this 2 engine JetStar that was part of the brochure they were giving out at an NBAA show. As soon as I find it I will scan it in and post it.

Topic: RE: Question For The Aeronautical Engineers ...
Username: 2H4
Posted 2006-07-21 03:08:45 and read 4520 times.




Quoting Jetstar (Reply 25):
Somewhere in my files I have an 8” x 10” glossy photo of this 2 engine JetStar that was part of the brochure they were giving out at an NBAA show. As soon as I find it I will scan it in and post it.

Excellent! It would be a perfect addition to the "Odd Aircraft" thread.




2H4


Topic: RE: Question For The Aeronautical Engineers ...
Username: Jetstar
Posted 2006-07-21 04:09:44 and read 4522 times.

Here are the photos of the 2 engine JetStar with the GE CF34 engines. It was called for marketing purposes the FanStar. I believe they used the same engines and nacelles from the Challenger 601. I didn’t keep the brochure but I do have a hat with the airplane as part of the FanStar logo that they were giving out at their booth. I believe the company's name was American Aviation. They had already flown this airplane quite a few times when GE pulled out of the program.

Big version: Width: 1508 Height: 1059 File size: 84kb
Big version: Width: 1502 Height: 1173 File size: 70kb


This airplane was an older turbojet powered model, not an updated model with the 731 engines. You can tell by the location of the auxiliary fuel tanks on the wings. On the original JetStar the tanks were placed halfway up on the wing. The wing passed through the aux tank and fuel was in the upper and lower sections of the aux tank.

On the 731 models and the JetStar 2, the aux tank was lowered to underneath the wing because of airflow problems. The 731 engines were wider and the outboard engines were in the slip stream of the older tanks so they had to lower the tank to get rid of the airflow problems. On this CF34 version the engines do not extend out as far, so they were able to leave the aux fuel tanks where they were.

On time on a trip to Los Angeles, I drove up to Van Nuys airport and I saw this airplane sitting by a hanger. The program had already been cancelled and the engines were removed so it became a derelict airplane. I would assume that because the turbojet version was obsolete, there was no value in the airframe and they probably parted out the rest of the airplane and scrapped what was left.

Topic: RE: Question For The Aeronautical Engineers ...
Username: 2H4
Posted 2006-07-21 04:17:20 and read 4508 times.



Wow. Great photos and even better trivia. Thanks very much for sharing, Jetstar!




2H4


Topic: RE: Question For The Aeronautical Engineers ...
Username: SlamClick
Posted 2006-07-21 04:27:53 and read 4506 times.

Quoting Jetstar (Reply 27):
It was called for marketing purposes the FanStar

That is actually a very nice looking airplane. I've always thought the Jetstar looked rock-solid but slightly dated.

Thanks for the info on the two engine version too. I've long wondered if it didn't have a lower TOGW than the garden variety.

Topic: RE: Question For The Aeronautical Engineers ...
Username: RaginMav
Posted 2006-07-31 21:24:55 and read 4356 times.

Quoting Bri2k1 (Reply 2):
It should be easy to see there's half again as much power with three identical engines compared to two of the same stock

Aahh... but the engines on a 3-hole Falcon are smaller than on a comparable sized twin.

Quoting Dw747400 (Reply 19):
Engine out takeoff abilities from short strips with obstructions are improved with a three-engine design. Twins need to meet obstacle clearance too, but at some more difficult airports they must be payload restricted to do so--the other options would be to install a massive amount of thrust that was rarely ever used. Thus, the tri-jet is more flexible in a small number of cases.

Yes, and reason being engine-inoperative take-off/climb performance.

A Falcon must meet climb requirements while operating on two of it's three engines. (That is, after loosing a 'critical [most powerful] engine', which would be number 1 or 3, as number 2 looses a small bit of thrust due to the 'S'-duct).

A comparable twin (let's say a Challenger) would have to meet the climb requirements after loosing one engine, thus leaving it with just one running.

The advantage of the 3-hole Falcon becomes evident when you analyze the one-engine-inoperative thrust-to-weight ratio. So let's study the numbers of the new Challenger 605 v. the new Falcon 900DX.

CHALLENGER: MTOW 48,200 lbs. Engines: 2x at 8729 lbs thrust each. Yielding an engine-out thrust-to-weight ratio of 5.521 lbs/lb. thrust.

FALCON: MTOW 46,700 lbs. Engines 3x at 5000 lbs thrust each. Yielding an engine-out thrust-to-weight ratio of 4.670 lbs/lb thrust.

This engine-out thrust advantage is extremely important at high-elevation airports like JAC or EGE. One place I have personally seen it's advantage is in RNO, where on a 105* day, a heavy twin goes nowhere if it can't 'see and avoid' the surrounding cumulogranite.

Topic: RE: Question For The Aeronautical Engineers ...
Username: Bri2k1
Posted 2006-08-01 06:46:58 and read 4283 times.

Quoting bri2k1 (Reply 2):
three identical engines



Quoting RaginMav (Reply 30):
the engines on a 3-hole Falcon are smaller

:-/

Topic: RE: Question For The Aeronautical Engineers ...
Username: Mrocktor
Posted 2006-08-01 17:28:39 and read 4250 times.

Quoting RaginMav (Reply 30):
This engine-out thrust advantage is extremely important at high-elevation airports like JAC or EGE. One place I have personally seen it's advantage is in RNO, where on a 105* day, a heavy twin goes nowhere if it can't 'see and avoid' the surrounding cumulogranite.

If that were important, the Challenger would have been designed with larger engines.

mrocktor

Topic: RE: Question For The Aeronautical Engineers ...
Username: RaginMav
Posted 2006-08-01 17:54:30 and read 4245 times.

Quoting Bri2k1 (Reply 31):
Quoting bri2k1 (Reply 2):
three identical engines



Quoting RaginMav (Reply 30):
the engines on a 3-hole Falcon are smaller

:-/

I realize he said identical, I just wanted to point out that they are certainly not identical.

Quoting Mrocktor (Reply 32):
If that were important, the Challenger would have been designed with larger engines.

But it is important! In fact, the majority of the time, it is the limiting factor when departing into IMC. Assuming there's enough runway to take off, the next limiting factor is climb performance. As another example, When it's 100+ in LAS, a Hawker or Citation VII has to be very light to make the climb gradient requirements of the departure procedures (assuming, once again, Instrument conditions).

I'm trying to point out that for business jets, there is still an advantage to 3 engines. As to whether or not it's worth the extra cost, that is the million dollar (literally) question.

Topic: RE: Question For The Aeronautical Engineers ...
Username: PlainSmart
Posted 2006-08-01 19:42:47 and read 4220 times.

I think take off minimums have not been addressed yet. Three or more engined aircraft have lower t/o minumums than single or two engined aircraft.
1-2 Engines 1 statute mile.
3-more 1/2 statute mile.
(Not part 91 operations)

Topic: RE: Question For The Aeronautical Engineers ...
Username: Mrocktor
Posted 2006-08-01 22:06:36 and read 4199 times.

Quoting RaginMav (Reply 33):
I'm trying to point out that for business jets, there is still an advantage to 3 engines. As to whether or not it's worth the extra cost, that is the million dollar (literally) question.

And I am pointing out that it is not an advantage of 3 engines. There is nothing stopping you from using bigger engines on your twin! The fact that manufacturers don't is a clear indication that other things - such as weight and fuel consumption - are more important than this.

I'll say it again: if climb performance were a driving design factor, twins would be build with larger engines and would still be more efficient than tri-jets.

The mistake here is assuming that aicraft are designed with "loose ends", like you are implying. Every performance aspect is taken into consideration when sizin the engine. If the Challenger can't take off without limitations under certain conditions it's because the manufacturer chose to build it that way not because of a metaphysical limitation inherent to the number of engines it has.

mrocktor


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