View Full Version : Concept Tri-Jet Airliner, need advice/aid

11th Oct 2010, 12:40

I have been around this site for many years and yet I have never made an account until tonight.

I've been designing over a few years in my free time a concept tri-jet airliner, the EC-30, in the 100-200 seat category for use primarily in the tropics and other 'hot and high' environments. The aircraft is powered by three relatively low powered engines with the belief in my view that it could use three smaller more efficient (and quieter) turbofans as opposed to two larger units.

The center engine is mainly to provide additional thrust during cruise and other critical flight phases where the MTOW is high. I also have in mind a variation of the aircraft, which I deemed the EC-20, to be an almost identical aircraft but without the center engine, therefore more designed for shorter routes. This adaptation I am considering could be just a simple design with the engine removed though I am skeptic whether that would work.

The EC-30 is also to be made primarily of composites with possible flow-molding used as the construction technique though I am not sure if it is possible to manufacture an entire airframe using this technique.

I have a drawing of this concept, here (http://fc05.deviantart.net/fs48/i/2009/156/b/7/Vulcanair_EC_30_TriJet_by_Denodon.jpg) and I must apologize for the poor quality of it. I am no real artist and that was done with a simple ruler and set square on an A2 sheet of paper that, due to its size, I could not scan in and had to photograph it instead.

I am curious therefore as to your opinion on the feasibility of a design such as this, whether it would be practical, and other ways that I could improve the design or get someone to do proper renderings for this airliner.

Best Regards,

11th Oct 2010, 18:17
The tri-jet concept (except for the biz-jet set, IE: Falcon) is dead as a doornail....for airline aircraft, IMO.
One must remember...the L1011 started design life as a twin, but was 'upgraded' to three engine status, as the available engines (at the time) were insufficient thrust.
Twins are the way to go now, in most cases.
Keep in mind that my entire jet flying has been in three or four engine types, which were the way to go...in past years.
Not any longer, however.

We have one specific contributor here, who was involved with the initial L1011 design and certification (glhcarl), and perhaps he can provide further comment.
One thing is for sure, however (regarding the L1011 design), two now retired Boeing design engineers told me personally that when they had a first close look at the L1011 avionics design, they admitted that Boeing, at that point in time, were 'waaaay behind' in systems automation.

11th Oct 2010, 18:33
Hi Raptorva,

No offense, but can I assume you have no formal qualifications in aircraft design?

Just a few of your comments seem to indicate a certain lack of knowledge wrt some fundementals.

For example, why do you suppose that, under any circumstances, three engines would be more efficient than two, assuming that two engines of sufficient thrust were available (which, in the size you are talking, is absolutley the case)?

You seem to have some vauge notion that having an "Extra" engine used only when needed (and, btw, aircraft power requirement is pretty much totally defined by power required to takeoff from an appropriate runway at MTOW) would be more efficient than using two, bigger engines.

Not so.

MUCH more efficient in every respect to have two, larger engines, and simply throttle them back to the required thrust.- the saving in weight and drag from the centre engine (ALWAYS an engineering nightmare), cost saving in maitenance and aquisition costs- the list goes on.

The only reasons we ever had tri-jets were because sufficiently powerful engines were not available, and twins were ham-strung pre ETOPS.

Sorry to pour cold water on your project- stick to the twin, and you look like you have something very like the new Mitsubishi RJ.

11th Oct 2010, 23:44
you're right, I'm only in my final year of secondary school with little flying time in a C152 and a Harvard so I've based the design primarily on what I've seen in the past.

I understand that tri-jets were only developed for reasons of power plants being under-powered at the time as you have said. Instead I might put more focus on my EC-20 design with the possibility of an APTU which if I remember correctly was considered in the Boeing 777 design during its preliminary design phase.

As for the L-1011, that is still perhaps my favorite airliner after the DH-106 Comet.

12th Oct 2010, 00:26
I understand that tri-jets were only developed for reasons of power plants being under-powered at the time

Probably more importantly, these older 3-holers predated ETOPS so the 3/4 engine configuration was necessary for longer range operation remote from suitable alternates so that the two failed engines cruise case could be addressed.

Another St Ivian
12th Oct 2010, 01:04
raptorva: First off, nice draftmanship on the drawing - Such skills will stand you in good stead when your theoretical knowledge comes on with time.

You obivously have an interest in such things, so have you thought about investing a little time in doing some reading and learning some of the fundamentals which the above posters hint at?

Generally the texts aren't cheap due to their specialisation, but if you can, try your local library, or even purchase, a book such as this;

Introduction to Aircraft Design (http://www.amazon.co.uk/Introduction-Aircraft-Design-Cambridge-Aerospace/dp/0521657229/ref=sr_1_2?ie=UTF8&qid=1286841459&sr=8-2)

You tend to find that the business of designing an aircraft is a huge task, composed of many comprimises influenced by factors as diverse as the size of your intended operating airports, efficiency of your engines, strength/mass/volume of your materials....the list goes on!

It takes many different branches of engineering to come together to reach a successful design - So start with an introduction, and perhaps start to hone in on the area which takes your interest and abilities. First and foremost though, you need someone with the vision to come up with a concept and set it all in motion!

Best of luck...it's an addictive pursuit! :8

galaxy flyer
12th Oct 2010, 02:59

In the 727 case, I believe the design was done because at the time the FAA imposed some tough take-off weather restrictions on twins, i.e. the DC -9. I'm guessing something like a twin required landing weather at the departure airport, that is, no reduced weather for takeoff. This was especially important to United and Eastern as launch customers operating in the NE US in winter. A tri-jet could depart more reliably.

I'll do some research when back from som recurrent, perhaps a discussion on Vmcg, too


12th Oct 2010, 03:03
well I'm currently completing the draft design for the EC-20 twin-jet so I'll post that too for critique once finished. I've substantially re-designed the nose, tail and wings as well as having removed the center engine so it almost looks like an entirely new design.

12th Oct 2010, 03:15
I believe the design was done because at the time the FAA imposed some tough take-off weather restrictions on twins

Certainly wasn't aware of that sort of consideration (and I've never been too involved with US operating rules as opposed to Design Standards). Can't really say that I see any great logic but, then again, I've been way off the mark in the past so no reason why I can't be again.

For interest, the current Oz requirements from the AIP are here (http://www.airservices.gov.au/publications/current/dap/GINNO05-117.pdf).

I'm only in my final year of secondary school

Go for it, mate. Give you, say, another thirty years and your name might have become a household word.

Now, are you looking at RMIT ? or something like Sydney or NSW ?

A budding Henry Millicer in our midst.

12th Oct 2010, 04:14
Well I finished the draft for the EC-20 Regional Jet which can be found Here (http://i267.photobucket.com/albums/ii294/raptorva/DSCF2316.jpg).

My actual life goal is to found a full-service style carrier in the likes of Ansett Australia though only for domestic routes. I just draw and design aircraft such as this in my free time though I have looked at the RMIT courses.

12th Oct 2010, 05:55
though I have looked at the RMIT courses

suggest you start off with an engineering degree (aero/mech/(maybe)elec) else you probably will have difficulty progressing past the sketching stage of things ...

12th Oct 2010, 14:40
Some mind food on Light Jet acft design..


let's start with an airbus a-380.. a huge plane that goes maybe 8,000 miles at mach 0.85 or so.
it weighs 1.2 million pounds and has length and breadth of 250 ft
now let's say we want to build a small jet.. a plane that is 1/10th the size.
this plane will have length and wingspan of 25 ft.

this light jet plane is
1/10 as long
1/10 as tall
1/10 as wide

that should give a plane that is one one-thousandth the volume
that should give a plane that is one one-thousandth the weight
that should give a plane that has one one-thousandth the thrust (to push one one-thousandth the weight)

so, we have a plane that is like an airbus a-380, but with one one-thousandth the weight and thrust and fuel-burn, right?

the frontal area and wetted area of our airplane is ONE ONE-HUNDREDTH THAT OF THE AIRBUS, NOT ONE ONE-THOUSANDTH!!!!!!!!

scale the airbus down by 10x and you have one one-hundredth the frontal and wetted area, not one-one-thousandth!
so, your new scaled-down plane has one-one-thousandth the thrust, but one-one-hundredth the (parasite) DRAG!!!!!

again: scale down an a380 by a factor of 10, and you have ten times the parasite drag per unit thrust, all else being equal!!!!!

speed goes with square root of drag, so we should expect to fly at a speed fraction of the square root of 10, or about one THIRD the speed.
if we have identical endurance (one-one-thousandth the thrust, one one-thousandth the fuel, would give the same endurance!) but one third the speed, we will clearly have ne third the range.
so, if we managed to do everything as well as an airbus a-380 scaled down, we would still only have one THIRD the speed and range!!!!!!!!!!!!!!!!!!!

this rules makes a very light jet impossible, since very light jets need to have near-airliner performance to perform like jets.

it gets worse.
jets do well because they have a high bypass ratio... teeny little turbines spinning at huge rpm driving giant, slow-turning fans
these teeny fast turbines give huge compression efficiency, these high bypass ratio fans give huge propulsive efficiency.. so we just scale it down, right?


if the turbine or compressor tolerance is 0.02" for blade-radius on the airbus, and we scale the engine down by 10x in every dimension, our part tolerance is STILL 0.02" since that is the best part we can make... that is now TEN TIMES THE ERROR on an engine that is one-tenth the size. in other words, the smaller engine has ten times the losses due to manufacturing tolerances. this means that you can NOT have a big fan with a small turbine.. the losses due to imperfections in the geometry of the engine are TEN TIMES GREATER, so the turbine can not be one tenth the size in each direction... that turbine is too small to work efficiently! remember, A FEW GRAINS OF SAND GOING INTO THIS ENGINE WOULD BE THE EQUIVALENT OF THROWING BAGS OF GRAVEL INTO THE ENGINE OF THE A-380.

so a small engine can NOT be as good as a big one, because the manufacturing tolerances become 10x as large, so the turbine must be larger, so the bypass ratio must be lower. (a larger turbine is by definition a lower bypass ratio, if the total air going thru the engine is held constant)

ok, so, our very light jet that is a scaled down airliner goes 1/3 the speed of an airliner,
going 1/3 the distance,
and actually does WORSE than the above because the the bypass ratio is lower because the turbine cannot be that small.
so now we are down to 250 knots or so.. maybe 300 knots if we put a bigger engine on and sacrifice even more range.

guess who flies alongside us if we are flying in a jet at 300 mph?
this brings us to our NEXT 2 fundamentals:


the thrust we get from air is the momentum-change: amount of air we grab times how much we accelerate it
the fuel flow we put into the air is the kinetic energy: amount of air we grab times how much we accelerate it SQUARED
therefore, for any propulsion system to be efficient, it must take a LOT of air and accelerate it a LITTLE.
thus, all else being equal, the HUGE prop of a lancair is inherently more efficient than the tiny compressor of a mini-jet

an internal-combustion recip engine gets the same compression ratio no matter how fast it turns. set the throttle to idle, take-off, cruise, descent, approach, or holding-pattern... it makes no difference: if the compression ratio of the engine is 7:1, you will get that compression ratio at all power settings: 7:1... the compression ratio is realized no matter how fast or slow the engine is turning... the piston still covers the same sapce in the cylinder, regardless of speed.

the JET engine, though, must turn at 100% rpm to get it's designed compression.. if the jet turns 1% less rpm than redline, compression is lost, and efficiency with it... the compression is caused by the dynamic pressure on the blades... 1% less speed on the blades is 2% less compression across them, with the resulting loss in efficiency. you can only run a jet on-design at 100% rpm... any speed less and the efficiency falls apart... no surprise that going to low power settings still involves huge fuel-flow... a jet engine at low power is losing compression! a jet engine at low power is like a recip engine that is losing compression and needs to have it's pistons replaced!!!!!!!!!!

so there you have it. 4 fundamental laws of physics that prove that a VLJ can't work:

-a plane that is 1/10th the size has 1/1000 the weight and thrust, but 1/100 the parasite drag, so will go about 1/3 the speed, all else being equal
-a plane that is 1/10th the size will have 10 times the manufacturing error in size-ratio, resulting in a larger turbine and therefore lower bypass ratio

so the small jet cannot go as fast as the a big jet, so we are down to 300 mph, so comparing to props:
-a plane with a jet takes a smaller bite of air than a plane with a prop, so cannot have the same propulsive efficiency
-a plane with a jet cannot run at lower power settings for much of the flight, like a prop can, without huge losses, because the compression ratio i only maintained at 100% rpm

Austin Meyer

12th Oct 2010, 15:51
I think many of us have started like you, with all kind of drawings in the margins of our school notebooks, then passing on to more seriously worked-out drawings. I certainly did....

I studied aeronautical engineering, but in my case I got more interested in performance, and stability and control, than in 'raw' design, so that's what I ended up specialising in.

So get yourself to university, then see which way your interests go!


12th Oct 2010, 17:43
Some mind food on Light Jet acft design..
Nice one.... hadn't seen that one before.
Go and tell that to Jim Bede, or Dick Rutan.

I remember, from way back, discussions/lectures that went in the opposite direction, demonstrating why aircraft much bigger than the ones flying at that time could either not be built, or would be so heavy as to have an insufficient payload, or would be grossly uneconomical.
Some of it was quite right, actually.... those much bigger aircraft didn't arrive until after a few quantum leaps in design and manufacturing methods.

What do you think? Is it worth moving Austin Meyer's "statement" to a separate thread and kick it around, rather than 'pollute' raptorva's thread?


Mad (Flt) Scientist
12th Oct 2010, 17:46

That analysis assumes that the designer is stupid enough to scale the entire aircraft down by a fixed ratio.

If the weight did indeed scale by [L]^3 - which i in fact doubt - then since wing area is definitely scaling by [L]^2 you'd end up with an aircraft with 1/10 the wing loading. That alone makes any further attempt to extrapolate the deign pointless.

It also assumes the designer, having been stupid enough to scale the whole aircraft, then ignores the drag of HIS aircraft and just scales the engine too. No-one does that either.

There probably aren't many aircraft at 1/10 the size of the A380 - but there's a whole class of medium sized business jets - such as the Lear45 to pick one example - which happily and successful fly around and are about 1/5th the size of a A-380, and they definitely are not restricted to propellor-class performance.

There ARE economic advantages of scale, to be sure - but to extrapolate from that to saying small aircraft "don't work" is a great deal of a leap.

12th Oct 2010, 18:09

One thing that keeps nagging me about your EC-20 is the size of the vertical tail.

Have a look at just about every existng twin-jet, big or small, with the engines under the wings.

While the area has come down a lot from the huge tail of the A300B to that of the A321 (as examples), you'll still see the area of the vertical tail is bigger than what you've drawn.

The "design case" that determines the minimum vertical tail and rudder surface is the case of an engine failure just after take-off, at which point the aircraft still has to be directionally stable, and controllable, with one engine pushing, and the other engine dragging....

The result, in nearly all cases, is a vertical tail that's a bit too large, but there's no way (yet) to get around that.

Oh, and one thing I really do like about your design is the "Comet" nose....
There must be little or no advantage in having a slightly "nicer-looking" nose, if you look at all the blunt noses that are all the fashion today (the piggy-eyed nose of the A380 really is the limit....).
But yours at least looks as if it wants to go somewhere!


12th Oct 2010, 23:53
With developing the design for the EC-20, I considered using turboprops or Open Rotor engines as its power source but later switched to more conventional turbofans.

I agree that the vertical stabilizer came out looking far too small in area though one thing I can't quite gather is that the design came out looking 'squashed' in the photo, in real life the plan is rather longer than the photo shows.

For the Comet nose, I pretty much adapted that straight from that airliner (still my favorite) and adjusted the windshield to provide better wrap-around visibility for pilots. Only thing I'm not sure of is whether a modern compact Doppler weather radar would still fit within the contours of the nose.

13th Oct 2010, 06:49
I was taught in an aerodynamics class long ago that the overriding factor in aircraft design is engine availability. No suitable engine; no plane.

The second factor is where to put the landing gear. Wing placement depends on that more than on aerodynamics.


14th Oct 2010, 16:15

If the weight did indeed scale by [L]^3 - which i in fact doubt

B734, L 36,5m, OEW 33,2T
B773, L 73,9m, OEW 167,8T

So B734 is half the length of the B773.
So 734 weight should be 1/6 th right?; 167,8 / 6 = 28T

Considering the 777 has more composites and is more modern overall
I'd say that weight does scale about L^3

but there's a whole class of medium sized business jets - such as the Lear45

Sure, but what bypass ratio do their engines have compared to their
larger cuisines?
And why don't airlines use them?


Mad (Flt) Scientist
14th Oct 2010, 17:26

B734, L 36,5m, OEW 33,2T
B773, L 73,9m, OEW 167,8T

So B734 is half the length of the B773.
So 734 weight should be 1/6 th right?; 167,8 / 6 = 28T

Considering the 777 has more composites and is more modern overall
I'd say that weight does scale about L^3

L^3 is length cubed. The cube of 2 is 8. So that "rule" would predict a 734 weight of 167.8/8=about 21 tonnes. The true value is 33.2T. That's an error of about 50%, indicating that the exponent, in this case, is closer to 2.5 than 3 - a pretty significant difference (it's actually 2.33).

That;'s not really surprising - airframe loads are probably closer to being dependent on area than volume, and loads drive structural mass. Things like fuselage skin are also area-dependent if not load driven (i.e. if you assumed a constant skin thickness, a tube scaled up by 2 would have 8 times the volume but only 4 times the surface area). A ratio a bit above 2 seems quite sensible.

Sure, but what bypass ratio do their engines have compared to their
larger cuisines?
And why don't airlines use them?


Airlines don't use small business jets due to all kinds of economic factors, some of which have nothing to do with aerodynamic efficiency (salary cost spread over very few seats, cost of slot used versus total revenue, being two examples) in addition to the fact that even if certified to part 25, being designed in most cases for Part 91 type operations there are many Part 121 regulations such small aircraft do not, and often cannot, meet. For example, fitting a reinforced cockpit door to a Learjet would be somewhat challenging, yet its a Part121 mandated feature.

14th Oct 2010, 18:00
L^3 is length cubed. The cube of 2 is 8

You are right offcourse, my mistake, 1/2*1/2*1/2 = 1/8

However, wouldn't that mean austins reasoning is even more true?
The shrunk smaller acft will have a higher "density" than the larger one...

So, a smaller aircraft has
- Larger relative weight
- More drag
- Less efficient engines (lower BR)
- Less airspeed (for the same relative thrust)


14th Oct 2010, 18:09

Wing warping was abandoned early on. Why did NASA recently modify an F-18 with warping wings to investigate the domain? Lunacy? Doubtful. It is arguable that the agency is looking for money, but are we that cynical? I'm not convinced that tri jets are done. ETOPS is not bulletproof, for example. By waiver, ESOPS is allowed. (Extended range, single engine).


14th Oct 2010, 18:37

When I was a student (not exactly yesterday...) we had sheets and sheets of 'parametric charts', with for instance, in this context, things like 'length vs weight' etc. etc. for dozens of widely different types of aircraft.
Some were really just 'scatter' charts with some effort at finding a trend, some were very interesting.
Probably mostly composed by student assistants with time to spare and a library at their disposal.

Do they still exist, are they still used as teaching tools, and are some of them on the 'net somewhere?

raptorva might like to see some of them.


Mad (Flt) Scientist
14th Oct 2010, 20:42
Such charts certainly exist; such parametric studies usually form the basis fort initial designs at the very early stages, before people start to spend serious amounts of money. But they are rather closely held trade secrets obviously (at least the ones OEMs have developed)

The problem with using these parametric methods for a more modern design is that they depend greatly on a large enough set of data to make the parameterization valid. As aircraft programmes have got fewer with each generation, there's less and less data to work with. But they are still better than nothing. "Benchmarking" is basically the same thing, and is used all the time as a sanity check for designs.

14th Oct 2010, 21:21
EMBRAER will be suing you for that EC-20.

Alber Ratman
14th Oct 2010, 21:35
Nice drawings.. And you are being introduced to all the basic fundermentals of aircraft design and certification!:ok:

Just make sure you get top grades in Maths and Physics!!:eek:

15th Oct 2010, 03:09
I'm still trying to work out what kind of weights and performance I could expect if both the EC-20 and -30 were powered by turbofans possibly such as the GeNX or LEAP-X engines or even open rotor models.

17th Oct 2010, 11:04
Whilst looking at regional jet design, its worth bearing in mind that the customer requirements are quite different to those of large, long range jets.

There is no point in SleazyJet and the like, flying for an hour or two at high speed, only to sit on the ground for twenty minutes waiting for a set of steps to appear. If the same journey can be flown a little more slowly, whilst maintaining the number of journeys per day, then some aerodynamic compromises can be made.

One Airbus A320-sized replacement being proposed by them looked very much like an Ilyushin 76 with rear, fuselage-mounted engines. The low fuselage meant integral 'Air Stairs' were feasible, and the two engines nearer the centreline presumably meant that the excess power requirement for single engine operation was less, as the dead engine was closer to the centreline, and less fin and rudder were required to overcome its drag. The high wing also allowed larger high lift devices, less constrained by ground clearance than, say, the A321's three stage flap. The high-lift devices would allow a smaller wing in the cruise and/or the opportunity to use cheaper airfields with shorter runways.

Taking this last point a step further, presumably if both engines were in the fin, either next to each other 'a la' TU-22 Blinder, or one above the other, which as far as I know, no one has tried yet, with the possible exception of the Trident 3B (albeit two different engines), then the engine-out power requirement would be even less. Yes, it would be ugly; yes, it would be a maintenance nightmare, compared to dropping a wing mounted engine, but if the sums add up otherwise, we might see it, as engines aren't removed that frequently.

18th Oct 2010, 00:49
Speaking as a maintenance engineer, those #2 engines in the tail can be a bit of a pig to work. Extra downtime for the airlines!

When I was studying for my CAA licence, I found The Mechanics of Flight an excellent book for aerodynamics, fairly easy to understand;

Mechanics of Flight: Amazon.co.uk: Mr A.C. Kermode: Books (http://www.amazon.co.uk/Mechanics-Flight-Mr-C-Kermode/dp/0582237408/ref=sr_1_2?ie=UTF8&qid=1287358722&sr=8-2)

Keep up the interest in aviation. You'll be a lot warmer, drier, better paid, less worked, less stressed, less tired in many other industries, but I wouldn't exchange aviation for any of them! Good luck