Trijet Stretch

Forgive me if this sounds like an inane question (I'm about as far from an engineer as you can get), but how long could you stretch a trijet with an all-rear engine configuration? Would there be an imbalance issue after reaching a certain length?

Could you illustrate your question with a sketch to visualize it? How far would you like to "stretch"? There might be various ways to balance the equation.

A.

Don't think, that it has much to do with the engine arrangement. The CG has to be kept
in its limits, If the engines for your stretched version aren't heavier, yopu can add a plugg
in front and behind the CG, I think.

The limit on stretching would probably be tail clearance on rotation. You need to be able to raise the nose some angle for take offs and landings, too long a tail would prevent this.

Thanks! I didn't have a specific length in mind, but the CG and tail clearance issues make sense.

I'm taking the liberty to move this topic to the "Theoretical..." section as we're not discussing industry programs here but speculating on possible aircraft shapes.

Wonder if there wouldn't be an issue with wing area too. Though not a trijet, here's a purely fictional (and not necessarily flightworthy) development of the DC-9 family I did a few years ago. You can see I added a second set of wings because the wings that were there seemed odd to me on their own given the length of that thing. Of course I don't have any technical expertise to back it up!

AFAIK, the tandem wing is said to a wider movement of the CG, so really a good idea.
Just wondering, if the rearward landing gear would have to be beefed up considerably, as it
would have to take the full weight during touch down.

Jemiba said:

AFAIK, the tandem wing is said to a wider movement of the CG, so really a good idea.
Just wondering, if the rearward landing gear would have to be beefed up considerably, as it
would have to take the full weight during touch down.

Click to expand...

I could be wrong but I think the full weight of an aircraft never rests on the main gear on touchdown because the aircraft is still moving and the jets remove some of the stress on the wheel trains. In this kind of configuration, you'd have TWO main landing gears under each pair of wings, touching down within a second one from the other, so the weight would not have much time to rest on the rear units anyway.

Well, that's just an academic discussion of course, but I think, the maximum strain for the
landing gear at the moment of touching down. There's still plenty of aerodynamic lift, of course,
so it has not to carry the full weight, but has to carry the dynamic load of the descending aircraft,
too. For such an aircraft there probably won't be carrier landings, but the rearward landing gear
probably would have to take a bigger load, than the forward one. And as you mention, that there
are two units nder each wing: Not always both are used, as shown in the photo (from http://www.toegel-web.de/fliegen/Seitenwind.html),
so the rearward units are much more likely to have to take the full load, something not very realistic
for the forward ones. But again, just a very academic discussion, maybe the best solution would
be, just to delete the forward landing gear and built the rear one sufficiently strong.

Plugs fore and aft of the wing is how to increase the length (for any aircraft regardless of engine location, see C-141B). Just look at all the planes in the past. Wings were modified in some cases for increased lift. Landing gear are also modified, even to include adding more (see DC-10).

http://www.boeing.com/boeing/commercial/out_of_production.page?

I hate to add harsh reality to this interesting thread, but here are some points to ponder.

The rear wing in a tandem wing arrangement works in the wake of the front wing. We would expect the rear wing to differ from the front wing, at least in its angle of incidence on the fuselage, and probably also in area, planform, and airfoil.

The load on the main gear in a worst case touch down can far exceed aircraft weight, as Jemiba pointed out. The design point for the main gear is usually the worst case touch down sink rate. Compare the F-18 main gear to the main gear of a business jet or small airliner of about the same weight.

The further the aft-most main gear is from the c. of g., the more elevator force you are going to need to rotate at lift off and while landing. This is one of the main draw backs of some recent flying cars, that use a car like tire geometry with the back wheels well behind the c. of g. There are two ways to over come this: both have drawbacks, and some of the flying cars seem to combine the worst of both solutions.

1. Increase the rotation speed, both at lift off and touch down. This results in longer required field lengths. Not much of a problem if a flying car field length is doubled from 1500 feet to 3000 feet, but more of a problem when an airliner field length doubles from 5000 feet to 10,000 feet.

2. Fit a bigger elevator and horizontal stab, capable of creating greater downforce for rotation at a lower airspeed. This is needed for just a few seconds at the beginning and end of each flight, but then you carry the dead weight and drag with you all the rest of the flight.

Bill Walker said:

I hate to add harsh reality to this interesting thread, but here are some points to ponder.

The rear wing in a tandem wing arrangement works in the wake of the front wing. We would expect the rear wing to differ from the front wing, at least in its angle of incidence on the fuselage, and probably also in area, planform, and airfoil.

Click to expand...

That why there are only biwings and not tandem wings.

Bill Walker said:

I hate to add harsh reality to this interesting thread, but here are some points to ponder.

Click to expand...

Quite the contrary, Bill, such educated and informative opinions are needed to bring some solid sense into our musings!

Thanks a lot for your contribution.

The problem with touchdown bending strength depends on the lever arm of the fuselage. The higher your fuselage, the stronger it is for bending.

There has been no widebody with tail engines only. But there almost was. BAC 3-11.

A300 was, for some reason, preferred to BAC 3-11. But A300, designed for initial length about 55 m, did get stretched to 75,3 m of A340-600. With 2 extra engines, too.

How about a BAC 3-11 stretch, with third engine in the tail, to match the length of A340-600?