Modern Biplane and Triplane Jumbojets

[MadRat]

Any reson why they don't use biplane and triplane arrangements for the mainwings in modern jumbojets? Tall justs like the Airbus 300 and the 747 could both have easily been designed around biplane arrangements to halve the length of the main wings. Likewise, taller jets with triplane arrangements could be used to reverse the trend for jumbo jets to have those enormous wingspans. I can find no aerodynamic reason other than an aesthetical reason why jumbojets have never fielded multiple levels of main wings.

 

[Yomicron]

Perhps bi and tri wing configurations are not used because the wings sway up and down, which could be a problem if multiple wings are stacked atop each other? Also, the air flowing over the lowerwing could hit the bottom of the upper wing, which would be ineffecient.

Boing and Lockheed Martin have been working on box wing aircraft though.

 

[pm]

Since my other hobby (other than computers) is R/C airplanes, aerial photography, and recently, model airplane design, I could guess. But I'd recommend asking your question again, here. Overall a very knowledgeable group.

My guess comes down to speed/velocity. Biplane (and multiple wings) create a lot of lift at the expense of adding a lot of drag. Early plane designs had relatively small power systems that generally created low-speed thrust by moving a relatively large prop at a relatively low speed. For comparison look at a vintage biplane's prop diameter and compare that to the size of the intake diameter on a modern jet plane engine. They aren't that different despite the massive change in scale. So a lot of low-speed air was moved limting the maximum speed of the aircraft to a fairly low speed - early biplanes travelled at ~50mph and some were as "slow" as 35mph . This low speed required a lot of lift and the effect of drag was less pronounced due to the relatively low airspeed. Contrast this will a modern jet which creates a lot of high-speed air by moving a relatively smaller prop (in terms of surface area) at a vastly higher rpm. This creates a lot of very high speed air moving the plane at a substantially higher speed. Drag becomes more pronounced and less lift is required due to the airspeed. I would imagine that bi-wing planes start to become a losing proposition around 80mph.

Edit: after reading some more, I found that the main point of the biplane was structural stability given the materials. Here's a link. And I found that biplanes could go as fast as 230mph. So my guess appears to be mostly incorrect. Although I don't think I'm that far off when I think about what biplanes are used for in modern times (low speed, high manoeuverability).

 

[MadRat]

The box wing is sort of a biplane, true, buts its designed around maximum strat strength for the amount of wing area. Connecting the wings together adds alot of vertical stability to the aircraft, which is a big plus for concepts like heavy-lift transportation. The main drawback is that the pilots cannot just jump in and fly one without retraining due to the radical departure from a conventional design.

The long single wing design is a triumph of materials like you said, pm, but it is not necessarily more efficient as speeds increase. Louvre designs for missile technology has been used for high manueverability and lower drag due to their ability to use thinner guide wings. The idea of a feathered wing tip has always been shown to be more efficient because of its ability to make airflow more orderly. The feathers of a bird's wingtip are the ultimate in efficiency with their ability to stagger at varying angles when the wing is outstretched, creating a great deal of orderly airflow which in turns smooths the airflow off the wingtip and lessens the drag. A straight wingtip actually creates drag due to the airflow separation which creates a negative pressure behind the wing, thereby adding drag.

I think alot of the appeal of a single thick main wing is because of tradition. Aerodynamic experts have traditionally used overwing airflow for their designs. Airbus has claimed that subsonic airflow is best for subsonic speeds while overwing airflow is better for supersonic speeds. Neither design was better than the other at the speed of sound due to the unique instabilities at that range of speeds. Most of the airflow in front of and behind a sonic pattern is unstable, which is the likely reason neither design would win out at that speed.

My guess is that multiple wings would increase the amount of lift for the limited amount of space in width while at the same time possibly reducing lag by ordering the airflow off the wings in a more pronounced way than a typical airplane design. Because more wings would be used the main wings could be thinner, thereby decreasing overall drag in the design. As an added benefit the biplane flies like a conventional design requiring no new pilot training.

 

[dkozloski]

Jumbo jets don't need more wing area which is the only conceivable reason for multiple wings. The most efficient wing design is the longest and most slender wing(high aspect ratio) that can be designed without too many structural compromises. Any wing area that exceeds that which is required to support the aircraft is just added drag and weight. As far as that goes, anyhing on an aircraft that is not absolutely required for function results in a loss of payload and is to be avoided.

 

[charrison]

Originally posted by: MadRat
Any reson why they don't use biplane and triplane arrangements for the mainwings in modern jumbojets? Tall justs like the Airbus 300 and the 747 could both have easily been designed around biplane arrangements to halve the length of the main wings. Likewise, taller jets with triplane arrangements could be used to reverse the trend for jumbo jets to have those enormous wingspans. I can find no aerodynamic reason other than an aesthetical reason why jumbojets have never fielded multiple levels of main wings.

Well if you want a big plane, take a look at this:




The boeing Pelican concept

 

[MadRat]

Originally posted by: dkozloski
Jumbo jets don't need more wing area which is the only conceivable reason for multiple wings. The most efficient wing design is the longest and most slender wing(high aspect ratio) that can be designed without too many structural compromises. Any wing area that exceeds that which is required to support the aircraft is just added drag and weight.

True if you select a single wing design, but not necessarily if you go with multiple wings. The raw volume of displacement also adds drag which is why multiple wings that stress thinness would offset the efficiency of the single slender wing. The most ideal wing is not only slender but it is also an impossibly high chord, which is limited by the materials used in its construction.

The real benefit of a single wing, and you can test this mathematically, is the minimal amount of surface area of the wingtip surface. Multiple wings create more surface area of the wingtip than a single wing. Realistic material limitations also decrease the chord of the wing, thereby making it equally difficult to make the single wing as efficient as an ideal wing. The problem with trying to model a miniature design to test this out is the idea that your materials would mimic what is possible with full-scale materials; models tend to be overweight by scale because they are much more dense than a full-scale project. The multiple wing design would increase the chord of each individual wing to offset the negative aspect of going multiple wings.

Orderly airflow is also a direct result of using multiple wings whereas one large (wide, not necessarily thick) wing creates separation of the airflow due to multiple vortexes along the surface of the wing. The ideal wing is mathematically designed for its high chord without any respect for the separation of airflow due to the width of the wing. Bird wings have feathers which help smooth the airflow over the wing and to act as thousands of trailing edges which smooth out the airflow. A large, single wing design is going to require some sort of slotted pattern of surfaces above it to decrease vortexes, which just made the surface area of perpindicular drag increase. If you are going to do that then you just made that single high chord wing function like it has a low chord!

The real reason behind my question is the relationship that as the size of the plane increases, the list of airports that it can operate decreases. Few commercial airports can already handle the largest of the 747s due to their width requirements, not necessarily for the length of the runway or the weight of the plane. Using louvre designs would decrease the width of the plane to accommodate use in more airfields across the globe.

If you really want to play with the aerodynamics experts then introduce the magnus effect. (Effect of a spinning tube held perpindicular to the airflow.) To a certain point you can use tubes, rather than flat wings, spinning so that its spinrate controls the lift of each "wing". This flies in the face of the traditional designs and is absurd to most of them because it is unthinkable! But realistically its more efficient use of space to design an active wing like this than a fixed wing but you will never see it happen.

 

[Armitage]

Subsonic wing efficiency is directly related to aspect ratio. With an infinitely long wing (or a finite wing spanning between two walls) you have 2D flow. As soon as you have a finite wing, high pressure air spills over the wingtip to the low pressure area above the wing. This is the wing tip vortex, and it accounts for a large portion of the pressure drag on a wing. So, a long tapered wing is the most efficient.

On top of this, two wings interfere with each other giving a lower L/D together then the sum of the seperate wings. You ideally want your wing flying in undisturbed air. If you have another large lifting surface in the vicinity, it screws up the airflow.

Another issue is that those huge wings are really only needed at low speed, especially takeoff & landing. That's why airliners have those huge flaps & slats that unfold during takeoff & landing ... to increase wing area. At cruising speed you don't need as much wing.

That said, long wingspans are certainly problematic, both structurally & operationally. One interesting concept is the Joined Wing design pioneered by Dr. Julian Wolkovitch. This desgn has a rear-swept, dihedral lower wing, joined along the span to a forward-swept anhedral wing that typically ties in to the top of the vertical stabilyzer. The forward/reverse anhedral/dihedral combination tends to reduce the mutual interference of the wings, and tying them together aids significantly in the structural rigidity. It's starting to be seen in some UAV designs, but I wouldn't count on seeing any airliners like this anytime soon. Airliner design is very conservative. The investment to roll out a new class of airliner is enormous. It's a "bet your company decision". And that's just for eviolutionary designs. A radically new design, with all the additional R&D involved is even bigger.

 

[dkozloski]

Madrat, the runway only needs to be wide enough for the aircraft's tires and some margin in width for the pilot to keep them on it. The airport where I work, Fairbanks International Airport, has runways that were built in the 1950's and have ben increased from 5000 ft. to 11,500 ft. to accommodate freighters going over the pole to Asia and Europe. Today we had an AN-124 which is the largest commercial aircraft in the world stop by to refuel. We have had Concordes, C-5 Galaxies, and Conroy Super Guppies here, all with no width problems. Maybe Podunk, Iowa, the hub of the universe, might have problems handling jumbo jets but C-5's are designed to operate out of relatively unimproved strips. I have seen a Lockheed Hurcules operate off an aircraft carrier. Width is not the big issue. The biggest annoyance here is that on some taxiways the engines overhang the sod alongside and tear up the grass and put a lot of dirt in the air with jet blast.

 

[charrison]

Originally posted by: dkozloski
Madrat, the runway only needs to be wide enough for the aircraft's tires and some margin in width for the pilot to keep them on it. The airport where I work, Fairbanks International Airport, has runways that were built in the 1950's and have ben increased from 5000 ft. to 11,500 ft. to accommodate freighters going over the pole to Asia and Europe. Today we had an AN-124 which is the largest commercial aircraft in the world stop by to refuel. We have had Concordes, C-5 Galaxies, and Conroy Super Guppies here, all with no width problems. Maybe Podunk, Iowa, the hub of the universe, might have problems handling jumbo jets but C-5's are designed to operate out of relatively unimproved strips. I have seen a Lockheed Hurcules operate off an aircraft carrier. Width is not the big issue. The biggest annoyance here is that on some taxiways the engines overhang the sod alongside and tear up the grass and put a lot of dirt in the air with jet blast.

The problem really does not exist with freighters, but commercial passenger planes.
They are packed tightly into the terminal where plane size does matter. Take the planned airbus 380, it will require major terminal modifications at the servicing airport along with widening of runways.

 

[dkozloski]

I suppose that one could make the argument that any terminal with waiting areas that would hold 5-600 people would have to have enough space to accommodate practically any size plane. Once again the wingspan might not be that big a factor.

 

[MadRat]

charrison alluded to the idea I was aiming at, that larger passenger planes will require a modification of alot of existing airports to service them. There is a demand for larger commercial transportation in the future. Don't let the recent downturn from 9/11 fool you. The 747 will some day look like a twig compared to some future monolith of a plane. I just presume that the vertical restrictions of aircraft design have been shunned since the convential monoplane became standard practice. Let me remind you that there was a day when monoplane design was deemed impractical.

ergeorge brought up an interesting point about the flaps. To some extent that is true, but that is not necessarily why they currently choose wings of a certain size and shape. Modern designs revolve around practical construction of the wings and the limits of the materials and their cost to assemble. I think multiple wings would actually be cheaper to make since the wing design would be simplified and exotic matierials wouldn't be necessary. There is no reason you'd have to worry about the multiple wings causing alot more drag then single wings because the multiple wings would have practically identical volume as a large single wing if done for maximum thinness.

Flaps on those planes require bulky hydraulic gear and add alot of weight to the wing but are necessary for low speed control because the bullet designs of these planes make them difficult to handle when slowing down. Less weight could be used if helicopter blades were mounted on the top of airliners for use in takeoffs/landings - you could also mount an overall much smaller wing for flight. The blade (using active lift) would spin up before takeoff then slowed down (to reduce drag) when the boost in lift was no longer necessary while cruising. Come to think about it, a magnus effect tube would be simpler to mount than a rotating blade. A simple scissors wing would be even easier yet. In any case, its not likely to happen. Flap designs are tradition. But remember, modern biplanes use flaps, too.

OT:

C-130's off a carrier? I suppose if you strapped on some AIM-9 boosters you can make just about any plane take off a carrierdeck. Sounds awfully dangerous nonetheless.

 

[dkozloski]

Operation of a C-130 off a carrier deck is in some ways easier than from dry land. The only accommodation made is to paint a line on the flight deck to indicate where the nose wheel should be to provide clearance with the island by the wing. JATO bottles, which are used in many dry land operations, provide the accelleration for takeoff. On landing the spin-up loads on the landing gear are reduced by the forward speed of the carrier into the prevailing wind which can give wind speeds over the deck of about 50MPH. On dry land I have seen civilian stretched C-130's land at Fairbanks International and make the first turn-off at 1500 ft. with no wind at all. An 1100 ft flight deck traveling at 46 knots is duck soup.

 

[Bleep]

I have not seen anyone posting to this thread mention aspect ratio which is very important in stability. Low aspect ratios such as found on bi-planes are very unstable, that is why a Pitts bi-plane is so areobatic. Just look at the U2 a very high aspect ratio, stability and lots of lift at high altitudes. Bi-Planes not good at high altitudes. Just simple as that.

Bleep

 

[Talon]

C-130 landing and takeoff from USS Forrestal w/video