Bjorn’s Corner: New aircraft technologies. Part 14. Airframe for lower friction drag

By Bjorn Fehrm

May 26, 2023, ©. Leeham News: This is a summary of the article Part 14P. Airframe for lower friction drag. The article discusses in detail a Blended Wing Body (BWB) type airframe and how it reduces the wetted area and, thus, air friction drag compared to a conventional tube and wing airframe.

Figure 1. JetZero Z5 250 seat BWB jet concept. Source: JetZero.

An airframe for lower friction drag

Last week we concluded that there were two ways to reduce the dominant drag component of an airliner, the air friction drag:

  • You can reduce the wetted area of the aircraft through efficient packaging or reduce the drag coefficient by increasing the amount of laminar flow.
  • Of the two, increasing the amount of laminar flow has proven difficult, as daily contamination with insects and dirt requires frequent washes to keep the aircraft leading edges from turbulent flow-tripping contaminations.

An aircraft architecture that reduces the wetted area is the Blended Wing-Body, BWB. This is the thesis of the JetZero upstart that proposes their Z5 BWB for carrying 250 passengers in Domestic two-class up to 5,000nm, Figure 1.

The passenger cabin is configured as three cabins side by side, as shown in Figure 2. We put the Z5 in our airliner performance model and looked at the drags and weights.

Figure 2. JetZero Z5 BWB concept with cabin principle. Source; JetZero.

To do this, we used the Z5 overlaid on a Boeing 767-200 from JetZero’s website, Figure 3, whereby we could deduce the dimensions.

Figure 3. JetZero Z5 overlaid on a Boeing 767-200. Source: JetZero.

We could conclude that the wetted area, and thus friction drag is lower, and the airframe architecture reduces the weight of the aircraft, but with did not reach JetZero’s values of half the weight of a 767-200 to fly 250 passengers a 5,000nm sector.

With the reduced wetted area and weight and the increase in span, the drag when flying the 5,000nm sector was substantially reduced.

It makes a BWB like the JetZero Z5 a very interesting architecture if issues such as evacuation times and passenger satisfaction can be solved.

33 Comments on “Bjorn’s Corner: New aircraft technologies. Part 14. Airframe for lower friction drag

  1. I remain sceptical as long as I don’t see real numbers (m² of skin per pax, kg of OEW per pax) but just futuristic pictures. The planform picture doesn’t show a real gain in projected area, but the frontal area would be much larger.

    The 767-200 is already an old airframe, the 787 would be a better comparison point, but for a 255pax/5000nmi it would have a significantly smaller wing as it is designed for a 255t MTOW for a 300 pax/7600nmi mission (and 259t tomorrow).

    Plus with such a short lever arm it would need enormous pitch control surfaces. I don’t see passenger satisfaction as a big problem though, it’s like being in the center bench of a current widebody. Lower fares are a good incentive.

    A double deck could have been a good packaging solution, but it obviously seems to be too big for the market.

  2. Seems very optimistic that they can reuse the A321neo engine of 34 000 lb T-O thrust for this 255pax/5000nm aircraft. Looks like they might need long runways and rotate at higher speeds. The 767-200 needed 52 000 lb engines, so having to compensate for the lack of 2 x 18 k thrust = 36k is alot. Still the US and EU has lots of airports with long runways and the 737-10 needs a bit more than todays A320/737. So reducing T-O mass and thrust with much shorter range might save the day? The USAF has long runways for the B-52’s so as a stealth taker it might fit better.

  3. electronic “windows” i.e. configurable screens on the sidewalls that offer a variety of real world and virtual views, bright colors and lighting will mitigate any concerns over no windows. everybody is watching videos these days, very few people looking out the window for more than a few minutes.

    on almost every flight I’ve been on recently better than 70% of the window shades have been pulled down anyway.

    it is time for a clean break from the traditional tube and wing designs.

    • That is my take as well. I would guess 50% of the people at the windows pull the shade and never look out. My only guess is no interruption by other pax to get out. Flip is you have to crawl over or get two people to move (I have found its easier to take the aisle seat and just move for my row as needed)

  4. Why use the 767-200. Airlines are using A321s and 737-9/10s to do the same job. How does the BWB compare to today’s NB?

    • The 767-200/-300 (non ER) was designed for the same mission. The A321XLR is getting closer. A new company designing a new type of aircraft usually underestimate empty mass, time to certification and cost. You need to add provisions in skin thickess for in service repairs like blends, corrosion and 2 oversize rivets besides manufacturing tolerances to get a 70 000 to 100 000 cycles/hrs cerrtified life and prove it in testing.

      • Should have paid more attention to the article A 250 pax plane, yet why compare it to a 767-200? Or as stated in another thread the 787? Unless the comparison is not as competitive.

        We know BWB work, the military has been and is flying them now. And not having the pax sit near the wings solves the excessive pitch the pax would feel.

        • william:

          BWB only in test articles.

          A 767 is the closest wide body comparison.

          • The military BWB probably means the B-2 and B-21. But those have only small ‘fuselage’ sections/pressurised cabin for the crew
            plus others spaces for electronics and weapons bay.
            And where does the baggage and cargo go ? Do they use main deck stowage like a turbo prop does?
            And box sections for the pressurised cabin shapes ? , that will be an interesting structural challenge to avoid weight that may negate the emphasis of reduced ‘wetted’ area
            Circular type shapes take advantage of a strong floor beam

          • Easy to be competitive against a 767 solution. ( just look at why B was staging the 787 vs the 767 )
            Look to the A300 … A340 cross section. for a comparison.

            How will BWB fuselages fare under a high cycles environment? Much stiffer structure creating higher load peaks.

  5. The percentage friction drag in the previous post was for a single aisle aircraft. I think we cannot assume it’s the same percentage for a BWB. I estimate the percentage friction drag for a BWB to be below 40%. The percentage of other forms of drag, (induced, pressure) are probably higher. Induced because of higher weight for the same (but flat) cabin volume, pressure because of the larger frontal area and lower wing aspect ratio.

  6. Even a BWB of 250 seater will bring constraints that we all know

    It takes a small BWB 1-class 100-110 seats flying 5.400 Nm IHMO.
    Folding wings for the same ICAO code 737/A32X wingspan

    A Freighter version with additional tanks at the rear to fly 5200 Nm with a quantity of Freight close to a 737-700F, and 4200 Nm for a Freight capacity greater than a 737-800F

    Airbus A230, Embraer E-X, or Boeing 808, which will do it first?…

  7. Knowing my limitations, I leave the tech specifics to Bjorn, he is the professional in that field and he has the tools to do the calcs.

    One of my questions has always been the moment arm where the further you get from the center, the more distance you move when you make a turn. I would like to see testing done in that area. It does not have to be a real BWB, just a mock up.

    The other of course is evacuation.

    I think its a very viable design for a freighter (floor loading would be interesting) and the military might well use it as a transport as the troops would be/could be trained for it.

    Having seen a Blackhawks turn itself on its side 90 deg to the ground carrying troops, clearly they love the thrill (its forbidden but I had a wood cutting permit on base and saw it several times). Granted not all troops are as nuts as young infantry but it would also be controlled motion.

  8. Looking at the graphic it seems more like an extreme flattened oval fuselage with some blending to what might be normal wings- and a tail less design but with vertical tail surfaces at the end of wings.
    Maybe the subscriber version goes into more detail and the possibility or not of some features

    • Duke:

      Unless the USAF changes setup (they could of course) cargo hold on current is pressurized and they carry troops as well as hospital setup for causality evac.

      I assume that is taken into account as its been discussed for passenger carry.

      • USAF uses circular or deep oval-egg shapes for its transport -tanker types. The heavy transport type doesnt have a mid fuselage floor beam but it doesnt have to compete economically with optimised passenger aircraft.

        The new Bomber design has a different approach because of radar cross section/reflection not the lower friction drag

  9. The other issue is the pressure shell. As you deviate from ideal shapes, the stresses grow and you need more structural material, leading to heavier aircraft.

    That’s why the USAF is focused on unpressurized applications (tanker/cargo) which don’t have that penalty. I would expect those to be developed first.

    • Rob,

      I think the wide cabin of the BWB could easily lend itself to the use of vertical structures in tension to alleviate the stress increase due to the oblate shape.

      • Agreed that the stresses can be mitigated, my point was that doing so adds weight to the structure.

        The ideal shapes are used for pressure vessels because they are the most structurally & space efficient. They require few structural members outside the pressure shell itself.

        • carbon fiber does extremely well in tension. likely any practical design would use carbon “cables” between top and bottom pressure skins which would be shaped like sections of a circle for structural efficiency, then there would be a non-structural fairing (as used for MLG and wing root fairings on current tube and wing designs) to give the smooth outside shape.

          the use of cables would make for a much more open feeling interior, which would again help mitigate the lack of “real” windows.

  10. An interesting concept that I think shows promise. Like others here, I need to see more details before I could be convinced.

    I’m not too concerned about the flight stability of this configuration. Going back to the work of Reimar and Walter Horten in the early 1930’s, the static stability of tail-less/flying-wing designs is well established.

    • Different rules for military and passenger aircraft. Plus the design would be a highly compromised version of a pure flying wing to fit the passengers so end up with some sort of vertical tail surfaces.

      • Point is that maintaining the stability of a BWB configuration is not that difficult, or novel for that matter.

        • Commercial passenger planes have to maintain positive static stability without any FWB assistance over the design flight envelope not just the usual places
          My point was the because military aircraft designs ( with miniscule central cabin/fuselage,) have worked is no guarantee they will pass the purview of FAA/EASA. Having room for 200 passengers/baggage changes everything
          Boeing got into its MCAS problem because a very tiny portion of the flight envelope, which was extremely rare occurrence, needed some horizontal tail assistance.

          Mostly likely a tail will be somewhere, even on a drone size
          A blended wing body airplane with a close-coupled, tilting tail

  11. Are the three passenger compartments walled to reduce pressurisation?

    • Likely to create structural support and rigidity across the wide oblate sections. Pressurization would be forcing the sections apart, the walls would be in vertical tension to resist those forces.

      • Could you simply include 3 tubes in parallel instead of boxes to satisfy the pressurization load requirements? And store other equipment and/or systems in the unpressurized gaps? E.g., fuel tanks, etc.?

  12. Though it’s only a schematic illustration, Figure 2 still shows a thorny problem: in an emergency evacuation, 250 passengers trying to get out through a small number of exits (4), all located at the front of the plane.
    Exits in the floor will be a no-go, since they can’t be used in the event of a landing gear collapse.
    Roof exits require passengers to climb in order to egress.

    Try selling that to regulators.

  13. You can increase laminar flow % of wing area by increasing wingspan and reducing chord. Like on sailplanes. You can also design clever highlift devices for T-O and landing not needed in cruise flight keeping the slender thin wing by using stiff carbon fiber wings that can have active flutter suppressionand slim electrical acutators for flight controls. Using this “wide body wing technology” on a narrowbody and you are ahead of the competition on performance. Then you need to certify and produce it with high quality for low cost and at high volumes.

    • For obvious reasons no company is going to prioritise the ways sailplanes work for commercial powered aircraft as its a highly specific niche product- for elites
      In fact many of your suggestions – stiff carbon fibre wings , long span and narrow chord are out of the question.

      Best to check back and start with Bjorns multipart series on Aircraft Lift rather than pie in the sky claims

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