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Pressure Thrust- The Future of Aviation?

By David Birkenstock
December 30, 2005

A nonstop, nonrefueled widebody jetliner flight around the globe?
An under-explored synergy that’s currently in development could finally make this goal reasonable…


Engineers have done fantastic things improving engines and airframes independently, but to generate aircraft with greater fuel economy we will need to combine airframe shapes and engine power in synergistic ways that are greater than the sum of its parts.

Since the very first scientific investigations into maximizing an aircraft’s performance, efforts to achieve that goal have been divided into two mostly independent specialties; thermodynamics and aerodynamics. The ‘engine people’ work to decrease engine fuel consumption and the ‘aero people’ work to decrease airframe drag.

It’s important to emphasize that this division is entirely manmade and is left over from aviation’s very beginnings, when such simplifications were necessary. It makes for convenient illumination of each subset of the bigger picture, but it also acts as a very effective limit on understanding and exploitation of synergistic interactions between airframe and powerplant.

Nature demands no such isolation. To the contrary, Nature's measurement of a machine’s performance, along with all of our various mathematical models of it, combines the two into one basic ratio of power efficiency: work done per unit energy. Cars and trucks have MPG, sailplanes have glide ratio and jets have the range factor section of Breguet’s Range Equation.

Some research has been done on ways to design an airplane without this forced disconnect between airframe and powerplant. For years, NASA and others have done research programs focused on something called Laminar Flow Control. By minimizing draggy turbulent-flow, the programs were highly successful from a fluids perspective, but those gains have yet to be realized in real-world flying due to prohibitively high system operating costs.

With the inexorable rise in the price of oil, the time is right to search for other methods of saving fuel. This article focuses on one such method: Pressure Thrust.

Recently, leading scientists have recognized the decreasing returns from ‘traditional’ aero research and the increasing attractiveness of less-common approaches to reducing fuel consumption. In 2000, NASA Langley chief scientist Dr. Dennis Bushnell wrote:

“In point of fact, Fluid Mechanics has not been particularly productive in the aero overall systems sense for decades. Almost all of the considerable improvements in aircraft performance since the early '70s were due to propulsion advances, primarily higher bypass ratio. As an example, the lift-to-drag ratio of transport aircraft has been nearly moribund since the early '60s.”

It’s worth noting that Dr. Bushnell was instrumental in uncovering this phenomenon, he was first to use the term “pressure thrust” and is a leading proponent of the ‘open thermodynamic’ approach to aerodynamic design, where energy is added inside the control volume. In traditional aerodynamic designs there is no energy added.

By removing the artificial restraint on exactly these types of aero/thermo synergies, major increases in aircraft efficiency are made available—some have been explored experimentally, and some have not yet even been conceived.

‘Open Thermo’ and Pressure Thrust

In terms of basic physics it remains quite simple: to accelerate any object or body, push it or pull it. In terms of aircraft design, it’s not much different. Engines traditionally push or pull, but with few (mostly experimental) exceptions, engines are rarely integrated into the airframe for the purpose of exploiting synergy between the two.

Pressure drag can be described by comparing the pressure acting on the rear of a body, with the pressure acting on the forward section of the same body. In the ‘open thermodynamic’ design philosophy, energy can be used to increase this ratio directly. What’s more, a clever design can increase both the pressure coefficient (Cp) of the aft section and the surface area on which that increased Cp will push.

Even if new designs can’t improve on the demonstrated stagnation region Cp of 0.4 – 0.6, the increase in available surface area will generate increased pressure thrust over that proven by Goldschmied/Bushnell. Indeed, the whole aft body portion can act as a ‘sink,’ with an increased pressure coefficient pushing the aircraft forward (traditional closed thermo aft stagnation Cp are usually at or below 0.2 and act on only one point).

What about Empirical Proof?

Tests performed on the Griffith Aerofoil and the Goldschmied body (see references below) both prove the merits of pressure thrust, but they neither sought to bring this proven benefit to current aircraft nor did they complete an exhaustive investigation of the phenomenon. A complete ‘top to bottom’ overview is (slowly) taking shape, and an aircraft designed to exploit this phenomenon is also evolving.

Making sense of how pressure thrust relates to aircraft performance requires some minor changes to the classic range equation.

Drag is traditionally made up of skin friction drag, induced drag (or drag due-to-lift), and pressure drag. Friction drag can be minimized by natural laminar flow shapes or an active laminar flow control system, induced drag can be minimized by winglets, and pressure drag is turned into thrust (negative drag) by the current combination of airframe and powerplant called pressure thrust.

A Computational Fluid Dynamics analysis of surface pressure on a modified blimp shape.

The above CFD image shows a large area of blue high pressure on the concave aft surface (left side) and pink very low pressure just in front of the suction slot. The greater force of pressure on the rear overcomes the lesser force of pressure on the convex forward end (right) and generates the “negative drag.” Of course, drag won’t actually be negative because friction drag has not gone down at all (it actually goes up). To be accurate, drag needs to be recalculated without pressure.

So instead of leaving the effects of pressure in the drag math where they were, the amount of pressure thrust and the fuel-flow required for the suction are included into engine performance as Thrust Specific Fuel Consumption (TSFC is defined as the amount of fuel required to generate one pound of thrust for one hour).

Why Bother?

Unlike active laminar flow control, which uses millions of microscopic openings to suck away ‘draggy’ turbulent flow, pressure thrust doesn’t suffer huge system costs and losses when operated in real world environments (like commercial airline service) and will bring comparatively large savings to myriad real world aircraft. With no more fuselage pressure drag and decreased overall TSFC, the performance of the resulting aircraft will exceed that of an unmodified ship.

From jetliners to turboprops and even the smallest piston planes, the traditional fuselage tailcone can be replaced with one housing a pressure thrust engine. The modification will save at least 20% of an aircraft's total fuel flow (assuming current technology can’t improve on what was done in the 1940s and '60s). Recent CFD tests have indicated that modern technology can improve on what’s been proven in those wind tunnel and flight tests, so the total savings could more than double, possibly more than triple.

Here’s a crude and exaggerated image of pressure thrust added to an Embraer 135 fuselage, which would use a suction engine of roughly 2,800 lbs thrust. Once at cruise speed most of the required thrust would be pressure thrust, so the main engines could run near flight idle.

Takeoff, climb and cruise performance.

With the added power of the suction engine in the tailcone, takeoff performance should improve some unless the operator would rather keep the same field length, in which case the main engine target fan speed would be set lower than the unmodified ‘standard power’ takeoff setting. Granted, the suction engine won’t put out its fully rated thrust because of losses in the ducts, but it’ll still be pushing somewhat.

At rotation and initial climbout speeds, the amount of pressure thrust generated will be at its lowest point in the flight, but it will still increase the total thrust available in the case of a main engine failure. Similarly, a failure of the suction engine won’t cause as much of a performance loss as a main engine failure would.

Since the amount of pressure thrust will increase with speed, the modified ship can cruise-climb at much greater airspeeds and climb rates than the unmodified version. Or, using current climb speeds and rates, the fuel required for the climb would be greatly diminished. With the main engines making less power than normal, their life cycle maintenance costs would also decrease.

Despite the added weight of the suction engine and the associated hardware, the payload of the aircraft will go up for flights over a certain stage length due to the reduced fuel requirements of the modified ship’s now-greater range. It might not help much on the short flights of some regionals, but on legs over roughly 90 minutes the savings should be significant.

What about that widebody flight around the world?

Essentially, this turns the entire tailcone of the aircraft into a powerful and very fuel-efficient engine. Given the wind tunnel and flight tests done in the past and the current computer modeling, it is entirely possible that a long-range widebody airliner modified to exploit pressure thrust could see a cruise power reduction of over 50%. With the recent 777-200LR flight of 11,664 nautical miles it’s not unreasonable that the same ship, with pressure thrust, might repeat the feat accomplished by only Voyager and the Virgin Global Flyer: a nonstop, nonrefueled flight around the world. Surely, these developments won’t come soon—much hard work still needs to be done—but it’s not an unreasonable goal.

Summary: It’s all about synergy.

Nature measures any mechanism by its efficiency. To generate an aircraft of maximum efficiency, it’s necessary to have a solid understanding of how to make shapes aerodynamic and engines fuel efficient, but independently these can only take us so far. To achieve the goal of maximum efficiency, synergy between airframe and powerplant is required.

As astonishing as it sounds, it’s true: current transports are not designed for this goal. They have fantastically optimized engines and equally perfected airframes but the whole is only equal to, not greater than the sum of its parts.

By exploiting airframe-powerplant synergies like pressure thrust, engineers are free to attack the issue on Natures terms, directly.

Click for large version
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Photo © Zdenek Ondracek

An incomplete list of references:

1. Goldschmied, F.R., “Airfoil Static-Pressure Thrust: Flight-Test Verification,”
AIAA Paper 90-3286-CP, September 1990. (available from http://www.aiaa.org/)

2. Goldschmied, F.R., “Fuselage Self-Propulsion by Static-Pressure Thrust: Wind-Tunnel Verification,” AIAA paper 87-2935, September 1987. (available from http://www.aiaa.org/)

3. Richards, E. J., Walker W. S. and Greening J. R., “Tests of a Griffith aerofoil in the 13 ft. x 9 ft. wind tunnel part 1, part 2, part 3, part 4, lift, drag, pitching moments and velocity distributions,” ARC/R&M-2148 ARC-7464 ARC-7561 ARC-8054 ARC-8055, 1944. (available for download at http://naca.central.cranfield.ac.uk/reports/arc/rm/2148.pdf)

4. Richards, E.J., Walker, W.S. and Taylor, C.R., "Wind-Tunnel Tests on a 30% Suction Wing," A.R.C. RBM 2149, July 1945. (available for download at http://naca.central.cranfield.ac.uk/reports/arc/rm/2149.pdf)

5. Richards, E.J. and Burge, C.H., "An Airfoil Designed to Give Laminar Flow Over the Whole
Surface with Boundary-Layer Suction," A.R.C. RBM 2263, June 1943. (available for download at http://naca.central.cranfield.ac.uk/reports/arc/rm/2263.pdf)

6. Bushnell, D.M., “Fluid Mechanics, Drag Reduction and Advanced Configuration Aeronautics”
NASA/TM-2000-210646 Available for download at:

Written by
David Birkenstock

David Birkenstock is a pilot for an east-coast charter and management firm, flying the TurboCommander 690B and the BeechJet 400A. When he's not flying, he's working on modifying a light sport airplane for pressure thrust, hoping to enter it into the NASA Personal Air Vehicle Challenge. He has posted more information about pressure thrust at PressureThrust.com.

5 User Comments:
Username: Efranrider [User Info]
Posted 2006-01-22 11:34:03 and read 32768 times.

With the recent addition of so many new models to the overall fleet, are there any planes that aren't adaptable to such changes?

Username: Kuhndogg [User Info]
Posted 2006-01-24 02:27:27 and read 32768 times.

WOW!! An easy way to bring troubled airlines to Freedom!! :)

Username: DBirkenstock [User Info]
Posted 2006-01-24 05:14:15 and read 32768 times.

Sure, some are not workable… but most civil transports, from the Eclipse 500 to the A380, are excellent candidates.

Scientists would say any aircraft that suffers from pressure drag can enjoy the benefits of pressure thrust, but the engineers would say that some real world problems are deal-breakers. For example, military transports must open at the rear, jet fighters are just too small. Even some widebody jetliners on short-haul trips (like the inter-island 747-400s used in Japan) might not be in the air long enough for the benefits to offset the added weight, complexity, etc. Ironically, the really high performance ships like the U2/TR-1, Voyager and the Virgin Global Flyer are perhaps the worst candidates because their fuselages are so small and their wings so long and thin.

I was going to use the Global Hawk as another not-very-workable example, but it too could benefit if the fuselage cross section were increased. Even if a simple shell is added over the existing Global Hawk fuselage, pressure thrust would bring a big increase in propulsive efficiency and by extension endurance or payload, or both. The convex curves on the tail would not be at all stealthy, but that’s just yet another trade-off for the designer(s) to manage.

Thanks for asking.

Username: OldAeroGuy [User Info]
Posted 2006-01-26 15:23:25 and read 32768 times.

To evaluate your idea, you need to provide more detail on how the suction engine proposal operates. How does it increase the pressure on the aft body, where is its inlet, where is it mounted etc..?

I can't detect any of these items from your verbal description or drawings.

Username: DBirkenstock [User Info]
Posted 2006-01-30 16:35:52 and read 32768 times.

The aft body pressure is increased by the combination of boundary layer suction, concave tailcone shape and forward speed. Neither suction, nor concave shapes will do much on their own but when combined they work together, creating a mechanism greater than the sum of its parts.

The suction inlet is located just before the beginning of the concave shape. In the CFD image of the modified blimp, the inlet is at about 75% of length. In that image, the blimp is moving from left to right as are the Embraer 135s. The top fuselage has been crudely modified with this type of convex tailcone. The suction engine itself is located essentially where the APU is on the un-modified ship.

Just like in the blimp analysis, the combination of suction, concave shapes and forward airspeed of the aircraft act in concert to dramatically increase total fuel efficiency. This is not any kind of perpetual motion thing because total drag is still positive and the suction system uses energy.

In short; we’re talking about propulsive efficiency. Pressure thrust has proven much more efficient than traditional propellers or turbofans because it combines engine power and airframe into one unified whole. Instead of using a comparatively large amount of fuel to propel a fuselage with lots of pressure drag, the idea is to use a bit of fuel along with the fuselage shape itself to eliminate pressure drag and replace it with genuine thrust, further reducing total power requirements.

Did I answer your questions?

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