Pressure Thrust

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 man made 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.

Clearly, optimizing each piece of the aircraft individually works well, but the combination of an airframe and powerplant that are optimized to interact directly can produce a synergy impossible from designs not built around each other.

One such synergy is an aerodynamic phenomenon called pressure thrust. This is the focus of this project and is important because flight tests and wind tunnel tests conducted to date have proved a decrease in total power required of up to 50%, and this was with low-tech BLC suction equipment from the 1960s. With today's powerful CFD and CAD/CAM technology, this phenomenon seems to be 'low hanging fruit' in the hunt for ways to decrease aircraft fuel consumption.

The goal of this project is to optimize the use of this phenomenon for current-generation aircraft and maximize the fuel savings possible. This web page will give a brief summary of how this goal will be met.

Examples of stock and modified wing sections

The image above shows a typical supercritical airfoil (top), designed for low drag. Again this works well, but there is not much room for improvement. By integrating engine power via Boundary Layer Suction and optimizing external geometry, the aerodynamic pressure thrust force can be created. This modified geometry is shown in the bottom image. A similar setup will bring the APT benefit to non-supercritical airfoils.

How do designers exploit this?

Set target aerodynamic pressure thrust roughly equal to the skin friction of the body. Iterative design of the different parameters (e.g. concave stagnation area and camber or 'bentness') required to create the pressure thrust phenomenon will yield a roughly optimized configuration for airfoil and/or fuselage implementation, then sophisticated CFD runs of the modified geometry will give Boundary Layer Volume & pressure differential required to keep airflow attached across this geometry. When combined with suction pump performance charts, this analysis gives an estimate of power required for volume & pressure differential, and can serve as an approximation of total power requirements of the modified aircraft.

The Aerodynamic Pressure Thrust phenomenon is of comparatively little use at very low speeds, so total aircraft performance should increase if we can use engine power for traditional reaction thrust (either propellers, turbofans, etc.) for the lower speed flying and divert engine power to the BLC system once the aircraft has enough speed to enjoy meaningful benefits from Aerodynamic Pressure Thrust (APT).

To achieve this goal, a hybrid approach to aircraft design must be employed.

For propeller-driven planes:

A high-output generator can be attached to the main internal combustion engine and electric motors to the suction fan for the wing, empennage and fuselage APT installation. This will allow the main internal combustion engine to power the suction system as well as the propeller. Current constant-speed propellers will maintain target RPM despite propeller loading that will vary as engine power is either sent to the generator / suction system or not. A preflight check not unlike those for other systems can be accomplished with ease.

During takeoff the generator/fan will not be engaged, allowing 100% of engine power to drive the propeller, which will accelerate the aircraft faster than if engine power were being sent to the suction system.

At some optimal combination of speed and altitude, the generator will be engaged and the suction fan will begin to operate and the Aerodynamic Pressure Thrust phenomenon will be created which will bring significantly increased performance to the aircraft.

If the suction system fails, the plane will still fly safely and under complete control of the pilot, albeit with higher drag from separated airflow. If one combustion engine, of an aircraft with more than one, the One Engine Inoperative (OEI) climb performance of the modified aircraft will exceed that of a non-modified ship. Such decreased performance is common to aircraft with some kind of propulsion system failure and aircraft are designed to operate safely despite such a failure occurring at the worst possible moment.

If the only internal combustion engine fails, the windmilling propeller will turn the generator and enable the airplane to glide farther than a stock ship would.

For multi-engine jet transport planes:

To increase efficiency, this configuration needs a mechanism to allow suction pump engines to produce reaction thrust for takeoff and critical climb segments (second segment climb and approach climb requirements are typically the most limiting) and then switch functions to create suction power for Boundary Layer Control (BLC) suction power. The mechanism that makes this switching possible can also be used to enable suction pump engines to create enough suction force to enable the aerodynamic pressure thrust phenomenon as well as meaningful reaction thrust, at the same time.

A typical type of passenger jet: The stock or un-modified aircraft has two traditional turbofan engines for reaction thrust, each reaction thrust engine is rated at 100% of stock power. This configuration gives adequate OEI second-segment and approach climb performance.

The modified aircraft has roughly 4* engines:

     Two traditional turbofan engines for reaction thrust
     Two dual purpose turbofan engines for reaction thrust and BLC suction power.

     These dual-purpose engines have doors that open to allow the engines to create reaction thrust for acceleration and second-segment and approach climb power. When reaction thrust is no longer needed or desired, these openings close allowing the dual-purpose engines to provide BLC suction to the APT system. By partially closing these doors, the suction pump engines can create enough suction force to create the APT phenomenon as well as meaningful reaction thrust at the same time. The amount of both reaction thrust and BLC suction power can be varied by the degree to which these doors are open to the surrounding environment or are closed and their fluid communication is limited to only the BLC suction ducting. These suction engines may be rated at roughly 50% of stock main engine power.

Each main reaction thrust engine may be rated at roughly 50% of stock main engine power due to decreased power demands of the modified aircraft.

Wing-mounted suction engine may be rated at roughly 50% of stock main engine power

(*if the existing wing design will not allow enough internal airflow for one central BLC suction engine to power the entire wing, the wing-mounted suction engine may be replaced by multiple engines or multiple electric fans across the span)

Tailcone suction pump may be rated at roughly 50% of stock main engine power.

In this example, the total takeoff reaction thrust is the same for both ships which will allow for comparable takeoff, climb and landing performance.

In the case of OEI second-segment and approach climb, the modified ship has reaction thrust equal to 150% stock ship's OEI reaction thrust. The modified aircraft will have higher drag than the stock ship (from separated flow on the wings and rear fuselage due to no BLC suction), but should exceed increased second-segment and approach climb requirements for aircraft with 4 engines, with 150% of stock OEI thrust at work.

Once the OEI second segment climb is over the aircraft can accelerate, the suction systems will create the pressure thrust phenomenon which will allow for either low power holding or climb and cruise to the landing airport.

On takeoffs where there is no engine failure, once the aircraft has exceeds height and airspeed conditions calculated for that takeoff, the dual purpose engines can be configured for BLC suction power and the aircraft will fly to it's destination with significantly less fuel consumption than the stock aircraft.

Effect on weight

     With a 25% reduction in fuel consumption; for flights over 2 hrs, equal payloads yield a lighter takeoff weight for the APT ship

Very Rough estimates:
     A stock 767 on a 2.5hr flight from Miami to NYC burns 25,000 lbs total (at 10,000 Lbs/hr).
     A modified 767 (with 25% savings) on the same 2.5hr flight burns 18,750 lbs total (at 7,500 Lbs/hr)
For a decrease of 6,250 lbs

     Each 60,000 lb thrust CF6-80 engine weighs roughly 9,760 lbs (times 2 = 19,520 lbs)
     Each CFM 56 engine weighs 5,700 lbs (times 4 = 22,800 lbs)
     For an increase of 3,280 lbs

Even if the modification adds 5,000 lbs total (suction engines, ducting, etc.) and saves only 25% of fuel consumption, the modified ship will take off lighter (with the same payload and fuel reserves) for flights over 2 hours long.

More about all this:

The Pickup Test Vehicle

     Pressure thrust is an aerodynamic phenomenon that, just like its better known sibling, aerodynamic lift, exploits Bernoulli's principle which relates fluid pressure and velocity. The flight and wind tunnel tests referenced below proved an over 40% reduction in total power required, including power for the suction system, against streamlined designs of comparable size.

     Bernoulli's principle tells us that as flow velocity accelerates, its pressure decreases and as the flow decelerates, its pressure increases. In much the same way as an airfoil generates lift by using a convex curve to accelerate the local flow, the pressure thrust engine generates high pressure by using a concave curve to decelerate the local flow. The local flow will separate if enough suction power is not used to keep it attached and the power required for suction will always exceed the power recovered as thrust.

     On aircraft wings in flight, decreased local pressure coincides with aerodynamic lift. On wings or fuselages modified to exploit pressure thrust the combination of powered suction, Boundary Layer Control (BLC) and optimized concave geometry create an area of high pressure acting on the aft end of the body. This distribution of pressure generates a thrust force that is real. Hence the term Pressure Thrust.

     Similar comparisons hold true for either Newtonian or Circulation explanations of aerodynamic lift.

     This is very different than the case on an unmodified wing or fuselage, where the pressure force recovered at the rear is less than the pressure force pushing the leading edge backward. This is called drag and it is the largest source of drag on aircraft at cruising speed and on cars and trucks at highway speed.

Economy of Power
     By combining engine power and aerodynamic design to exploit pressure thrust, the total power required can be reduced by at least 40%. Repeated testing in wind tunnels, in CFD analyses and in numerous flight tests have proven the economy of power (see references below). With today's sophisticated CFD and CAD/CAM tools at their disposal, designers should be able to increase this benefit.

BLC and Real World Operability
     Unlike active laminar flow control, used on the X-21 demonstrator, the BLC for pressure thrust is less susceptible to the clogging problems that plague active laminar flow control.

     With refined quality assurance programs like Six Sigma, the operability and reliability of a pressure thrust engine and its related systems should match that of legacy propulsion systems.

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 here.)

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 here.)

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 here)

6. Bushnell, D.M., "Fluid Mechanics, Drag Reduction and Advanced Configuration Aeronautics" NASA/TM-2000-210646 (available for download here. )

Pressure Thrust

From Aerodynamic Drag to Aerodynamic Drive

A significant increase in fuel economy is possible from a powered aerodynamic design.