41

ESTOL (Extremely Short Take-Off and Landing)

S. Tsach* and L. London†

Israel Aerospace Industries (IAI), Ben Gurion Intl. Airport, 70100, Israel

D. Kleiman, L. Abush and A. Tatievsky

Israel Aerospace Industries (IAI), Ben Gurion Intl. Airport, 70100, Israel

The use of propulsive lift provides STOL performance that

significantly reduces field-length capabilities and lowers approach speeds

for improved safety. ESTOL aircraft could use runways at much smaller

airports than the conventional aircrafts, allowing expansion of commercial

flights to many more locations .Enabling commercial flights to take off and

land in ever-shorter distances is an ongoing goal for aircraft designers, and

several approaches are under development.

I. Introduction

Airport limited capacity is a current problem in US and Europe aviation traffic. Airport

activity has increased considerably over the last few years. As a result airports have become

extremely congested and have undergone many flight delays and cancellations. With expected

growth in traffic demand, this problem will get even worse.

One way to alleviate this growing problem is to utilize the runways, airport ground, and

infrastructure that are already in existence at major airports, but are too small for large aircraft

all while maintaining current air traffic patterns. By making use of the smaller runways for

ESTOL vehicles will greatly relieve current airport problems.

Figure 1. Air Traffic Forecast [7]

Numerous aerodynamic and mechanical methods have been proposed and evaluated for

such applications to fixed-wing aircraft, including the use of very low wing loading, passive

leading- and trailing-edge mechanical high-lift devices, boundary layer control on leading-

and trailing-edge devices, and redirection of the propeller or jet engine exhausts on trailing-

edge flap systems.

Using engine exhaust to augment wing lift is known as powered lift, and this approach

differs from vertical takeoff and landing (VTOL) systems where power is used for direct lift.

* Director, Advanced Programs Department, [email protected] † Engineer, Advanced Programs Department, [email protected]

During the middle 1950s, intense research efforts on several powered lift schemes began in

Europe and the United States, resulting in dramatic increases in lift available for STOL

applications. The accompanying figure no. 2 shows history of maximum lift development.

Figure 2. History of maximum lift development

[12]

In the period after World War II, when the U.S. NASA conducted researches on a fairly

large number of approaches to ESTOL flight, the deflected slipstream approach was

investigated through models, wind tunnel tests and construction of full scale aircraft.

The concept of the ESTOL 60's studies is increasing maximum wing lift to CLmax=6 to

8 by the means of:

x Induced airflow of the propeller on the wing/flaps.

x Advanced two-staged flaps with advanced profiles.

Figure 3. Effect of tail on longitudinal aerodynamic characteristics

[2]

Late research, wind tunnel and flight tests have shown that using Upper Surface Blowing

(USB) technology can produce maximum lift coefficient near 10.

A great class of concepts have been tried to increase maximum lift using high pressure air

from the engine. Some examples of powered lift concepts are:

A. Propeller Slipstream Deflection

The basic principle is to deflect the slipstream from propellers to create an upward thrust.

employing the efflux of engines to increase wing lift using the jet- flap concept to remove the

limitations of conventional high-lift devices. The magnitude of maximum lift obtained in this

approach can be dramatically increased—by factors of three to four times as large as those

exhibited by conventional configurations—permitting vast reductions in field length

requirements and approach speeds.

Wind tunnel conducted in NASA and reported in report TN D-4448 – concludes that the

deflection of the air stream generated by front mounted propellers, using high angle (80º-

100º) large flaps, causes a significant increment in lift coefficient value.

Figure 4. Wind tunnel model

[1]

Figure 5. Model geometry for NASA wind tunnel tests [1]

This report points that a combination of running propellers that cover most of the wing

can increase up to 4 times the maximum lift coefficient. In addition, a combination of running

propellers and lowering flaps can increase up to 8 times the maximum lift coefficient. The

same maximum lift coefficient can be achieved in higher angle of attack by using slat. This

report also describes a high pitching moment Cm that obtained by the combination of flaps

and running propellers. It is necessary to trim this moment.

Figure 6. CLmax Vs. CD for different Propeller Thrust [1]

The Breguet 941 was a French four-engine STOL transport aircraft developed by Breguet

in the 1960s.

The Breguet STOL concept used four Turbine engines to drive a common power shaft,

which, in turn drove four oversize propellers, which were evenly spaced along the leading

edge of the wing with large, full-span, slotted flaps, with the arrangement known as blown

wing.

The Breguet Max takeoff weight: 58,422 lb proved Take-off run (at 48,500 lb) of 185 m

(607 ft).

Although widely evaluated, it was not built in large numbers, with only one prototype

and four production aircraft being built. In service: 1967-1974

Figure 7. The Breguet 941

[6]

Figure 8. The Breguet 941

[14]

Figure 9. Positioning the Breguet 941

[14]

23.75 m 23.40 m

9.65 m

B. Upper Surface Blowing

One of the most promising powered-lift concepts is the upper surface blown (USB).

Figure 10. Upper Surface Blowing [12]

In the USB system the engine is arranged over the top surface of the wing, blowing over

the flaps. When the flaps are lowered the Coand�* effect makes the jet exhaust attach to the

wing and turns it downward over the trailing-edge flap for lift.

Boeing YC14

The Boeing YC-14 was a twin-engine short take-off and landing tactical transport.

The upper-surface-blowing design for high aerodynamic lift used two jet engines that

blew high-velocity airstreams over the inboard portion of the wing and over special trailing-

edge flaps. The large multi section flaps extended rearward and downward from the wing's

trailing edge to increase the wing area, thus creating extra lift, which was further augmented

by positioning the engines so their jet blast across the upper wing surfaces created still more

lift.

The first Boeing YC-14 (serial number 72-1873) flew on 9 August 1976. The first flight

of the YC-14 demonstrated superior STOL performance and low-speed maneuverability.

Even with its 27,000-pound STOL payload, the YC-14's takeoff run was 1,000 feet, and it

could land in a slightly longer distance.

Figure 11. Boeing YC-14

[25]

NASA QSRA

In the late 1970s and early 1980s, NASA used C-8A Buffalo in the Quiet Short-Haul

Research Aircraft (QSRA) program.

Its experimental wing was designed, fabricated and installed by Boeing, and was a swept,

supercritical design incorporating a boundary layer control system. Instead of the standard

engines, this aircraft was powered by four prototype Avco Lycoming YF102 high-bypass

turbofan engines mounted above the wing to take advantage of the Coand� effect.

In 1980, this aircraft participated in carrier trials aboard USS Kitty Hawk, demonstrating

STOL performance without the use of catapults or arrestor gear.

This project proved that CL of 10 could be achieved.

Figure 12. NASA QSRA

[6]

C. Externally Blown Flaps

In the Externally Blown Flaps (EBP) system the engine is arranged under the wing,

where compressed air produced by the jet engine is forced in the wing flaps of the aircraft

when the flaps reach certain angles. Injecting high energy air into the boundary layer produces

an increase in the stalling angle of attack and maximum lift coefficient by delaying boundary

layer separation from the airfoil. Typical flap designs are split near the engine such that they

don't deflect the thrust; however, with sufficiently powered engines, the effect of the flaps

being in the path of the exhaust can be tremendous.

Figure 13. Externally Blown Flaps [19]

NASA first carried out exploratory research on the EBF. In 1956 NASA started years of

intensive wind-tunnel research which in the end matured to corporation with McDonnell

Douglas on the YC-15 Advanced Medium STOL Transport (AMST) prototype in the 1970s,

and then with Boeing on C-17 transport project. This aircraft is the only U.S. production

powered-lift fixed-wing airplane

McDonnell Douglas YC-15

Figure 14. McDonnell Douglas YC-15 [21]

The McDonnell Douglas YC-15 was a four-engine STOL tactical transport. It used

externally-blown flaps to increase lift. This system used double-slotted flaps to direct part of

the jet exhaust downwards, while the rest of the exhaust passed through the flap and then

followed the downward curve. Two YC-15s were built, one with a wingspan of 110 feet (#72-

1876) and one with 132 feet (#72-1875). Both were 124 feet (38 m) long and powered by four

Pratt & Whitney JT8D-17 engines, each with 15,500 lbf of thrust.

The first flight was 26 August 1975. The second prototype followed in December.

On 1979 the project was upgraded and developed into C-17 Globemaster III.

Boeing C-17 Globemaster III

Figure 15. Boeing C-17 Globemaster III [6],[23]

The Boeing (formerly McDonnell Douglas) C-17 Globemaster III is a large military

transport aircraft. Developed for the United States Air Force from the 1980s to the early

1990s by McDonnell Douglas. The C-17 Globemaster III, is a derived from the YC-15, shares

a similar configuration. Compared to the YC-15, the new aircraft differed in having swept

wings, increased size, and more powerful engines.

The C-17 is 174 feet long and has a wingspan of about 170 feet. It is powered by four

Pratt & Whitney F117-PW-100 turbofan engines. Each engine is fully reversible and rated at

40,400 lbf of gross thrust. The C-17 is designed to operate from runways as short as 3,500 ft

(1,064 m) and as narrow as 90 ft (27 m). In addition, the C-17 can operate from unpaved and

unimproved runways.

Status: In production, in service

D. Comparison of Powered Lift Concepts

The aerodynamic performance of the USB concept is roughly equivalent to that of the

EBF concept. Thrust performance is usually higher for the USB because of the thrust loss

caused by the direct impingement of engine efflux on the trailing-edge flap in EBF concept.

This thrust loss will limit the flap setting selection flexibility. The USB also offers a critical

advantage because objectionable noise levels generated during powered-lift operations are

much lower for observers below the airplane.

An additional operational advantage of the USB concept is the reduction of foreign object

ingestion into the high-mounted engines.

Figure 16. Comparison of powered lift concepts [6]

E. Future Programs

NASA’s CESTOL Concept

Cruise Efficient Short Takeoff and Landing (CESTOL) aircraft that cruise at high speed

(Mach 0.8+) with low environmental impact yet can take off and land on very short runways.

Research in support of CESTOL concepts seeks to develop powered lift technologies to

increase lift on an aircraft at low speeds, such as during takeoff and landing, while decreasing

drag at high speeds to maintain efficient cruise. Powered lift and active control concepts can

enable significantly shorter field lengths for a given vehicle weight

Figure 17. NASA CESTOL Concept [9]

NASA examines short take-off and landing (STOL) airliners able to operate from under-

used runways shorter than 3,000ft. The problem with traditional STOL aircraft is they are

slow, and don't integrate well with conventional jet traffic in en route or terminal airspace.

NASA has been working with the California Polytechnic University on the concept of a

"cruise efficient" STOL - or CESTOL - aircraft. This would take off within 3,000ft, but cruise

at normal jetliner speeds. In January 2012, a 10ft-span model was tested in NASA's NFAC

wind tunnel. The model, called AMELIA (Advanced Model for Extreme Lift and Improved

Aeroacoustics), scaled 1/11th was designed as a 150-passenger regional cruise efficient, short

takeoff and landing airliner.

The 737-sized CESTOL design uses a combination of upper-surface blowing (USB) and

a circulation-control wing (CCW) to increase lift and reduce takeoff and landing distances.

Figure 18. CESTOL [18]

German Aerospace Center, has been focusing its research an A320-sized quiet STOL

aircraft - QSTOL, that uses upper-surface blowing. Simulations of airport operations are

showing the advantages and disadvantages of STOL.

Figure 19. QSTOL [18]

To date, the NASA ESTOL vehicle sector has set forth goals for a state-of-the-art 100

passenger airliner to be operational by the year 2022: a take-off and landing distance of less

than 2,000 feet, a cruise Mach number � 0.8, a 1,400 to 2,000 mile range capability, noise

containment within an airport footprint, and low speed maneuverability.

Figure 20. CESTOL [11]

F. IAI RA100 ESTOL

Israel Aerospace Industries Ltd. (IAI) is a prominent worldwide leader in wide scope of

aerospace market sectors, from unmanned aviation through business aircraft and satellites. It

is involved in evaluation of potential configurations and technologies, which will be

applicable for the increased commercial aviation traffic in the future. ESTOL is defined as a

critical capability for the future aircraft, due to expected air traffic congestion and inability of

the major hubs to cope with increased traffic flow. Thus, IAI researches the opportunities for

entering this field of interest, by investing in ESTOL research and development, mainly in

USB direction. One of its major design points is aimed at the future regional market, at the

niche of the regional aircraft, sized at about 100 passengers. This segment is expected to

grow, while ESTOL capability will give it the competitive edge, allowing it to operate in the

complicate future air traffic environment. IAI as the leader of the Israeli aeronautical industry

and one of the few companies in the world, capable of dealing with the challenge of designing

such an innovative concept, is currently evaluating the market and technologies applicable for

this sort of aircraft.

Figure 21. IAI RA100 ESTOL

G. Powered Lift Challenges

Longitudinal trim is a major challenge because of large nose-down pitching moments

produced by powered-lift flaps at high thrust settings. Very large horizontal tails are required

for trim at high-lift conditions,

Aircraft structure must be designed to withstand large Aerodynamic loads. Wing and flap

structures must be analyzed to static and fatigue loads cased by impinging jet flow.

Structural heating issues caused by the engines mounted in close proximity to the wing’s

upper surface require attention.

Another big challenge of the power lift systems is special procedures needed to maintain

safety in case of an engine failure.

Higher fuel consumption must be taken into consideration in overall system performance.

The aerodynamic flow field of power lift system is complex. More researches and wind

tunnel tests are needed during development process.

H. Conclusions

It is forecasted that air travel demand will continue to grow, cause airport congestion.

The use of powered-lift technology provides STOL performance that significantly

reduces field-length requirements and can alleviate the problem of airport congestion.

Use of a USB configuration for civil transports may be compatible with shorter field

lengths while retaining low noise characteristics.

Technology challenges to develop medium and large transport aircraft having STOL

capabilities are being evaluated, to introduce new generation aircraft..

References

1. V. Robert Page, Stanley O. Dickinson and Wallace H. Deckert, NASA TN D-

4448, "Large-Scale Wind Tunnel Tests of Deflected Slipstream STOL Model

With Wings of Various Aspect Ratio", Ames Research Center, March 1968

2. Richard J. Margason and Garl L. Gentry Jr.,NASA TN D-4856, "Aerodynamic

Characteristics Of Twin Propeller Deflected slipstream STOL Airplane Model

With Boundary Layer Control On Inverted V-Tail", Langley Research Center

November 1968

3. V. Robert Page and Thomas N. Aiken, NASA TN D-6393, "Stability and Control

Characteristics of Large-scale Deflected Slipstream STOL Model with A Wing of

5.7 Aspect Ratio", Ames Research Center, October 1971

4. Oliver Schneider, Stefan Kreth and Lothar Bertsch, " Towards a Quiet Short

Take-Off and Landing Transportation System: Concept Evaluation and ATM

Integration"

5. "Mobility", NF200903491HQ, nasa.gov

6. W.H. Mason, "Some High Lift Aerodynamics, Part 2 Powered Lift Systems",

Virginia Tech

7. Global Market Forecast 2011-2030, Airbus, 2011

8. Aero Gizmo By Jeff Salton– 21:20 November 8, 2010

9. (C)(E)STOL in Airport Operations - Technische Universität München - 2011

10. Nicolai Fundamentals of Aircraft Design, 1976, attributed to Boeing

11. NASA Cruise Efficient Short Takeoff and Landing, Craig Hange, April 2011

12. Joseph R. Chambers, "Innovation in Flight Research of the NASA Langley

Research Center on Revolutionary Advanced Concepts for Aeronautics", NASA

SP-2005-4539

13. Craig Hange, "Short Field Take-Off and Landing Performance as an Enabling

Technology for a Greener, More Efficient Airspace System"

14. Jacques Noetinger, " Breguet Br 940/941, The Airplane with the "Deflection

slipstream concept"

15. Donald, David "The Encyclopedia of World Aircraft". Aerospace Publishing,

1997

16. Borchers, Paul F.; Franklin ,James A. Fletcher, Jay W.. "Flight Research at Ames,

1940-1997, Chapter 8 Boundary Layer Control, STOL, V/STOL Aircraft

Research NASA SP-3300". http://history.nasa.gov/SP-3300/ch8.htm. Retrieved

2007-07-02

17. Hartman, Edwin P. "Adventures in Research: A History of Ames Research Center

1940-1965, Part III: THE LEAP TO SPACE : 1959-1965, 1963-1965, NASA SP-

4302". http://history.nasa.gov/SP-4302/ch3.9.htm. Retrieved 2007-07-02.

18. Aviation week, The Commercial Aviation Blog , 11.04.10

19. Boeing YC-14 on GlobalSecurity.org

20. YC-14 Military Transport page on Boeing.com

21. YC-15 Military Transport on Boeing.com

22. Kennedy, Betty Raab. "Historical Realities of C-17 Program Pose Challenge for

Future Acquisitions." Institute for Defense Analyses, December 1999.

23. C-17 page on Boeing.com

24. DOD-Future Need for ETOL/VTOL Aircrafts – Jul 2007

25. Aviation week, The Defense Technology Blog , 10.15.09