NASA Facts VSTOL Aircraft 1964

8/6/2019 NASA Facts VSTOL Aircraft 1964

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NA - A FACTS PagAn Educational.Services Publication of the.,

National Aeronautics and Space Administration

V/STOL ~ i r c r a f t @ol. II, No.3

N64 27070

C-142 Tr iServ ice transport (art ists 's concept ion ).

Aircraft capable of landing and taking offvertically, or with a relatively short ground run,are being studied by the National Aeronauticsand Space Administration in a program calledV/STOL (pronounced VEE-stoll), fo r vertical orshort take-off and landing.

The helicopter is an example of such aircraft;abil i ty to operate from a small airfield is thebasic advantage.

NASA's role in the program consists of basicand exploratory research on behalf of the mili-tary an d the aircraft industry, and applied reosearch for development of specific V/STOLtypes.

A vertical take-off and landing aircraft (VTOL)is defined as on e that takes off vertically,changes from hovering to forward flight , cruisesto its destination, then hovers again and landsvertically.

A short take-off and landing aircraft (STOL)one that takes off an d lands-cruising to its destnation meanwhile-from a relatively short runwathat one expert has defined as a 500-foot runwawith a 50-foot-high obstacle at each end.

Scores of possible V /STOL configurations havbeen studied in this country, and development oseveral of them has been carried as far as thflight-test stage. On e way to classify the possible types, so that they may be compared, is btheir method of converting from vert ical (o r neavertical) fl ight to horizontal flight .

The first method is to TILT THE ENTIRAIRCRAFT.

The second method is to TILT ONLY THROTORS, PROPELLERS, OR OTHER SOURCEOF THRUST. The wings , i f any , can be tiltealso, but the fuselage and the p ilo t remain in thsame position as when the aircraft took off.


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Five VTOl concepts : (A) tilt rotor, (B) deflected slipstream ,(C) tilt duct, (0) deflected jet , an d (E) t ilt wing .

The third is to DEFLECT THE THRUST; the airswept back by the propellers or exhausts is bentdownward, with wing flaps, fo r example , ornozzles.

The fourth is DUAL PROPULSION-to have different engines (o r sets of engines), one for l iftingand lowering the aircraft and one for driving ithorizontally .

Another way to classify V/STOL aircraft is according to the source of thrust . The source ofthrust of a particular a" rcraft may be- ( 1) rotor(s),(2) propeller(s), (3) ducted fan(s), (4) jet exhaust(s), (5) a combination of some of thesemeans.

Aircraft have been considered that would paireach of these source-of-thrust possibilities withthe previously mentioned methods of conversionfrom vertical to horizontal flight. Before a discussion of some of the particular types, however,let's glance briefly at some history of V /STOl research in this country.

In 1921, Dr. Albert F. Zahm patented a machine with a special wing and flap arrangement todeflect downward the propeller slipstream (the airmoved by the propeller). Here note the two requirements fo r vertical take-off: first, the propellerslipstream must be directed straight down, to produce the vertical thrust to lift the airplane straightup; and second, this upward thrust must begreater than the weight of the aircraft.

Dr. Zahm's airplane wa s never built. It metthe first of these requirements, but not the second.There was then no airplane engine powerfulenough to produce a deflected propel ler slipstream that could lift the aircraft. And becauseduring the 1920 ' s and early 30's no big improvements in engine power were expected, designers

and in v entors in that period put aside anythoughts of tilt-wing propeller V/STOL aircraftand turned to the autogiro and the helicopter,whose rotors could lift aircraft powered by theengines then on hand or expected .

A rotor, in general, is a propelle r that is largerthan usu a l. Its blades are longer and broader.Its l i ft varies with how much ai r it can move andho w fast it can move this air. The same amountof l i ft can result whether you move a large massof air at a low speed or a small mass of ai r at ahigh speed.

BUT the power consumed va r ies with (1) themass an d (2) the SQUARE of the speed. So, byreducing the speed of the air and proportionatelyincreasing the mass of the air being moved, thedesigner was able to ge t his airplane up with theengines then in existence . This is why the firstV ISTO l aircraf t - the autogiros and the helicopters-had large, slow-moving rotors .

In the late 1940 ' s the introduction of turboprop and turbojet engines prompted another lookat V ISTOl airplanes other than helicopters.

V ISTOl research by the National AdvisoryCommittee fo r Aeronautics (predecessor ofNASA) began in 1950 with wind-tunnel testsand flight research with small-scale models, andit has increased rather steadily since. NASA'stwo largest wind tunnels-one the 40- by 80-foot tunnel at Ames Research Center in California and the other the full-scale tunnel atlangley Research Center in Virginia-are nowdevoted largely to VTOl studies . Anotherfacility is the 17-foot test section built into oneof the langley 7- by 10-foot tunnels.


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Vol. II, No.3 PagHELICOPTERS

A substantial part of NASA's VISTOL work ison helicopters. At present the helicopter is theonly operational VTOL aircraft, at least in theUnited States.

Because of its relatively low slipstream velocities and hovering power requirements, the helicopter is best suited for missions requiring lengthyperiods of hovering-such as rescue work involving the lifting of people from th e ground in theopen air, and for operation from unpreparedbases, where higher slipstream velocities causetrouble from ground erosion and flying dust anddebris.

under certain weather conditions, (5) inefficiecruising operation, and (6) slow cruising spee(less than 200 m.p.h. maximum).

NASA is doing research to reduce helicoptvibration and maintenance problems and to improve the flying qualities. One study at Langledeals with factors involved during the transitiofrom steep approach to vertical touchdown anduring blind hovering with the Vertol YHCl A, large modern twin-turbine helicopter. This aicraft is fitted with variable-stobility equipmenwhich allows wide variations in flying and handling characteristics, and with special navigatioand pilot-display instruments that should produc

For this reason the helicopter will probably continue to be the best vehicle for certain missions inspite of th e disadvantages inherent in its design.Among these disadvantages are (1) mechanical A - - ; - ; ; - ~ - _ ; ; : : ~ ; ; ~ ~ ; ; ; . : : , complexity, (2) higher maintenance costs, (3) vi-bration and noise, (4) difficult flying qualities

Vertol YHCIA helicopter.

McDonnell XV-l compound helicopter.significant data on blind or instrument-flight coditions-for other V/STOL aircraft as well ahelicopters.

The "compound" helicopter, which has a coventional fixed wing to improve its cruise peformance and a separate propulsion unit for foward flight, is again receiving serious attentiofrom the U.S. military services. The concept habeen flight evaluated in the past with the MeDonell XV- 1 and the Fairey Rotodyne vehicles, anis being studied currently by Bell Helicopter withmodified UH- 1 helicopter.


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VTOl research models (propeller types).

PROPELLER CONFIGURA iONS

The four-part illustration above shows some ofthe models NASA has used in exploratory re-search in the propeller VTOL transport field. Thetwo models on the left are deflected-slipstreamVTOL configurations, and have large flaps to deflect the slipstream downward and produce thelift for vertical take-off and hovering flight. Thistype is probably no t very promising fo r VTOL use,the research indicates, because of the thrust lostwhen the flaps deflect the slipstream and becauseof certain ground-effect and trim problems, butit works very well in STOL operations.The model at the upper right is a tilt-wing type.The wing and propellers are in the position shownat take-off an d landing, an d they rotate down tothe normal pasition in cruising flight. This con f iguration is good for VTOL use, but is not asgood for STOL aircraft as the type with largeflaps.

The model at the lower right represents a combination VTOL-STOL machine with a tilt wing and

a moderate-size flap that is effective in low-speedflight. It combines the best features of the othermodels and is a promising V/STOL design.

The illustration on page 5 shows a large-scale,tilt-wing general-research model being studied inth e Langley full-scale tunnel to investigate sta-bility and control characteristics, wing and flaploads, and effects of wing and propeller changesfor hovering, transition, and forward flight.Similar studies on other research models havebeen completed or are scheduled in the Ames40- by aO-foot and the Langley 7- by 10-foottunnels .

Wind-tunnel research model studies of the tiltwing VTOL concept have received much ofNASA's V/STOL efforts, and the resulting datacontributed to the design of a t i lt-wing for theTri-Service C-142 transport, fo r the Army, Navy,and Air Force. This aircraft, being built by theVought.Ryan-Hiller Companies in combination, isscheduled fo r initial flight in 1964.


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Vol. II, No.3 Pag

I largescale tiltwing model in wind tunnel.DUCTED FAN CONFIGURATIONS termine performonce, stobility, and control

/ scheduled.A ducted fa n is a propeller surrounded by a

. ring, or duct, which changes the shape of the slipstream. Behind the conventional propeller thestream tapers to about half its original area, achange that reduces the thrust. Use of the ductprevents this tapering, so that a given thrust canbe produced with a smaller propeller. The ductalso serves as a safety device.

The Navy feels that the duded-fan conceptmay have special advantages fo r aircraft carrieruse because aircraft with ducted fans tend to besmaller. For this reason, another Tri-Servicevehicle that is now under development is the X-22tilt-duct aircraft. l imited NASA research, including general wind-tunnel model studies and a han.dling-quality study, has been conducted on thisconcept. Additional wind-tunnel research to de-

JET CONFIGURATIONSTurbine engine technology has reached th

point that turbojet engines capable of l iftintwenty ti mes their own weight are now feasiblA l ift as great as that would make hovering posible with almost any aircraft configuration thdesigner wished. Because of this and of the at icipated military requirement for higher-speeV/STOL aircraft, a great many new designs havbeen proposed recently fo r je t VTOl types.

However, the use of je t engines for VTOll i fing and hovering has some considerable disavantages. Among them are extra weight, higcost, complexity, a high rate of fuel consumptioa great volume of noise, and the fact that the higvelocity and temperature o f the exhaust gase


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Page 6may require special landing pads or groundprocessing.

Two basic jet types are being studied. In one,the engine provides both the l if t for take-off andhovering and the thrust for forward flight. Anexample of this type is the Hawker Pl127 deflected-jet aircraft. In the second type, twokinds of engines are provided. The DassaultBalzac is an example; l i ft is provided by eightRolls-Royce RB 108 engines an d thrust by one BSOrpheus engine. NASA wind-tunnel studies ofsmall-scale general research models are underway.

The General Electric Company has developed a wing-fan configuration in which theexhausts of the jets necessary fo r high-speed flightare used to drive large fans and produce verticall ift. Extensive large-scale wind-tunnel studies,conducted at th e Ames Research Center, havehelped development of the GE-Ryan XV-5A aircraft, which began flight-testing in 1964. Thelockheed Aircraft Corporation has developedanother system of fuselage je t thrust augmentersfo r VTOl operation. This has been incorporatedin the XV-4A, also undergoing flight evaluation.

FLIGHT RESEARCH PROGRAMUnder a 1958 agreement, seven research

vehicles which the military services had built tostudy various V ISTOl l ift-propulsion conceptswere later turned over to NASA for research in-vestigations at langley or Ames, following functional demonstrations by the m anufacturers.

These airplanes were sponsored and financedby the military services to obtain preliminary in-formation on VTOl airplane flying qualities. Re-search airplanes of this kind are sometimes called"f lying test beds." Simple and usually crudelooking, they provide a relatively inexpensivemeans of gathering f l ight research data.

Most of these aircraft were studied in theNASA full-scale tunnels and several received (orare receiving) extensive flight research evaluation.

The i l lustration above shows the Vertol VZ-2ti lt-wing test be d in flight at langley. NASA

Vol. II, No.3studies of this and other flying test beds providedby the military services have thrown much lighton V/ STOL flying and handling-quality requirements.

The first part of NASA's program was completed in 1961 . Wing-stall an d control deficiencies encountered in that study led to additionalwind-tunnel research, which resulted in modifications to the aircraft. The results of these programs should aid in the development not only ofthe C-142 bu t of future, more-advanced tilt-wingaircraft as well.

Vertol VZ-2 t ilt-wing model being flown ot Longley.

Particularly valuable information has been obtained from the X-14 deflected-jet test bed.Modified to include variable stability an d controlfeatures, it continues to be used as a fl ight simu-lator of the characteristics of other aircraft not yetflown or built. Other test-bed aircraft flighttested by NASA are the Doak VZ-4 tilt-duct, theRyan VZ-3 deflected-slipstrea m, and th e BellXV-3 ti lt-rotor aircraft.


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GE-Ryan XV-5A wing-fan aircraft (artist's conception) .

Bell X-22 tilt-duCT aircraft (artist's conception) .

STOl

Although only a small percentage of the NASAfunds now appropriated for V/ STOL researchgoes into STOl work specifically, it should beremembered that much of what is learned aboutVTOl aircraft applies to STOl aircraft also. Forexample, the tilt-wing VTOl type is expected to

be an excellent STOl aircraft as well, an d it wbe used as such, for economy an d safety reasonunless the particular operating site demandVTOloperation.

Specific STOl flight research programs arunder way at Ames on two aircraft. One is thVZ-3 deflected-slipstream mentioned earlierThe other is the C-130C transport airplane tha


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Lockheed C- 130C transport .

utilizes boundary-layer control (in this case, blowing of air over the flaps to prevent separation ofthe airflow) to obtain higher l i ft and steeper takeoffs an d landings. In the current program, handling qualities at very lo w speeds (abou t 60knots) are being evaluated.

An especially promising example of a STOlvehicle is the Breguet 941 deflected-slipstream,inter-connected-propeller aircraft. Recent NASAf l ight and simulator studies indicate that this concept may have a good deal to offer in applications for various military and civil uses.

NASA FACTS Number- Volume I of NASA FACTS consists of all issues publishedprior to July 1964 and running from A-62 to 8-2-64. Volume 1/ begins withNASA FACTS, Interplanetary Explorer Satellites, Vol. II, No.1 .

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NASA Facts VSTOL Aircraft 1964