NASA Facts VSTOL Aircraft

8/6/2019 NASA Facts VSTOL Aircraft

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VOl. II,

C - 1 4 2 Tri-Service transport (one-ninth scale model)

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

The helicopter i s an example of such aircraft;abil ity to operate from a small airfield i s thebasic advantage.NASAs role in the program consists of basicand exploratory research on behalf of the mili-tary and the aircra ft industry, an d app lied re-search for development of specific V/STOLtypes.

A vertical take-off and landing aircraft (VTOL)i s defined as one 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) isone that takes of f and lands-cruising to i t s desti-nation meanwhile-from a relatively short runwaythat one expert has defined as a 5OO-foot runwaywith a 50-foot-high obstacle at each end.

Scores of possible V/STOL configurations havebeen studied in this country, and development ofseveral of them has been carried as far as theflight-test stage. One way to classify the pos-sible types, so that they may be compared, i s bytheir method of converting from vertical (or nearvertical) flight to horizontal flight.The f i r s t method i s to TILT THE ENTIREAIRCRAFT.

The second method i s to TILT ONLY THEROTORS, PROPELLERS, OR OTHER SOURCESOF THRUST. The wings, i f any, can b e tiltedalso, but the fuselage and the pi lot remain in thesame position as when the aircraft took off.


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Page 2 Vol. II, No. 3

FiveVTOL concepts: (A ) tilt rotor, (B) eflected slipstream,(C ) tilt duct, (D) deflected jet, and (E ) tilt wing.

The thir d i s to DEFLECT THE THRUST; the air> swept back by the propellers or exhausts i s bent

downward, with wing flaps, for example, ornozzles.

The fourth is DUAL PROPULSlON-to have dif-ferent engines (or sets of engines), one for li ftingand lowering the aircraft and one for d riving ithorizontally.

Another way to classify V/STOL aircraft i s ac-cord ing t o the source o f thrust. The source ofthrust of a particular aircra ft may be-( 1 ) rotor(s),(2 ) propeller(s), (3) ducted fan(s), (4) jet ex-haust(s), (5 ) a combination of some of thesemeans.

Aircraf t have been considered that would paireach of these source-of-thrust possibilities withthe previously mentioned methods of conversionfrom vertical to horizontal flight. Before a dis-cussion of some of the particular types, however,lets glance briefly a t some history of V/STOL re-search in this country.

In 1921, Dr. Albert F. Zahm patented a ma-chine with a special wing and flap arrangement todeflect downward the propeller slipstream (the airmoved by the propeller). Here note the two re-quirements for vertical take-off: first, the propellerslipstream must be directed straight down, to pro-duce the vertical thrust to lift the airplane straightup; and second, this upward thrust must begreater than the weight of the aircraft. It metthe first o f these requirements, but not the second.There was then no airplane engine powerfulenough to p roduce a deflected propeller s l ip-stream that could li ft the aircraft. And becauseduring the 1920s and early 30s no big improve-ments in engine power were expected, designers

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Dr. Zahms airplane was never built.

and inventors in that period put aside anythoughts o f tilt -wing propeller V/STOL aircraftan d turned to the a utog iro a nd the helicopter,whose roto rs cou ld lift aircraft powered by theengines then on hand or expected.

A rotor, in general, i s a propeller that i s largerthan usual. Its blades are longer and broader.I t s lif t varies with how much air it can move andhow fast i t can move this air. The same amountof li ft can result whether you move a large massof air at a low speed or a small mass of air a t ahigh speed.

BUT the power consumed varies with ( 1 themass and (2 ) he SQUARE o f the speed. So,byreducing the speed of the air and proportionatelyincreasing the mass of the air being moved, thedesigner was able to get his airplane up with theengines then in existence. This i s why the firstV /STOL aircraft-the autogiros and the heli-copters-had large, slow-moving rotors.

In the late 1940s the introduction of turbo-prop and turbojet engines prompted another lookat V/STOL airplanes other than helicopters.

V/STOL research by the National AdvisoryCommittee for Aeronautics (predecessor ofNASA) began in 1950 with wind-tunnel testsand flight research with small-scale models, andit has increased rather steadily since. NASAstwo largest wind tunnels-one the 40- y 80-foot tunnel at Ames Research Center in Cali-fornia and the other the full-scale tunnel atLangley Research Center in Virginia-are nowdevoted largely to VTOL studies. Anotherfacility i s the 17-foot test section built into oneof the Langley 7- y 10-foot tunnels.


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Vol. I I , No.3 HELICOPTERS Page 3

A substantial pa rt of NASAs V/STOL work i son helicopters. At present the helicopter i s theonly operational VTOL aircraft, at least in theUnited States.

Because of i t s relatively low slipstream veloci-ties and hovering power requirements, the heli-copter i s best suited for missions requiring lengthyperiods of hovering-such as rescue work involv-ing the lift ing of people from the ground in theopen air, a nd for operation from unpreparedbases, where higher slipstream velocities causetrouble from ground erosion and flying dust anddebris.

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For this reason the helicopter wil l probably con-tinue to be the best vehicle for certain missions inspite of the disadvantages inherent in i t s design.Among these disadvantages are (1 mechanicalcomplexity, (2 ) higher maintenance costs, (3) vi-

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

NASA i s do ing research to reduce helicoptervibration and maintenance problems and to im-prove the flying qualities. One study at Langleydeals with factors involved during the transitionfrom steep approach to vertical touchdown andduring blind hovering with the Vertol YHCl A, alarge modern twin-turbine helicopter. This air-craft i s fitted with variable-stability equipment,which allows wide variations in flying and han-dling characteristics, and with special navigationand pilot-display instruments that should produce

brati on and noise, (4) difficult flying qualities0

Vertol YHCIA helicopter.

McDonnell XV-1 compound helicopter.

significant data on blind or instrument-flight con-ditions-for other V/STOL aircraft as well ashelicopters.

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The compound helicopter, which has a con-ventional fixed wing to improve i t s cruise per-formance and a separate propulsion unit for for-ward flight, i s again receiving serious attentionfrom the U.S. mil itary services. The concept hasbeen flight evaluated in the past with the McDon-nell XV-1 and the Fairey Rotodyne vehicles, andIs being studied currently by Bell Helicopter with amodified UH- 1 helicopter.


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Page 4 Vol. I I , No. 3

VTOL research models (propeller types) .

PROPELLER CONFIGURATIONSThe four-part illustration above shows some of

the models NASA has used in ex plo ratory re-search in the propeller VTOL transport field. Thetwo models on the lef t are deflected-slipstreamVTOL configurations, and have large flaps to de-flect the slipstream downward an d produce theli ft for vertical take-off and hovering flight. Thistype i s probably not very promising for 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 i s a tilt-wing type.The wing and propellers are in the position shownat take-off and landing, and they rotate down tothe normal position i n cruising flight. This con-figuration i s go od for VTOL use, bu t i s not asgood for STOL aircraft as the type with largeflaps.

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

a moderate-size flap that i s effective in low-speedflight. It combines the best features of the othermodels and i s a promising V/STOL design.The illustration on page 5 shows a large-scale,tilt-wing general-research model being studied inthe Langley full-scale tunnel to investigate sta-bilit y an d control characteristics, wing a nd flaploads, and effects of wing and prope ller changesfor hovering, transition, and forward flight.Similar studies on other research models havebeen completed or are scheduled in the Ames40- by 80- foot and the Langley 7- by 10-foottunnels.

Wind-tunnel research model studies of the tilt-wing VTOL concept have received much ofNASA's V/STOL efforts, and the resulting datacontributed to the design of a ti lt-wing f or theTri-ServiceC-142 transport, for the Army, Navy,and Air Force. This aircraft, being built b y theVought-Ryan-Hiller Companies in combination, i sscheduled for initial flight in 1964.


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Vol. II, No. 3 Paae 5

Large-scale tilt-wing model in wind tunnel.

DUCTED FAN CONFIGURATIONSA ducted fan i s a propeller surrounded by a

ring, or duct, which changes the shape of the slip-stream. Behind the conventional propeller thestream tapers to about hal f i t s 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 ducted-fan conceptmay have special advantages for aircraft carrieruse because aircraft with ducted fans tend to besmaller. For this reason, another Tri-Servicevehicle that i s now under development i s the X-22tilt-duct aircraft. LimitedNASA research, includ-ing general wind-tunnel model studies and a han-0 dling-qua lity study, has been conducted on thisconcept. Additional wind-tunnel research to de-

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termine performance, stability, and cont rol isscheduled.

JET CONFIGURATIONSTurbine engine technology has reached the

point that turbojet engines capable of liftingtwenty times their own weight are now feasible.A lift as great as that would make hovering pos-s ib le with almost any aircraft conf igura tion thedesigner wished. Because of this and of the an-ticipated military requirement for higher-speedV/STOL aircraft, a great many new designs havebeen proposed recently for jet VTOL types.

However, the use of jet engines for VTOL lift-ing and hovering has some considerable disad-vantages. Among them are extra weight, highcost, complexity, a high rate of fuel consumption,a great volume of noise, and the fact that the highvelocity and temperature o f the exhaust gases


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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 lift for take-off andhovering and the thrust for for ward flight. Anexample of this type i s the Hawker P1 127 de-flected-jet aircraft. In the second type, twokinds of engines are provided. The DassaultBalzac i s an example; li ft i s provided b y eightRolls-Royce RB108 engines and thrust b y one BSOrpheus engine. NASA wind-tunnel studies ofsmall-scale general research models are underway.

The General Electric Company has devel-,oped a wing-fan configuration in which the

exhausts of the jets necessary for high-speed flightare used to drive large fans and produce verticallift . Extensive large-scale wind-tunnel studies,conducted at the Ames Research Center, havehelped development of the GE-Ryan XV-SA air-craft, which began flight-testing in 19 64 . TheLockheed Aircraft Corpora tion has developedanother system of fuselage jet thrust augmentersfor VTOL operation. This has been incorporatedin the XV-4A, also undergoing flight evaluation.

FLIGHT RESEARCH PROGRAMUnder a 1958 agreement, seven researchvehicles which the milita ry services had bu ilt to

study various V/STOL lift-propulsion conceptswere later turned over to NASA for research in-vestigations at Langley or Ames, fol lowing func-tiona l demonstrations b y the manufacturers.

These airplanes were sponsored and financedby the military services to o bta in preliminary in-formation on VTOL airplane flying qualities. Re-search airplanes of this kind are sometimes calledflyin g test beds. Simple and usually crude-looking, they p rovid e a relatively inexpensivemeans of gathering flight research data.

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

The illustration above shows the Vertol VZ-2tilt-wing test bed in flight at Langley. NASA

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

The first p art o f NASAs prog ram was com-pleted in 1961. Wing-stall and control deficien-cies encountered in that study le d to add itionalwind-tunnel research, which resulted in modifica-tions to the aircraft. The results of these pro-grams should aid in the development not only o fthe C-142 but of future, more-advanced tilt-wingaircraft as well.

t

Vertol VZ-2 tilt-wing model being flown a t Langley.

Particularly valuable information has been ob-tained from the X-14 deflected-iet test bed.Modi fied to include variable stability and controlfeatures, it continues to be used as a flight simu-lator of the characteristics of other aircraft not yetflown or built. Other test-bed aircraft flight-tested by NASA are the Doak VZ-4 tilt-duct, theRyan VZ-3 deflected-slipstream, an d the BellXV-3 tilt-rotor aircraft.


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Vol. II, No. 3 Page 7

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

Bell X-22 tilt-ducr aircratf (artist's conce ptio n).

STOL be an excellent STOL air cra ft as well, and it willbe used as such, for economy and safety reasons,unless the particular operating site demands

Specific STOL Aight research programs areunder way at Ames on two aircraft. One i s theVZ-3 deflected-slipstream mentioned earlier.The other i s the C-l3OC transport airplane that

Although only a small percentage of the NASAfunds now appropriated for V/STOL research VTOL operation.goes into STOL work specifically, it should beremembered that much of what is learned about

Forexample, the tilt-wing VTOL type i s expected to

0 VTOL aircraft applies to STOL aircraft also.


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Vol. II.No.3age 8 . ~-

f

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Lockheed C-l3OC transport.

utilizes boundary-layer control (in this case, blow-ing of air over the flaps t o prevent separation ofthe airflow) to obtain higher l if t and steeper take-offs and landings. In the current program, han-dling qualities at very low speeds (about 60knots) are being evaluated.

An especially promising example o f a STOLvehicle i s the Breguet 941 deflected-slipstream,inter-connected-propeller aircraft. RecentNASAflight and simulator studies indicate that this con-cept may have a good deal to offer in applica-tions for various military and civil uses.

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