Flight records are presented from an early flight test of a wing-tip mounted tilting-ducted-fan, vertical-take-off and landing (VTOL) aircraft configuration. Time histories of the aircraft motions, control positions, and duct pitching-moment variation are presented to illustrate the characteristics of the aircraft in hovering, in conversion from hovering to forward flight, and in conversion from forward flight to hovering. The results indicate that during essentially continuous slow level- flight conversions, this aircraft experiences excessive longitudinal trim changes. Studies have shown that the large trim changes are caused primarily by the variation of aerodynamic moments acting on the duct units. Action of the duct-induced downwash on the horizontal stabilizer during the conversion also contributes to the longitudinal trim variations. Time histories of hovering and slow vertical descent in the final stages of landing in calm air show angular motions of the aircraft as great as +/- 10 deg. about all axes. Stick and pedal displacements required to control the aircraft during the landing maneuver were on the order of 50 to 60 percent of the total travel available

Kelley, Henry L.

Tapscott, Robert J.

English

NASA Technical Reports Server

NASA TN D-372 
TECHNICAL NOTE 
0-372 
A FLIGHT STUDY OF THE CONVERSION MANEUVER 
O F  A TILT-DUCT VTOL AIRCRAFT 
By Robert J. Tapscott and Henry L. Kelley 
Langley Research Center 
Langley Field, Va. 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON November 1960 
https://ntrs.nasa.gov/search.jsp?R=19980227756 2020-06-15T22:49:44+00:00Z
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
TECHNICAL NOTE D-372 
A FLIGHT STUDY OF WE CONVERSION MANEUVER 
OF A TILT-DUCT VTOL AIRCRAFT 
By Robert J. Tapscott and Henry L. Kelley 
SUMMARY 
Flight records are presented from an early flight test of a wing-tip 
mounted tilting-ducted-fan, vertical-take-off and landing (VTOL) aircraft 
configuratidn. 
and duct pitching-mbment variation are presented to illustrate the char- 
acteristics of the aircraft in hovering, in conversion from hovering to 
forward flight, and in conversion from forward flight to hovering. 
Time histories of the aircraft motions, control positions, 
The results indicate that during essentially continuous slow level- 
flight conversions, this aircraft experiences excessive longitudinal trim 
changes. Studies have shown that the large trim changes are caused pri- 
marily by the variation of aerodynamic moments acting on the duct units. 
Action of the duct-induced downwash on the horizontal stabilizer during 
the conversion also contributes to the longitudinal trim variations. 
Time histories of hovering and slow vertical descent in the final 
stages of landing in calm air show angular motions of the aircraft as 
great as +loo about all axes. Stick and pedal displacements required 
to control the aircraft during the landing maneuver were on the order 
of 50 to 60 percent of the total travel available. 
INTRODUCTION 
In a continuing effort to provide data for developing flying qual- 
ities criteria and to define the basic characteristics of various types 
of aircraft, the National Aeronautics and Space Administration is cur- 
rently obtaining flight data during the early trials of the VTOL test 
aircraft recently constructed. 
flight data were obtained was a tilt-wing type of aircraft. 
of some of the early flight trials with ttie tilt-wing aircraft are pre- 
sented in reference 1. Some preliminary data have now been obtained on 
a second VTOL configuration to reach the flight-test stage. 
reported herein are from manufacturer's flight tests of a tilting ducted- 
fan type of aircraft (the Doak 16) . 
The first of these vehicles on which 
The results 
The data 
2 
One problem area being studied in the VTOL concept is the conversion- 
flight region between hovering and conventional airplane flight. 
flight characteristics of the tilt-duct VML test bed during conversion 
from hovering to forward flight have been documented by data obtained in 
flight with recording instrumentation. Some of khese flight records were 
analyzed and are incorporated herein to show characteristics that must be 
considered in future designs of this type. 
The 
The flight records utilized herein were obtained with the coopera- 
tion of Doak Aircraft Coaany, Incorporated. 
APPARATUS AND PROCEDURE 
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Aircraft 
The vertical-take-off-and-landing flying test bed shown in figures 1 
and 2 is similar in configuration to a conventional airplane, with the 
exception that a tilting-ducted-fan assembly is mounted at the tip of 
each wing. 
tion perpendicular to the wing-chord plane f b r  hovering to a position 
essentially parallel to the wing chord for high-speed flight. 
cal dimensions of the aircraft for this investigation are given in 
table I. 
The thrust axis of the ducted fan can be rotated from a posi- 
The physi- 
The ailerons, elevator, and rudder are actuated in normal fashion 
for the forward-flight configuration. A single switch on the top of the 
control stick is used for duct rotation during the conversion. For r o l l  
control in fiovering flight and during the conversion process, the lateral 
stick motions actuate guide vanes arranged radially in each duct inlet. 
The actuation of these guide vanes changes the effective angle of attack 
of the fan blades, and thereby changes the thrust output. 
of the duct inlet-guide-vane control is such that lateral stick motions 
cause a maximum guide-vane pitch change when the ducts are set for verti- 
cal flight, and the same motions cause correspondingly less change as 
the ducts are rotated toward the forwaratflight position. When the ducts 
are rotated fully to the forward-flight position, lateral stick dis- 
placements cause no guide-vane motion. 
The design 
The cruciform tail vanes in the engine-exhaust exit (shown in fig. 2) 
are used for pitch and yaw control during hovering. 
vanes are not phased out as the ducts are rotated. 
steel tailpipe directs the turbine exhaust gases over the tail vanes. 
These vanes are of three-piece articulated dqijign. 
control is accomplished in the hovering configuration by variation in 
rotational speed of the fixed-pitch ducted f w .  
is provided at the engine power control for this purpose. 
These cruciform tail 
A long stainless- 
(See fig. 2.) Height 
A vernier adjustment 
The horizontal 
3 
stabilizer can be varied through 1l0 to help offset the nose-up pitching 
moment encountered during conversions. 
Instrument at ion 
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The airspeed, pressure altitude, angle c;f attack, duct angle, engine- 
output shaft speed, horizontal-stabilizer angle, and engine gear-box oil 
pressure (provides torque output reference) are recorded by two motion- 
picture cameras photographing the pilot's instrument panel. 
forces and moments are sensed by strain-gage bridges mounted on the duct 
support trunnion and recorded on a 14-channel oscillograph as an axial- 
force component (thrust), normal-force component, and moment tending to 
rotate the duct (pitching moment). 
the aircraft angular velocities about the roll, pitch, and yaw axes, as 
well as lateral-, longitudinal-, and directional-control positions. 
The duct 
Also recorded on the oscillograph are 
Test Conditions and Procedures 
The data obtained during test flights of the tilt-duct aircraft 
included approximately level-flight conversions from hovering to forward 
flight, from forward flight to hovering, and a hovering descent to the 
landing condition. The ground altitude at the test site was approxi- 
mately 2,200 feet. The data on the conversion-flight regime presented 
herein were obtained during t@ first conversion flight at low altitude 
(between 50 and 100 feet above the ground) representative of VTOL opera- 
tion. Earlier conversion flights had been made, however, by the same 
pilot at altitudes of several thousand feet. These flights and conver- 
sion flights made by NASA pilots subsequently to the one for which data 
are presented herein indicate qualitatively similar characteristics. 
is believed, therefore, that the data obtained are typical for the 
aircraft. 
It 
Time histories of representative conversion-type maneuvers were 
obtained by direct readings of the records at regular time intervals plus 
between-point fairings based on detailed inspection of the records in the 
zones between the points. The time histories with the data points shown 
were obtained f r o m  motion pictures of the pilot's panel taken at two 
frames per second; other time histories were taken directly from contin- 
uous oscillograph records. 
It should be noted that the magnitudes of the duct moments shown 
herein include the relieving moments resulting from the duct drive shaft 
torque and are thus applicable from a standpoint of structural load in 
the support mechanisms. In order to obtain the aerodynamic moment acting 
on the aircraft, for design needs such as estimating aircraft trim varia- 
tions caused by each duct, the values of duct moment shown must be 
increased in the nose-up direction by an amount equal to the torque in 
4 
the duct drive shaft. 
maximum engine output speed, the relieving moment amounts t o  approxi- 
mately 475 foot-pounds. 
For rated power (430 horsepower per duct) at 
RESULTS AND DISCUSSION 
Conversion From Hovering t o  Forward Flight 
Time h is tor ies  during the conversion from hovering t o  forward f l ight  
of velocity, duct angle, horsepower, pressure al t i tude,  elevator position, 
duct moment, angle of attack, and horizontal s tab i l izer  angle are shown 
in  figure 3. The conversion from hovering t o  forward f l i gh t  w a s  completed 
i n  11 seconds, a minimum amount of time for  this a i r c ra f t  (it takes a 
minimum of 11: seconds t o  ro ta te  the ducts goo); power changes w e r e  smooth, 
and the control deflections, although judged t o  be excessive, were not 
intolerable. The a l t i tude  against time curve shows a continuous increase 
throughout the accelerating condition, indicating that the a i r c ra f t  i s  
capable of complete conversion during an uninterrupted clinib-out from the  
take-off point. During t h i s  conversion only about 30 percent of the 
available longitudinal control was used t o  maintain continuous level  
f l i gh t .  It should be noted, however, t ha t  the maximum nose-down trim 
set t ing of the horizontal s tab i l izer  w a s  used t o  help offset  the nose-up 
pitching moment. 
Conversion From Forward Flight t o  Hovering 
In  the conversion from forward f l i gh t  t o  near hovering, the time 
h is tor ies  of velocity, duct angle, horsepower, pressure al t i tude,  eleva- 
t o r  position, duct moment, angle of attack, and horizontal s tab i l izer  
angle are shown i n  figure 4. 
f l i gh t  t o  near hovering w a s  completed i n  approximately 1 minute, much 
slower than the  accelerating conversion. During this  conversion a large 
amount of forward s t ick  trim change w a s  experienced i n  an attempt t o  
maintain continuous leve l  f l igh t .  
60 seconds, the  s t i ck  was  against or close t o  the forward stop, and the 
angle of attack was  loo t o  l5O. 
set t ing of the horizontal s tab i l izer  was used t o  help offset  the nose-up 
pitching moment. The nose-down a t t i tude  of the a i r c ra f t  (negative angle 
of attack) over a period of 25 seconds, i n  which the a i r c ra f t  drag w a s  
kept low, resulted i n  a re la t ive ly  long time for  deceleration during 
this  conversion. If a faster deceleration were made ,  however, it would 
be expected that higher duct pitching moments would resu l t  and require 
more forward change i n  the position of the s t i ck  than did the nose-down 
technique. 
1,700 foot-pounds and lasted f o r  about 31 seconds through a velocity 
range of 100 t o  50 knots and a duct angle range of 30° t o  60'. 
be noted again that the measurements of duct pitching moments shown herein 
The decelerating conversion from forward 
It can be seen tha t  a t  a time of 
The meximum a i r c ra f t  nose-down trim 
The maximum moment encountered on each duct w a s  about 
It should 
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5 
include the relieving effects of the torque in the duct drive shaft; thus 
the magnitude of aerodynamic moments tending to cause the aircraft to 
nose up are greater than the values shown in this figure by amounts up 
to about 47'3 foot-pounds at rated power and rpm. From these data it is 
indicated that the most severe design condition from a standpoint of 
design loads and trim requirements of a ducted-fan configuration occurs 
during the decelerating conversion where the ducts experience a large 
angle of attack at relatively high airspeeds. Although the aerodynamic 
moments acting on the ducts appear to be the primary cause of the longi- 
tudinal trim variations, another factor is the action of the duct-induced 
downwash on the horizontal stabilizer. 
Hovering and Vertical Descent to Landing 
During the hovering and vertical descent to the landing condition, 
the time histories of control position movements and angular velocities 
about the three axes are shown in figure 5. It should be noted that even 
though these data were taken under calm air conditions, large control 
motions (sometimes as much as 50 to 60 percent of the total available 
control travels) were used to control the attitude of the aircraft and 
hold it over the intended landing spot. By integration of the angular 
velocity traces it can be found that angular displacements of the air- 
craft as much as -1-10' from the trim attitude are experienced about a l l  
axes. 
control motions and aircraft angular velocities whether the large, erratic 
aircraft motions are induced by excessive control manipulation in this 
first conversion at low altitudes (i.e., 50 to 100 ft) or whether the 
large control motions result from an effort by the pilot to correct exces- 
sive uncontrolled-for (not pilot induced) motions of the aircraft. 
flights by NASA pilots, however, tend to show that the large control 
motions were necessary to correct uncontrolled-for motions of the 
aircraft. 
It is not possible to determine from the time histories of the 
Later 
CONCLUDING RENARKS 
The preliminary study of flight records taken during conversion 
maneuvers performed by the tilt-duct aircraft under favorable wind con- 
ditions indicated the following conditions: 
1. Results obtained during conversion from hovering to forward 
flight indicate that this condition provides the most desirable flying 
qualities of the three conditions studied. The ducts were rotated to 6' 
continuously in the minimum amount of time, power changes were smooth, 
and the control deflections, although judged to be excessive, were not 
intolerable. The altitude against time curve shows a continuous increase 
6 
throughout the accelerating condition, indicating that the aircraft is 
capable of corqplete conversion during an uninterrupted climb-out from 
the take-off point. 
2. The results obtained during conversion from forward flight to 
hovering indicate that this maneuver represents the largest problem area, 
insofar as the practical operational standards for transition flying are 
concerned. 
maneuver was the excessive longitudinal trim change caused by variation 
of aerodynamic moments acting on the duct. 
dawnwash on the horizontal stabilizer also contributes to the longitudinal 
trim variations. 
The most apparent of the difficulties encountered in this 
Action of the duct-induced 
3.  During hovering and vertical descent to landing, results indicated 
Plots L 
8 
9 
1 
angular displacements of the aircraft of flOo about the three axes. 
of control positions show that as much as 50 to 60 percent of the total 
available control travel was used to control the aircraft during hovering 
in calm air. 
Langley Research Center, 
National Aeronautics and Space Administration, 
Langley Field, Va., January 26, 1960. 
REFERENCE 
1. Thomas, Lovic P., 111: A Flight Study of the Conversion Maneuver 
of a Tilt-Wing VTOL Aircraft. NASA TN D-153, 1959. 
7 
TABU I.- PHYSICAL CHAEUCCTERISTICS OF TBE AIRCRAFT 
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1 
Ducted propellers: 
D i a m e t e r , f t . .  . . . . . . . . . . . . . . . . . . . . . . .  4 
Number of blades (each fan) . . . . . . . . . . . . . . . . .  8 
Inside diameter, f t  . . . . . . . . . . . . . . . . . . . . .  4 
Chord, f t  . . . . . . . . . . . . . . . . . . . . . . . . . .  2.75 
Rotation, deg . . . . . . . . . . . . . . . . . . . . . . . .  92 
Span (exctuding ducts), f t  . . . . . . . . . . . . . . . . . .  16 
Overall span, f t  . . . . . . . . . . . . . . . . . . . . . . .  25.3 
Mean aero$ynamic chord, f t  . . . . . . . . . . . . . . . . . .  5.89 
Air fo i l  skction (modified) . . . . . . . . . . . . . . . .  NACA 2418 
Taper r a t i o  . . . . . . . . . . . . . . . . . . . . . . . . .  0.747 
Sweep, deg . . . . . . . . . . . . . . . . . . . . . . . . . .  0 
Dihedral, deg . . . . . . . . . . . . . . . . . . . . . . . .  0 
Area, s q f t  . . . . . . . . . . . . . . . . . . . . . . . . .  96 
Area of each aileron, sq f t  . . . . . . . . . . . . . . . . .  6.2 
Height (approximate)', f t  . . . . . . . . . . . . . . . . . . .  4.8 
Average chord, f t  . . . . . . . . . . . . . . . . . . . . . .  2.55 
A i r f o i l  section (modified) . . . . . . . . . . . . . . . .  NACA 0012 
Area, s q f t  . . . . . . . . . . . . . . . . . . . . . . . . .  13.9 
k e a ,  s q f t  . . . . . . . . . . . . . . . . . . . . . . . . .  28.5 
Air fo i l  section (modified) . . . . . . . . . . . . . . . .  NACA 0012 
span (projected) . . . . . . . . . . . . . . . . . . . . . . .  11.6 
Dihedra1,deg . . . . . . . . . . . . . . . . . . . . . . . .  10 
Fuselage length (approximate), f t . . . . . . . . . . . . . . .  28.7 
Overall length (approximate), f t  . . . . . . . . . . . . . . . .  31 
Engine . . . . . . . . . . . . . . . . . .  LYCOM~MT YT-?~-L-~ 
Weight as flown (approximate), l b  . . . . . . . . . . . . . . .  3100 
Forward, percent M.A.C. . . . . . . . . . . . . . . . . . . .  25 
Rearward, percent M.A.C. . . . . . . . . . . . . . . . . . .  32 
Ducts : 
Wing : ' 2  4 3  
Vertical  tail: 
Horizontal tail: 
Center of gravity: 
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A Flight Study of the Conversion Maneuver of a Tilt-Duct VTOL Aircraft

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