Flight-Testing For The Amateur By G. Jacquemin, EAA 3618 7408 Granite Ave., Orangevale, Calif. EST FLYING a new airplane, whether it. is a new design T or simply reproduced from plans is a subject which seems to have been somewhat neglected. In general, the amateur does not always have a clear understanding of what is meant by flight-testing. There is a tendency for him to assume that a couple of hours flight time flown by a competent pilot is all that is necessary to close this subject. This, in fact, constitutes only a very small part of flight-testing as it involves only the initial flight and a rather cursory check-out of the flying qualities. Most amateurs recognize the need for another phase of flight-test necessary to ensure that the aircraft will not present any bad behavior in stall. As a rule, this is where test flying ends so long as the aircraft does not exhibit any objectionable features. Although the above test flights constitute a bare minimum, they should not, by any means, be considered as sufficient to examine the characteristics of a new aircraft. Flight-testing is a vast subject which can lead to very complicated procedures and require a considerable amount of time. However, the amateur is not interested in obtaining data which would be requested by an engineer for the purpose of designing or modifying an existing design. Except in a few rare cases, test flying for the amateur is rather a matter of calibrating his instruments, checking out elevator position in the flight cases of practical interest and for the purpose of obtaining a more accurate estimate of the performance of his aircraft so that he may compare these with the calculated estimates or with the figures claimed by the designer. The procedure to carry out these tests is neither complicated nor expensive. The amount of flying time required is small and can easily take place during the hours of restricted flying that all new aircraft must perform before obtaining their final certificate. The tests can be conveniently divided into three groups: (1)

Performance tests.

(2) Stability tests. (3) Controllability tests.

In all cases, it is assumed that initial test flights have been carried out by a competent test pilot and that, having satisfied himself that the flight characteristics of the aircraft are generally satisfactory, he has released the aircraft to the care of its owner. It is not superfluous to emphasize the importance of having a competent test pilot to do the initial flight of an amateur aircraft, especially in the case where the builder does not have at least 100 hours flying time and experience with various types of aircraft. It is well to remember that, even with all due care, mistakes can be

1.

PERFORMANCE TESTS.

During the initial flights, the test pilot takes note of some reference speeds such as, take-off speed, cruising speed at some specified rpm, speed at maximum rpm, approach speed, etc. Stalling speed is read later when stall

tests are performed. However, the test pilot can only record speeds read from the instruments provided in the cockpit. In other words, what he reads is the indicated speed which can be very different from the true speed duo to pressure error in the Pitot installation (assuming the instrument has been properly calibrated). These speeds are sufficient references for the pilot so long as he does not intend to navigate. A calibration is necessary in order to find out what are the true speeds corresponding to the indicated speed near ground level. The true speeds can be obtained very simply by checking in calm weather the speed over a chosen course and using a stop-watch to obtain the time required to cover the course. Since most country roads are set at one mile intervals (in flat countries) it will usually be easy to select a tenmile course (ten is a convenient number but five would do just as well) and have someone place some appropriate marker at each end. The course will be flown several times (at least five times) each way in very calm weather so that a time average can be established easily. Then, the speed can be calculated from the following formula with the time in seconds: „ , , length of course in miles ._ _ Speed mph = ——-———————————— 3600 time average (sec.) (See Figure 5) The test should be carried out at three different speeds: minimum cruising speed, speed at maximum continuous rpm, and at a speed between the two. At the end of each run, the pilot will record both the indicated speed and the time required to fly the course. The flights should be carried out at a safe altitude, at least 300 ft., but not too high, otherwise the accuracy of the test would suffer: 600 ft. seems to be the maximum height. The aircraft will be carefully trimmed about half-a-mile before crossing the first marker so that it will enter the course at a stabilized speed. The pilot will start and stop his stop-watch when the markers disappear under the wing or pass below a strut or some other convenient reference on the aircraft. The end product of this test is a curve of true speed versus indicated speed which usually looks like Figure 1,

being drawn between test points and the origin. nt vr SPff*

made and that the assistance of a test pilot in the final

check-out is invaluable from a safety point of view. The importance of this procedure is very obvious whenever the aircraft to be test flown is a prototype of a new design or a modification of an existing one, but it is nevertheless just as important for aircraft reproduced strictly according to plans. 8

APRIL 1965

trttit J*>r

FIG. 1

In most instances, it will bs found that the Pitot installation reads low. Although it is not of great importance to the amateur, he should record also atmospheric temperature, barometric pressure and flight altitude above sea level so that the performance data may be reduced to sea level standard

HAX.

K/C

conditions. This may be important to an engineer if his

help is required at some later date. This extra data is

necessary if the flights have been carried out from an airport lying at high altitude, then correction of the speed data for altitude must be done in order to compare with the values quoted by the designer which are usually given at sea level in standard air. Correction method is given in the appendix. FIG. 2

Stalling Speed.

It is rarely possible to obtain an accurate reading of stalling speed. It is of course not possible to check it out by flying at that speed over a course. On the other hand, a standard Pitot installation will not read accurately outside of a cone of approximately 10 degrees angle. If the Pitot is set to read correctly at cruising speed, it will rotate some 15 degrees at stalling speed and thus read low. On some aircraft, it is possible to see the indicated speed fall off to zero at the stall. It is therefore difficult to obtain, with any good precision, the true value of the stalling speed. Fortunately, the stalling speed can be calculated with fair accuracy for small airplanes of this category from the

Service Ceiling.

The practical ceiling is defined as the altitude at which the rate of climb is reduced to 100 fpm. The rate of climb tests should be stopped when the aircraft R/C is reduced to this value at each indicated speed V,. then, a curve of practical ceiling versus V,. can be drawn and the speed for maximum practical ceiling read from it as shown on Figure 3.

usual formula:

•/ where W is simply the wing loading. We do not know ~S~ c i.max' but fortunately, for most amateur-built aircraft having no flaps, C I m n x is approximately equal to 1.2 with the airfoils in current use. C Lnillx can also be obtained with good accuracy from airfoil data provided it is corrected for aspect ratio, Reynolds number and surface roughness. Hence, an approximate value of V stan can be calculated and checked out on Figure 1. Rate of Climb.

The stop-watch will again be the instrument used to provide data from which the rate of climb can be calculated. The aircraft will simply be flown at a steady indicated speed and the time to pass every 300 ft., 500 ft. or 1,000 ft. mark recorded. The test should be repeated at least at three indicated speeds, but preferably five. The rate of climb in fpm is simply found by the following formula: R/C =

FIG. 3 Minimum Gliding Angle.

In case of engine failure, it is important to know what is the speed for the minimum gliding angle. By definition, it is the angle for which C, /C D is maximum. Unless the engine is fitted with a starter, power-off flights required for these tests will have to be followed by dead stick landings and may require some practice. Power-off flights will be made during which speed and rate of sink will be read, then plotted in the following manner which is familiar to sailplane pilots — Figure 4.

altitude element (300', 500' or 1000')

X 60 time at end — time at start (See Figure 6) It will be found that at some indicated speed V,., the

rate of climb is greater than at the others. The data can be

plotted to determine the best climbing speed at each altitude as shown on Figure 2. It will be found convenient to use two stop-watches mounted on a small board and placed in such a manner that their buttons can be pushed simultaneously so that

one watch can be stopped and the other started affording

convenient recording for each altitude element. Thus, simultaneous reading of altimeter and stop-watch can be avoided.

FIG. 4

A tangent to the curve drawn through the origin will

determine the speed for the best gliding angle. This speed should be marked on the speed indicator

dial or placarded in the cockpit as it will permit in case of emergency to stretch a glide to its maximum. (Continued on next page) SPORT AVIATION

V

FLIGHT TESTING FOR THE AMATEUR . . .

(Continued from preceding page)

2.

STABILITY TESTS.

It will be necessary to obtain an appreciation of the damping in pitch. The stop-watch will again be used. The procedure is quite simple. First, the aircraft is trimmed to fly level at some cruise rpm. Stick Fixed.

The stick is pushed forward about two-thirds of its course and brought back immediately to neutral and held there. The nose will drop, then rise, overshoot, drop again,

rise again, etc. The oscillation will die out after a few cycles. Count the number of times the nose dropped and record the time required for the oscillation to disappear. If the oscillation does not die out quickly, something is wrong, the CG is probably too far aft. Repeat with a pull on the stick rather than a push. Repeat the test at various cruising speeds to explore as much as possible the speed spectrum. Stick Free.

The procedure is the same as above except that once the stick has been pushed or pulled, it is released free to return to neutral. Again, the oscillation must die out quickly. If it does not, the CG may be too far aft or the geometry of the tail inadequate. These tests should be performed, at least, at two different CG locations and in calm weather. Stability in Yaw.

Similar tests can be carried out using the rudder instead of the elevator. It will be found that homebuilt aircraft frequently exhibit some instability in yaw with rudder free. This should be corrected.

s. f e t i . 1 1 1 1 I. I. /*"• • '•*

What to Do in Case of Instability.

Instability exists when the amplitude of an oscillation increases with time instead of dying out. It occurs also when it diverges suddenly as is often the case in yaw with too little fin (rudder lock). Instability will show up more frequently with control free than with control fixed. In all cases, it should be corrected; no aircraft should be allowed to fly with such undesirable characteristics. A badly damped oscillation will usually make the aircraft hard to manage in turbulent weather. If an oscillation does not vanish within a few seconds, steps must be taken to correct it. In pitch, it will be generally sufficient to move the CG forward and set an aft CG limit which will ensure satisfactory characteristics at all times. If this does not work, then the design of the tail should be examined

and the amateur will be wise to seek help from a specialist. 3.

CONTROL TESTS.

It is necessary to know what is the elevator angle for various flight conditions. This is important to determine the best stabilizer setting and to ensure that enough control is provided in all flight cases. The instrumentation required is a simple measuring tape, preferably the fabric type used by dressmakers. The spool of tape is attached to the stick and the end of the tape to a hook behind the firewall. By pulling on the stick, the spool of tape unwinds and it is possible to read the stick position in inches from the firewall. Using a protractor, a calibration is made of the elevator angle versus stick position and the elevator zero angle duly noted. Flight tests are carried out, noting stick position versus speed. The elevator zero angle should correspond to normal cruising speed. If it does not, the stabilizer setting should be altered until correct elevator position is obtained. It is important to check also that sufficient elevator power is available in case of balked landing when the aircraft is near stalling speed and full power is suddenly applied. This is particularly important with aircraft having a large pitching moment due to power. Elevator Control Force.

The aerodynamicist will derive some useful information regarding the stability of an aircraft from a plot of stick force versus speed. This is quite important in the case of tailless designs. Unfortunately, some special equipment is necessary: viz., a dynamometer handle from which the stick force can be read. A dynamometer handle must be an accurate instrument. It is a rather complicated piece of equipment. For convenience, it should be able to give both push and pull loads within at least one-quarter of an ounce. It seems it is an instrument which should be made as a club item and available on a rental basis to the amateur. Speed Correction for Altitude and Temperature.

This correction will be applied to a full power speed test. It applies to the true speed obtained by the stopwatch method described in the performance tests. The speed at sea level can be found by using the following formula:

Air. 2 10

APRIL 1965

where h is the altitude in feet and A t the temperature in centigrade above or below standard temperature.

FLIGHT-TESTING . . . (Continued from page 10)

where a (sigma) is the air density ratio. It is given in many text books for standard atmosphere where temperature at sea level is 15 deg. C (50 deg. F). These tables give also the standard temperature at various altitudes. If the test has been carried out at a temperature different from the standard temperature, a can be found from an air chart such as the chart given on page A3-3 in Technical Aerodynamics by K. D. Wood (1955). If such an air chart is not available, a can be calculated from the following equation: 1. 1 "•

f 288 -. i 268

—/

Data Recording.

In most cases, the test pilot will appreciate being relieved of the chore of recording the data by writing. This can be accomplished, if there is aircraft radio equipment in which case, recording can be made at a receiver station on the ground or in a chase plane. If the aircraft does not have radio, Citizen Band transmitter/receivers may be used for the same purpose, but may require some testing to ensure that the interference level is not too high and allows good reception of the data. If radio communication is not possible, the use of a small portable tape recorder aboard the aircraft will provide a means of simplifying data recording and keep permanent record. *

THE BEDE XB2 AND BD3 . . . (Continued from page 7)

able since they form individual air pockets. The ability of the skin to give instead of ripping gives the structure very high impact absorption characteristics as has been proven by numerous accident studies. Controls arc accessible from a removable panel on the bottom of the fuselage. All electronic gear can be housed in the nose section, which will be quickly removable for easy maintenance. The prototype presently flying is of the four-place XBD-2. The philosophy behind the XBD-2 was to build a comparatively simple ship to examine the basic factors involved in a design of this type. From it should come data used to refine the design of the production models. Performance was compromised to provide ease of con-

With the airspeed probe on the Bede XBD-2 thrust forward, the Bede Aircraft Co. challenges the aviation world with its new concept in aircraft design. This XBD-2

is the prototype for the projected BD-3 model.

BEDE XBD-2 and BD-3

Specifications:

BD-3

XBD-2

Gross weight . . . . . . . . . . . . . . . 3 , 3 0 0 lbs. . . . . . . . Useful load . . . . . . . . . . . . . . . . . — ......... Wing area . . . . . . . . . . . . . . . . . . 150 sq. ft. . . . . . . . . Length . . . . . . . . . . . . . . . . . . . . .23.7 f t . . . . . . . . . . Span . . . . . . . . . . . . . . . . . . . . . . . 3 7 . 5 f t . . . . . . . . . . . Height . . . . . . . . . . . . . . . . . . . . . 12.4 ft. . . . . . . . . . . Fuel ..................... — ...........

Propeller . . . . . . . . . . . . . . . . . . . H a r t z e l l , 3-blade . . aluminum alloy constant speed Powerplant . . . . . . . . . . . . . . . . . 2 Continental O-300, 145 hp each Performance (computed):

Maximum cruise (65% power, 9,000 ft.) Normal cruise (50% power, 10,000 ft.)

179 mph —

Range (with 45-min. reserve) and fuel consumption Maximum cruise . . . . . . . . . . . . . . . . . . . 16 gph Normal cruise ............. — Service ceiling . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 , 0 0 0 ft. Single-engine absolute ceiling . . . . . . . . . . . 14,500 ft. Rate of climb 2 engines (max. gross wt.) 1,050 fpm Rate of climb 1 engine . . . . . . . . . . . . . . . . 720 fpm Stall speed with boundary layer control 42 mph Stall speed without boundary layer control 64 mph Take-off Ground roll . under 300 ft. Over 50 ft. obstacle under 500 ft.

.4,300 lbs. .2,254 lbs. .192 sq. ft. .30.8 ft.

38.5 ft. 12.4 ft.

116 gals. .Hartzell, 3-blade aluminum alloy constant speed .2 Lycoming 10-540-BIA-5

290 hp each

300 mph 280 mph 1,080 miles, 27 gph 1,270 miles, 22 gph 28,000 ft. 21,000 ft. 2,260 fpm 1,200 fpm 42 mph 74 mph

under 300 ft.

under 500 ft.

struction by using a fixed-gear, 145 hp engines, standard Goettingen airfoil, and rivets in some areas instead of bonding. This ship is the only one for which performance figures can presently be released and are those quoted in the listing of this article. Since the engines are coupled to a common drive shaft and propeller in line with the longitudinal axis of aircraft, no assymmetrical thrust problems complicate engine-out operation. Indications have been received from the FAA that multi-engine ratings will not be necessary to pilot this aircraft. Thus, single-engine rated private pilots making up 85 percent of this group, will have available a ship with twin-engine performance that can take off from an improvised short strip at their own door. Competitively priced with other mediumpriced twins and with superior speed and range, the Bede line should find a ready and enduring general aviation

market.

#

SPORT AVIATION

11