communicated by the accidents investioation committee...

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R E P O R T O N A C C I D E N T S TO C E R T A I N A E R O P L A N E S ,

W I T H S P E C I A L R E F E R E N C E TO " S P I N N I N G . "

Communicated by the ACCIDENTS INVESTIOATION COMMITTEE.

Reports a~d Memoranda, No. 592. December, 1918.

SUMMARY.--(a) Introductory (masons for inquiry).--The Accident Com- mittee were requested to investigate the considerable number of accidents which had occurred during " spinning " and which related mainly to one type of aeroplane A,.

(b) Range of the investigation.--A thorough investigation into the peculiarities of aeroplane A compared with other types B, C, D and E was undertaken. Longitudinal stability experiments to compare the spinning tendencies of the machines of type A and C, which were both good single seater machines, were made, the air speed recording accelerometer being used. Measurement of the period of spin and the force on the control stick were also recorded. Consideration of these results led to an inquiry of the effect of the slip stream on the flying of an aeroplane.

(c) Conclusions.--The main conclusion reached was that an increase in rudder and elevator urea would render the aeroplane safer, whilst possibly increasing its handiness at high levels. A change in the training routine from E, a machine not greatly sensitive to its controls, to A, which is very sensitive, was considered undesirable, a recommendation being made that the pilots should have special training before flying the latter, and that the machines should be fitted with dual control.

From a comparison between A and C, which are respectively unstable and stable aeroplanes, it undoubtedly appears that an equally good fighting machine can be designed which is stable, and that it is unnecessary and dangerous to design an unstable aeroplane with a view to obtaining great controllability, a feature which can be as readily obtained with a stable machine.

I. I~TRODUCTO~Y.

1. T h e C o m m i t t e e was r e q u e s t e d t o i nves t i ga t e the con- s iderab le n u m b e r of acc iden ts wh ich h a v e r ecen t ly occur red dur ing " S p i n n i n g , " a n d p a r t i c u l a r l y as t o h o w fa r these acc iden t s m a y h a v e been caused b y some phys ica l d isabi l i ty of t he pilot , such as dizziness.

2. I n t he m a j o r i t y of cases e x a m i n e d u n d e r th is reference, i t was f o u n d t h a t the pi lots c o n c e r n e d h a d c o m p a r a t i v e l y l i t t le expe r i ence on this or a n y o the r t y p e of machine .

8. I f a c o m p a r i s o n is m a d e be tween aerop lanes A, B, C, D , it will be f o u n d t h a t the t o t a l n u m b e r of acc idents occur r ing on A is g r e a t l y in excess of t he t o t a l on each of t he o ther types .

855

This is also borne out when the comparison is made on the basis of the percentage of aeeidents to the number of machines on charge.

Further, if this comparison be extended to the percentage of " Spinning " accidents to the total accidents on each type, the machine A stands out again most conspieuously. (See table, Appendix I.)

II. BEHAVIOUg 0~ ~ " k."

4. During the course of the investigation tile Committee had the advantage of evidence on tile behaviour of various aeroplanes from fourteen pilots with wide flying experience.

5. On many of the most importanb points their opinion was unanimous. Where differences oeeurred the contradiction was more apparent than real, and further evidence showed that such difference did not affect the actual man(mlvre. For instance, some pilots make use of the lateral control, bu t all agree that this does not materially affect the spinning of an aeroplane.

6. All the pilo~s agreed that ~he machine A had certain peculiarities which may be summarised as follows : - -

GI]?~ERAL CKARACTI~P~ISTICS.

7. (a) Stalt.b~g.--A as usually rigged is tail heavy and longitudinally unstable. This is accentuated when a light Le t~hone engine is substiguted for a heavier Clerget and/or where guns and ammunition are removed.

During normal horizontal flight, it is necessary to keep a continuous forward pressure of about 14 lbs. on the control column. (See Appendix II., R.A.E. Report.) Any relaxation of the pilot's effort is therefore likely to end in a stall. .Because of the longitudinal instability of the aeroplane engine failura will be followed by a stall unless the pilot dives the machine.

8. (b) 5Turning.--Most of the pilots, in the course of their evidence, drew attention to the fact that a steeply-banked right-lmnd turn requires full left rudder. Thi~ is accounted for by gyroscopic effect. (See Apl?endix III . (A) and (B).)

9. The unsymmetrical forces and couples on an aeroplane arise from engine torque, slipstream and the gyroscopic couple. The first two, torque and slipstream, may produce an unsym- metric setting of the ailerons and rudder respectively. They do not produce effects which depend on the rate of turning to any appreciable extent. The gyroscopic couple alone is capable of explaining the effect mentioned above.

10. The propeller, viewed from the pilot's seat, turn~ clockwise, and when the aeroplane is turning to the right it tends to put its nose down as a consequence of the gyroscopic eouple. The tendency is countered by left rudder and baek (or top)

856

elevator. Turuing to the left, the nose of the aeroplane tends to go up, and this effect is countered by left rudder and forward (or bottom) elevator.

11. As will be seen from the t,~ble in Appendix I I IB, the effective couple produced for a given setting of the rudder is, in A, 40 per cent. less than in C, whereas the gyroscopic couple to be counteracted is more than twice as great. To make the two machines equM in this respect the area of the rudder on A would require to be doubled.

From the evidence of the witnesses it is ~pparent that praeti- cMly all flyers feel strongly the ~eed of a larger rudder surface.

12. (c) (i) Steep Diving a'nd Divi,ng over the Vertical.--Several accidents appear to have originated from the pilot falling forward on his controls, particularly when diving at a target in firing practice. Owing to its longitudinal instability machine A needs to be carefully controlled in order to keep on the target, and is liable to dive suddenly if the stick is merely held. The effect produee({ is an acceleration which throws the pilot from his seat on to his belt, and the belt as fitted is not effective in keeping him from falling forward on to his controls.

When once the verticM has been passed the aeroplane will go over completely on to its back, as a machine which is tail heavy in normM flight will also be tail heavy in inverted flight.

13. (ii) S(tfcty Belts.--Safety belts were considered in regard to their efficiency for retaining the pilot in position and preventing him falling on his controls.

Two types appear to be in use :-- (t) Standard web belg. (2) Elastic belt.

The first is attached to the seat bearers by rope, and fastens more or less over the pilot's thighs and lower part of the stomach. While this allows great freedom of action of the shoulders, and the abil i ty--of great importance in fighting--to look behind, it does not hold the pilot securely in place and prevent his forward movement. In iustruetional work it is advisable for the pupil to be fairly positioned in his seat, and all possibility of falling forward on his controls r emoved . This could be effeeted by some modification of the seat and belt at tachment in order to bring the belt higher up the pilot's body.

The elastic belt has the advantage of being fitted round the seat, and therefore higher up, but the effect is nullified by the elasticity of the belt.

14. (d) Looping.---In the case of A all violent use of the controls must be avoided. With the stick pulled too hard back, the machine will fail to complete the loop. Further, if a strMght loop is required, left rudder must be used to counteract gyroscopic effect, and it was stated by one witness that the rudder was barely sufficient for this manoeuvre.

857

15. (e) Spinning.--The general method adopted when getting into a right hand spin is : - -

(i) Cut off engine. (fi) Bull back control stick.

(ifi) Pu t on right rudder. This describes the action usually taken, but the essential

feature is that the machine is slowed down until it stalls, when the normal sequence is a spin.

16. There is general agreement as to the position of the controls when in a spin. The essential feature is that the stick is kept hard back ; the position of rudder and ailerons is relatively unimportant.

17. The normal method of coming out of a spin is with the control stick slightly forward and other controls neutral. In this position the spin will soon stop, but the nose of the machine must be held down until flying speed is attained, when the stick should be pulled gently back, and the machine is flattened out.

In the ease of an emergency, the spin can be stopped more rapidly by putting the stick forward and reversing the rudder, but in this ease a violent manceuvre is induced, and want of skill may lead to the machine dropping back into the reverse spin.

18. I t appears to be established that the rotation cannot be reversed without an intermediate period, during which the machine is restalled. This does not require the fuselage to be brought to ~ horizontal position.

19. As usually rigged, the aeroplane A will not come out of ~he spin with hands off, and will only come out slowly, if at all, with all the controls held strictly central.

20. In training, the usual transition is from a machine E to A, and this transition appears to be too rapid. E, though very suitable for ordinary trainhlg, is much more difficult to get into ~ spin, and comes out much more easily, attaining normal flying position without appreciable effort. I t does not require to be held down at the end of the spin in the same essential manner as A. Its controls are not so sensitive, and there is plenty of time to recover in case of mistakes. The spin is slower, and there is less tendency to dizziness.

~i. There is no aeroplane at present in use which spins faster than A ; its time period is 1½- to 2 seconds, and the loss of height per turn 150 to 200 feet.

~ . A machine with large span and a large amount of inertia around the longitudinal axis will be slow in starting to spin, and may get up flying speed in the nose dive before a regular spin is established. The recovery of flying speed is assisted considerably by longitudinal stability ; in fact, a stable machine will automatically come out of the spin and recover speed when- ever the controls are left free. This probably accounts for the large difference in the number of spinning accidents in A and C.

858

The time of spin in the two machines is practically identical, and hence there cannot be any great difference in the physiological effect.

By reference to the Table in Appendix I it will be seen that the number of spinning accidents during the months of May and June were 28 for A and 3 for C. This fact must be attr ibuted, therefore, to the difference in stability between the two types.

9"3. Accidents due to ~pinning.--The Committee have considered 41 ~ccidents to machi~m A due to stalling and spinning. These accidents can be divided into two groups : -

Group (i) includes accidents due to stalls at low level. Group (ii) includes accidents due to spins continued to the

ground from a high level. A classified list will be found in Appendix IV. 9,4. On all aeroplanes loss of control occurs when stalled.

At a low level this results in a nose dive or spin, and an accident ensues as lack of head room precludes the completion of the manoeuvre before the ground is reached. Accidents in group (i) belong to this category. The common feature is tha t the stall was unintentional, the primary cause of the stall varying (e.g., engj~m failure, faulty turn, general inexperience).

25. In group (ii) there are six accidents (lgos. 284, 310, 346, 409, 534,-484) in which the flyer has spun in one direction and then the other, finishing by hitting the ground. In these it is reasonable to suppose that the second spin was involuntary.

9"8. Also in three cases (Nos. 308, 57.83, 153), where a spin has been restarted and continued to the ground, u like presump- tion ~rises.

9"7. Thirdly, there are three cases (Nos. 322, Y.52, 338), where an unsuccessful a t tempt at recovery was noticed when the aeroplane was approaching the ground.

9"8. Finally, there were 6 cases in which there was apparently no at tempt at recovery (Nos. 131, 536, ¥.32, 307, 151, 344). Such cases must be due either to mistaken action, caused by inexperience, or to inaction caused by temporary physical disability of the pilot.

I I I . PHYSIOLOGICAL ~]~FECTS OF SPIlg:NI~G.

9"9. In a manoeuvre requiring accurate judgment and skilled manipulation the attributes of the human organism are not less important than the properties of the machine.

80. In considering the mental ~nd physiological effects that spinning and other evolutions m~y produce on the pilot, the Committee have taken evidence both from pilots and from medical witnesses. All are unanimous in considering that spinning produces an effect which may roughly be described ~s dizziness. The acuteness of the sensation varies greatly with the experience

859

of the pilot, his state of health and the rate at which the machine spins. Three factors contribute to this condition :--

(i) The continued high acceleration. (ii) The rotating field of view.

(iii) The effect of the rotation on the semi-circular canals which normally determine the sense of equilibrium.

81. Owing to its rapidity in the spin, A is much more likely to induce dizziness than other and slower spinning types. The pilot spills, becomes dizzy, and loses full control of himself. He is then possibly able to make the necessary movements to come out e r a spin, but is unable to rcalise when the machine is in its normal position. In other and slower machines this would not be of great importance, but in A, with its extreme sensitiveness and tendency to stall, it is not only necessary not to do the wrong thing, but it is imperative to do the right one.

g2. The pilot, in his somewhat eonfused state, frequently does not exert the proper control, the machine again stalls, and goes into another spin. This results in a crash, as the second spin--usually in the opposite direction--inevitably causes the pilot Go lose all consciousness of control. The tendency to do the right thing when in a confused condition can only be at tained by h~bit. Should habits have been acquired for the controls of another type those will govern the pilot's action when his mind is confused. This points to the dual control machine of type N as essential for instruction.

IV. CoxcLIJSlO~. 38. I t appears that the number of accidents due to spinning

might be reduced by : - - (i) Alterations to the machine.

(ii) Modifications of the method of trMning.

34. (i) Alterations to the Aeroplane.---Most of the features of the design which determine the ease with which an aeroplane drops into a spin, and its rate of revolution during the spin, also effect its quickness on a turn and ready response to a light touch o n the controls. I t is not advisable to suggest modifications which, while decreasing the number of accidents during practice flights, would render the aeroplane less efficient and hence less safe on active service.

The Committee consider, however, that an increase in rudder and elevator area would render this aeroplane safer, whilst possibly increasing its handiness at high levels. Experiments on this question are in progress at ehe I~.A.E.

35. (ii) Tmining. - - I t is noticeable that out. of 41 fatal accidents due to spinning the majority occurred to pilots having little experience with this particular machine, and not very long practice on any other. The accidents due to spi~s near the ground are mostly the sequence of an accidental stall caused

860

by an unskilful turn, or by engine failure; several of these accidents occurred during the first or second solo on machines of type A.

The spins to the ground from a high level must mostly be at tr ibuted to the dizziness induced by the high speed of revolution, and to the rather high degree of skill required to right this machine after a spin. In several cases the aeroplane appears to have been pulled out of the dive before it had acquired sufficient speed, when a second spin followed.

86. Owing to the extreme sensitiveness of A, and also to certain distinctive peculiarities referred to above, the Committee considers that special training is necessary in this type.

87. The transition from E to A, unless the pupil has been very skilfully trMned on the former, is too rapid. As an intermediate type another machine is suitable, but the safest course is clearly to be found in the use of type A fitted with dual control. This was strongly advocated by several of the instructors who gave evidence.

By the use of instructional dual machines, the pilot can be made acquainted not only with the differences which exist between this type and less sensitive machines, bu t has also an opportunity to overcome any tendency to dizziness which the rapid spinning of A sets up.

35. In eases where previous dual instruction on A is impossible the movements necessary in a spin must be impressed so forcibly on the pupil that they will return to his mind subconsciously when the diNculty arises. The routine method is : - -

(i) Stick forward and central and rudder central. (if) Wait for the machine to acquire flying speed.

(iii) Pull out gently. 39. The Committee considers that in view of the considerable

risk of loss of control, pupils should be instructed that long spins are not to" be undertaken until the pilot has been accustomed to them by easy stages. That in no ease are spins to be started in the early stages under a fixed height of say 5,000 feet. I t should be borne in mind that owing to its physiological effects, spinning differs from other manoeuvres, and ~ long spin Mways involves an element of risk.

(Signed) ME~VY~ O'GoR~AN, Lt.-Col.,

Chairman of Committee.

861

A P P E N D I X I.

C O M P A R A T I V E T A B L E OF A C C I D E N T S D U R I N G MAY, 1918.

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A P P E N D I X I I .

R O Y A L A I R C R A F T E S T A B L I S H N I E N T R E P O R T NO. ]B.A. 251;

EXPERIMENTS TO COMPARE THE ~PINNING T E N D E N C I E S OF A AND C.

1. A reques t was recent ly received f rom the Air Minis t ry to ob ta in informat ion for the Accidents Commit tee relat ing to the longitudinal period, wi th fixed elevators, of cer ta in aeroplanes which are liable to get out of control during prolonged spins or when coming out of a spin. I t was t hough t t h a t the tendency to stall, which is one aspect of longi tudinal instabi l i ty, migh t account for some of t he invo lun ta ry spins which have been observed to precede an accident. The aeroplanes ment ioned were 13, C and A, the last being the worst in this respect.

2. Much da ta has previously been published respecting the longitudinal period of var ious aeroplanes wi th free elevators (see Advisory Com- mi t t ee Repor t s R. and l~I. 326 and 505*), bu t hardly any th ing has been done on C a n d A. I n addi t ion to the phugoid experiments , a series of spinning tests has been done on each of these aeroplanes. FreSher, in t h e case ot A, measurements of the force on the control stick necessary to t r im in vary ing condit ions of speed, tai l setting, height and th ro t t l e have also been obtained.

One machine of t y p e C and two of t ype A were used. I n every case tnll mil i tary load was carried. At has a 130 h.p. Clerget engine, Aa a I~.R.I: 'engine. The weight dis t r ibut ion in the la t te r was arranged so t h a t its weight and centre of g rav i ty coincided wi th those of the s tandard machine A with Clerget engine.

8. Longitudinal Stability Ezperimenls.--(a) l~[achine C. These experi- men t s were done a t heights va ry ing f rom 5,000 to 10,000 ft., the engine being switched off. As a tail adjus t ing gear is f i t ted to this aeroplane, two positions of the tai l were used, of wbich one was wi th the tail Ia l ly back (to t r im slow) and the o ther about t he middle of t he tai l settling range. W i t h each of these tai l set t ings, an oscilIation was done at th ree indicated speeds---45, 70, 85 m.p.h.

* See al~o T. 1038. Unpublished.

862

Two series of these were carried out. I n t he first, the air speed was recorded every five seconds by the pilot by means of a s top watch and air speed indicator. I n the second, the air speed was recorded by the airspeed recording accelerometer. Al though in the la t te r t i le t ime scale is liable to small var iat ion, i t is cer ta inly to be considered the more reliable method.

The actual m e t h o d of exper iment was as follows : - - T h e pilot would t r im the aeroplane a t the speed required wi th his engine switched off and lock tile elevators. The engine was then switched on for a m o m e n t to provide the init ial displacement and then off again, the aeroplane being subsequent ly left to itself.

Examples of the oscillations obtained are given in Fig. 2 and the results in Table 1.

I t will be not iced tha t the periods obta ined by the two recording methods are in fair agreement. The whole of the periods plot ted against indica ted speed (Fig. 1) lie modera te ly well on one curve, a l though small var iat ions are to be expected from the fact t h a t the period is no t a func t ion of the indicated speed alone, but also of the density, which var ied to some ex ten t in these experiments.

Tile period a t a g iven indicated speed shows no great var ia t ion wi th tail setting. This, of course, confirm.s the " s t ra ight line " law of varia- t ion of tail l ift wi th tai l incidence. I t m a y be remarked tha t these experi- nlents show C under wha t is probably its condi t ion of m a x i m u m stability. Previous exper iments have shown tt lat when the eIevators are free, this machine is m u c h more stable gliding than wi th engine on, and it is well established that , o ther th ings being equal, t he period is reduced by locking the elevators.

(b) Machines A 1, A ~..~The air speed recording accelerometer was used in these experiments , and as the aeroplanes are ve ry unstable, special a r rangements were m a d e for a quick release of the control stick. T h e m e t h o d was to t r im the aeroplane a t a g iven speed, lock the stick and leave it to itself, Records were obtained bo th for engine off and engine on. The tail set t ing was normal. Examples of the records obta ined are given in Figs. 4, 5.

A l though i t is difficult to draw exact conclusions f rom motions so unstable as these, there are clear indications, especially in the case of A~, t h a t t he aeroplane is more unstable wi th the engine running t h a n when i t is switched off. The mot ion is in all cases of the nature of an increasing oscillation, the nose u l t imate ly going down in a dive. Once in the case of each aeroplane, wi th engine off and a t an indicated speed of about ('0 m.p.h., a qui te regular oscillation of this t y p e was obtained (Fig. 5). The mot ion tends to become more violent as the speed increases.

I t is not easy f rom the records to make any differentiat ion be tween /tie effects of t he Clerget and B.R.I . engines. Indeed, since the weight and centres of g rav i ty were adjus ted to be roughly the same, t he two aeroplanes should only differ, so far as longitudinal period is concerned, because of the differences (1) on the rates of var ia t ion of the s l ipstream over the tail, and (2) in the height of the thrus t l ine above the centre of gravity. The centre of grav i ty of /k 1 having been m.easured, a rough calculat ion was made to discover if any reasonable increase of e levator area would ensure its stability. I t is es t imated tha t the present total tail area (23 square feet) would have to be about doubled to make it at M1 satis- factory in this respect. The present ra t io of tail to wing area (0"1) is, of course, abnormal ly small, and i t would seem tha t radical changes oi design are necessary to make this aeroplane stable.

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a t the beginning of the first t u rn and s topped it a t t he end of ti le last, before he moved the controls. The number of tu rns in each spin varied fronl 5 to 10. The height was observed a t t i le beginning and end of Lhe spin.

I n one ease, on Ax, the air speed and accelerat ion were recorded by the accelerometer. This record shows t h a t the speed on this par t icular spin was by no means constant , but increased, as the spin proceeded, in a series of jerks. The accelerat ion " K " appears to h a v e a rapid var ia t ion on ,each turn, reaching a m a x i m u m of abou t 3"5.

C spins wi th equal ease in ei ther direction, as does also A2, the period ~>:f a t u rn being abont 2 seconds in all cases.

A1 appears to spin smoothly to the left (period, 1"8 seconds), but badly to the right, when it goes round in sharp jerks and tried to come out of the sp in (period, 2"25 seconds).

These results are t abu la ted in Table 2.

5. Force on Control Stick (A).---These exper iments were carried out pr imari ly to determine wbether it is possible to arrange t i le tail set t ing of A so as to make it t r im for a large speed range wi th l i t t le or no effort on the pilot 's part. The var ia t ion of tt le necessary force wi th indicated speed is Mso useful in giving a rough indication of the stability of the aeroplane.

A2.--In the first instance the normal tail setting was used. The scheme was to measure the force at 5,000 ft. a t speeds f rom 50 to 100 m.p.h. wi th engine off, and then wi th engine on. This series was then repeated a t 10,000 ft. There are two sets of these. I n t i le first t i le control stick was f i t ted with elastic which took 6,} lbs. of force. I n the second this was removed.

Final ly, t he tai l set t ing was increased by 2 ° in t i le posit ive direction in .order to decrease the push required, and tile series was repeated with m) elastic on the stick.

The results are t abu la ted in Table 3 and plot ted in Fig. 3.

A v - - A similar series of tests was carried out on this aeroplane with normal tai l setting.

W i t h the normal tail set t ing the force curves for bo th these aerot)MnL, s (see Fig. 3) under all conditions are, roughly, s t ra ight lines parallel to the axis, the var ia t ions from these mean lines being a lmost within the order of accuracy of t he observations. This indicates neutral s tabi l i ty in lhe rough sense tha t i t takes t i le same force to hold the aeroplane s teadi ly :at a n y speed. Thus with this tai l se t t ing the aeroplane does not t r im a t al l be tween 50 and 100 indicated m.p.h.

The var ia t ions of force wi th height and condi t ion of engine, as shown by the re la t ive positions of t he var ious curves, agree fair ly well wi th what theore t ica l considerat ions would suggest.

Thus, a t a g iven height and indicated speed, the push required is less wi th the engine off t h a n wi th the engine on. For the s l ipstream increases t he down load on the fixed par t of the tail, and as the e levator has to pro- v ide an up-load to counte rac t this, the push is increased.

Again, wi th the engine off, t he force should be independent of he ight a t a g iven indicated speed, as t i le curves show. But wi th the engine on, a t a g iven indicated speed, the s l ipstream effect decreases wi th height, because the advance per revolu t ion increases. Hence the push will be g rea te r a t 5,000 ft. t h a n a t 10,000 ft. This is also borne out by tile curves.

W i t h the tai l set t ing increased by 2 ° t he curves m o v e up towards the axis, bu t this a l tera t ion .is still no t great enough to produce a t r imming speed in t he r~nge of ti le experiment . W i t h this set t ing t i le aeroplane would t r im at abou t 105 m.p.h, indicated speed with engine off a t 6,5011 ft. The push has a m a x i m m n va lue a t abou t 70 m.p.h., and decreases wi th increasing speed, indicat ing instabi l i ty in t he sense referred to above.

864

TABLE 1 .

L O N G I T U D I N A L P E R I O D S O F C.

P r o p s top w a t c h . B y aeeoleromoter .

' l ' ~ ' i l pull Airspeed. Period. Airspeed. Period. back. M.p.h. Sees. M.p,h. Sees.

Tai l central .

56~ 58 69 69½ 8a½

57 58 70 84 85

15"5 16"5 19"2 19"1 21 "4

14'0 16"7 19'2 22"1 23"1

64 70 84

61{ 73

85½

16 18 21

16 18

22

TABLE 2 .

RESULTS OF SPINNING TESTS. ENGINE OFF.

R i g h t h a n d spin . Lef~ hand spin.

Acre- No. of drop per Ver~ieM Time of |)liute. %urns. turn. velocity, turn.

dght No. ef p per Vertical Time of turns, velocity, turn. irn.

l ft . ft. pe r

soc. see.

- ; o K - . . . . 10 1 ~

A~ 5 - - ~ ! 2"0

10 ' - - - - i 2"1 i

A, 1 0 190 95 2"0 10 200 80 2"5

J

I _ f~7.

10 210

5 10

10 i 130 10 130

ft . per i see. see.

2.17

- - - - 2 '0 - - 2 " 0

72 1 '8 72 l "8

8 6 5

TABLE 3

RESULTS OF CONTROL FORCE EXPERIMENTS, Aa, A~.

N o r m a l tai l se t t ing .

E l a s t t c on stick.

Engine off. E~ ine on.

Ind. 5,000 ft. 10,000 ft, 5,000 ft, 10,000 ft. i /

Experiment. speed m.p.h. - - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . .

Force Force Force Force lbs. lbs. lbs. l~evs, lbs. lh.vs.

r

n ~ .

N o r m a l ta i l se t t ing .

E la s t i c r e m o v e d .

6 ,000-7 ,000 ft.

A 2 . Tai l s e t t i n g

increased . 2 ° e las t ic

r e m o v e d .

50 ½ puI1 1~ p~)ll

60 11 , 70 80 1 ,, ½ ,, 90 -} . . . . -}

100 1 -} ,,

60 5 p u s h

7 0 s , - - - 8 0 ,, ,, 5½ ,,

90 - - 100 6 __, 6 ,,

5 0 2 p u s h 6 0 2 ~ ,, - - 70 3 ,, - - 80 3 ,, - - 90 2 ,, - -

100 1 ,, - - 110 1 ,, - -

8 p u s h ,s

7 j ,

7 , ,

7 , ,

15 p u s h

15 ,,

15 ,,

12 p u s h 11 , ,

11 ,, 10 ,,

9 ,, 7 , ,

1,200 4½ p u s h 1,225 4 ,, 1,2,50 3} ,, 1,275 3-} ,, 1,300 4 ,,

1,2001 14 p u s h 15 ,,

1 , 2 5 0 ' 1 4 - - 14 ,,

1,300 I !4 ,

m

m

1,150 1,175 1,200 1,240 1 , 2 7 5

I 1,31o

1,200 1,225 1,250 1,275 1,310

Experiment.

A 1 •

N o r m a l tail set- t ing. N o elastic.

Ind. speed m.p.h.

50 60 70 80 90

1O0

Engine off.

5,500 ft. 9,500 ft.

:Force lbs. Force Ibs.

3 p u s h 2 p u s h 4 ,, 3} ,, 4 ,, 4 ,, 4 ,, 3½ ,, 5 ,, 4 ,, 4} ,, 4} ,,

Engine on.

5,500 ft. 9,500 ft.

Force Ibm Force Ibm.

10 p u s h 8 p u s h 1 % ,, 8 ~ ,,

11 ,, 9 , ,

11 ,, 8 5 ,, lO-} ,, 9 ,,

11 ,, 9 ,,

866

APPENDIX IIIA.

THE EFFECT OF SLIP STREAM ON THE FLYING OF AN AEROPLANE.

1. General note. ~. Longitudinal trim. B, Lateral trim. a~. Summary of effects of slip stream.

1. The trim of most high-powered aeroplanes is considerably ~,ffected by the position of the throttle, i.e., the aeroplane behaves differently with engine on and off. The aeroplane usually trims differently both longitudinally and laterally with engine on and off.

2. Consider first the longitudinal trim. Imagine the aeroplane to be gliding at any definite speed. There will be a certain air force on the tail which maintains the trim of the aeroplane. This force is usually downwards, and is nearly always so in the case of stable aeroplanes. I t will be spoken of as " tail l i f t " nevertheless, a downward force being a negative tail lift.

Now suppose the engine is suddenly opened out, two main effects will be produced : - -

(i) The propeller thrust will exert a pitching movement which will tend to put the nose of the aeroplane up or down, according as the line of thrust passes below or above the C.G. This effect is usually small.

(ii) The slip stream over the tail plane will increase the tail lift. As was explained above, the tail lift is downwards, therefore a greater force will be produced ; this will tend to make the aeroplane trim at a lower speed, i.e., will tend to stall if the effect is very large, as it often is.

8. Consider next the lateral trim. Again imagine the aeroplane to be gliding at any definite speed; there will be no slip stream and no engine torque, and the aeroplane will glide straight. Now suppose the engine to be suddenly opened ou t ; again two main effects will be produced : - -

(i) The torque applied to the engine bearers owing to the reaction of the engine will tend to make the aeroplane drop one w/ng. This effect is small, and usually so small as not to be noticeable.

(if) A slip stream from the propeller will be created and will strike the fin and rudder. Tiffs slip stream does not flow straight back over the fuselage, but is given a helical or swirling motion by the rotation of the propeller. If the fin were disposed equally above and below the centre line of the slip stream the air would strike the fin on one side at the top and on the other side of the bottom, and a torque only would be produced, which would be opposite to, though much less than, the engine torque. However, in the great majority of aeroplanes, the fin is almost entirely above the centre line of the slip s t ream; this means that the air strikes one side of the fin only. This force on the fin has, of course, exactly the same effect on the aeroplanes as turning the rudder ; the aeroplane swings round one way, and commences a sharp under-banked turn. In order to fly straight now with the engine on, the rudder must be put over so far that the air force on it is equal and opposite to that on the fin.

867

4. Thus i t is seen that there are two very important effects produced by opening out the engine : - -

(i) The aeroplane alters its trimming speed, and in most cases c6mmences to stall.

(it) I t swings round one way, often very violently.

Both these effects are produced almost entirely by the slip stream and not by the position of the propeller and the engine torque, as is usually .supposed.

,APPENDIX IIIB.

TURNS.

A, ~c. Witnesses' Statements may be Summarised as . - -" In a banked %urn of 45 ° bank or more, it is necessary to use fu l l left rudder for either tu rn after the tu rn has been fully started. The rudder is used to keep the body of the aeroplane level. The phenomenon is not connected with Side-slipping, as one witness specifically stated that the bubble in the cross level was central."

General Explanation.--The unsymmetrical forces and couples on an ~aeroplane arise from engine torque, slip stream and the gyroscopic couple .due to propeller when the aeroplane is turning. The first two, torque a n d slip stream, may produce an unsymmetrical setting of the ailerons a n d rudder respectively ; they do not produce effects which depend to any .appreciable extent on the rate of turning of the aeroplane. The gyro- • scopic couple alone is capable of explaining the pilot's observations.

Viewed from the pilot's seat, the propeller turns clockwise (in A), and when turning to the right, the aeroplane tends to put its nose down as a consequence of the gyroscopic couple. The tendency is countered by ~left rudder and back (or top) elevator.

Turning to the left, "the nose of the aeroplane tends to go up, and this effect is countered by left rudder and forward (or bottom) elevator.

Numerical Estimate of Couples.--The estimate will be based on a 45 ° banked tu rn with body horizontal. If V be the speed of the aeroplane in fit/see., ~ the angular velocity of the airscrew in rad/sec., tF the angular velocity of turning in rad/sec, and ¢ the angle of bank, the gyroscopic • couple is

G = I n~F . (I)

about a horizontal 8~xis perpendicular to the propeller axis. Of the tots/ .couple G a part G sin ¢b is t akenby the rudder and G cos ~ by the elevators.

For a steady turn with a bank of 45 °

'~ = g l r • . (2) Example.

V = 1 1 0 f/s and . ' . ~ = 0 " 2 9 ; r.p.m. =1 ,250 and .'. ~ = 1300 I for propeller = 2"2 slug it. 2. "/ Rough estimate for A with 130 Clerget I for engine = 7 slug ft.% j engine.

,.Couple taken by rudder = 9"2 × 0"29 × 130 × 0"707 lbs~ ft, = 235 lbs. ft.

B~200~ z

R E L A T I O N OF S I Z E OF R U D D E R TO G Y R O S C O P I C C O U P L E IN A, C A N D B MACHINES.

For airscrews the following approximate figures are used : - -

A - - R a d i u s of g y r a t i o n ---- 1.60 ft. Wt. = 28 lbs. 1 ---- 2"2 C . . . . 1"65 ,, 35 , 3"0 B . . . . 2 '0 ,, 50 ,, 6"2

r.p.m. = 1,250 1,500

900

(t)

Aero- plane.

A.

C.

B.

(2)

Propeller.

2 blades, diam. 8' 6", ro ta ry engine.

4 blades, diam. 7' 9", s ta t ionary engine

4 blades, diam. 8' 8", s ta t ionary engine.

(3)

Rudder area

(sq. ft.).

4"9

5"8

10"2

(a)

Eleva tor area

(sq. ft.).

10"5

15"8

22

(5)

A p p r o x i m a t e d i s t ance

be tween CG and cont ro l

(f~.).

13

13"5

17

(6)

R u d d e r effectiveness

(3) x (5).

64

78

173

(7i

Eleva to r effectiveness

(4) × C5)

136

213

374

(8) ! (9) Pressure on

Components rudder to of couple on , balance gyro-

scopic couple rudderelevatorand (lbs. p e r sq.

(~t pbs.). (8) ~t.)~ (6)

255 3"9

100 1"28

124 0"7

(lO) Pressure on elevator to

balance gyroscopic couple (lbs. per sq. ft.)

(8) +(7).

1"9

0"47

0"33

00

The figures in column (9) show tha t t he rudder control a t a g iven speed and angle of rudder is three t imes as great in C and 5½ t imes as great in B as in A.

The force on the rudder in A is nearly 4 lbs. per square ,1oot , whilst a t 80 m.p.h, a force of about 10 lbs. per square foot can be supplied bef,gre Che max imum lift coefficient of t he rudder is reached~

869

I t will be seen that in a steady banked turn at 45 ° the force on the rudder of A, due to gyroscopic action alone, is nearly half that which can be applied_ The manoeuvre is not extreme, and records taken on an aeroplane sh~w centrifugal forces more than four times as great as the weight of the acre* plane, whilst in the case of the steady turn the centrifugal force was iust' equal to the weight.

I t is possible to make an estimate of the gyroscopic effect in the mere extreme turn. At any chosen speed the air force on the wings, which cam produce the centrifugal force necessary for turning, is greatest at the critical angle, and to produce an acceleration of 4g the speed will need t~ be twice the stalling speed of the aeroplane, say 80 or 90 m.p.h. A~ ~@ m.p.h. ~he value o f t is 0"97, i . e . , one turn in about 6 seconds, and the gyroscopic couple of 1,170 lbs./ft, would require 18 lbs. per square f~a~: of rudder area. I t is probable tha t full rudder would not suffice, and it seems safe to conclude that manoeuvres of not uncommon occurreuce will call for full left rudder on a right hand turn apart from any effec~ arising "lrom side-slipping,

APPENDIX IV,

SUMMARY OF SPINNING ACCIDENTS. 1. iV~ACHINES A. (a) DURING MAY, 1918.

GROUP I. GROUP II.

Spins started near the ground. Spins started at a high levol.

Engine failure at 300 ft. stall Nearlyrecovered. T.31, N,~ and spin. T.55. A.13. Accidental spin diving at target. T.57. A.23½. Intentional spin, nearly recovered. 2".42. A. 10,

0/192.

0/195.

0/199.

o/2o5.

0/206. 0/209.

0/'232.

o/2s2.

Low spin. Doing wireless. T.69. A.31.

Low spin. T.38, A.15. Low spin. Possible faint. T.47. A.1. Stall and spin from 150 ft. T. " S " turn and spin from 40 ft. T.29. A.½.

0/268. Stall, and spin from 50 ft. T.45. A.11.

0/283. Spin from 200 ft. Partial recovery. T.26. A.5.

0]294. Spin from 200 It. T.62. A.14.

0/318. Choked engine. Spun, T.18½. A.2.

0]320. Nearly recovered. T.30. A.6½.

0/345. Spin from 200 ft, T.37. A,14.

0/362, Spin from 200 f t T.41. A.I0.

0/243.

0/284.

0/308.

0 ]307.

Left, then right spin. T. '2~. A.11½. Righted at 500 ft. right turin and spin to ground. To3.% A.1½. S!ow spin to ground, no at- tempt at recovery. T.21L A. Considerable expexie~ea

0]310. Right then le f t spin. TAI& A.16.

0/322. Spin to ground from 2,N)@ ft. T.28. A.4.

0/338. Spin from 2,000 ft. neatly' recovered. Medically unfi~ T . - - A . - -

0/244. Roll and spin into sea~ T.29. A.8.

0/346. Right then left spin. T.SF. A,3.

0/357. Nearly recovered. T.5~ A.10.

Y.52. Nearly recovered. TAXI. A.18.

Y.60. Nearly recovered. T.37~ A.½.

870

(b) AT VAmOUS R s c E ~ Ti~s,

GRouP I. GRouP II.

Spins started near the ground. Spins started at a high level.

0/148. Spin a t 80 ft. in straighten- ing out after dives. T.42. A.3.

0]158. Stall and spin a t 100 ft. Engine failure. T.33. A.10.

0]402. Spin from 500 ft. T.35. A.15.

¥.23. Nearly recovered.

0/131. No a t tempt a t recovery. Capable pilot.

0]151. Pilot passed as unfit for al- t i tude above 3,000 ft. by Med. Bd. Seen flying a t 5,000 ft., from which height he spun to the ground. No a t t empt a t recovery. Com- petent pilot.

01153. Temporary recovery a t 400 It. T.25½. A.1.

0/409. Left right. Left right to ground. T.29. A.0.

0]484. From 1,000 ft. right then left, pulls out near ground, but before regaining speed, re-spins. T.118. A.30.

0 ]534. Spin from 1,500 ft., left then right. T.56. A.11½.

0/536. Spin from 1,000 ft. (un- known). T.137. A.8.

Y.31. Nearly recovered. T.331. A.2.

Y.32. Spinfrom2,500 it. toground. Pilot with lame leg flew without permissiorL

Y.83. Partial recovery, roll and spin to ground. T.24. A.~.

I I . MACHINES B.

~) ]190. Spin, crash, burn. T . - - B . - - 0/246. Stall a t 400 It. spin. T.10.

B.1~ 0]258. Stall a t 150 ft. spin. T . - -

0]327. Spin from 150 ft. T.34½. B.3½.

<)/358. Spin from 300 ft. T . - -

'0/233. Spin from 500 ft. T . - - B . ~

DURING MAy, 1918.

0 ]328. Spin from 2,000 ft. "1".82. B. 0/368. Spin from 700 ft. T.21½. B.

01271. Spin from 3,000 ft. T . - - B . - -

0]263. Stall a t 1,000 ft. spin. %17½. B.8½.

III. MACHINES C.

GROVP I.

DURING iVIAY, 1918.

GRouP II.

0/221. Spin, recovered, dived. T.44½. C.~.

0]253. Spin, then dive. T.35~. c.11-~.

871

IV. MACHINES I). Du~I~G MAY, 1918.

Spins s tar ted near ~he ground. Spins started at a high level.

0/166. Spin ~rom 250 ft. T.32. D.3.

0/235. Spin from 500 ft. T.37. D.O.

I~oTE.--Nos. after T. denote total number of hours flown. . A. ,, number of hours on type A,.

,, B . . . . . . . . . B.

. C. . . . . . . . . C.

,, I) . . . . . . . . . D.

communicated by the accidents investioation committee...

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