Aircraft ski research in Canada

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Aircraft ski research in Canada

1

NATIONAL RESEARCH COUNC IL OF CANAD/\

CQMMONWEALTI-! ADV1S08YAER0NAUT1CAL RESEARCH COU:'iCIL.

COPY NO.

AIRCRAFT SKI RESEARCH IN CANADA

REPORT NO. MM - 225

Li(.;

90

BY

G . J. KLEIN

DIVISION OF MECHANICAL ENGINEERING

OTTAWA

15 AUGUST 1950

T HIS RE POR T MAY NOT 8€ PUBL I SH ED I N WIIOLE OR IN P ART WI THOUT THE WRITTEN CONSENT O F

l

NATIONAL RESEARCH LABORATORIES Ottawa, Canada

REPORT

Division of Mechanical Engineering Engineering Section Pages - Preface - 2 Text - 25 Figures 4 For: Internal Re~o rt: MM-225 Date~ 15 August9 1950 File : M17-5S - l

Subject: AIRCRAFT SKI RESEARCH IN CANADA Submitted by: G. J. Klein

Section Head Approved by: SUMMARY J. H. Parkin Director Author~ G. J. Klein

This paper discusses the comprehensive research programme aimed at the development of improved skis for aircraft which has been carried out in Canada during the past twenty years. The investigations o f (a) the sliding

resistance and adhesion 0£ skis on snow, (h) the aerodyn

a-mics of skis with particular reference to their i nstability in pitch, and {c) the dynamic loads on skis while landing are described . The hitherto unpublished results of recent trials of skis on deep, soft snow throw new light on the problem and stress the importance of designing aircraft

skis to suit the conditions or the snow on which they are

intended to operate. General information on the relevant

features of snow is also included. By bringing the various

researches together in a paper of th is kind, it has been

possible to draw general conclus ions which differ in a number of respects fro~those previously published o

I ｾ＠ MM-225 TABLE OF CONTENTS SUMMARY l o IWrRODUCTION 2. SNOW

3o SNOW CHARACTERISTICS OF SKIS

4o _{AERODYNA MI C CHARACTERISTICS OF SKIS}

5. DYNA MIC LOADS ON AIRCRAFT SKIS 6. SKI CONSTRUCTION 7o GENERAL CONCLUSIONS BIBLIOGRAPHY FIGURES 1 TO 4 Page ( i ) 1 2 4 16 18 21 22 24

AIRCRAFT SKI RESEARCH IN CANADA lo INTRODUCTION

Page - 1

MM=225

In Canada9 transportation by rail and highway is

limited to a comparatively small part o f the total areao This is mainly due to the very large size of the country in relation to its population" Most of the country rs mineral and timber r esources lie in districts where surface forms of transportation are limited to the waterways9 which are seldom completely free

of rapids and other barriers to navigation s and to a few r o ads o Fortunately9 these districts contain a great many lakes which

4 fo r m natural airports and make practically any point readil y

accessible by airo Air transpor t therefore plays a very impor = tant part in the development of Canadais mineral and timber

resources. Throughout the summer the a i rcraft are equipped with floats; during the winter the same aircraft are fitted with skis and operate from the snow covered frozen surfaces of the lakeso

The problem of developing improved types of aircraft s kis is therefore one of national significance and considerable

work in this directio n has been undertaken in this country with

noteworthy successo

Ski research began in Canada in 1922 when Glidden

measured the sliding resistances of a number o f small model skis a t McGill University (Refo 1)o The first aerodynamic tests of skis were conducted in 1928 by Parkin at the Un i versity of Toronto

(Refo 2)o

These early tests~ though limited in scope~ were the beginning of the important ski research carri ed out by the Natiohal Research Council of Canada after its Aeronaut ical Laboratories were established in 1930 0

The purpose of this paper i s t o give a general aGcount

of the various phases of Canadian ski research in order to bring it to the attention of those who have not had the opportunity to study the reports given i n the Bibliography , By bringing the various researches together in a paper of this kind9 i t h a s been possible to draw general conclusions based on the work as a whole and to add some remarks which were not evident at the t i me the reports were writteno It a l so give s an opportunity t o present t he main results of recent trials of skis on the DeHavilland Beaver which have not appeared in report formo

An excellent description of various types of airc r aft skis and general information on aircraft ski pract i ce has been given by Ferri er in the Journal of, the Royal Aeronautical Soc iety

(Refo 4)o . \

MM =225

It should be mentioned that, although two ~· s k i landing gear (i ae . special ski undercarr i ages w1th no tail ski), retrac ~ table skis and the appllcation of skis to tricycle landing ge a r have been discussed from time to ti~e ; none of thesei as far a s

the author is awar e, have b e e n used jn Canada m These arrange= ment s impose spe cial problems which requlre development and possibly some further research berore they can b e a pplied sue = cessfully o

SNOW

The following account of snow is included in order tha t the ski problem ~ay b e more fully apprec1ated o At the b eginning of the tests described in

Reference

9 there was very little in the literature on snow that was useful in deal i ng with t he ski problem . The information given here isp to a large

extents a result of the sk i r6sear ch and subsequent Canadian snow s t udleso

The

crystals of snow

are

formed by

the

sublimation of wat er vapour onto solid nuclei in the atmosphere and ·fall ~o e a r th i n a great varie~y of rorms v Hexagonal plates~ stars9

nee dles~ short hexagonal columns and combinations of these basic f orms are quite common ,, The crystal a general! y range in size

up to a bout 8 nnn a The form

or

each crystal depends upon its rate or series of rates of growth which is different for each crys tal - hence the great variety of crystal forms o

Other types of ·winter precipitation are ~ ha!~ snow heavily coated with rime₉ clusters of partly mel t ed crystal s 9 s now mixed with rain s.nd r'ain >

A layer of dry fres hly fall~n snow which has been deposited durin~ l i t tl e or no wind i a very soft and

r1urry .

Its specific gravity is low = between 0 . 03 and O a07 = and therefore

a very large part of its bulk i s air o

Snow con~inually undergoes changes not onl y while

it

is being formed in the atmosph ere but also I'rom the time of its deposition until i t disapp ears by evaporat i on or runs away as

t haw water in spring .

At temperatur es below freezing ) vapour pressure differ

-ences cause the poln~s and ~lender b~anches o f the crys~al s to e vaporate and sublime again on the larger part s whic:t. oec:ome

small 1rre gularly shaped gr ains o As the crystals change shape t he y become more compact and the snow settles o At the end of the inl tial settli ng stage ,· when no trac.e s of the o riginal crys ~ tals remain9 the specific gravity wi ll be be~ween Oo2 and 0 . 359

---Page 3

r.:r.1~ 2 2 5

the snow grains will be about Oo2 mmo in size and some bo ndi ng b e tween gra i ns wi l l have taken placeo Beyond thls stage a gradual increase i n grain s i ze takes place with very little change in s pecific gravity, the larger gra ins growing at the expe nse of the smaller grain s o

Wind greatly accelerates the s ettling of snow and i ncreases t h e bonding betwe en gra i ns which give s the snow con= siderable strength and har d ne ss o In extreme cases ~ suc h as on the Canadian Barrens north of t he tree line whe r e winds are very persistent, the hardness

or

wind packed snow may be as great as 6~000 gmo per sq . cm.-lf Since the hardness of' dr y new snow is seldom a bo ve 20 gmo per sqo cm v thi s represents a remarkabl e increase in har dness o

A snow layer located well be low the su rface and

i solated from the snow above by a cr ust thr ough w~ich a ir flow is extremely ·limi ted will1 after a pro l onged period of low t emperatures9 develop into a layer of

11_{dep th hoar '\, L , e o;,}

weakl y bonded9 ho llow cup-shaped crystals from 1 to 8 nuno in

size. Depth h oar layers are ve ry weak , the usual range of hardness be ing from 20 to 100

gmo

per sqo cm .

Thawing condit ions also accelerate the ch anges which take place in snow and9 if followed by freezing1 result ln very str ong bonding between the gra i ns = snow hardnesses exceeding 20,000 gmo per sq o cm. h ave frequently been recorded a nd specific gr a vity values between Oo 3 and 0.,5 are very common o Ra i n followed b y freezing produc es very hard i c y crusts o Cycles of t h awing

and freez i n g cause the grai n size to increa se at t ne expense of t he smaller grains and produce the coars e gra~ular snow corr.mo nly f ound i n spring o

Slightly wet snow usually has no s olid bonds between 1 ts grains ·but 9 due to surface tension forc e s at t he air=to ~we. ter

bound ar iesp has approclab l e coht: slon a nd some other prope rt ies

unlike those of dry snow o Very wet snow (slush) has virtual l y no cohe si on and its properti es approach those of a liquid o

Most types of snow have a remarkable capacity to absorb mechanical shock such as the impact of skis on l andingo The very h ard types possess little of this c harac ter istic

e specially when the y o ccur as a fa irly thick layer

*

Throughout this paper snow hardnes s is give n as the pressure on a flat pla te at which the snow begins ~o fall in compres sion - see Refere nc e 12 0

MM=225

Since snow is an excellent therma: insulatcr9 the

temperature near the ground is generally not far from the freezing pointo Deep snow-covers on lakes greatly retard the thickening of the lake ice and , if a slush layer occurs directly above the ice9 may prevent the slush from freezing even in fairly cold weather o

Snow surface irregularities9 such as wi nd erosion

ridges and drifts J> complicate the ski problem especially when the y

are large and

consist of hard snow as is generally the case north of the tree line in Canada o

A snowcover is usually co~posed of l ayers of differ

-c n t types of snow which are sometimes separated by thin ice sheets o The total depth varies appreciably throughout the winter and in most parts of Canada reaches a maximum of bet

-ween 20 and 36 inches , The maximum depth is usually about

1/0 the total snow fall given in meteorological records where9

by international agreements the average specific gravity of new snow is taken as Oo1o

In most parts of Canada9 thawing conditionp are fairly wel l confined to the spring break up period whereas

winds are common throughout the winter seasono The influence of the sun's radiation on snow conditions has been fou~d to

be l imited to the snow immediately a.t the surface wherep during mi l d temperatures it may promote a small amount of melting or e vaporationo Therefore9 the primary factors affecting snow conditions in a particular locality are (a) the a mount of' snow ~ rall and (b) the local strength of the windo

3o SNOW CHARACTERISTICS OF SKIS

Although the structural9 aerodynamic and snow

characteriatics

or

Bkls are or equal importance, tho snow characteristics of' skis will be discussed most because many aeronautical specialists are unfamiliar with snow research whereas the other phases of the ski pr oblem follow well ~~own

linesc

There are times when the sliding resistance of air-craft skis is so great that it is impossible to reach flying speed, There are other times when the skis v after remaining stationary for even a few seconds9 adhere to the snow to such

an extent t h at drastic methods have to be used in orde r to break the skis freeo Theref ore9 the ob j ect of the tests a bout

to be described was to deter~ine what changes in ski design

should be made ln orde r to achieve a substantial reduction in both the slidi ng resistanc e and adhe sion

or

skis ~

Page ~ 5 MM~225 It is worth noting that ? in the special case of sport skis9 fairly high adhesion is requi re d to avoid back=

slip during climbing9 and sport ski waxe s are therefore de ~

signed to provide high adhesion comb ined with low s l iding resistance o Si nce ski waxe s gradually wear off and have to be re placed from time to time t h ey are generally unsuitable for aircraft sk is ~

The towing dynamometer shown in Figure 1 was used to measure the adh esion and sliding resistances of more than

40 ski modelso Fairly l arge models (ap proximately hal~ scal e)

were used to avoid large ·scale e ffects o In one series the

models differed in shape ; in another they differed in s h oeing material. Only three of the models were of flexible construc-tion and a few others incorporated some special feature ~ such as a glass window in the s k i bottom= t o aid in the

investiga-tiono

The tests were conducted at Ottawa during ·the winters from December 1935 to May 1938 and at Sioux Lookout f in Northe rn Ontari o9 during the following wintero Towing speed ~. unit loading

and l ong i tudinal position of the ski axle were varied o Some

further tests in which the unit loading was extended to l pOOO lb o per sqo ft o were carried out at Arnprior (near Ottawa) during 19480 All three of these test areas were exposed to moderate winds and onl y one= Sioux Lookout - was on a frozen lake .

Early in the tests i t was f ound that slidi ng r e sis = tance and adhesion were far more dependent upon snow condit i ons than on the design of the ski o Furthe~ s the skiing qual i ty of the snow changed continually and it was not unusual to observe marked changes taking place in less than an hour1_{s time ~ This}

made it difficul t to compare tr.e resistances of two different skis unless they were tested at very nearly the same time o Therefore one of' the models wa s taken aa a r e i'e rAnce and was

always tested simultaneously with one of t h e other mo del so Even with this procedure it was necessary to carry out a very large nunber of tests in order t o obtain reliable informat i on on the effects due to var i ations between the different models o

The resu l ts of these tests have b e en given i n

Reference 9o

It was found that the sliding resistance of a ski could be divided into (a) the resistance component due to compacting

MM-225

in ae rodynamic s - and (b) the resistance component due to frict iono When these components were express ed as coe ff i ~ cients :

= component due to compaction load on the ski J

component due to frlction µf = load on the ski

it was found t hatµ increased with increasing unit loading while µ f decreased pto such an extent that , fo r a ll snow condi= t i ons and r ange of unit loadi n g ( 100 t o 500 lbo per sqo ft o)

covered b y the t ests, t he total resistance coefficientµ + µ1' decreased l i nearly with increa s ing unit l oadi n go Later , Pwhen the tests were extended t o 1,000 lb. per sq o f t" it was found that the above rel_at io n still he ld a.nd that the un it loadir_g for minimum µp + ~f h a d n ot ye t been re achedo

It was - disco ver ed that µf varied with aspect ra t io

(bearing surface length/width) and since high as pe ct ratio models were longer than those of l ow a spect r atio (the b earing surface

a rea being the same for all mod els )

µf

al so var ied wi th bear ing s u rface lengtho Thi s suggested that the mode of friction was

not constant along the l ength

or

the ski andp together with f u rther analysis~ lead t o the following theory of ski fric tio n which

gives a qual itative explanation of the r esults obtai ned in all o f t he s liding resistance tests of which there were mor e t han

2 ,00 0o

The the ory is based on the idea t hat the contact

between ski and s now takes place at a large number of very small areas and tr.at the heat generated by f r iction causes sufficient me l ting to provide wa te r lubricat ion-,1- at the points of' contact o It is further con sidered that each contact area is enclosed in a water dro p which does not rrove appreciably with respect to the snow a nd that the rema inder ot' the ski bottom 1 s in contac t wit11

air ~ Thus, in the case of dry snow9 the frictional component of

*

The film of water is only present when the ski is siid~ ing or, for a stationary ski, whe n the snow is wet ;

therefore pr essure appears to play onl y a seeondar y role in providing water lubricat i on c Bowden and ~ughes ~ work-i ng work-i ndependently of the present author 9 also conclud ed that l u brication caused by frict ional melt i ng occurred during sli ding (Refo 17). I t should be noted that a fairl y high pressure (1~000 lb . per sqo lno) is required to lower the melting point o ~ ice 1°P~

Page ·~ 7

MM~225

ski resistance is considered to be made up o f ( a) sol id friction, most of which occurs near t h e toe of the ski~ (b) viscous drag due to shearing in the very thin fil m of water between the ski and the areas of contact which9 because of the burnishing action

of the ski 9 became small polished flats , and ( c ) ·drag due to

surface t ension forces acting at the perimeters of the water drops in contact with the ski bottom o

A glass window was set flush in the bottom of one of the ski models so t ha t the contact betwe e n the ski and the snow could be examinedo When the ski was in mo tion, a narrow meniscus of water surrounding each small flat contact area was clearly · vis ibl e9 and it was estimated that the total area of conta c t

for settled snow of medium hardness was of the order of 20 per cent at a unit loading of 200 lb o per sq ~ ft o and 50 per cent at

500 l b o per sq o ft o It was further note d that, whene ver ·there

was evide nce of wear, it always occurred near the bottom of the curve at the t oe of the ski which supports the supposi t ion that, when the snow is dry9 solid friction takes pla ce on the forward

part of the ski o These observations9 to gether with the

correla-t i on becorrela-tween correla-the resulcorrela-ts of correla-the sliding correla-tescorrela-ts and correla-theory9 while

not an absolute proof9 show that the theory is at l east a very

good working hypothesis o

Under ce r tain snow conditions of fairly common occur-ance9 the drag due to surface tension is a surprisingly large part of the total ski resistance o During sliding, the lead ing angle of contact of the water drops is greater tha n the ~rail-ing _{angle of contact 9 therefore} the resultant of all surface tension forces acting on the ski has a component9 tange ntial to

the ski bottom, which resists the motion of the ski o 'I'hts com-ponent, which will be called "surface tension drag11

9 is propor-tional to the difference between the l eading and trailing angles of contact o It is a l so urouo rtion&l to the sum of the perimeters

of all the water drops 1~ c~ntact with t h e ski bottom. The angl es

of contact depend primarily upon tee ma te rial used for shoeing~ while the sum of the perimeters d epend s mainly upon the t e xture9

hardness and free-water ~content of the snow and on the unit load-· i ng of the skio Some simple experiments have shown that~when the water drops are small a nd very closely spaced p surface tension

d r ag may be of the order of 20 lbo per sq o ft o of be aring surface

area.,

The l ocation of the transiti on fro~ solid frict ion to film lubrica tion depends mainly upon the temp e rat ure of the snowo As the temperature falls9 more frictional heat i s required to establish the water film a nd the transition moves further aft o The location of the transition is more or less independent of the length or width of the skio

MM- 22 5

The sliding resistance of a ski is quite high at very l ow temperatures because the area over which so l id friction takes place is relatively largeo Since this area is approximate ly proportional to the width of the ski , the sliding resistance can be effectively reduced by using a

high aspect ratio ski. Sliaing resistance at low temperatures may be further reduced by using shoeing of low thermal con-ductivity because it helps to conserve frict ional he at o In

th is conn ect i on

it

is interesting to note that t he sleds

used by the Eskimos to carry fai rl y heavy loads in the Arctic are fr om 14 to 20

rt.

long and have runners only 2 to 3 ln.

wide wh ich they coat with a smooth layer of ice - a poor thermal conductoro

In very wet snow the entire bottom of the ski is wet and most of the resistance is due to visc ous drag which varies as the s quare of the speed. Increasing the uniL loading9 i oe.,

decreasing the bearing area, seems to be the only method of reducing sliding resistance in very wet snow u

Wheneve r the snow is slightly wet a large part of ski friction is due to surface tension drag o II' the snowt besides being slightly wet , retains some o~ the feathery struc -ture of freshly fallen snow and is therefore very soft~ the areas of contact wi.:th the ski bottom will be extremely numerous and of elongated form with the res ult tha t surface ter.sion drag will be very large indeedo Surface tension drag may be r~ duced

by increasing the uni t loading which not only decreases tl1e

number of contacts by virtue of the smaller be a ring surface but

also causes adjacent water drops t o join together u In this wa y, increasing the unit loading reduces the sum of the perimeters of all the water drops in contact with the sk i o Surface t e nsl on drag is also influenced by the wetting properties o f the material used for shoeing the skic

All three components of ski friction - solid fric tion , viscous drag and surface tension drag - increase wit~1 a n in-crease in the number of contacts between the ski and the ~now and are therefore greater for sof t snow tnan for hard snow o They are also greater for low unit l oading than for high unit

loading.

In order to evaluate the component of resistance due to compacting the snow, a recording type penetrometer wi th a f la t circular pressure plate of 0.4 sq c f t ~ area was used tfl obtain curves of pressure against penetrationa Although t here

was an appreciable variation f rom one curve to another it was found that9 as a general rule, the energy represented by tl•e

Page ~ 9 MM- 225

area under the curve was approximately equal to half t h e

pres-sure multiplied by the penetrat ion~ From this general relation

it can be shown that µp i s approximately equal to 1/2 °d/l in other words9 the part of the ski resistance due to forming a tra ck in snow is approximately equivalent to climbing a grad e having a rise equal to the depth ( d) of the ski track in a dis

-tance equal to twice the length (1) of the bearing surface of the ski u

From tne same genera l relati o n it can also be shown that, r or a gi ven total load on the ski a nd a given snow condi -tion 9 the resistance due to forming the tra ck is invers el y pro ~ por tio nal to the square root of t h e aspect ratio a nd direc~ly proportional to the unit load ing ~ Consequently9 wnile an

increase in unit loading reduces the resistance co mpone nt due to friction it inc re ases the compo nent due to compact1on c There-fo re, £or a particular set of conditions of snow9 total load9 aspect rat io9 etc o, there will be a value of unit loading at

which t h e total resistance will be a minimum o

In the tes ts

at

500 lbo pe r sqo ft g loadlng8 the value

of µp rarely approached Ool a nd was generally le ss than Oo06 o Consldering that the bearing surface length of most of the models was only 4 ft o9 these low values were due t o the fact that the test areas were exposed t o moderate winds o This does not mean that the snow during the t ests was generally f a irly hard c In a

l arge proportion of the test there wa s a mode r ate l y thick layer of soft snow at the surfaceo However9 the underlying sno w was

usually moderate l y hard with the re sul t that the skl t rack depth

rarely exceeded 10 inches c

It is true that most a ircrart landing areas are fairly large and therefore more or les s exposed to winds i but there are notable exceptions y particularly on the smaller lakes in the

heavi l y wooded parts o r Northern Canada where the forests give

fa i r shelter fr om the wind and the low temperatur e s retard the settling and hardening of the snowo The ful l significance

or

this fac t was not realized at the time Repor t AR - 2 J Reference 9 , was written o

A summar y of the res ults of ~he model ski tests is given belowq

When the ski ing qual ity of t he snow was good there was

MM-225

but when it was very poor the sliding re sistance was reduced to less than one-half

that of a conventional ski

b y

using ~

(a) high unit loading ( 5 00 lb ~ per sq ~ ｾｴ ｾ＠ r ecomn:ended)s (b) laminated bakeli te * s hoeing,

( c) high aspec t ratio (value of 6 or more recon.nend ed) ;

(d) fine entrance angl e at the toe o f the ski (not greater than 20 degrees to 25 degrees relative to bearing sur -fac e recommended), and

(e) an axle position such that the result ant of the vertical load~ resis tance reac t ion and moment due to ski trimming gear , will pass approximately through the centre of lhe bearing surface le ngth - see Figure 4 o A low axle

heights not greater than one-fifth the bearing surface length, was recommended o

It was also found that (a) end ( b ) abo ve reduced t he maximum adhesion to less t han one - t hird that of a c onventional

skio

Ski flexibility was found to ~ave no effect on slid-ing res istanceo Howevers the force re quired to ove rcome the adhesio n of a flex ibl e ski was fo und to be from 80 to 85 per cent of that of a similar ski of r igid constructiono Apparently a slight amount o~ flexing took place whi le breaking free o

Bakelite9 because of its toughness and hardness , has

remar kable r esistance to wear particularly when there is any water present t o provide fi lm or incomplete film ( boundary) lubri cation . Experience haR shnwn that bakelite shoeing will

ou tlas t sheet me tal shoeing and rarely ne eds to be replaced during the l ife of the sk i . Bakel i te shoeing on an Arctic type sled, after three winters' service on the dry i very cold sr-ow in the Barrens ha s shown o n ly minor signs of wear o Tne low the rmal conductivity and smooth surfa c e of bakelite ap pea r to be mainly res pons i ble for its lo w sl i ding resistance and low adh esiono However, its surface tension drag ~ while not hlgh₉

*

Phenol ic resin sheet with c l oth o r paper laminations sold under vari ous trade name s such as Bakelite r Pheno ~ lite9 Dilecto, etc o Sheets hav i ng high resin co ntent and high cure gave a subst ant i al redu ctlon in res ista nce as compared with o rdinary comme rci al s he ets ,

Page ~ 11

MM- 225

is not as low as that of some other materials (such as wax ) so that there is a possibility that a s ti ll better ski shoeing material may be foundo

In 1942, service trials of experime ntal skis were carried out on the Fleet "Fort11 _{aircraft (Refo}

7)o Three sets of' skis, designed for unit loadings of 5009 600 and 700 lb o

per sq. ft., were tested. All of the skis were shod with bakeQ lite applied as a liquid and cured in place at 300°F. This produced a glossy coating, about 1/32 inches thick , which was

not nearly as hard nor as tough as laminated bakelite sheet which, unfortunately, could not be procured in time for the

tests .

During these tests the snow was quite hard, and although the snow-cover was moderately deep~ the ski track depth was generally not more than 4 inches . The snow charac-ter! stic s of' all the skis were quite satisfactory 9 but, since

the range of snow conditions was very lim ited9 the tests were inconclusive with regard to the desirable unit loading for aircraft skis.

It was found that bakelite applied as a liquid and cured in place had poor resistance to wear and was therefore not suitable for ski shoeing. Although the skis were designed to r esist the stresses specified in C.AoAo Report Noa 20,

December 1940

9

they were not strong enough to with stand the

harsh treatment imposed by the hard, rough snow conditions e~countered in the tests and, as a result9 one of the skis

failed in bending. These trials served to emphasize the neces-sity for tests to determine the stresses in skis during taxi-ing, take-off and land ingo

During the winter 1949-50~ the Department of Lands and Forests of Ontario in cooperation with the National Research Council of Canada carried out service trials of skis on the

DeHavilland 11_{Beaver" which has an approved all~up-weight of}

4820 lb o The tests were ma.de on a number of the small lakes

in the area between Sudbury and Kapuskasing9 Ontario . This

fairly flat area is covered with a dense forest which ~ to a con-siderable extentt shelters the snow on the lakes rrom the wind.

Two practically identical aircraft were used in nearly all of the tests ; the one fitted with NoRoCo skis as shown in Figures 1 and 2~ the other fi t ted wj_th Elliott Brothers E=13

skis which are one of the best forms of conventional skis and,

for many years, have been very popular in the northern, wooded

MM-225

The principal features of the two types of skis are given below ~

Length overall9 inches

Length of bearing surface9 inches

Width9 inches

Unit Loading~ l ba/sq n ft u

95 70 l0 - 5/8 465 108 80 22 195

Shoeine Laminated bdkel1te sheet brass

Weight/ski, without pedestal9 l b? 5C 100

The NoR oGo skl had a be ar i~g surf&ce with a sli3ht

rocker wh lch was intended to assist in breaking the sk i free "

The 1/8 inch thick bake lite shoe and the l e.mina~1ons of

1/8 inch yellow birch veneer1 of which the ski was made ;, were bo nde d together with a cold setting wa te rproof adhesive n The E-13 ski had no rocker but had two gro oves~ 2 - 1/2 in . wid e by

3/8 in o deep9 r unning the length of the ski9 and was ~ade of white ash lamlnations9 approximate ly 1/2 in o thick assembled

with copper rivetso Both the NuRoCo and E~13 skis had flexible pedes tal s of the pneumatic rubber-bag type shown in Figure 1 ~

I n the preliminary trials on bare ice 1t was found that the narrow skate, which can be seen in Figure 29 fre quently

caught in cracks in the ice and gave the aircraft a strong ten-dency to gr ound- loop o The skates were cons i dered dangerous and were therefore removedo A wider skate might have ove rcome this

dlfficulty . On be~e ice the N. R.c. ~ki 3 had extr~mely low resis

-tance and a very long landing run was required to bring the air = craft to r esto A braking device~ sue~ as a reversible pltc~

propeller9 would reduce the landi ng run considerablyo

The most significant result of the Beaver ski t rials was that the snow conditions on the smal19 fa i rly we ll sheltered

lakes were found to be considerably different from those which occurred during the mode l ski tests (Ref n 9) and at the observa=

tion stations in the Canad ian snow=cover survey (Ref9 12 )o In

the earl i er work the underlying settled snow was usua ll y fai r ly hard, whi l e in the Beaver ski trials the underlying snow s though

settled, s e ldom had a hardness exceeding 200 gm o per sq~ err.a

Furthers due to the prevalence of fairly low ter.1peraturesp

Page ~ 13

[v'iM - 225

occurred it was noted that the snow between the crust and t he lake ice had developed into a weak layer of depth hoaro The thickness of this layer was sometimes as much as half the t otal depth of the snow-cover.

In several of the tests a layer of slush, about 5 ino thick, was found directly on top of the lake i c e < Exuerienced pilots of the Department o f Land ~ and Forests of Onta~io have est i mated that they encounter slush laye rs on about 15 per cent of the lakes ei:en in very cold we ather and, when spring thawing

conditions set in, this figure rises to about 70 per cent or

moreo They also re por t that, wh en a conventional ski re mains stationary on a snow covered slush layer i t h e water is drawn up by the compacted snow a nd freezes onto the ski to such an extent

that it is extremel y difficult to broak the skis free o

Since the whole snow- cover was relatively soft, the highly loaded NoRoC. skis penetrated to a considerable depth o The greatest penetratio n was 20 inches in a snow=cover 28 inches deept consisting of 14 inches of dry new snow on t op of a weak

crust 1-1/2 ino thick1 which covered depth hoar for the remainder of the snow~cover de p th c In this case

2 0

µp

=

2 X 70

=

0. 1 43

which is quite high. Unfortunately, an aircraft on E-13 skis

was not ava ilable ror comparative trials at the time o Ho wever,

if we assume penetration proportional to unit loading we obtain

a penetration of 8.3 inches and a value of µp = 00052 for the E-13 skis. Since the fric t ional component would be appreciably higher for the E-13 skis than for the NoRoCc skis ~ it is not possibl e in t h is case to say which of the two woul d have the lower to ta l resistance .. However9 thi~ test did l nd1cate that

changes in the design of the NoR.C. skl were required in o rder to reduce µp to a reas onable value o For example) if the bearing surface were made 88 in . long by 12 in o wide ~ the penetration would be about 14 ol inches and µp would be about 0 ~08 . It should be noted that an increase in bearing surface length is a more effective way of reducing µD than a correspond ing percentage

in-crease in width . ·

Since one of the primary objects in ski design i s to obtain low sliding resistance p both µf and µ p must be reduced to as low values as practical o The first requires that the surface in contact wit~ the snow be small, while the second requires tha t

t he length of the bearing surface be not less than say 6 times the maximum pe netration of the ski . It appears9 the refore9 t h at

Page - 14

MM-225

---··

for deep sof t snow conditio ns, ther e is a desir~bl e mi n i mum

l ength of bearing surface wnich is inc.ependent of the weight of t he airers.ft but is mainly d e pende nt upon the snow depth and hardnes s d istribution and on t he cholce of unit load ing n

Although the pe netration of the NoR J C o skis was gene r ally from 2 to 2 - 1/2 times that of the E~13 skis ~ the

total sli ding resistance of t~e NnR oCv skis W~S ge n erally as

l o w as or l ower than that of the E=13 sxiso I n o ne test the

NoR oCo skis had higher re sistance because they penetrated to snow of ~uch poo rer skiing quality t han that 0f t he snow near the surfaceo In another te st on deep soft snow the r esistance o f the NoRoCo skis was about 60 per cent of the resistanc e of

the E .. 13 skls o

The adhesion of' the NoR oCo ski s was generally appre = ciably less than that of the E- 13 skiso KoRcCo skis could even be brought to rest on snow coveree 8l ush wi thout any d i fficu lty in breaking free agai n = a pro cedure which the pi l ots would not risk on E=13 skis o However~ very careful observations showed that the skis did not tilt t he slightest amount forward while breaking free and it was there f ore con-cluded that the rocke r on the bearing surface did not assist in freein g the skis and may have only tended to inc re ase the penetrati on of the ski because of the result ing non=uniform distribution of the lo ad c

After the short s kates we re removed from the N,Ro Co

ski, t he ski had a tendency to sidesli p on snow ; therefore two l amin a ted bakelite strips9 2 in . wide by 1/8 in o thick1 exte ndm

ing along t h e entire l ength of the ski were added to overcome t h is tendency o These were located symmetrically about the ski

centre line leaving a 2 in , wide groove b etween tne stri ps o The

stri ps el i minated the sldeslipping tende ncy and improve d the control of the aircr aft on snow with no noticeabl e increase i ~ minimum turning radiuso It was conclud ed that the edges of flat bottom skis did not provide adequate l ongitudinal guidi ng and that these strips9 or their equivalent , were nec essaryo Furthe r ~ there a ppeared to ·oe no object in us i ng strips more than 1/8 ino thick o

Some features of the KoR oCo ski a s compared with the E-13 ski were found to be ~

(a) the ski weight9 without pedestal, was about half that of the E-13 ski ~

(b) since the deck was narrower9 the r e was much less

possi-bility of collecting a large weight of slush on t he forward part of the ski which could cause dangerous

extensio n of the ski trimming gear, and

Page - 15

MM - 225

{c) the aerodynamic d rag was considerably l e sso Level fl i ght tests made on the same a i rcraft gave airspeed indicator read ings a t cruising r.p . mo o f 108 mvpoh o f or E- 13 skis and 124 mop . h. for NoR. C. skis .

The results of t h e Beaver ski trials may be s u m-marized as follo ws ~

(a) The snow on sma l l fairly well sheltered lakes in moderately cold areas, s uch as Northern Ontari o,

is ge n erally fairly s oft a t all levels in the snow-cover , and skis fo r aircraft whi ch operate from t h ese lakes should be designed to t1eet this snow condition.

(b) Th e leng th of the bearing surface of a ski should be at least 6 times t he expected maximum pene t ration of the skl . The

N.R.Co

s k is , considering t he moderat ely deep, soft snow c ondi tion s on the l ake s9 were ther e fore

too short for best performa n ce o

(c) Lami na ted bakelite shoeing is very s atis factory and ma y h ave been respo n sible for the low adhe s ion of

the N.R . C. skis on snow- covered slush .

(d) Laminated bakelite runner s, 1/8 in o thick9 or t h eir

equiva l e nt~ give a ski sufficient di re ctional stabi

-l ity o

(e ) A s light r ocker on the bearing sur face has no advan -tage whereas a slight arch may tend t o d i stribute the load more uniformly al o ng the l ength of the ski ~

(f ) If a skate i s used it shoul d be wide e nough to avoid dro pp i ng into crack s i n the ice .

(g) Skis for use on small landing a reas shoul d be de signe d for low sliding resis tanc e eve n a t t he expense of

increas ing adhes i on, because safety J whic h is of para-mount i mportance, r equires that the take - off r un be

sho r to

Pl ans have been made to continue the Beaver ski trial s next winter usi ng new skis designed on t he basis o f

MM-225

4. AERODYNAMIC CHARACTERISTICS OF SKI S

The aerodynamic characteri stics of skis have been discussed in such a complete manner in nefere~ce 5 that only a brief account of this phase of ski research will be give n here.

There are two aerodynamic problems in connection

with aircraft skis (a) the reduction of drag and (b) the

re-duction or complete elimination of the aero dynamic instability in pitch.

Since skis are unstable in pitch it is necessary to adopt some form of trim-~ing gear in order to maintain the skis in their proper attitude in relation to the aircraft in f l ight. The t ri mm ing gear must also permit cons iderable free-dom in rotation about the axle in order to provide for taxiing over uneven snow surfaces o

The mo st common for m of trirr.ming ge ar is two or more strands of s hock - cord in series with a length of ste el cable and in parallel with a short length of steel safety cable which limits the extension of t he shock-cord . One end o f this

assembly is connected to a deck fitting near the toe of the ski while the other end is connected to some conve nient point on

the aircraft, A steel cable, having a length which assures the proper at t i t ude of the ski in fl i ghts is also connected to a deck f i tting near the heel of the ski and to a second convenient point on the aircrart . Sometime s a seco~d shock- cord asse~bly is used in place of the s i mple rear cable o

The aerodynamic drag of this type of trirmning gear

is quite high and, in the case of moderatel y high speed aircraft, t he moments to be controlled are so large and the trimming

gear consequeut., ly so st1.r:r that there is insu1'f'1c1ent fl e

xi-bility when taxiing over fairly uneven snow surface~ .

So~e pltching moment measurements (Refo 3) were obtai ned for a full size ski with vari ous mod ifications o One of thes e

was a tail plane mounted above and a little aft of the hee l of the ski. It was .found that t he very large wake from t he non-streamlined body and pedestal of the s~i made a considerabl e area of the tail plane ineffective and the tail plane span re ~ quired to give aerodynamic stability was considered to be too l arge to be a prac tical solutiono Streamlining lhe s~i and pedestal woul d have reduced the required span~ but unfortun-ately this expedient was not tried - perhaps because the wake of the ski at large angles

or

at t a ck would still be large . However, it was found that a. swept upward extension at the

heel of the ski appr eciab l y reduced the slope of the pitching moment curve 9 although it did not eliminate instability in

Page .. 17

MM= 225

The experi~ents described in Refere n ce 5 showed that₉ with reference to D1 _{(the ae rodynamic drag of the corresponding}

wheel) ~ the distri bution of the drag of a typical non-stream= lined ski and pedestal was as fol l ows ~

ski body C O O O O O O C O O O O O O C C 1 , 07 D•

pedestal 0 0 0 0 0 0 oo c O O O ~ <I C, (I 0 088 01

trimming cables 0 0 0 0 0 0 0 0 0 1o 05 D 1

Total 3o00 D~

Thls clearly shows the large drag component due to the trimming cableso The experiments further showed that1 by streamlining the ski and pedestal and using an internal trimming gear~ the drag could be reduced to Oo55 D1_{o The slope of the}

pitching moment curve for the streamlined ski v which had a swept-up extension at the heel9 was about half that of a

con-ventional skio Skis of this type are shown in Figure 3 o

Reference 6 describes experiments aimed at I'ur t her aerodynamic improvement of skis and rollows much the same lines as Reference 5 o Some improvement was realized9 but the most

interesting result of these tests was that an appreciable amount of aerodynamic interference was found on the drag and pitching moment of a ski mounted near a wing o

I t was shown in the se c tion o n 11_{Snow Characteristics} of Skis11 _that

9 for good snow performance ~ a ski should b e long and narrow and should have a smaller bearing surface than a con-ventional ski o These features a lso have real aerodynami c advan= tageso It was further shown that9 if t h e entrance angle is not greater than 25 degree s , the best longitudinal posit i o n of the

axlo9 f'rom th e snow p e rf'ormance po i nt o.f' view :i1 i.:5 near the c entrt:,

of le ngth of the bearing surface9 i oe c~ about 10 per cent of the bearing surface length farther forward than the position c ommonly employedo II' a ski incorporating the above features we r e streamm lined9 its air resistance would be very low and its aerodynamic

instability as represented by the slope of the pitch i ng mom ent curve would be relatively small o The u s e

or

an aero d ynam i c sta-bilizer of reasonable size may therefore be qu i te practi cal for a streamlined ski of these proportions and would seem to be the logical solution f or skis of high s p eed a t rcrart and for r etrac ~ table skis9 since stability is maintained at all speeds and at all

MM=225

5o DYNAMIC LOADS ON AIRCRAFT SKIS

Earlier in this paper i t was mentioned that structu~ ral failure of one of the skis occu rred during the ski trials

on t he fleet "Fort'' (Ref, 7), Although the ski was designed in

accordance with the strength requirements in effect at that time, it was not strong enough to with s tand the harsh tr eatme nt

imposed by the rough9 hard snow conditions and failed in bend

-ing while c r ossing a deep icy rut o

The design regulations~ of necessity s have been largely

based on experience and9 at times , have been sub jec t to question

and change o Prior to 1947 the regulations di d not specify a

loading condition for t he bent up part at the toe and₉ although

manufacturers applied their own arbitrary loading conditions to this section, tension f ailure s in the upper surface near the toe

were reported from time to time~ The need for expe r imental data.

on t he loads encountered by skis lead the National Research

Council of Canada to extend their ski researc h to include dynamic

lo a d and bending moment measurements o The results of these

measure-ment s have been given in References 10 and 11 and were made avail =

ab le to the International Civil Aviation Organization as par t of

Cana da's contribution to the group setting up strength require = mentso

A Noorduyn Norseman Mk uVI was used in all the flight

tes t so This type of aircraft is very popular in Nort h ern Canada.

It has an approved all=up~weight of 7400 lb u

Three types of skis were used in the tests ~

(a) USAAF Ski o Length 101 inches p effecti ve bearing length

about 80 inches9 black iron shoeir-g o

(b) Noorduyn "Bear Paw" Ski o Length 116 inches ₉ ei'f'ective

bearing length about 92 inchesv width 32 inches tapering

to 18 inches at the heel v brass shoeing ,

{c) North-West Industries MoSo 7500 Skio Length 98 inches9

effective bearing length about 75 inches. wid th 21 inches,

brass shoeing~

Skis (a) and (b) were used in the bending moment tests. Electrical res i stance strain gauges were cemented to the ski at f i ve stations along the ski length and continuous records were

obtained on film by means of amplifiers and special recording

Page = 19

MN1"225

Sk i s (b) and (c) were used in t h e dynamic load tests o They were fitted with a special load measuring pedestal₉ shown in Figure 39 which utilized electrical resistance strain gauges

to obtain continuous records on fil~ of t h e verti c al load? dragy side load1 and the torque about the axis normal to the ski

bottom.

Records of vertical acceleration of the fusela g e were obtained with suitable electrical acceleromete~s a

The vertical velocity immediately before landi n g was determined b y a method described in Reference 1 0a

In some of the tests the p i lot was guided on a pre = determined glide-path, by observations and signals made fro m the ground9 in an attempt to control the vertical velocity immediate-ly before landingo Vertical velocities up to 10 ft o per second we r e obtained by this methodu The regulations spec i fy a

verti-cal velocity of 9 08 ft o per second for the land i ng g ear of this particular aircrafto In other tests the pilot carried out a

ttnormal landing11 _o

Most of the controlled glide =path and normal landing t ests were made on the rolled snow landing strip a ~ Arnprior p Ontario1 during the three winters prior to Marc h 1949 c The snow surface was generally qui t e hard and reasonably l evel ,. but each spring, when thawing conditions began to set in9 the surface on the landing strip became icy and fairly rough e A fair number of normal landing tests were also carried out on the very hard

and consiaerablv wind eroded snow surface of a lake near Churchill n Mani toba9 during March 1949 ~ in order to include the severe snow ·

conditions peculiar to the Canadian Barrenso The d y nami c loads on skis while taxiing ? taking off and during normal ground handling, were also measured at both Arnprior and Churchill o

The bending moment records showed s trong os c i llations and bending moment reversals over the ent i re length of t h e skio The se were most pronounced for landings on ice but were also p r esent when t h e landings were made on fairly deep snow , Since the envelope of the maximum bending moments was found to extend to

t he very toe of the ski, the necessity £or including a toe

load-ing condition in the design requirements was very clear o The Canadian and PIGAO regulations were therefore changed by specify= ing that the loads in the "concentrated up loads11 _{case be applied} at the extreme ends of the ski instead of at the ends of the bear-ing surfaceo The empirical strength requirementsi as modifi ed ~ give a bending moment curve which has the same general s h ape as the

envelope of maximum bending moments recorded in the t e sts - see Figure 16 of Reference 10 0

Mi\~- 225

Th e c on s ide r able amount o f d at a obtained i n the dynam i c l oad tests h as been pre s e nted in t h e fo r m o f his to -gr ams p graph s and tab l es in Referenc es 10 a nd 11 0 The max i -mum re co r ded values, expre sse d as a percenta ge of t h e u l t i mate l oad r equ ire rr.ents o f PICAO documen t 3031 AIRJ 18 19 1 9 47P are give n bel owo

MAXIMUM RECORDED VAL UES,% OF REQUIRED ULT I MATE

Dyna mic Load Vert ical Dr ag Side

I

Tor que

I

PI CAO, 19 47 1 9, 9 00 4 1980 69980 7938 0

I

Re q 'd Ultimat e l b o l b ~ lb ., ft., l b o I MS 7 500 Sk i 56 48 25 46 a t Ar nprior Bea r Pa w Sk i

_I

68 53

I

50 41 at Ar n p rio r

I

t I MS 750 0 Ski

I

118 3 6 48 4 2 at Chur chill

Th e abo ve table is based on the a pprove d all ~up ~

weigh t of 7 , 400 l b . In the tests s the we i gh t o f t h e air c raf t a s f l own va rie d s omewh at a nd ave r aged a bou t 6 3 00 lbo

Th e ex tre me ly hi g h v a l ue

or

vertical l otid r eco r d ed at Churchill o ccurr ed when a l arge s now hu mmo c k wa s s t ruck ap pro -x ima tely 100 yard s after tou ching down o Th is produced a ve r t i -c al load of 1 5, 620 l b o o n the starboard sk i -a t t he i n stant of impact and 2393 90 l b o on the port ski after t h e sub sequent

bounc e . Tho se fam i l i a r wi th t he seve r e condit i o ns in the

Cana-di a n Bar r e n s wi ll appre c i a t e that the snow sur fa ce is no t alway s

cl earl y visible to the pilot du e to t he pre sence of a l aye r 9 fr om

1 to 5 f t o t h i ck , of "blowing snow " abo ve the sur f ac e o :it is

t herefo re c onsi der ed l i ke l y that a small pr opo rt i on of al l land = ing s made in t he Barren s wil l be as se vere a s the one de s cribe d a bove a nd a n oc casional ve rti c al l o a d o~ the order quoted c a n b e exp ected . Opposed to this po int o f view is the fac t t ha t s k i fa i l ur es are quite rare e ven in the Barr ens9 bu t th is may be

Page - 21 t..r,r-22s this area. Admittedly, the margin of safety appears to be small . However, the problem is not onl y concer~e~ w~th the strength of the skis but also with the shock absorbing charac-teristics of the undercarriage.

The margin of safe ty provided by current design

requirements appears to be quite adequate for skis of aircraft which operate south of the tree lineo

It is interesting to note that the records obtained with the load measuring pedestal clearly showed that$ even for fairly hard snowg the sliding resistance of narro w skis was

less than that of wide skiso 6. SKI CONSTRUCTION

A few remarks on ski const r uction appear t o be worth including in a general paper of this kind.

The skis shown in Figures 1 and 2 were easy to make o The bakelite shoeing and yellow birch veneers were bonded

together in one operation by assembling and clamping t hem o~to a form shaped to fit the ski bottom. A cold setting synthetic resi n adhesive was used. The deck was shaped by a power d riven cutter guided by a sirr.ple prof ile jig made of plywoodo Surpr is -ingly little hand work was required in the shaping and finishing of these skis. They were designed in accordance with the PICAO requirements of 1947 and showed no constructional weaknesses in the test even though they were purposely submitted to a normal amount of rough treatmento

The deck fairing of the streamlined skis shown in Figure 3 was not intended to carry any of the bend i n g loads on the sk1S o l t was there.fore constructed o f light plywood on

wo oden forms . Howe ver9 the fairi~g did take sone of th e bendin~

stresses and developed a number of crackso A fairing designed to carry the stressesj or a flexible fairing v woul d have overcome this difficultyo This ski had a special flexible pedestal incor -porating a cam which automatically locked the ski in the flight position when the pedestal was extended~ but whi ch unlocked the skis when the pedestal was compressed by a vertical load o~ the ski o The front and rear cables, shown in Figur~ 3 . were safety cables which wer e only used during the flight t ests , Althou~h the pedestal performed satisfactorily it was c ons ide red somewhat complicated for general us e. Some variati on of t hi s type of pedestal appears to bP worth furt her consideration

MM-225

7 o GENE.'RAL CONCLUSIONS

While the conclusions given in previous NoRoCo re ports on aircraft ski investigations are applicable to the range of the tests to which they refers the results of recent tests and experience have shown that a few of the earlier concl usions r e quire some mo di fic ation in order to

be of more general application. The conclu sions give n below are based on the fore goin g discussion of the ski research carried out in Canada up to the present time and are believed to be of general applicat iono

(a) There appears to be some justification for twoy or possibly three9 types of skisg each designed to meet parti =

cular snow conditions such as~

(1) skis for use on fairly large landing areas which are exposed to moderate winds, where the soft snow depth

seldom exceeds 12 inches.

(2) skis for use on fairly well sheltered landing areas where temperatures are fairly low and the snow- cove r is soft at all levels and may reach a depth of 20 to 36 inches.

(3) skis for use on open areas exposed to strong and persis-tent winds and low temperatures where hard9 wind eroded

snow is very commono The main difference between types (3) and (1) is that type (3) would be of stronger construe= t i ono

The "general purpose type" of ski would be type (2) above. (b) For improved sno·N performance, the skis shoul d be

longer, narrower and have somewhat higher unit loading than conventional skiso The ski pr oportions and unit loading will de pend primarily upon the snow conditions on which the ski is expected to operateo For soft snow conditions9 it is suggested

that ski designers assume the following as maximum values ~ d = unit loading9 lb . per sq. ft c

_s_n_o_w ___ c_o_v_e_r~d-,-e-p~t-h

700

= 1 d

2 •

I

( see Fig o 4 .)

=

20 lb. per sq. ft , of contact surface vertical load on ski

( 1 )

(2)

Page -· 23

M1'. "'225

The maximum ski penetration obviously cannot exceed the depth of the snow-cover and equation (1) is therefore

hypothetical o However, equation (1) g i ve s values which roughly agree with those observed during the Beaver ski trials and may be considered useful for design purpo ses only up to a unit load~

ing of 500 lbo per sqo fto ~rom the above equations the designer

can derive curves of the maximum expected s l iding resistance against ski dimensions and select the most suitable proportions and unit loadi ng compatible with othe r d e sign limitations .

{c) The ski should have laminated bakelite shoeing and

one or two laminated bakelite runner strips e Shoeing and

runners~ 1/8 ino thick ~ are adequate and should have high resin content and hi gh cureo

{d) The ski should have a fine entrance angle (not

exceed-ing 25 degrees relative to the ski bottom ) and should be such that the resul tant of the forces acting on the s k i pas s es close

to the centre of the bearing surface - see Figure 4 o

(e) Streamlining a ski and using an internal type

or

ski

trimming gear greatly reduces the air resistance of the

instal-lation. The high air resistance of an exposed , shock - cord type

trimming harness may be reduced by enclosing the shock~cord in .

a cellular rubber sheath of streamlines section. Streamlining

the ski also reduces the aerodynamic instability of the skl o

(r) A ski may be aerodynamically stabilized in pitc h by

the use of a f ixed tai l plane mounted high enough above the

hee l of the ski to clear the snow. Although the size of the

stabilizer required for a conventional non~ streamlines ski is excessive, the size for a s t reamlined ski of long and n arrow proportions appears to be quite reaso nable.

(g) The PIGAO design requirements of 1947 appear to be

adequateo Iri the case of skis intended for use in the Canadian

Barrens, designer s may prefer to increase the 9~rength of the

skis over and above that required9 or to use a longer travel

•

MM-22 5

BIBLIOGRAPHY

1. Investigation into the effect of weather conditions on the friction of sleigh 1•unners on snow - Gliddono Postgraduate Thesis, McGill University, 1922.

2. Undercarriage resistance - Parkin9 Coates and Klein o

Aeronautical Research Paper Nao 23~ School of Enginee ring Research, University of Toronto₉ Oct . 1 928.

3o Wind tunnel tests on the trimming of aircraft skis - Green . National Resear ch Council of Canada~ Report No. PAA- 13, August 1933 0

4o Winter landing gear - Ferrier. Journal of the Royal Aeronautical Society9 June 1934.

5o The aerodynamic characteris tics of aircraft skis and the development of an improved design= Green and Kleino

Journal of the Royal Aeronautical Society$ August 1935 .

6. A wind tunnel investigation to develop imp roved stream~ li ned skis for air c raft= Levy and Richmond. Nat ional Research Council of Canada Report Nao MA0 102

1 September 1942 .

7o Snow loadin_g ski tests at Porquois Junction, 4-16 Aprily

1942 - Levy o National Research Council of Canada₉

Report Noc MM-62, June 19420

8. Shock strut deflections and the corresponding angle of thrustline to horizontal of two-ski landing gear for alrcrart - Mandl. Nat ional Research Council of Canada ,

Report No. MA-1721 March 19460

9. The snow characteristics of aircraft skis = Klein. National Research Council of Canada₉ Report Noo AR~2₉

19470

10. Interim report on measurement of landing loads on a ski plane - Uffen and Wood . Na tional Research Counci l of Canada1 Report Noo MM~2109 October 1948 .

•

Page - 25 MM- 225

11. Flight tests on undercarriage l oad s for a sin gl e-engined aircra ft equipped with skis - Wo odo Nat ional Re se a rch

Counc i l of Canada, Report Noo MR - 79 June 1 9 4 9 v

12. Canadian survey of ph ysical charac teris t ics of snow-covers - Klein. Associate Committee on So i l and Snow Mechanics of t he National Research Council of Canada₉ Technica l Memorandum No. 1 5, Apr i l 1950.

13. Investigations on ski friction - S~derberg .

IoVvA:s Flygtekniska Kommi t tee, Report No. l s Sto ckh o l m, 1932.

14. The physics of skiing, the preliminary and general survey -Nakaya et al . Journal of the Faculty of Science, Series II9

Volume 1, No . 9 , Hokkaido Imperial University, 19 36 .

15. Snow structure and ski fields - Seligma n .

MacMillan and Company, London, 1936 .

16. Wind tunnel measur ements o f aircraft ski s = Kohler . Luftwissen, Vo lume 4 , No. 1, Berl ini 1937 .

17. The me chanism o f sliding on ice a nd snow ｾ＠ Bowden and

Hughes . Pro c . Royal Society9 Lo ndon9 193 9.

18. PICAO Document 1548 AIR/62 , 1946 . 19. PIGAO Docume nt 3031 AIR/ 181 , 1947 0

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SKI DYNAMOMETER

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DEHAV I LLAND BEAVER ON N.R.C . SKIS

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LOAD MEASURING PEDESTAL (NEAR SKI) AND

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