Dynamic problems of unmanned tethered rotor platform Sea-Kiebitz with

SECOND EUROPEAN ROTORCRAFT.AND POWERED LIFT AIRCRAFT FORUM

PAPER NO,

8

DYNAMIC PROBLEMS OF UNMANNED TETHERED ROTOR PLATFORM SEA-KIEBITZ

WITH SPECIAL REGARD TO THE LANDING

W. Benner

Dornier GmbH

Friedrichshafen, Germany

September 20 - 22, 1976

BUckeburg, Federal Republic of Germany

Deutsche Gesellschaft fUr Luft- und Raumfahrt e.V.

Postfach 510645, 0-5000 Koln, Germany

I NTRODOCT I ON

Unmanned tethered rotor platform~ differ in their dynamics from manned helico~ters of the sam~ size because of

the strong influence of the tethering cable on the dynamic behaviour of the syste~.

Dornier has developed such a ph.tfom system, the opcratior.al Kiebitz: which will soon be ready for r:l!Jltiplc pro-duction.

ln the case of using tethered rotor platforms on a ship as for example the SEA-KIEBITZ (fig. 1), a rr.odified form of the operational Kiebitz, further dynamic problems arise from the move:r:ent of the ship which causes strong disturbance of the system of tethering cable and Kiebitz at low altitudes. Based on a brief explanation of the system, its structure, its function, and its equipment the standard mission phases are analysed.

The main flight characteristics of the system are described in these phases. The operational limits of the syste~

will be shown and explained. Possible ways to realize an optimum system configuratio.n are discussed, with sper.:ial

regard to the landing.

1HE SEA·KIEBITZ SYSTEM

The SEA-KIEBITZ system comprises (fig.

2):

- Carrier and radar

- Tether cable and winch drum on deck

- The deck landing pad Hith protecting cover

- Airborne unit monitoring and control panel

- Radar Control and Display unit. Radar information may also be presented on ship•s tactical display

Fuel supply and data to and fro:;; the carrier and radar are provided via the tethe1·.

The carric:r is a rotor supported platform with torque fr~e blade tip drive and an automatic flight control syste:;..

The platform is controlled in a fashion similar to a helicopter in pitch and roll and by deflection of turbine ex-haust gases in yaw. The cable ~enslon is automatically controlled by using the collt:c":ive pitch and by the power manageMent of the turbine.

Tl1e Sf:A-Kl£BITZ can be operated from the smalJ Janding platforms of non aviation st.ips or FPBs in r.;ooerately high seas.

SEA-KIEBITZ Hith a special radar is capable of almost trip1ing the rada1· horizon of current ships and of extendir.g their surface surveillance capability by almost ten times (fig. 3).

The performance of current on board missile guidance or target detection equipment is severely limite.:l by the

height of masthead aerials.

The strike range of medium range missile syst~ms is therefore limited.

Thanks to its mission height of 300 metres, the SEA-l<IEEITZ extends the radar horizon to approxir.tately 60 km or 35 n:

For e>:ample, a FPB equipped with SEA-KIEBITZ \-1i1l b~ able to overlook the entire sea area of the \\estern Baltic

fro~ coast to coast.

SEA-KIEBiiZ \·lith for exar;;;:>le a FERRANTI SEAS?RAY radar could provide 24 hot:r su1·veil1ance over th~ horizon even in adverse 1-:ea ther cor.dit ions ~:i t.hout the probl err:s of a data link, and reaCi 1y integra ic:s l'iith existing ship;;' .,.:;::apon:. syste:::s.

SEA-KI£BITZ may be d~ployed from ships and also fro~ those vessels t~at are too small to support a helico~trr, and g~eatly enhances their offensive capability.

Some specifications of the carrier and the radar are listed in fig. 4.

The main supposition for all SEA-KJEBITZ activ1ties was to use the carrier of our o~erational KIEBITZ witho~t change~

in its hardwa~e and the structure of the controller if possible.

MMIN MISSION PHASES

The main mission parts of a SEA-KIEBITZ operation from a ship are

- rotor start

take-off and ascending to mission hei9hts of about 300m. pulling up the tether cable by a controlled tether tension

- surveillance at mission altitude for 24 hours maximum

hauling down and landing on the deck

- rotor stop

Analysing these operations the environ:nent in which the.Y take place must be considered. The problems cre:ated by the

motion of the ship and its landing platforn (Hhich may be very restricted in size), th~ relative wind and the air

turbulence around the ship must be discussed in relation to their effect on the performance of the platform.

In relative wind from ahead, the airflow around the shi~ and its superstructure in the vicinity of the landing

platform, is variable and complex in character and depends on the type of ship. It has not yet been conside-red for the landing cases analysed here because too few data were available.

DYIW·\1 CS OF 'THE SYSTEM

As said bE"fore unmanned tethered rotor su;:>;-orted platforns are differing in their dyr.arr,ics from manned helicopters

of similar size because of the strong influence ofrthe (tensioned) tethering cable. Because of their configur~tion

and their flight performances requir·ed by their cargo as for example special radars, tethered helicopters drones like the KIEBITZ can be looked at in some other 'rfay as rr.anned helicopters.

Only a few dynamical aspects cf the SEA-K!EBITZ shall be presented here with special regard to the landing. Because it is there that the actual proble;;JS of tethered rotvr platforms operated fi-oo moving vehicles arise. Fig. S shows three time histories of simJlated SEA-KIESITZ flights at a mission heig~.t of 300m. The systei:l is disturbed by a horizontal headwind step gust of 5 rn/sec at the beginning of the flights. The co:nparison of thr

un-controlled tethered and untcthered flight may show the strong influence of the tensic:-~ed cable. The pitch attitude

time history of the untethered flight sho.,.,·s the normul oscillatory divergent long term response of the hovering helicopter. The curve of flying tethered under the same assumptions shows a similar initial pitch response of the platform. But after about 1,5 sec the tension force of the tether cable increases ar.d declines according to the tether cable curve. It fonns a pitch moment and tries to turn back the helicopter against the gust-induced pitch up rotor r:1oment.

The most important and characteristic factors that detern:ine the dynamics of the SEA-t~l£BlTZ-tether cable system

are the own natural f1·equencies in the rotatorial and translatorial axes of the helicopter, wrot ar.d wtr'

espe-cially under consideration of the ship's pitching, heaving and rolling. wtr is much h.Qr~ critical than wrot because

of the ship's roll mode at rr.oderate se:a state. The cause is that the ship can roll w~th the sa::~e frtqu~nc.y and

usually ship~ roll at their own natural rcll fre~uency - as th~ own natural translatorial frequencies of the

SEA-Y..IESJTZ are. This is the case \oihen the drone is hauled d:.;.m and a;."~proaches the deck to a distance of or,ly a fe;.: meters for landing-on, or if it is op;rating at very low altitudes .. For frigates the roll period can be taker. to be 8 .;. 10 sec, FPBs roll more quickly.

Fig. 6 shows 1n which way wtr depends on some of the main parameters 1nfluer.~ing KIEBITZ dynamics.

as

the ca~l~ length (lcable)' the rotor thrust

{PA).

and the distance of the platform

CG

to the tethering point,

1

1 •

PA is a function of the relative wind, the Available power and the commanded value of the cable tension force,

if tethering cable tension is controlled. The relative vertical wind results fro~ the hauldown speed and the

ver-tical gusts.

The moments of inertia do not influence-the own natural transl.:otorial freqOJencies but the rotatorial frcc;:Jcncies as shown in fig. 7. Fig. 7 s~:.. .. -s wrot =-- f {lcable' PA, 1: and zs"' canst). wrut mainly depends on the rot.or thrust

PA and the moments of inertia in the pitch and roll axes. It limtts therefore the maximal utilizable PA because

of the cyclic pitch frequency boundary.

Considering these param~ters, the equations and data of the automatic controller. the rotor system, the actuators

and the envirCJnmental factors, it is possible to get optimum configuration:of the platform-cable system.

To discuss these problems math~~atical digital and hybrid simulation models of the SEA-KIEBITZ have been derived

from the models of our operational KIEBITZ. Complete SEA-KIEBITZ operations from ships can be simulated. including

the ship and the interaction of shi~ and helicopter through the tethering cable.

The wave induced motions of ships are statistical and related to those induced by a random sea. In the sim:.~1ation

models they are assu:!led to be harmonic. They are functions of the significant \:ave height and the heading angle. For the simulations, the ship has bee':! assumed to be moving at constant heading and speed with respect to the seaway and the prevailing wind and wind direction. The motions of the ship are restricted to heave, pitch and roll.

Physical interaction of the ship and the rotor platform occur through the teth.cr cable and in addition, by the

ship's perturbation of the win.:: vt-:locity profile. The cable tension force is considered in the aircraft 11:odel but not the airflow around the ship. The model of the helicopter is a G-OOF-unsteady model. including a r.:odel

of the automatic controller end the actuators and a model of the tet~er cable.

fig. 8 shows some simulation results of digital sif:lulated flights at low altitudes to get an optimu'll ccnfig•Jration. For selected heavy ship motic:1s ~nd several low flight altitudes resp. cable lengths, '1\.h.e cunstant ro11 attitud~s intlucer:l by the ship's rolling are plotted over

h.

The left figure is plotted for z₅ "' const, the right figut·e for z

5 + 12

=

cor.st, that m!:ans Ix and IY

=

f{12 }. Fig. 10 shows the time history of one of these simulated

f1 ights.

These results r.ny show that thr:re are ways to get optim!lm dynamics by discussing the inf1;Je of the most

in-portant dyna~ical parameters of a tethered helicopter like the SEA-KIEBITZ.

The dynamical behaviour of the SEA-KJEBITZ and therefore the safe flight envelopes in the phase of hauling dol'm depends on the haul d01m speed, the ship motions, the relative horizontal and vertical wind and the resulting rotor

thrust under consideration of the cable tension force control. It depends too on the reduce of sink rate before

on-set and the touch dol'm rate.

Fig. 11 sho·fss safe flight- • take-off- and landing-envelopes for the tethered SEA-KJEBITZ in thP. case of a

Gern~an FPB {5143). The take-off and mission flight envelopes are satisfactory but for landing on the deck the

~aximal roll am~litvde is li~iteC to 0,2 m. For the landing er.velope it was assu~ed that the sink rate is 2 to.3 rn/sec and that the touch do•,T, ve1ocity must be very small because of the safe structural loads of the lar.ding gee!r of the K1E£1T:Z. The h.cul down envelope could be extended by decoupling the moving of the tethering poin: on the deck fro:n the ship's r::.ving, for example by a roll-stZ:bilized landing p11d as shown in fig. 2.

Tre cause for the strong restrictions of the p~rmissible roll att1tu~es of the ship is the coincidence of the frcc:;e .. :y

of t~e ship's rolling and the translatorial own natural frequency of the platfor~-~ether cable-system in flight ~~1~·!s

sc:::e meters above the deck as discussed before. If the ship rolls too hard the platform is disturbed too much by !te

oscillating tension forces of the cable which in aCdition is much more deflected fro:n the vertical than in higher

altitudes. Fig.

12

shows a haul

down and too big roll

attitudes

of 'the

S£A-K1£DJTZ which

are caused by the

ship

1

s

rolling.

The cable tension depends on the ship motions, the' haul down speed and the way of controlling haul down speed.

In some recently completed investigations we have tried to find out the best landing procedure and so to extend the landi"ng envelops, by increasing the haul down velocity and the onset velocity, by dir:1inishing more quickly the sink rate before touch down and by controlling the haul down velccity relatively to the sea level to maintain the cable tension forces at constant values.

The haul down velocity resp. the rate of descent of the SEA-KIEBITZ showed to be limited by the following reasons.

- the influence of the relative horizontal and vertical wind on the system of tether cable and rotor platform

- the flight idle of the turbine which is used in the KlESITZ and which delivers the co:npressed air for the

torque free blade tip drive (even at flight idle) and the electrical energy for the automatic flight con-trols. the rotor controls with actuators, the ya>tt controls and the cable tE-nsion control (as said before

descent and landing-on by autorotation will not be discussed here)

- the flutter boundaries and the beginning of the vortex ring state.

Fig. 13 shows the iink rate capability of the aircraft as a function of the relative horizontal wind.

Based on these results and the limits of rotorthrust, haul down sir.1ulations sho>tt that the landing procedure can be improved and the roll boundaries can be extended. As fig. 12 shows the rotor platform SEA-Kl£BlTZ can utilize higher rates of descent at lower levels of relative horizontal•winds.

In general i t can be sho'rm that thS: capabilities of a tethered landing rotor platform to stand higher roll

a~plitudes of a ship or FPB, can be broadened by a quic¥ haul down at high rotor thrust and a relative high

on-set velocity. The average rotorthrust should be less at 1ow altitudes ~bove the 1unding pad becaus~ of the

coincidence of the translatorial frequency of the tethercable-platfonn pendulu:n and the ship's roll

fre-quency but in the {..Ontrary it should be high or even increased at this altitude b~cause of the limited touch

dmm speed.

F1g. 14 shows a time history of the variables of motion of a sim~lated (6DOF) SEA-KlEBITZ quick haul do·.·m opera-tion for landing on a FPB. The boat is moving according to sea state 4 at 5 Beaufort head•nind. The sigrt1ficant

wave period is abo:..tt b sec, the significant >t1ave height is 2915 m, ship speed is 15 knts end the relative

head-wind is about 30 kn.

CABLE 1ENS!C.~ CC.'ffROL

lt has been shov;n before ho1~ landing dynamics of tethered rotorplatform lif:e the SEA-KIEBITZ can be

in-fluenced and im~roved by the dynamical important parameters and by seeking an optimu~ landing procedure.

But as pointed out before there is still another 't.'ay of improving landing.

If the haul down speed is constant relatively to the sea level the disturbances of the platform induced by the

cable tension can be minimized. There are quite good results if e shipside cable tension central can be realiz~d.

A selected and cooroo:anded tensior1 of the tether cable is r:J:!intained at a constant value during ship r.1otion and unCt:r' landing tension by co:::paring the actual to the selected tension. The resulting error signal is utilized to

drive a .hydrostatic transmission \;:~ich rr.aintains the selccttd tension 'tiithin Marrow limits by turning a winch dn.:-..

lh:: landing platform motions of SEA-KIEBITZ capable ships in moderately high seas have a very strong effect on

the safe landing capability of tethered rotor supported platforms.

C;:~:-:J::t dynamics during the landing procedure can be realized by an optimum configurution, by a high haul do~<vn anC tvv:.h d~:on speed and by controlling the cable vr:locity and tension.

Tr.e rotor. stop ar.d start 1 imits under consider~tion of the airflow around the ship have not been 'discussed here b~t

the; are as important as the landing envelope and may too restrict the operational freedom of the ship.

e~t only trials on

a

moving landing platform and later on actual sea trials could validate the calculated

opera-tional limits.

lr.JTAT!Oil

•

f

amplitude

frequency of moving ship Fast Patrol Boat

mo~ents of inertia about aircraft body axes

distance rotor-tethering point distance CG-tethering point

1

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~il,lcable length of tethering cable

~: torque rr,owent of \'!inch drum

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t time

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~.

e

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w frequency

IEFERENCES

m

Hz

mkp sec2 m m m mkp kp kp kp m sec m degr. 1/sec

[l) Simulation des schiffsgestUtzten Einsatzes

der

Rotorplattform Kiebitz

Oornier-Bericht EP 20/ST-49/72 [2) Sche:1zlc, Blume Sub:;~ripts g A

s

rot

tr

Vorausberechnung der

zu

erwartenden Bel':~gungen im Seegang fUr das Projekt S-Bcot 143

vor und nach de~ u~rUstung

{3]

Jnstitut fUr Schiffbau der Universitat Ha~burg

Benner, Ehe!drcher. Herberg. Hi rlh, Lutz

Fl ug:;;cche:-: i sch/rotoraerodynami sche U:ltersuchungen ZI.!;;J Schne 11 einzug gfrfe:sse 1 ter R8~<>rplatt"'·::n<en r:-,it !,e;·lt;;gtE::l BDd~r.fe:sse1punl;t

ZiL-Endb~rich: 75/50 3 1975 geodetic tethering point CG of aircraft I . rota tonal translatorial

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