Thesis (MScEng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: A need exists for the development of an unmanned rotorcraft capable
of autonomous flight, as would be required for the survey of high
voltage electricity supply lines. A program was initiated at the
University of Stellenbosch in December 2002 in order to develop such
an aircraft.
The first goal of this thesis was the development of software that could
calculate the stability and control derivatives of a model helicopter.
These derivatives could then be used in the formulation of an
appropriate helicopter control strategy. The second goal of the thesis
was an investigation of the stability and control characteristics of model
helicopters.
The trim settings of the helicopter were required in the calculation of
the stability and control derivatives. A computer program was
developed to determine the trim settings of a helicopter in forward
flight. Another program was developed to calculate the stability and
control derivatives, using the results of the trim analysis.
The trim analysis was based on the assumption of negligible coupling
between the longitudinal and lateral modes of motion. The method
proposed by Bramwell (1976) was used to perform the trim analysis.
The stability and control derivatives were calculated by obtaining the
trim settings from the trim analysis. These derivatives were then used
to solve the roots of the characteristic equations of the longitudinal and
lateral modes of motion. The stability of the helicopters were
investigated firstly by examining the stability derivatives and secondly
through root-loci analyses.
The most important results were the following:
• The root-loci analyses indicated that a helicopter without a
horizontal stabiliser suffered from instability of the phugoid
mode. It was also found that the short-period motion of these
helicopters was heavily damped. Fitting a horizontal stabiliser to
these helicopters caused the phugoid motion to become stable
even at low speeds. This was achieved at the cost of a reduction
in short-period motion damping.
• The periods of the lateral and longitudinal motions were smaller
than those found on full-scale helicopters. This was attributed to
the small mass and inertia properties of the model helicopters. An increase in speed is generally accompanied by an increase in
the stability of the helicopters. This could be attributed to the
effective operation of the tail surfaces at higher speeds.
• The axial climbing speed of a helicopter is influenced by the rotor
speed. A low rotor speed allows higher climbing velocities at a
given power setting. This was due to lower induced power losses
at low rotor speed, assuming that no blade stall occurs.
• The rotor speed does not influence the incremental amount of
power (M:,) required to achieve a certain climbing velocity, due
to the fact that the profile power losses are constant for a certain
rotor speed.
• The simplified horseshoe-vortex theory can be used to analyse
the downwash angle at the horizontal stabiliser if the helicopter
is in high-speed forward flight. / AFRIKAANSE OPSOMMING: Daar is tans 'n vraag na die ontwikkeling van onbemande rotor-vlerk
vliegtuie wat die vermoë beskik om hulself te beheer. Hierdie tipe
vliegtuie sal gebruik word om byvoorbeeld hoë-spannings elektrisiteitverskaffingsdrade
na te gaan. 'n Program is in Desember 2002 by die
Universiteit van Stellenbosch begin om sulke vliegtuie te ontwikkel.
Die eerste doel van hierdie tesis was om sagteware te ontwikkel wat
die stabiliteit- en beheerafgeleides van 'n model helikopter kon
bereken. Hierdie afgeleides kan dan gebruik word om 'n gepaste
helikopter beheerstrategie saam te stel. Die tweede doel van die tesis
was om die stabiliteit- en beheerseienskappe van model helikopters te
ondersoek.
Die berekening van die stabiliteit- en beheerafgeleides van die
helikopter berus op die beheerinsette benodig om die helikopter in
ewewig te hou (trim). 'n Rekenaarprogram is ontwikkelom hierdie
beheerinsette vir 'n helikopter in voorwaartse vlug te bereken. 'n Ander
program is ontwikkelom die stabiliteit- en beheerafgeleides te bereken
met behulp van die ewewig beheerinsette.
Die analise van die helikopter in ewewig berus op die aanname dat die
grootte van die koppeling tussen die longitudinale en laterale
beweginsmodusse weglaatbaar klein is. Die beheerinsette van die
helikopter in ewewig tydens voorwaartse vlug is bereken deur van
Bramwell (1976) se metode te gebruik. Die stabiliteit- en
beheerafgeleides is bereken deur van hierdie beheerinsette gebruik te
maak. Die afgeleides is gebruik om die wortels van die karakteristieke
vergelykings van die longitudinale en laterale bewegingsmodusse te
bereken. Die stabiliteit van die helikopters is eerstens beoordeel deur
die stabiliteitsafgeleides te ondersoek en tweedens deur middel van 'n
wortel-lokus analise.
Die belangrikste resultate is as volg:
• Die wortel-lokus analise toon dat 'n helikopter sonder 'n
horisontale stabiliseerder phugoid-onstabiliteit (Iangperiode
onstabiliteit) het. Die kort-periode beweging van hierdie
helikopters het verder groot hoveelhede demping aangetoon. Die
phugoid-beweging kon selfs teen lae snelhede gestabiliseer word
deur 'n horisontale stabiliseerder aan te heg. Hierdie stabiliteit is
egter bereik ten koste van die demping van die kort-periode
beweging wat verminder is. • Die periodes van die longitudinale en laterale bewegings is
kleiner gewees as vir volskaal helikopters. Dit kan toegeskryf
word aan die klein massa en inersie van die model helikopters.
• Die stabiliteit van die helikopter is in die algemeen verbeter soos
die snelheid verhoog. Dit kan toegeskryf word aan die beter
werking van die stert teen die verhoogde snelhede.
• Die klimtempo van die helikopter word beïnvloed deur die
hoofrotor snelheid. 'n Lae hoofrotor snelheid laat 'n hoër
klimptempo toe teen 'n spesifieke drywinginset. Dit is as gevolg
van die laer geïndusseerde drywingsverliese teen die laer
hoofrotor snelheid. Daar word aanvaar dat die lugvloei oor die
lem nie staak nie.
• Die hoofrotor snelheid beïnvloed nie die inkrimentele drywing
(M,,) wat benodig word om 'n sekere klimtempo te bereik nie. Dit
is as gevolg van die konstante drywings verliese teen 'n sekere
hoofrotor snelheid.
• Die vereenvoudigde perdeskoenwerwel teorie kan gebruik word
om die afspoel hoek by die horisontale stabiliseerder te bereken
indien die helikopter in hoë-spoed voorwaartse vlug is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/50102 |
| Date | 12 1900 |
| Creators | Visagie, Jonathan Gerhardus |
| Contributors | Pienaar, D., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 206 p. : ill. |
| Rights | Stellenbosch University |
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