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Radio
Control Model World - Nov '95
by
Stan Yeo
INTRODUCTION
During
the non-flying periods of my trips to the slopes I regularly lapse
into my 'Desmond Morris' mode (you know, him of the Naked Ape and
Human Zoo fame) and observe the way people fly their models. This
is quite enlightening as it is often possible to highlight areas
of weakness in a modeller's flying skill and predict the type of
accident that will befall them (watch out Stans about!). This is
not always the case as some models demand so little of the pilot,
except on landing, that it is difficult to form an opinion. As a
result of these observations I have come to the conclusion that
most modellers fly very conservatively, or at least the ones I meet.
Very few flyers 'throw' their models around as if there were no
tomorrow. Most are content with stooging around and doing the occasional
loop and the odd roll. This is in no way meant as a criticism of
this style of flying, each to his own. The object of the exercise
is to enjoy ones-self and I have spent many a happy hour cruising
up and down a slope in light lift on a balmy summer's evening. But
when the lift is good I often ask myself why is it that modellers
are not a little more adventurous in their flying. Is it because
they have no interest in aerobatics or is it, as I suspect, a lack
of confidence to have a go, after all nobody likes to take their
model home in bits. Whenever I bend a model two thoughts race through
my mind. One, how am I going to repair the model so that it is still
presentable, and two, how am I going to find the time to carry out
the repair?
Unfortunately
'conservative' flying does little to improve overall flying skill
or increase confidence in your flying ability. This was brought
home to me a few years ago when I took up full size gliding again
after a lapse of almost 30 years (I learnt to fly in the ATC on
T31s and Sedburghs). I was never a confident flyer and in fact I
gave it up originally because someone scared the living daylights
out of me on a check flight, consequently the things that frightened
me then came back to haunt me again. Spins in the 1960s were not
a compulsory part of the ab initio training but they are now so
the closer I got to re-soloing the more apprehensive I became until
one day when I could put it off no longer I bit the bullet so to
speak. After doing a couple I was left wondering what was all the
fuss about. Now at the end of a good soaring flight, if I have height
to burn off, I will do a few spins, not only does it keep me in
current practice but it gets the blood circulating before I attempt
a landing! The same applies to performing aerobatics with a model,
it develops flying skill and builds confidence. It could be the
difference between staying up in marginal conditions to ending up
with a pile of bits or not flying at all.
WHERE
TO START
The
first point to consider before attempting to fly aerobatics is the
safety aspects of models, people and property. If the sky is crowded
or there are a lot of people wandering around or the conditions
are marginal it would be prudent to postpone the 'adventure' until
conditions are more favourable. After all you do not want to 'bend'
your model let alone hit someone with all the consequences that
may bring.
Points
to remember to remember before carrying out aerobatics are:
1.
Have you got sufficient height?
2.
Is the model structurally sound and capable of carrying out the
manoeuvre?
3.
Where are the other models?
4.
Have you made sufficient allowance for the model being blown back
on the wind? i.e. is the model far enough forward.
5.
Have you learnt the sequence of control movements required to carry
out the manoeuvre? Practice them at home with a friend holding the
model and simulating the manoeuvre in response to your control inputs.
6.
Plan the manoeuvre so that the model exits the manoeuvre pointing
into wind. If the model does not exit the manoeuvre into wind it
is likely there will be a panic control input to turn it into wind
away from the slope.
7.
Where possible have an experienced aerobatic flyer on hand to talk
you through the manoeuvre and highlight areas of weakness.
8.
Always remember what manoeuvre you are performing and the stage
the model has reached. This may sound a bit fundamental but you
would be surprised how many flyers forget when things start to go
wrong and let 'instinct' take-over with often disastrous results.
CARRYING
OUT THE MANOEUVRE
there
are four distinct stages to performing an aerobatic manoeuvre:
1.
The preparation.
2.
The dive.
3.
The manoeuvre.
4.
The recovery.
Preparation
This
consists of carrying out all the necessary safety checks, some of
which are mentioned above and positioning the model in the right
place in the sky to carry out the manoeuvre. Positioning is the
most important part of any aerobatic manoeuvre. If
it is not right at the start of a manoeuvre then it will get progressively
worse during the manoeuvre. Most aerobatic competitions, even at
club level require that the manoeuvres are carried out crosswind
i.e. along the face of the slope. Into wind loops do not score points!
Initially flyers will find this difficult so until you are confident
in performing a manoeuvre into wind do not attempt it crosswind.
Some may disagree with this advice but the object of the exercise
is to practice the manoeuvre and keep the model in
one piece. Into wind manoeuvres do not look as pretty as those carried
out crosswind but they do reduce the risk of a serious accident.
The
ideal position to start most manoeuvres is with the model straight
in front of you, at an angle of approximately 60 degrees to the
horizon and at a height 150 - 200 feet (50 - 65 metres). This is
of course a subjective measurement as it is very difficult to judge
a models height without some form of external reference. The 60
degree angle to the horizon is the critical yardstick as it helps
to ensure the model is a sufficient distance away from the slope
to carry out the manoeuvre.
The
Dive
Once
the model is in the correct position it can be dived to build up
speed to carry out the manoeuvre. Diving was discussed at some length
in an earlier article ( July '94 RCMW) but essentially the most
efficient way to build up speed with the minimum height loss is
to dive at an angle of 15 - 30 degrees. The more efficient the model
the shallower the dive angle. Diving at an angle greater than 45
degrees is definitely not a good way to build up speed as so much
energy is lost during the recovery that often there is insufficient
left to carry out the manoeuvre properly. Most models need to be
dived for at least 4 seconds before they have built up sufficient
speed to carry out the manoeuvre. The dive angle and dive time is
very much dependant on the type of model and the prevailing conditions
and can only be determined by experience.
Some
manoeuvres such as the roll require that the model be returned to
level flight before the manoeuvre is carried out. On some models
returning the elevator to neutral is sufficient whilst on others
a small amount of up elevator may be required. Again experience
will dictate the amount of control required.
The
Manoeuvre
At
the start of an aerobatic manoeuvre the model has a certain amount
of energy in excess of that which it has in normal flight. It is
this excess energy that is used to carry out the manoeuvre. If too
much energy is used at the beginning of the manoeuvre by erratic
or excessive control movements then it is likely that the model
will fail to complete the manoeuvre successfully. It will either
'flick' roll out of it in the beginning or fall out of it at the
end through lack of speed. A successful aerobatic manoeuvre is one
that is performed smoothly with energy used at a uniform rate throughout.
Being able to fly smoothly is a pre-requisite of a good aerobatic
flyer. It also pays dividends in marginal lift conditions when the
slightest hint of over-control would mean a premature arrival.
The
Recovery
On
completion of the manoeuvre the model must be returned to normal
level flight either flying in the same direction or in the opposite
direction as in the case of a stall turn. Ideally, for novice aerobatic
pilots, the model should finish up pointing into wind, hopefully
above slope height! This means that before carrying out the manoeuvre
the exit from it should be planned in advance i.e. the model should
be correctly positioned at the start of the manoeuvre.
THE
MANOEUVRES
The
basic aerobatic manoeuvres most pilots would like to master are
the Loop, Roll, Outside Loop (bunt), Inverted Flying, Stall Turn,
Cuban Eight and the Spin. There are others of course but they are
not within the scope of this article (it is long enough already!)
As mentioned before my approach to performing some of the manoeuvres
may offend some dedicated aerobatic flyers but please remember the
purpose of this article is to encourage the inexperienced flyers
to have a go at slope aerobatics by minimising the risks.
The
Loop
This
is the most basic of aerobatic manoeuvres and one that I recommend
is initially practised into wind. Position the model as previously
described i.e. straight in front, at an angle of 60 degrees and
approximately 150 to 200 feet high. Dive the model, straight, wings
level, to build up speed, again as previously outlined and then
slowly apply approximately 25 to 30% of the available up elevator
to smoothly enter the loop. If the model has insufficient speed
or not enough up elevator has been applied the model will stall.
Recover and try again, this time either building up more speed or
using more elevator control as the situation dictates. This time,
assuming the model has sufficient speed and you have applied the
right amount of up elevator continue with the loop until the model
is upside down. When the model is upside down and has just started
the recovery dive slowly release some of the applied up elevator.
This will help to prevent an inverted stall and to allow the model
to build up speed for the recovery to level flight. For the recovery
slowly re-apply the up elevator until the model is again in level
flight. To prevent the model zooming up into a stall it may be necessary
to apply a small amount of down elevator until the excess speed
has been 'burnt off'.
Common
faults when performing a loop are:
1.
Incorrect initial positioning i.e. model off to one side, too low
or too close to the hill.
2.
The dive is either too steep or not held on long enough to build
up sufficient speed.
3.
The loop is not entered with the wings level and the model corkscrews
off to one side during the loop.
4.
Too much up elevator at the start of the loop resulting in either
a very tight loop that could overstress the model, a flick roll,
or the speed being 'scrubbed' off leaving the model with insufficient
speed to complete the manoeuvre.
5.
Too much up elevator at the top of the loop resulting in an inverted
stall or poor recovery.
6.
Too much elevator during the recovery resulting in a zoom climb
and stall.
7.
Inadvertent introduction of rudder or aileron control when applying
up elevator resulting in the model 'screwing out' of the loop.
Once
you have successfully looped the model a few times try a loop with
a little more speed and less elevator. This will open the loop out
and should the model start to screw off to one side there will be
more time to apply control correction, that is once you have worked
out which way to apply it! Experiment with the manoeuvre until you
have got the feel as to how much speed and elevator is required.
This will take a few sessions and when you are confident that you
have mastered into wind loops try crosswind loops. These are often
easier because you can see what the model is doing and there is
less wind variation during the manoeuvre. Take care though that
you leave enough space between the hill and the model because all
the time the model is performing the manoeuvre it is drifting back
towards the hill. Remember also, which direction you need to turn
to get back into wind after you have completed the manoeuvre.
The
Roll
The
roll is another manoeuvre, the rudiments of which are best learnt
flying into wind. the secret to performing a good roll is to:
1.
Have sufficient speed.
2.
Recover the model to a level attitude before commencing the manoeuvre.
3.
Good co-ordination between the aileron and elevator controls.
To
perform a roll first position the model as for a loop but a little
lower. A roll does not require quite as much speed as a loop consequently
the dive is shorter and can be started closer in. Dive as before
to build up speed, recover to a level attitude and apply full
aileron. As the model starts to roll inverted progressively apply
a small amount of down elevator to keep the nose up during
the inverted phase of the manoeuvre. The amount of down elevator
required will vary widely from model to model; the less symmetrical
the section the more down elevator. Once the model is fully inverted
and starts to recover to the upright position so the elevator is
returned to the neutral position. Sometimes it may be necessary
to apply a small amount of up elevator to stop the nose dropping
at the end of the roll but be careful as too much up applied at
this point will cause the tail to 'wag'. For initial attempts it
is often easier if you start the manoeuvre with a slightly nose-up
attitude this way there is often no need to apply down elevator
correction.
As
you have probably realised the most difficult part in performing
this manoeuvre is the synchronisation of the elevator control input
with the lateral rotation of the model and this is one reason full
aileron control is used during early attempts at rolling. Another
reason is a large proportion of models do actually require full
aileron to successfully complete a roll.
Minor
control input errors will manifest themselves as rear end waggling.
More severe errors will result in loss of control. If this happens
let the controls go and after a suitable pause recover the model
from the ensuing dive. Novice pilots will find it virtually impossible
to fly their way out of these situations. It does require
a certain amount of height and it may sound a bit drastic but, a
common failing among tyro pilots is not to admit defeat in these
circumstances and try to fly their way out of trouble, often with
disastrous results. Most models when out of control will, if left
to their own devices, end up in a dive, recovery from which is usually
straightforward. Try it. Position the model at a safe altitude and
briskly apply full control on all control surfaces. Hold for a second
or so and then let the sticks go. The model may flick roll
and start to spin but when the controls are neutralised a dive will
result, honest!!
Inverted
Flying
Very
few slope flyers that I meet fly inverted for any length of time,
possibly because very few have aeroplanes the symmetrical sections.
It is possible to fly inverted with non symmetrical sections but
it requires much better lift conditions. Once again initial attempts
at inverted flying are best practised into wind. To enter the inverted
position build up a small amount of speed and perform a half roll.
As with the roll, slowly apply down elevator to prevent the nose
dropping when the model is upside down. The amount of down elevator
required will be slightly more than that for a complete roll. Once
inverted keep the wings and nose level. This will require a steady
hand and the suppression of your natural instincts! Developing the
ability to suppress these natural instincts i.e. not to over control
is invaluable in tight situations and could mean the difference
between an undamaged model and a bag of bits. Incidentally whilst
the Rudder and Elevator controls are reversed upside down the Ailerons
are not (fortunately).
Should
the nose of the model start to 'nod' or should the model sink rapidly
release some of the 'down' (up actually!) elevator and allow the
speed to increase slightly. The model is at the point of stalling
and in a high drag situation. The most common fault when flying
inverted is over control, particularly the elevator. To overcome
this try flying with reduced control throws i.e. use rates. Recover
from the inverted position by completing the other half of the roll,
not forgetting to use up elevator on the recovery.
As
your confidence increases practice gentle 45 degree. turns left
and right, always recovering into wind. During these turns increase
the 'down' elevator to prevent the nose dropping i.e. just as you
would apply up elevator in a turn with the model the right way up.
It is my experience though that slightly more 'up' elevator is required
when inverted. Once you have mastered these turns try a complete
360.
If
you find the model close to the slope, inverted and sinking rapidly
DO NOT PANIC. Gently turn the model into wind and inch it forward
away from the hill, avoiding any sudden control movements. DO NOT
attempt a panic recovery, it is a recipe for disaster. If you keep
your nerve the worst that will happen is the model will land inverted.
So what, the forward speed will be low because the model is flying
into wind, sink will also be low because the model is still in lift
albeit insufficient to keep it airborne. I have been in this predicament
a number of times and the most damage the model has sustained is
a broken fin. The alternative, a panic recovery attempt, invariably
results in a much more severely damaged model.
The
Stall Turn
The
stall turn is a simple manoeuvre that is sometimes difficult to
get right because timing, speed and positioning are all important.
A stall turn looks similar to a skate board zooming up a ramp, performing
a 180 degree turn at the top before zooming back down again. The
manoeuvre must be performed crosswind with the 180 degree turn INTO
wind i.e. a stall turn performed from left to right would require
LEFT rudder.
Start
the manoeuvre a little way out from the slope, off to one side at
a height of 50 to 100 feet (15 - 30 metres). Enter a shallow dive
to build up a little speed. Pull up into a wings level almost vertical
climb, and as the speed drops off apply FULL rudder. The model should,
with the aid of the wind (hence the into wind turn) perform a 180
degree rudder turn!! The difficulty in performing this manoeuvre
is judging:
1.
The speed on entry (not too fast or too slow).
2.
The angle of climb (approximately 80 degrees).
3 When
to apply full rudder.
It
is possible to cheat a little if the manoeuvre is not working out
as desired by applying a small amount of aileron at the start of
the turn and a 'dab' of down on the exit. The stall turn is a very
pretty manoeuvre when performed properly but it does require good
judgement and a model with an effective rudder.
The
Cuban Eight
We
are now getting into the more advanced aerobatics by combining two
or more manoeuvres into one. If viewed from the side a Cuban Eight
looks like a figure eight laid on its side with a twist in the middle.
Hence the eight in the name, I do not know where the Cuban
came from perhaps someone could enlighten me. It consist of two
loops sharing the same entry and exit that are joined together at
the crossover by two half rolls.
Sounds
complicated, so how do you do it? Firstly, perfect your cross-wind
looping technique because this manoeuvre must be performed cross-wind.
Next position the model off to one side, a good distance out from
the slope (the model will get blown back during the manoeuvre) at
a reasonable height. Dive to build up speed and start the first
loop. When the model is coming down vertically during the second
half of the loop, perform a half roll away from the slope and start
the second loop. Again, when the model is coming down vertically,
carry out another half roll into wind and recover, hopefully in
the same position as when you started the manoeuvre.
When
performing a Cuban Eight there are three main problem areas:
1.
The model was incorrectly positioned and or aligned at the start
of the manoeuvre resulting in the model getting too close to the
ridge.
2.
Incorrect speed on entry (normally too slow).
3.
Insufficient speed for the second loop because of a poor first loop.
Alignment
is probably more of a problem than positioning. The model should
enter the manoeuvre wings level and flying parallel to the slope.
The speed on entry into the first loop should be a shade faster
than that for a normal loop. After completing the first half roll
do not 'snatch' at the up elevator on the start of the second loop
as this will 'kill' the speed making it virtually impossible to
complete it satisfactorily. Likewise the up elevator applied during
the recovery should be gradual to allow the model to regain normal
flying speed on exit. It is possible to cheat a little by stretching
the manoeuvre at the crossover. This will allow the model to regain
the speed lost during the first loop making the second loop easier
to perform.
The
Outside Loop or Bunt
The
outside loop or bunt is similar to the conventional loop except
of course it is performed below the entry flight line as opposed
to above it. The other main differences are the type of model that
will perform a bunt and the speed on entry to the manoeuvre. To
perform an outside loop satisfactorily a model with a fully symmetrical
section is required. Semi-symmetrical or asymmetrical sectioned
models will perform outside loops but with varying degrees of difficulty
depending on how 'clean' and efficient they are. With flat bottomed
sectioned models it is almost impossible. Another factor which affects
the quality of manoeuvres is the drag / volume to weight ratio
of the model. With a light model it is often difficult to build
up enough speed / energy to complete the manoeuvre satisfactorily.
Start
the bunt as per initial attempts at looping i.e. high and into wind
but a little further out. The main reason for carrying out the first
attempts into wind is that the wind will assist the model during
the recovery phase. Start the manoeuvre with a dive to build up
speed and then level out. Gradually re-apply down elevator to enter
the manoeuvre taking care to keep the wings level. As the manoeuvre
progresses continue to gradually increase the down elevator until
the model is again in level flight where upon the elevator control
is returned to neutral. Too much down elevator at the bottom of
the bunt could result in the model rolling out of the manoeuvre.
Most embarrassing if the advice high and wide has not been followed.
Fully symmetrically sectioned models will require less down elevator
than less symmetrically sectioned models. The main problems encountered
when performing a bunt are similar to those for a loop i.e. not
entering the manoeuvre wings level and as a consequence screwing
out.
The
Spin
The
spin is best described as a stalled spiral dive. It requires plenty
of height and for the model to be a good distance out from the slope.
A lot of height is lost in a spin and the model gets blown back
towards the slope during it. For a model to spin satisfactorily
it must have a minimum of rudder elevator control. Aileron elevator
controlled models generally do not spin very well as it is difficult
to establish the required spin speed necessary to keep the model
in the fully stalled condition. Spins with aileron elevator controlled
models generally end up with the model in a spiral dive and an even
greater loss of height.
To
enter the spin point the model into wind and slowly ease the elevator
back to gently enter a stall. At the point of the
stall apply FULL up elevator, FULL left or right aileron and FULL
rudder in the same direction as the aileron. To exit the spin return
the controls to neutral and recover from the ensuing dive. Most
models will come out of a spin without needing to use anti-spin
rudder or aileron. If however nothing is happening after half turn
of returning the controls to neutral apply opposite rudder until
the spin stops and then use up elevator to recover from the resulting
dive. The secret to a good recovery is knowing how many turns or
fractions of a turn the model takes to stop spinning so that the
model can be recovered into wind. This angular movement is then
used to determine when the controls are neutralised i.e. if a model
takes half a turn to recover then the controls are neutralised when
you can see the top of the model. This is very important as a downwind
recovery will often require a panic turn into wind to avoid colliding
with the hill!
Inverted
spins are similar to upright spins except that the rudder and aileron
controls are crossed i.e. applied in opposite directions
as the rudder control is reversed when upside down. Recovering inverted
is quite difficult and will require considerable practice as the
normal control neutrals cannot be used as a reference points.
SUMMARY
The
essence of good aerobatic flying is planning, positioning and empathy
with the model. Smooth control movements and efficient use of the
available energy are also fundamental ingredients. I am not a competition
aerobatic pilot so my methods may differ from those of the experts.
Aerobatics are an important part of flying, they build confidence
and increase flying skill. The purpose of this article is to remove
some of the mystique of aerobatic flying and encourage you, the
average modeller, to broaden your aerobatic horizons and have a
go. The manoeuvres described hear are not the only ones there are
many more. Try inventing some of your own and stringing them
together to form a montage. It goes without saying however that
when performing aerobatics safety must be uppermost in your mind
particularly if there are other flyers and spectators about.
One
final point. Aerobatic flying will show up any inaccuracies in a
model's state of trim such as warps of the flying surfaces, asymmetric
control response and out of balance wings etc. so it is worth spending
a little time checking for these problems and sorting them out if
you want the model is to perform at its best.
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