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The
back side of the power curve is an arse of a place to be. Dec31
02.
In this article I hope to explain the
effects and outline some safety issues of powered paragliding PPG. I will
start with the most common question I get asked and then lead us deeper into the
more technical areas.
Q: Will I need a bigger wing to go paramotoring?
A: If you are in the upper half of
your wings weight range for free flight, it should be
safe for PPG.
If the wing is not certified for PPG then
you should ask if it has any bad habits under power, this is a stronger case for
changing wings as some respond badly under power.
The next question you should be asking but
never do is
Q: What effects will flying too heavy or
too light have.
A; Lots, so lets run through the
pros and cons.
First lets consider flying heavy
on a wing.
PPG pilots generally fly with considerably
more weight under their wings than PG. Expect a wing rated for both PG & PPG
to have two weight ranges, one being about 40Kg higher for PPG. Remember,
most paragliders are tested to 7g or more so the extra weight does not damage
them.
The main advantage of more weight is extra
speed, and pilots like speed.
Unfortunately extra speed also means
faster take off and landing speeds, so in nil wind conditions you can expect a
strenuous run & heavier landings with the added weight of a motor on your
back. It's
worth noting, most wings considered good for PPG have "power
risers"
with trimmers to adjust speed.
The power required to achieve take off and
climb increases with weight.
You could reach a point where the weight
exceeds the motors limits. Combine this with adverse conditions or an unsuitable
wing and you might end with a long take off run and a poor climb rate!
Flying heavier and faster requires more
thrust to maintain level flight.
This increases fuel consumption, or put
another way reduced flight duration.
Where ever you fly, you get there quicker,
but have to land sooner!
In flight every thing is more heavily
loaded so in the unlikely event of a collapse it will be faster and harder. But,
the advantage of being heavily loaded is the likely hood of getting a collapse
in the first place is hugely reduced.
A PPG pilot sitting upright with a big
round cage on his back adds considerably more drag to the total aircraft drag (pilot+wing),
than an aerodynamic supine free flier.
This can degrade the overall Lift Drag
ratio by 0.5 or more
A heavier pilot is usually a bigger pilot.
A physically bigger motor or pilot adds more drag. This further degrades the LD
ratio, so the steeper you glide down.
So don't
expect the same glide with the power off, as you did free flying your
wing.
All things considered, the improved speed,
maneuverability, response, and resistance to collapse from flying heavily loaded
on a smaller wing more than compensates for the reduced efficiency. (most
of which can be offset by applying more power anyway.)
Now to look at the effects of flying
lightly loaded on too large a wing, or too much power to weight ratio
The first annoying thing is, with less
weight you fly more slowly. Applying more power will not make you fly faster, in
fact as we have no elevator controls to control pitch the extra thrust will
simply rotate the aircraft into a steeper climb.
This hides a far more sinister effect of
power on paraglider flight.
A simple analogy helps explain.
Relative to the wing a PG pilot provides
the same loads as if sitting on a garden swing. All his weight is carried by the
line and so loads the wing.
If a person pushes our pilot forward on
the swing, he starts to carry more of the pilots weight as he pushes him further
round. If he pushes the pilot to a horizontal position he will now supports all
the pilots weight. Subsequently no weight is on the lines.
We know that the speed of a wing is
determined by the weight it carries.
If the weight reduces the speed reduces.
If the weight reduces to Nil the speed
reduces to Nil.
Now substitute the person pushing for a
paramotor. Any alarm bells ringing yet?
It would take a lot of push to swing the
pilot into this position. How much? Well if the combined motor & pilot
weight were 80Kg and the motor pushed 80Kg we are there.
Let our pilot first take off on a heavily
loaded wing, and moderate thrust; he will fly fast.
To climb out he applies full power. As he
swings round, the resultant weight carried by the wing reduces causing his
airspeed to decreases, but not by too much.
The BIG problem comes if our pilot
takes off lightly loaded on a wing that is too big, and loads of thrust.
When he takes off he will be flying
considerably more slowly. Things can only get worse. As he applies power he
slows down even more and finds him self in an area known as the Back side of the
power curve.
Depending on our pilots weight, the wing
size, and crucially how much thrust is available, our pilot is running a real
risk, as he applies more power. As he swings further round, the weight on
the wing reduces and his airspeed falls away. Additionally the wings own weight
increasingly acts against its airspeed. As the weight approaches the minimum
limit required to keep it flying, it becomes unstable, and is reached before the
pilot swings all the way up in front of the wing. He is flying closer and
closer to the stall.
With enough thrust applied for level
flight the pilot is pushed round, the resultant weight left to load the wing
determines how much slower the aircraft will fly.
At full power the amount of excess thrust,
which further reduces the weight on the wing, determines how close to the stall
our aircraft is capable of getting.
Don't
be too alarmed, this example demonstrates a problem at the real extreme limits.
But it illustrates 2 very important
factors that should be considered when flying PPG.
1 Don't
fly light on a large wing with a big powerful motor, particularly if a lighter
pilot.
2 Although most PPG don't
fly at these extreme parameters This tendency towards the stall effects ALL
PPG to varying degrees, and is dependent on the wing size, weight and thrust
available.
Seek advice In paramotoring bigger
is not always better, either for the wing or motor. If you are unsure talk to
some one who can help recommend a motor suitable to your pilot weight, and match
the total weight to a suitable wing.
A safety issue worth considering is
this.
In a situation flying with lots of power
and not enough weight you could be closer to a stall than you think. The amount
of brake you are used to for a quick tight turn when free flying, could now be
enough to stall parts of your wing. A stalled wing tip could induce a flat spin
under power. And when do we often find ourselves liberally applying large
brake inputs? During take off and landing.
SO, IF YOU FLY PG AND PPG AND ARE LIGHT ON
THE SAME LARGE WING BE AWARE THIS CONCERNS YOU.
DON'T
GET CAUGHT OUT
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