Articles of Interest
Wing Size selection for Powered Paragliding
Here's a question I frequently receive from new and experienced powered paraglider pilots alike:
"Is it best to fly a smaller or a larger wing when under power?"
Definitely smaller, but within limits.
My feeling is that a paraglider pilot should fly the same wing under
power that he would safely fly unpowered, even if adding the motor places
the wing above the prescribed weight range; provided the wing can
structurally tolerate the higher wing-loading.
Wings with competition lines (unsheathed, thin lines) should probably be kept within their
certified wing loadings; however, most other wings are likely to be
strong enough to carry the extra weight.
There are enough performance benefits to powered-paraglide with
higher wingloadings, to identify those wings that can handle the
extra weight without the potential downfalls that some wings exhibit
when flown "too heavy".
Assuming you select a suitable wing that lends itself to powered
flight (not all paraglider wings make good powered wings), then the
benefits of having a wing that is smaller, rather than too large, are
numerous. Of course, there are limits.
What is too small? As a rough guide, I find that many PG wings may
be safely flown at the coast, under power, up to 30kg (65lbs) over their
placarded unpowered weights. At higher take-off altitudes, this
figure should be adjusted downwards.
Avoid wings with comp lines (as already mentioned), alsoavoid wings
that tend to develop a spiral dive after a severe collapse, as this
characteristic may be many times amplified when flown heavier than
certified (tested).
Remember, that a paraglider wing is not necessarily "unsafe" when flown outside its certified weight range; however, it does mean the wing is un-tested outside this range. This means its performance, safety and handling is untested, hence unknown outside the certified weight range.
If uncertain, ask your instructor, or a few experienced PPG pilots or dealers to advise you on suitable wing brands and models. Everyone has their own preferences, but most will advise you against known unsuitable wings.
Essentially, if you stick with DHV1 or DHV1/2 wings, or AFNOR
Standard rated wings, then you are likely to be safer (under power,
with heavier wing-loadings) than the more advanced wings.
This is not a hard and fast rule, but is a good starting point.
Many paragliding instructors have argued against flying smaller rather than larger, to which I
usually reply:
The proof is in the pudding!
The new Xplorer Sting, designed specifically for powered paragliding while retaining exceptional paragliding performance, is certified by both DHV (for soaring) and DULV (for PPG), with the SAME lines and risers, as follows:
Sting 140 DHV 80-105kg all up (soaring)
Sting 140 DULV 80-140kg all up (for PPG)
Sting 160 DHV 90-125kg all up (soaring)
Sting 160 DULV 80-160kg all up (for PPG)
Another well-known wing designed specifically for powered flight, is rated by AFNOR for paragliding
flight (unpowered), 80-105kg all up. Yet for powered
paragliding, it is rated by BCAR (British Civil Aviation Reg), for 80 to 270kg!!!
That is rather an extreme example, as that wing is specifically
re-inforced to handle such an incredible load, yet it goes a long way
to prove that small wings perform well under power.
What are the actual benefits of smaller powered-wings?
Well, a smaller wing offers far less drag than a larger wing. This
has a number of benefits for powered flight.
To understand why, we need to realise that powered paragliders (and
powered parachutes) are significantly different to almost any other
powered aircraft, in one MAJOR respect: The thrust line (propeller) is SEVERAL
meters below the centre of drag (wing).
Even on conventional propeller driven aircraft where the propeller may be below the
wing's centreline, such aircraft usually have a horizontal stabilizer
flying several meters behind the wing, which helps to prevent the
prop thrust from pulling the wing out of trim.
On powered paragliders, we have no such horizontal stabilisers flying
behind the wing.
Visualise this:
- The wing (and lines) offers a considerable amount of drag to the air.
- The thrust (propeller) is several meters below the wing.
- The thrust can only be transferred to the wing via its suspension lines.
- As the motor thrusts the pilot forward, the wing tends to resist this due to its drag.
- The wing tends to "drag" slightly behind the pilot under power.
- Because of the suspension lines and their rigging, as the wing drags behind, it remains at a fixed angle to the lines. The lines themselves are tilted back above the pilot, so the wing is tilted upward relative to the horizontal.
- This tilting up of the wing (aka pitching up), also increases the angle the wing strikes the air, even though it may be climbing.
This causes an increase in the wing's angle of attack (AoA).
- The increased AoA causes a further increase in drag, which will tend to make the wing drag even further behind the pilot, further increasing the AoA, until a state of equilibrium is reached.
- The end result, is that the wing sits fairly far behind the pilot, with an uncomfortably high Angle of Attack.
Now, we all know that a wing stalls when its AOA approaches a certain
angle. (and... we all want to avoid inadvertent stalls!)
Sure, we think of stalling at a certain AIRSPEED, but that is
really just a SYMPTOM we use, which is much easier to guage and
control, than the actual AOA.
If flying under power causes an excessive AOA, then we may be flying
VERY close to the stall - not a healthy situation. Even if you do not
actually stall the wing, the closer to the stall you fly, the more
sluggish the controls, the less responsive the wing, the more power
you need to get out of trouble. Trouble is, the more power you apply,
the more critical the situation becomes.
Now read the points 1 to 9 above again, and envisage the same
scenario with two different wings: One smaller, the other larger.
The larger the wing, the greater its drag, the worse the AOA problem is. Sure, if the two wings
have the same L/D ratio, then the larger wing offers greater lift too, but it is the drag component that causes PPG problems.
The smaller wing offers less drag, so will fly "not so far behind"
the pilot. It also offers less lift; however, should the lift be
sufficient for safe and easy takeoffs and you can cruise at a
reasonable power level with the smaller wing, then it may be a far
better wing to fly under power.
If you go overboard and fly a wing that is far too small, it simply
will not lift you off the ground with your available power. Its
flying speed may also be too high for safe approaches and landings.
There is a happy medium between a smaller wing which flies well, and
a "too small" wing which does not get you airborne.
Convinced yet?
WAIT! There's more!
A smaller wing may require a faster takeoff run than a similar larger
version; however, due to the lesser drag, you may find it easier to reach
that higher takeoff speed, than to reach the lower takeoff speed that a
larger wing may require, but its extra drag prevents you from getting to
that (lower) takeoff speed.
Want more? Why not (you want everything, don't you...)
A smaller wing is much lighter, hence easier to pullup in zero wind,
and will easier and quicker rise above you to the flying position.
Still hungry for more? (Insatiable, are you not?)
A smaller wing will offer less resistance to the wind during stronger
wind ground-handling, so you get dragged around your field much less
(and retain your dignity in front of all those spectators that appear
from no-where).
I cannot believe you need further convincing, but here is a last
(but not least) benefit.
Smaller wings simply fly faster! You may fly safely in slightly
stronger winds, you can fly faster than you would on a larger wing,
and you will cover more ground on a smaller wing.
Your fuel consumption per hour may not be better on a smaller wing,
but measured per mile covered, you will have further reach and range
on a smaller wing.
The extra response a smaller wing offers (not as close to the
stall), makes for improved manoeavrability, safer flying when showing
off near the ground, and surprisingly enough, often exhibits better
climb-rates than a larger equivalent.
My final argument for smaller rather than larger wings, is the
increased cell-pressure and stability (resistance to tucks) that a
higher wing-loading offers. This infers not that a heavier loaded
wing will not collapse at all, just that it may collapse less often,
and/or less severely, in the same conditions. How the wing recovers from large collapses when flown heavier, varies from model to model - get advice from your instructor.
Your primary concern when choosing a smaller wing, is the structural
strength of the wing at these higher wing-loadings.
So don't go too small - just stay away from too large.
Once again, my personal preference is to paraglide (unpowered) near
the top end of a wing's rated weight range, and use that same size
wing under power, in spite of the extra weight of the motor and fuel
(but not with those old monster motors that weighed almost as
much as the pilot!)
I would like to see the paraglider certification agencies testing the
wings for powered flight too, and specifically testing their safety and
flying characteristics at these higher wing-loadings (for power-flight only).
This article brought to you by Xplorer Powered Paragliders, in the interests of safety and pilot education.
Address comment or questions to Keith Pickersgill at keith@xplorer.co.za
