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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 weights. At higher take-off altitudes, this figure should be adjusted downwards.
Avoid wings with comp lines (as already measured), and avoid 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" 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. A well-known British wing designed specifically for powered flight, is rated by AFNOR for paragliding flight (unpowered), 60-85kg ( 132 to 187lbs) all up. Yet for powered paragliding, it is rated by BCAR (British Civil Aviation Reg), for 65 to 270kg (143lbs to 594lbs) !!!
That is rather an extreme example, as that wing is specifically reinforced to handle such an incredible load, yet it goes a long way to prove that small wings perform well under power.
Want another example? A newly developed German wing, also developed specifically for powered paragliding, is rated to 140kg powered flight, yet is only 28 square meters in size, which is the typical paraglider size for around 90kg max.
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 realize 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 aircraft where the propeller may be below the wing's centerline, 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 stabilizers flying behind the wing.
1. The wing offers a considerable amount of drag to the air.
2. The thrust (propeller) is several meters below the wing.
3. The thrust can only be transferred to the wing via its suspension lines.
4. As the motor thrusts the pilot forward, the wing tends to resist this due to its drag.
5. The wing tends to "drag" slightly behind the pilot under power.
6. 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.
7. This tilting up of the wing (aka pitching up),
also increases the angle the wing strikes the air, even though it may be
8. The increased ATA causes a further increase in drag, which will tend to make the wing drag even further behind the pilot, further increasing the ATA, until a state of equilibrium is reached.
9. The end result, is that the wing sits fairly far behind the pilot, with a 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 gauge 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 its 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.
WAIT! There's more!
A smaller wing may require a faster takeoff run than a similar larger version, but due to the lesser drag, you may find it easier to reach that higher takeoff speed, than to reach the lower takeoff speed a larger wing may require, but its extra drag resists you 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 maneuverability, 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).