Niviuk Takoo tandem - less affected by turbulence than a solo wing
About this article
The fact that paragliders of different sizes react in different ways has been common knowledge for years. But do we really know how they react differently and why? Bruce Goldsmith explains why bigger gliders are better.
Expert. Paragliding World Champion 2007. Three times British Champion, most recently 2004. Airwave paraglider designer for 20 years
03 Mar 2006
Original post on ParaglidingForum.com
09 Dec 2015
Added to the Knowledge Base with Bruce's kind permission
l Effect of size on performance l
l Alex Hofer l
l Effect of size on safety l
l Why is size so important? l
l A fact of life l
Effect of size on performance
I have a good friend who is a small competition pilot, He weights it at 52kg, and he always flies a Small glider or XS if he can get hold of one (it is rare for a manufacturer to make an extra small competition glider). He tells me that he has been competing on small gliders for years and is painfully aware of the difference in glide performance between sizes. He says ‘a difference of two sizes gives around the same performance difference as a whole class of paraglider’. That means that someone flying a Small competition glider would have the same glide performance as someone flying a Large Intermediate wing. If true this would be a really very large difference in performance indeed.
Bigger pilots should therefore have an advantage in competitions. I have noticed some evidence to support this. If you look at the results of the World Championships for example and look at the glider sizes being flown we have:
1) Alex Hofer – Large
2) Frank Brown – Large
3) Masataka Kawachi – Medium
And the PWC 2002 results give
1) Alex Hofer - Large
2) Scotty Marion - Medium
3) Jean marc Caron – Medium
PWC 2003 is not yet finished, but it will be interesting to see.
To put this into perspective I compared how many competitors where flying which sizes of wings in the PWC this year. I found that 15% were on Smalls (including XS, S and MS), 68% were flying mediums and 23% were flying Larges (including L and ML, there were more pilots flying the ML than the L but significantly the winners were the ones flying the L!).
Pilot skill is obviously the main factor but if your glider goes a bit better because it is bigger, then that for sure helps. It is precisely for this reason that ballast is so popular in competitions and the FAI and PWC have had to bring in rules limiting the use of ballast. Pilots found that they perform better if they fly a bigger glider and carry ballast, rather than fly a glider that is the correct size for them. Most people do not even bother to drop ballast when the conditions are getting weaker, as the strength of the conditions is not the main reason for carry the ballast.
The fact that Alex Hofer is doing so well in all major competition also makes it look like large gliders are winning more, so I asked Alex Hofer for his view on the subject.
Bruce: ‘do you feel you have a performance advantage because of your size’
Alex: ‘ Yes, though it does depend on the conditions, generally the big gliders go better, but if conditions are weak with small thermals then the tighter turning radius of small gliders can give them an advantage, this was the case with Jimmy Pacher at the Europeans when he came 3rd on a small glider.’
Bruce: ‘What weight are you and what size glider do you fly?’
Alex: ‘ 92kg naked and all up between 120 and 125kg depending on how much ballast I carry, which depends on the conditions. I always fly a Large’
Bruce: ’Why do you think Large gliders go better’
Alex: ‘When the air is moving and turbulent I think that a bigger glider is less shaken about by the air than a small one. Also I have noticed that pilots flying larger wings generally fly with a greater wing loading. On Large gliders the wing loading is around 4.3kg/sq m and on small gliders it is around 4.0kg/sq m. Gliders go better when heavily loaded.’
At Airwave we did a theoretical study of the difference between Medium and Large competition gliders and the prediction was that at trim speed the Large glided around 1% better than the medium but at 54km/h the Large had a 5% advantage over the medium. Quite a significant difference if you are racing.
Effect of size on safety
But it is not only glide performance that is influenced by size, the safety is also effected by size. When preparing a glider for DHV certification it is always much easier for the manufacturer to get the Large gliders to pass the tests than the Small ones. This often means that the small gliders need to be trimmed more to pass the certification which could mean for example that they are trimmed slower to get the same certification grade as their bigger brothers. Some Small or XS gliders cannot obtain certification at all, where as their bigger brother pass with no problems.
So smaller gliders are less safe and have worse performance! Unfortunately this is inescapable and applies to all paragliders, even though the manufacturers try their best to produce the safest and best performing small gliders that they possibly can.
Why is size so important?
There are several factors that this can be put down to.
1) Reynolds Number effect.
The Reynolds number is a relative measure of the inertial forces in a fluid compared to the viscous forces. The Reynold’s number itself is dependant on the speed and cord of the wing. As similar sizes of paragliders should be flying at similar speeds it is only the cord that changes. Therefore a wing with a bigger cord will have a higher Reynolds number. Any Sailplane designer will tell you that at a higher the Reynolds number the thinner the boundary layer becomes relative to the cord of the wing. Put very simply this means the flow around a wing is relatively more smooth on bigger wings compared to smaller ones.
Big paragliders are made from the same materials as small ones. On modern paragliders smaller gliders are simply scaled down versions of the large one, but nobody scales the cloth. Typically there is 4% difference in linear scaling between any two sizes of paraglider. A Large glider is therefore 8% larger in linear scaling than a small. If the large is made with 42gm/sqm cloth then it would be logical to make the small from cloth that is around 8% lighter, which would be 38.5gm cloth. However this is not practical for several reasons. The lighter cloth would be more fragile and would not last as long as it is thinner, and also you would need to load test every different size of glider in the range, which would put the cost of certification up. Also there are simply not so many different weights of cloth commercially available from the cloth manufacturers. But the net result is that small glider use relatively heavier cloth. This also applies to the lines and risers as well of course.
3) Pilot drag.
About 10% of the total drag on a competition paraglider flown with a low drag supine harness is created by the pilot when flying at trim speed, on an intermediate glider this percentage is higher. This percentage also goes up the faster you fly. However the drag on a large pilot is relatively less than the drag on a small pilot. This is due to the fact that his frontal area does not go up as much as his weight (frontal area is the square and weight is the cube of the linear scaling factor).
So bigger pilots have relatively less drag than smaller pilots.
4) Scaling time.
If everything is scaled on a small glider, then you may also expect that time should be scaled to match the smaller size of the glider. This means you can expect things to happen faster on a smaller glider. As all certification is measured by factors such as the speed with which the glider turns or rotates you can only expect that smaller gliders will do it faster. This is one reason why smaller gliders will receive worse certification grades than larger ones. In other words smaller pilots may need faster reactions, because their gliders react quicker.
A fact of life
Unfortunately there is no solution to this problem, it is a fact of life we all have to live with.
When writing this article I looked closely at the results of the PWC for 2003. I was surprised to see a remarkable lack of pilots doing well flying small gliders. This was even more obvious than the number of pilots doing well on big gliders. Perhaps the top of the PWC will end up looking like a team of American footballers in the future!
l Knowledge Base l CSC Home