___________________ SPEED ________________>

My new DHV2/3 glider flies at 73km/h.

There. I've got your attention - it's like printing 'sex' or 'easy money' . You want it. Speed. Lots of it.

If only it were this simple, that you could take the claims at face value. But almost all the gliders I have reviewed yield a lower top speed than what is qouted in the promotional literature. My Brauniger trailing speedprobe is correctly calibrated, so how can this be?

1. Speed varies greatly depending on air density, as I recently discovered when testing the Firebird Matrix at the coast (800m msl), in cool, dense air. Top speed - 43km/h. But Firebird claim 52km/h, as per DHV test? On another (much warmer) day, I clocked the Matrix at 46km/h. Take the Matrix to a high site in the Alps in mid-summer and carry another fifteen kilograms of test pilot, and you'll be whizzing along at over 50km/h.
What matters to you is not the number, but the relative performance against other wings flown in identical conditions - I flew five wings on the day of the review within three hours to get fair comparatives.

2. The highest speed achieved by any one of the gliders sent for testing (S, M, L) seems to be snatched up and advertised as 'the top speed'. This is misleading, for you are unlikely to be flying at the maximum allowable weight, and may not be flying the particular size of wing that achieved the best result.

3. There may be spikes of higher speed as the glider swings through air currents - I use a twenty second average to determine 'highest sustained speed', which is what matters to you as a pilot fighting a headwind.

4. How accurate is a propeller-driven speedprobe?
The following question was posed to Brauniger (from their website - www.brauniger.com )
"If you fly at high altitudes in thinner air the True Airspeed must be higher than in denser air to get the same speed displayed on the vario. This is because in denser air you have more air molecules hit the propeller. GPS o n the other hand always displays the correct Groundspeed. Thus we should get a difference of TAS and GS (assuming no wind) of about 2% per 300 m altitude difference. How can the wind speed indicator work with this phenomenon?"

Brauniger's answer puts the issue to rest :
This is basically right but only if you fly with a Pitot-Tube System which measures Indicated Airspeed. We use a propeller driven sender which always displays True Airspeed. This means there is no difference in Groundspeed and TAS when the re is no wind, no matter how high you fly. For L/D calculations we have to use TAS. Tas has one disadvantage, though: The stall displayed stall speed changes with altitude. The vario shows true rates of sink or climb, they are compenstated according to IC AO standard atmosphere. Your theory with air molecules is correct for the wing of an airplane. but incorrect for the propeller. Because the propeller runs practically friction-free, the angle of attack is always 0, it always follows the speed of the air mo lecules, no matter how many air molecules flow through. Even when you dip the speed probe in water flowing with 50 km/h, it shows 50 km/h. Following your theory it would have to show 1000 times faster speed because the density of water is 1000 times higher than air. The lift of a wing in water is 1000 times higher than in air if the angle of attack is the same.

5. How does the DHV do it?
I sent the following e-mail : "The DHV tests have in the past quoted trim speed and maximum speed achieved during flight. No information is given about the atmospheric conditions during the test (altitude, temperature, pressure) which all affect the airspeed attained. Does the DHV standardise the test results, and if so, what is the formula you use?"

I was writing in my office in South Africa, when my mobile phone rang and yielded a cheerful germanic voice :
"Hallo, this is Harry Buntz, from the German testing centre."
A hasty check confirmed that I was, in fact, awake, and had no hallucinogenic products in sight.
"Um, hello!" I answered, unsure of what I'd done. "Can I help you?"
"I am phoning because it is too much to discuss in an email," said Harry. "You had a question wiz ziss speed recording."
Ah. What a great organisation! Where else do you get such a personal, professional attitude than the DHV?
"Yes, we have a problem!" said Harry Buntz. "There can be up to 10% error in speed recording."
[conversation continues for a few minutes ... 10% can really damage your performance ...]
Harry : "No we don't standardise the results. We test safety, not performance, so speed measuring is not important to us."
Greg : "You're doing a great job of testing safety. Great job! The problem is that manufacturer's are advertising the high speeds achieved in some tests, and it is misleading."
Harry : "Ya, this is why we are cutting out the speed readings from the tests now."
[more conversation ... ]
"Are there different test sites?"
"Yes, we could use Stubai (very high) for one test, and Monaco (coastal) for another."
This is probably the most important fact, because it means one can never compare speeds between DHV tests.
"Who tested the Matrix L at 52km/h?"
Harry searches through his records. "Mike Kung. He uses ze Aircotec speed-device."
We both agree that if Mike Kung says it was 52km/h, then it was 52km/h. The legendary test pilot is surely accurate. But boy, he must have been high and heavy.

So we have a problem. His Matrix L flew at 52km/h, mine flew at 43km/h. To bring the tests into line with the new European Standard for Certification, the DHV will not be quoting speed measurements in the future. No prizes for guessing why.

6. Who do we believe?
You may be nervous about believing the manufacturers claims - they are trying to sell you something! This shifts the burde n of providing accurate performance data onto the reviewers. No one pilot can fly all the wings in the world. But there is a universal need for comparative data, and the need is going to get even greater with the DHV ceasing to measure performance completely.
The best we can do, is to standardise the speed and performance data that we achieve in our reviews to the same air density, and always test the glider against as many other wings as possible before mentioning anything about performance. I echo Jerome Daoust's proposal of 1013hPa Qnh, 15degrees C air, 1000m altitude as the standard (see his website for a great standardised comparison of wings) http://www.mindspring.com/~sylvie_jerome/Jerome/PG/Perf/Compare_English.htm

Gerard Florit has collated an immense amount of technical data for paragliders on www.para2000.org., though unfortunately there is no standardisation of the performance data as yet.

7. Is speed really what you need?
Watch any pilot reading a glider review article, and where do their eyes go first? Zoom .. straight to the maximum speed. We all do it - we want to know that the glider we are going to buy is f aster than everything else. Some manufacturers know this, and will simply increase the speedbar travel to the point where the glider fails its DHV test on accelerated asymettric collapse, then back off by one centimetre. What you end up with is a glider that wobbles along with a terrible glide angle but impressive speed. What you really need to know is the glide angle at various speeds, which is the polar diagram. We're back to the same fundamental problem - an accurate collection of speed and glide data t akes many, many flights in perfectly still air, a task which very few people can undertake. A polar diagram can be hopelessly wrong if the airmass you test in is descending at 0,1m/s, or your speeds are not standardised. So what can we do?

8. Review with caution
Pull up next to 'the other glider' and compare your performance. But be careful, it can be misleading. You can't have one flight and say that the Bagheera performs better than the Matrix, because it may beat the blue one, and not the red. Different sizes of the same glider will perform differently. Different pilots will perform differently on the same wing if their weights are not identical. Different harnesses will produce altered drag, changing glide and speed. All that is possible is to compare against many gliders present in the same air, and comment only once all the relevant details about each specific pilot and wing have been collected.

This makes the reviewer's job an honerous and lengthy task. So when the guy next door says he has the fastest glider on the block, you may be forgiven for grinning and answering "Prove it!".

It may be the one that does 73km/h, straight into the forest.