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Subject: 200th PW-5 glider left the PZL Swidnik factory
Date: Fri, 10 Apr 1998
From: Ian Strachan <Ian@ukiws.demon.co.uk>
This thread seems to contain a lot of emotion and speculation, and little discussion of more factual points. Here is an attempt to inject a few facts into the debate, although my first para is an opinion. Some of this has appeared before but I thought it worth repeating on this thread, as the readers may be different. If nothing else it may change the direction of some of the discussion rather than concentrating on alleged differences between US and European soaring organisation, funding, and practices.
1. Any cost-effective glider with safe flying characteristics which is introduced in large numers into the world gliding community, has to be good.
2. I understand on the PW5 winch launch safety issue that a design change has already been agreed where the winch hook position is moved further forward. Although the World Class is "one design", and generally mods are not allowed, this was considered a safety issue and one having no effect on any future World Class championships. I have only flown the PW5 on aero tow where the handling is very light and responsive, and due to its low weight the climb rate is high compared to other gliders. But I have flown gliders which, with a powerful winch, rotate almost automatically into a very steep climb unless you take positive action to put the stick forward to control the angle in the critical early stage of the launch. This is apparently what happens with the original PW5 hook position. It appears that the early part of a high-acelleration PW5 winch launch is controllable (or it would not have got a Type Airworthiness Certificate which includes winch launching) but its characteristics need understanding and positive control action is needed to avoid over-rotation. The first part of PW5 winch launches which I have seen at Lasham (which has powerful winches) have looked no steeper than usual with other gliders. Clearly if the first part of a winch launch is not properly controlled in pitch, and the cable breaks at the critical moment, there can be a disaster. As with any glider. As with anything in life, knowledge, training, and correct practise will keep you out of trouble. Meanwhile a small design change has already been agreed to reduce this effect. Very sensible.
3. Some comments on performance aspects:
In article <firstname.lastname@example.org>, Andreas Maurer <email@example.com> writes:
'it looks and fits well. I bet it fly as well. Hmmm. I'm sure people said the same thing of hundreds of other planes,
What did the proposed test pilot say about the first Constellation? "Wings are much too small. It will never fly."'
There are various rules about aircraft design (including gliders) which are pretty much "laws" rather than choices.
First "looks" may not translate into good aerodynamics, stability, control, economics, etc. Many customers including the military and airlines have been seduced by pretty looking aircraft which then did not perform as expected. You have to employ well-organised independent testing to establish the practical position with a production aircraft (not a specially polished prototype). Of course, having been employed in flight test, I would say that, wouldn't I! But even with this handicap, I believe it to be true.
"Small" wings (in comparison with the load to be carried or the role of the aircraft). Yes, you can have small wings, but you always need the "CD half ro vee squared" to produce the lift required to equal the weight. So you either need a high coefficient of lift (CD), or more Vee (higher stall speed if you look at the low-speed case). The ro and the half are not under designer control! CD definitely is under design control and with aircraft of vast weight like Jumbos you can see the complicated slotted flaps which produce the very high CDs needed to get threshold speeds down to reasonable values.
Applying this to gliders, lift is a little less than weight, but not by much. To have a small wing supporting the weight you need a higher-than-normal CD if you still want that low stall speed. If you look under the PW5 wing you will see some reflex curvature, and having flown it I can say it goes up in a thermal like the proverbial dingbat, better than most, and the responsive controls make shifting your position very easy. For flapped gliders, the Jumbo flap design is not an option because aside from mechanical complexity and expense, such flaps also have very high drag coefficients which is why glider flaps are less exotic.
The trade-off is high speed performance because small span, low aspect ratio, and airfoils designed for slow speed, are factors unlikely to give high max L/D (say 1:40 or above). Also, the L/D at higher speeds may be just as important as the max L/D achieved at low speed. Some older gliders with thick airfoils had quite good max L/Ds but the polar curve then became rather steep and so they did not "penetrate" very well in conditions of strong wind and through areas of sink. Indeed, the most significant aspect in glider performance since I started in the late 50s is the increase in high speed performance.
Getting back to the PW5, in my flights at Lasham (a demonstrator is stationed there), I recorded a sink rate of 5.9 knots at 80 knots indicated (3.04 m/sec at 148.2 km/h). In many countries with good thermals and high cloudbases, such a performance will be no problem in terms of not landing out (aux vaches). In others with lower cloubases and gaps or areas of sink to cross, it may be. As usual in life "you pays your money and takes your choice". No one solution to enjoyable soaring is wrong, just different.
In terms of "rules" about glider design, one often quoted is "there is no substitute for span". Of course cost comes into it and not many people can afford the exotics. Syndication helps, and in my syndicate there are six shares. The question is "cost-effectiveness", or "cost-perfomance" in the gliding case. For instance, the 15 metre span design seems to have proved itself over the years, in both flapped and unflapped (Standard Class) variants. I recall that back in the early 70s when IGC were discussing the introduction of the 15 m class to complement the existing Standard class, some manufacturers said that 15m was not at that time the "cost-effective" span level, but 16 metres was. IGC thought that the two figures were so close together that the already well-known 15m span was used for the new class. Commonality of trailers was also a factor.
Last year I heard a lecture from Gerhard Weibel (the W in ASW) on future design. He described the possibility of an over 30m span design with a max L/D of 80 or so. The wheel has turned full circle because to support such a wing, struts are needed! (my Silver was done in a strutted glider). However, even if such a glider is ever built, few will be able to afford it. So I asked Gerhard "in terms of cost against performance, what span do you see as the best with today's technology". I was expecting him to say 22, 23, 25 metres. His reply was "about 19 metres". This debate was not relevant to the PW5 design criteria where cost and simplicity were overriding parameters. But interesting all the same.
Best wishes to all r.a.s. readers for good soaring over the Easter period.
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