Possibilities of Combining Various Wings and Winglets
A frequently asked question:
Your winglets on the DG-800/18 are obviously very good.
Could they be added to a DG-400 or DG-500/22?
Unfortunately the answer generally has to be “No” for three reasons:
- The depth of the winglets would have to be exactly identical to the depth of the wing at the point of extension. This never happens.
- The profile of both parts would have to be identical, which also is not so. One could force a match with much filler, but the results would not be very satisfactory.
- The wing has to be able to withstand statically and dynamically the extra mass of the winglets.
The first two points could be overcome by designing new winglets. We conducted extensive research to see if the 280 DG-400 could be equipped with winglets, which would bring their owners considerable advantage, which they probably would be glad to pay for.
However, the third point proved to be an insurmountable obstacle.
The wings are designed with considerable safety margins according to the design rules, but not with reserves over and above the requirements. The wings are designed to withstand a stress of 6 g plus a safety margin, but not more.
A winglet at the end of a wing generates additional lift - it is the equivalent of extending the span, and the longer leverage adds a bending moment to the spar root. However, it has not been designed for that.
It is entirely possible that a destructive test, which is required for new wing designs, would prove sufficient strength. But the theoretical proof would also be required, and for that there are not enough reserves in the design.
Why should it be?
The calculation of the static loads showed the addition of winglets is impossible. Even more complicated are the dynamic loads with regard to flutter. Obviously a winglet at the end of the wing could change the internal resonance of the whole structure, and only extensive tests would show if the wing is still flutter free. The cost is not worth the result.
We have reluctantly come to the conclusion that the addition of winglets will never be possible. Some aero technical companies offer “homemade” winglets for installation. As manufacturers we can only warn against this.
Sailplanes with such winglets no longer comply with the design rules. Probably nothing will happen because the design rules do contain considerable reserves, but would you trust your life to a possibility?
It is better not to try.
But there is one exception:
It is possible to build winglets for the DG-300, as it is done for the newer DG-303.
Similar it is possible and very advisable, to fit the DG-600 with winglets and even extend the wing span up to 18 meter.
Specialized for such a job is Güntert & Kohlmetz in Bruchsal.
Extending the Wing Span.
In this connection the answer to the question: is it possible to extend the wing of a DG-400 17 m by 0.5 m and get a 18 m wing? One pilot asked if it is possible to extend the wings of his DG-800 by one meter and get a 20 m span.
One could play with these ideas forever, and before you know it a DG-400 becomes an ASW-22....
But as mentioned before it is not possible. If a wing is extended beyond the original design span, the center section would have to have a considerable reserve strength. Without that it cannot be done. If it should be there, the center section would be quite heavy and much more expensive. Anyone who helped assemble an ASH-25 knows how incredibly heavy the relatively small center section is. There is so much carbon fiber in it that you can over stress your back. You would not want that for an 18 m span.
Every wing is optimized for a certain wing span. Variations within limits are possible, such as a 15 m / 18 m wing, or an 18/20/22 m wing, but that is about the limit of possible variations.
As a rule span extensions are not possible.
translated by Albin Schreiter, CDN
It is unbelievable that in spite of these clear statements a repair shop in Great Britain developed winglets for the DG-400.
That is why Wilhelm Dirks issued the following warning to the manufacturer:
Warning
Please be informed that the results of our stress analysis don't allow to install winglets to the DG-400 17m tips.
So such winglets can't be certified in any country and operating the glider with such winglets is illegal.
Please stop advertising 17m winglets immediately.
with best regards
Wilhelm Dirks
DG Flugzeugbau GmbH
Very simple:
The wing of the DG-400 is not strong enough for the additional forces of a winglet.
To fly with one is dangerous for your life!
A pilot in Japan flew with homemade winglets. After having read our warning, he cut them again!