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FAQS WINGGRID
(frequently asked questions)

THE FAQS WINGGRID are intended to help in positioning applications and focusing on the specific problems to solve and ask for.

last update 03.11.1998

Test Conditions Full-scale Proof_of_Concept

1. Type of aircraft used for proof_of_concept fullscale ?
Two-seat, twin-jet sailplane with Stemme wing demountable at 10 m span.
Configuration with Winggrid mounted:
-span 12 m, aspect ratio 12, specific wing load 70-80 kg/m^2, max weight 1'000 kg
-stall speed 115-125 km/h, max. authorized speed for tests 270 km/h
-Winggrid with 4 winglets, individual aspect ratio 5, relative halfspan 17%
-optimum glide_number 25+ at 155 km/h

2. What were the flight test conditions & measurement setup ?
There were 2 test programs:

-airworthiness tests prior to permission for experimental performance tests
These tests essentially confirmed the resistance of the Winggrid_device to wingtip-stall.

-performance tests
The latter consisted in in_flight measurements of the glide_number measured:
-horizontal speed with GPS 8 parallel channels (e.g. minimum 6 satellites)
-vertical speed with barometric measurement all recorded on the same data-logger
-levels for these measurements chosen between 12'000 feet and 5'000 feet, weather permitting.
-straingauge measurements of the root-moments of the winglets for check of lift on the Winggrid
-all measurements recorded simultaneously on the same datalogger

A simultaneous comparison with e.g. conventional elliptic wingtip was not made, because the span_efficiency expected and confirmed exceeded values of 1.5 by a large margin (actual values are around 2.0 compared to elliptical wing, which would give values around 1.0 as borne out by an earlier version of the identical airplane fitted with conventional wings of 12 m span)

3. What is the speed range tested ?
Stall-limit of IAS 125 km/h and authorized max speed of IAS 270 km/h

4. What are the Reynolds-Numbers in the tests ?
The speed range covered the envelope given by stall-limit of IAS 125 km/h and authorized max speed of IAS 270 km/h. On the main wing chord of 1 m this would be equivalent to Re > 2'000'000.

5. Can you give live demonstrations with your test_aircraft ?
Yes, please ask for details

Applications & Design

1. What is the range of speeds, where the device will improve operation ?
Actually this question should be put in a different form, because improvement is (within limits of subsonic operation) dependent on the balance between friction drag and induced drag of the airplane at the operational point considered.
If the ratio of friction drag to induced drag exceeds 3, no big contribution will be gained by reduction of the latter.
The device allows on the other hand to reduce span without increasing induced drag, what as a by-effect can also be used to reduction of friction drag.
The design data of the airplanes polar are a good start for evaluation (see How to evaluate an airplane’s performance and the Winggrid).

2. What is the difference of the Winggrid_device compared to other known devices ?
It is the breakthrough in combining a very modest addition to vertical dimension with very high span_efficiency (e.g. compare a design study on the C-Wing:
-Winggrid has vertical extension below 0.05 b with span_efficiency of 2.0 or higher
-C-Wing has vertical extension of 0.2 b with span_efficiency of 1.5

Also Cl_stall for the Winggrid is rather high: above Cl = 2.5 due to grid_configuration. the useful Cl_range is therefore typically Cl = 0.1 to 2.5. This is incidentally also the reason for the improved resistance to wingtip-stall which the winggrid provides.

There is a very important „BUT" often misunderstood by would-be evaluators of wingtips:
The Winggrid gives its potential with an essentially rectangular lift distribution over span only. Trying to apply it (and compare it) on an essentially elliptic lift distribution is foregoing the effect possible. As nature shows, either you can afford big span solutions as the albatross, or you are looking for short, limited span solutions as, e.g., a stork, a pelican or a vulture, the latter being the regime of real worthwhile winggrid applications.

3. Is there a lower limit using the Winggrid for Reynolds-number effects, e.g., using for very low speeds and/or small chord ?
For conventional smooth profile surfaces, yes, because Reynolds number of the Winglets will be, for example, 6 times smaller compared to main profile.
If necessary another experimentally verified development applying oblique grooves on part of the laminar upper profile surface will allow to go practically as low as you wish. (details available)

4. What about behaviour in compressible flow conditions ?
We did not make yet any tests to date in this flow regime. Due to the characteristics of a lift producing grid, we expect rather less problems compared to wing profiles.

5. Is it easily applied for retrofit on existing wings ?
Not in general because the Winggrid_device develops the same lift per span unit as the main wing would have in undisturbed two-dimensional flow. Conditions for effective retrofit are:
-rectangular wing (constant chord along span) with no taper (maximum benefit)
-root-moment of Winggrid can be absorbed by actual wing-structure at attachment point

6. Is there an optimum number of winglets in the Winggrid_device ?
No, because one will have to make a choice between mechanically robust design and maximum aerodynamic effect. The span-efficiency attained is roughly equivalent to the ratio (sum of halfspan aspect ratios mainwing+winglet)/(halfspan aspect ratio mainwing).

7. Have you publications on the Winggrid_device available ?
Yes, at present you will find a complete engineering description of its operation in:
WING-GRID, a Novel Device for Reduction of Induced Drag on Wings, by U. La Roche and S. Palffy, Proceedings ICAS 96, Sorrento, ITALY, September 8-13, 1996 , ISBN 1-56347-219-8.

8. Have you a pre_engineering routine available for screening potential applications and or airplanes for suitable Winggrid-application ?
Yes, see the "How to evaluate an airplane’s performance and the Winggrid". This appendix gives a routine, how to establish the coarse performance profile of any subsonic airplane and assess the potential of the winggrid device on improving its performance.

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Updated: 23 February, 1999