First outer-span test (2/28/2004)

Figure 16. Lower surface 6" in from aileron on Standard Cirrus #60
After a spell of bad weather, we finally got in the air again Saturday the 28th. This time we installed two drag probes on Standard Cirrus #60, one at the usual 53" station on the lower surface of the right wing and the other 6" inboard of the aileron (158" station) on the lower surface. Probe #1 (Figs. 1-3) was used on the right wing and probe #2 (Fig. 4) was used on the left wing. We wanted to test a new way of making the FCSD material on the right wing, by comparing the results to previous data. On the left wing we were taking drag data for the first time at the outer span where the airfoil has essentially fully transitioned from the Wortmann FX S 02-196 to the Wortmann FX 66-17 A II-182. We need data at both ends of the transitioion zone in order to know where to place the FCSD throughout the zone.

The results were mixed. First the good news. As indicated in Fig. 16, we got pretty good drag reductions at high speeds on our first try at the outer station. It should be noted that drag probe #2 is a bit too tall for the 158" station where the wing chord is 25.32". We would like the highest probe Pitot to be 0.02 times the chord above the wing surface. That would be 0.5". However it overreaches by 0.16" to 0.66". This, and the smaller chord, have the effect of flattening the curves, compared to measurements at the inboard location. However, the probe size also makes changes in the boundary layer flow appear smaller. Imagine, for instance, that the three highest Pitots were fully in the free stream flow, than only the lower three would see any change. Averaging them together with the higher three Pitots prodeuces a value that changes less than if they were averaged alone. Therefore, the drag reduction in Fig. 16 is less than what would have been measured with a properly sized probe. How much less? I don't know. However, it should be noted that the probe is 32% taller than ideal. So it is reasonable to assume that the highest Pitot saw very little change at all and the next one saw significantly less than it should have. On the other hand, these Pitots would ordinarily see only small changes anyway, so this ameliorates the error a good bit. I would like to think the probe hight error is good for an additoinal few percentage points. In that case, high speed performance at the 158" span station is in line with the results inboard at the 53" station.

The poor low speed results are not well understood at this point. Perhaps the chordwise placement is not right, perhaps the material width needs to be scaled down commensurate with the chord length. Or perhaps it's due to flaws in the construction of the FCSD material.

The bad news is that, whereas we obtained spectacular results two times running at the 53" station, this time we saw essentially no change at the same station at any airspeed. Evidently, we have a problem maintaining the close tolerances needed for consistent results. In the past, poor results were identified to flaws in the construction of the deturbulator. This time too, the problem appears to be linked to the hand construction methods used to produce the composite surface. Dr. Sinha is working on improved tooling to address this issue.

These results illustrate the challenge before us and indicate that progress will be slower than we wish.

Jim Hendrix
Oxford Aero Equipment

Airspeeds shown in graphs are instrument calibrated. The aircraft airspeed system is not calibrated. Errors in the Standard Cirrus static/Pitot system bias the data towards higher speeds. This makes polars seem better than they really are. However, this is not an issue when the purpose is only to show comparitive data on the same glider.

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