• HQ2/9A-PT (Fig. 12.42)
• HQ2/9A-PT u. s.t. xjc – 20%,h/c = .17%, w/c = 1.0% (Fig. 12.43)
• HQ2/9A-PT u. s.t. xjc — 40%, h/c = .17%,wjc = 1.0% (Fig. 12.44)
• HQ2/9A-PT l. s.t. xjc = 50%, h/c = .17%, tv/c = 1.0% (Fig. 12.45)
Before discussing the lift-drag characteristics of the HQ2/9, one very important point needs to be made. The differences between the nominal HQ2/9, RG15, and S2048 are small. Moreover, these differences are of the same order as the differences between the nominal airfoils and the models of the HQ2/9, RG15, and S2048 actually tested.
To illustrate, Figs. 11.3 and 11.4 compare the nominal HQ2/9 with the nominal RG15 and S2048. As for the actual airfoils tested, Figs. 11.5, 11.6, and 11.7 compare the HQ2/9B-PT with the HQ2/9A-PT, RG15-PT, and S2048-PT. Looking at the figures, one arrives at the conclusion that the nominal airfoils were not really tested; rather a group of very similar airfoils was tested instead.
Another important point, which immediately follows, is that the performance differences between these models are probably not meaningful (at least as they apply to RC sailplane performance) despite the fact that some differences in the overall shape and appearance of the polars are apparent. When all the other components of drag are factored into the performance of the sailplane—induced drag, wing/body interference drag, fuselage drag, etc.—the subtle differences between similar polars tend to be lost. And even though the airfoils were not tested with flaps (though flaps are usually used) it is doubtful that any one of them would have a decided advantage, even with a flap.
Two versions of the HQ2/9 were tested. The first model, version A, was found to be inaccurate and was later modified to become version B. The polar taken on the plain A model is shown in Fig. 12.42, while Figs. 12.43-12.45 show data taken with trips.
What stands out is that the drag is quite low, especially when compared with the Eppler airfoils. There are two reasons for this:
1. First, this type of airfoil, which is popular in F3B type flying, is intended for use with flaps in order to achieve a wide lift range, while the Eppler airfoils (E205, E193 type) are designed for a wide speed range without flaps. If an airfoil is intended to be used with flaps then a certain amount of lift range (at any given position of the flap) can be traded for lower drag, since the flaps will be used to recover the range. This is the approach taken by Quabeck.
2. The second reason was described in the overview of the HQ-series airfoils; that is, the upper surface pressure gradient is rather gradual, which improves the management of the laminar separation bubble.
Trips were placed on the upper surface at 20% and 40% chord. For the 20% case shown in Fig. 12.43, there is an improvement only for Jin’s of 150k or less. For the 40% case shown in Fig. 12.44 the break-even point seems to be between 150k and 200k. For the HQ2/9, therefore, it is generally advisable to use trips on any portion of the wing that usually operates at Rn less than 150k-200k. With a tapered wing, the trip location near the root should be further aft (in percent of chord) than it is at the tip, since the tip operates at a lower Rn.
Figure 12,45 shows the effect of a trip at 50% chord on the lower surface. If anything, the drag has increased around C of 0.5.
• HQ2/9B-PT (Fig. 12.47)
• HQ2/9B-PT u. s.t. x/c = 50%, h/c = .17%,w/c = 1.0% (Fig. 12.48)
The untripped HQ2/9B-PT has performance much like the (less accurate) HQ2/9A-PT. An upper surface trip at 50% improves performance below Rn of 200k, and only slightly hurts the performance at 300k.
• HQ2/9B-PT u. s. blowing xfc — 50%, type В (Fig. 12.49)
• HQ2/9B-PT trips, Rn = 200,000 (Fig. 12.50)
The A/В model was hollow so that tests involving boundary layer blowing through a row of spanwise holes could be conducted. The blowing configuration is shown in Fig. 5.7. As depicted in Fig. 5.8 ram air from a single “total head tube” fixed to the lower surface of the model was used for the air supply.
A complete polar for the type В blowing is shown in Fig. 12.49. Even though tripping the boundary layer with a trip strip improved the performance of the airfoil (Fig. 12.47) at low Rn, tripping by blowing degraded it. Perhaps the amount of blowing was too large.
Figure 12.50, which compares the blowing data with the cases with and without trips, shows that all of the trips are worse than the untripped case at 200k. Since this model arrived late in the experiments, extensive testing was not possible.
Also see: RG15, S2048, SD8000, S2030, S2055, SD2083, S3021
Digitizer plot: Figs. 10.17, 10.18
Airfoil comparision plot: Figs. 11.3-11.7
Polar plot: Figs. 12.42-12.45, 12.47-12.50
Lift plot: Figs. 12.46, 12.51
Thickness: 8.97% Camber: 1.99%