Category AIRFOILS AT LOW SPEEDS

• FX60-100-PT (Fig. 12.39)

From a quick glance the FX60-100 looks like a good airfoil; there are no signs of excessive bubble drag, and the width of the low-drag portion of the polar is similar to other 10% sections. Despite these qualities, the FX60-100 has not re­ceived wide acclaim. Possibly the unusually prolonged, thin trailing edge presents more of a construction problem than is warranted by the good performance. Or perhaps it was simply overlooked when the E214 became popular.

Also see: SD7037, SD6080, E387

Digitizer plot: Fig. 10.15

Polar plot: Fig. 12.39

Thickness: 9.97% Camber: 3.55%

• Flat Plate-PT (Fig. 12.37)

This airfoil was the only one machined from solid metal. It was A in thick, had a rounded leading edge, and the aft 3 in were tapered to a ^ in trailing edge. Since this model was completely symmetrical, the data served to check several aspects of the instrumentation.

The geometry of the flat plate is typical of that used for slab, sheet-balsa stabilizers. The polar data is representative of a full-flying stabilator. In other words, there was no elevator deflection, only angle of attack changes.

As shown in the polars, the lift and drag coefficients are practically the same over the Rn range tested. From both theory and many previous measurements it is known that for a flat plate developing lift, the recovery pressure gradients on the suction side are both early and steep. This promotes a rapid transition to turbulent flow. The independence of drag with Rn and the relatively high value of the drag are commonly found when the flow is turbulent, as it is here.

Comparing the data with the symmetrical SD8020, for example, shows that the drag of the flat plate is considerably higher. Consequently we recommend a more streamlined airfoil like the SD8020 for stabilizers.

Also see: SD8020 Polar plot: Fig. 12.37 Lift plot: Fig. 12.38

Thickness: 2.08% Camber: 0.00%

• E387A-PT (Fig. 12.31)

• E387A-PT repeated (Fig. 12.32) .

The E387A-PT built by Bob Champine was used to compare our data with E387 data taken in other facilities. It also served as one of two calibration models used to check for measurement repeatability over the course of the six-month testing schedule.

The first data set (Fig. 12.31) was taken very early in the schedule and the second (Fig. 12.32) was taken near the end. In general the agreement is good. However, at low-lift coefficients the disagreement is larger than we found else­where. The discrepancy may be related to a spanwise twist in the model. For the first run, the twist was removed by applying an opposite twist over a two day period. No attempt was made to untwist the model for the repeated runs or for the tripped and high-turbulence runs.

• E387A-PT u. s.t. xjc = 20%, Vе = 0.17%, w/c = 1.0% (Fig. 12.33)

Figure 12.33 shows that the addition of an upper-surface trip at 20% lowers the drag over the entire range except at high lift and at 300k.

• E387A-PT high turbulence (Fig. 12.34)

To test the E387 under high turbulence conditions, a screen was placed in the test section 3 ft upstream of the model leading edge. As expected, the high turbulence leads to lower drag because the turbulence shortens the bubble in a way much like that produced by the upper-surface trip. No turbulence measure­ments were taken under these conditions as this was done merely to compare the data with that taken under the normal, low-turbulence test conditions.

• E387B-PT (Fig. 12.36)

The E387 version В data provides some measure of the need for accuracy. Since the camber of this model is substantially less than that of the prototype, large differences in performance can. been expected and were measured. An inaccurate copy of an airfoil will probably share few traits with the parent section, as this model illustrates. While it may be possible in some cases to improve an airfoil by modifying its shape, clearly if the airfoil is a good one to begin with, any modification will usually degrade its performance.

Also see: E193, SD6080, S4061, SD7037, S2091 Digitizer plot: Figs. 10.13, 10.14 Polar plot: Figs. 12.31-12.34, 12.36 Lift plot: Fig. 12.35

Thickness: 9.06% Camber: 3.80%

• E374A-PT (Fig. 12.24)

• E374B-PT (Fig. 12.25)

The E374 is often selected because its low camber (2.2%) makes it more of a high-speed type than a floater, and because its thickness (10.9%) allows it to be used for the large spans found in cross-country sailplanes. Recently, for example, Joe Wurts flew the E374 to a new cross country distance record of 141 miles. As with the E205, E193, and E387, the E374 shows the effects of a laminar separation bubble through the mid-lift range at the lower Jin’s.

Two models of the E374 were produced for these tests. Overall, both models were very accurate; the A version has slightly more aft camber and is slightly thinner rearward of 50% chord than the B. For reasons that cannot be satis­factorily explained, the A version is marginally the better of the two in terms of performance, even at the lower C’s where it would be thought that the additional camber would be detrimental.

• E374B-PT u. s. bumps xjc = 50%, type A (Fig. 12.26)

• E374B-PT u. s.t. xjc = 20%, А/с = .17%, w/с = 1.0% (Fig. 12.27)

On the more accurate E374B model, two kinds of trips were tested: upper – surface, three-dimensional bumps (type A) and the more commonly tested two­dimensional trip strip. It is a widely held view that three – dimensional boundary layer disturbances produced by bumps or zig-zag tape are more unstable than the initially two-dimensional disturbance produced by a continuous trip strip28. Because the trips were placed at different locations, comparisons of their relative effectiveness cannot be made. What can clearly be seen from these results is that a trip strip at 20% leads to improved performance for Rn less than 200k. For the bumps, the 150k case is most interesting. Below Ci of 0.5 the bubble is effectively tripped; however, above 0.5, there is no benefit. Most likely the leading edge of

the bubble is upstream of the trip, which makes it ineffective because it is in the recirculating region of the bubble.

• E374B-PT u. s. wavy clay, x/c = 0% to 15%, h/c = .20% (Fig. 12.28)

To investigate the sensitivity in performance to gross, upper-surface waviness near the leading edge, a generous amount of artist’s clay was smeared over the forward 15%. The wavelength, which was random, was on the order of 7% with a maximum height of 0.2%. All edges were carefully smoothed. Although the waviness was greater than anything expected in typical construction, the results do provide clues to potential performance losses due to, say, hasty repairs near the leading edge. Interestly, as Fig. 12.28 shows, there are no large adverse effects.

• E374B-PT thickened trailing edge (Fig. 12.29)

The effect of a thickened trailing edge was also measured. The shape is given in Fig. 5.2. As with the DAE51, there is a small drag increment everywhere, indicating once again that a thin trailing edge is better.

Also see: SD6060, SD7090, E205, NACA 2.5411, CLARK-Y Digitizer plot: Figs. 10.11, 10.12 Polar plot: Figs. 12.24-12.29 Lift plot: Fig. 12.30

Thickness: 10.91% Camber: 2.24%

• E214A-PT (Fig. 12.16)

• E214B-PT (Fig. 12.17)

By inspection of the E214 contour, it is clear that it not designed like the E193, E205, E374, or E387. The large amount of aft camber (sometimes called aft loading) markedly distinguishes the E214 from the others. On a more sub­tle level, the E214 is designed with a laminar separation bubble ramp—a mild pressure recovery on the aft upper surface.

While the bubble ramp is good for performance, the large aft camber leads to some trouble. Notice that at lift coefficients below 1.0, the lift at a given angle of attack decreases as the Rn is reduced. This loss in lift is due to a large, upper – surface separation over the aft end of the airfoil. (A more thorough discussion is given below in the discussion on tripping the E214.)

It should be noted that for the E214 this loss in lift at low Rn’s does not necessarily hamper the performance of a sailplane using this airfoil. Since the operating Reynolds numbers of a RC sailplane decreases (along with the speed) as the lift coefficient increases, the mid-range lift coefficients at low Rn are not used. Low drag is, however, desirable at high lift and low Rn, and this the E214 has. ■

Two models of the E214 were constructed. Version A had a little less aft camber than the true E214, and version В a little more. The additional camber of version В produces a slight increase in lift, much like that produced by a small, positive (downward) flap deflection. Aside from these small differences the two models are excellent reproductions of the E214.

The E214 is used primarily for thermal-duration flying, owing to the excellent low-drag, high-lift capability. To improve the high speed characteristics full-span flaps are often employed.

• E214C-PT 3° flap (Fig. 12.18)

• E214C-PT 0° flap (Fig. 12.19)

• E214C-PT -3° flap (Fig. 12.20)

• E214C-PT -6° flap (Fig. 12.21)

A 22% flap with a lower-surface tape hinge was added to the В model (see Fig. 5.4 for flap configuration). Tests were performed for flap settings of 3°, 0°, — 3° and —6°, covering a range typically used in practice. The flap setting was accurate to within ±-| .

It is instructive to start with the 0° case. Compared with the unflapped version, there are no detrimental effects on performance. At the location of the hinge line the upper-surface boundary layer is turbulent and therefore insensitive to the small, 0.03% (0.004 in) step of the flap seal. Although the boundary layer on the lower surface may be laminar at the location of the flap hinge at high angles of attack, a small step at this location typically has little influence on the drag. Note that the lift range is shifted downward slightly, probably because the flap was set at a small, negative angle rather than exactly zero.

For the 3° setting, the lift curve is shifted upward by about 0.2, but the increase in maximum lift is hardly noticeable. There is little change in the drag. In short, for the E214 positive flap deflections do not lead to improved thermalling performance.

On the other hand, the —3° flap deflection leads to improved high-speed performance by shifting the polar downward. In some areas the drag is reduced, while in others it is increased. Interestingly, for the —6° case nothing is gained over the —3° deflection.

• E214C-PT u. s.t. x/c = 20%, hf c = .17 %>,w/c = 1.0% (Fig. 12.22)

For Rn’s less than 200k, a two-dimensional trip strip or turbulator placed on the upper surface at 20% chord leads to substantial improvements in the E214’s performance. The advantage is seen by comparing the 100k data with Fig. 12.19. (It can be reasonably assumed that at 60k a similar performance gain takes place.) As will be discussed later (see SD7090) the height of the trip is an important parameter. The height,0.17% of chord, was found necessary in order to achieve the indicated drag reduction.

A trip or turbulator is not a panacea; its purpose is to promote transition which shortens the separation bubble. This decreases drag provided that the trip drag is smaller than the bubble drag. For some airfoils tripping the flow either had no effect at any Reynolds number or actually increased the drag. In fact, 200k is the break-even point for most of the airfoils tested; only in a few cases, such as the MILEY, did a trip reduce the drag at 300k. It appears that at 300k the longer length of turbulent flow behind the trip generates more drag than the untripped bubble. For the E214 we recommend that trips be used only for Rn’s below 200k.

Recalling that for the untripped E214 at a given angle of attack the lift de­creases with decreasing Rn, the tripped E214 offers insight into this phenomenon. By tripping the boundary layer, transition to turbulent flow takes place sooner than without tripping. As a result, the boundary layer approaches the trailing edge with more energy to negotiate the sharp pressure recovery over the aft 5% or so of the airfoil. With the trip, the flow can stay attached and maintain the lift (compare Figs. 12.19 and 12.22). Only below Ci of 0.35 does the lift at 100k begin to depart from the other curves, probably because of separation on the lower surface which effectively decambers the airfoil. In summary, the trip reduces not only the size of the separation bubble but also the extent of the turbulent trailing-edge separation.

Also see: SD7032, SD7043, S2091, AQUILA Digitizer plot: Figs. 10.9, 10.10 Polar plot: Figs. 12.16-12.22 Lift plot: Fig. 12.23

Thickness: 11.10% ■ Camber: 4.03%

• E205A-PT (Fig. 12.13)

• E205B-PT (Fig. 12.14)

The popularity of the E205 grew tremendously with the introduction of the Airtronics SAGITTA standard class sailplane. Since this time, the E205 has become one of the most favored RC soaring airfoils. The SAGITTA, which was designed to compete effectively in multi-task events as well as F3B competition, owes much of its success to the E205. As compared with the E193, the better low-lift performance of the E205 offers improved wind penetration.

Of the two models constructed, the В version was the more accurate, es­pecially near the leading edge. Not surprisingly the drag of the В version is everywhere lower than the A.

From the polar it can be seen that the drag curves bunch at the lowest drag point near Ci of 0.25. Compare this to the AQUILA, where not only is the minimum drag at any Rn reached at a Ci of about 0.6, but it is also higher there than the E205 is at 0.25. Because the lift coefficient of a RC sailplane is about 0.2 when flying at high-speed cruise, the E205 is clearly better here than the AQUILA. This illustrates the general rule that good multi-task airfoils have the low drag region shifted to lower C;5s than thermal soaring airfoils.

Also see: S3021, RG15, E374, E193, SD5060, DF101, SD7080, SD7084 Digitizer plot: Figs. 10.7, 10.8 Airfoil comparision plot: Fig. 11.12 Polar plot: Figs. 12.13, 12.14

Lift plot: Fig. 12.15 .

Aircraft polar: Fig. 5.9

Thickness: 10.48% Camber: 3.01%

• E193-PT (Fig. 12.11)

The E193 has been used on a wide variety of designs from hand-launch gliders to large cross-country ships. In this sense, it may be considered as a multi-task airfoil. What makes the E193 a popular choice is not the performance in terms of drag, but rather the low-drag range from C; of 0.1 to 1.1. With this range a sailplane will have good wind penetration and thermal performance under most weather conditions. Of course an airfoil with the same lift range but with lower drag will always be a better choice.

In terms of the overall accuracy of the E193-PT, it should be noted that the E193-PT matches the E205 better than the intended E193 (Fig 11.12).

Also see: E205, S3021, E387, E193MOD Digitizer plot: Fig. 10.5 Airfoil comparision plot: Fig. 11.12 Polar plot: Fig. 12.11

Thickness: 10.22% Camber: 3.57%

• E193MOD-PT (Fig. 12.12)

Dale Folkening generated the E193MOD by scaling the ordinates of the E193 by a factor of 1.19, effectively adding camber and thickness. The result increases the lift (as indicated by the change in the zero-lift angle of attack) and the width of the polar. There is an associated slight increase in drag, owing mostly to the added thickness.

Also see: E193, S4233, E205, E387 Digitizer plot: Fig. 10.6 Polar plot: Fig. 12.12

Thickness: 11.85% Camber: 4.15%

This series of airfoils was designed for very large sailplanes where thin airfoils are not structurally possible. The starting seed was the SD5060; however, the DF-series is thicker and has a different thickness distribution.

The design requirements were:

1. 11% thick

2. flat bottom aft of 30%

3. minimum drag at C; = 0.2.

The DF101 is the initial design; the DF102 and DF103 are variations on the basic airfoil. The DF102 has an increased thickness (about in max., 0.5%) between 2% and 30% on the upper surface. The DF103 has its thickness decreased by approximately the same amount in the same place. See Figs. 11.1 and 11.2.

The purpose of the variations was to test the effects of a controlled contour change in a region previously recognized as important to performance. To make the test more precise, the same physical section was used for all three; it was modified once and became the DF102, and modified a second time to become the DF103. Besides demonstrating the effects of minor, local changes in the airfoil contour, these airfoils as a group provide a measure of the sensitivity of the airfoil performance to surface modifications, whether deliberate or inadvertent.

• DF101-PT (Fig. 12.8)

As the polars show, the initial design is the best, suggesting that deviations either way from a good nominal design are as likely to hurt performance as to help it. Compared to other airfoils, the DFlOl’s performance is quite good; in particular, it has a remarkable lift range for the 11% thickness. At low speed it performs better than the NACA 2.5411 and not quite as well as the CLARK-Y, something that could be expected from the CLARK-Y’s much higher camber (3.55% for the CLARK-Y vs 2.30% for the DF101). At high speeds it is the equal of the NACA 2.5411 and better than the CLARK-Y.

Also see: SD5060, DF102, DF103, CLARK-Y, S3010, S3021

Digitizer plot: Fig. 10.4

Airfoil comparision plot: Figs. 11.1, 11.2

Polar plot: Fig. 12.8

Thickness: 11.00% Camber: 2.30%

• DF102-PT (Fig. 12.9)

Contrary to what might be expected, the addition of upper-surface thickness did not change the low-lift characteristics. Instead, the lift range was extended by 0.1 at the high end. But this is not the aerodynamicist’s “free lunch,” since overall the drag has increased for lift coefficients above 0.5. Furthermore, at Rn’s of 60k and 100k the stall occurs earlier than at the higher Rn’s, a condition that can cause the wing tips to stall before the inboard sections, leading to “tip stall” problems. In conclusion, the DF101 is a better airfoil.

Also see: DF101, DF103, CLARK-Y, S3010 Airfoil comparision plot: Fig. 11.1 Polar plot: Fig. 12.9

Thickness: 11.00% Camber: 2.30%

• DF103-PT (Fig. 12.10)

As compared with the DF102, removing thickness has produced the opposite effect. The maximum lift is reduced, but the stall behavior has improved over the DF101. Still, the DF101 seems to be the best choice of the three.

Also see: DF101, DF102, NACA 2.5411 Airfoil comparision plot: Fig. 11.2 Polar plot: Fig. 12.10

Thickness: 11.00% Camber: 2.30%

• CLARK-Y-PT (Fig. 12.3)

The famed CLARK-Y airfoil needs no introduction; it is perhaps the most popular airfoil ever used on both full-scale and model aircraft. As compared with the AQUILA, the high-speed, low-lift performance is superior. What is surprising is the small price for the improved performance—the maximum lift coefficient (Cimai) is a mere 0.1 less than the AQUILA.

The effects of a laminar separation bubble are apparent only at a Rn of 60k through the mid-lift range, where the drag coefficient reaches a maximum of 0.032 at a Ci of 0.5. However this bubble does not necessarily detract from the performance since the drag rise occurs mostly in the mid-lift range, a range not used by the vast majority of sailplanes at 60k.

At the high-lift, low-Rn regime of the RC sailplane, the drag reduces as the bubble shortens, reaching a minimum of 0.027 at a Ci of 0.9 for 60k. No doubt the CLARK-Y will stay popular for some time to come.

Also see: DF101, S3010, S3021, SD5060 Digitizer plot: Fig. 10.2 Polar plot: Fig. 12.3 Lift plot: Fig. 12.4

Thickness: 11.72% Camber: 3.55%

DAE51

• DAE51-PT (Fig. 12.5)

The DAE51, along with several other DAE airfoils, was designed by Mark Drela using the ISES code developed by him and Giles21’22. As was mentioned in Chapter 1, this same code was used to analyze the new SD-series of airfoils.

The DAE51 was designed for the propeller of the DAEDALUS, human-powered aircraft25. Thus it was not designed for one operating point, but rather for the range of anticipated conditions. The requirements were26:

1. CiM > 1.2

2. Transition ramp optimized for Rn = 125k, 0.5 < Ci < 1.0

3. No bubble “bursting” for Rn greater than 75k.

4. Thickness less than 9%

The achievement of these design goals was demonstrated by no less than a record 74-mile flight from Crete to Santorini across the Mediterranean, a flight that duplicated in reality the mythical flight of Daedalus and his son, Icarus. Although only the third airfoil to be discussed, the variety of performances and the trade-offs made to achieve them are becoming clear. The drag of the DAE51 at Rn of 300k is considerably lower than the AQUILA and CLARK-Y, but the range of lift is significantly less. For application to RC sailplanes a broad lift range can sometimes be recovered by use of a full-span flaps, while maintaining the low-drag characteristics of the unflapped airfoil. With a 20% flap, the low- drag range could probably be extended up to С/ of 1.1, which would make it competitive against the unflapped AQUILA and CLARK-Y.

• DAE51-PT thickened trailing edge (Fig. 12.6)

The DAE51 was tested with a substantially thickened trailing edge (see Fig. 5.1 for contour) to measure the possible loss in performance. Over most of the low-drag range, a 4-5% increase in drag was found. As further examples, the E374 and SD6080 were also tested with thickened trailing edges, and the same trends were observed. Similar results were observed by Althaus at Reynolds numbers 1-3 million27.

Also see: S4061, SD6080, E387, E193 Digitizer plot: Fig. 10.3 Polar plot: Figs. 12.5, 12.6 Lift plot: Fig. 12.7

Thickness: 9.37% Camber: 3.98%

AQUILA

• AQUILA-PT (Fig. 12.1)

With the level of sophistication in F3B today, it is hard to believe that in 1977 Skip Miller flew his “modified” Airtronics AQUILA sailplane to a first place finish at the first F3B RC Soaring World Championships. One can only speculate on the performance differences between Skip Miller’s “modified” AQUILA and the stock version; nevertheless, the stock AQUILA was a formidable RC sailplane from the mid 70’s to early 80’s.

In comparison to other popular airfoils of the time, the AQUILA airfoil gave excellent thermal performance. However, the high camber was a handicap at high speed. Recognizing this deficiency, Airtronics introduced the SAGITTA with the Eppler 205 airfoil, which had the desired high-speed qualities. The popularity of the AQUILA waned, and production was finally discontinued.

Also see: S2091, SD7032, SD7037, FX60-100 Digitizer plot: Fig. 10.1 Polar plot: Fig. 12.1 Lift plot: Fig. 12.2

Thickness: 9.38% Camber: 4.05%