• 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%