• MB253515-PT (Fig. 12.53)
The MB253515 (designed by Michael Bame) was one of the most intriguing of all the airfoils tested. The relatively high drag of this 15% section leaves much to be desired; nevertheless, it is favored by some.
It may be that the attraction has more to do with the lift characteristics than the drag. Under most types of flying conditions the RC sailplane spends considerable time climbing in thermals, with the wing operating very near the maximum lift coefficient. Unfortunately for most airfoils, just beyond C(max the airfoil stalls, posing handling problems. In a thermal the turbulence is quite high and, for a sailplane operating close to its stall angle of attack, the turbulent conditions can cause portions of the wing to stall intermittently. The problem is further aggravated by the lower tip chord Rn’s because with most airfoils the stall angle of attack decreases with Rn. The net effect of the local stalling and tendency to tip stall makes efficient thermalling difficult.
Although the drag characteristics of the MB253515 are hardly dazzling, the airfoil may make up for this deficiency with good handling. (See also Section 5.2.) In Fig. 12.55, the lift characteristics of the MB253515 are shown for Rn from 30k to 100k. Note that the stall angle of attack is at least 18°—very far from the thermal operating point. This large angle of attack margin gives the MB253515 section a docile feel in thermals and ultimately helps the thermaling efficiency, not through low drag but through handling—by decreasing the work load of the pilot.
This characteristic of the MB253515 separates it from most low-Rn airfoils. Usually the lift increases smoothly with angle of attack and finally breaks away, with a stall following shortly thereafter. The highly desirable stall characteristics of the MB253515 may explain why it is favored by some flyers.
Referring to Fig. 12.55 for Rn = 100k, the lift increases rapidly between —2° and 0°, flattens between 0° and 3°, then becomes more typical above 3°. When separation begins to take place, maximum lift (Cimai = 1.0) is reached at 10°, followed by a dip and a long plateau which is maintained down to as low as 30k.
Note also that as the Rn is decreased the lift characteristics change markedly. Comparing the 100k, 90k and 40k cases, two important observations can be made. First, below the angle of attack at Cimax (a « 10°), the lift decreases with Rn, which indicates the presence of a large bubble or large trailing edge separation or both. Some type of separation may also be deduced from the high drag below Cimax shown in Fig. 12.53. The second observation is the well-defined hysteresis loop in lift which is clear indication of laminar separation. Hence the relatively high drag can be attributed to a laminar separation bubble, suggesting that the airfoil could be improved with a trip.
• MB253515-PT u. s.t. xjc – 20%,hjc = .17%,w/c = 1.0% (Fig. 12.54)
Figure 12.54 shows that a trip at 20% chord improves the performance for Rn less than 200k. For airplanes using this airfoil, therefore, a trip should be used at least on the wing tips, and possibly on the entire wing, depending on the expected speed range.
Also see: S4233, SD7062, E193MOD, WB135/35, WB140/35/FB
Digitizer plot: Fig. 10.20
Airfoil comparision plot: Fig. 11.11
Polar plot: Figs. 12.53, 12.54 Lift plot: Fig. 12.55
Thickness: 14.96% Camber: 2.43%