IMPACT OF WIND AND SINK ON GLIDER PERFORMANCE*

Doug Cline

*Based on topic suggested by Tom Roberts

FLSC 2006 Safety Seminar

Polars for 1-26 and ASK 21

• Figure 1 shows the polars for the Schweizer 1-26 and ASK-21 taken from the evaluations by Dick Johnson.

• The 1-26 assumes the minimum mass pilot leading to a wing loading of 3.06 lbs/sq ft.

• The ASK-21 shows 574KS at minimum pilot weight and maximum weight with corresponding wing loadings of 5.39 lbs/sqft and 6.84 lbs/sqft respectively.

• Glider wing loadings range from 3 lbs/sq ft to 11 lbs/sqft for racing glider.

• The 1-26 has a maximum L/D of 21.6 at 37kt

• The ASK21 has a best L/D of 31.7 at 48kts (light) and 56kts (heavy)

• The ASK21 best performance occurs at higher speeds due to the higher wing loading since the polar plot, of forward speed versus vertical speed, scales as the square root of the wing loading. Jets have wing loadings around 100lbs/sq ft shifting the polar by a factor of about 4 to the right and downward.

• The ASK21 has about a 50% better L/D compared to a 1-26

GLIDING PERFORMANCE FOR 2KT THERMALS FOR ZERO WIND AND SINK

• The average gliding speed with respect to the parcel of air is given by the intercept on the x-axis of the tangent to the polar from the 2kt lift point

• 2kt overall climb rate corresponds to instantaneous climb rates of up to 4kt.

• The average cross country speed of the 1-26 is 20kt if it cruises at 46kts

• The average cross country speed of the ASK-12 is 30kt and 32kt flying at the two wing loadings cruising speeds of 60 (68) kts.

• Like the polars, the average cruise speed of the ASK-21 is roughly 50% better than the 1-26.

GLIDING PERFORMANCE FOR 2KT THERMALS WITH 20kt HEADWIND, ZERO SINK

• The average gliding speed with respect to the ground is obtained by subtracting 20kts from the airspeed shown on the x axis

• The 1-26 makes zero headway against a 20kt headwind for 2 kt lift whereas the ASK21 averages 10kts ground speed

• The best L/D for the 1-26 drops to 10.8 at 46 kts

• The best L/D for the ASK 21 drops to 18 (21) at 55 (59) kts.

• The headwind accentuates the difference in performance, that is the ASK21 now is 100% better than the 1-26

GLIDING PERFORMANCE WITH 40kt HEADWIND, ZERO SINK

• The average gliding speed with respect to the ground is obtained by subtracting 40kts from the x axis.

• The best L/D for the 1-26 drops to 4.6 at 65 kts

• The best L/D for the ASK 21 drops to 10 (12) at 74 (76) kts.

• Note the best L/D speed to fly is increased by roughly half the wind speed

• CONCLUSION:A headwind greatly accentuates the difference in performance, that is the penetration of a ASK21 now is 160% better than the low wing-loading 1-26

GLIDING PERFORMANCE WITH 10kt SINK, NO WIND

• With a 10kt sink (1000ft/min) the L/D of the 1-26 drops to 4.2 at 67 kts.

• The best L/D for the ASK 21 drops to 5.8 (6.3) at 90 (97) kts. This is equivalent to using full dive brakes.

• CONCLUSION:Sink greatly decreases the L/D with the greatest impact on the higher performance ship. They all have the glide ratio of the Space Shuttle

GLIDING PERFORMANCE WITH 10kt SINK PLUS 40kt HEADWIND

• The best L/D of the 1-26 drops to 2.0 at 79kts

• The best L/D for the ASK 21 drops to 3.5 (3.9) at 109 (114) kts.

• CONCLUSION:All ships soar like bricks under these conditions.

• (By comparison with maximum dive brake plus flaps the Jantar has a worst L/D of 14 and Pegasus a worst value of 5 which seems very steep)

HYPOTHETICAL RIDGE DAY WITH 40KT WINDDrift back from lift into sink in lee of hill. • To recover the 1-26 has to dive 60 ft to gain the best penetration

speed of 80kts in order to penetrate against the wind• To recover the ASK21 has to dive 131ft to gain the optimum

penetration speed of 110kts• To penetrate 500m upwind to lift requires altitude clearance above

top of hill of >420ft for the 1-26 and >200ft for the ASK21CONCLUSION:

ASK21 has a much better chance of penetrating across the ridge to reach the lift if there is sufficient altitude to clear the top of the hill

40KT WIND SHEAR• For a constant low ground speed the airspeed drops with altitude

leading to stall at 100ft if the ship does not accelerate• Slowing the airspeed to try to extend the glide exacerbates the

problem• Dive 60ft will provide sufficient ground speed to maintain airspeed

throughout the wind shear• Diving to accelerate also will enable the ship to penetrate further at

lower altitudes where the headwind is lower.

DIFFERENT BEHAVIOUR AT LOW AND HIGH WING LOADING

Low wing loading: 1-26, 1-33Advantages: Forgiving and safe flying qualities, low airspeeds allow high climb

rates in thermals. Can land on a dime using a slip.Disadvantages: Penetration against headwind is very poor leading to poor cross

country capabilities

High wing loading: ASK 21, Grob 102Advantages: Excellent penetration against head winds. Cross country flight more

viableDisadvantages: High kinetic energy requires longer landing distances

Transitions between high and low wing loading:Very different flight characteristics requires great care when transitioning

students taught in the ASK21 to the 1-26. Invariably the new student will land the 1-26 off field if not careful. Fortunately the 1-26 is easy to land off field. The former teaching ship sequence [ 2-33 to 1-26 to ASK21 to Russia] produced less surprises than the [ASK21 to 1-26 to Russia] sequence.

SUMMARY• Wind and sink have an enormous impact on

glider performance

• Large difference in flight characteristics between low and high wing loading ships requires caution. They require different flying techniques.

• Always conserve total energy; extra altitude and airspeed are money in the bank