Problem: If your altimeter is set correctly to sea level pressure, how high must it read at point H to reach the airport A in still air if you are a distance D from A. Assume the glider has a glide ratio of 30/1 at 45 mph and that the airport is 530 ft. ASL and that D is 20 statute miles and that you want to be at the airport at a height of 1000 ft. AGL for a safe circuit.
With No Wind
Hn = ((D x 5280) / (GRg)) + 1000 + h
= 3520 + 1000 + 530
= 5050 ASL.
As you reach an altitude of 5050 ft. ASL you note that the time is 2:47 EST and you set off for the airport A. What is your ETA to airport A.
time = 20 miles/45 mph. = 0.44 hrs = 26.7 minutes.
ETA = 2:47 + 0:26.7 = 3:13 EST.
With a 15 mph Headwind
GRg = 30((45-15)/45) = 20
Hh = ((20 x 5280)/20) + 1000 + 530 = 6810 ft. ASL
ETA = 2:47 + (20/(45-15)) x 60 = 2:47 + :40 = 3:27 EST.
With a 15 mph Tailwind
GRg = 30((45+15)/45) = 40
Ht = ((20 x 5280)/40) + 1000 + 530 = 4170 ASL.
ETA = 2:47 + (20/(45+15)) x 60 = 2:47 + :20 = 3:07 EST.
Feet Lost Per Mile - Glide Ratio 30/1 at 45 mph.
No wind = 5280/30 = 176 ft.
Head wind (15 mph) = 5280/20 = 264 ft.
Tail wind (15 mph) = 5280/40 = 132 ft.
Distance covered per 1000 ft. Height Loss
30/1 = 30 x 1000 ft./5280 = 5.7 miles
20/1 = 20 x 1000 ft./5280 = 3.8 miles
40/1 = 40 x 1000 ft./5280 = 7.6 miles