The following provides simple mathematical models to demonstrate the most efficient altitude to leave the last thermal in preparation for a final glide to the home field. The base assumptions assume a sailplane 20 naut miles from the home field at an altitude of 2000 feet above the home field. It is further assumed that there is no wind and that there will be no up/down drafts during the final glide. The calculations are done so as to have the sailplane arrive home at zero altitude. If it were desired to have extra altitude for a landing pattern or for "insurance" the numbers would change, however, their relative difference would still remain. Also, the presence of a headwind or tailwind will change the numbers, but not the relative difference.

All calculations are based on the McCready speed ring values for a modern Standard Class sailplane. The cruise speed during the final glide is based on using a McCready speed ring value equal to the rate of climb in the last thermal. Therefore, for each value of rate of climb, there will be a minimum altitude that must be achieved before the final glide can be started.

A time to climb to the minimum required altitude (minus the initial 2000ft) is calculated, as is the time to cruise to the home field. The sum of these times is the total time to complete the task of last climb and cruise to the home field. This total time for different rates of climb will be compared to ascertain the most efficient altitude to leave the last thermal (i.e.: minimum time is most efficient).

Cases 1, 2, 3, and 4 are for rates of climb of 2, 3, 4, and 5 kts. As can be seen in the following tables, a higher cruise speed (with its associated greater sink speed) is used for higher rates of climb. This results in the need to climb to a higher altitude before starting the final glide. Therefore, as the rate of climb increases, the required altitude increases. However, the total time becomes less as the rate of climb increases. Note that the rate of sink due to using the higher recommended cruise speeds does not increase proportionally with the rate of climb. From the tables it can be seen that the time spent climbing to each higher required altitude actually decreases with higher rates of climb.

The total time to climb to an altitude higher than required is also shown for each case. This will also increase the total time and is therefore less efficient. However, those pilots not interested in maximizing their speed should note how little extra time in the last thermal it takes to gain the extra altitude so as to be able to make the final glide comfortably.

Case 2 (a) illustrates the effect of climbing higher than required in a weak thermal so as to be able to be able to use a faster cruise speed on the way home. This takes longer than leaving at the proper altitude and speed. It is therefore less efficient. Case 4 (a) illustrates the effect of leaving a strong thermal early and cruising at a slow speed to conserve altitude. This also takes longer than if a correct, higher speed were used.

The proper altitude to leave the last thermal in preparation for a final glide and the most efficient speed to cruise, are directly dependent upon the rate of climb achieved in the last thermal. This correct altitude and speed can only be determined with the use of a proper Final Glide calculator.

GIVEN: Sailplane 20 naut mi from home field, 2000 ft above home field elev.

After climb, cruise speed (V_cr ) based on McCready value of last thermal.

Sailplane performance provides Speed Ring Values (SRV) according to the following table.

PROBLEM: Determine minimum time for (climb + cruise home).

ASSUMPTIONS: No wind and No up/down drafts, during cruise.

1 kt = 100 ft/min

Rate of climb: v_cl (kts) Given for each Case:

Altitude required for cruise (in ft) =

Time to Climb =

Time to Cruise =

Cruise: MC = 2 V_{c r} = 65 kts Sink = v_sink = 1.72 kts

Min Alt Reqd = 3218 ft

CLIMB ALT (REQD - 2000)	TIME TO CLIMB	TIME TO CRUISE	TOTAL TIME
1218	6.1	18.5	24.6
1500	7.5	18.5	26.0
2000	10	18.5	28.5
2500	12.5	18.5	31.0

Cruise: MC = 3 V_{c r} = 75 kts Sink = v_sink = 2.39 kts

Min Alt Reqd = 3875 ft

CLIMB ALT (REQD - 2000)	TIME TO CLIMB	TIME TO CRUISE	TOTAL TIME
1875	6.3	16.0	22.3
2000	6.7	16.0	22.7
2500	8.3	16.0	24.3
3000	10.0	16.0	26.0

Cruise: MC = 5 V_{c r} = 86 kts Sink = v_sink = 3.27 kts

Min Alt Reqd = 4624 ft

CLIMB ALT (REQD - 2000)	TIME TO CLIMB	TIME TO CRUISE	TOTAL TIME
2624	8.7	14.0	22.7
3000

Cruise: MC = 4 V_{c r} = 83 kts Sink = v_sink = 2.98 kts

Min Alt Reqd = 4366 ft

CLIMB ALT (REQD - 2000)	TIME TO CLIMB	TIME TO CRUISE	TOTAL TIME
2366	5.9	14.5	20.4
2500	6.3	14.5	20.8
3000	7.5	14.5	22.0
3500	8.8	14.5	23.3

Cruise: MC = 5 V_{c r} = 86 kts Sink = v_sink = 3.27 kts

Min Alt Reqd = 4624 ft

CLIMB ALT (REQD - 2000)	TIME TO CLIMB	TIME TO CRUISE	TOTAL TIME
2624	5.2	14.0	19.2
3000	6.0	14.0	20.0
3500	7.0	14.0	21.0
4000	8.0	14.0	22.0

Cruise: MC = 2 V_{c r} = 65 kts Sink = v_sink = 1.72 kts

Min Alt Reqd = 3218 ft

CLIMB ALT (REQD - 2000)	TIME TO CLIMB	TIME TO CRUISE	TOTAL TIME
1218	2.4	18.5	20.9
1500	3.0	18.5	21.5
2000	4.0	18.5	22.5
2500	5.0	18.5	23.5