True short field landings

[Ultralights]

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[27/09]

Originally Posted by john_tullamarine

The Vs figure quoted in the POH is at MAUW.

Depends on the POH performance section. The better documents will have more data and schedule the speeds/distances according to the weight.

Interestingly, if the approach speed is solely for MLW, the landing distance required usually then increases with decreasing weight, largely due to the increased float .. but it saves dollars in the work up side of things for the OEM or subsequent modifier.

Small bugbear of mine, I'm afraid ..

Agreed using the MLW approach speed for light weights will increase the airborne section of the landing distance due to the increased float though the landing roll should be shorter since there is less energy to to bring to a stop. Over all the impact of the increased float will be greater than the shorter ground roll.

All of this leads to an oxymoron of landing at a lighter weight than that at which the Vref was calculated at will result in a longer landing distance. Most pilots would expect the opposite.

It's not hard to calculate a new Vref based on the reduced weight.

Some rough calculations

C 172 Empty weight 635 kg
Fuel 100 litres 72 kg
Two occupants 165 kg
Landing weight 872 kg

Max Landing weight 1043 kg

Load Factor = 872/1043 = 0.836

Sqr root of Load Factor = 0.914


Vs at 1043 kg = 59 KIAS
Vs at 872 kg = 59 X 0.914 = 54 KIAS

[john_tullamarine]

Couple of points to keep in mind ..

(a) there are other considerations in the certification side of things apart from stall speed and weight. Just figuring a reduced speed might not keep one sweet unless the rest of the Design Standard requirements be checked.

(b) if one is going to do sums with speeds, first convert from IAS to CAS, then do the sum, then convert the answer back to IAS, lest the answer be in error.