본 연구는 태풍이 주로 도래하는 시기에 사과 ‘후지’(Malus domestica Borkh) 과실의 부착력 변화를 조사하고, 풍동 및 진동대를 이용한 시뮬레이션을 통해 낙과와 관련된 정보를 획득하고자 수행하였다. 과실 부착력은 과실을 잡아당겼을 때 낙과하는 시점의 인장력을 조사하였다. 생육기(7–9월) 과실 부착력은 성숙기에 가까워질수록 감소하는 경향이었으나 통계적인 유의성은 없었다. 반면 과경축을 기준으로 당기는 각도를 0, 30, 60도로 측정했을 때의 부착력은 각도가 커질수록 유의하게 감소하였다. 가속도센서를 활용한 주지의 기부, 중간과 바깥쪽의 고유진동수는 각각 52.0cycles·min<sup>-1</sup>, 332.2cycles·min<sup>-1</sup>, 346.5cycles·min<sup>-1</sup>로 측정되었으며, 주지의 운동 방향은 수직(443cycles·min<sup>-1</sup>)보다 수평(874.2cycles·min<sup>-1</sup>)적인 변위에서 더 높았다. 과실의 고유진동수는 496.9cycles·min<sup>-1</sup>로 측정되었다. 추가적으로 주지의 60cm 부위(원줄기로부터)를 결속하여 고유진동을 측정한 결과, 무결속보다 고유진동수가 낮았으며, 상대적으로 수직변위가 수평변위보다 낮았다. 풍동을 활용한 실험에서 0–9m·s<sup>-1</sup>의 풍속에서 과실은 낙과하지 않았으며, 풍속별 평균 변위는 풍속이 강해질수록 증가(최대 17mm)하는 경향을 보였다. 진동대를 이용한 실험은 풍동 실험에서 산출된 변위계수를 근거로 5–30mm까지 변위값을 적용하였으며, 진동방식은 ‘흔들림(pendulum)’ 형태로 과실이 착과된 결과지에 진동을 주었다. 과실 낙과는 변위계수 10mm(고유진동수 57.0-72.6cycles·min<sup>-1</sup>)부터 낙과가 시작되었고, 변위계수가 증가할수록 낙과까지의 소요시간은 단축되었다. 변위계수와 풍속과의 관계식에서 10mm의 변위계수는 풍속 6–9m·s<sup>-1</sup>를 의미하지만, 이와 같은 결과는 재배 특성(수형, 잎, 과중, 착과량, 과실 이층부의 성숙도 등)과 자연적 바람의 특성(돌풍, 와류, 풍향 등)이 적용되지 않아 보다 낮은 풍속에서도 낙과가 발생할 수 있다. 따라서 바람은 과실을 포함한 수체에 1차 변위를 발생시킨 후 과중과 가지의 탄력성에 의해 2차 변위를 유도할 수 있으며, 이러한 1, 2차 변위에 의해 낙과하는 것으로 추정된다.
This study was conducted to investigate changes in the adhesivity of ‘Fuji’ apple (Malus domestica Borkh) fruits during the period when typhoons mainly occur and to obtain agricultural engineering information related to fruit drop through a simulation using a wind tunnel and shaking table. Fruit adhesivity was assessed by measuring the tensile force at the time of fruit drop when pulling the fruit from the tree. Fruit adhesivity during the growing season (July to September) tended to decrease as maturity approached, but this was not statistically significant. On the other hand, fruit adhesivity when measured at artificial drop angles of 0°, 30°, and 60° based on the fruit stalk decreased significantly as the angle increased. Using an acceleration sensor, the natural frequencies of the inner, middle, and outer parts of the scaffold branch (in the direction of the main stem) were found to be 52.0 cycles·min<sup>-1</sup>, 332.2 cycles·min<sup>-1</sup>, and 346.5 cycles·min<sup>-1</sup>, respectively. The natural frequency according to the movement direction of the scaffold branch was higher for horizontal displacement (874.2 cycles·min<sup>-1</sup>) than for vertical displacement (443 cycles·min<sup>-1</sup>). The natural frequency of the fruit was measured and found to be 420.5 cycles·min<sup>-1</sup>. Additionally, as a result of measuring the natural frequency after binding 60 cm parts (from the main stem) of the scaffold branches, the natural frequency was found to be lower than the control (no binding), and the vertical displacement was lower than the horizontal displacement. Using a wind tunnel, fruits did not fall at wind velocity of 0 to 9 m·s<sup>-1</sup>, and the average displacement (up to 17 mm) tended to increase as the wind velocity was increased. In an experiment using a shaking table, displacement values in the range of 5 to 30 mm were applied based on the displacement coefficient calculated in the wind tunnel experiment, and the bearing branch of the apple tree was shaken using a ‘pendulum’ type of vibration method. The fruit drop began with a displacement coefficient of 10 mm, and the time required for fruit drop shortened as the displacement coefficient was increased. In the relationship between the displacement coefficient and the wind velocity, a displacement coefficient of 10 mm indicated a wind velocity of 6 to 9 m·s<sup>-1</sup>, but this result may lead to fruit drop even at lower wind velocities because certain cultivation characteristics (e.g., tree form, number of leaves, fruit yield, abscission layer maturity of fruit) and natural wind characteristics (e.g., gusts, eddies, wind direction) were not taken into account. Wind can cause primary displacement in apple trees containing fruits and can then induce secondary displacement due to the weight of the fruit and the elasticity of the branches, and it is presumed that these primary and secondary displacements cause fruit drop.