Undercarriage Retraction and Stowage
Retraction is required for aircraft operating at more than 150 to 200 knots. A rapid increase in drag starts building up for speeds of more than 150 knots. There are basically three situations, as shown in Figure 7.6:
1. No Retraction. The fixed undercarriage is primarily for smaller aircraft or larger aircraft that have a high wing and are operating at low speed (e. g., the Twin Otter and the Shorts 330).
2. Partial Retraction (Kneeling Position). A large wheel bogey with restricted stowage space would have to sacrifice full retraction; however, partial retraction helps considerably to reduce drag.
3. Full Retraction. Stowage space must be provided for a wheel bogey (i. e., for higher-speed aircraft).
AJT retraction kinematics
Figure 7.7. Undercarriage stowage space and retraction
Provision for stowage must be made early in the conceptual design phase. Only the space provision, after consultation with structural and undercarriage designers, is sufficient at this early stage of the project. Typical extended and retracted positions of civil and military type aircraft are shown in Figure 7.7. Following are areas where the undercarriage can be stowed:
1. In the Wing. If wing thickness is sufficient, then a maximum of twin wheels can be retracted. Provision for the wing recess is made as early as possible in the design phase. For a thinner wing, if the strut is mounted on the wing, it can go through the wing recess and the wheel to reach the fuselage stowage space (e. g., Learjet 45; although it has a single wheel, the wing thickness does not have sufficient space).
2. In the Fuselage. This is the dominant pattern for a large undercarriage because the fuselage underbelly could provide generous stowage space. If not, then it can be kept outside encased by a fairing that appears as a bulge (e. g., Antanov 225). For fighter aircraft with a very thin wing, the entire undercarriage is mounted on and retracted within the fuselage (e. g., the F104). The coursework example is a high-wing aircraft (see Figure 7.7) and the undercarriage is stowed in the fuselage.
3. In an under-the-Wing Nacelle. High-wing turboprop aircraft have a long strut; therefore, stowing the undercarriage in the nacelle (see Figure 10.19) slung under the wing reduces the strut length (e. g., the Fokker27 and Saab340).
Once the gear is extended, it must be locked to avoid an inadvertent collapse. A schematic retraction path of an AJT also is shown in Figure 7.7. Retraction kinematics is not addressed in this book. It is assumed that during the conceptual design phase, designers have succeeded in retraction within the stowage space provided by
Figure 7.8. Three possible wheel positions
the aircraft engineers. See the references for more details on undercarriage retraction kinematics.