Ventral Fins and Strakes, a Multi purposed Aerodynamic surface
Ventral is defined as the area relating to the underside or belly. Any fin located on the back portion of the aircraft towards the belly is called a ventral fin or strake. Strakes are short but very wide, while fins can be more like winglets, shorter but longer in span.
You may have noticed aircraft design evolving on many aircraft to gradually incorporate these modifications. For example, the original EMB 145 and CRJ 900 were strakeless until recently when the Next Generation models have included strakes on the raise back portion of the fuselage.
Interestingly, strakes in some situations are used on aircraft that are just too laterally stable and have difficulty rolling. By introducing Strakes or fins some lateral instability is introduced to improve responsiveness. An example of this use of ventral strakes or fins can be seen on T plane empennage's. The horizontal stabilizer being on top of the vertical stabilizer pulling it out of the down wash of the wing makes the aircraft very stable laterally, so the strakes on the bottom of the fuselage are used to combat this.
Conversely, aircraft that are not directionally stable, are given ventral fins to increase directional stability. Shorter aircraft that are naturally unstable, or airplanes that experience vertical stabilizer shadowing at high AOA, where the relative airflow is blocked by the fuselage being in the way are often given ventral fins or strakes to improve directional stability.
Dual strakes in the aft portion of the airframe, streamline and control the turbulent air. I suspect somewhat similarly to vortex generators(but not sure). This produces a high pressure area and reduces drag, particularly on aircraft with a substantial raise back on the fuselage. Beech Duke and King Air airplanes are examples of aircraft that use strakes for this purpose. Aircraft that carry pods (these pods create a raise back on the fuselage) also see a noticeable improvement in fuel economy and TAS from these modifications.
Some good examples.
Aircraft with Ventral Strakes
In Summary -
Ventral Strakes and Fins
- Reduce Lateral stability, causing the aircraft to be more responsive in the roll
- Increase Directional Stability, could be of benefit to a twin engine aircraft with engine failure, by reducing Vmc.
- Reduces drag caused by fuselage up sweep, improving TAS and fuel burn