Rotorcraft Tail Boom Protuberances for Negative Lift Reduction
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Solution Overview
Problem
Conventional tail booms generate significant negative lift during hovering and climbing flights due to air stream impact, which reduces rotorcraft climbing speed and payload capacity, and modifying them to minimize this effect while maintaining structural integrity and avoiding additional drag is challenging.
Innovation Solution
A tail boom with elongate protuberances extending downwards from the bottom zone, securely attached to the structure, reduces negative lift by controlling the air stream and minimizing wake formation, with specific dimensions that balance weight increase and drag reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tail boom has a large relative thickness to provide mechanical strength and reduce lateral drag, then structural strength and lateral drag reduction are improved, but negative lift increases during hovering and climbing flights
Solution Approach 1:
The tail boom is segmented into a main body and separate protuberance elements. The protuberances are attached to the bottom surface of the tail boom, creating distinct functional zones: the main body provides structural strength and lateral drag reduction, while the protuberances specifically address negative lift generation by modifying the air stream flow pattern underneath the tail boom.
Solution Approach 2:
The solution adds a new dimensional element (protuberances extending downward) to the existing tail boom structure. This vertical dimension modification allows the air stream to flow smoothly underneath the protuberances, preventing flow separation and reducing negative lift without compromising the horizontal cross-sectional dimensions that provide structural strength.
2Force
If the tail boom has a large relative thickness to carry tail fin and secondary rotor, then load-bearing capacity is improved, but negative lift increases
Solution Approach 1:
The tail boom structure is divided into load-bearing components (main body with adequate thickness) and aerodynamic components (protuberances). The main body maintains the necessary thickness for carrying the tail fin and secondary rotor, while the protuberances are added as separate elements that specifically address the negative lift issue through their aerodynamic shape and positioning.
Solution Approach 2:
The protuberances serve multiple functions: they modify the air stream flow to reduce negative lift, they do not significantly increase weight, and they maintain compatibility with the existing structural design that carries the tail fin and secondary rotor. This multi-functionality allows simultaneous achievement of load-bearing capacity and negative lift reduction.
3Stability of the object's composition
If the tail boom has large surface area for bending stiffness, then aerodynamic stability is improved, but drag in elevation direction increases
Solution Approach 1:
Instead of increasing the horizontal surface area of the tail boom (which would increase lateral dimensions and potentially increase drag), the solution extends the structure vertically by adding downward protuberances. This dimensional change allows the air stream to follow the contour of the protuberances, preventing flow separation and reducing pressure drag in the elevation direction while maintaining adequate bending stiffness through the extended vertical profile.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The protuberances effectively reduce negative lift and drag, enhancing rotorcraft performance during hovering and climbing flights without compromising structural strength or increasing lateral drag, and can be retrofitted onto existing tail booms.
Implementation Method 1
The air stream passing through the main rotor in flight can impact against the top portion of the tail boom... This air stream thus impacts against the top portion of the tail boom and splits so as to flow round the tail boom in the form of two air streams. These two air streams may separate from the tail boom... This separation gives rise to considerable wake being created.
Data Source
AI summary
A tail boom having a structure and a protuberance, the protuberance being secure with the structure. In each section of the tail boom, the protuberance extends in elevation over an extension height (Hext) and it extends laterally over an extension thickness (EPext), the extension height (Hext) lying in the range 0.05 times the maximum thickness (EPmax) of the structure, included, to 0.5 times the maximum thickness (EPmax), included, of the section, the extension thickness (EPext) lying in the range 0 to 0.4 times the maximum thickness (EPmax), included, of the section.