ViabilityStability and Control Propulsion Performance and Handling General Issues The Carter Hummingbird aerobatic aircraft is an original configuration with no flying precedent. It is therefore quite reasonable to ask the question: But will it work? This section is an effort to address this question by identifying potential problems along with possible solutions. No attempt is made here to hide flaws in the concept; to the contrary, Nature cannot be fooled, and since Hummingbird is to be built and flown, it is preferable that any potential problems be addressed from the beginning. To simplify the treatment of the subject and provide some order to it, we will address the viability of the Hummingbird configuration under five headings:
Some of the following points have been called Objections, since the perceived problem is not a real problem at all. Real problems are offered solutions.
StructuresObjection: The stators look too flimsy to support the aft fuselage.
Answer:
Problem: The duct will deflect with wing bending, forcing large tip clearances, which defeats the propulsive purpose of the duct.
Solution:
That being said, we are not relying on optimum tip clearances to achieve hover performance. Any propulsive benefits attainable from tight tip clearances will be a bonus, not a requirement. We will have a more accurate estimate of how far we can limit duct deflection when an FEA study is performed.
Problem: Synchronous belts require tight limits on sprocket alignmentwithin 1/4 degree to meet spec, and never more than 1 degree. How can the engines be kept so closely aligned to the propeller axle?
Solution:
In the unlikely case that sufficient structural stiffness cannot be created to maintain sprocket alignment within limits, the small sprockets will be mounted directly to the axle and driven through a flexible coupling.
Problem: The propeller blades may strike each other during violent maneuvers.
Solution:
Stability and ControlProblem: At idle power and flat pitch, when the propellers are creating drag, dirty air behind the duct will disturb airflow over the tail surfaces, leading to loss of stability and control.
Solution:
Hummingbird will be more stable with partially blanketed tail surfaces than are conventional aircraft. The duct, stators, propellers, and winglets are all behind the CG and contribute to static stability. There is no huge propeller far forward of the CG destabilizing the aircraft. Propeller pitch will be limited under normal conditions so that the blades cannot be set finer than that required for hover. Drag mode will require a positive action from the pilot. In the event of propeller pitch control failure, the blades are designed to automatically feather under aerodynamic and centrifugal loads when the system is turned off.
Problem: If the duct is allowed to stall before the wing, the airplane will pitch up, leading to a region of pitch instability approaching the stall.
Solution:
Due to the positive camber of the duct (inlet area greater than outlet area) the upper duct surface will have positive incidence, the lower duct surface negative. The inboard wing is at zero incidence and will lower the ducts effective angle of attack. Thus the duct outer surface and inboard wing airfoils are designed for a soft, simultaneous stall, minimizing any pitch trim changes. The duct section in particular has an extremely soft stall, which when combined with the circular geometry will ensure that the duct remains flying well after the outboard wings have stalled, meaning that the aircraft will need to be held in a stalled condition with up elevator.
PropulsionObjection: You will never get enough thrust to hover.
Answer:
The fact is that Hummingbird has sufficient power to weight and propulsive efficiency to hover without a duct at all. Some conventional aerobatic aircraft do have sufficient thrust to hover, but they waste energy trying to control the torque. Hummingbird will be comfortable and efficient in the hover. It will not be a struggle. The real issue for Hummingbird is not whether it will hover, but how fast it will climb vertically. The best current estimate for vertical climb rate is about 25 knots at sea level, dropping off to zero at a hover ceiling of about 6000 feet.
Objection: According to Russian experience with counter-rotating ducted fans, the two fans must be synchronized for the system to work.
Answer:
Objection: Those belts look too small to take all that power.
Answer:
When, as expected, Hummingbirds engines are uprated to 150-160 hp, the belts will be upgraded to a new generation of synchronous belts presently in field testing and due for release in 1998. The new belts are promised to improve power density by 50% over existing types, allowing the present 62 mm belt width to be maintained with the larger engines. This is important for Hummingbird, since any increase in belt width would force major changes in the surrounding structures, and may even require an increase in duct chord.
Performance and HandlingObjection: With all those surfaces, this thing is going to be slow.
Answer:
Since there is so much surface area on the airplane, the drag of that surface area must be reduced as much as possible. The wing airfoil is only about 11% thick outboard of the duct, and is being designed for laminar flow back to about 25% chord (60% chord inboard of the duct). Analytical results of current airfoils show significant drag reductions over competing aerobatic airfoils. The duct outer surface is predicted to have laminar flow to about 50% chord and relatively low drag. All airfoil intersections are at 90 degrees, while fuselage cross-sections transition smoothly through the duct, minimizing interference drag. Fuselage, landing gear, and cooling drag are low. Power to weight ratio and propulsive efficiency are high. Hummingbirds speed will be surprising.
Objection: The thing may not snap worth a damn.
Answer:
Objection: The thing is so symmetrical, you wont be able to tell whether it is knife-edge or upright, so it wont make any difference.
Answer:
The long landing gear legs will be the visual cue of whether the aircraft is upright or inverted.
General IssuesObjection: I accept that the concept will work, but it is very complex.
Answer:
For the sake of comparison, the Lancair 235/320 is representative of a popular, successful, experimental aircraft kit of moderate cost. The following compares the complexity of the two aircraft by counting molds and major fabricated parts (considering just airframe, landing gear, and control systems): |
Lancair | Hummingbird | |
Mold Count | 41 | 61 |
Parts Count | 119 | 144 |
Objection: The aircraft will be very difficult to build. Answer:
In summary, Hummingbirds construction processes are more involved than for conventional aircraft and require careful planning and clear thought. Taking each step at a time, however, the process is no more difficult. |
© Copyright 1992-2009 Philip Carter
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