Propellers

No propellers currently exist which satisfy Hummingbird’s requirements. Existing propellers are too heavy and/or flexible, and they will not accommodate the axle-mounted counter-rotating configuration. For many reasons, propellers must be designed and built from scratch.

The specification for Hummingbird’s propellers is challenging. They must function 100% reliably in an extremely hostile environment. To accommodate the axle there must be a large hole through the rotational axis. For optimum propulsive efficiency, from hover to high speed, they must be fast-acting constant speed units, preferably controlled by software. Blades must be stiff enough to prevent their striking each other during violent maneuvering. A preliminary weight goal for 85 inch propellers is 30 lbs each.

The units are each three-bladed. Dynamic effects are relieved by using an odd number of blades, since only one blade (per propeller) passes through fin/wing turbulence at a time. Since the two propellers each have three blades, the aft blades will simultaneously pass through the wake from the forward blades about 190 times per second, creating a unique low-frequency sound.

While this could be corrected by using different blade numbers on each propeller, that approach turns out to be impractical. Two and four blades are undesirable for reasons stated above. Five blades would lead to blades of significantly smaller chord, which would be structurally more flexible, and the blades and hub would turn out heavier, negating any aerodynamic benefits. So we will live with the 190 Hz hum - likely it will become one of Hummingbird's trademarks.

The hubs are of large diameter (around 24 inches) in order to satisfy structural and engine-packaging concerns. This large-diameter hub does cut into disk area slightly, but the penalty can be negated by increasing duct diameter fractionally if required. Also, since blade twist can cause the blade roots to stall at low and zero airspeeds (when disk area is most valuable), the large-diameter hub may actually enhance propulsive efficiencies.

The propellers rotate relatively slowly. A 3.2:1 reduction from 6000 rpm leads to a propeller speed of just 1875 rpm. There are several reasons for turning the propellers so slowly:

• The gyroscopic inertias of the propellers are reduced at lower rpm, lowering resistance to attitude changes and loads on propellers, bearings, and axle.
• Centrifugal forces, and thus loads on hubs and blades, are significantly reduced.
• Tip speeds are kept well below the transonic realm.
• Blade area is inversely proportional to the square of the rpm. Thus, reducing rpm results in blades of significantly larger chord. Large-chord blades, properly designed, will be significantly stiffer than narrower blades of the same weight—a very important consideration for this airplane. Furthermore, wide-chord blades enhance the ability of the propellers to create drag power-off.

Propeller Structures

Hummingbird’s propeller hub geometry is so unique, it has been necessary to rethink how a propeller hub should be configured and built. The opportunity is to take advantage of the structural performance, fatigue resistance, and ease of manufacture of carbon composites in these highly-stressed structures.

If Hummingbird were to lose a propeller blade the aircraft could very well break up in flight, since the duct and stators are primary structure. Even if the duct and stators were to survive, the tail surfaces remain vulnerable. Every effort is being made to create fail-safe propeller structures because any risk of throwing or contacting blades in Hummingbird is considered unacceptable. In the case of a large bird passing through the propellers, a ballistic recovery system is standard equipment.

A hub design has been developed which is built up from seven simple carbon/epoxy laminations, requiring just three molds. The only metal parts are the bearings and titanium sprockets, bearing housings, blade retention hardware, and fasteners. The structural configuration renders delamination impossible; the only possible mode for hub failure is to break carbon fibers in tension.

The blade structures are similarly innovative, being more damage tolerant and of higher structural efficiency and than existing composite blades.

The creation of light, reliable, efficient propellers is key to Hummingbird’s success. Further details on our propeller structures are not being disclosed at this time.

Pitch Actuation

Propeller pitch actuation has proved to be one of the most difficult design problems in the airplane. Hydraulic, pure electric, and electric/mechanical systems have been investigated. Propeller pitch is a critical system in Hummingbird because the aircraft may not be flyable if an actuator fails at flat pitch.

The location of the propellers slightly aft of the CG allows great flexibility in using propeller pitch to control both thrust and drag. A decision was made early on to take maximum advantage of this opportunity by having software control blade pitch.

While the presence of two independent electrical systems on the airplane enhances the reliability of an electric pitch actuation system, creating sufficiently fast electric propellers is a challenging problem. Hummingbird will be capable of changing airspeeds and power settings very rapidly, and the blade pitch actuation system must be able to keep up.

An early actuation system placed three servos off-axis in the propeller hub, but centrifugal forces turn out to preclude this simple approach. After considerable effort an electric propeller pitch actuation system has been devised which is as innovative as the rest of the airplane, as fast as a hydraulic system, and intrinsically fail-safe. Further details are proprietary at this time.