Personal Helicopter Airframe - Early concept

Structural analysis for personal helicopter airframe

Introduction

Recently we have discussed the type of flight control and navigation equipment that will appear in our experimental personal helicopter. Of course, there are still a few components to model like: landing skids, pilot’s seat, wind shield. Nevertheless, we are slowly approaching the next stage of importing all components to Unreal Engine 4 and we will start work on materials and functionality. Today’s theme is the airframe of a helicopter, whose task is to withstand stresses during the flight, and also its impact for “One Man Helicopter” project, so let’s get started!

Personal Helicopter Airframe - Early concept
Main chassis with pre-designed components

Airframe

Chassis is the most critical constituent in keeping the integrity of a vehicular structure. Likewise, helicopter chassis is like its skeleton. According to Wikipedia, the free encyclopedia the airframe of an aircraft is its mechanical structure. It is typically considered to include fuselage, wings and undercarriage, exclude the propulsion system. Airframe design is a field of aerospace engineering. That combines aerodynamics, materials technology and manufacturing methods to achieve balances of performance, reliability and cost.

Structural stresses

The primary factors to consider in aircraft structures are strength, weight, and reliability. These factors determine the requirements to be met by any material used to construct the aircraft. Airframes must be strong and light in weight. Many forces and structural stresses act on an aircraft when it is flying and when it is static.

When it is static, the force of gravity produces weight, which is supported by the landing gear. The landing gear absorbs the forces imposed on the aircraft by takeoffs and landings.

During flight, any maneuver that causes acceleration or deceleration increases the forces and stresses on the wings and fuselage. Stresses on the wings , fuselage, and landing gear of aircraft are tension, compression, shear, bending, and torsion. These stresses are absorbed by each component of the wing structure and transmitted to the fuselage structure. The tail section absorbs the same stresses and transmits them to the fuselage. These stresses are known as loads, and the study of loads is called a stress analysis. Stresses are analyzed and considered when an aircraft is designed.

TENSION

Tension is defined as pull. It is the stress of stretching an object or pulling at its ends. Tension is the resistance to pulling apart or stretching produced by two forces pulling in opposite directions along the same straight line.

COMPRESSION

If forces acting on an aircraft move toward each other to squeeze the material, the stress is called compression. Compression is the opposite of tension. Tension is pull , and compression is push. Compression is the resistance to crushing produced by two forces pushing toward each other in the same straight line. For example, when an airplane is on the ground, the landing gear struts are under a constant compression stress.

SHEAR

Cutting a piece of paper with scissors is an example of a shearing action. In an aircraft structure, shear is a stress exerted when two pieces of fastened material tend to separate. Shear stress is the outcome of sliding one part over the other in opposite directions. The rivets and bolts of an aircraft experience both shear and tension stresses.

BENDING

Bending is a combination of tension and compression. For example, when bending a piece of tubing, the upper portion stretches (tension) and the lower portion crushes together (compression). The wing spars of an aircraft in flight are subject to bending stresses.

TORSION

Torsional stresses result from a twisting force. When you wring out a chamois skin, you are putting it under torsion. Torsion is produced in an engine crankshaft while the engine is running. Forces that produce torsional stress also produce torque.

VARYING STRESS

All structural members of an aircraft are subject to one or more stresses. Sometimes a structural member has alternate stresses; for example, it is under compression one instant and under tension the next. The strength of aircraft materials must be great enough to withstand maximum force of varying stresses.

Vibrations

Sources of vibrations in the helicopter originate from: main rotor, tail rotor, engines and other rotating systems such as hydraulic pumps and air forces acting on the fuselage. The frequency of vibrations caused by the main rotor is at integer multiples of the rotor RPM – 1 per revolution (1/rev) is the rotor RPM, then 2/rev, 3/rev, etc. In addition to the main rotor, other sources of vibrations are the engine/fan system, the main rotor transmission/driveshaft/gear system, the tail rotor and its transmission system and loose components that are a regular or external part of aircraft. Examples are out of balance rotor blades, loose tail fins, loose engine shaft mounts, unsecured canopy, the landing gear system or external weapons or cargo systems.

In addition to dynamic coupling, significant amount of aerodynamic interference or coupling exists between the main rotor, the airframe and the tail rotor structures. The flow around the fuselage affects the aerodynamics of the main rotor and the tail rotor. The downwash from the main rotor changes the aerodynamics of the fuselage, the tail rotor and the horizontal tail and stabilizers. Under certain low speed conditions, the vortex wake from the main rotor impinges directly on the tail boom that gives rise to fuselage vibrations at the blade passage frequency.

Main rotor vibrations arise especially in forward flight. The rotor experiences varying fluid velocities and angles of attack at the advancing and retreating blade. Varying span wise distributions of lift and drag excite the blade bending modes. This results in alternating rotor hub loads, especially vertical forces and lateral and longitudinal mast moments. The occurring vibration frequencies are typically a multiple of the blade number and the revolution frequency.

Personal-Helicopter-Airframe-Vibration-damping-solution
Mechanical dampers.

Ground resonance is an imbalance in the rotation of a helicopter rotor when the blades become bunched up on one side of their rotational plane and cause an oscillation in phase with the frequency of the rocking of the helicopter on its landing gear. The effect is similar to the behavior of a washing machine when the clothes are concentrated in one place during the spin cycle. It occurs when the landing gear is prevented from freely moving about on the horizontal plane, typically when the aircraft is on the ground.

CHASSIS SPECIFICATION

There are different types of chassis which comprises of space frames, monocoque, semi-monocoque etc. Among all these different types of chassis, space frame is often used in motorsport event and high performance vehicles. Space frame chassis is commonly used due to its rigidity and minimalism of construction. Usually in space frames, circular or square tubes are amalgamated together in order to form a lattice structure. Usually, for a space frame the load is distributed in axial direction, this ensures that no part of the frame experience severe bending forces.

Monocoque is a structural system where loads are supported through an object’s external skin, similar to an egg shell. The word monocoque is a French term for “single shell” or (of boats) “single hull”. A true monocoque carries both tensile and compressive forces within the skin and can be recognised by the absence of a load-carrying internal frame. The true Monocoque construction uses formers, stress skin, and bulkheads

Semi-monocoque is a hybrid combining a tensile stressed skin and a compressive structure made up of longerons and ribs or frames. In order to build bigger and stronger airplanes, a hybrid of the two construction techniques was put forward and remains in use today.  A semi monocoque airplane’s skin supports much of the load, with some internal bracing and bulkheads in place to maintain structural integrity. This design works surprisingly well, and remains in place on most modern aircraft from single engine pistons to the brand new Boeing 787 Dreamliner.

Materials

An aircraft must be constructed of materials that are both light and strong. Early aircraft were made of wood. Lightweight metal alloys with a strength greater than wood were developed and used on later aircraft. Materials currently used in aircraft construction are classified as either metallic materials or nonmetallic materials. The main group of materials used in aircraft construction has been:

  • wood,
  • steel,
  • aluminum alloys,
  • titanium alloys,
  • fiber reinforced composites.

In aeronautical applications, strength allied to lightness is most important in material selection, when the material is stable in environment conditions. The airframe, or fundamental structure, of a helicopter can be made of either metal or organic composite materials, or some combination of the two.

Composite: Higher performance requirements will incline the designer to favor composites with higher strength-to-weight ratio, often epoxy reinforced with glass, aramid, or carbon fiber. Typically, a composite component consists of many layers of fiber-impregnated resins, bonded to form a smooth panel. Composite components can be formed into complex shapes that, for metallic parts, would require machining and create joints.

Tubular and sheet metal substructures are usually made of aluminum, though stainless steel or titanium are sometimes used in areas subject to higher stress or heat. To facilitate bending during the manufacturing process, the structural tubing is often filled with molten sodium silicate.

Standard aerospace aluminum – 6061, 7050, and 7075 – and traditional aerospace metals – nickel 718, titanium 6Al4V, and stainless 15-5PH – still have applications in aerospace. These metals, however, are currently ceding territory to new alloys designed to improve cost and performance. While carbon fiber reinforced polymer (CFRP) represent the lion’s share of composite material in both cabin and functional components, and honeycomb materials provide effective and lightweight internal structural components, next-generation materials include ceramic-matrix composites (CMCs), which are emerging in practical use after decades of testing.

Ceramic-matrix composites (CMCs) are comprised of a ceramic matrix reinforced by a refractory fiber, such as silicon carbide (SiC) fiber. They offer low density/weight, high hardness, and most importantly, superior thermal and chemical resistance. Like CFRPs, they can be molded to certain shapes without any extra machining, making them ideal for internal aerospace engine components, exhaust systems, and other “hot-zone” structures – even replacing the latest in heat resistant super alloys (HRSA) metals listed earlier. 

Gussets

The latticed beam structure is nowadays preferred for low speed small aircrafts, with low serial of production for the fuselage, empennage and flight control surfaces. The beams employed on the fuselage construction are usually of Pratt or Warren type. Because of weight constraints, the wall thickness of tubes is kept to a minimum, thus, buckling is becoming a major design constraint. In order to improve the general buckling behaviour of members and to achieve a better stress distribution, joints are stiffened by using gussets. Gussets are used in aerospace welded structures in “T” joints or bolting areas to decrease the level of stress or to improve rigidity.

Summary

The above knowledge allowed me to pre-design tubular lattice beam structure airframe with gussets to increase stiffness. I think about using the standard aerospace aluminum, or even magnesium square tubes. In real conditions, the thickness of the walls of our profiles should have great significance, however, considering that we operate in the virtual world at this stage, we will omit this fact partially. However, we cannot omit it completely. We need to calculate the weight of this structure and we will return to this issue at a later stage once the airframe will be completed to get final dimensions.

Also chassis is designed to absorb and reduce vibration generated by engine and rotors with help of mechanical dampers to decrease level of vibration. The final structure should be stronger then previous builds, to allow us to try some acrobatics.

Personal-Helicopter-Airframe-sections
Divided to 5 sections

The whole frame will consist of 5 sections. The 1st for the cockpit space for a pilot seat and devices for measuring flight and navigation. Sector 2 is intended for the main rotor shaft with actuators and an additional vibration dampener, the remaining space will be allocated for placing a lubricant system, electronics from the engine, battery, etc. Next, space 3 is for the engine and its systems like exhaust, water, fuel. Sector number 4 is for the anti-torque device. At the same time, this sector is designed to be able to exchange and apply with other solutions in the future. At the beginning, I will start with the traditional tail rotor solution. And the last 5th section is devoted to landing gears and this is the next component to model.

Subscribe

Hit the red bell in the left corner of your screen to get notification on upcoming updates and releases. Thank you!

Reference: