Aspects of the Light Aircraft Engine Mounts Strength analysis

What is an Aircraft Engine?

As it is said in common articles about the Wing Structure and Aerodynamics , Engine in heavy than air flying is used to create Thrust – that’s the main difference of the Airplane from the glider!

Modern commercial and military aircraft use 3 main types of engines: Turbojet, Turboprop and Piston.

Turbojet engines have the most powerful Thrust-to-Weight Ratio which allows them to develop the highest speeds – that’s why they  are typically used on the long-range commercial (cargo and passenger) airplanes – like Boeing 737/747/767/777/787, Airbus A320/330/350 etc., military airplanes: fighters, bombers, transports (Lockheed-Martin F-22, Northrop B-2, Lockheed C-5 Galaxy and many others).

Turboprops are now may be met on almost all types of airplanes, except only Fighters which strongly require supersonic speeds: from super-heavy transports like Antonov An-22 and strategic heavy bomber/missile platform Tupolev Tu-95 to light attack airplanes like EMB 314 “Super Tucano” and modern utility airplanes like Quest Kodiak 100 or Cessna 208 Caravan designed for multi-purpose usage.

Antonov An-22 with 4 × Kuznetsov NK-12MA turboprop engines


EMB 314 “Super Tucano” attack airplanes with Pratt & Whitney PT6A-68/3 turboprops

Why are turboprops so “popular”? The answer is hidden in the questions “what is a turboprop?” and “how does a turboprop engine work?” Without going into details, the turboprop uses a gas turbine core to rotate a propeller. It burns the fuel to release the hot exhaust gas which rotates the turbine blades and a propeller on the same shaft. More complex engines (An-22) have 2 shafts rotating the propellers on opposite sides. That is it’s difference from the Piston engine which work principle is same as for the car combustion engine (gas burning in a cylinder and pushing a piston which is connected to a shaft with attached propeller) and jet engine which operating principle is based on burning of fuel in air to release hot exhaust gas which rotates the turbine blades, but without propeller.

Turbojet and Turboprop engines illustration


Obviously turboprop is more fuel-efficient than a turbojet because it requires less energy to rotate the propeller than a turbojet to create the powerful jet stream. Generally, turboprop airplanes have lower service ceilings than turbofan or turbojet powered airplanes, they are also unable to reach supersonic speeds, but it is obviously not necessary for, i.e., light multi-purpose airplanes, military transports and light attack airplanes working on low altitudes. The era of piston engines is still not over: of course, nobody will now equip by them Fighters or Bombers (as it was being done in 1930-40s) because of their very low power at the same dimensions in comparison, for example, turboprop vs piston, however, Piston engine now takes its worthy place in very light airplanes segment and even in the aircraft segment with MTOW up to 7 tons. The reason is simple: piston engines are fuel-efficient (in some ranges and flight modes they may be even more efficient than Turboprops) due to the aid of modern technologies of combustion engine design; they also have very “user-friendly” servicing possibilities, - almost same as for the automotive engines: any mechanic without expensive special equipment can refuel the engine, change the oil or replace the spark plug in field conditions, - no need of ferry flight to the manufacturer’s site, - that’s why piston engines are still used on airplanes with the max speed up to 300 km/h and service range up to 1500-2000 kilometers. Cessna “Skyhawk” 172 and “Skylane” 182, some modifications of Vulcanair P68 and many others are living examples of immortality of the use of piston engines in aviation.

How does an aircraft engine mounted and what is an engine mount?

The Engine Mount is intended to transfer all forces from the engine, propeller, nacelles (hoods) and other installation units to the aircraft structure. The engine mount must be able to take all loads that occur in flight; absorb vibrations of the engine and propeller; be robust with minimal weight; compensate for the thermal deformations of the engine housing; provide ease of structural assembly and disassembly.

The structural scheme of the engine mount to the aircraft structure depends on the type of engine, as well as on the layout of the propulsion system itself. The modern trend is to create quick-detachable propulsion systems which design allows the replacement of engines on an aircraft in a short time during the repair process. In most cases, turbojet engines are mounted on the aircraft using separate support units, brackets, rods or special units located along the engine mountings. Structural elements are made of mechanically and heat-treated special steels, aluminum (magnesium) alloys, hot stamping, forgings or castings. Without going into details of the commercial airplanes turbojet engine mounts design, let us say that they are typically mounted on pylons out of the wing:


Turbojet engine mounting structure of a modern commercial airplane

Propeller-driven aircraft engines are mounted with use of spatial rod systems, support belts-rings or Beam plus Rod combinations. A feature of these systems is the presence of powerful shock absorbers to mitigate vibrations induced by the engine and propeller.

Small aircraft Engine Mount system

1 – Aircraft Engine (Piston engine is shown, Turboprop may be also installed)

2 – Fire barrier wall

3 – Engine mount system (frame rods)

4 – Fuselage part

5 – Engine Mount Beam-Rod system

6 – Engine Mount Brackets

7 – Engine mount attach bolts + rubber dampers – “mounting pads”.


Engine mount analysis steps

The first step of any structural analysis is to define the loads. Aircraft engine mount loads are the following and acting at once:

- Action of the engine weight with the overloads according to FAR/CS23 or FAR/CS25

- Propeller thrust with required margin of safety

- Engine Torque reaction, also with required margin of safety

- Loads induced by the Engine and propeller gyroscopic moments

All loads exerted on the engine and propeller are distributed through the engine crankcase and reacted at engine mount fittings and finally – at mounting pads.

Typically the Engine is assumed to be a rigid body with inertial, thrust, torque and gyroscopic loads derived at different locations on the crankcase. To simplify the analysis all load factors are derived to one point called “engine focal point” which typically lies on the engine center of gravity (C.G.). This point is defined as Loads Application Point. Loads derivation procedure is described by the following equations:

Where:

MM – pitching moment of the engine to the middle value of C.G.

MV – pitching moment of the propeller to the middle value of C.G.

GM – engine weight

GV – propeller weight

xM – distance between engine C.G. and middle value of the airplane C.G.

xV – distance between C.G. of the propeller and the middle value of airplane C.G.

g – gravity acceleration (g = 9.81 m/s2)

Analyzed aircraft C.G. dimensions


Engine mount load due to engine torque:

According to CS23, torque moment should be recalculated as follows:

Where:

M K – engine torque moment

M Kp – recalculated engine torque moment according to FAR/CS23

P – maximum engine power

k – coefficient from FAR/CS23

ω – shaft rotation rate

n – engine RPM with full power

Gyroscopic moments from Engine rotor and Propeller:

where:

J – propeller mass moment of inertia related to the axis of rotation

m V – propeller mass

L – propeller diameter

M Y – gyroscopic torque related to Y-axis

M Z – gyroscopic torque related to Z-axis

ω X , ω Y , ω Z – angular velocities

n – propeller RPM

m – engine and propeller mass

All these factors, separately calculated for each load case according to FAR/CS23 regulations are to be put in the table of proposed format:

Table Step 1 – Default conditions for different load cases

LC

Roll (ω X )

Pitch (ω y )

Yaw (ω z )

Fwd

Side

Upw

% Thrust

1

0

1

2.5

0

0

2.5

1

0

0

0

40

0

1

0

After calculations of all force factors in Table Step 1, go into Table Step2 with force/moment values:

Table Step 2 – Force/Moments for all cases after recalculation:

LC

Inertial Loads at Engine C.G.

Propeller Gyroscopic Moments

Thrust Load

Engine Torque

Fx

Fy

Fz

Mx

My

Mz

Mx

My

Mz

Fx

Mx

1

295

-20

-954

-67

156

17

0

17526

4789

-1245

-6547

-14254

0

-421

0

0

0

0

0

0

0

0

Finally, the loads which should be put into finite element analysis software (i.e., MSC.Nastran , ANSYS or CosmosWorks ) are to be summarized in Table Step 2 and shown in Table Step 3 as 6 components – 3 forces and 3 moments:

Table Step 3 – Final loads to be used for the analysis in FEM Software:

LC

Applied Loads at Load Point

Fx

Fy

Fz

Mx

My

Mz

1

-758

-17

-964

-6217

-15789

5978

-14275

0

-458

0

15475

0

Finite element model is typically created using BAR/ROD elements, Rigid-body elements are representing the rigid connections of an engine body with the Mount frame. Some engine mount brackets may be modeled using 2D/3D elements and analyzed locally. What is finite element analysis and how finite element analysis works you can read in the article about the work of stress engineer .

Analysis results are typically shown in deformations and Von Mises Stress:

FEM Analysis results: Von Mises Stresses and Deformations

It is necessary to say that the vibrations take a special place in engine mounts analysis process: natural frequencies of the Engine and Engine Mount should not match the frequencies which occur in the Engine during flight. FEM Analysis tools like MSC.Nastran allow to analyze the natural frequencies or the frequencies induced by the periodical impact. Frequency of periodical forces from the Engine in flight may be taken as the Engine RPM in regular flight as the “first trial”.

Results of stress and vibrations analysis are compared with allowable values and if some of them are not acceptable – the engine mount design has to be changed in the areas where the problems occur.