February 21, 2022

Engine bypasses for micro turbines

0. Introduction

I have worked on jet engine bypasses for several decades.

Form CFD involved, to manual calculations, to testing on bench and in-situ.

I designed my first engine bypass around a JPX260 for the first Jet World Masters in 1995 ( Ulm, Germany ). This bypass was fitted in an Aviation Design Mirage 2000 C.

Very few companies and engineers have the knowledge and data backing to design a proper bypass. One of them was Bob Violet who was the pioneer in this filed in the 1990. My first design was inspired by his work. As the years passed and I graduated as an aerospace engineer and studied aerodynamics, I tried to understand and then simulate how a bypass system works.

In this blog, I will share with you some initial considerations and main principles to make an efficient bypass system.

1. General

Before diving into practical considerations, here are the pros and cons about a bypass system ( assuming it is properly designed )

Pros:

Greatly reduced drag
Increased thrust
Reduced noise
Cooler engine
Cooler thrust tube operation
Less risk of internal component sucking ( Screws left over, etc...)
No risk of internal fire by fuel drip during a hot start
More scale noise ( if intake and ducts are scale )

Cons:

Requires calculation to be properly setup
Requires to make room in the plane for the ducts
Requires high temperature and fire extinguishing materials
Heavier
Requires more work to install
Requires more attention during service inspections

But one of the most important point is the following one: a bypass system can actually be detrimental to the proper operation of the jet engine ( hot running ) and can pose risks to the airframe ( inlet or thurst tube collapsing under vacuum ) if not properly calculated and installed.