制作涡喷发动机(4)

sustains. It can get hot when starting if too much fuel is fed in to quickly. With practice starting is simple and undramatic , there should be no flames visible either during starting or running.

Controlling the speed

控制速度

This is achieved by the use of a speed controller that controls the fuel pump motor. When bench testing it is more convenient to control the speed by a hand operated controller such as a minidrill speed controller rather than use a transmitter receiver and speed controller. When mounted in a plane a motor speed controller is used or if available a fully programmed ECU. Overspeed should be controlled by a combination of battery power and pump pressure so that whatever happens to the receiver or transmitter the supply of fuel is restricted.

Underspeed can be controlled via a pressure switch that stops the engine if pressure drops below a pre determined point.

What is a sensible rev limit?

转速控制

In the Kamps book there is full detail on how to calculate the critical speed for the shaft. The speed will depend on the material the shaft is made of and the thickness as well as the weight of the compressor and turbine and their positions. In practice engines with a 316 or 310 stainless steel turbine should have the revs restricted to about 75,000 rpm. Above this speed the material will expand and cause it to bind on the housing. The Kamps engine is limited to about 103,000rpm and the new Schreckling at 117,000rpm. The higher the revs the more need there is for the engine being well balanced.

How do I balance the engine?

如何做动平衡

Simple static balancing has proved to be adequate. The method in the Kamps book produces good results. He uses a tube that contains 2 new bearings and the shaft to be balanced. This is laid on a flat surface such as a piece of glass and is rocked to and fro until the heavy point reaches the bottom. This is marked and a little is ground away on the heavy side. When the shaft is balanced the compressor is balanced. Though this is from the factory the balance has in practice found not to be accurate enough. The compressor is removed and the turbine wheel is fitted and this is balanced . The blades can be thinned or the roots can be ground away slightly to reduce the weight.

How do I make the turbine wheel?