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I cannot fully understand in simple words the differences between the axial flow engine (Von Ohain) and Whittle's centrifugal turbojet.

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  • $\begingroup$ According to Wikipedia it was the other way around, Von Ohain centrifugal and Whittle axial $\endgroup$
    – sophit
    May 15, 2023 at 17:09

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The centrifugal flow engine sucks air in the center of an impeller and flings it out by centrifugal force as it spins. The energy added to the air by flinging it out compresses it, and the compressed air is passed along to the burner cans. If you've ever seen the impeller of a vacuum cleaner or a water pump, it's pretty much the same thing, except the impeller is being spun by an electric motor instead of a turbine, and instead of a burner can, you have the vacuum bag or water tank.

An axial flow engine's compressor is more like a series of propellers with a shroud around them. It's a large number of spinning wings driving air straight back, like a ducted fan. You could think of the shrouded fan on your computer case as a single stage axial compressor (driven by an electric motor). Put ten of those fans in a stack, with each one boosting the air sent to it by the one ahead of it, with the air passage getting smaller and smaller as you go, and you have a multi-stage axial compressor.

So in a nutshell, a centrifugal compressor forces air to the perimeter of the compressor housing by spinning, then redirecting the air to the burner. The need to fling air at 90 degrees to its original flow path makes the engine quite fat. However the centrifugal flinging action makes the compressor relatively insensitive to flow disturbances or restrictions.

The axial compressor just drives the air straight back to the burner can down a narrowing passage with smaller and smaller sets of "propeller" blades. A much skinnier configuration, and a lot more efficient at compressing the air because you can incorporate many stages easily (you can make multiple stages with a radial compressor, but you have to stack the impellers and it's not desirable to do more than two at most), but since the axial compressor is basically spinning wings driving the air straight back, much more sensitive to flow disruptions, because they are wings that can stall like regular airplane wings.

Crude fuel control systems on these 1st generation engines being little more than a faucet operated by the throttle admitting fuel to the burner can, the Whittle engine was somewhat easier to manage by the pilot thanks to its relatively insensitive centrifugal compressor. The axial engines, while making more thrust with a smaller frontal area, were much more temperamental had to be very carefully monitored and managed when making power changes.

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    $\begingroup$ 1. There are more instances where axial and radial were confused. 2. The Ohain engines also used radial compressors; you describe the difference between Whittle/Ohain (Heinkel) designs on one hand and Franz (Junkers)/ Oestrich (Bramo-BMW) designs on the other hand. $\endgroup$ May 15, 2023 at 5:37
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    $\begingroup$ @PeterKämpf I believe the question already contains this misconception. Von Ohains first engine was a radial turbojet engine... $\endgroup$
    – U_flow
    May 15, 2023 at 6:41

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