# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DbkIESYAldU
- **Gondola URL:** https://gondola.cc/posts/68785338-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/e7c1d0f388.jpg
- **Posted:** 2026-08-03T04:26:11.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

Pull the cowling off a King Air and the PT6 looks like somebody installed it backwards.

They did not. It is a reverse-flow engine, and every bit of that layout is deliberate.

Air enters a scoop near the propeller, travels aft along a duct, then turns 180 degrees through a screened annular inlet at the very back of the engine. From there it flows forward, against the direction of flight.

The internal running order, front to back:
- Propeller and epicyclic reduction gearbox
- Exhaust stubs, sitting just behind the spinner
- Power turbine
- Compressor turbine
- Annular reverse-flow combustor
- Centrifugal impeller
- Three or four axial compressor stages
- Inlet screen and accessory gearbox

Air moves forward through the axial stages, then the impeller flings it radially outward into a ring of diffuser tubes. Those tubes turn the flow aft into the combustion liner. Fuel is added, the mixture burns, and the gas reverses a second time to head forward into the compressor turbine nozzle.

Two reversals sound wasteful. They buy a very short, very stiff engine, and they put the hot section at the front where a technician can actually reach it.

The second trick is the free turbine. The compressor turbine drives only the compressor and the accessory gearbox. The power turbine sits on a separate concentric shaft with no mechanical link to the gas generator whatsoever. Nothing but hot gas connects the two.

Typical PT6A figures:
- Gas generator around 38,000 rpm at 100 percent
- Power turbine around 30,000 rpm
- Reduction gearbox roughly 15:1, propeller near 2,000 rpm
- Around two thirds of the extracted energy goes straight back into driving the compressor

Because the shafts are independent, the propeller can be feathered while the core keeps turning, and the starter only ever has to spin the compressor.

Sometimes the elegant answer is to turn the whole thing around.

#stemantics #turboprop #aviation #engineering #aerospace

## Stats

- **Views:** 11,046
- **Likes:** 164
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- **Comments:** 1

## Tags

engineering, aviation, stemantics, turboprop, aerospace

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