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Windmilling?

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Sometimes i see, jet turbines slowly turing. How does this happen? Is the engine at really low RPM? or is it the wind? Lots of pilots call it windmilling.

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It's the wind (I assume you mean when at the gate). Jet engines use ball and roller bearings and the "break-away" friction is actually quite low. Interestingly, if there is too much of a tail wind (up the exhaust pipe) the engine cannot be successfully started without danger of a "hung start" or "hot start", and the aircraft must be pushed back and turned to better align with the breeze.It takes about 100-200 horsepower to start the engines on a large aircraft. 727 for example used an ATS100-129 100 hp starter made by Garrett (now Honeywell).The term wind mill is also used to describe an "air start" or "windmill start" if the engine flames out and the on-board cross-bleed from a remaining engine doesn't work. You must put the aircraft into a dive sufficient to achieve enough airspeed to get both spools turning, but particularly the HP (inner) spool. If it's speed decays to zero ("core lock") you may be screwed.

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It's the wind (I assume you mean when at the gate). Jet engines use ball and roller bearings and the "break-away" friction is actually quite low. Interestingly, if there is too much of a tail wind (up the exhaust pipe) the engine cannot be successfully started without danger of a "hung start" or "hot start", and the aircraft must be pushed back and turned to better align with the breeze.It takes about 100-200 horsepower to start the engines on a large aircraft. 727 for example used an ATS100-129 100 hp starter made by Garrett (now Honeywell).The term wind mill is also used to describe an "air start" or "windmill start" if the engine flames out and the on-board cross-bleed from a remaining engine doesn't work. You must put the aircraft into a dive sufficient to achieve enough airspeed to get both spools turning, but particularly the HP (inner) spool. If it's speed decays to zero ("core lock") you may be screwed.
Geez 100hp to start a engine! I know about windmill starting, i was refering to at the gate. Do you know how much HP the APU turbine/unit has on a typical airliner 737/757

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As I recall, the 747 apu (GTCP600) provided the equivalent of 600 hp in pressurized "bleed air" (at 350 degrees F) and shaft power at the electrical generator pads. For smaller aircraft it would be proportionately les, but still pretty significant.

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The term wind mill is also used to describe an "air start" or "windmill start" if the engine flames out and the on-board cross-bleed from a remaining engine doesn't work. You must put the aircraft into a dive sufficient to achieve enough airspeed to get both spools turning, but particularly the HP (inner) spool. If it's speed decays to zero ("core lock") you may be screwed.
In the C-130 when faced with a bad starter valve and a remote location we have fast taxied the aircraft to get a windmill start. Obviously the length of runway has to be sufficient and care has to be taken with assymetric handling as the engine starts. I'd be interested to know if any Capt Sim Herc drivers out there can simulate that in FS?

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In the C-130 when faced with a bad starter valve and a remote location we have fast taxied the aircraft to get a windmill start. Obviously the length of runway has to be sufficient and care has to be taken with assymetric handling as the engine starts. I'd be interested to know if any Capt Sim Herc drivers out there can simulate that in FS?
Nope. No windmill starts. :( That reminds me, though. I was witness to a nincompoop trying to "wind start" a Cessna 337's rear engine--on a 5500' long runway (KRUQ). Anyway, it didn't work. He came back to the fuel pumps while I was on the ladder refueling and asked if I could help hand prop it... I suggested someone with "bigger arms" :( .

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