Engines turning at gate?

[Chock 17,769]

The infamy was largely the volume invlolved, but the event that made them infamous in the true sense was several DC-9 crashes owing to icing on the wings, since using reverse thrust could kick slush up onto the upper wing surface, which would of course be occurring after the walkaround. Powerbacks also throw grit about all over the place, and equipment if it is light enough, and are noisy as hell too, plus they use up fuel unnecessarily. Technically, they could also make a skid in icing conditions more likely, since the old clamshell reversers deflect a lot of thrust downwards, and that does actually make the aircraft lighter on its wheels, which was a problem particularly on B737 models with JT8D engines. But in the seventies it was certainly not uncommon for aircraft to have their engines running at the gate, I recall walking out to several old airliners and boarding them whilst the engines were running at the gate back in those days, mostly DeHavilland Comets and that sort of thing, back then there was no such thing as Health and Safety and all the airliner cockpits had ashtrays in them LOL. Al

[Tierborn 191]

The infamy was largely the volume invlolved, but the event that made them infamous in the true sense was several DC-9 crashes owing to icing on the wings, since using reverse thrust could kick slush up onto the upper wing surface, which would of course be occurring after the walkaround. Powerbacks also throw grit about all over the place, and equipment if it is light enough, and are noisy as hell too, plus they use up fuel unnecessarily. Technically, they could also make a skid in icing conditions more likely, since the old clamshell reversers deflect a lot of thrust downwards, and that does actually make the aircraft lighter on its wheels, which was a problem particularly on B737 models with JT8D engines. But in the seventies it was certainly not uncommon for aircraft to have their engines running at the gate, I recall walking out to several old airliners and boarding them whilst the engines were running at the gate back in those days, mostly DeHavilland Comets and that sort of thing, back then there was no such thing as Health and Safety and all the airliner cockpits had ashtrays in them LOL. Al

thanks for the clarity .. and i agree, what you describe would put them in the category of infamy.

[vololiberista 205]

This is a temperature and eflux velocity diagram for just "one" Conway jet on the VC10 at break away thrust!!!vololiberista

[jbassue 181]

This is a temperature and eflux velocity diagram for just "one" Conway jet on the VC10 at break away thrust!!!vololiberista

I've never understood how thrust moving at 300mph, can push the plane way faster.

[Tierborn 191]

I've never understood how thrust moving at 300mph, can push the plane way faster.

Dont anyone say its simple, its not rocket science .. because it is Its not just about the speed of the air being moved .. its also about the mass of the air being moved that creates the thrust. The thrust of the engine is equal to the fluid mass multiplied by the speed at which the engine emits this mass: (thrust) = mc

[jbassue 181]

Dont anyone say its simple, its not rocket science .. because it is Its not just about the speed of the air being moved .. its also about the mass of the air being moved that creates the thrust. The thrust of the engine is equal to the fluid mass multiplied by the speed at which the engine emits this mass: (thrust) = mc

OH, I partially understand, so it's not how fast the air goes but the amount and the speed that this amount travels at? If I got that correct. Thanks ;)Now to my other question, if air gets thinner with altitude, why are more aircraft more efficient at higher Altitudes? To my understanding if there is thinner air = Less air mass = Less mas of air moved by engines = Less efficient?Can the increase in efficiency be attributed toward less Air molecules = Less friction? EDIT : Can you make your answer so the effect of the Jet Stream is negligible, even though it's far from negligible in RL? :(

[Tierborn 191]

OH, I partially understand, so it's not how fast the air goes but the amount and the speed that this amount travels at? If I got that correct. Thanks ;)Now to my other question, if air gets thinner with altitude, why are more aircraft more efficient at higher Altitudes? To my understanding if there is thinner air = Less air mass = Less mas of air moved by engines = Less efficient?Can the increase in efficiency be attributed toward less Air molecules = Less friction?EDIT : Can you make your answer so the effect of the Jet Stream is negligible, even though it's far from negligible in RL?

It is true that you will experience a power drop off at high altitudes ... However, the reason aircraft become more fuel efficiency at higher altitudes is because drag on the aircraft decreases faster than the power drop-off. For example, an aircraft needing an 85% N1 setting to maintain 350kt at 12,000 ft. might only need a power setting of 60% at FL300 to maintain the same 350kts.

[vololiberista 205]

I've never understood how thrust moving at 300mph, can push the plane way faster.

The velocity of 300ph as shown in the diagram is only "break away" thrust. That thrust needed to initiate movement on the ground. Which equates to 68%n2. You can imagine that full power is significantly higher!!!! As is cruise and climb. Climb is 93-96% n2 depending on ambient conditions. Cruise is about 84% n2. The power of the Conway jets are well demonstrated in these videos here

and here

vololiberista

[hontzt 14]

You very rarely see powerbacks anymore. American was infamous for them with their Super 80's.

Yes they were. Especially at DFW. VRTodd

[jbassue 181]

It is true that you will experience a power drop off at high altitudes ... However, the reason aircraft become more fuel efficiency at higher altitudes is because drag on the aircraft decreases faster than the power drop-off. For example, an aircraft needing an 85% N1 setting to maintain 350kt at 12,000 ft. might only need a power setting of 60% at FL300 to maintain the same 350kts.

Wow, thanks guys, you never know what else you learn everyday you check these forums.