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PostPosted: Mon Mar 20, 2006 19:16 
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Hi mole,

I think we're tripping over the definition of the word 'power' here; I agree that it seems trite and simplistic to simply state that a reaction engine is producing no power (in the sense that we traditionally understand the word to mean) just because it is sat stationary. After all, an engine running at max RPM strapped to a 747 sat at the end of the runway looks so powerful, surely something is going on? A hair drier must be producing some power even though its just sending a stream of hot air out of it...
I have a few ideas to play around with, but I need to go and do a bit of research. I'll get back to you.


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 Post subject: Re: Aircraft power
PostPosted: Mon Mar 20, 2006 21:11 
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Mole wrote:
Nah! I've never studies aeronautical engineering so I can't argue that this isn't what's taught in the profession but it really doesn't work in automotive engineering. The engines ARE doing work, they're moving something with a mass through a distance in a particular time.


OK, got it...just need to hunt around the wood to find some trees :lol:
To find the answer I had to look away from the standard use of a reaction jet engine (to move an aircraft) and to look elsewhere, i.e. where might a jet be used in another application. Aha :idea: In an MRD of course. A what you ask :?
Oh sorry, an MRD - Machine Runway Deicer. The military are great for acronyms :roll:
Basically, 2 * Derwent turbojets strapped to a chassis with a diffuser nozzle in place of the jet exhaust nozzles pointing roughly towards the ground pushed along by a fuel bowser to which it was attached. This heath-robinson device was used to clear runways and aprons of snow and ice during the 70s and 80s; basically a 'kin enormous hairdryer. So, this device must develop power even though its not pushing the bowser, its being pushed itself right? Yes! Providing we know the amount of thrust being produced and the actual exhaust gas velocity at the nozzle exit we can work out the (new term to me here), e(equivalent)hp. This isn't something normally considered partially because, for a given engine RPM we can change the exhaust exit velocity simply by changing the shape of the exit nozzle. In fact, we can almost reduce it to zero by designing a crappy stupid nozzle that simply slowed the exhaust gas down and passed it to atmosphere, but pedantic arguments aside the basic idea works so hurrah for that discovery!

BTW, picture the scene...its 05:00 hours on a freezing February morning and you are sat in a plywood cabin behind two 30 year old turbojets which were recalcitrant SOBs to start due to their tendency to overtemp, and 10 ft behind you are several thousand gallons of aviation fuel :shock: Eee we earned our corn back then lad... :lol:


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PostPosted: Mon Mar 20, 2006 23:59 
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I don't envy the military the risks they end up having to take - even in peacetime!) but I Do envy them the toys they get to play with! My brother in law is an air traffic controller in the RAF and he has some good tales to tell now and again. For me, the closest I've ever been to a working jet engine (well, OK, gas turbine) was a tractor-pulling contest. Some mad SOB had one of these planted on a tractor chassis and then had a saddle on it! I really didn't fancy sitting astride some knackered old jet with turbine blades spinning at unpteed quadzillion RPM right under my "family allowance" but he seemed to enjoy it!


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PostPosted: Tue Mar 21, 2006 09:25 
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Mole wrote:
I don't envy the military the risks they end up having to take - even in peacetime!) but I Do envy them the toys they get to play with!


Oh yeah!!!! Ground running a Harrier jet or a C130 Hercules up to full power is one hell of an experience. Even managed to get a trip in a 2 seat Harrier once....now THAT my friend is something you don't forget in quick time. When you're zipping across mother earth at 250 ft and 500 kts and you have to fly DOWN the side of a valley...wow :shock:

Mole wrote:
IFor me, the closest I've ever been to a working jet engine (well, OK, gas turbine) was a tractor-pulling contest. Some mad SOB had one of these planted on a tractor chassis and then had a saddle on it! I really didn't fancy sitting astride some knackered old jet with turbine blades spinning at unpteed quadzillion RPM right under my "family allowance" but he seemed to enjoy it!


Yeah, that'll be a Gnome turboshaft engine used in Seaking helicopters and others. There are a fair few kicking around for sale, trouble is most enthusists who get a hold of one don't always manage to get all the ancilliary kit they need, the High Energy ignition system for starting for example. One guy we saw was starting his Gnome by spinning the starter and chucking a fuel soaked rag into the intake :shock: It didn't last very long..


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PostPosted: Fri Apr 21, 2006 02:23 
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When I went up to Finningley I was told about a little mishap that occoured when they were a dispersal field for the Vulcans, the CO was taking a new guy out for a check ride and called in for a long field approach (one that uses the whole runway, normally reserved for a plane that is battle damaged), gets the ok and comes in using all the runway to roll to a halt never having deployed the braking parachute.

About half an hour later an irate farmer phones up complaining about this bloody parachute on his field.

Turns out the braking chute was deployed instead of the under carraige (the levers are next to each other) so when the Vulcan stopped in mid air they cut the chute loose.

Hairiest moment for me was hot refueling (filling up with the egine running) :shock:

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