[paul cotter (OP)]

About a month ago, outbound at Dublin airport terminal #2, the boss points out signs at all aircraft stands saying "pilots use minimum power when breaking away" or similar wording. I had never seen these before and wondered what significance they held. On return I thought I had figured it out: a number of very large windows had cracks, some with multiple cracks. I concluded that the architect had not foreseen possible resonance effects on these large sheets. Outbound at Dublin airport terminal #1 last Friday I noticed similar signs but truncated to just "pilots use minimum power". Terminal #1 is an older building, more traditional and less flashy than terminal #2. Well Alancalverd, can you enlighten me?

[alancalverd]

Pretty much as you guessed. You are required to start the engines before being towed onto the taxiway (to save embarrassment if they don't start, or catch fire) , and to run the engines up to about 75% power at some point before beginning your takeoff run, but the intervening bit between the terminal and the designated runup point should be taxied at minimum necessary power to avoid all sorts of damage to buildings, vehicles, following aircraft, and any pedestrians who may be going about their lawful business.

I'm surprised that airline captains need reminding, but when flying from a small strip run by an aerobatic school I'm frequently dishevelled by young lads determined to scramble straight from the fuel pump to meet the incoming Hun at full throttle.

[alancalverd]

PS there's often a fair bit of loose crap from sweet wrappers via scavenging birds and their nests  to dustbin lids, around the terminal. The technical term is Foreign Object Debris (you will see FOD bins near the gate) and you really don't want to suck that into a million-dollar engine. 

[Pseudoscience-is-malarkey]

Alancalverd, is Cotter being facetious with his "paragon of aeronautical erudition" business?

[paul cotter (OP)]

Absolutely not. It was meant as a compliment to his obvious knowledge on the subject and I sincerely hope it was received that way. If I got the slightest inclination that it was inappropriate I would delete the expression without hesitation.

[alancalverd]

Dunno about paragon, but it's certainly a subject I take seriously - the alternative being a fiery death. The Wisdom of the Aeronauts says:

After 100 hours, you know everything

After 1000 hours, you realise you don't know everything

After 10,000 hours you know you will never know everything.

Currently hovering around the 1000 hour mark, so on the steep part of the learning curve.

[paul cotter (OP)]

Two more aeronautical questions for you, Alancalverd: ( a ) the perceived wisdom is that a propeller is limited to about 400kts and surely the fan of a turbofan is a propeller by nature, albeit a ducted prop. ( b) in 1983 high over the rocky mountains on a heathrow to lax flight boredom got to me and I went to the last available( rear ) window and peered down at the unfolding landscape. Every now and then, guess it was about 10 seconds, something would flash past my eyes. Curious about this I watched very carefully and eventually determined that there were football sized lumps of soot being ejected in the exhaust plumes of both engines. Obviously at the speeds involved only the faintest glimpse of such an object is possible even with very careful scrutiny. Ever heard of this? or a possible explanation? This was on a Rolls Royce  rb211 powered 747.

[alancalverd]

One limiting factor with propellors is tip speed. As it approaches Mach 1 you get lots of noise and stress for very little additional thrust. Anyone who has heard a T6 Harvard plodding round the sky at 200 mph will know the buzz-saw noise of its huge 2-blade prop. You can get more thrust at high speed by having more blades, but each blade is working in the turbulent wake of its predecessor at low airspeeds, where you want lots of  thrust for takeoff and landing. The compromise rarely exceeds an airspeed of Mach 0.6 (~ 400 mph) with the prop tips just beginning to scream.

The compressor fan of a pure jet isn't required to produce thrust but to stuff lots of subsonic air into the combustion chamber. Interceptors like the Lightning have prominent passive nose cones to slow down the airflow, and the intakes of the Concorde Olympus engines are still something of a trade secret. Thrust mainly comes from the exhaust gases.

Subsonic airliners are optimised for a different mission: long efficient cruises at Mach 0.9 or thereabouts, so you can use a multi-blade bypass fan with short blades and a very coarse pitch, and tolerate the extra fuel burn needed to get off the deck as it's a small part of the mission.  The "jet" bit is mostly there to drive the fan rather than the other way around!

Not sure about your sooty RB211s. There are all sorts of resonances happening in the cruise and it wouldn't surprise me if this leads to occasional pulsatile overrichness and a bit of smoke - jet fuel is like diesel but cheaper! I recall as a passenger of Ryanair (my favorite airline) being intercepted by a couple of French jets who left impressive smoke trails like old tractors as they climbed towards us at full thrust: the smoke gradually thinned as they accelerated and the mixture optimised around 20,000 ft and 600 kt.

[Zer0]

Where did Captain Calverd disappear?

Long time no see!

Anybody know Anything?

[paul cotter (OP)]

I hope he has not fallen into a black hole, it would be a dreadful loss to the forum and also to the "boss". Seriously I hope all is ok with him.

[alancalverd]

I paid someone else to fly me (and the boss) in a fairly serviceable 737  to somewhere I didn't have to think about physics for a whole week. Croatia is recommended.

[paul cotter (OP)]

Ah, my fears have been allayed. I was tortured by the most ghastly images( too graphic to relay on a family friendly forum ) of Alancalverd being spaghettified by said black hole. Panic over.
 

[Zer0]

" somewhere I didn't have to think about physics for a whole week. "

But you did...lol...We know it!
(u just hid it from the boss)

Difficult to take a Scientist out of the Lab, nearly Impossible to take the Science out of a Scientist.

Welcome Back!

[alancalverd]

We did try, but the main harbor road in Porec is named after Nikola Tesla and the first bloke we met at dinner was a retired physics teacher, then somebody started dropping parachutists out of a Cessna Caravan and somehow it all seemed a bit like being at home.....