Aviation technical quiz

[Great Ozzie]

it's something I'm just reading about, but found it quite interesting...

Hey Nick,

By chance did this come out of Kershner's Instrument Flight Manual?

[Nick - A2A]

... if your static source was blocked, and your vsi case had a leak towards the cabin (say the glass was broken) ...

Vladimir, yes this is exactly the sort of scenario I was thinking about. Reading this post by 'UKJim' got me to thinking about it. Being rather more observant than me, he noticed that there's no alternate static selector in the Comanche cockpit. I've known for a while that breaking the VSI glass in the event your static port is blocked should help restore some functionality to the ASI and altimeter if you don't have an alternate static source. In fact, I probably even tried it when I had a certain Diamond Katana installed which simulated this eventuality.

However, I rather blithely assumed that you'd just biff in the glass and all would be good. However, seems there's a bit more to it than that, as this video demonstrates...

[youtube]http://www.youtube.com/watch?v=ERP2nKgWpxk[/youtube]

So not only is it necessary to break the VSI glass, but one would also need to puncture the diaphragm to avoid unacceptable lag in the indications and (if I understand correctly which is always far from guaranteed ) eliminate the VSI reversal. Presumably in the process the VSI would be more-or-less totalled, but I'd wrongly assumed that just giving the glass a whack with a fire extinguisher or similar hefty object would be enough. (Actually, I say 'just' but in practice it probably wouldn't be that easy: at least this author found as much.)

Afraid my understanding of all this is hazy at best and I believe there are some types of VSI where the above wouldn't apply?

By chance did this come out of Kershner's Instrument Flight Manual?

Rob, no I was just digging around reading random bits and pieces that I unearthed on the net I'm afraid. However, in glancing through the U.S. Navy NATOPS Instrument Flight Manual I did come across this statement on the subject of VSI reversal under 'normal' use...

The vertical speed indicator is also subject to reversal error. This error is caused by inducing false static pressure in the static system and normally occurs during sudden or abrupt pitch changes. The reversal error is not synonymous with lag error; however, both may occur simultaneously. The magnitude of this error varies with the aircraft and the abruptness of pitch changes. The reversal error can be minimized by making small and/or smooth pitch changes.

Cheers,
Nick

[Great Ozzie]

So not only is it necessary to break the VSI glass, but one would also need to puncture the diaphragm to avoid unacceptable lag in the indications and (if I understand correctly which is always far from guaranteed ) eliminate the VSI reversal. Presumably in the process the VSI would be more-or-less totalled,

Kershner discusses this... that the rate would be accurate but the direction reversed. And there would be lag in the other two (until the diaphragm was punctured). This is why he mentions cracking the glass on the airspeed or altimeter (if nothing is damaged in the process) that all three should still be about as accurate as an alt cabin source.

I think the important thing would be not put oneself in that position i.e. fly something that has an alternate static port when going into the muck. Flying "Partial Panel" (loss of the vacuum instruments) is *real work*. Loss of the Pitot / Static instruments would be tough. This is where the sim would shine - to practice such failures.

Another thing - wrt that article on AVweb... the author made the statement:

I did know, however that 2,100 RPM in level flight in the Cherokee would give us about 90 KIAS and 1,700 RPM on a glideslope would also give us 90 KIAS and the proper rate of descent.

So important to know stuff like that. You need to know "what you need" and what the instrument indications will look like under normal operation. (btw that 727 accident is oft' quoted when one gets into instrument training).

The vertical speed indicator is also subject to reversal error. This error is caused by inducing false static pressure in the static system and normally occurs during sudden or abrupt pitch changes. The reversal error is not synonymous with lag error; however, both may occur simultaneously. The magnitude of this error varies with the aircraft and the abruptness of pitch changes. The reversal error can be minimized by making small and/or smooth pitch changes.

I have never really thought about why that is... I guess it is due to "inertia"?

But yeah unless you are trying to disorient someone ("the student" ) small & smooth is the way to go.

[Nick - A2A]

Kershner discusses this... that the rate would be accurate but the direction reversed. And there would be lag in the other two (until the diaphragm was punctured). This is why he mentions cracking the glass on the airspeed or altimeter (if nothing is damaged in the process) that all three should still be about as accurate as an alt cabin source.

Thanks Rob - interestingly, in the online advice/discussions I've looked at, I don't think I've seen anyone suggest breaking anything in addition the VSI. I just looked up Kershner's Instrument Flight Manual and maybe I should treat myself to a copy.

I think the important thing would be not put oneself in that position i.e. fly something that has an alternate static port when going into the muck. Flying "Partial Panel" (loss of the vacuum instruments) is *real work*. Loss of the Pitot / Static instruments would be tough. This is where the sim would shine - to practice such failures.

Yeah, the only problem is that this rules out our nice new Comanche for instrument work. I definitely agree with practicing failures though. I've never been quite sure though whether Accu-Sim static port blockage is analogous to the FSX static port blockage or whether it's coded separately. I've yet to experience the former, though I rather hope to see it blocked during pre-flight one of these days.

I think it's been noted by a few on these forums that it would be great to have the ability to actively fail various Accu-Sim parts as well as fix 'em via the maintenance hangar. The tinkerer within me often wishes it was possible to delve into those log.dat files with a text editor or something and set about various systems with a virtual lump hammer (or roll of sticky tape)!

Cheers,
Nick

[Great Ozzie]

I don't think I've seen anyone suggest breaking anything in addition the VSI.

You mean, "in addition" as in "other than"?

Yeah, the only problem is that this rules out our nice new Comanche for instrument work.

Not necessarily.

And you could always have one installed.

[Nick - A2A]

You mean, "in addition" as in "other than"?

Yes - rather poor phrasing there. I guess it wouldn't pay to start bashing away at multiple instruments. The point was I'd really only seen people suggest smashing the VSI glass, it being less critical than the ASI and altimeter. Presumably unless one is pretty familiar with the construction of these things, it's hard to judge the likelihood of breaking the glass without wrecking the instrument completely. And even trickier to execute this 'measured destruction' whilst flying partial panel in IMC without access to suitable implements.

As you say, best avoided altogether for stress-free flying. Perhaps rather a moot point in the sim too, as I'm not aware the static port can ice up or otherwise become blocked during flight, unless you specifically cause it to with the FSX failures menu.

Nick

[ft]

Ft has the honor of doing the next one

What I was trying to avoid by being vague and incomplete...

I've got something planned, but it'll be a few days before I'm posting from a computer again. The thread seems to be kept alive regardless though.

[Nick - A2A]

I've got something planned, but it'll be a few days before I'm posting from a computer again. The thread seems to be kept alive regardless though.

Sounds good! Yeah, this is an interesting thread and a nice change from the typical "A2A - why don't you make us such-an-such an aeroplane?" ones that run into several pages. I've learned some good stuff, what with the direct drive turboprops shafts which go saggy* unless you turn them by hand after landing, and those perplexing 'compressor maps'. (Not pretending to understand the latter just yet though!)

Nick

*Well - sort of anyway.

[Levkovvvv]

Ok, lets have another one while ft comes up with his devious question

Is jet engine fuel-air mixture typically set to stoichiometric or is it kept richer or leaner and why? Shouldn't be too difficult

[AKar]

Is jet engine fuel-air mixture typically set to stoichiometric or is it kept richer or leaner and why? Shouldn't be too difficult

This is a bit of a trick question actually. In gas turbines most of the air doesn't take a part in combustion process, so one could say they burn very 'lean'. This is not really the case however, if one is very pedantic, because the fuel nozzles attempt to atomize and mix the fuel with the air in an optimal way. I'd figure within the actual flame pattern the 'mixture' will be quite close to stoichiometric. There is excess air going through the engine as a whole but it 'bypasses' the actual flame pattern even in 'straight jet' or in a turboprop/-shaft for example. Functionally, it is used to keep the flame away from combustion chamber walls and cool the 'hot flow', and it serves as mass flow of course, heating up and expanding in the process.

The idea is not entirely unlike in diesel engine that uses excess air likewise, and achieves extremely lean apparent mixtures at low powers especially, but that is only apparent because the fuel-air mixture that is combusted doesn't actually fill the whole cylinder.

The excess air that passes through the jet engine can be used in afterburner, where the remaining oxygen is mixed with fuel and ignited. The fact that afterburner can even work should make it immediately obvious that the jet engine operates 'leaner', if one can say so, than stoichiometric if one thinks about it. Otherwise there wouldn't be any oxygen for combustion remaining!


-Esa

[Levkovvvv]

Well AKar, you got 90% of it right, your explanation of cold air flow protecting combustion chamber walls is spot on, but inside the actual flame pattern, mixture is kept rich because at stoichiometric mixture, jet fuel combustion temperature is about 2250 degrees Celsius (4000 Fahrenheit) and your turbine blades would get damaged. Yes,turbine blades are cooled with air, and yes, exhaust gasses are mixed with cold air to protect the turbine, but you don't have to heat metal up to melting point to damage it.

Does anyone know the name of the phenomenon that would result in turbine blade damage if they were heated excessively but not up to melting point?

[Nick - A2A]

Does anyone know the name of the phenomenon that would result in turbine blade damage if they were heated excessively but not up to melting point?

Blade 'creep' is the word you're looking for I think, and it's one of the main reason why engine manufactures produce fantastically expensive single crystal blades using secretive techniques.

Here's a good BBC documentary I watched on this kind of stuff a few years ago: How to Build... A Jumbo Jet Engine. It follows some of the workers and production methods at Rolls Royce's plant at Derby...

[youtube]http://www.youtube.com/watch?v=RHGbWKnhrp4[/youtube]

Cheers,
Nick

[Levkovvvv]

Nice one Nick, might I ask if you have any formal education on the subject, I really don't expect someone to just "know" about material creeping And on the matter of jet engine cost, a professor of mine once said that a jet engine is more expensive than its weight in silver

[AKar]

but inside the actual flame pattern, mixture is kept rich because at stoichiometric mixture, jet fuel combustion temperature is about 2250 degrees Celsius (4000 Fahrenheit) and your turbine blades would get damaged.

Could be, though I don't completely buy the use of 'rich mixture' as an intentional way to control TIT without some further verifications. The biggest reason is that the unburned fuel is a significant hazard for the very reason you mention, the high flame temperature. As long as mixing takes place, the excess fuel tends to burn and extend the flame pattern all the way into the turbine stator (an effect known as 'streaking' and it typically causes some significant damage very quickly). In normal operation, the flame pattern never reaches the 1st stage stator and is significantly cooled before that. Especially at very high flow rates some of the fuel thrown through the nozzles doesn't necessarily burn completely which results in smoke being exhausted, that is visible in most jet engines at takeoff power. This also shows in diesel engines. This is actually quite unavoidable because of inefficient mixing of the fuel and air in the engines where the fuel is just 'dumped' into the air. However, when the power is reduced (which results in direct reduction in fuel flow), I'd guess the 'mixture' (I still don't like using that word because it is not how the things work in these engines! ) immediately gets leaner. With proper atomization the combustor would tend towards stoichiometric naturally due to presence of excess air in the environment. Of course, with spray nozzles a 'perfect' mixing cannot be achieved, whether one wants it or not, but I'd figure the engines get very close today, at least when operating at reduced power, measured in fuel flow. The possibility of 'rich mixtures' in jet engines requires inefficient mixing, which in turn is relatively hard to control. They are known to be present, but cannot be created by intentionally dumping extra fuel into the engine as in ones using Otto cycle. Bad spray patterns are a very common reason for turbine engine issues.

As Nick mentioned already, blade creep can happen especially at high powers where the temperatures are the hottest. Against a common misunderstanding, this is precisely the reason for reduced power takeoff in the airliners, not the fuel savings as is often quoted. Actually, reduced power takeoffs cost some fuel when compared to full power.

-Esa

[Nick - A2A]

Actually, reduced power takeoffs cost some fuel when compared to full power.

Hadn't realised that, though I was aware the purpose of reduced thrust take-offs was to increase engine life. I'd guess it's the derated climb in particular which could burn off more fuel than a full power climb to the same altitude?

Nick

P.S. No materials science training Vladimir, but I've always enjoyed documentaries on this sort of stuff. These generally prompt a foray into the internet to find 'further info' and fill in some of the gaps. Such is how large amounts of time can be happily wasted!