Today's Tech Trivia #2

[USMCmech]

Man, I'm learning a ton in A&P school, here are some more questions that I think most pilots should be able to answer.


What is the first place that ice will form on the airplane?


Why do airplane engines have two spark plugs? (it's not what ya think)


At what point in the engine cycle has combustion finished? (when has all the fuel that will burn finished burning?)


What is the hottest part of the Cylinder?


If your throttle cable breaks, what will happen and how can you get back on the ground safely?


Answers to follow tonight. [img]/ubbthreads/images/graemlins/cool.gif[/img]

[averyrm]

I'll give ‘er a go

1) Small protrusions, such as the OAT probe, strut-mounted steps on Cessnas, etc.

2) Redundancy and more complete fuel burn.

3) It usually starts before TDC and ends after the down stroke - still burning in the exhaust manifold sometimes.

4) I would guess the aft side of the cylinder?

5) Not sure - if it's like a car, it would close to idle, but I would imagine it would remain at the same power setting or go to full throttle. Power can be reduced by using only one mag or cutting the mixture

[roundout]

i'll let myself be publicly humiliated .

1. antennae or OAT probe. possibly stall warning flapper if it's piper style?
2. complete fuel burn
3.when you shut off the engine [img]/ubbthreads/images/graemlins/wink.gif[/img]
4. spark plug electrode
5. control engine speed as best possible with a combination of mixture, carb heat, and aircraft attitude/airspeed. i wouldn't mess with the mags.

[ananoman]

1. I'll go with what the others have said about the airframe, but you can get ice at much higher temperatures in the carburetor.

2. There are several reasons for multiple spark plugs, one is reliability as in most systems there are separate magnetos for each set of plugs. There are some notable exceptions to this such as the Lycoming 'dual magneto' that incorporated both magnetos into one big block with only one drive connected to the engine. Also, quite a few of the old radials had separate distributors, but shared a common magneto to generate current. Another is more power, two plugs lead to more complete combustion, but you can also run a higher compression ratio in a big bore engine if you use two plugs instead of one, without encountering detonation (this is more important in two valve engines that cannot use a centrally mounted spark plug). I think this is probably the best answer, more power and increased detonation margins.

3. I will go with the exhaust stroke. It was common practice in the old days to lean by the color of the exhaust flames coming out of the stacks. So, depending on the mixture setting, it may not be until after the exhaust exits the stack.

4. The spark plug electrode is very hot as is the exhaust valve, but the exhaust port is the hottest part of the actual cylinder. Most of the heat dumped into any engine comes from this location.

5. I believe that the throttle is supposed to go wide open in this case. You would have to modulate engine power with the mixture to land. On a twin you could just feather the engine once established on final. The same happens with constant speed props, they are supposed to go to full rpm if the cable breaks.

[USMCmech]

Great responses! Thanks guys!

[ QUOTE ]
What is the first place that ice will form on the airplane?

[/ QUOTE ]

The Carburator. Carb Ice will form well before any surface ice, and if you are picking up ice on the outside, you'll be picking up some in the carb. Carb heat/Alt air is or should be part of any checklist for icing conditions.


[ QUOTE ]
Why do airplane engines have two spark plugs? (it's not what ya think)

[/ QUOTE ]

Ya'll got this one right! Complete combustion. The piston/cylinder of an airplane engine is so huge (each one has more displacement than most small car engines) that if the flame starts just on one side, it won't burn completly before it should. Since each cylinder needs two spark plugs, engine designers decided to take advantage of the available redundency and use two magnetos.


[ QUOTE ]
At what point in the engine cycle has combustion finished? (when has all the fuel that will burn finished burning?)

[/ QUOTE ]

The spark plugs fire on the compresion stroke about 15-12 degrees before Top Dead Center. All combustion is complete at TDC. The expanding gasses then push down the piston on the power stroke.


[ QUOTE ]
What is the hottest part of the Cylinder?

[/ QUOTE ]

The hottest part of the cylinder is the exaust port. The exaust port is identifyed by the greater number of cooling fins around it. The exaust valve gets extreemly hot too.


[ QUOTE ]
If your throttle cable breaks, what will happen and how can you get back on the ground safely?

[/ QUOTE ]

If the throttle cable breaks the throttle will spring to wide open. If this happens you can reduce power by reducing the mixture. You can depower an engine by starving it for air (closed throttle) or you can starve it for fuel (lean mixture).

Try it sometime, even if you don't buy LOP operations (and I can show you plenty of evidence that LOP is perfectly OK), doing it once or twice won't hurt a thing. Broken throttle cables do happen, and knowing how to controll you airplane like this could save your life.


I'll be back next week with more. [img]/ubbthreads/images/graemlins/cool.gif[/img]

[Cutlass1287]

What does LOP mean? Im sure if I think hard enough It will come to me....something about Low, power, maybe operations?

[MDPilot]

Lean Of Peak

[Cutlass1287]

[ QUOTE ]
Lean Of Peak

[/ QUOTE ]

(expletetive) [img]/ubbthreads/images/graemlins/grin.gif[/img]

[USMCmech]

Here's some more. [img]/ubbthreads/images/graemlins/cool.gif[/img]


Why are partial power takeoffs in piston airplanes extreamely bad? (even if you do have 15,000ft of runway)


What happens to the mixture when you add carb heat?


What indications should you see if your idle mixture is adjusted properly?


Where is the fuel mixed with the air in a typical fuel injected engine?


This one's a toughie! Some A&Ps might have to think a while. [img]/ubbthreads/images/graemlins/confused.gif[/img]
Your engine isn't producing full power, vibrations have increased, fuel flow is higher than normal, one cylinder hits peak EGT well before the others, what's wrong?

[USMCmech]

Correction.

[ QUOTE ]
This one's a toughie! Some A&Ps might have to think a while. [img]/ubbthreads/images/graemlins/confused.gif[/img]
Your fuel injected engine isn't producing full power, vibrations have increased, fuel flow is higher than normal, one cylinder hits peak EGT well before the others, what's wrong?

[/ QUOTE ]

[RiddlePilot]

1. I have no idea, but I've done partial-power takeoffs plenty of times in turbocharged piston aircraft.

2. Enrichens due to the fact that less dense air is now entering the carburator.

3. I'm a little confused on this one. Is this what you're looking for when you're leaning, or when you've already leaned? Anyhoo, I lean to peak (or roughness), then bump the mixture up a bit to make it run smooth again.

4. I'd like to say the intake manifold, but it's been a while since I've gone over this...

5. At first I was thinking you had a clogged injector or something, but that wouldn't explain the higher fuel flow. I dunno. [img]/ubbthreads/images/graemlins/smile.gif[/img]

[RPM]

[ QUOTE ]
Correction.

[ QUOTE ]
This one's a toughie! Some A&Ps might have to think a while. [img]/ubbthreads/images/graemlins/confused.gif[/img]
Your fuel injected engine isn't producing full power, vibrations have increased, fuel flow is higher than normal, one cylinder hits peak EGT well before the others, what's wrong?

[/ QUOTE ]

[/ QUOTE ]

fuel injector partially clogged, cylinder running lean due to lack of fuel..so the EGT goes up [img]/ubbthreads/images/graemlins/confused.gif[/img]

[RPM]

[ QUOTE ]
Where is the fuel mixed with the air in a typical fuel injected engine?



[/ QUOTE ]

depends on what type of FI system it is...the cylinder/combustion chamber, intake port of the cylinder
head, or the manifold.


[ QUOTE ]
What indications should you see if your idle mixture is adjusted properly?



[/ QUOTE ]

hmm, EGT and cylinder head temp guages reading in the middle green...and no roughness, and you should be able to check it by setting the engine at around 1,000-1,200 RPM and pulling the mixture out slowly...the RPM should rise slightly, but no more than 50 RPM increase before dropping RPM's...if it rises more than 50, its set too rich....no rise at all, set too lean.

[SunkenLunkenCFI]

On the fuel injected engine - lack of fire in one cylinder?

On the icing question- depends what type of aircraft --

[ananoman]

[ QUOTE ]
Why are partial power takeoffs in piston airplanes extreamely bad? (even if you do have 15,000ft of runway)

[/ QUOTE ]
Using partial power will often result in a lean mixture setting at high power, which is not a good idea. Many engines enrichen the mixture when the throttle is fully open. It is never a good idea to use partial power in any aircraft that does not have this procedure in the flight manual. Many mistakenly think that they are 'saving' their engine. This is false economy. The engine will last to TBO if it is operated correctly, and it is safer to use full power. The faster you get off the runway and in the air, the better. Only large jets and turbo-props use partial power takeoffs, and they have the data to support this in their flight manuals.

[ QUOTE ]
What happens to the mixture when you add carb heat?

[/ QUOTE ]

When you add carb heat, you lessen the density of the intake air. Since most systems meter fuel/air by volume and not density, you will enrichen the mixture. If you intend to run for a long period with carb heat, you should lean the mixture.

[ QUOTE ]
What indications should you see if your idle mixture is adjusted properly?

[/ QUOTE ]

When you pull the mixture to idle cutoff to shut down, you should see about a 25 RPM increase in RPM before the engine quits. This shows that you are running slightly rich at full throttle, which is what you want. If you get no increase, you are probably running lean when you use full power for takeoff, which is not good for your engine's, or your longevity.

[ QUOTE ]
Where is the fuel mixed with the air in a typical fuel injected engine?

[/ QUOTE ]
Most fuel injected engines have constant flow injectors at each intake valve. So the fuel and air is mixed in the intake port. Some of this pools as liquid fuel when the valve is closed, but vaporizes as it is sucked into the cylinder, or as it hits the hot intake valve. In the past some of the largest radials had direct injection, but this is not common. Tne new diesels also inject fuel directly into the cylinder.

[ QUOTE ]
Your engine isn't producing full power, vibrations have increased, fuel flow is higher than normal, one cylinder hits peak EGT well before the others, what's wrong?


[/ QUOTE ]

One of the fuel injectors is plugged, specifically the one on the lean cylinder. This engine does not really have a 'fuel flow gauge' it has a pressure gauge labelled to show fuel flow. The problem with this is that if one of the injectors is plugged, it will show up as higher than normal fuel pressure. Since the gauge is labeled in GPH instead of PSI, the pilot thinks that fuel flow has increased. This is common on Aztecs, Arrows, older Barons and Bonanzas, and many other aircraft. Most of the high end new production aircraft have had these gauges replaced by fuel flow transducers, which actually measure fuel flow through the line leading to the flow divider on the top of the engine. On some aircraft like older Barons and Bonanzas, the only way to tell what you have is to pull the cowling off and look for the transducer. If you see a dual gauge that shows MAP on one side and Fuel Flow on the other, you probably have one of the older gauges that is actually indicating pressure.

[USMCmech]

ananoman pretty much covered it! [img]/ubbthreads/images/graemlins/smile.gif[/img] [img]/ubbthreads/images/graemlins/cool.gif[/img]

Just a couple of nit picks.

At WOT (wide open throttle) the power enrichment valve is opened in the carburator. This increases fuel flow into the venturi. This helps keep the engine cool durring take off and climb. Running at less than WOT is seriously hurting your engine. I personally recomend climbing to cruise altitude at WOT unless your POH says different.

For those flying turbocharged engines with fixed wastegate turbos that can't be opened to WOT. Use the recomended max power setting, this will keep your engine happy for years to come.


At idle the fuel flows on a seprate path than when the throttle is open. It has a seperate metering jet that should be adjusted by a mechanic. This is what I was refering to as Idle Mixture Setting. If you are leaning for ground ops, make sure you run very lean so that you can't inadvertnaly try to take off with a lean mixture.

[CaliforniaSurfer]

I'm too distracted my RPM's avatar....nice.

Surf

[USMCmech]

Here's this weeks bunch. It's kind of short because we've been into complicated stuff that I don't expect most pilots to know. Pressure carbs are just too complex to get into here. [img]/ubbthreads/images/graemlins/confused.gif[/img]



Why can't auto gas be used in high altitude aiplane engines? (even if the timing has been adjusted to prevent detonation)


You leave the mixture rich and turn the mag key to OFF. The engine keeps running, what's wrong?


Where does the magneto get it's electricity?


Who is ultimately responsible for the mechanical condition of the airplane?


Next week we do troubleshooting, so I should have some good stuff. [img]/ubbthreads/images/graemlins/smile.gif[/img]

[Dazzler]

I'll have a try....

[ QUOTE ]

Why can't auto gas be used in high altitude aiplane engines? (even if the timing has been adjusted to prevent detonation)


[/ QUOTE ]
Octane level too low.
[ QUOTE ]

You leave the mixture rich and turn the mag key to OFF. The engine keeps running, what's wrong?


[/ QUOTE ]
Broken p-lead (not grounding properly)
[ QUOTE ]

Where does the magneto get it's electricity?


[/ QUOTE ]
From the turning of the engine (i.e. dynamo)
[ QUOTE ]

Who is ultimately responsible for the mechanical condition of the airplane?


[/ QUOTE ]
The owner/operator

[roundout]

[ QUOTE ]
Why can't auto gas be used in high altitude aiplane engines? (even if the timing has been adjusted to prevent detonation)

lead acts as a lubricant. without the lead in the gas, the engine doesn't get all the lubrication it's designed to receive. i THINK there were some cases of people converting to mogas and then not making TBO because of valveguide problems dealing with insufficient lubrication. don't quote me.

You leave the mixture rich and turn the mag key to OFF. The engine keeps running, what's wrong?

busted p-lead

Where does the magneto get it's electricity?

it generates its own electricity. the mag is driven by the engine, which in turn, helps the engine run (way simplified, obviously).

Who is ultimately responsible for the mechanical condition of the airplane?

the owner/operator

[USMCmech]

[ QUOTE ]
lead acts as a lubricant. without the lead in the gas, the engine doesn't get all the lubrication it's designed to receive. i THINK there were some cases of people converting to mogas and then not making TBO because of valveguide problems dealing with insufficient lubrication. don't quote me.

[/ QUOTE ]


Urban Ledgend/Old Wives Tale Alert !!!!! [img]/ubbthreads/images/graemlins/banghead.gif[/img] [img]/ubbthreads/images/graemlins/banghead.gif[/img]

Not slaming you Roundout, but this myth needs to be corrected.

Tetraethel Lead is the antiknock additive added to unleaded gasoline to boost it's octane number up to 100. This is to decrease the amount of detonation in the cylinder

It has nothing to do with lubrication of the valves! The only thing it does to the engine is to foul the spark plugs.

Running a airplane engine on mogas can be safley done on certian models. Low compresion, small displacement engines aren't really prone to detonation. These don't need all that octane value. The EAA researched several STCs for common aircraft engines in the 1980s acording to the instructions the timing of the magnetos must be adjusted so that detonation dosen't occour. If this is properly done, a small four cylinder engine can run just fine on 92 octane auto fuel. I personally know a guy who flys pipeline patrol in a C-172 who usses auto fuel to keep his costs down. His engine has 7000 hrs on it, and shows no sign of any problems.



Now to the answer of my origional question.
[ QUOTE ]
Why can't auto gas be used in high altitude aiplane engines? (even if the timing has been adjusted to prevent detonation)

[/ QUOTE ]
Auto gas has a lower vapor pressure than 100LL. At higher altitudes this can cause vapor lock.

[ananoman]

[ QUOTE ]
Why can't auto gas be used in high altitude aiplane engines? (even if the timing has been adjusted to prevent detonation)

[/ QUOTE ]
Most 'high altitude' engines are turbocharged and most available mogas STC's are for lower compression engines which are normally aspirated. But I am guessing that the reason you are looking for has to do with the vapor pressure of auto gas. At high altitudes, it will tend to evaporate due to the lower atmospheric pressure. This can cause all sorts of interesting fuel problems, but vapor lock would be the most common. Avgas is formulated to help avoid this, but it can be much more important to follow correct procedures when switching fuel tanks at higher altitudes. Some manuals specify using the electric boost pumps when switching tanks. At low altitudes it is usually no big deal if you omit this, but at high altitudes, you might get a suprise.


[ QUOTE ]

You leave the mixture rich and turn the mag key to OFF. The engine keeps running, what's wrong?

[/ QUOTE ]
You either have a broken 'P-lead' or lots of carbon deposits in the combustion chambers.

[ QUOTE ]
Where does the magneto get it's electricity?

[/ QUOTE ]
The magneto generates its own current. It is kind of a combined generator/distributor, which unlike a car, will allow the engine to run after a complete electrical failure.

[ QUOTE ]
Who is ultimately responsible for the mechanical condition of the airplane?

[/ QUOTE ]
The owner/operator is responsible for keeping an aircraft in airworthy condition. The pilot is responsible for making sure it is safe for flight.

[USMCmech]

Hey anonaman, how about you throw some questions out there? I've tried to include some that will be informative to newer pilots, but I'm sure you can think of some I've missed. You obviously know your stuff.

[ananoman]

[ QUOTE ]
lead acts as a lubricant. without the lead in the gas, the engine doesn't get all the lubrication it's designed to receive. i THINK there were some cases of people converting to mogas and then not making TBO because of valveguide problems dealing with insufficient lubrication. don't quote me.

[/ QUOTE ]
So much for not quoting you! This has some truth, but not in the example you mention, and not usually in aircraft engines. When leaded auto gas became unavailable there were some problems with farm equipment and some cars, due to the valve seats wearing out. Most of the farm tractors did not have high compression engines and did not need the octane, but the cylinder heads were somewhat 'soft' and the lead helped cushion the valves and prevent wear. Some cars also had this problem, and some of the older muscle cars had difficulty with the lower octane ratings. The fix for this was to machine out the valve seats and press in new hardened seats. These engines would have had cast iron cylinder heads that were fairly soft. Newer engines have induction hardened seats (a rapidly oscillating electric field is used to heat the metal in a localized area to heat treat the metal) or have aluminum heads with pressed in valve seats made of steel. This is the reason aircraft engines usually do not have this problem, they have aluminum heads with pressed in valve seats of sufficient hardness to avoid erosion.

You are more likely to have problems with valve guides from too much lead than not enough. If you run rich, you will have lead deposits build up on the spark plug electrodes and on the valve stems, since the engine is not running hot enough to burn it off. This lead build up can cause a 'sticky valve' which can be bad news.

As USMCmech says, lead causes as many problems as it solves. The only reason it is still around is because it is the only successful way they have found to raise the octane rating to the levels needed in some high performance engines.

[JHines]

[ QUOTE ]
... The only reason it is still around is because it is the only successful way they have found to raise the octane rating to the levels needed in some high performance engines.

[/ QUOTE ]

I wonder about this. Automotive gasolines are available in unleaded formulations up to at least 104 octane. Of course, that's pump method (R+M)/2, so it probably doesn't square up exactly with aircraft lean/rich ratings.

I know there are other problems with automotive fuels (vapor pressure, alcohol, water, etc.) but it seems like the technology out to be out there for a no-lead aircraft fuel.