To Prime Or Not To Prime? That is the question!
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If the aircraft flew only 20-30 minutes ago, I'd be astonished if it required priming. You'll usually get away with a certain degree of over-priming, but one day you won't! IMHO it's a good idea to use the absolute minimum amount of priming you can get away with.
Also, in my (limited) experience, you'd be surprised how many engines like being started with the bare minimum of throttle. A quarter-inch is often too much... try just a couple of millimetres. This will also mean that you need less priming, because you're letting less air into the engine - hence you need less fuel to achieve the correct mixture. You'll also avoid the 2000rpm scream
Also, in my (limited) experience, you'd be surprised how many engines like being started with the bare minimum of throttle. A quarter-inch is often too much... try just a couple of millimetres. This will also mean that you need less priming, because you're letting less air into the engine - hence you need less fuel to achieve the correct mixture. You'll also avoid the 2000rpm scream
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I've had a bit of experience of starting troubles on a PA-28-161 (O-320-D3G) that highlighted the 'black art' approach to starting engines that seems so common in the aviation community and is illustrated in places on this thread.
This particular PA-28 became obstinate to start over a period of time and various pilots evolved 'home brew' approaches to starting. Unfortunately one of these appears to have involved throttle pumping and one day the aircraft caught alight, damaging hoses, cables and, thank goodness, emptying the fire extinguisher of a nearby pilot who knew what to do!
Without that quick action, the aircraft might have been a total loss.
Throughout this period, various engineers protested that there was nothing wrong and that the starting problems were 'normal'. Each engineer also had a 'home brew' starting philosophy.
After the fire, we investigated the possible causes and found that throttle pumping introduces fuel directly into the carburettor bore and runs straight back down the throat after only 2 or 3 pumps, flooding the air changeover valve box (where the fire started).
The primer introduces fuel into the inlet manifold and seems much less likely to result in fuel running out of the carb. I was told (by yet another engineer) that there is a bowl shaped depression in the manifold that holds the primer fuel but I have not seen this with my own eyes.
The carburettor and airbox were replaced on the engineers theory that worn slides were the cause of the starting problems but not so. Next, the primer nozzles and pipes were replaced with no apparent change. All this time the aircraft was tempramental to start with on average 2 or 3 attempts at cranking before it would go. Even the CFI volunteered this was to be expected with an engine of 'this age' (1500 hrs).
Eventually, it was discovered by a new engineer that the mags which were shown as overhauled at last annual had in fact not been opened and required replacement. Voila! Instant starting without funny proceedures - 4 or 5 primes from cold, none when hot, goes every time.
So everything that went before - agonising cranking, back firing, over priming, pumping, near disastrous fire - seems attributable to weak or mis timed ignition.
Now this might not be the case with your aircraft. But if starting is poor, needing non-standard proceedures, please find out why before something nasty happens. Engines are repeatable mechanical creations that behave consistently when properly maintained. Otherwise we'd all be walking (or swimming).
This particular PA-28 became obstinate to start over a period of time and various pilots evolved 'home brew' approaches to starting. Unfortunately one of these appears to have involved throttle pumping and one day the aircraft caught alight, damaging hoses, cables and, thank goodness, emptying the fire extinguisher of a nearby pilot who knew what to do!
Without that quick action, the aircraft might have been a total loss.
Throughout this period, various engineers protested that there was nothing wrong and that the starting problems were 'normal'. Each engineer also had a 'home brew' starting philosophy.
After the fire, we investigated the possible causes and found that throttle pumping introduces fuel directly into the carburettor bore and runs straight back down the throat after only 2 or 3 pumps, flooding the air changeover valve box (where the fire started).
The primer introduces fuel into the inlet manifold and seems much less likely to result in fuel running out of the carb. I was told (by yet another engineer) that there is a bowl shaped depression in the manifold that holds the primer fuel but I have not seen this with my own eyes.
The carburettor and airbox were replaced on the engineers theory that worn slides were the cause of the starting problems but not so. Next, the primer nozzles and pipes were replaced with no apparent change. All this time the aircraft was tempramental to start with on average 2 or 3 attempts at cranking before it would go. Even the CFI volunteered this was to be expected with an engine of 'this age' (1500 hrs).
Eventually, it was discovered by a new engineer that the mags which were shown as overhauled at last annual had in fact not been opened and required replacement. Voila! Instant starting without funny proceedures - 4 or 5 primes from cold, none when hot, goes every time.
So everything that went before - agonising cranking, back firing, over priming, pumping, near disastrous fire - seems attributable to weak or mis timed ignition.
Now this might not be the case with your aircraft. But if starting is poor, needing non-standard proceedures, please find out why before something nasty happens. Engines are repeatable mechanical creations that behave consistently when properly maintained. Otherwise we'd all be walking (or swimming).
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Is that eninge still running in said aircraft?
It seems to me (Back on the subject of new technology) that a choke would be far more efficient, and in some cases a lot safer than the primer? Why don't they use chokes? The Lycoming philosophy of using 30's technology at work again?
WF.
It seems to me (Back on the subject of new technology) that a choke would be far more efficient, and in some cases a lot safer than the primer? Why don't they use chokes? The Lycoming philosophy of using 30's technology at work again?
WF.
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This thread has really shown the bizzare ways of engine starting... I bet a lot of them are unnecessary!
A question worth asking is whether priming is EVER needed. The engine ought to start eventually if cranked for long enough. I have certainly seen this "procedure" with fuel injected engines; it tends to start after a lot of cranking but no more cranking than the "textbook hot start procedure" normally requires.
Re a choke - if this (being simply a mixture enrichment) worked then the process of squirting liquid fuel into parts of the engine (which is what "priming" is) would not be required. (BTW I am NOT saying that squirting liquid fuel into parts of the engine IS required
) I don't think that a choke would help because the carb (or a fuel injection system) in an aircraft engine is already set up for a relatively (relatively to a car engine) rich cut, about 150F rich of peak with all 3 levers fully forward. Once an aircraft engine starts, there is normally no problem running it, so I don't think a "permanent" richer mixture would help.
A question worth asking is whether priming is EVER needed. The engine ought to start eventually if cranked for long enough. I have certainly seen this "procedure" with fuel injected engines; it tends to start after a lot of cranking but no more cranking than the "textbook hot start procedure" normally requires.
Re a choke - if this (being simply a mixture enrichment) worked then the process of squirting liquid fuel into parts of the engine (which is what "priming" is) would not be required. (BTW I am NOT saying that squirting liquid fuel into parts of the engine IS required
) I don't think that a choke would help because the carb (or a fuel injection system) in an aircraft engine is already set up for a relatively (relatively to a car engine) rich cut, about 150F rich of peak with all 3 levers fully forward. Once an aircraft engine starts, there is normally no problem running it, so I don't think a "permanent" richer mixture would help.
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A choke would be just another place for ice to form !.
Ice forms because the reduction in pressure inside the venturi causes the air temperature to drop. This causes water vapour to to condense out and form ice crystals, which are then deposited on the throttle butterfly, which is downstream of the venturi.
A choke butterfly is upstream of the venturi and would therefore not suffer from the problem.
Mike
Why do it if it's not fun?
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I think there are lots of ways of designing a choke. The choke on my car works by moving the jet and the needle up and down - there is no butterfly at all, but the choke is alongside the venturi, not upstream of it.
No idea what type of choke my Rotax engine has, but I've never heard of any icing problems with it.
FFF
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No idea what type of choke my Rotax engine has, but I've never heard of any icing problems with it.
FFF
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Mike
Assuming we're talking about a conventional choke mechanism (i.e. a second butterfly, similar to the throttle one), then I would have said that the butterfly itself causes a reduction of pressure, and thus can contribute to icing. Both butterflies come to think of it.
How the restriction is achieved for the Venturi effect to take place, surely can be either by reducing the diameter offered by the walls, or by introducing a restriction 'from the centre out' - a butterfly for example. Either way, for a given volume of air to move past the restriction, the pressure and temperature will drop. Boyles' law P=VT? Was that his name?
Rich
Assuming we're talking about a conventional choke mechanism (i.e. a second butterfly, similar to the throttle one), then I would have said that the butterfly itself causes a reduction of pressure, and thus can contribute to icing. Both butterflies come to think of it.
How the restriction is achieved for the Venturi effect to take place, surely can be either by reducing the diameter offered by the walls, or by introducing a restriction 'from the centre out' - a butterfly for example. Either way, for a given volume of air to move past the restriction, the pressure and temperature will drop. Boyles' law P=VT? Was that his name?
Rich
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Interesting and good to know.
I think i'll check with one of the instructors what they recommend as there are a lot of mixed views.
I've never had a 2000rpm scream before, it usually starts up ok. If anything I usually under prime as I can always prime more.
__
I'd hate a choke on an a/c like the one in the car i drive. Of a morning its like cruise control.
I think i'll check with one of the instructors what they recommend as there are a lot of mixed views.
I've never had a 2000rpm scream before, it usually starts up ok. If anything I usually under prime as I can always prime more.
__
I'd hate a choke on an a/c like the one in the car i drive. Of a morning its like cruise control.
Why do it if it's not fun?
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@ Spikee - I know what you mean about cruise control! But I don't even taxy my aircraft, let alone fly it, until the choke has been pushed all the way back in. In fact, unlike the choke in a car, it's a sprung knob which needs one hand constantly on it to hold it out, so it wouldn't be practical to move the aircraft whilst having to hold the choke out.FFF
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An engine for the PA28 I fly costs about €25,000. A new plane €200,000???
My Toyota car cost €24,000 in total!
Now the airplane is a bitch to start.
The car? Well, you just make sure it's not in gear, the hand brake is on, and do nothing more than just turn the key. Does't matter if it's hot or cold, up a mountain or down a valley. Just turn the key, and it starts effortlessly, every time. Time after time. It's so straight forward, I don't even need a checklist! I don't have to think about it, it just works.
Can't help but think that there is something wrong here. The cheap one is the one that's not a problem to start......not to mention, quiter more fuel efficient, more comfortable.....but then again it's ground based
dp
My Toyota car cost €24,000 in total!
Now the airplane is a bitch to start.
The car? Well, you just make sure it's not in gear, the hand brake is on, and do nothing more than just turn the key. Does't matter if it's hot or cold, up a mountain or down a valley. Just turn the key, and it starts effortlessly, every time. Time after time. It's so straight forward, I don't even need a checklist! I don't have to think about it, it just works.
Can't help but think that there is something wrong here. The cheap one is the one that's not a problem to start......not to mention, quiter more fuel efficient, more comfortable.....but then again it's ground based
dp
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In a conventional carburettor the choke butterfly is at the intake end of the venturi and the throttle butterfly is at the engine end.
The throttle butterfly therefore controls the volume of fuel/air mixture while choke only controls air. The effect is that if you close the choke the suction on the jet is greater and proportionally more fuel flows, creating a richer mixture. The choke is called the choke because it chokes off the supply of air.
There are other methods of enrichening the mixture, e.g. by opening up an additional jet or by using some sort of tapered needle valve to adjust fuel flow. The "tickler" on an old motor bike carb and the primer are other ways.
Re the icing issue:-
If ice were to be forming on the choke butterly it would also be forming on the airframe it's not the airflow hitting an obstruction that causes the ice to form, it's the temperature reduction as it expands through the venturi. The ice crystals that have already formed then hit the throttle butterfly and accumulate. The butterfly forms a much larger obstruction at low throttle openings which is why it is more prone to icing at low power settings. At higher power settings the ice crystals are swept into the cylinders as part of the mixture.
Try these two exercises.
1. Pump up the tyre on your bike. The pump will heat up because you are compressing air.
2. Give a prolonged spray from an aerosol can. The can will grow colder in your hand as the gas in it expands.
Mike
The throttle butterfly therefore controls the volume of fuel/air mixture while choke only controls air. The effect is that if you close the choke the suction on the jet is greater and proportionally more fuel flows, creating a richer mixture. The choke is called the choke because it chokes off the supply of air.
There are other methods of enrichening the mixture, e.g. by opening up an additional jet or by using some sort of tapered needle valve to adjust fuel flow. The "tickler" on an old motor bike carb and the primer are other ways.
Re the icing issue:-
If ice were to be forming on the choke butterly it would also be forming on the airframe it's not the airflow hitting an obstruction that causes the ice to form, it's the temperature reduction as it expands through the venturi. The ice crystals that have already formed then hit the throttle butterfly and accumulate. The butterfly forms a much larger obstruction at low throttle openings which is why it is more prone to icing at low power settings. At higher power settings the ice crystals are swept into the cylinders as part of the mixture.
Try these two exercises.
1. Pump up the tyre on your bike. The pump will heat up because you are compressing air.
2. Give a prolonged spray from an aerosol can. The can will grow colder in your hand as the gas in it expands.
Mike
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Mike Cross
You have totaly mis-understood carb icing.
I would recommend that you go back and read the post by richyrich three or four times it is very well written and the theory cant be faulted.
I would recommend that you go back and read the post by richyrich three or four times it is very well written and the theory cant be faulted.
