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Cowl flaps in hot climate

Old 27th Mar 2015, 20:18
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Cowl flaps in hot climate

Hi,

Just wondering what everyone does with the cowl flaps (specifically C206) in a hot climate (30+). Getting differing opinions.

Using open for takeoff/climb, and generally closed for descent (though most flying is fairly low level anyway (3000 and below)).
More interested in cruise position; whether they are open or closed all temps and pressures remain well within limits. The gain in airspeed from closing is pretty minimal over relatively short legs.

Do the rear cylinders receive enough cooling in this climate if cowl flaps are closed?

Cheers.
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Old 27th Mar 2015, 21:41
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For start, taxi, TO, and climb....open.


If unusually short time to warm up or open cowls drastically retard oil warm up then perhaps nearly closed during initial warm up.


Cruise, set where experience and common sense dictates.


Descent, closed until circuit height.


Circumstances and individual aircraft will dictate slight variances to the above, these principals apply through winter down to ~10 degrees-ish. The biggest variance in winter summer cowl flap operation is the warm up and taxi period, there after the open until TOC applies.


Here is a tip, might be an old Deakinism, but when you cycle the cowl flaps from open to closed (assuming cable operated) you will feel an area of light load on the lever. This is where the cowl flap wants to "fly" and is balanced between pushing against external ram pressure and internal ram/static pressure.


This spot, around half way is not a bad spot to set for cruise if you are not sure where to set, it also gives you a bench mark to a setting that is either restricting cooling or forcing extra cooling. The problem you face is the industry on the whole is not much interested in aircraft engine operation, being such a largely insignificant component to aviating, that likely you're flying a machine with a compromised single CHT probe placement. If you are blessed with an all cylinder setup hopefully you got the training to go with it, judging by your question though this appears not the case.


"Do the rear cylinders receive enough cooling in this climate if cowl flaps are closed?"


It's not that straight forward, the hottest cylinder can move from left to right bank from middle to front to rear cylinder depending on cowl design, speed of flight, angle of attack and amount of power being used. It can help to think of the airflow being more dictated by pressure differential between top and bottom deck (above/below the cylinder line) and hence flowing downward rather than front to back although it does flow front to back as well.




If you want to see a case study of IO-540 abuse in AC50 Shrikes watch the GAM pilots the country over get cowl flap operation drastically and consistently wrong.


Fuse lit, runs for cover.

Last edited by Obidiah; 27th Mar 2015 at 22:43.
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Old 27th Mar 2015, 21:56
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Maybe start with the questions, what should I try and achieve by changing the cowl flap setting?
If you understand why you move them, then asking for opinions will seem as rediculous as it really is.
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Old 27th Mar 2015, 22:06
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..........

Last edited by Radix; 18th Mar 2016 at 01:35.
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Old 27th Mar 2015, 22:28
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"Maybe start with the questions, what should I try and achieve by changing the cowl flap setting?
If you understand why you move them, then asking for opinions will seem as rediculous as it really is." (
Quote thingy won't work)


A very good point.


Although often we will not know when we have achieved what we set out to as the level of engine instrumentation is so crude and basic.


Warm up and taxiing for example, the instrumentation will tell you that the cowls should be closed but hard won engineering knowledge will often point to problems with closed on the ground. In the cruise with one CHT probe, where to from here?
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Old 27th Mar 2015, 23:06
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What I want to achieve:
Well, all the poh says for cowl flap position in cruise is 'as required'. What I want to know is, what is the best for the engine (given available temps and pressures are all well within limits regardless of position).
Inclined to go with closed during cruise, if no other factors. Mostly just wondered if rear cylinders receive enough cooling of closed given high ambient temps.
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Old 27th Mar 2015, 23:09
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You blokes ever heard of cylinder head and oil temperature gauges?

Edit .... Bones seems to have a general idea.
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Old 28th Mar 2015, 00:17
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Could someone with multi cylinder CHT probes please tell old mate which ones run the hottest.? And when?
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Old 28th Mar 2015, 00:46
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Open for everything but the Cruise and Descent IMHO. Yes the speed gain is small but it is still a gain maybe 3-5kts depending on your Aircraft, if you're flying for an hour that could mean about 1minute off the flight, not much I know but could mean the difference between a 0.95 flight and a 1.00 flight in the Techlog, do that same flight a few hundred times, say over 400hours that would equate to around 20hrs at an operating cost of say $400/hr and that's $8000 you cost the company just by not operating a simple lever. If you have a fleet of 3-4 Aircraft or more it all starts to add up and that could be your wage or your mates wage for the year! It could mean the difference between the Boss being flush enough to spend some money on extra maintenance or fancy toys like a decent GPS, JPI Gauges, an Upgrade to IO-550s etc...etc...

Of course this very much depends on the Aircraft and Climate you're in, I've had some Singles that you just cannot close the Cowls as they'll start to get real hot, real fast, so don't just do it and expect something, do it and make sure everything is working as expected.

Incidentally, it could save your Exhaust one day, I had a guy who had the Exhaust become disconnected and the only thing that stopped it from completely dropping out the bottom of the Aircraft was the fact he had his Cowl Flaps closed!! the sound it created scared the crap out of him, haha.
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Old 28th Mar 2015, 01:29
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open, open, open and open. This is Australia, not Alaska!

Cowl flaps are made for colder climates like the USA where engines need to be kept WARM, not Australia's conditions.

Leave them open for ALL operations except possibly a long descent on a cold day.

There are a number of approved modifications available that fix cowl flaps in the open position for the Australian temperatures, and this does not effect engine life or operation.
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Old 28th Mar 2015, 01:58
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Originally Posted by Obidiah View Post


If unusually short time to warm up or open cowls drastically retard oil warm up then perhaps nearly closed during initial warm up.



r.
The Cessna 206 POH does not permit the cowl flaps to be set to anything but open when the aircraft is started, taxied, runup or for takeoff.
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Old 28th Mar 2015, 02:15
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open, open, open and open. This is Australia, not Alaska!
Struth, words fail me.
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Old 28th Mar 2015, 03:29
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BPF,
At least somebody has the good sense to mention the AFM/POH.
Tootle pip!
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Old 28th Mar 2015, 03:51
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In the absence of common sense and experience talk to your chief pilot and familiarize yourself with the POH.
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Old 28th Mar 2015, 05:37
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The silliest thing you can do is open them after long descent at low power settings in the circuit.

Keep them closed until on the ground and tie them into the flap sequence. Flaps retracted cowl flaps open.

The AFM is the best resource, but it's a one size fills all proposition. Watch the ts and ps and act accordingly.
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Old 28th Mar 2015, 06:37
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Problem is the c206 flight manual just says during cruise open 'as required'
What is 'as required'? If ChT And oil temp/pressures green does that mean ok? (Given cht is hottest cylinder?)
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Old 28th Mar 2015, 08:12
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Bones, the CHT in most 206's is a rather rudimentary gauge. If yours is any good, and assuming it is fitted to the hottest cylinder, I would aim to cruise with the CHT's always lower than 380.

So you could try cowl flaps shut under 370, approaching say 375, cowl flaps open or part open. You will need to experiment. Watch the oil temp as well but here I would just monitor the red line. Approaching oil temp red line, cowl flaps open regardless of CHT.

Unfortunately most CHT gauges are not that good. If it was my aeroplane it would have an engine monitoring system that looked at all cylinders.

Opening the cowl flaps after a long descent will do no harm whatsoever, my preference would be to open them on downwind or according to POH.

Closing them will give a few extra knots and may provide more uniform cooling but again without a monitoring system you really have no idea.
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Old 28th Mar 2015, 10:30
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Thanks Aussie Bob.

My CHT reads 340-350 cowl flaps open and 355-365 closed (never seen it exceed 370 in cruise). Oil temp varies between 170 and 185 depending on cowl flap position, so well below 240 red line.

Cheers
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Old 28th Mar 2015, 13:33
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In the absence of common sense and experience talk to your chief pilot
Just because he has risen to the post of chief pilot does not mean he is an expert in engine handling. He will have his own personal ideas on engine handling and is in a position to insist his employees do what he says.
Every pilot has opinions on engine handling usually handed down from who ever checked him out.

The AFM and POH give the manufacturers recommendations. Stick to those publications. Finally, read the advice by John Deakin, the author of the "Pelican's Perch" His advice is gold. Google his name.
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Old 28th Mar 2015, 15:34
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What about high powered, high performance machines, like the humble Aerostar, which is completely devoid of cowl flaps?! They seem to get by in hot parts of Australia just fine without them!
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