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Cowl flaps in hot climate

Old 28th Mar 2015, 17:55
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Last edited by Radix; 18th Mar 2016 at 01:34.
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Old 28th Mar 2015, 22:31
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I think I'd be worrying less about cowl flap etiquette and more about flexible baffle condition, chafe strip condition and holes, gaps in the aluminium baffling to the engine.

Engine cooling depends on about a 4 psi differential between the top of the engine and below it. Cowl flaps serve to reduce the pressure below the engine by virtue of the venturi effect.

Job 1 is to make sure all the elements of the cooling system are in good condition and doing their job.

This goes for the oil cooler too. A significant proportion of the total cooling of the engine is done by engine oil. Make sure the oil coolers are clear with no bent fins and (one again) properly sealed.
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Old 28th Mar 2015, 23:18
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All baffling and the oil cooler look good in my engine. Only concern really is whether the indicated CHT is actually the hottest and that the rear cylinders are actually sufficiently cooled.

Cheers for all the replies!
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Old 28th Mar 2015, 23:45
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Bones13
Old Akro has covered it !!
Good to see some helpful responses as apposed to some of the smart a$$ comments you get on here.
I flew 182s and 206s years ago meat bombing and we always left the cowl flaps open for climb, then closed for jump run and descent. Once on the ground, retract the wing flaps and open cowl flaps.
Never had to replace a cylinder so it seemed to work well.
If you're flying a 206 with a pod they modify the cowl flap range as part of the fitment.
Cheers
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Old 29th Mar 2015, 00:47
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Bones,
Common sence is the rule with cowl flaps. Watch your CHT. Heat is related also to power, more power more heat. So cowl flap settings are linked to power setting. If on one side you are doing a lot of high power low level work on a hot day your CHT will say leave flaps open, if you are at low power settings in the cruise at altitude then your CHT will say close flaps. The POH is right "as required". Don't lock yourself into doing something because some expert says do this or that unless you can see a logical argument to change. Think about what is happening under the cowl what the CHT,s are telling you. Get a EDM that monitors all cylinders as how do you know what the others are doing.
I urge all pilots to understand what's happening under the cowls and how to make your engine happy. That's the best and safest and most economical way to operate. Understand what CHT your engine is happy at, set yourself a mental alarm eg happy at 350-375 ROP but when we start heading to 400 keep an eye on it, over 400 maybe crack cowls a bit to keep under 400. Now that is only an example as every engine type is different. Talk to an engineer for specific or god forbid look up the POH.
I wish EDM,s and how they relate to good operations were taught as part of the flying syllabus.
If not make it a priory and read up on stuff John Deakin, Walter Atikinson and Goerge Braly have been saying for years. First came across their management ideas 15 years ago and started running our aircraft with their management styles and get all our pilots to understand why engine management and understanding of what they are seeing in front of them is so important to a healthy happy engine.
By the way I have nothing to do with APS. But recommend their courses.
MS
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Old 29th Mar 2015, 01:24
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Yes,talk to your chief pilot but also talk to the Head of aircraft maintenance at your organisation. Hopefully they will be on the same page re engine management. (God help you if they are not).

Otherwise Mick Stup is on the money. Maybe try & convince the boss that an APS course is a good idea (because it is). Otherwise try & find some spare coin and do it for yrself. Not sure if it's tax deductible as part of work related self funded education.

Also not related to anyone at APS but do use the knowledge I learned there every time I fly, with the CP / Haamc / Engineers approval.
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Old 29th Mar 2015, 03:03
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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!"


If they do then they are simply unnecessarily overcooled in the cruise and suffering from excessive cooling drag.


Talk to any high performance non cowl flapped kit plane owner about spending undue time on the ground with the engine running.


If you want to see what a 540 donk install looks like on an aircraft that spends its time at high power and low speed and lots of ground idling then look at the size of the cooling outlet on a Pawnee. And still you can get them so hot you can actually feel the powering dying.
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Old 29th Mar 2015, 06:42
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In PNG years ago when flying some rather well used A model C402s the Company policy was cowl flaps open for all ground ops, T/O and climb, closing them about 200' short of cruise altitude.

They remained closed until after landing when opening them was part of the After Landing checks; ie Flap lever located, identified and flaps Up, cowl flaps Open etc
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Old 29th Mar 2015, 10:18
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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!"


If they do then they are simply unnecessarily overcooled in the cruise and suffering from excessive cooling drag.
Bit oversimplified here. High performance machines generally do a few things differently like:
1. Climb faster (higher TAS - more cooling airflow)
2. Cruise higher (cooler air)
3. Climb quicker (less time to get hot)

My Aerostar, when it's hot, can't climb at anything near best angle or best rate without the CHTs creeping up. Fortunately, I can still climb at >500fpm at Vy + 30kts. Ted Smith has been accused of many things but 'unecessarily overcooled in the cruise' isn't one of them. Never have any problems with CHTs on the ground.

Your Pawnee example is true because they're designed to fly slow and low - and need to compensate for a lack of cold ram air.

UTR
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Old 29th Mar 2015, 12:58
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"Bit oversimplified here. High performance machines generally do a few things differently like:
1. Climb faster (higher TAS - more cooling airflow)
2. Cruise higher (cooler air)
3. Climb quicker (less time to get hot)

My Aerostar, when it's hot, can't climb at anything near best angle or best rate without the CHTs creeping up."


1. It is not the TAS that cools it is the IAS
2. Higher = less air density which reduces cooling capability, not sure where the cross over of reducing OAT and air density is but suffice to say at FL's (with turbo/super charging) you will struggle with cooling


Your last comment sums it up, it is either overcooled in the cruise or under cooled for low speeds and ground ops, hence why cowl flaps were invented, you can't have it both ways.
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Old 29th Mar 2015, 20:23
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At the moment the centre 2 cylinders on our TSIO360's are running hottest. However, despite our baffles LOOKING good, you can see an airflow pattern and location of 2 leaks on the inside of the cowl. These can be traced to 2 issues in our case. Firstly, an area where the baffles don't conform around a strengthening rib adequately and secondly where a LAME has taken some shortcuts in fitting new baffles and not running replacement baffle strip far enough forward to seal to the nose bowl.

I expect that when we correct these deficiencies that we will have quite even CHT's
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Old 29th Mar 2015, 23:46
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I have just returned from a packed out (above MTOW) APS class this weekend. Boy am I wrecked! Owners, commercial operators and everything from a Jabiru powered U/L to a C421 owner, Chieftan and Airvans……what a great mix!

I am most pleased to see this thread and some good advice in it. I have a few thoughts, first of all to anyone reading it,PM me your details and I will keep you informed when the next class will be.

First of all a little mind game. A previous student reports that after the class and learning the "really old way" of operating his C182 (IO540) he was able to do the following things. Burn way less fuel, have more range, cooler CHT's and actually have a higher IAS/TAS. How is this possible when WOT/2400 and ROP produces more HP than when he was WOT/2400 and appropriately LOP?

The simple answer is he could now contain CHT to a reasonable level with the cowl flaps closed and they remained lower. A win / win I would say.

The secret to all this lies in the following. First of all you need to get your Fuel/Air ratios sorted out. This then balances up the heat IN part of the equation. Then you can work on a level playing field at getting the baffles etc sorted and thus the heat out part of the equation fixed. What is left is the cruise CHT, and if you have the first two sorted the cowl flaps can be closed.

DO NOT go monkeying around with the injectors unless you work for GAMI or your last name is Denyer and you have a flow bench.

SLIGHT CORRECTION FOR A STATEMENT ABOVE
It is a commonly held belief that the higher the power higher the CHT. this is not true. In fact at 500 feet running WOT/2500 and 80dF LOP generating 82-84% power my CHT's are 30 or so degrees cooler than FL130 WOT/2400 and appropriately LOP. Someone above (not radar identified) mentions the air density.

It is also true that even generating the SAME horsepower at the same altitude but using two different methods of doing so will result in different CHT's. This is all to do with the effective timing of the engine controlled by mixture, resulting in a different placement of ThetaPP, which results in a higher or lower peak pressure. It is the magnitude of the peak pressure that determines the heat into the cylinder head, and not the HP or fuel flow. This is not what we are taught in flying schools.

In the photo on the front cover of the APS class manual you can see if you look closely some interesting data. Besides weather, look at the engine instruments. Both engines were generating the same HP, the pilots feet were flat on the floor and the skid ball in the middle


Any questions drop me a line!
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Old 30th Mar 2015, 01:12
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Interesting stuff there ^

A bit off topic, but what sort of CHT (if so?) can tend to cause cylinder glazing.. Experienced this and someone brought up cowl flap use (without explanation). My general understanding is that it is caused by light engine load or improper breakin.
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Old 30th Mar 2015, 02:50
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You were correct. It is from babying it during break-in. During break in it is important to have high MEAN(average) EFFECTIVE PRESSURES and not necessarily high peaks.

In other words, engine power settings at greater than 75% is generally considered the goal. So lets assume that is 225HP on a 300 HP engine. You want the high MEP rather than a high peak for two reasons, one has a greater sweep of piston ring travel with a highish pressure and you want the one that has the lower CHT.

This is where many LAME's and all but a few engine builders come unhinged, because the best way to achieve the better break in is using a LOP power setting. Of course some engines straight out of the box will not have good F/A ratio's and thus will not run smoothly LOP. If you can get that sorted at the beginning that is the best way. If you can fit a set of GAMIjectors and go for it, even the first cut is usually good enough, and you can tweak them during the break in period. Which by the way is really 5-6 hours and certainly by 10.

If you can't get a smooth engine (good F/A's) then do the break in at say 75-80%, which generally equates to 5000' or 24" and 2600 RPM in most engines and FULL RICH.

Vary the RPM occasionally and don't be afraid of it. Not a good time to baby it.

One method has cooler cylinders, better mean effective pressures over a greater sweep and is by far cleaner on the combustion chamber etc. One is lighter on your wallet too!
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Old 30th Mar 2015, 03:16
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Babying the engine is a false economy. I recently watched a top overhaul on an O-470 at 1000 hours due to glazed bores. The fact that the owner insisted that the plane was warmed on the ground until the oil was "in the green" then insisted on reducing power as soon as the flaps were retracted was considered a contributing cause.

My gauge of when to take off has always been CHT. Once the first cylinder hits 300 I am away regardless of oil temp. In my POH (for an O-360) it clearly states that full power can be applied as soon as the throttle can be advanced without hesitation or rough running.

I recommend a Jabba (APS) course to any piston aircraft owner.
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Old 30th Mar 2015, 03:36
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Last edited by Radix; 18th Mar 2016 at 01:33.
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Old 30th Mar 2015, 03:47
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What about S/E ops in a Chieftian.

Cowl flaps open you will most likely not be able to maintain height.

Cowl flaps closed, you should be able cruise and maybe climb but from my experience doing endorsements and also according to the very experienced pilot who did my endorsement in BK last century, the live engine will most likely over-temp.
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Old 30th Mar 2015, 03:54
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If I had an engine failure in a Chieftan for real, I don't think I'd be worry to much about high CHT's.
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Old 30th Mar 2015, 06:01
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I had two genuine engine failures in PNG flying 'Bongo vans' and I can assure you that high CHTs on the remaining live engine were not even a passing consideration, at least for the first few minutes anyway!

Once I had everything dealt with, and was headed back to base I then considered the state of the remaining operating engine.

Going down in the approximate area of where one initial failure occurred could possibly have resulted in another aircraft missing and never to be seen again.

Last edited by Pinky the pilot; 30th Mar 2015 at 06:24.
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Old 30th Mar 2015, 13:16
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"A bit overstated again. The venerable GA8 Airvan for example has 'smart' cowling that is designed to produce different cooling airflow at low speeds versus high speeds. All because angle of attack will be different. So, have it both ways without those pesky cowl flaps! And it works very well."


Ok I'm in.


How does it work?


Jabawocky,

"It is a commonly held belief that the higher the power higher the CHT. this is not true. In fact at 500 feet running WOT/2500 and 80dF LOP generating 82-84% power my CHT's are 30 or so degrees cooler than FL130 WOT/2400 and appropriately LOP."

That statement is likely trip off a whole new round of incorrect myth and confusion for some low time/experience pilots.


It is imperative to apply sound scientific principals in your above comparison and that means you must have exactly the same configuration bar one change, the power output. Same AoA same mixture same OAT same altitude same IAS same cooling configuration.


If you do that and increase power you will see greater heat loading and therefore a CHT rise. I point this out for the benefit of others as I have no doubt you know all this and it was just a somewhat rushed statement at the end of a long day.


Certainly using the full mixture spread capability you can do many things with temperature including increasing power whilst reducing CHT and EGT, but this is not the same as saying it is not true that the higher the power the higher the CHT.

Last edited by Obidiah; 30th Mar 2015 at 23:25.
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