Car engines start at the first turn but the aircraft engines dont most of the time, years ago i had a renault 12 with a choke on it, it showed the same behaviour with my 2008 aircraft engine
Car engines i am sure have the same shock cooling, why is it a problem for aircraft engines,
Why change oil every 50 100 hours for aircrafts while this period in cars is around 1 year or 10000 kms,
I really wonder why, is it a problem of additional weight and cost?
Properly maintained aero engines start just fine with the correct technique. I've had plenty of car engines that were mongrels to start.
Car engines *don't* have the same shock cooling issues - they're liquid cooled with a thermostat to control coolant flow into the engine so that the temperature is relatively constant.
Car engines don't operate at full power (or some high percentage of full power) then 55%-75% for hours on end. Car engines rarely see full power, let alone operate continuously above 55%. My car specifies 3000miles between changes. Conservatively choosing 30mph average speed (and for easy calculation) that's 100 hrs of use - and not even above 50% power for any of it.
Big Pistons - I also fly geared engines, but the GTSIO are especially highly strung. I always quiz my old Commander mechanic (he knows serial numbers by heart after 30 years of only doing Commanders) if the old GTSIO-powered 685 would be something I should step up to at some point, fully knowing it's his least favourite of them all. Always gets him going. He says they rarely make TBO and often have to have some top work done before hitting 1000hrs. Sluggish take-off as well, even though they have all that power. They do perform like turbines up high, though.
I think the part of the recommendation in the manual you talk about is probably due to gearbox limitations rather than a cooling thing, no? The gearbox chatters and doesn't like to be pushed by air. I always try to fly mine with positive drive and have been cautioned to never pull to idle ever, except in the flare. It sometimes means she's little harder to slow down, but if you dump gear early, it's normally not a problem.
Unlike the geared lycomings which uses a planetary reduction gear the Continental GTSIO 520 has one just massive spur gear to provide the reduction in prop RPM. It is pretty unbustable, the problem with this engine is the cylinders. They are basically the same as the 285 hp I0 520. Pulling another 90 hp (Cessna 421) or worse another 115 hp (Commander 685) is asking a lot. High CHT's or shock cooling will just kill this engine. However the last 421 I flew was sold with 1400 hrs on both engines and both had all original cylinders. It will hold up just fine if you treat it right.
I was flying tugs at Lasham when the cylinder cooling studies were done. Shock cooling does exist, and the changes made to the flight profile made a big difference to our maintenance costs. Does anyone perchance have a copy of the original study, or a link? Thanks for thinking about your engine!
My personal opinion is for the simple 4 cylinder engines fitted to your typical club trainer/tourer it is pretty much a non issue.
I think the reason for that is elsewhere:
Most flight training (UK, anyway) is done with full-rich mixture all the time, so the engine doesn't get that warm to start with.
Also many of the higher performance aircraft have less air going into the cowling, because air going down those holes means drag.
And the slower aircraft have less potential for shock cooling because the air at 100kt has a much smaller cooling effect than at 150kt+.
So I don't think it is exactly engine type or size related. For example the 360 is the same as a 540; just 2/3 of the length. It has the same cylinders, AFAIK. Maybe the 0-200 type engines are more robust, due to smaller cylinders but the metal is the same thickness as the bigger cylinders?
I am surprised to see no mention of the cowling vanes (or what do you call them?) ; I always understood Lycosaur-powered planes have them, and proper technique includes closing them during descent (sic!) for the exact reason of avoiding, or at least reducing, shock cooling?
Jan Olieslagers
* Join Date: Jul 2010 Location: Near EBGB Posts: 967 I am surprised to see no mention of the cowling vanes (or what do you call them?) ; I always understood Lycosaur-powered planes have them, and proper technique includes closing them during descent (sic!) for the exact reason of avoiding, or at least reducing, shock cooling?
Cowl flaps, and it depends. None on the 152/172, Pa 18 supercub, and many others.
Unlike the geared lycomings which uses a planetary reduction gear the Continental GTSIO 520 has one just massive spur gear to provide the reduction in prop RPM. It is pretty unbustable, the problem with this engine is the cylinders. They are basically the same as the 285 hp I0 520. Pulling another 90 hp (Cessna 421) or worse another 115 hp (Commander 685) is asking a lot. High CHT's or shock cooling will just kill this engine.
Car engines i am sure have the same shock cooling, why is it a problem for aircraft engines,
As Tinstaafl mentions, car engines are liquid cooled. The cooling air flows over the radiator, rather than the engine. The engine maintains a much more uniform temperature throughout, and that temperature changes much less quickly when conditions change. If something shock cools, it would be the rad, not the engine. This is a happy characteristic of liquid cooled aircraft engines. Having done 64 three minute climbs these past four days, in a DA42, with the required descent following each climb, I was delighted that the liquid cooling allowed somewhat more rapid power changes.
Quote:
I always understood Lycosaur-powered planes have them, and proper technique includes closing them during descent (sic!) for the exact reason of avoiding, or at least reducing, shock cooling?
Yes, if you have cowl flaps, their proper use it very important. Any aircraft which has cowl flaps will also have a cylinder head temperature indicator. As mentioned by Big Pistons, some lower powered aircraft engines are less sensitive to shock cooling, and thus do not have cowl flaps. It is still and excellent habit to treat those engines similarly gently with power changes. It is still better for the engine, and forms a good habit, which will be reassuring to the check pilot when you are hoping they will sign you off in the bigger engine plane. That said, I have see cracked O-200 cylinders...
Car engines don't operate at full power (or some high percentage of full power) then 55%-75% for hours on end. Car engines rarely see full power, let alone operate continuously above 55%. My car specifies 3000miles between changes. Conservatively choosing 30mph average speed (and for easy calculation) that's 100 hrs of use - and not even above 50% power for any of it.
--- My bike specifies 10,000 km between changes, is a small, light engine producing 160 HP, runs at ~50 - 60% power a lot of the time (on motorways) and gives no problems... but it is new technology, fly by wire throttle, electronic injection, etc.
it is possible with airplane engines too - sure the new rotax engines will prove very reliable for instance... just need more competition and volume in engines to spur the manufacturers on - what's the point in adding 20kg to make an engine that only needs an oil change every 200 hours, when regulations will make you inspect it every 50 anyway.
At the club where I tow our procedure, after the glider releases, is to take 15 seconds to go from full climb at 65-70 kts, typically, to the descent airspeed of about 110 kts whilst maintaining more or less constant RPM (not constant speed props) and then a further 15 seconds to reduce to 2300 which gives about 1000' a minute rate of descent. At about 500' the tug is levelled off, without moving the throttle, allowing the speed to reduce into the white flap band and the rpm will of course drop further. Power is gently reduced on final. Since this procedure has been in place the number of cracked cylinders has become negligible - before it was not.
The descent procedure for the Scouts (180 HP Lycoming) at my club in Alberta was to throttle back to 2100 rpm, maintaining the tow speed ~70 mph and dump full flap. After one minute or CHT below 300, reduce to 1700 rpm (below the resonance zone) and descend at 90 mph with full flap.
I'm not aware of any of the Scouts at Alberta gliding clubs ever having any cracked cylinders.
"A lot of the time..." is still not the same duty cycle as a typical aero engine. An aero engine can typically climb at, or near, full power for some time then cruise at 65 or 75% for several hours. It only experiences power at less than 45% during the approach & landing.
As an example, I flew a Navajo yesterday for about 6 hours. A single leg of that flight consisted of full power for take off, 85% for 25 mins followed by a continuous 65% for another 2 1/2 hours then finally low power for the las 5 minutes for the approach & landing. It's quite capable of doing that day after day, hour after hour. I'd have no qualms about doing a 5 hour sector in it, with the engine running continuously at that 65% for the cruise. I wouldn't like my car to have to do that.
The piston engines in most aircraft don't need an ancillary electrical system to run, unlike cars with their battery, alternator, coil & distributor (or electronic equivalent of the coil/distributor) system. If the engine is turning the ignitions (there are two) will provide spark without any involvement of the ancillary electrical system. Hell, one whole ignition system can fail and the aero piston engine will still run with only a minor loss of performance.
Is it possible to wring greater economy or performance? Of course! But always at a price. Cost, weight, complexity, reliability & increased maintenance issues all get a look in. One thing I'd like to see is magneto timing adjustment system. It could be electronic or mechanical but I'd want it to have a failsafe reversion to a simple fixed timing magneto if the whiz-bang bit went wrong.
Thinking about it, when I took the bike on a track day, when the profile more closely fits your useage, the engine gave up half way through... so not sure i'm on a winning argument here.
On the commander I fly, I've been taught to set the engine monitor (EDM700) to show shock cooling - it shouts if cooling exceeds 50F/min anyway, which makes life easier (albeit meant a descent at 160Knots - cruise is 130) to keep in enough engine power to avoid cooling too quickly.
I don't know much about engine design but as a general engineering description its more about the temprature gradient across a lump of solid.
You can also get shock heating.
Basically when you get a temprature difference across an object there will an associated stress/strain gradient set up by the differences in the expansion of the material at the different tempratures, if this stress or strain is more than a number of engineering events the material will fail.
Now even if you don't exceed the value which would cause a immediate failure you might very well be into an area of the stress / cycles of a fatigue graph which was never designed for by the engineer. So even if it doesn't fail you have used up the life of the material. The amount used will have a none linear quanity related to the amount of temp gradient.
And example of shock heating is pouring boiling water into a old style pint glass and it shattering
And an example of shock cooling is a compressed cyclinder exploding when the contents are expanded in a none controlled manner.
modern cars specify at least 10,000 mile oil change intervals.
A large part of that is due to water cooled engines (close piston clearances), and better air filtering (aero engine air filters generally drop only about 0.5" MP across them, or less).
Not silly at all. Aero engine service intervals are not massively different to car engine service intervals, and the oil in the latter benefits from a cleaner environment.
One other factor I forgot to mention: in a car engine, after you park the car, it is harder for water to get into the engine and condense there, into the oil. Aero engines have big breathers, which is one reason why corrosion due to infrequent use is such a problem. Another reason is that the large amount of blow-by past the loosely fitting pistons (air cooled engines) dumps a lot of corrosive muck into the oil, which will accumulate if the engine does only short flights.
There is a well known article by a gliding tug operator who reported consistent cracked cylinders, until they did a bit of a low power section before the (rapid) descent, and it cured the problem.
Can someone point me in the right direction for the article? I can't find it. I've read it a few years ago, but did not bookmark it. It's some kind of scan of a magazine article. Your help is appreciated!
