Cessna 182...a SLICK airplane?

[3WE]

I was reading Flying magazine about yet another Diesel engine conversion, this time on a 182.

The article implied that the plane was hard to descend and that you needed to slow to 140 kts and drop 10-degrees of flaps, and said that the glide ratio was 11:1.

That was surprising- wing struts, fixed gear, Omnivision, and plenty of rivets- but the plane is slick and slow to descend? I also didn't think the wings were excessively long and glider-like.

The article makes no mention of over-cooling or cowl flaps, I could see where that would make for a slower descent, but not the airplane.

[Gabriel]

Is 140kts within the envelope of the flaps 10 config?
And is 11:1 the glide ratio at 140kts and flaps 10?

The answers to these questions would make a difference. 11:1 is not a very good (not very bad either) glide ratio for a clean airplane. At 80kts (if not exactly at least close to the best glide) it would be making some 750fpm. However, at 140kts (about twice best glide speed) and flaps 10(nothing good for a good glide) 11:1 would be quite remarkable.

If 11:1 is for best glide, then you should know that at the relatively slow speeds those airplanes glide "slikness" (in terms of rivets, struts, gear...) is not a huge factor. Wing shape is (aspect ratio, airfoil, geometric and aerodynamic tapper, and planform shape, in order of importance). Because this defines the drag you have to make just to lift the weight.

At higher speeds, with parasite drag growing with the sqare of the speed, things sticking out in the airflow start to be more relevant.

Think for example of the PA-11 or J-3. Hard to imagine a less slick airplane, but they glide Ok.

[3WE]

I'd assume the 11:1 is best glide near the 80 kts you describe and was supplied in the article as background information to suggest that the plane is "hard" to descend.

No it's not a glider, but sure beats the heck out of the space shuttle

But I'd figure at 140 kts & CLEAN that, descending would not be an issue in strutted/fixed-gear Cessna, unless you have to carry a very high power level for some other reason.

[Dmmoore]

The reason no cowl flaps were discussed is because they do not exist on the Diesel, it's liquid cooled.
The engine cowling is aerodynamically cleaner giving the aircraft a different descent profile over the air-cooled version.
A very nice conversion / option for the sturdy Cessna four place.
The 182 has always been one of my favorite moderately well performing, fixed gear, aircraft.

[3WE]

The reason no cowl flaps were discussed is because they do not exist on the Diesel, it's liquid cooled.
The engine cowling is aerodynamically cleaner giving the aircraft a different descent profile over the air-cooled version.
A very nice conversion / option for the sturdy Cessna four place.
The 182 has always been one of my favorite moderately well performing, fixed gear, aircraft.

I was thinking "liquid cooling", even though the cowl has some conventional looking inlets right where they've always been.....however that muddies the water even more. With liquid cooling, you have thermostatic control of coolant flow and much more constant engine temperatures allowing you to chop the power, point the nose at the ground (pitch controls altitude) and redline the aircraft without the risk of "shock cooling" an air-cooled cylinder head?!?!?.

To dot all "i"s, yes, a "full" power reduction would still affect the temperature of many things (especially turbochargers which do exist on these engines).

On some diesel engines it is important to idle the engine for a few minutes before shutdown to cool the turbocharger, but that's because if you shut down totally from a high-power situation, the turbo overheats.

My bottom line remains: I am still amazed that it's a "challenge" to make a fixed-gear, strutted airframe properly descend without dropping flaps and flying faster than a 172 can go. (unless there is some issue with the engine having to run at a very high power setting- which was not mentioned in the Flying article.)

[Gabriel]

No it's not a glider, but sure beats the heck out of the space shuttle

Yeah, evn the Tomahawk beats the space shuttle (barely)

[Gabriel]

The reason no cowl flaps were discussed is because they do not exist on the Diesel, it's liquid cooled.
The engine cowling is aerodynamically cleaner giving the aircraft a different descent profile over the air-cooled version.

Since I know very little about Diesel engines, be it in palnes, cars or whatever, perhaps you can help me.

Ok, the engine is liquid cooled. But the liquid must be cooled too or it would boil. I guess that, like in gas engines in cars, the liquid cools the engine and the air cools the liquid in a radiator (I also gues that. unlike cars, there won't be a fan). But in the end, you must have cooling drag due to air going through a heat exchange system, be it to cool the cylinder or to cool the liquid that cool the cylinders. So the question is, why would liquid cooling be cleaner from an aerodynamic point of view? (I can only guess that maybe the heat exchange is much more efficient so much less air has to go through).

And very good point made by 3WE: Being liquid cooled you would not shock-cool the engine, so you could just idle the engine and dive down as steep as you want (with the limit of the redline). Is there any flaw there?

[3WE]

Why does a liquid-cooled engine have lower drag than an air-cooled engine?

Your question is valid as to some extent you have to pass X amount of air over Y amount of hot metal to transfer Z amount of heat, and that may not radically change just becuase you have coolant in the mix. Yes, there's going to be some drag in the colling system of a liquid-cooled engine.

I think the answer to that on an air-cooled engine, you have to design "a thing" that both produces power and cools itself. So, air has to flow around big finned cylinders and cylinder heads which are cylinders and not sleek aerodynamic ovals.

However a radiator can be better engineered to have air "flow straight through it" and better designed for optimal heat exchange, without having to consider how to withstand the pressures of combustion and contain a piston that's flying back and forth 2000+ times a second, etc.

So, you can set the radiator "off to the side" somewhere and have a nice clean airflow, while the engine cooling air also encounters push rod guides, wires, fuel lines, alternator belts, suction pumps and all sorts of non-aerodynamic things.

As to cowl flaps- I could make an argument that on a cold winter day, you may need minimal airflow over a radiator and could theoretically reduce drag by running less air through a radiator.

But whether you get a significant "bang for your buck" by doing that is another question- it's probably less critical with liquid cooling vs air.

The "trouble" with liquid cooling has traditionally been weight.

[Dmmoore]

Well, it turns out it's an Aircooled 5 liter, turbocharged, 4 opposed cylinders.

[3WE]

Whoops!

I will say that most diesel cowlings have no air inlets immediately to the side of the spinner, but the 182 pictured DID have very normal-looking inlets, but also had some OTHER inlets.

The one photo of the "power area" did not show a cowl flap control which you might expect, but then again, in the pickup-truck world, diesels are considered to run cooler than gas?!?!?

At this point I'm going to assume that this engine is in "some way" more sensitive to shock cooling and that you can't throttle back too much lest you cool it too much.

And- just for grins- being expensive is one genuine way of being sensitive to shock cooling.

[PurduePilot]

However a radiator can be better engineered to have air "flow straight through it" and better designed for optimal heat exchange, without having to consider how to withstand the pressures of combustion and contain a piston that's flying back and forth 2000+ times a second, etc.

Damn, that's some engine!

[RadarContactLost]

Yeah, the highest reading tach I've seen only went to 20,000 and was redlined at 15,000.

[3WE]

However a radiator can be better engineered to have air "flow straight through it" and better designed for optimal heat exchange, without having to consider how to withstand the pressures of combustion and contain a piston that's flying back and forth 2000+ times a second, etc.

Damn, that's some engine!

Minute....second.....quit being so pedantic....and where in the hell did the edit button go?

[PurduePilot]

Yeah, the highest reading tach I've seen only went to 20,000 and was redlined at 15,000.

Was that the fach for a turbo??

[PurduePilot]

However a radiator can be better engineered to have air "flow straight through it" and better designed for optimal heat exchange, without having to consider how to withstand the pressures of combustion and contain a piston that's flying back and forth 2000+ times a second, etc.

Damn, that's some engine!

Minute....second.....quit being so pedantic....and where in the hell did the edit button go?

aoppppppppparently with the new forum it disappears after a little while of making ht eoriginal post. argh!

[flyboy2548m]

So the question is, why would liquid cooling be cleaner from an aerodynamic point of view? (I can only guess that maybe the heat exchange is much more efficient so much less air has to go through).

That's a pretty good guess, Gabriel. Think P-51 vs P-47.

[Digger]

Didn't the P-51 actually realize some thrust from the cooling system?

[Dmmoore]

Didn't the P-51 actually realize some thrust from the cooling system?

Off Topic
No. The radiator is inside the scoop on the belly. It's pure drag.

The DeHaviland Mosquito used leading edge surface radiators which caused very little drag but also provided little cooling for ground operations.

[RadarContactLost]

The P-51 and ME-109 cooling system ducting produced thrust, but it wasn't more than the drag produced by the radiators. The planes would have been much worse off if the cooling systems were not "tuned."

The 20,000 rpm tach was off a RR Dart, the Brits were more worried about engine speed than prop speed.

Of course the 182 is a slick plane, it doesn't have any guns mounted! (Probably a meaningless statement to anyone under 55.)

[Gabriel]

So the question is, why would liquid cooling be cleaner from an aerodynamic point of view? (I can only guess that maybe the heat exchange is much more efficient so much less air has to go through).

That's a pretty good guess, Gabriel. Think P-51 vs P-47.

I'm afraid I don't get it. If the pictures in my mind are right, the main factor about aerodynamic cleanless between those planes is that the p-47 is a radial engine while the p-51 has cylinders in line?

But I guess that's not what you meant. So I'd appreciat if you explained what's the p-51 vs p-47 thing you had in mind.

[VectorForFood]

Big fat flat surface area creates drag, not streamlined by any means...



Put that in comparison with the P51, liquid cooled, so less surface area meeting airflow, streamlined, thus less drag.

[flyboy2548m]

Big fat flat surface area creates drag, not streamlined by any means...



Put that in comparison with the P51, liquid cooled, so less surface area meeting airflow, streamlined, thus less drag.

Thank you, Multiman, hopefully Gabriel gets it now.

[3WE]

But the P-51 does have a radiator scoop that creates some drag.

And, in the fancy case of using the aircraft skin/wing leading edge as the radiator (i.e. no added surface for parasite drag), the coolant likely adds weight and induced drag.

And, don't forget there's a little horsepower used to run a water pump.

Just want it clear that liquid cooling is not a zero-drag affair and that there's reasons why the majority of piston aircraft engines use air cooling, even though there's issues with drag and less-optimal cooling.

But, yes, the pictures make a striking case that you can have a much lower-drag engine cowl with liquid cooling.

And yes, Gabe- all the cylinders in a straight line makes it easier to design a long, narrow, low-drag cowl, as opposed to cylinders pointing different directions, that scoop in air and run it through hundreds of fins & other plumbing.

[3WE]

For completeness, I should mention that engine cooling is pretty important and a coolant pump failure, or a significant coolant leak (possible from a very large number of places) could require that you have a glider rating to legally fly the plane.

That may be a bigger reason for the predominace of draggy air cooling.

[Dmmoore]

For completeness, I should mention that engine cooling is pretty important and a coolant pump failure, or a significant coolant leak (possible from a very large number of places) could require that you have a glider rating to legally fly the plane.

That may be a bigger reason for the predominace of draggy air cooling.

We are getting off topic since the Diesel engine in the aircraft in question is air cooled.

However The difference between the aerodynamic drag of the P-47 and the P-51 can be shown in performance and specifications. The R-2800 engine powering the P-47 produced 2000+ HP and by augmenting the engines built in two speed supercharger with two external turbo chargers allowed the engine to produce that power above 20,000 feet.

The P-51's V-1650 Merlin produced 1400 - 1600 HP (depending on engine model) without external turbochargers. The performance of both fighters is essentially the same. The lower HP requirement is one reason for the lower fuel burn of the P-51.

In combat, the P-47, designed for high altitudes, excelled in low altitude strafing because the aircraft was impervious to small arms fire. You had to badly damage the engine its self, the pilot (who sat in front of a thick armor plate) or set the self sealing fuel tank on fire to down the aircraft. A single shot into the coolant system of the P-51 would down the aircraft.

Several P-51's were lost when the coolant system was damaged when they flew through the debris of an aircraft they had just shot down.