Weight vs. Horspower

[Cardiff Kook]

It has been a while since I polled the cognescenti.

All things being equal (which they never are) would you rather have a 185 that weighs 1795 lbs with a 350 hr io-520 or a 185 that weight 1955 lbs w a io-550 and 950 hrs on the engine. Why?

Pros for the ligher one are obvious- shorter landings and possibly better handling. Lower engine hours it MAY actually perform better than a 550 with 600 more hours on the engine.

550 is faster with less fuel burn. Maybe climbs better- but the HP to weight ratio would probably be about the same with a 550 lbs load.

If you were considering floats in the future how would that play?

I realize this a very broad question- and maybe not answerable- but I needed an excuse to engage with the brain trust.

 

[skywagon8a]

I've had both engines in my 185 on floats. The IO-550 is the better choice by far. The IO-550 produces a minimum of 300 horsepower at 2700 rpm and the IO-520 produces a maximum of 285 horsepower at 2700 rpm. The question you need to address is "Why is one 185 160 pounds heavier than the other?" Can you remove that 160 pounds, or most of it? Is there some other reason which makes the IO-520 airplane more desirable than the IO-550 one? If the IO-520 airplane is otherwise more desirable, change it to a IO-550 when overhaul time comes.

I thought your 185 has an IO-550?

 

[hotrod180]

.... and the IO-520 produces a maximum of 285 horsepower at 2700 rpm. ......

I believe the IO-520 as installed in the C185 is rated at 300hp at 2850 rpm for takeoff (5 minutes), 285 hp at 2700 rpm continuous.

 

[Cardiff Kook]

I believe the IO-520 as installed in the C185 is rated at 300hp at 2850 rpm for takeoff (5 minutes), 285 hp at 2700 rpm continuous.

Correct.

Apparently the 550 though was rated using a newer/different standard than the 520 where every 550 was required to make 550 from the factory. The 520 I think was only batch tested and it had to be within some percentage of 300 like 5% (there is a post covering this somewhere on the site.). Net result being that some 520's never could make 300 hp at all while all 550's had to so they were forced to put in some margin where most made over 300.

 

[skywagon8a]

I believe the IO-520 as installed in the C185 is rated at 300hp at 2850 rpm for takeoff (5 minutes), 285 hp at 2700 rpm continuous.

That is correct. I was comparing both engines at their maximum continuous rated rpm. Also the method of rating the engine changed between the time the two engines were certified. The IO-520's rated power is a maximum number, while the IO-550's rated power in a minimum number.

 

[Charlie Longley]

I was pretty happy with an O470R in my 180 with a seaplane prop. I had to pull the power back quite a bit when doing formation takeoffs with my buddy. Our empty weight was similar except he had worn off O470J with a standard propeller.

There is some happy weight to HP ratio for each aircraft. That's my theory anyway. On my Super Cub project, I am looking at every ounce that gets added.

 

[mvivion]

I've never met a pilot who thought more power was a bad idea. Unfortunately, as is the case often with pilots, as aircraft age, they often gain in mass. The good news is, it's generally easier to reduce that mass on an airfame than it is to reduce the scale reading for a pilot. That said, nothing is free. I'd be looking to put that heavy 185 with the IO 550 on a diet, and see if I can reduce its weight some. Or install a 550 on the lighter one.....

I believe the IO-520 as installed in the C185 is rated at 300hp at 2850 rpm for takeoff (5 minutes), 285 hp at 2700 rpm continuous.

As Pete noted, the older rating scheme that Continental used until they certificated the IO 550 allowed for demonstrated power on the dyno of plus or minus (I believe) 3 % of rated horsepower. I've also heard that it was +/- 5%, but result is similar. So, a 300 hp rated engine could actually make 291 hp (if -3% is the number) or 285 hp (if -5% is) and pass muster and be mounted on your airplane as a "300 horsepower engine".

Under the new rating system, under which the IO-550 was certificated, the engine MUST make at least its rated horsepower or up to 5% more than rated: Plus 5%, minus 0.

So, as Pete noted the IO-520 could pass muster producing significantly less than its rated 300 hp, whereas the 550 WILL make at least 300 hp.

Now, consider that, as a manufacturer, you have quality control. A QC inspector pulls an engine off the line, puts it on the dyno, and it MUST meet the chosen rating criteria. If you know that any given engine could be tested, and failing that test could be very painful, and that test says you MUST make rated power, in fact, you're likely to design an engine that actually makes significantly MORE than minimum rated power, just to be sure every engine tested passes the test. That IO-550 may in fact make 305 or more power.....just to be sure.

And, btw, Lycoming has used this later rating scheme for decades.

MTV

 

[hotrod180]

....All things being equal (which they never are) would you rather have a 185 that weighs 1795 lbs with a 350 hr io-520 or a 185 that weight 1955 lbs w a io-550 and 950 hrs on the engine. Why?.....

Might be useful to calculate the power-to-weight (aka power loading) of these two.
1795 / 285 = 6.3 pounds per horsepower (assuming full 285hp)
1955 / 300 = 6.5 lbs per hp.
So those two are very close-- on paper.
But you need to take into account that the heavier airplane is gonna feel heavier,
and (assuming the same gross weight) is gonna have 200 pounds less useful load.
Personally I think I'd go for the lighter airplane, the lower engine time would just be a bonus.
BTW I'm curious as to why this "poll"... as I recall, you already have a 185.
You thinking about trading it in for another one?

 

[StuBob]

550 is faster with less fuel burn.

How is that possible?

 

[BC12D-4-85]

I'd poll a few overhaul facilities or commercial operators to determine which typically makes TBO without replacement of major components, and shows less wear at overhaul. Dependability is important.

Gary

 

[mvivion]

How is that possible?

Both engines CAN be run Lean of Peak, but the 550 really loves LOP ops.

MTV