Rated Horsepower Q

[airdale]

One of life's little mysteries:

The Lyc O540/IO540 is used in a lot of airplanes, rated variously IIRC at 230, 235, 260, and 300 hp. I think I even saw a conversion shop advertising it at 310.

So if I am Cessna and building a 182, why do I rate the engine at 230 instead of advertising 260 or even 300MTOP like in a Saratoga? Or if I am Piper back in the day, why 235 instead of 260 when I have the 260s sitting right there for the Six? Speed sells and I can't imagine that the price of a 300hp rated IO-540 is much more than a lesser one. It's basically all the same parts.

I'm guessing that it must be some kind of certification issue, but I see Diamond DA-40's advertised cruise speed is in the yellow arc, so it would seem that 260hp putting a 182 a bit into the yellow (if it does) would not be an issue.

Facts, anyone?

 

[Dan Thomas]

Lycoming's 540s aren't all the same engine. They have 540 cubic inches, but there are many internal and external differences. If the 182 is designed for 235 hp then Cessna will risk either damage to the airplane if it's overpowered, or they'll have to beef it up and make it heavier, and they'd also need bigger tanks to take more fuel for more horsepower. And the 540s that make more power do it at a higher RPM, too, so noise from the engine and prop gets worse, the exhaust system has to be capable of more waste heat, and so on. It can get ugly. More power also costs more.

A Saratoga is a six-seat airplane and is built to take such power. It also weighs a whack more and needs more power.

Lycoming talks of the engine differences here:
http://www.lycoming.textron.com/support/tips-advice/key-reprints/pdfs/Key General.pdf

Scroll down to page 28 of the Flyer, or page 22 of the pdf. Look for "The Same Engine Myth."

Dan

 

[Ddayle]

There are several STC's for engines up to 300 hp in the 182 ( and amazingly the wings don't fall off!) but most the 182's i have seen have a 470 stock. Dave

 

[PilotAlan]

Well, for one, manufacturers derate engines to differentiate models. The 172R and 172S each have IO-360 engines, one rated for 160hp and one for 180hp.
The Cherokee 6 has a carbuerated O-540 at 260hp, and an injected IO-540 at 300hp.
It might make sense that injection might bump up the power, but the Comanche used an IO-540 rated at 260hp.

In the case of the Cherokee 235, I am pretty sure the power was derated for airframe reasons, not anything to do with the engine. IIRC, they had to thicken the airframe skins to get 235 certified.
In other cases, manufacturers limited power to prevent cannibalizing other model lines. In Cessna's case, a 260hp or 300hp Skylane would have directly competed with the 210 (RG) and 207/208 (fixed gear).

There are only so many engines out there. Piper uses Lycoming, so there's only the O-320 (max 160hp), the O-360 (max 200hp), and the O-540 (max 310hp). So, whatever hp they want or can get certified has to fix in those three engines.
Also, more HP costs more. Higher compression, more work and more complexity in cooling the engine, etc.

It's just like cars. How many versions of the small block V-8 has GM come out with, with different hp rating? Or Ford 4.6L V8?

 

[Dan Thomas]

In other cases, manufacturers limited power to prevent cannibalizing other model lines. In Cessna's case, a 260hp or 300hp Skylane would have directly competed with the 210 (RG) and 207/208 (fixed gear).

Not 207/208. You mean 206/207. 208's the Caravan, a 675 hp turboprop. A PT-6 turboprop, versions of which run between 580 and 920 hp. Slightly larger units produce up to 1920 hp. See, even the turbine makers use a base engine to create numerous models of various horses.

Dan

 

[Dan Thomas]

There are several STC's for engines up to 300 hp in the 182 ( and amazingly the wings don't fall off!) but most the 182's i have seen have a 470 stock. Dave

The 182s had the 230-hp Continental O-470 up until Cessna stopped making piston singles in 1986. When they went back to building 182s in '97 or so they used the 230-hp Lycoming O-540, which they'd been using a 235-hp version of in the R/TR182s of '78-86. Textron owns Cessna and Lycoming (and Bell helicopters).

Dan

 

[denverpilot]

The Skylane is about as close to a flying pickup truck as one can get in a four-place single, and the main concern isn't so much engine size/power or fuel tank size (to feed a hungry engine), as it is Cessna shooting for the right "combination" of power, load, and fuel burn to make the venerable old airframe look good compared to the competition of the day, as far as I can tell.

Skylanes can always do one thing at a time a little better than average, but never two at the same time. True airspeed is usually 130-135. Not zippy but not a slouch. Fuel flow is usually 13 GPH down low to 11.5 GPH up high. Not great, but not that bad. Useful load is good, but if you have the extended fuel tanks you'll eat it up quick. No Utility Category, but other than tending to be nose-heavy CG-wise, it's hard to load it outside the envelope. Climb is great, lightly loaded but becomes anemic at altitude with a heavy load... Etc.

The only problem common to all Skylanes is that if you land them on the nose wheel you're going to do major firewall damage, and it won't be cheap to fix.

I mention this here because engine weight compounds this problem when someone transitions from a Skyhawk and has to learn that the trim wheel is probably going to be all the way nose-up for landing it properly and you're still going to have to haul back on the yoke.

If you didn't trim, you'll feel like your arms are getting a nice upper body workout. All that iron hanging out there and a tendency to be at forward CG limits with full fuel and two big folks up front, add up to making a heavy nose. Many Skylane owners leave a toolbox in the baggage area to get the CG back a bit.

The big fat beautiful, but heavy, Continntal O-470 is typically 1500 hour TBO (some O-470U models are 2000) and they do need care to make it that far without a cylinder needing topped. The IO-540 was probably chosen for similar weight and to get back to the more common industry "standard" 2000 hour TBO, by Marketing more than Engineering. The IO-540 derated has a 2000 hour TBO and probably makes it most of the time.

Later airframes of the 70s were either certified for 3100 lb takeoff weight or many early airframes can get it today via a cheap paper STC, up from the book number of 2950. The airframe is hellaciously strong. 2950 is still the landing weight though, so you'll be circlinYg a while or throwing someone/something overboard if you have to return "right now".

Aftermarket engines for the airframe abound, all the way up to the IO-550 from Texas Skyways, and I assume the Soloy turbine conversion is also still available but haven't checked. That ones a ways outta my league. So it's definitely not a "structural" thing.

And of course the turbocharged versions ytypically put out some more ponies at the cost of fuel burn...

The folks at P.Ponk have also done some nice work on prop sizes and number of blades with actual static "pull tests" to see how hard the aircraft pulls on the ground with different props on the pointy end. (They also make a nice engine upgrade.) Again an argument that it's not a structural limit. Changing the prop size can significantly change the static pull by a couple hundred pounds at a dead stop.

So... while I'd normally agree that engines are sometimes derated to deal with structural limits of the airframe, the sheer number of higher horsepowoer approved STCs for the Skylane model line, kinda makes me think that's not the limiting factor Cessna was working with. Just taking all things into account as a pragmatist and not an aircraft engineer.

They probably just wanted to sell more IO-540s after the restart, from their sister company under the Textron umbrella, and chose a similar horsepower and weight Lycoming to match the past Continental O-470 for consistency. They could have gone much bigger in that airframe. But, keeping fuel and other numbers similar probably makes the airplane "safer" for those upgrading from the O-470 to the newer birds. And remember after the liability lawsuit for the seat-track stupidity, the place was (is?) crawling with lawyers. There's what, 15 fuel sumps in a restart 182? 16? I forget. Enough to be completely ridiculous, I do know that!

Toss in the turbo of the T182T and fuel flow bets are off the table though, as well as watch out if jumping into the R182 with the retract... And the TR182... that's a completely different engine, selected to lug around the additional weight of the gear mechanism and also try to keep up with the other retracts of the day...

Pretty sure the choice of horsepower numbers on the Lycoming powered birds was just to match whatever targets Marketing wanted.

 

[flyingcheesehead]

Not 207/208. You mean 206/207. 208's the Caravan, a 675 hp turboprop.

I was thinking he meant 205/206, as even the 207 is a much larger airplane than the 182.

The 182s had the 230-hp Continental O-470 up until Cessna stopped making piston singles in 1986. When they went back to building 182s in '97 or so they used the 230-hp Lycoming O-540

IO-540 - Apparently Cessna went all-injected after the production restart because they really really really didn't want to hear anything about carb ice, ever again.

The big fat beautiful, but heavy, Continntal O-470 is typically 1500 hour TBO (some O-470U models are 2000) and they do need care to make it that far without a cylinder needing topped.

Ours made it to 2451 SMOH before the last overhaul. Yes, 1500 TBO. Needless to say, we went back and had Poplar Grove Airmotive do the overhaul AGAIN - They do an absolutely fabulous job.

But, keeping fuel and other numbers similar probably makes the airplane "safer" for those upgrading from the O-470 to the newer birds.
...
Pretty sure the choice of horsepower numbers on the Lycoming powered birds was just to match whatever targets Marketing wanted.

Well, it's all about practicality too. Jack your engine up to 300hp, and suddenly you are burning a bunch more fuel, and your range and endurance go to hell. Or you add bigger tanks so the range and endurance match, and your useful load goes to hell.

Cessna probably (and correctly) realized that the 182 sold well because it's a damn good balance between all the design compromises that must be made on an airplane, and it sold extremely well that way. It meets many missions. Why mess with a good thing?

 

[denverpilot]

Ours made it to 2451 SMOH before the last overhaul. Yes, 1500 TBO. Needless to say, we went back and had Poplar Grove Airmotive do the overhaul AGAIN - They do an absolutely fabulous job.

Nice! We'd hope ours does that too, but if it doesn't we get to consider upgrades sooner. Dilemma! A few extra HP up here or a turbo would be useful...

Cessna probably (and correctly) realized that the 182 sold well because it's a damn good balance between all the design compromises that must be made on an airplane, and it sold extremely well that way. It meets many missions. Why mess with a good thing?

Judging by the number of "restart" 172s sold vs "restart" 182s, I think they missed the sweet spot price-point by at least $50K. Your DA40 your club bought is a better deal today, methinks. The T182T is a wicked smooth, powerful beast, but not done at the right price either. Not until they're old and the current 30-35 year old fleet is starting to really wear out in another ten years.

I wail and moan that my generation missed the heyday of piles of new Cessnas being flown here from Wichita, KS. But they can keep 'em at over $300K! Skycatcher doesn't fill that void, and Corvalis??? Cessna has lost touch with their roots. I hope they find their way back someday.

 

[flyingcheesehead]

Nice! We'd hope ours does that too, but if it doesn't we get to consider upgrades sooner. Dilemma! A few extra HP up here or a turbo would be useful...

Be prepared for a delay. We did consider other options, but nobody had an engine, or parts, or whatever available. The fastest two options were going to be G&N or Poplar Grove, and with Poplar Grove's already-awesome track record on our equipment, we went back to them.

I'm not sure that Texas Skyways conversion would have really done much except make us look cool and burn fuel faster, tho.

Judging by the number of "restart" 172s sold vs "restart" 182s, I think they missed the sweet spot price-point by at least $50K.

Ugh... More than that. Hell, they missed the 172's sweet spot by $50K too. In terms of private owners, I know of a LOT more who have bought new 182's than new 172's. I know of only one post-restart 172 that was bought by a private individual - The rest are all getting beat up on rental lines.

Your DA40 your club bought is a better deal today, methinks.

The DA40 is a great plane, but it can't do everything the 182 can - Especially in terms of being able to haul a load. With full (long range) fuel, I can really only do two people and bags in the DA40, or 3 people with some fuel already burned off.

 

[Ted]

The O/IO-540s I'm aware of come in: 235, 250, 260, 290, and 300 hp forms.

The 235-260 hp forms use the parallel valve heads. The 290-300 hp forms use angle valve heads. The IO-580s make 315 hp (only available in angle valve). Parallel valve engines save about 50 lbs or so over angle valve engines, so if you don't need 290-300 hp, the weight savings is worthwhile.

RPM and therefore noise has a lot to do with the rating. Why the manufacturers chose a lower power instead of a higher power in some cases, I don't know. I'm a big fan of aircraft that were STC'd with higher power engines. I've yet to find a plane that I actually consider to be "overpowered." The 310 with 300 hp engines instead of 260 hp has great performance, and I think is how it should have come from the factory (especially since I still get a respectable 175 kts @ 25 gph combined that way). Same goes for the Aztec.

The airframe manufacturer did not derate any of the engines. Lycoming and Continental provide the engines the way the manufacturer requests them. Engines will sometimes have two ratings (such as 235 hp @ 2400 and 250 hp @ 2575, which is the case on some O/IO-540s), but typically they have one rating, or a takeoff rating and a maximum continuous power rating (my IO-520s are 300 HP @ 2850 RPM takeoff, 285 HP @ 2700 RPM continuous).

 

[airdale]

Thanks guys. Seeing the discussion prompted me to do a little more reading, including the two TCDSs that apply to the Lyc 540s. I did not realize that the heads were different in a way that would increase the costs of the higher horsepower engines and I did not realize that the weight difference among the models can be as much as 77#.

I think the weight explains the C182 decision. Adding 70# at station -25 really moves the CG to the edge of the envelope and even out of it with just two people in the front seats. I wonder if P-Ponk et al add rear ballast to deal with this or maybe the Continental engines are lighter?

Re airframe strength, I'm not sure I buy that one. The forces are not that much larger and, as several have pointed out, there are a lot of STCs for bigger engines in the 182s. There is also that 8-cylinder Dakota with IIRC 400 hp.

Same-o on fuel capacity. The 182 already carries more fuel than a 300hp Cherokee Six -- both bladder busters anyway.

Market segmentation, again as several have pointed out, is probably a factor. Not for the big motor PA-28 but the logic makes sense for Cessna keeping the 182 in its niche. I hadn't thought of that.

Re fuel burn I don't see that one either. Unlike a car, where the engine size does change the fuel economy, in an airplane you can just fly at lower power levels. So if the customer complains to the sales guy that his 300hp 182 burns too much fuel, his easy response is that if you want fuel economy just fly it at 60%, almost like a 230 at 75%. But if you need extra power you have it. That's also another argument against fuel capacity motivating against a 300hp offering.

All IMHO anyway.

 

[Dan Thomas]

Re airframe strength, I'm not sure I buy that one. The forces are not that much larger and, as several have pointed out, there are a lot of STCs for bigger engines in the 182s. There is also that 8-cylinder Dakota with IIRC 400 hp.

A heavier engine will need a stronger engine mount to meet the FAR 23 strength requirements, especially if the original mount only meets the requirements for the original engine. Higher HP also usually requires a different and often heavier prop, and at the higher RPMs there are higher gyroscopic forces to deal with, too. Then, if ballast is required to get the CG back where it belongs, there may be a need for some mounting structure in the tailcone to take that weight. All of it adds up to more airframe weight.

Dan

 

[PilotAlan]

Re airframe strength, I'm not sure I buy that one. The forces are not that much larger and, as several have pointed out, there are a lot of STCs for bigger engines in the 182s. There is also that 8-cylinder Dakota with IIRC 400 hp.

I don't know anything about a 400hp Dakota. Are you thinking of the Comanche 400 with the IO-720?
But airframe certification can and does play into things. The Comanche 400 has a different skin that the 260, and that cost a lot of money to certify.

Remember, there's only three basic motors to choose from on the Lycoming side, and three on the Continental side.
If Piper wanted a bigger Cherokee, the only engine option was the O-540. If the FAA said "We'll certify to 235hp, any more requires a new certification process", then I can see Piper saying OK.
Remember, it's not about what the aircraft will handle, it's about what the ACO will sign off on for their production certificate.

 

[Hollada]

Replaced the O-470 in our 1968 Skylane with TX Skyways O-520-U/TS (derated to 280hp by a red dot on the MP) plus three-blade Scimitar prop and beefier engine mount. Improvements are immense: smoother, quieter, 1200+ fpm climb at 100kts, 150 KTAS cruise, 15gph at 10,000, 250ft short-field t/o, 2500hr TBO. The BEW increased by about 30 lbs (mostly from the prop) CG moved forward so little as to make no difference.

The bigger engine really makes the plane everything we always wanted in a 182. It's now truly a 150kt, 4 seat, luggage, full fuel, 4+ hour comfort cruiser.

The O-470 we replaced had 1800 hrs and was still reliable but burning a quart every full tank so it was time for the swap.

My guess is Cessna's original match of airframe to engine is simply that it worked really well and there was no reason to change. The balance of expense, useful load, fuel burn, systems complexity, stability and toughness have great staying power. Add to that the O-470 was virtually bombproof and easy to maintain. Cessna tried designing different faster, sexier birds that were cool, but didn't stand the test of time all while keeping their solid, all-around dependable performer in the paddock. The simplest reason has to be why mess with something that works?

 

[PittsDriver]

I used to say "there's no such thing as too much power." Not true. The reliability, weight, and economy of operation trade-offs for some high amount of output just isn't worth it. Sure I could hang an AEIO-580 on my Pitts and the weight would spoil the plane. I could put a pumped AEIO-540 on it making 385 hp and it would last about 100 hours if I'm lucky. And that's an application where I don't car about fuel burn. In a traveling plane, that's a big consideration too. So my 260 hp AEIO 540 with it's 2,000 TBO is just fine though I have to admit I still sometimes wonder what 310hp would feel like in that plane