Engine overhaul

[Aztec Driver]

So when it comes time to have my engines overhauled, how likely is it to have FSDO sign off on installing two IO360 in my Twin Comanche, as opposed to replacing my IO320's with the same engine?

Some of the Twin Comanche's had an STC for replacement of the original engines with the IO360, but mine, of course does not. I would either have to purchase an STC and all of the available parts, which are no longer available, from what I can find, or purchase another bird with it, and I am not willing to do that.

My mechanic believes that he can do all of the necessary work, and then get the FSDO to sign off OKing the return to service. Is that a viable option, or is he just very optomistic?

I like the idea of slightly better performance, with a tradeoff of more fuel burn and, possibly, more weight. Depending, of course, on how much weight and fuel burn penalty we are talking about.

Anyone out there with the Miller conversion that could tell me a little about it?

 

[Tom-D]

So when it comes time to have my engines overhauled, how likely is it to have FSDO sign off on installing two IO360 in my Twin Comanche, as opposed to replacing my IO320's with the same engine?

Some of the Twin Comanche's had an STC for replacement of the original engines with the IO360, but mine, of course does not. I would either have to purchase an STC and all of the available parts, which are no longer available, from what I can find, or purchase another bird with it, and I am not willing to do that.

My mechanic believes that he can do all of the necessary work, and then get the FSDO to sign off OKing the return to service. Is that a viable option, or is he just very optimistic?

I like the idea of slightly better performance, with a trade off of more fuel burn and, possibly, more weight. Depending, of course, on how much weight and fuel burn penalty we are talking about.

Anyone out there with the Miller conversion that could tell me a little about it?

A couple questions.

Is there any STC for Twin Comanche, to do this?

does any Twin Comanche use these engines?

 

[Aztec Driver]

A couple questions.

Is there any STC for Twin Comanche, to do this?

does any Twin Comanche use these engines?

yes and yes.

Unfortunately, the STC holder decided not to continue and noone else purchased it from him. Therefore, the STC pretty much died.

 

[d.grimm]

I just went through this with tires. No longer supported STC
so I had to go through the whole Field Approval Process.
If you are serious about this I would hire a PowerPlant DER
to do this. You would be time and money ahead in the long run.
IMHO
Dave

 

[Tom-D]

yes and yes.

Unfortunately, the STC holder decided not to continue and noone else purchased it from him. Therefore, the STC pretty much died.

Call your local FSDO and ask if the STC is in public domain, if it is, you can use it with out buying the engineering.

Ask your A&P-IA if they are familiar with the process to gain approval thru the deviation to the STC process.

 

[Capt.Crash'n'Burn]

On a slightly related subject, is there an STC for higher HP engines on the Aztec??

 

[RotorAndWing]

On a slightly related subject, is there an STC for higher HP engines on the Aztec??

SA179CE:
PA-23-250
Installation of the following: Lycoming IO-540-A1A5or IO-540- G1A5 engines, HC-A3XK-2C/8433-10 or HC-A3XK-2B/7636D-4 propellers.

This gives the Aztec 290 hp.

 

[bbchien]

Bryon, much as I hate to suggest this- I think you need to BUY a Miller equipped Twinkie. It's be cheaper than to do the reverse engineering and signoff.

 

[Ted]

Bryon, I'm one of those believers that it's hard to go wrong with more power. The one question would be what it would cost to swap your 320s for 360s, and if you could swing it with the paperwork.

Outright, it's probably cheaper and easier to find one that's already equipped as Bruce suggests. But I would probably do the same thing you're interested in doing. Aviation is not always about being cheap or easy.

SA179CE:
PA-23-250
Installation of the following: Lycoming IO-540-A1A5or IO-540- G1A5 engines, HC-A3XK-2C/8433-10 or HC-A3XK-2B/7636D-4 propellers.

This gives the Aztec 290 hp.

Hey, I want that STC!

Now I know what to start looking into.

 

[bbchien]

40 gph, 50 at departure.....for 580 hp.

 

[Ted]

It'd be more than 50 gph at takeoff. My engines as they are burn 48 to make 500 hp. But somehow I manage to get 21-22 gph out of them for the trip at 155 KTAS. I'd expect to get a bit faster on a bit more fuel burn - something in between where the Aztec is now and the 310.

 

[poadeleted20]

Without an existing STC, it's a long and expensive process to get approval to put a bigger engine on a production-certified airplane. Based on the work involved for several folks I know who've done it, figure at least $30K and six months' time for the flight testing on top of all the engineering and installation costs.

 

[Tom-D]

Without an existing STC, it's a long and expensive process to get approval to put a bigger engine on a production-certified airplane. Based on the work involved for several folks I know who've done it, figure at least $30K and six months' time for the flight testing on top of all the engineering and installation costs.

There are several things that can vary this turn around time. What FSDO you are working with and what method you choose to gain approval, and the credibility of the facility doing the mod.

The DER can take their own sweet time, The manufacturing or modification of parts can some times be a night mare.

But when the engineering already exists as in a STC on a different model of the same production certificate, or of the same make and a different model, it can be done quickly,

the STC for replacing the C-145/0-300A with a 0-300-D on the early 172 was completed in about 2 weeks. Basically a paper work drill that the FSDO was willing to do to see a vac system on the aircraft.

My point in all of this, it depends upon what you are doing, and who you are working with.

 

[flyingcheesehead]

Without an existing STC, it's a long and expensive process to get approval to put a bigger engine on a production-certified airplane. Based on the work involved for several folks I know who've done it, figure at least $30K and six months' time for the flight testing on top of all the engineering and installation costs.

Depends on your definition of "Existing." A number of Twin Comanches, probably in the high hundreds, were upgraded to 200hp IO-360's via the Miller STC, which is no longer available.

Bryon,

Where you'll see the biggest performance improvement is climb, on either one engine or two. Using the old cube-root trick, you can expect about a 12-knot increase in cruise, but you should see your max climb rate with both engines turning and your single-engine service ceiling increase significantly. (I might try to calculate those later, but I don't have the formulae handy right now.)

I think the biggest issue in this will be that I think you need more than just an engine swap. I'm pretty sure the Miller conversions required an enlarged dorsal fin as well - There are three types of dorsal fins that you'll find on Twin Comanches:

Standard Twin Comanche dorsal fin: (note that it's small to nonexistent)

Miller/Arapaho/Knots2U dorsal fin: (note that it's a few inches higher and much longer)

Robertson STOL dorsal fin: (nice, big and curved)

I don't think I've ever seen a Miller 200hp Twinkie without either the Miller or R-STOL dorsal fin on it - When increasing HP, you need to either raise Vmc or somehow mitigate the effect of more asymmetric power. Luckily, Knots2U still sells the Miller-style dorsal fin. But, that's an extra $1,600 plus labor to add to the project.

One other thing you may want to consider is doing the PA-39 conversion as well - Since you're already going to have to buy new engines and props, it probably won't cost much extra to get an LIO-360 and left-turning prop for the right side. And that *MAY* mean you won't have to do the dorsal - Though if I were you, I'd do both. Making the plane tamer on one engine can't hurt.

Finally - I'm not sure if the engine mod requires anything else besides the dorsal fin. It'd sure be interesting to see an installation manual for the Miller conversion. Also, do you have tip tanks? If not, you might want to consider what the increased fuel burn will do to your legal IFR range.

I'm definitely interested in what you find out, and how it goes. Good luck!

 

[Ted]

Where you'll see the biggest performance improvement is climb, on either one engine or two. Using the old cube-root trick, you can expect about a 12-knot increase in cruise, but you should see your max climb rate with both engines turning and your single-engine service ceiling increase significantly. (I might try to calculate those later, but I don't have the formulae handy right now.)

That is probably correct. On the 310, I end up getting about the same cruise speed running the 520s at economy cruise as most people got with the 470s at power cruise, on about the same fuel burn. The takeoff and climb are the big advantages.

I can run at 190-200 KTAS (15 kts or so faster than my economy cruise), but burn 40% more fuel to do it.

 

[flyingcheesehead]

Where you'll see the biggest performance improvement is climb, on either one engine or two. Using the old cube-root trick, you can expect about a 12-knot increase in cruise, but you should see your max climb rate with both engines turning and your single-engine service ceiling increase significantly. (I might try to calculate those later, but I don't have the formulae handy right now.)

Here we go, Bryon... I'm making some assumptions on the current numbers for now. I'd be happy to get more accurate if you let me know your actual performance.

According to the Lyc TCDSs, the IO-320-B1A that you currently have weighs 259 lb and the IO-360-C1F used for the conversion weighs 293 lb. So, an increase of 34 lb/side, all else being equal. That'll knock 68 pounds off your useful load unless you add tip tanks as well.

Adding the tips wouldn't be a bad thing, the bigger engines will burn more fuel and having tip tanks will allow for a longer legal IFR range. EdFred got the tips installed on his Comanche, hopefully he can give you some more info. With the tips, MGW increases to 3725 lb, so you add 125 pounds to max gross and 125 minus the weight of the tanks to your payload. Note that any weight above 3600 must be fuel in the tips. Many Twinkie owners will simply leave their factory aux tanks empty and fill the tips instead if they need the extra weight capability, or fill 'em all if they need the range. (Both factory aux and the tip tanks are 15 gal/side).

Now, for performance. Like I mentioned, you should see maybe a 12-knot increase in cruise speed.

For climb - At a 3600lb gross weight, your single-engine rate of climb should improve from 260 to 612 fpm. Based on other calculations, I'm not sure I buy the 260 to begin with, but it gives you some idea what the extra HP could do for you. Even if your SE ROC is 0, converting to the bigger engines should give you 360 fpm. With both engines turning, climb should improve from 1460 fpm to 2344 fpm. All of these calculations are accounting for the extra weight of the bigger engines, too, though not the weight of the dorsal fin or the tip tanks that would make it legal to carry the same payload.

Single-engine service ceiling should *improve* by about 7,000 feet. (IE if you can get 50 fpm ROC with OEI at 5000 now, you should be able to get 50 fpm ROC OEI at 12,000 after the conversion.) Both-fans-turning service ceiling should improve by several thousand feet as well.

Another way to look at all this is by weight. Let's say you go flying now with the mains topped off and the aux tanks empty. Assuming a 2400 lb. empty weight and a 200 lb. pilot, and nothing else on the plane, that gives you a gross weight of 2960 lb. for a power loading of 9.25 lb/hp. With the new engines, that same power loading (and same climb performance) will be seen at a gross weight of 3700 lb, almost up to the new gross weight if you get the tip tank mod. That's another 740 lb. 68 of that will be engines, but that will gives you 672 pounds more useful load to have the same performance as you'd see out flying around by yourself right now.

So... Have you found out any more information about this yet? I'm curious!

 

[rotorhead1026]

You can go the LIO-320 route still, I'm sure; I don't think the LIO-360 was ever approved for the Miller STC. I've never seen one. The engine used in the conversion I did was the IO-360-C1C. This was in 1981. I asked about using the -C1C6 (no rpm restriction), but they couldn't use it under the terms of the STC. As far as a dorsal fin addition - yes, it's required. I'm not sure what the STC says if you already have a Nuts-to-You or Robertson fin installed.

Finally - I'm not sure if the engine mod requires anything else besides the dorsal fin.

No more cast prop extensions, and the cowls are modified (gills added). Less range overall, but not much less. Unless you regularly fly 3+ hour legs under IFR you'll be fine. Miller had an STC for in-wing tanks too, rather than tips.

 

[flyingcheesehead]

You can go the LIO-320 route still, I'm sure; I don't think the LIO-360 was ever approved for the Miller STC. I've never seen one.

Hm... I could have sworn that I've seen a C/R Miller for sale. What did they do when they converted a PA39?

As far as a dorsal fin addition - yes, it's required. I'm not sure what the STC says if you already have a Nuts-to-You or Robertson fin installed.

I've definitely seen some Miller + Robertson birds (In fact, the example picture I posted above for the Robertson dorsal fin has the 200hp engines), but who knows which was done first.

Miller had an STC for in-wing tanks too, rather than tips.

And some of 'em have both - 158 gallons usable! That's a LOT of fuel for such an efficient bird... *almost* enough to make Hawaii if you've got turbos... But not quite. Damn.

 

[rotorhead1026]

Hm... I could have sworn that I've seen a C/R Miller for sale. What did they do when they converted a PA39?

I don't doubt your word - I've just never heard of one. Yeah, PA39 - makes sense. There must be some.

I've definitely seen some Miller + Robertson birds (In fact, the example picture I posted above for the Robertson dorsal fin has the 200hp engines), but who knows which was done first.

I have, too. I just don't know the STC language.
...
You mentioned "in the high hundreds" conversions. I don't think that many got the engines. There were, of course, many modified w/o engines (tanks, or brakes, or nosewheel taxi light, or tips, or long nose, or nacelle lockers ... etc.).

 

[flyingcheesehead]

You mentioned "in the high hundreds" conversions. I don't think that many got the engines. There were, of course, many modified w/o engines (tanks, or brakes, or nosewheel taxi light, or tips, or long nose, or nacelle lockers ... etc.).

Whoops. That's not really what I meant. "High hundreds" does kind of imply somewhere near 1000, and only slightly over 2000 Twinkies were built, which would mean nearly half were converted if you read what I wrote. No way.

What I *meant* was, I would guess high in the 100 range - IE, somewhere north of 150 but less than 200. Another way to put it would be, less than 10%.