Beginning the search for a Commander (560F, 500x, 680E/F)? Not sure yet

[aerocheck]

Good morning all,

I just posted a quick intro about myself in the Introduction sub forum.

Just now beginning a preliminary search of Twin Commanders in the hopes of buying one some time down the road for a family airplane.  We are looking for something to travel around the Bahamas and the Caribbean for scuba dive trips.  Usually 4 people but occasionally 7 (6+1 child) with dive gear
I want to have the payload for all that but still try and retain good short field and climb performance while keeping fuel burn and maintenance costs from getting out of hand.
I am very open to any input from anybody regarding opinions, knowledge, etc on various models but here is my current thinking on model choice.

560F - Great payload, but the geared engines are a bit of a concern (operation, overhaul cost, parts availability)
680F/680F(P) - Better payload, but geared and supercharged (more cost, etc and for mostly sea level flying, I don't see the need for the added complexity of supercharging)
680E - Same as 560F, but adding supercharging
680FL/680FL(P) - Bigger cabin would be nice but same engine issues as 680F and I haven't looked at performance yet.

560E - Average Payload, 223 gal fuel
500Series - Average Payload, but i'd like the added fuel capacity for flexibility (I know there are some STC's for added fuel)

And then what I think might be my ideal choice
500series with Merlyn 350 conversion - Cost is the major issue with this choice - but all the benefits of 560F without drawbacks of geared engine.

So as I mentioned, please feel free to point out gaps in my logic, better choices, etc.

Also any insight, suggestions, personal experience, unique knowledge etc that anybody would be willing to share would be most welcome.
I'm going to be going through the forum as well as the back issues of the matronics list over time and trying to harvest every bit of knowledge that I can.

One item in particular that I'd like to start working on is a pre-purchase inspection checklist.  I've got generic ones, but I want to customize one for the Commander
with items that are unique to the type.  (corrosion focus points, unique system anomalies, things to look for, warning signs, etc, etc, etc).

Thank you all in advance for any assistance and input.

Ray Wolfe

[Willis]

Hey Rey.  As you can tell, Im a fan of big motors.

the Merlyn Conversions cover a range of Horsepower.  http://www.merlynproducts.com/350c.html

The highest is "350", but that may not really be the top.  Ive since learned that a TurboNormalization (using high compression pistons) can get you more power.  Its just not thought of widely.

The 680 conversion to IO720's is the biggest piston motor Ive seen so far.  It will take a doughty FL or FLP and regain some of its short field capabilities.

Adam Frisch has a lot of time in an early 520 and they behave exceptionally, but the parts are getting difficult to source.  maybe he can tell you more.

Bud

[donv]

I think, for the mission you describe, that a 500 series would be a good choice. The Merlyn conversion sounds great if you don't mind the initial expense, but even a stock 500B/U/S would be a great airplane for you.

[Adam Frisch]

Hi Ray - welcome to board!

The geared engines are not to be afraid of. Lot of misinformation out there, but if you fly them right they'll last as long as anything else. I loved mine and wouldn't hesitate to own geared engine again one day. The great thing about them is they have much more torque. You'll find that when you compare performance, all the Commanders that do really well on short fields are the geared engined ones. They just get off quicker.

That said, depending on TBO of your geared engine, they are all orphaned and getting harder to maintain as cheaply as a direct drive engine. The GO-480's are still very plentiful and it will take a long time before they run out of parts, but for the oddball ones like my GO-435 and the fire breathing GTSIO engines, it can be a bit more challenging. Also, overhaul is specialized and costs more. And they all have lower TBO, so that has to be taken into account. But because people are a little afraid of them, you also get a healthy discount on purchase price, so they can still be a good deal.

But yeah, you can't go wrong with a 500B. Bulletproof and cheap to maintain. And they keep their resale value.

One last thing in regards to turbines. Don't fully discard the early cheaper models like the 680V and the 681 - they might be had for close to the same money as a good 500B/U/S. They give a lot of bang for the buck. As an example, I bought my 680V with the newer Century engines for $115K. I've spent another $46K on her. She's almost ready now, so should be up and running for less than $170K out the door. And per nm mile and hr, they'll cost the same as a legacy piston. Per nm, the turbine is actually cheaper than an avgas burner these days with the low Jet A1 price. Anyway, just a thought - I don't know your budget, so it might be a stretch.

[donv]

I don't know about this-- I think the number of 680 and 681 turboprops which have flown in the US in 2016 can probably be counted on the fingers of one hand... And once they've sat for years, the initial price may be low, but the cost of making them functional can be really high.

Are there any for sale now?

What I'm saying is, if you can get Montana's one owner from new 680W, or your friend's 681, then go for it, but as far as I know neither are for sale...

One last thing in regards to turbines. Don't fully discard the early cheaper models like the 680V and the 681 - they might be had for close to the same money as a good 500B/U/S. They give a lot of bang for the buck. As an example, I bought my 680V with the newer Century engines for $115K. I've spent another $46K on her. She's almost ready now, so should be up and running for less than $170K out the door. And per nm mile and hr, they'll cost the same as a legacy piston. Per nm, the turbine is actually cheaper than an avgas burner these days with the low Jet A1 price. Anyway, just a thought - I don't know your budget, so it might be a stretch.

[Adam Frisch]

That's true Don, they are far and few between. So not sure it's a real option. I would also not consider anything without at least a -1 engine. The original engines are not worth the hassle. There is a 681 for sale in Canada with the rare Super Dave or Super One conversion. Basically a -6 compression section mated to a -1 engine. Makes for a great performer at 280kts, but the guy still wants too much for it. Which is why it's sitting.

I don't know Ray's budget, but some 690A's can be had for higher end piston money as well, albeit they will need a little investment soon after purchase, probably.

[Steve binnette]

I have a little experience in the 680 with IO720s and I was not impressed with the performance.  Thet did burn less fuel than the IGSO-540.

At sea level the supercharging will help and is an advantage.  These engines pull more than atmospheric pressure on takeoff so they help  with short field, single engine, anywhere more power is good!

The IO720s were limited to atmospheric and did not help at sea level.

The geared engines are great! Better power and quieter than a non geared.  The cost to oerhaul is more but dont buy one that needs overhaul or factor it in.

I would not worry about them being fragile, Every Turbo prop is geared and producing much more power through the gear box.

[Willis]

I have a little experience in the 680 with IO720s and I was not impressed with the performance.  Thet did burn less fuel than the IGSO-540.

At sea level the supercharging will help and is an advantage.  These engines pull more than atmospheric pressure on takeoff so they help  with short field, single engine, anywhere more power is good!

The IO720s were limited to atmospheric and did not help at sea level.

The geared engines are great! Better power and quieter than a non geared.  The cost to oerhaul is more but dont buy one that needs overhaul or factor it in.

I would not worry about them being fragile, Every Turbo prop is geared and producing much more power through the gear box.

Im wondering about the power/noise comparisons for these engines.

Attached is a spreadsheet calculation of the engine/prop speeds and tip speed and mach numbers.

From first priciples, the MrRPM has more power, regardless of supercharging or Turbo/normalizing.  so at sea level, if there is a difference in performance, then it must be in the propeller efficiency.

Prop efficiency falls off exponentially as the tip speed approaches mach 1.0

The 80.625" dia. prop on the Mr. RPM should be more efficient and a lot quieter.

My 210D is loud  using an 88" diam prop spinning at 2700 rpm at take off.  Great power, but illegal in Santa Monica and San Diego after 10pm.

Im guessing that there is something odd with the props on Mr.RPM engines.  Bad selection, less efficient (even though it consumes less fuel).

There are a few MrRPM fitted aircraft for sale. Im looking hard at the one in Seattle as a project and it looks like it has really small props. 

I see no reason it couldnt handle 84-85" diameter props, unless there is a stress/fatigue factor.  Putting 400 HP through a long blade causes much higher stress than a shorter blade.

[Adam Frisch]

It is well known the props on the 685 is way to small for the aircraft, contributing to it's bad short field and climb capabilities. Dunno of the 680 suffered from same problem, but since they have shorter wing box I shouldn't think so.

[aerocheck]

The geared engines are great! Better power and quieter than a non geared.  The cost to oerhaul is more but dont buy one that needs overhaul or factor it in.

I would not worry about them being fragile, Every Turbo prop is geared and producing much more power through the gear box.

How about parts availability and GOOD shops that still do the overhauls?  Any problems with either of those?

[aerocheck]

The highest is "350", but that may not really be the top.  Ive since learned that a TurboNormalization (using high compression pistons) can get you more power.  Its just not thought of widely.

Bud

I think 350 would be plenty...I would just like the 7500 GW of the 560F.  Seems like empty weight on most 500/560/680 commanders is in the 4500-4700# range? (limited data so far so that may not be a very accurate range)
With the 7500# GW you can fill the seats and still just about fill the tanks (223gal). 

I know the 500S/B/U work out about the same payload because of the lower fuel capacity, but I'd like the bigger tanks
(probably won't always fill them, but I'd like the option when fuel availability could be an issue)

[aerocheck]

That said, depending on TBO of your geared engine, they are all orphaned and getting harder to maintain as cheaply as a direct drive engine.

One last thing in regards to turbines. Don't fully discard the early cheaper models like the 680V and the 681 - they might be had for close to the same money as a good 500B/U/S. They give a lot of bang for the buck. As an example, I bought my 680V with the newer Century engines for $115K. I've spent another $46K on her. She's almost ready now, so should be up and running for less than $170K out the door. And per nm mile and hr, they'll cost the same as a legacy piston. Per nm, the turbine is actually cheaper than an avgas burner these days with the low Jet A1 price. Anyway, just a thought - I don't know your budget, so it might be a stretch.

That's my biggest concern on the geared engine is the parts and expertise availability.  Not a deal killer but I don't want to walk into a parts vacuum

It sounds like you were the lucky guy to find and catch the unicorn!  I'm jealous.

[aerocheck]

I don't know Ray's budget, but some 690A's can be had for higher end piston money as well, albeit they will need a little investment soon after purchase, probably.

Trying to stay in the lower end of the piston range as far as budget and then put some money into the airplane over time bringing things up to the way I want them.

[Willis]

Ive been mulling some advice Glenn Hancock gave me. 

Paraphrasing as best I can remember:  "Find a solid airframe with as few structural issues as possible.  Engines can be overhauled". 

so Im searching for airframes with little, few or no accident histories,   OR ones that can easily fixed.

N800T, a 680F with Mr.RPM IO720's.   For Sale up in Seattle (Tumwater).  It has seen a lot of work, doing aerial photography missions. No mention of TTAF, TSMO engines.  I may just call them today.  Its raining here.....

It has a BUTT-Ugly  Camera Hole, installed by a ham fisted moron.  (IMHO).  BUT, this STC can be cured with a new 337: some judicial sheet metal, New floor panels and its back in-specifications =  "Bob's yer Uncle".  A lot less than Overhauling an engine.

[Adam Frisch]

Yeah, the 560F was the one I was going to go to after my 520, as I was doing so much long range flying. You just can't beat great range. But couldn't find one when I was ready to buy so in the end I went for an Aerostar as I wanted full FIKI and pressurization. But I do think the 560F is a good choice. Yes, the engine is unique to that one, but I do think it shares most of the parts with the sister engine. But if you plan on going west much, or mountain flying, the perhaps a 680F with the superchargers is a better choice.