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Piston Vs. Turboprop Heavy Twins

Tue Jun 12, 2001 1:39 pm

A variety of heavy twins later saw life as turboprop twins. The two that I am most interested in are the Navajo/Cheiftain and Cheyenne and the C-421 and C-425/441 Conquest I/II.

What, if any, advantages was there to putting a turboprop on these aircraft? Were there any marked improvements in performance or dramatic reduction in operating costs? As far as I can tell from looking at and Trade-a-plane the Conquest I pulls a significant price advantage over Golden Eagle.

So was there really a big advantage to using the PT-6A/Garrett 331 over the big geared Continental 520 or the Lycoming 540?
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia
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RE: Piston Vs. Turboprop Heavy Twins

Tue Jun 12, 2001 2:28 pm

1. Overhaul costs on turbines while more expensive are much further apart;

2. Fuel availability for turbines is far superior throughout the sparsely settled parts of the world;

3. Thrust to weight ratios are superior when discussing turbine engines;

4. Cold weather operations for a turbine is a non-event.

Four quick "off the top of my head" answers...

Best Regards,

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RE: Piston Vs. Turboprop Heavy Twins

Tue Jun 12, 2001 3:59 pm

To add to Buff's response, turbines are also much more reliable than pistons are.  Big thumbs up
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RE: Piston Vs. Turboprop Heavy Twins

Wed Jun 13, 2001 1:42 am

Also, you need to add to what the other guys mentioned high power, comparative light weight and small size. I used to fly an MU2 Marquise that had Garrett TPE731-10 engines. They weren't much larger than a very big water melon, but they had thermodynamic ratings of over 1,000 HP! Although it's been alluded to, single-engine performance in these turbo-props is usually more than adequate. In most of the piston powered aircraft it can be abysmal. Couple this with what Ralpha said - turbines are much more reliable - and you've got the best of both worlds. The drawbacks to the turbines are few, when it comes to maintenance costs they can take your breath away and while they are typically much simplier to operate (no mixture controls to screw around with and very little to do with the propellor controls) they don't tolerate pilot mismanagement as well. Screwed up starts, over temping, and/or over torquing the engines can quickly become very expensive affairs. All in all, turbine engines are probably the best thing to ever happen to non-recreational aviation.
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RE: Piston Vs. Turboprop Heavy Twins

Wed Jun 13, 2001 8:25 am

Thanks for your input guys. I figure since these planes are earning their keep by flying charter, air taxi, corporate transport... efficiencies and costs of turboprops are better than pistons.

Now, let me throw another one at you, if you will. There have also been several turboprop conversions for single-engine aircraft. There is a mod for the Cessna P210, and presumably the T210 as well to fit an Allison 250 in the nose, and the same engine has been put into A36 Bonanzas. Plus there is the turbine Malibu conversion that puts a PT6A-35 in the nose. And now there is the production Piper Meridian with a PT6A-42.

Now is there any advantage in these applications to the turboprop?
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia
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RE: Piston Vs. Turboprop Heavy Twins

Wed Jun 13, 2001 8:55 am

Buff forgot to say how cool they sound too..JT
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RE: Piston Vs. Turboprop Heavy Twins

Wed Jun 13, 2001 6:05 pm

I flew both the 404 Titan and the 441 Conquest II with my previous employer.They were 2 completely different planes,believe me!Speed,range and comfort was alot better with the 441.Our fleet of 404's counted 5 planes,and there were 20 unscheduled engine changes over a 3-year period.Trying to start the Continental engine during a Norwegian winter (-20 C or even colder) was an EVENT!
"Luck and superstition wins all the time"!
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RE: Piston Vs. Turboprop Heavy Twins

Wed Jun 13, 2001 6:21 pm

In defense of the big pistons they do better down low then the turboprops do. They have to go high or else they are gulping fuel like mad. Pistons often mean that you can stay low, below the ceiling and make into non-approach equipted airports. A important consideration where I live.

Generally speaking too, you are less likely to pick up rocks with a piston then with a turboprop propellor. That is because you can vary the RPM a piston propellor is spinning at. Most turbine props only very their RPM over a much smaller range.

What I want to know is why when they develop a turboprop version of a piston aircraft, say going from a Navajo to a Cheyenne, do they keep the piston systems.

Look at the Cheyenne II. It has to PT-6 motors, which produce gobs of extra bleed air, that could be used for cabin heating or de-icing. But from what I have been told rather then do this a Chyenne still uses a Janitrol heater and de-ice boots pressurized by engine drivin vacume pumps.

Why the hell would you choose to keep these systems?

RE: Piston Vs. Turboprop Heavy Twins

Thu Jun 14, 2001 12:25 pm

They keep the same systems for usually just one reason - economics. They've already paid to certify the "piston" systems and why spend more money? It used to really bug me when I flew the Piper Cheyenne II that had a Janitrol heater in the nose when it had two perfectly good PT6s on the wings. I also was annoyed by the fact that they had to add two "55 gallon" drums on each wing tip so that the airplane could carry enough fuel to go somewhere. It was ugly as sin.

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