Turbo vs Normal?

I have another question for you guys, I have also considered Turbos, would love the p337. but that falls outside my budget. I read somewhere that the turbos on earlier models have automatic wastegates, how do they compare with the newer ones? and with the normally aspirated? is there too much more maintenance? if they are automatic, what manifold pressure do they keep?

I am mostly considering them to have the capability of going over the weather ocasionally, but must of my flying will be below oxigen altitudes anyway.

Regards,

Ignacio

[Richard]

I've owed both. I like the non turbo for it's economy. It's a fuel mizer and matenaince is less...but... I like the turbo. Great climb rates, good cruise. Best of all. I've lost an engine over mountains. It made me nervous in the non turbo. With a turbo you have 23000 on one engine ceilings...so... my two cents. If you don't care too much for the cost.. I take the turbo.

Richard

[Bob Cook]

one added comments

Agree with the above with following added.

1) great to get over weather.
2) climb out of ice (heaven forbid) rather than decending.
3) great for high altitude airports.
4) hp is hp. cost is same (IMHO) for fuel, however, you pay in the end for additional costs such as exhaust maintenance and possible lower tbo.
5) you need to manage the engines!
6) speak to Kevin about ground speeds east bound in the winter --- yahoooooo.

bob


bob

[kevin]

It was spring. 314 kts at FL200. At 65% power too.

[SkyKing]

Ignacio,

If you're willing to put some 'elbow grease' into the equation, right now there are at least two P337's in the market place, one at $85K and another at $100K, the latter of which is deiced with intercoolers and both with a fair compliment of IFR avionics. Yes, they may need some work, i.e., rubbing out the oxidated paint, replacing parts, etc., but under the current economic order of things (planned BTW), you might just surprise yourself at what you'll find if you look closely.

There is much 'boo-hoo' in mechanic circles about turbos, but if the equipment isn't abused by the operator and you run them with some sensibilities, they'll do what they're supposed to... and then some, and in the long run you will be rewarded time and time again. Overall, your cost of operation with the turbos won't be that big of a 'boo hoo'. Go back through the threads and read the article from AvWeb that Bob Cook recommended on valves... you'll get the idea. The problem with turbos is the ignorance of the operator and ham-fisted throttle operation... which you just don't do with turbos. As long as you don't overtemp them, allow sufficient cooling & spool down you'll be fine.

BTW, the two P337's I mentioned are on The Controller. One is out here in Oregon and has been owned by the same owner since he took delivery of it from Cessna when it was new... that's the one I'd look at. It's the $85K unit.

Skying

[SkyKing]

Ignacio,

Looking back through this thread, I guess we all missed the 'real' meat of your question which was the operation of the 'automatic' wastegate.

Insofar as the 1973 through 1980 P337 series, the wastegate is controlled by oil-pressure which is varied through separate oil-line plumbing to a device called the 'VAPC' which is the acronym for Variable Absolute Pressure Controller, located on top of the engine. It is mechanically interconnected to the throttle body in such a way that upper deck pressure from the compressor section of the turbocharger is sensed by a vaccum sealed aneroid mechanism, similar to your home barometer, that regulates or modulates the amount of oil pressure in an oil-line from the outlet side of the wastegate actuator to the controller and to a scavenger pump back into the engine's oil supply. That's a simplified explanation.

Manifold pressure is maintained at whatever throttle position you select by the VAPC sensing compressor discharge pressure at the throttle valve inlet, which is called 'upper deck pressure.' The set-up is such that the controller setting varies proportionately to the amount of power selected by the throttle through the mechanical linkage to the throttle arm.

When the engine is started, the oil pump located on the lower back side of the accessory case builds pressure and there is a capillary tubing attached to the bottom of the accessory case in the sump area from where oil is drawn into the wastegate inlet chamber which controls a valve and ultimately a mechanical butterfly inserted in the exhaust pipe.

At idle throttle there is very little compressor discharge pressure, so the VAPC is allowing all of the oil flowing through the capillary tubing to the wastegate, through the outlet side up through a larger oil return line to the VAPC and back into the engine, thus the wastegate is 'open'. Now, when the throttle is advanced, the throttle body calls for more fuel to be injected, the engine driven fuel pump increases fuel pressure, the compressor discharge pressure momentarily decreases as the air enters the engine and the engine begins to accelerate. As the air pressure decreases, the VAPC senses the decrease and closes off the oil pressure in the return line from the wastegate and oil pressure builds in the wastgate chamber which closes the valve and thus repositions the butterfly to a more closed position. There is also a spring that holds the wastegate 'open' and the oil pressure must build to a sufficient level to overcome the resistance of the spring.

As the engine RPM increases more exhaust is diverted from the exhaust pipe through the exhaust turbine end of the turbocharger and the increased exhaust pressure blowing on the turbine increases the compressor speed (remember they're on the same shaft). As the turbine speed increases, more compressor discharge pressure is created by the impeller at the ram-air end of the shaft until it balances with the manifold pressure and throttle setting selected. As the airplane climbs the VAPC senses the lower pressure changes and closes the wastegate butterfly further by restricting the oil flow until eventually reaching what is known as "critical altitude" where the system reaches equilibrium. The 'automatic' part of the wastegate is actually the VAPC sensing differences in upper deck pressure which in turn repositions the wastegate butterfly.

I've simplified this as much as possible, and it's a lot easier with pictures to show the flow of things, but I think you'll get the basic idea. Climbing to higher altitudes, the VAPC 'automatically' senses lower air pressure and continually closes off the oil pressure in the return line from the wastegate, but descending to lower altitudes is not automatic... you must continually reduce throttle settings as you descend as air pressure increases in the lower atmosphere.

Since you're new to this realm, what I suggest is you order a copy of a POH (Pilot Operating Handbook) from from one of the Cessna dealers for a 1978 P337H and go through the "Airplane & Systems Descriptions" in addition to the other tabbed areas on "Limitations", "Normal Procedures", "Performance", etc. The reason I suggest the 1978 version is because they come with the original Cessna cover and they're assembled such that the punch holes are placed on an extra width margin which won't impair the printing and your reading. The 1978 manual is more refined than earlier versions and you'll find that with the exception of 4724 maximum ramp/taxi weight, all other parameters are the same as the 1973-1980 versions of the P-model.

Hope this helped you udnderstand the 'automatic' wastegate. It's really the VAPC that's the heart of the entire system.

SkyKing

[kevin]

If the P337 in Oregon you are talking about is being offered by Salem Air Center, beware, I'm told it has a cracked case on one engine. Both engines are original, total time on the airplane is somewhere close to TBO. Registerd as N198.

Kevin

[SkyKing]

Kevin,

I think that's the one, with 2120 TT, 1225 SMOH & 1250 SMOH on the engines. This is the first I've heard about a cracked case and if so, it's more than likely the rear position. Well, what's a guy to do now? I guess the answer would be to find a serviceable engine or spend the money for a reman... that changes the price equation... no wonder its $85K.

SkyKing

[kevin]

Interestingly, they were asking 85K for it *before* it was determined a case was cracked.

Kevin

[Frank Benvin]

Dad & I had a 1973 P337G Pressurized and Two 1967 337B non turbo. For low level short hops the non turbo is the way to go.If you want to get up and go places then turbo is the way to go. Get a newer model turbo with the larger tanks. We found the P model to lack range with the smaller tanks 128 US gallons if I recall. We commuted between Nanaimo BC (45 minutes north of Seattle Wa and Phoenix with a stop over in Reno) in both aircraft. It was nice to get up high and go but the P model was lacking fuel. We had a way better range in the Non turbo. Also all our flying was VFR so we had to worry about alternates due to weather. For short hops around here at 7000ft and below I think turbo is out of the question. You have to way out what type of flying your going to do

[Ernie Martin]

I know from another posting that you're at KTMB near Miami. For what it's worth, I've flown two 337s (a 337D and now a 337G) out of KTMB and never felt I needed a turbo. From my experience, the weather here is typically circumnavigatable or not, but basically never of the type where you can get above it. BTW, I flew the 337D to Idaho and back, and did quite a bit of mountain flying on that trip; you've got to learn about mountain flying (there are good resources on the Web), do proper planning (taking into account your airplane's performance) and be aware of weather (including high winds), but the normally aspirated 337 was a joy to fly on that trip, typically at 11,500 and 10,500 altitudes.

Ernie

[Paul Sharp]

I believe where and how you fly become the largest questions to answer. All 337's have "automatic wastegate" control so far as I know - it's in the design. There are after-market turbos that some aircraft have on them (such as a Twin Comanche I owned) that utilize a manually-adjusted wastegate, but I've never heard of a Skymaster that didn't use the standard automatic wastegate setup that comes on the TSIO 360.

In Florida you might not have much reason or justification for a turbo. I live the intermountain west, fly there, during winter, in IFR, at night, and often have my family with me. I wouldn't want anything else but a turbo.

Many thanks to you all for the huge response, I think that left no doubt in my mind of what to expect from both planes and understood the turbo's wastegates operation (at least basically, I think I need a couple of courses to really pick all that up).

Yes I'm based in florida and do not expect that the local weather would need a turbo unless you could climb to FL410 to top it and even so, it would probably take less time to circumnavigate it.

I do a trip to Pennsylvania every two months or so, and that would be very nice at the upper teens.

I guess a normally aspirated would do the job more than fine, as a matter of fact my Cherokee 235 does it pretty good with just one stop and about 7-8.5 hrs flight time depending un my luck with the wind. (I almost always is 8.5 going up and down, at least it was so last time with 20-30 kt headwinds almost all the way).

I guess also having about 500 hrs total SEL and 250 with retractable with no IFR the insurace would be a factor in the P and probably also on the Turbo.

Anyway Still looking for a plane, willing to trade or sell my 235 in order to get the Skymaster, if any of you guys know somebody that would like the small load hauler, I can tell you that I'm confident is one of the very best in the market. if somebody wants details write me to ignacio@commconcept.com or call me @ 786 271-3436.

Thanks again to all

BTW Earnie,

It's been a long time since I flew in a 337, if you need company for a flight around I'll be more than happy to be your copilot. Would love to see your bird anyway. I park at CR Aviation on the north side of the field.

Regards,

Ignacio

Paul,

I'm curious, how do you compare the twin comanche against the 337? When I started looking for a twin I seriously considered the twin comanche for it's efficiency and low maintenance (not sure about this one) but later I am almost convinced of the skymasted for its added safety (basically the centerline trust).