Center Thrust in GA (Why not?)

[docmirror]

That must not be too big a problem, because the C-336/337 performs better OEI on the rear engine than on the front engine. So much so that it inspired Cessna to investigate a twin-boom single-engine pusher design in the C-150 category, to see if better performance could be coaxed from a low-power engine. Performance was okay, but it too had issues with noise, vibration, rear-engine cooling and crashworthiness, control system friction, overall weight and projected construction cost.

I never mentioned OEI. Turn off the front engine, and lots of things change. However, with both engines on, it's a problem. There were two 337s at my little airport for many years, and after talking to the owners of both, it was the most underperforming plane either of them ever had.

Someone mentioned the Twinkie, and if memory serves, the 337 required 420HP to get the same speeds and climb that the Twinkie could do on 360HP. Also, if you get the C/R option when you rebuild the Twinkie engine so it rotates the correct way, no more critical engine, and you gain 3Kts as well.

So, it's 1963-66. You have a choice between the Twinkie and the Skymaster. AvGas is only 58 cents a gallon or something like that, but the Skymaster is several thou more than the Piper, and goes the same speed, and you have extra cylinders to deal with. It'll carry more weight if that's an issue but the rear engine didn't help much in cruise.

 

[jkgoblue]

Adam A500

another winner

 

[weilke]

By potential buyers.

During the time it was produced, Cessna could sell a good-size bunch of almost anything--for a while.

They sold 5700 C310s but had 10 years more to do so. Obviously an abject failure, just like the Twin Comanche that sold 2100 total.



From what I can tell it performs as expected from a 420hp twin. No better or worse than a Seneca II, the only aircraft comparable to the T337. Some people like them for long overwater legs, e.g. from NC to the Bahamas.

Would I buy one ? Probably not. The way twin prices have dropped and how fuel and maintenance dominate overall cost, there is no benefit in skimping on the entry price.

 

[flyingron]

From what I've read about the 336/337, it failed to live up to the promise of being a safer airplane than the traditional twin. This is largely in part because it suffers from low power engines, so single engine performance is poor. I have read that the Riley Super Skyrocket conversion, with two TIO-520s on it, it performs as it should.

The 337 is not unique in suffering from bad SE performance. Many popular (non-CLT) twins are successful without suffering from the problem.

My guess is that the normal operation performance just didn't compare with the contemporary competition. Nobody buys the plane just because they like how it will operate on one engine. It's like buying a Cirrus for the sole reason that it has a chute.

 

[Dav8or]

The 337 is not unique in suffering from bad SE performance. Many popular (non-CLT) twins are successful without suffering from the problem.

My guess is that the normal operation performance just didn't compare with the contemporary competition. Nobody buys the plane just because they like how it will operate on one engine. It's like buying a Cirrus for the sole reason that it has a chute.

You're kidding right? Absolutely people buy the Cirrus because it has a parachute!

In the case of the Skymaster, the promise was that engine out ops were simple and easy to deal with, therefore this twin was to be safer than the competition. I'm very sure many, if not most buyers of the brand new Skymaster chose the model for this very reason. When the stats came in over a few years... well, combined with reliability problems, cabin noise, smaller baggage area and less than amazing performance, buyers chose otherwise.

More powerful engines, or an airframe that wasn't a test bed for drag inducing features, it may well have been a success.

 

[3393RP]

You're kidding right? Absolutely people buy the Cirrus because it has a parachute!

He said the sole reason. That makes sense to me. Indeed the parachute is a solid plus for many people, but some choose a different aircraft with similar performance and price characteristics.

 

[RotorAndWing]

In the case of the Skymaster, the promise was that engine out ops were simple and easy to deal with, therefore this twin was to be safer than the competition.

They are if flown correctly.

combined with reliability problems,

What "reliability" problems?

cabin noise

Cabin noise is on par with any high performance single, no more.

smaller baggage area

That's a given. Cessna did offer a baggage pod.

less than amazing performance

The Skymaster will make book performance if operated correctly. The other advantage the Skymaster had over the other "conventional" twins was the ability to operate in and out of smaller and unimproved strips.

Both my '66 and '74 NA models would do 160-165 knots on 20gph block to block and had a 1800 lb payload. Using a TCM IO-360 (210hp) that's not "less than amazing" performance.

And the Air Force operated the O-2's as FAC's. Those were flown from Wichita to South Viet Nam and back with only one loss enroute.

 

[RotorAndWing]

From what I've read about the 336/337, it failed to live up to the promise of being a safer airplane than the traditional twin. This is largely in part because it suffers from low power engines, so single engine performance is poor.

So 335 fpm climb on one engine isn't good enough?

And that's on the front engine, on the rear engine only it's 415fpm.

Front engine only 7500ft service ceiling, Rear engine only service ceiling is 9500ft.

So what are the numbers for the Twin Comanche, Seneca and Travel Air?

 

[Dav8or]

I couldn't agree more. And who could forget my favorite center thrust car of all time, the flying Ford Pinto?



Back half of a Skymaster + 1972 Ford Pinto = Mizar Flying Car. Flew on test flights, but just before going on sale, the wings separated in mid air, killing the engineer. What a sad day for the center-thrust world!

Don't believe it's real? Check out the 1972 promo video: http://www.youtube.com/watch?v=rzv4q5EEy1k

Doesn't really count. It's not really center line thrust. Center line requires two engines. In a single like that one, it is just considered a pusher.

 

[Skylane81E]

So 335 fpm climb on one engine isn't good enough?

And that's on the front engine, on the rear engine only it's 415fpm.

Front engine only 7500ft service ceiling, Rear engine only service ceiling is 9500ft.

So what are the numbers for the Twin Comanche, Seneca and Travel Air?

The Seneca I did my multi in was,

Opens envelope

Anemic, even 300+lbs under gross. What do you expect from a light twin?

 

[VWGhiaBob]

Doesn't the Pinto car engine provide the counter rotation?

Oh well.

 

[Dav8or]

They are if flown correctly.

Can't that be said about any airplane? They are all really safe... if flown correctly. The point is, the center line thrust concept was supposed to make twins easier and more accessible to single engine pilots. The record shows not much difference from the traditional twin. Apparently, there is more to this "correct" flying than in the sales brochure.

What "reliability" problems?

Reputations usually get earned for a reason. There is a whole host of aftermarket mods available having to do with rear engine cooling issues. Obviously, some people were having problems. I'm not saying that the problems were never overcome, I'm saying they existed long enough to get a reputation and to scare people away. What happens today in the used market is another story.

Cabin noise is on par with any high performance single, no more.

OK, if you say so, but are they noisier than their competition, the traditional light twin?

That's a given. Cessna did offer a baggage pod.

It is what it is. Not all planes work for all people. Unfortunately, buyers of planes seem to envision hauling around the load of a UPS truck in their decision making, even if they are unlikely to do so.

The Skymaster will make book performance if operated correctly. The other advantage the Skymaster had over the other "conventional" twins was the ability to operate in and out of smaller and unimproved strips.

There's that correctly business again. Lots of planes will do book numbers if operated correctly. The short field performance is definitely a big plus in the Skymaster's corner. The plane is not without merit. It should be pointed out that it can also be parked pretty much anywhere a single can be.

Both my '66 and '74 NA models would do 160-165 knots on 20gph block to block and had a 1800 lb payload. Using a TCM IO-360 (210hp) that's not "less than amazing" performance.

I guess it depends on what amazes the buyer of a new plane. Fuel burn? Payload? Speed? Like I said above, the Skymaster is not without merit, it just looks as though the numbers it had were not good enough to sway enough new buyers, nor inspire competition in the market place.

And the Air Force operated the O-2's as FAC's. Those were flown from Wichita to South Viet Nam and back with only one loss enroute.

Well, with professional crews flying them correctly... anyhow my understanding is that the O-2 is significantly different from the civilian version, how much that equates to reliability I don't know.

I'm not in this thread to bash the Skymaster, or center line thrust. Quite the opposite. I'm a big supporter of the center line thrust concept and I like Skymasters. Aside from the Leza Aircam, the Skymaster is the twin I would personally buy.

The OP was asking, why don't we see more center line thrust in GA and I was saying that buyers, the market, GA media, all seem to judge the merits of this configuration on one airplane and I think that's very wrong. Then the topic of why wasn't the Skymaster a bigger success than it was and I offered potential reasons.

If I'm all wrong, then why didn't buyers in the '60s and '70s flock to the Skymaster. Why didn't Beech, Piper and others build a competitor?

 

[ron22]

Doesn't the Pinto car engine provide the counter rotation?

Pinto engine . And people were saying the skymaster was under powered

 

[Dav8or]

So 335 fpm climb on one engine isn't good enough?

And that's on the front engine, on the rear engine only it's 415fpm.

Front engine only 7500ft service ceiling, Rear engine only service ceiling is 9500ft.

So what are the numbers for the Twin Comanche, Seneca and Travel Air?

I think they all are underpowered and lead to many accidents because of it. The common wisdom these days seems to be, never operate light twins and max gross and actually stay well under. So given that, I think perhaps GA may have been better served by either limiting the number of seats in twins with small engines, or increasing the size of engines on the same airframe. This of course goes contrary to what the buyers actually want.

It's not that the Skymaster is any worse than others, it's just that it hasn't proven to be much better either and that was kind of the promise held by Cessna.

 

[Tony_Scarpelli]

Probably noise and performance mixed with a not spectacular record because it was often bought and operated by less than competent pilots.

Yes, great idea but the law of unintended consequences (novice pilots flying them them and assuming them to have not gotchas). Also I have heard that the inattention to rear engine not working on take offs caused them to get a bad wrap as well. Then the issue of maintaining them and the line gets a black eye in reputation. I have read the factory iteration of the rear engine cooling was not sufficient. Later there are STCs which greatly improve it.

I would love to own one.

 

[jasc15]

Searching for images of that Moynet Jupiter yielded this picture:

That's a pretty sexy inline twin. Very scant info on it, but the N-number is a giveaway.

I also found this writeup:

http://www.davemorss.com/Starcraft.html

6-foor carbon fiber driveshaft for the rear engine.

 

[RotorAndWing]

Well, with professional crews flying them correctly... anyhow my understanding is that the O-2 is significantly different from the civilian version, how much that equates to reliability I don't know.

The O-2 was an "off the shelf" aircraft. There is not that much significant difference between the O-2 and the 337.

 

[weilke]

Well, with professional crews flying them correctly...

Read an article about that operation the other day. Ferrying the aircraft was contracted out and the crews willing to ferry those planes across the pacific were neither professional nor did they operate the aircraft correctly.

 

[RotorAndWing]

Reputations usually get earned for a reason. There is a whole host of aftermarket mods available having to do with rear engine cooling issues. Obviously, some people were having problems. I'm not saying that the problems were never overcome, I'm saying they existed long enough to get a reputation and to scare people away. What happens today in the used market is another story.

The aftermarket mods are there to fix a problem that really doesn't exist. Both of my 337's the rear engine ran cooler than the front engine and I have seen this on others I flew.

What "scared people away" were the OWT's that people kept (and keep on) circulating about the airplane.

OK, if you say so, but are they noisier than their competition, the traditional light twin?

The 337 props turn 2800 rpm at takeoff power. During cruise the props turn 2300 to 2400 rpm, just like other twins. And like older twins with little to no insulation they could be perceived as noisy. I've flown in Travel Airs and Twin Comanches that were far noisier.

As far as my 2 337's, I replaced the interiors as well as the insulation. The result was in cruise I could take my headset off and hold a normal conversation with my passengers.

 

[weilke]

The 337 props turn 2800 rpm at takeoff power. During cruise the props turn 2300 to 2400 rpm, just like other twins. And like older twins with little to no insulation they could be perceived as noisy. I've flown in Travel Airs and Twin Comanches that were far noisier.

From the outside, the 337 does sound very different from other twins. A bit like the Mu2 that makes a terrible racket on the outside but is no different from other TPs on the inside.

 

[RotorAndWing]

From the outside, the 337 does sound very different from other twins. A bit like the Mu2 that makes a terrible racket on the outside but is no different from other TPs on the inside.

True, the comments from people on the ground when taxing in on a Garret powered airplane is usually along the line of "wow, that airplane must be real noisy in the cockpit!"

People hear a Skymaster flyover and it sounds different, so then it's assumed it's "noisier".

 

[Skylane81E]

Trying to find the picture of the O-2 with the horizontal stab nearly split in two but not having any luck

 

[wabower]

Having watched the story play out from the beginning, I always thought the fixed-gear 336 poisoned the well for many potential 337 buyers.

 

[RotorAndWing]

Having watched the story play out from the beginning, I always thought the fixed-gear 336 poisoned the well for many potential 337 buyers.

Yep, the 336 had issues that were resolved on the 337.

 

[JHW]

Everyone wants to find a "deal". But there are no bargains in general aviation, the market is very efficient at assigning value. And the market says you can't give away a skymaster. Of course that doesn't prevent a few cheerleaders from soldiering on. Even the ford pinto has a fan club.

 

[RotorAndWing]

Everyone wants to find a "deal". But there are no bargains in general aviation, the market is very efficient at assigning value. And the market says you can't give away a skymaster. Of course that doesn't prevent a few cheerleaders from soldiering on. Even the ford pinto has a fan club.

And it doesn't prevent those who have an axe to grind from poking in either.

 

[poadeleted21]

And it doesn't prevent those who have an axe to grind from poking in either.

Ahhhhhhggbbb you let him bait you but didn't take mine... I gotta tighten up.

 

[JHW]

the only axe I have to grind re:skymasters is that I spent far too much of my flying youth in one. The funny thing is, since it was the plane i first solo'd in, I thought it was normal. It wasn't until I got stranded somewhere and someone loaned me a 310 to get home that i saw the light.

 

[RotorAndWing]

the only axe I have to grind re:skymasters is that I spent far too much of my flying youth in one. The funny thing is, since it was the plane i first solo'd in, I thought it was normal. It wasn't until I got stranded somewhere and someone loaned me a 310 to get home that i saw the light.

Look at the "quality" twin in your avatar. Seriously, you remind me of a guy driving a '72 Vega rolling down your window and heckling a guy driving a Pinto.

 

[JHW]

Look at the "quality" twin in your avatar. Seriously, you remind me of a guy driving a '72 Vega rolling down your window and heckling a guy driving a Pinto.

You've nailed it. The one in my avatar is the kind of quality that has seen the same model in continuous production in various forms since 1958.

 

[weilke]

Everyone wants to find a "deal". But there are no bargains in general aviation, the market is very efficient at assigning value. And the market says you can't give away a skymaster.

The same can be said about earlier B55 Barons. Little difference in prices, the ones with workable engines sell, the ones with runouts turn into beer-cans.

 

[RotorAndWing]

You've nailed it. The one in my avatar is the kind of quality that has seen the same model in continuous production in various forms since 1958.

Nice twist Jeff. The TA was so successful they built a whopping 720 of them.

An oxymoron is "quality" and "beechcraft" used in conjunction.

 

[JHW]

The same can be said about earlier B55 Barons. Little difference in prices, the ones with workable engines sell, the ones with runouts turn into beer-cans.

Yep. Virtually anything with a pair of 6 cyl continentals falls into that category. That's why there is still some demand for twinkies and travel airs. The 337 is the worst of both worlds. All the space of a Twinkie but with the expensive engines of a larger plane.

 

[LJS1993]

So if a guy has plenty of cash and finds a 337 that has solid engines and gear he would still be massively disappointed with this aircraft? Would a person go deaf from the noise? Is the design inherently dangerous?

 

[weilke]

So if a guy has plenty of cash and finds a 337 that has solid engines and gear he would still be massively disappointed with this aircraft?

Not if he knows what to expect.

Would a person go deaf from the noise?

A good set of ANR headsets will take care of the noise issue.

Is the design inherently dangerous?

Last week some knucklehead in California crashed a 337. The plane was in the shop due to an engine issue. He was unhappy with the shop and tried to ferry the plane with one hinky engine. Made it a mile past the airport until he hit a couple of powerlines and crashed into a fedex truck. Not the planes fault in my book.

The problem (VMC roll-over) that centerline thrust fixes is a uncommon cause for twin accidents. Centerline thrust twin pilots will still:
- fly into the side of mountains
- fly into thunderstorms
- fly into IMC when their IFR skills are not up to the task
- fall out of the sky during instrument approaches for no good reason
- botch landings
- botch take-offs
- do the 25 other things that cause the 80% share of the accidents that are pilot induced.

 

[JHW]

So if a guy has plenty of cash and finds a 337 that has solid engines and gear he would still be massively disappointed with this aircraft? Would a person go deaf from the noise? Is the design inherently dangerous?

Yes, you will be once you have a chance to make an objective comparison with the other light twin alternatives. OTOH if you just want to be different for the sake of being different, it might be just your thing.

 

[RotorAndWing]

So if a guy has plenty of cash and finds a 337 that has solid engines and gear he would still be massively disappointed with this aircraft?

Go fly it and make up your own mind.

Would a person go deaf from the noise?

If the cockpit is noisy replace the insulation with the newer style. I did and had a very quiet cockpit during cruise.

Is the design inherently dangerous?

Nope.

 

[Henning]

So if a guy has plenty of cash and finds a 337 that has solid engines and gear he would still be massively disappointed with this aircraft? Would a person go deaf from the noise? Is the design inherently dangerous?

Nah, you get one that's been taken care of and you'll like it.

 

[Dav8or]

Back to the original question- Center Thrust in GA (Why not?), I think this thread sort of sums it up. The whole concept is judged in our time based on one airframe that wasn't designed all that great. It seems to have been a collection of compromises and the net result wasn't a terrible airplane (although Burt Rutan thinks so), but not a great one either. Sadly, neither the Rutan Defiant, or the Adams A500 ever reached any sort of production numbers to make them significant. Reports from both planes are favorable though.

If one looks before the Skymaster and even before the end of WWII, you'll see the layout has been used before quite successfully. The Dornier company made it practically a trademark and their seaplanes were world renown to be excellent.

And then there is the case of the fastest piston fighter of WWII, Dornier 335. From all accounts by pilots from both German and Allied, an amazing airplane that would have ruled the skies had it not been for the turbo jet engine.

 

[LJS1993]

Go fly it and make up your own mind.



If the cockpit is noisy replace the insulation with the newer style. I did and had a very quiet cockpit during cruise.





Nope.

Come pick me up man!!!