Why isn't the push/pull twin more popular ? What you say.

[Pops]

With all the problems of pilots being proficient in handing a light twin on single engine, why isn't the push/pull twin design used more ?
Pro's and Con's

 

[Vigilant1]

I wonder about that, too. Single-engine ops are much simpler and safer when all the thrust remains on the centerline.

The C-337's had a reputation for being a maintenance hog and for not being as fast as people thought a twin should be, maybe that has hurt the cause. (The speed complaint rings hollow to me-- the 337:is faster than a Seminole an carries more). Neither of those shortcomings were the result of the push-pull layout, but were due to other design decisions made by Cessna.

Maybe the challenges of asymmetric single engine operations get insufficient consideration from the buyers of twins. After all, we are all above average pilots and would never allow ourselves to be caught below Vmc

 

[gtae07]

I suspect a big reason is just that the piston twin in general is not all that viable in the market; on the top end they're squeezed by turbine singles and on the lower end there's pressure from higher-performance piston singles. The operating expenses are high and they require a new rating just to fly them, so there aren't many twins out there to begin with. And I'd bet that the majority of the piston twin market is therefore training--and nobody's going to want to get a multiengine rating with a centerline thrust limitation. A push-pull twin is therefore only going to be used for non-training purposes. It will be competing in the market against aircraft that are desirable for training as well (and can therefore access a larger market with corresponding higher sales).

From a technical standpoint, integrating it into the airframe is probably a bit more challenging due to CG concerns, cooling, and airflow to the rear engine. And then you most likely need a twin boom arrangement (read: more weight/complexity). But I think the market considerations outweigh the technical challenges.


In short, I think there's very little demand compared to the already small demand for piston twins in general, so it's just not worth developing one for so small a market.

 

[Niels]

A certain mr Rutan once said that he would never again put something in front of propeller

 

[Bill-Higdon]

I can think of 1 reason a "conventional" twin provides immediate feedback as to which engine has gone "south" by the old dead foot = dead engine. Where as a center line twin AKA push pull doesn't provide that feed back.

 

[Vigilant1]

I suspect a big reason is just that the piston twin in general is not all that viable in the market; on the top end they're squeezed by turbine singles and on the lower end there's pressure from higher-performance piston singles. The operating expenses are high and they require a new rating just to fly them, so there aren't many twins out there to begin with. And I'd bet that the majority of the piston twin market is therefore training--and nobody's going to want to get a multiengine rating with a centerline thrust limitation. A push-pull twin is therefore only going to be used for non-training purposes. It will be competing in the market against aircraft that are desirable for training as well (and can therefore access a larger market with corresponding higher sales).

In short, I think there's very little demand compared to the already small demand for piston twins in general, so it's just not worth developing one for so small a market.

We should probably also note that, as far as certified small aircraft are concerned, there's not much development right now of anything by way of new airframes.


So, are there factors we could use to identify where a niche/market might justify development of a centerline twin? Likely attributes to consider:
- Barriers to entry: High certification costs and a small potential market make it more likely to come from the E-AB world than certified.
- Mission: The higher marginal cost of a twin vs. a single is more justifiable if the risk of engine failure is greater (e.g novel engine type, lack of certification safeguards, etc) or if the consequences of engine failure are greater (e.g. frequent flight over unlandable terrain or water).
- Pilot certification costs: Is there a case where there's no FAA requirement for additional certification to fly a twin? (Yes, apparently a multiengine rating may not be required for solo flight in a multiengine E-AB . Doing your own annual?

- Cases where a centerline twin has especially significant advantages over other twins:
-- If the pilot is likely to be of low total experience or expected to have relatively low annual flight time in type or low amounts of engine-out training currency.
-- If low speed flight is required. A twin with a Vmc below the aircraft's stall speed can be safely flown on approach much like a single. Similarly for climbouts. With a "regular" twin, a climbout at a steep angle that puts the plane below Vmc can result in a very bad situation very quickly in case of engine failure.

 

[Riggerrob]

The biggest problem is streamlining the aft engine cowling. A typical light twin needs to rapidly taper from a 4 foot wide cabin rear bulkhead down to zero thickness in only 4ish feet. This short cowling is driven by the need to install the rear engine as close to the rear spar for balance.
That is why Cessna 337 climbs worse after the rear engine quits ... it loses smooth airflow over the aft cowling.
This problem is not unique to push-me-pull-yous. All pod-and-boom pushers suffer similar problems (Republic RC-3 Seabee, Glass Goose, Taylor Coot, etc.).

 

[ToddK]

Because it has all the same problems a twin has. Extra weight, extra expense, extra maintenance, extra complexity, extra fuel, extra training/checkride.

The value proposition is just not there.

 

[Richard Schubert]

I suspect the view over the nose is a large but unspoken factor.

 

[TFF]

Cessna had competition between the single and twin parts of the company. When they decided to make the 337, it was going to be made in the single factory to bump up the production, and the twin side wouldn’t help with engineering. The single engine side ended up making all the mistakes the twin side knew to avoid. They were snobby and thought it was funny. What popped out had enough issues that it became an also ran. It’s also not very fast. Lots of people love them. I had a friend that had one, and there was one locally with every STC including two 550s.

 

[Vigilant1]

Lots of people love them.

That's definitely true. But, apparently, not enough people to keep their prices high in the used airplane market. And that's another problem, in a way: I suspect the low market value results in a disproportionate number of people who buy 337s and then aren't able/willing to keep up with the maintenance. That results in a bad maintenance cost spiral, griping, and a (not entirely deserved) reputation for high mx costs.

 

[Pops]

1 --- with both engine weights in the fuselage the wing has to be designed for more stress than if the engines were on the wings.
2--- cooling problems with the rear mounted engine.
3--- Maintenance of retractable landing gear in the small space of under the floor along with everything else is a headache.
4--- When retracting the gear, the main gear doors are like two air brakes as built from the factory. Read the manual on when to retract the gear to not get in trouble.
5--- Poor engines. Cont- 360's are not as good as Lyc- 360 in several ways.

Have several hours fly the 337 and have helped with the 100 hr inspections several times.

I do like flying the 337. Flys like a C-182 with with two engines. Identifying an engine on an engine out is simple . Look at the MP, RPM gauge when there is a power loss.
When adding power on take-off, lead with the rear engine on powering up for extra indication you have a good engine behind you.
A 337 with a pair of 550's would be a nice airplane.

 

[TFF]

Just like car owners, there are two general types of owners. The kind who trade in every three years or the kind who buys and runs it until the end. Both have their merits. One keeps value up by trading in, and one is after the longest usage. Airplanes are the same. You can hound every squawk or you can run them until it’s a core to be rebuilt.

One issue is the lapse time of TBO on things. Very hard to judge that value when each person has their balance point of good or bad. Lots of people buy in at thinking they will never reach it, and they can get out at what they got in with.

A twin costs twice as much for a plane worth as one plane. Which would you rather have, one 337 or two 182s? You can buy a 337 for 182 money, but when it comes to prop and engine OH, it will be as much as two 182s. There isn’t enough of most types of planes to nibble at the value and trade, you have to ride the value down as it ages and components wear. The choice is always market value. If you sell at the back quarter of life, it’s bottom value. You take the money, or you might get some added value after you overhaul it before putting it up for market.

That 337 with the 550s had $400,000 in STCs. There is a lot more airplane I would buy if I had $400K. If not a fleet. Heck you could buy an 1/8 in on a P51 and really strut around. To me the 337 isn’t a bad plane, but it doesn’t match up for what it is.

 

[PMD]

I think I have posted the story before, but any time the question comes up I go back to my own second hand 337 and first hand 336 experiences. We had a customer who was a bush charter operator with a 185, a 336 and in the end a Bell 206. When he bought it (used in USA) we thought it was a silly thing, as EVERYONE in the North seemed to have a 337 and all of the maintenance issues they came with. What we quickly learned was the 336 was dead reliable and lacking all of that mickey mouse (sorry Walt) gear also had a lot better payload. Compared with a 180 or 185, it was extremely easy to load passengers and/or freight. It was more like a 400 horse 206 (Cessna that is).

Regarding the 337, though, my pilot/engineer was also chief engineer for a charter operator that leased my 3NM. They had a 337 go missing on a medical charter out of YYL one winter night in a snowstorm. A hunter found the wreck and bodies after summer snow melt just off the departure end of the runway. The accident report blamed the pilot, but there is a back story I will try to keep short. We made one of our marks with the genav community when I bought my Expediter with an outstanding gear retract on taxi insurance claim. The adjuster insisted that all that had to be done was dial the prop flanges and if in spec, put back in service. I was very uncomfortable with that so asked our MoT maint overseer what could be done. He said to see what P&W said in the engine overhaul manual, so we had a good look. It read that the engine SHOULD be torn down for internal inspection. The regional office issued a letter that said when interpreting a manual when it says "should" it must be taken as you "SHALL" etc. A few people were POd with us and a few were supporters, but we soon got to known as the prop strike guys. (BTW: on teardown, my 985s both NEEDED new cranks, even though dialed out just fine).

The crash 337 belonged to an ex MoT inspector (also ex-RCAF 3NM pilot) that he landed gear up about the same time we were rocking that boat. Can't say there were personal favours or relationships involved, but the rear engine crank was dialed up and the engine went back into service. We never got our hands on the crank, but my pilot/AME friend was able to get his paws on the rear prop and it was in FLAT pitch producing no power. The report IIRC claimed possible fuel exhaustion (all tanks compromised in crash). Our feeling based on a LOT of questions, study, debate, etc. was that the rear crank cracked letting oil pressure to prop hub leak back to the crankcase and giving full flat pitch. Takeoff into heavy snow, pilot gets air brake on rear engine before he has time to deal with it and has departure stall at low altitude and game is all over for him and nurse on board.

It is all to likely that an engine failure with a side-by-side twin could have the same outcome, but this on was supposed to be so safe because of coax thrust line. Also why I take ANY accident report with a HUGE grain of salt.

 

[llemon]

Rutan said he went with the boomerang arrangement to avoid the noise and vibration problems of push-pull.

 

[Vigilant1]

Which would you rather have, one 337 or two 182s?

It would depend entirely on the flying I was doing. I can't say that one would always be better.

If my flying was overwater search in international waters with the occasional need to throw out a raft or bundle with radio, drinking water, etc, then I'd prefer the 336/337 to a 182. Likewise, I can understand why the 337 is popular for pipeline survey work, esp over rough or heavily forested terrain. That second engine on the O-2 got some guys back to have a beer at the club rather crashed in the jungle and spending a few years being beaten senseless in a POW camp. That's worth something.

OTOH, if I was doing charter work and operating cost was key to paying the rent, then I'd almost surely choose the182 over the Skymaster.

Of course, resource constraints also factor in. For that overwater search mission a turbine engine plane would be great, especially if I wasn't buying the plane or paying the fuel bill.

Different horses for different courses.

 

[Pops]

It would depend entirely on the flying I was doing. I can't say that one would always be better.

If my flying was overwater search well offshore in international waters with the occasional need to throw out a raft or bundle with radio, drinking water, etc, then I'd prefer the 336/337 to a 182. Likewise, I can understand why the 337 is popular for pipeline survey work, esp over rough or heavily forested terrain. That second engine on the O-2 got some guys back to have a beer at the club rather crashed in the jungle and spend a few years being beaten in a POW camp. That's worth something.

OTOH, if I was doing charter work and operating cost was key to paying the rent, then I'd almost surely choose the182 over the Skymaster.

Of course, resource constraints also factor in. For that overwater search mission a turbine engine plane would be great, especially if I wasn't buying the plane or paying the fuel bill.

Different horses for different courses.

When you are running a business trying to make the most money the choice of airplane for the mission can mean a lot. At one time I was thinking of upgrading to a C-206. Found a nice 206 but after doing a lot of looking at the details, my profit would have been less.

 

[TFF]

If the mission is “you will loose an engine to ground fire”, the mission statement clearly says the other side be fine. An engine clearly only means one.

There are a couple of O-2s in the area. Still all military. One was normal. The other was some type of electronics warfare setup. Still has all the switches and knobs except the one box that matters.

All the pipeline work I know is in single engine helicopters 500 ft or lower. Pistons and turbine. Sometimes over swamp with gators. If we had offered airplanes, the companies would have cheaped out and used the airplane.

 

[Vigilant1]

There are a couple of O-2s in the area. Still all military. One was normal. The other was some type of electronics warfare setup. Still has all the switches and knobs except the one box that matters.

About 95% of US O-2s were the A models, used for the forward Air Control (FAC) mission. Two seats, radios, and hardpoints for a WP marking rocket pod.
The other 5% were B models used for PSYOPS (leaflet drops, loudspeakers). It's possible the unusual one near you was a PSYOPS bird.

 

[Pops]

At one time I was about to sign a contract for a job during pipe line patrol. Then the bottom dropped out of the price of NG and the company almost went bankrupted. They had 7K miles of pipe line from PA to TX. After the company got back on it feet they call me and wanted me to do the work, but just sold the company and retired.

Local man that I have known for many years used to do an aerobatic routine with a C-337. " Bodacious "
And it was impressive.