Why was the Fokker D23 built as a pusher puller?

Draconis,

Don't know if they were retrofitted to the original, but there were proposals to combine the heat exchanger packages into the fuselage behind the engines and incorporate an exhaust pumped ejector. The entire heat output of the Allisons would then, to some extent, participate in propulsion.
Dynasoar

Would you mind elaborating on this? I don't understand what it means.

I like the idea of the hydraulically driven cooling fans. They should've been made featherable to get them out of the way when the plane is at a sufficient airspeed for the slipstream to provide adequate cooling. Reducing the auxiliary loading on the engines.
 
Draconis,

As I understand the description, the fans did not cover the full face of the heat exchangers. Hydraulic power transmission systems are inherently reversible and characterized by high efficiency. Here, I'm speculating but if it was my project, I'd permit the fans to idle, extracting very little power from the system. Having designed turbomachinery in the past I'd estimate that we're looking at maybe 80 horsepower in at max standstill cooling, half that during climb and maybe less than ten cruising at altitude.

About exhaust eductors: These devices have been used in the past on aircraft as different as the Convair 240-440 transports, Piper Apache and Beech Duchess and Helio Courier as well as several helicopters (also on some models of the Tatra automobile. Basically the principle is at least as old as the steam locomotive- a high velocity gas (exhaust from a gas or steam engine) is conducted into a larger diameter duct connected to the volume that you wish to evacuate. Viscous mixing of the high velocity gas transfers kinetic energy to the flow in the surrounding duct. Complications arise if the high energy stream is intermittent; this determines the required length of the surrounding duct since at least two exhaust events are required for efficient pumping.

I'm sure that you are familiar with the legend that the "Meredith Effect" (Spelling has never been a strong point) utilizes properly shaped ducting to provides negative cooling drag for the P-51. The analogy of a low temperature ramjet often applied. The process actually does work to significantly reduce cooling drag. Visualize a P-51 style heat exchanger complex upstream of an exhaust eductor. Virtually all the waste heat- more than twice the shaft horsepower, participates in the propulsion process in the "ram jet" cycle. _Thermodynamic eff.is relatively low, but in this example there would be positive thrust- quick calculation at 375 MPH, at least 800 HP/lbs thrust- probably more.

Dynasoar
 
091014 223.png Just a thought.
 
As Stenz suggested, push-pull airplanes handle better after one engine quits. Most light twins suffer several yawing problems after one engine quits, too often overwhelming the pilot. Combine that with marginal climb performance on one engine and it is little wonder the light twins cost far more to insure than complex singles offering similar performance (e.g. Pilates PC-12). Push-pull Cessna 337 Skymasters are so much easier to fly - after one engine quits - that the FAA issues (limited) multi-engine ratings to pilots who only fly 337s.
See my rant about Blohm unt Voss 141 for a detailed explanation of handling with assymetrical thrust.

As Dynasoar suggested, streamlining around pusher engines is a huge problem. On lighter airplanes (e.g. Fokker D 23) the rear propeller needs to be close to the trailing edge to maintain balance. Unfortunately that dictates rapidly tapering the fuselage over the last half of the wing chord ...... a recipe for separated airflow and massive drag ..... poor propeller thrust .... and cooling problems.
See my rant about Republic RC-3 Seabee flying boat to better understand airflow around the aft end of short nacelles.

Many twin-boom pusher prototypes (Anderson-Greenwood, Vampire, Adams, etc.) have been test-flown, but few reached production. Cessna's 337 Skymaster was the most numerous, but rapid tapering - and poor airflow - around the aft engine hampered climb performance when it quit. With the rear propeller no longer pulling the boundary layer, it became turbulent, dramatically increasing aft-body drag.

Sarcasm alert!
Even Burt Rutan - the master of bizarre configurations - only designed four airplanes with pusher props in the aft fuselage. His Vari-Eze and especially Long- Eze pusher singles revolutionized the Homebuilt scene. These tiny two-seater pusher canards extracted amazing speed and range from comparatively small engines. His similar push-pull Defiant twin was built in small numbers.
Burt Rutan's last push-pull airplane: Voyager really only flew one mission. His brother Dick Rutan (and co-pilot Jeana Yeager) barely circled the planet once, landing with mere fumes in its fuel tanks!
Hah!
Hah!

Mind you, Voyager's rear engine hung a long way aft of the wing, easing streamlining. Also consider that Voyager's fuselage cross section was not significantly larger than its engines.
 
Last edited:
Riggerrob,

Have to disagree with some of your conclusions relative to pushers in general and the Cessna 336/337 in particular. Climb performance on the rear engine is markedly improved relative to the front engine. (Time logged in a 336 back in the sixties including caging each engine individually). Discussions with the owner of a Turbo 337 (also an aero engineer so we spoke the same language) revealed that in addition to climb performance, top speed and %power required in cruise were both superior on the rear engine. Jack Norris, Burt Rutan's Director of Propulsion on the design and the flight or the Voyager has written an excellent paper for the AIAA on the loss of propeller efficiency, relative to spindle performance, when the prop is installed in a conventional tractor aircraft. Another long time friend, Joe Brownlee, an FAA test pilot was assigned to the LearFan certification program was amazed at the performance of this pusher, relative to similarly powered conventional twins. Don't have all my stuff accessible as I type, but if you are interested I'll dig out the the title and number of Jack's paper.. My earlier posts on this topic go into why pusher props, facilitate attached flow over upstream surfaces and, operating in an accelerated boundary layer demonstrate higher propulsive efficiency. In fact, look up "Wake propulsion" One of the examples, a long range small cruise missile with pusher counterrotating fans showed an installed propulsive eff. exceeding 100% (this is relative to the propulsor operating in a free field).

The original Consolidated intercontinental bomber proposal involved four engines , two tractor, two pusher. Powered model wind tunnel tests conclusively demonstrated that four, and later six pusher props made the B-36 a design success.

I'd be interested in seeing disagreeing arguments.

Dynasoar
 
I'd be interested in seeing disagreeing arguments.

Not really disagreement, but Steppenwolf once sang "God damn the pusher".

The Cessna 337 never equalled the 310 as a sales success. The Starship, Avanti, and a string of others, including the un-certified Lear Fan, never rivaled the Beech King Air. The B-36 was only a limited design success with 4 burning engines, and suffered from engines not designed for pushing. Even the humble Walrus was superceded by the humble Sea Otter.
 
Dear Dynasoar,

I do not see a major disagreement between our positions.
I focused on un-powered airflow while you talked about powered airflow ..... two different conditions experienced by the same airframe.
I was talking about airflow around nacelle/fuselage after-bodies .... mainly after pusher engines quit. I largely ignored how propeller-generated thrust affects boundary layers.
Many of my conclusions are based on a variety of attempts to improve airflow around the aft end of the cabin on Cessna 337s, Seabees and other small amphibians (e.g. Glass Goose).

OTOH You focussed on how propeller thrust re-energizes boundary layers increasing thrust while simultaneously reducing drag.
 
Not really disagreement, but Steppenwolf once sang "God damn the pusher".

The Cessna 337 never equalled the 310 as a sales success. The Starship, Avanti, and a string of others, including the un-certified Lear Fan, never rivaled the Beech King Air. ......
-----------------------------------------------------------------------

Dang those salesmen messing with engineering data!
Hah!
Hah!

Seriously, Beechcraft King Airs dominate the light twin turboprop market. King Air was the first decent light twin turboprop and it quickly dominated the market. KAs now represent half of all the light turboprops sold .... despite Cessna, DeHavilland, Embraer, Mitsubishi, Piper, Shorts, etc. trying hard to sell competing airframes.

DeHavilland and Embraer aimed for the slower STOL market sector.

Beechcraft's exotic Starship failed because the FAA insisted on so much redundancy that it became so heavy that Starship competed directly with KA, negating the advantage of its sleek composite airframe.

Both MU-2 and Piaggio Avanti compete for a faster market sector that overlaps light jets ... and requires longer runways. Despite testing dozens of different propellers, Avantis still have a distinctive sound caused by propellers cutting wake.
 
Top