YESTERDAY'S WINGS

The Boeing Monomailby PETER M. BOWERS / AOPA 54408

Flight view of the Model 200 shows the incompletely retracted wheels and the streamlinedfairings behind them. Boeing retouched this photo, adding windows to show what theunfinished passenger.carrying model, the 221, would look like.

BOEING 200/221

~

-

158 mph **135 mph

* * compromise fixed-pitch propeller

Performance

High speedCruise speed

(60% power)Landing speed 56 mphInitial climb 850 fpmService ceiling 14,000 ftRange 530 sm

Specifications

Powerplant Pratt & WhitneyHornet B

575 hp @ 1,950 rpmSpan 59 ft 11/2inLength 41 ft 21/2in *Area 535 sq ftEmpty weight 4,7581bGross weight 8,000 lb* stretched 27 in for Model 221A

~~

THROUGHOUT aviation historythere have been many examplesof new airplane designs that

failed to live up to their potential be­cause the airplanes got ahead of theavailable powerplants. In many cases,an airplane would be designed to usea promising new powerplant that wasthen on the drawing board or even inthe development stage. All too often,the engine was delayed for various rea­sons and the new airplane had to flywith a lower-powered, or otherwi~eunsatisfactory, substitute engine.

What at first appears to be anexample of the airplane getting aheadof the engine can be found in theBoeing Monomail of 1930. However,this was not really the case. The power­plant was the Pratt & Whitney Hornet,a well-proven air-cooled radial withextensive military and civil operationdating from 1927.

Taken by itself, the Model 200,which the builder named Monomailfor being a single-wing mail plane at atime when traditional biplanes domi­nated the airmail routes, was a trulyrevolutionary airplane. In spite of theheavy influence it exerted on subse­quent designs, the Monomail is virtu­ally forgotten today because of thegreater fame of its descendants and thefact that the design it~elf was not acommercial success and did not estab­lish a reputation on the mail routes.

No single feature of the design wasnew-the revolution came about byputting several different preexistingfeatures together in one airplane forthe first time.

From a distance, the most notablefeature of the Monomail was that itwas a clean low-wing monoplane at atime when most commercial single­engine monoplanes were high-wingswith strut bracing. Cantilever wingsthemselves were not new, going backto World War I and being conspicuouson the contemporary Fokker and Fordtrimotor transports and the famouswooden Lockheeds.

The reason for the wing being in thelow position was most significant-itwas down there so that it could supportand house a retractable landing gear.This tripod assembly rotated backwardabout the front spar and into a wheelwell just ahead of the rear spar. Sincethe wing at this point was not thickenough to house the full diameter ofthe wheel vertically, the lower half of

62 THE AOPA PILOT I DECEMBER 1978

The first Boeing Monomail. Model 200.Note the three separate mailcompartments ahead of the cockpit, thetransition from a circular fuselagecross section forward to a vertical knife

edge aft. and the large fillet betweenthe wing and the fuselage.

the wheel projected into the slip­stream. While this arrangement didnot realize the full drag-reducingpotential of the retractable gear, it wasstill a big step forward, since no otherdesigns were using anything like it atthe time.

Retractables had been around sincebefore World War I, but were usedmostly on amphibians with no con­sideration of drag reduction. The air­planes of the time weren't fast enoughto benefit significantly from such afeature; the handicap of the addedweight and machinery outweighed thebenefits until speeds approached 150mph.

The low cantilever wing and retract­able gear of the Monomail resulted ina very clean airplane, which was fur­ther enhanced by two other innovativefeatures. One was a ring cowlingaround the radial engine, which wasactually the newest feature of all whilealso being an oldie. The apparent con­tradiction comes from different appli­cations at different times.

Back in World War I, many air-

YESTERDAY'S WINGS continued

planes were powered with rotary en­gines, which threw off great quantitiesof unburned castor oil. On pushers,with the engine behind the crew, thiswas no problem. With the engine upfront, such devices had to be shielded.As the speed of the planes increased,these shields, or cowlings, took on amore streamlined form to reduce air­plane drag. When the rotary enginewent out of production after the Armis­tice, the cowling disappeared with it.The static radial engines that replacedthe rotaries operated entirely in theopen to get maximum exposure to thecooling airstream.

As radial engines became morepowerful, their high drag became aserious handicap to high-performanceaircraft. The National Advisory Com­mittee for Aeronautics (NACA, nowNASA, for National Air and Space Ad-

The speed range of the Monomail (right)was seriously hampered by the need to use

a fixed-pitch propeller. Here the Model200 is tested with a special geared Hornet

engine that drives a more efficients/ow·turning, three·blade propeller.

The Model 221 (below) was completed with asix·passenger cabin in place of the two

forward mail compartments. later. it wasstretched 27 inches to add a seventh

passenger seat and the original Model 200was reworked to become a duplicate Model

221A. Note that the wheel fairingshave been removed.

ministration) went to work on the prob­lem and developed a new cowling thatat first glance looked like the oldrotary cowling of 1918. However, itdid much more than contain oil sprayand streamline the nose of the air­plane. It did smooth out the flow of airpast the engine, which was a signifi­cant drag reduction in itself, but it alsoaccomplished the seemingly bootstrapoperation of adding thrust.

By careful contouring, the NACAcowling acted in effect as a small wingbent into a circle, a wing that had asignificant forward vector of thrust.This was borne out when designers,expecting a rearward load to be appliedagainst the cowling, found the cowlingactually pulling forward against theattach fittings and hitting the pro­peller. The performance gains possiblewith the new cowling were demon-

I

strated very spectacularly at the 1929National Air Races and designershastened to adapt it to existing radial­engine models and to design it in tonew ones. Many designs merely cut theforward portion of the full NACAcowling back to form a ring-and thiswas widely used as the "TownendRing;" the Monomail used Boeing'sown version of the latter.

The final "new" feature of the Mono­mail was its structure-all-metal semi­monocoque, with smooth skin in placeof the traditional corr~gations, andflush riveting. The wing had the twobolted, square, aluminum tube spaisused on some previous Boeing designs,but the torsional stiffness was providedby the metal skin, which substitutedfor the fabric used previously. Thefuselage had longerons to take themajor bending loads but the torsion

was again taken by the smooth metalskin. The fuselage cross section wasan oval, which further improved thestreamlining but required an elaboratefillet between the fuselage and thewing.

For all its advanced features, how­ever, the Monomail retained one realanachronism-the open cockpit behindthe wing for the pilot. Established air­mail pilots had a say in the design ofthe new ship and insisted on this par­ticular detail. In further keeping withold design concepts, the wing loadingof the Monomail was light enough toavoid the need for wing flaps.

The Model 200 Monomail was asingle-seater with three separate mailand express compartments totaling220 cubic feet. It was awarded Ap­proved Type Certificate A-330 on June24, 1930. A sister ship, the Model 221,which differed only in having a six­passenger cabin in place of two of themail compartments, received ATC-366on September 16, 1930. After brief useon Boeing Air Transport routes, theModel 221 was stretched 27 inches toincrease the passenger capacity toseven as the Model 221A, and theModel 200 was converted to a dupIi~cate 221A.

Since the two Monomails were a tonheavier than the Boeing Model 40B-4biplanes, had double the passenger andmail capacity and were over 20 mphfaster with the same engine, whydidn't they enjoy commercial success

commensurate with their advancedconfigurations?

It wasn't a case of the airplane beingahead of the engine this time, but acase of being unable to take full ad­vantage of what should have been afully compatible powerplant at bothends of the performance scale. TheMonomails needed something that wasnot available at the time-a good con­trollable-pitch propeller. The ground­adjustable models of the period couldbe set for the most effective takeoffand climb performance, but the high­speed performance then suffered. Ifthe blades were set for efficient cruis­ing, the takeoff suffered, particularlyat places like 6,000-foot-high Cheyenneon the San Francisco-Chicago route.

Boeing had the same trouble withits 247 twin-engine model of ] 933 un tilHamilton-Standard perfected a con­trollable-pitch model, which went intoservice on the improved 247D modelof late 1934. The Monomails eventu­ally got controllable propellers, but bythat time other designers had caughtup with and improved on the revolu­tionary Boeing, so there was no pointin perpetuating the 1930 model.

The Mdnomails left their mark, how­ever, by· contributing their aerody­namic and structural features to theBoeing B-9 that revolutionized theArmy bomber business and the 247transport that itself inspired and wasquickly surpassed by the Douglas DC-],-2 and -3 models. D