JETS VER­SUS TURBOPROPS

When it comes to per­for­mance, both jet and tur­bo­prop air­craft in the regime of busi­ness avi­a­tion have at­tributes and lim­i­ta­tions that a prospec­tive buyer would need to take into ac­count while se­lect­ing and eval­u­at­ing the plat­form

SP's Airbuz - - Front Page - BY B. K. PANDEY

THE CHOICE OF JET OR TUR­BO­PROP AIR­CRAFT BY THE AIR­LINE IN­DUS­TRY IS IN­FLU­ENCED LARGELY BY THE ECO­NOM­ICS OF THEIR BUSI­NESS MOD­ELS

AN IS­SUE THAT PLAGUES the minds of those who are new to the field of avi­a­tion is of­ten re­lated to choice of air­craft pow­ered by jet en­gines as against one pow­ered by a tur­bo­prop en­gine. Es­sen­tially, the is­sue boils down the type of power plant the air­craft in ques­tion is equipped with.

HIS­TORY OF DE­VEL­OP­MENT OF EN­GINE TECH­NOL­OGY. A jet en­gine, also re­ferred to as ‘tur­bo­jet’, rep­re­sents a quan­tum jump in tech­nol­ogy over the pis­ton en­gine, which was the only power plant for aero­planes till the end of World War II. The tur­bo­jet is an air breath­ing, gas tur­bine en­gine that has an in­ter­nal com­bus­tion cy­cle dur­ing its op­er­a­tion. The credit for the in­ven­tion of the jet en­gine, a feat that was achieved just be­fore World War II, goes to Sir Frank Whit­tle who worked on this project sin­gle-hand­edly and had patented the con­cept in 1930. Sir Frank Whit­tle, re­garded as the fa­ther of the mod­ern tur­bo­jet en­gine, was an en­gi­neer in the British Royal Air Force and re­tired from ser­vice in 1948 in the rank of Air Com­modore. A few years af­ter the suc­cess recorded by Sir Frank Whit­tle, Hans Von Ohain, an aero­nau­ti­cal en­gi­neer in Ger- many, came up with a design that evolved into the first op­er­a­tional jet en­gine. Soon af­ter, in 1938, a lit­tle-known Hun­gar­ian aero­nau­ti­cal en­gi­neer named György Jen­dras­sik, came up with a design of the first true tur­bo­prop (short for turbo pro­pel­ler) en­gine, des­ig­nated the Cs1. Through­out the pe­riod of World War II, tech­no­log­i­cal in­no­va­tions in air­craft en­gines con­tin­ued to take place.

How­ever, it was the tur­bo­jet and not so much the tur­bo­prop en­gine that was gen­er­ally re­garded as a sig­nif­i­cant ad­vance­ment in tech­nol­ogy in the do­main of power plants for air­craft. The air­frame plat­forms for test­ing tur­bo­jet en­gines un­der de­vel­op­ment were pro­duced by Heinkel Flugzeug­w­erke in Ger­many and the

Gloster Air­craft Com­pany in Bri­tain. How­ever, it was only af­ter World War II that the jet en­gine be­came a widely used as the power plant for aero­planes.

DESIGN FEA­TURES. Tur­bo­jet En­gine. A tur­bo­jet is es­sen­tially an in­ter­nal com­bus­tion, air breath­ing en­gine with a ro­tat­ing air com­pres­sor laid out in sev­eral stages and mounted on the drive shaft. Cold air from the at­mos­phere is drawn into the en­gine through the air in­take and by means of the ro­tat­ing com­pres­sor blades, the mass of air is com­pressed to high pressure as much as 10 times the nor­mal at­mo­spheric pressure, as it passes through the stages of the ax­ial flow com­pres­sor and is mixed with fuel. The fuel and air mix­ture then is ig­nited in the com­bus­tion cham­ber and the rapidly ex­pand­ing hot gasses exit the rear end of the en­gine where the ex­haust is lo­cated, im­part­ing the en­gine and in turn the air­craft on which it is mounted, sub­stan­tial re­ac­tionary for­ward thrust which pro­pels the aero­plane for­ward.

Tur­bo­prop En­gine. The tur­bo­prop en­gine is an ad­vance­ment in design over the tur­bo­jet en­gine around which it is en­gi­neered. Es­sen­tially, the en­ergy gen­er­ated by the com­bus­tion of the fuel and air mix­ture as in the tur­bo­jet en­gine is used to im­part ro­ta­tion to the drive shaft which through a re­duc­tion gear mech­a­nism, drives a multi-blade pro­pel­ler mounted at the front end of the drive shaft. The for­ward thrust is gen­er­ated by the mass of air pro­pelled rear­wards by the pro­pel­ler. Thus the tur­bo­prop en­gine is a hy­brid be­tween two de­signs as it com­bines a jet en­gine with a con­stant speed, vari­able pitch pro­pel­ler that was de­vel­oped as a part of the large or high pow­ered pis­ton en­gines.

JET OR TUR­BO­PROP AIR­CRAFT IN BUSI­NESS AVI­A­TION. When it comes to per­for­mance, both jet and tur­bo­prop air­craft in the regime of busi­ness avi­a­tion have at­tributes and lim­i­ta­tions that a prospec­tive buyer would need to take into ac­count while se­lect­ing and eval­u­at­ing the plat­form. An im­por­tant char­ac­ter­is­tic of tur­bo­prop pow­ered air­craft is that it is de­signed to fly at medium speeds at which it de­liv­ers the most op­ti­mum per­for­mance in terms of econ­omy in op­er­a­tions. This is be­cause the ef­fi­ciency of a pro­pel­ler is the high­est at a par­tic­u­lar speed in the medium speed range. The pro­pel­ler is ef­fi­cient at flight speeds be­low 725 kmph. How­ever, the ef­fi­ciency of the pro­pel­ler re­duces with in­crease in speed be­yond the design pa­ram­e­ters. Tur­bo­jets on the other hand are rel­a­tively much less ef­fi­cient at low speeds and their ef­fi­ciency im­proves sig­nif­i­cantly in a speed range that is higher than air­craft that are pow­ered by of tur­bo­prop en­gines.

One ad­van­tage that the tur­bo­prop air­craft of­fers is that it is gen­er­ally more ca­pa­ble of op­er­at­ing from grass air­fields. Jet air­craft on the other hand, re­quire a con­crete run­way for take-off and land­ing. Tur­bo­prop air­craft are also ca­pa­ble of op­er­at­ing from smaller airstrips as com­pared to jet air­craft of sim­i­lar size and class. This is par­tic­u­larly rel­e­vant for busi­ness houses whose ex­ec­u­tives need to visit ci­ties and towns that have smaller air­ports. Just as an exam- ple to il­lus­trate this point, a King Air 250 tur­bo­prop air­craft can op­er­ate from a 2,400 feet run­way whereas a Ci­ta­tion CJ4 busi­ness jet re­quires a much longer run­way, one that is 3,410 feet in length.

From the point of view of pas­sen­ger com­fort, while jet en­gines have higher noise lev­els, tur­bo­prop air­craft are less noisy; but may have slightly higher vi­bra­tion lev­els. How­ever, in mod­ern tur­bo­prop busi­ness air­craft, vi­bra­tion lev­els are gen­er­ally within tol­er­a­ble lim­its. Mod­ern twin-tur­bo­prop air­craft are as com­fort­able and well equipped as busi­ness jets of the same cat­e­gory.

From the fi­nan­cial per­spec­tive, tur­bo­prop air­craft have a lower cost of op­er­a­tion as com­pared to busi­ness jets of the same class. For fly­ing in sec­tors where dis­tances to be flown are short, the dis­ad­van­tage of tur­bo­prop air­craft in terms of time taken to reach the des­ti­na­tion on ac­count of its lower cruis­ing speed com­pared to jet air­craft, is neg­li­gi­ble. How­ever, the cost savings with the tur­bo­prop air­craft are high. Just to il­lus­trate, for a dis­tance of up to 400 nau­ti­cal miles, cost sav­ing on fuel with an Avanti is around $195 per hour as com­pared to a Cessna Ci­ta­tion CJ3 busi­ness jet. The sin­gle-en­gine tur­bo­prop busi­ness air­craft are also ex­tremely com­fort­able and have lower op­er­at­ing costs on ac­count of the fact that they only have one en­gine re­sult­ing in lower main­te­nance costs com­pared to twin-en­gine air­craft. Some other ex­penses such as main­te­nance costs per flight hour and per­haps in­sur­ance, tend to also be lower for turboprops.

The two com­pa­nies of global fame man­u­fac­tur­ing en­gines for tur­bo­prop air­craft are Pratt & Whit­ney Canada whose main prod­uct is the PT6A and is vari­ants that are used widely on sev­eral plat­forms across the world. The other en­gine man­u­fac­turer is Honeywell Aero­space whose prod­uct TPE 331 and its vari­ants are also fit­ted on a num­ber of tur­bo­prop air­craft used in busi­ness avi­a­tion. Both these prod­ucts have built up a con­sis­tent rep­u­ta­tion for reli­a­bil­ity and are fit­ted not only busi­ness air­craft, but also on mil­i­tary, agri­cul­tural and small air­craft used by the air­line in­dus­try.

JET OR TUR­BO­PROP IN THE AIR­LINE IN­DUS­TRY. The choice of jet or tur­bo­prop air­craft by the air­line in­dus­try is in­flu­enced largely by the eco­nom­ics of their busi­ness mod­els. As air­lin­ers pow­ered by two or four jet en­gines are gen­er­ally of large size, fly at close to su­per­sonic speeds at al­ti­tudes be­tween 35,000 feet and 40,000 feet and carry large pas­sen­ger and cargo loads, these of­fer bet­ter econ­omy of op­er­a­tions when de­ployed for long dis­tance flights both within the coun­try or across the oceans to other con­ti­nents. Com­pared to jet en­gine pow­ered air­lin­ers, air­craft equipped with tur­bo­prop power plants are smaller in size and have lower seat­ing ca­pac­ity gen­er­ally un­der 100. As these air­craft op­er­ate be­tween al­ti­tudes of 20,000 feet and 25,000 feet as also have lower range and en­durance ca­pa­bil­ity, these are bet­ter suited for op­er­a­tions over short dis­tances as these air­craft pro­vide far bet­ter econ­omy of op­er­a­tions. It should be ob­vi­ous as to why tur­bo­prop air­lin­ers such as the ATR and the Q400 dom­i­nate re­gional avi­a­tion.

THE FI­NAL WORD. The ex­pres­sion Jet vs Tur­bo­prop in the ti­tle of this ar­ti­cle is likely cre­ate an im­pres­sion that these two seg­ments of the air­line or busi­ness avi­a­tion fleets are ri­vals and are com­pet­ing against each other in the avi­a­tion in­dus­try. The fact of the mat­ter is that this is not the case. In fact the two are mu­tu­ally com­ple­men­tary as each one takes care of a seg­ment of the in­dus­try that the other is not bet­ter equipped to do so. While rapid tech­no­log­i­cal ad­vance­ments are tak­ing place in both the sec­tors, there is no pos­si­bil­ity what­so­ever of ei­ther one dis­plac­ing or re­plac­ing the other.

THE TWO ARE MU­TU­ALLY COM­PLE­MEN­TARY AS EACH ONE TAKES CARE OF A SEG­MENT OF THE IN­DUS­TRY THAT THE OTHER IS NOT BET­TER EQUIPPED TO DO SO

Most Ef­fi­cient New-gen­er­a­tion Jet:

Mod­ern Tur­bo­prop:

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