The Big Things About Little Engines

The Big Things About Little Engines

The Big Things About Little Engines




Bigger, faster, stronger…it’s always better, right? Well, maybe so and maybe no. 

In aviation, smaller can bring some real benefits if there are size constraints. The airplane that can’t fit in your hangar will not do a lot of good. And parasite drag is directly commensurate with the size of the object being hurled though the air. The only thing that overcomes drag is thrust, and thrust can cost you a bundle. So, smaller can be better from an efficiency standpoint.

Consider faster. Faster sure seems better, but those 30 extra knots that your next “dream airplane” may provide can come at an exponential cost. Not only do the bigger engines require more fuel, they also have a larger engine reserve cost (the engine overhaul cost divided by the TBO expressed in a cost per hour). Speed is expensive.

Stronger also appears better, but strength usually brings his “best friend” weight along for the ride. And weight is not a friend in aviation if you are interested in a good climb rate, fast speed or higher useful load.

Yet, the market almost always prefers bigger, faster and stronger. And a premium is frequently paid by those who simply gravitate to the biggest, fastest and strongest. But herein lies an opportunity for those who choose to actually analyze their mission, do the math and approach an airplane purchase without bias. In the world of aviation, there is oftentimes a better deal found in smaller, slower and weaker options, especially when aircraft engines are considered. A puffed-out chest in the airplane buying process can end up costing money for no other reason than feeding an ego.

Let’s consider a practical example. I fly both a -6 powered King Air B100 and a -10 powered King Air B100 regularly. The engines are the exact same design, but the -6 engine is slightly smaller, developing less thrust at higher altitudes. I keep very accurate data for both airplanes and fly both over 100 hours per year. What’s the difference? At the end of the analysis, the difference is 14 knots of cruise speed (262 KTAS vs. 248 KTAS), two minutes of climb to FL230, five-minutes on a 400 nm flight, 10 gallons/hour in fuel burn and no change in payload. But, any -6 powered King Air B100 will cost hundreds of thousands less than the -10 variants.

Consider the JetPROP. The most popular engine on a JetPROP is a PT6-35 engine, but a PT6-21 version is also available. The -35 version probably outsells the -21 version by a 10-to-1 ratio but the differences are relatively minor: 17 KTAS (260 KTAS vs. 243 KTAS) of cruise speed, 3 gallons/hour (32 gph vs. 29 gph) in fuel burn and the time to climb to FL270 is only about a three-minute difference. But, the -21 JetPROP can be purchased for $90,000 less (on average). Personally, I think the -21 JetPROP is one of the best values in the PA46 world, mainly because it can be purchased at a lower price point and it still flat-out performs.

And, what exactly is the difference between a Continental 520 and a Continental 550 engine? The difference is in the stroke of the piston. A 550 engine has a 4.25-inch stroke and the 520 has a 4-inch stroke. However, both engines produce the same amount of horsepower as the 520 engine has a higher manifold pressure limit. So, the 550 engine breathes a bit better at high altitude, but the 520 is still a remarkable engine that serves many owners on many airplanes well. Still, the 550 version of any airframe will sell better than the 520.

My point in all of this is to consider the value that is found in an airplane that has the smaller engine. Most buyers in a particular category will dismiss the small engine airplanes purely based on “numerical prejudice,” meaning they simply feel that the bigger number must mean that it is “better.” The bigger number does not mean it is better, it just means it is different. If you are a buyer of a twin or turbine airplane, I recommend taking a serious look at the smaller engines because a deal can oftentimes be found.

For instance, if you want a King Air 90, you’ll find lots of examples on the market, but the best deals are found on an earlier King Air 90 with the PT6-21. Yes, the versions with the bigger engines will go a little faster and climb a little quicker, but the small engine versions can be bought for a song and they “sip” fuel compared to the bigger engines. The fuselage size, panel real-estate and cabin differential pressure are all the same. The wise owner of the smaller-engine version can smile all the way to the bank.

Most popular flight planning software (FltPlan.com, Foreflight, etc.) even have profiles set up for the various engines on the various airframes. Whenever I help a customer purchase an airplane, I always create a spreadsheet with the most common flights and contrast the block-to-block times and the fuel burn expectations. The difference is
usually negligible.

One other interesting thing about the smaller turbine engines: they usually develop similar torque at low-density altitudes. This translates into the nearly identical ground rolls on takeoff, and the initial climb rate is also almost identical too. Since both the larger and smaller engines have similar max torque
available values, the difference in power is only seen when the engine becomes “temp-limited,” meaning that the limit of power available is limited by the ITT, not torque.

Every turbine driver knows that an engine will become temp-limited at a certain altitude (dependent upon temperature), and the smaller engines will become temp limited at a lower altitude. It is only then that the difference in torque between the larger and smaller engine becomes apparent. As an example, the -6 powered B100 I operate will develop the same torque as the -10 when departing my home airport (KJSO, 670 ft. field elevation). Both airplanes will takeoff equitably, and both will climb equitably through about 10,000 MSL. However, the -6 version will become temp limited earlier than the -10 version, and then the -10 version will develop more torque for the rest of the flight, climbing better and cruising faster.

The altitude where the engine is temp-limited becomes critical for the pilot who operates from high-density altitudes. I’ve departed Santa Fe, New Mexico (KSAF, 6,348 ft. field elevation) in the summer heat and was temp limited on the takeoff roll in the -6 powered B100. Had I departed on that same day in the -10 powered B100, I probably would not have been, and the takeoff performance and climb would certainly be better with the bigger engine. So, if you operate out of high-density altitude airports, you might be one of the few who would want to more strongly consider the bigger engines available on a
particular airframe.

But there is a catch here. Remember, what comes around goes around…if you purchase that smaller-engine airplane for a lesser price, the market will almost always want to pay you a smaller price when you sell, and there will be fewer buyers for that smaller-engine airplane when you do sell. My advice? Seek out the best example of whatever type airplane you wish to purchase and nearly disregard the type of engine. Find the one with the right avionics, nice paint/interior and excellent maintenance pedigree. If that airplane happens to be a small-engine version, great! There will be some wonderful efficiencies that you’ll grow to appreciate.

Stay tuned for my next article where I will explore the reverse: why bigger engines are worth considering.•T&T

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