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Why You Rarely Fly At Best Range Speed In A Prop, But You're Close To It In A Jet

Boldmethod

Your "Best Range" or "Maximum Range" condition is a term you've probably heard. But, in a propeller-driven airplane, it's a setting you will probably never use. In fact, many piston aircraft don't publish a best range speed. That's because best range in a prop is often impractical. But - in a jet - it's more useful than you'd imagine.

Even if you took two identical airframes and outfitted one with a jet and the other with a prop, their best range performance would be very different. And it all comes down to the difference between power and thrust. Jet engines create thrust, and propeller spinning engines - reciprocating or turboprop - create power. That changes best range performance.

The Difference Between Thrust and Power

Thrust and power are not the same. Thrust is a pure force. And that's what a jet engine produces - a force that pushes the jet forward. So, those levers in a turbine cockpit aren't throttle levers or power levers, they're thrust levers. (And your captain might correct you if you use the wrong name.)

But, reciprocating engines and turboprops do not directly create thrust. Instead, they spin a shaft. The more fuel they burn, the faster the shaft spins. So, they're applying a force at a velocity. Power = Force * Velocity. The faster they spin the shaft, the more velocity it has, the more power they generate. Of course, the shaft connects to a propeller, and the propeller creates thrust. But, the rate of fuel burn is directly related to how fast the shaft spins.

Turboprop engines do actually create some thrust. Some of the exhaust gasses passing out of the turbine create thrust. But, it's a small amount of thrust compared to the power generated by the turbine. And that's why turboprops are considered to be power generating engines.

How Range is Affected by Thrust and Power

Best or maximum range occurs where the proportion between the aircraft's velocity and the engine's output is the greatest. Essentially, you're getting the most velocity per unit of fuel burned. Since jets produce thrust, you use the thrust required curve to find their best range. And since propeller driving engines create power, you use the power required curve to find their best range condition. Let's start with the jets.

This is a typical thrust required curve (below). It's made by adding the induced and parasite drag curves - that becomes total drag. And total drag equals the thrust required for level flight.

Finding the greatest proportion between velocity and engine output is actually very simple on a graph. Draw a straight line from the origin of the graph, tangent to the engine output curve. On a thrust required curve, that tangent point occurs a little faster than the lowest thrust required point. So, your best range speed with a thrust producing engine is faster than your minimum drag speed. That minimum drag speed is called Lift / Drag Max, or L/D max. It's where induced drag equals parasite drag. In fact, with a thrust producing engine, this best range tangent point occurs where induced drag makes up about 25 percent of total drag, and parasite drag makes up the other 75 percent.

The Propeller Problem

Propeller driven aircraft use the power required curve, which is easy to calculate - simply multiply the total drag by the velocity at each point along the curve. That becomes a power required curve. L/D max is no longer at the lowest point of the curve - it moves up. The L/D max speed doesn't change as you transform a thrust required curve to a power required curve. The curve simply shifts around it. And, if you draw a line from the origin tangent to the power required curve, you'll see that it reaches the curve right at L/D max. In a propeller driven airplane, best range happens at the point where induced drag equals parasite drag. You can quickly see that the best range speed for a prop happens at a much slower point on the drag curve than it does for a jet.

In a propeller driven aircraft, L/D max is often associated with VY - best rate of climb speed. In a Cirrus SR22T, our indicated cruise speed is often 30 to 40 knots faster than VY with 75% power. Cruising at VY would be a slow way to fly.

But, in a jet, you'll often fly close to your best range speed. In fact, the long-range cruise (LRC) thrust setting for a jet usually provides 99% of the maximum, or best range. At LRC, you trade one percent of range for three to five percent faster airspeed. Not a bad tradeoff.

But, when jet fuel prices are high, airlines often fly at economy cruise settings instead of LRC. These settings, called a "cost index," balance the cost of fuel against the other hourly costs of operating the aircraft. And, these indexes use a thrust setting between maximum and long range cruise. Of course, ATC requirements and departure delays can change a jet's enroute cruise speed. But, their planned speed usually falls between LRC and max range.

Propeller driving engines create power, and jet engines create thrust. And because of that fact, a jet usually cruises very close to its maximum range speed. A prop rarely gets close to it.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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