by Jeb Burnside
An aircrafts range is just another parameter, like gross weight or horsepower, based on the results of design and engineering choices. In this case, cruise airspeed, fuel consumption and fuel capacity combine to tell us how far we can fly before the fan stops turning. The wind velocity at our chosen altitude will have some impact, also. Range is distinctly different from, say, how fast we can fly or for how long. But, together, these variables-and a host of others-comprise an aircrafts performance specifications, making one example more suitable for a certain task than another.
Of course, pilot technique can have an amazing impact on whether and how the choices made by the designers and engineers are maximized and the aircraft used to its greatest efficiency. A simple example is the Cessna 150 driver with sloppy airspeed control who lands long and uses most of a 5000-foot runway while a Skylane pilot lands behind him and turns off before the 150 can reach the taxiway at the end.
Another, more relevant example comes from WWII and the legendary Charles Lindbergh. While serving in the South Pacific as a technician, Lindbergh advised pilots flying Lockheeds P-38 fighter to extend their range by reducing rpm and increasing manifold pressure. The results were widely acknowledged at the time for increasing the twin-engine fighters combat radius by at least 50%, from 400 to 600 miles. In this instance, the aircrafts effectiveness was increased through pilot technique without changing the airframe or the engines.
Similarly, we can use some basic and well-understood techniques to plan and fly a max-range trip. Its not difficult, but getting the greatest range from an aircraft does require us to think a bit differently in the planning and flying phases of a given trip. And it will definitely require a bit more flying time to get to our destination. Often, though we will actually save time by eliminating a fuel stop. We wont be able to get that Cessna 150 from Wichita to Honolulu on factory tanks, but we can definitely find ways stretch 800 miles of range into 900, or 1000.
Flying a max-range flight starts well before we turn the key. Of course, we must have a destination reachable on the fuel carried, the weather-especially the winds-must cooperate and the aircraft must be up to the task, among other considerations. We also must have backups available in case the weather or winds are not as forecast, a mechanical problem develops or the fuel we planned to use turns out not to be available. This is normal stuff and shouldnt be any different for any other flight.
However, and especially when trying to squeeze the most miles from full tanks, there is no substitute for being familiar with the aircraft. Owners and others who fly only one airplane have the luxury of great familiarity with their steed; renters who are lucky to get the same N-number twice in a row simply cannot confidently employ max-range techniques in the same ways.
For example, how full are the tanks when the fuel is spilling out of the filler cap? Some airframes are notorious for being difficult to fill under any but ideal conditions. Also, how accurate are any electronic totalizers or fuel-flow instruments? If the fancy electronics say well have 15 gallons on board when on the ramp at our destination, how accurate are they?
We must also have some way to monitor and ensure our en route performance-are we achieving the fuel consumption and groundspeed values we need to complete our maximum-range trip as we planned it? Is that 15 gallons we planned to have on the ramp at our destination going to be adequate? Planning a max-range trip is slightly more complicated than a normal hop around the pattern, perhaps, but none of this is rocket science.
To achieve maximum range, you need to fly at your airplanes most efficient angle of attack (AOA). Thats fairly intuitive: We want to generate the most lift at the same time were minimizing drag. Unfortunately, most GA aircraft are not equipped with a sensor giving the pilot accurate information on AOA so that he may adjust the aircrafts pitch attitude accordingly. Instead, we have to rely on the most accurate pitch/AOA indicator we have remaining, the airspeed indicator. So, instead of pitching the airplane for the most efficient AOA, we will fly it at the most efficient airspeed, the one that maximizes lift while minimizing drag. This speed is known variously as Vmr, for maximum range velocity, or as (L/D)max, for maximum lift over drag. How we find this airspeed is the trick.
The problem is two-fold. First, Vmr doesnt necessarily correspond to, say, the power-off stall or best rate of climb speeds. If it did, all we would have to do is compute that indicated airspeed for our temperature and barometric pressure and off wed go. The ideal speed also doesnt necessarily correspond to our airplanes best glide speed, the one we use when our single engine fails. Secondly, the ideal Vmr will vary with the airplanes weight: Higher weights will require a higher airspeed to maintain the best AOA. Conversely, as our weight decreases during a flight due to fuel consumption, so must our airspeed be reduced to maintain the ideal AOA. Thats okay, though; we can easily slow down.
Determining this magic airspeed is the tough part. For example, each airplane design is different when it comes to the amount of drag produced at a specific speed. We can spend a lot of time and effort performing flight tests designed to establish Vmr. We can also spend a lot of time and effort poring over our airplanes performance charts to come up with an estimate of it. Thats a lot of work, however, and errors can creep into our computations. Fortunately, failing to nail this precise airspeed will have a negligible effect on our maximum range.
Rules of Thumb
To simplify all of this, we need to come up with some rules of thumb. To do so, we researched various materials to develop an idea of how such rules should be developed and applied. The best recommendations we can make revolve around a specific multiple of the airplanes clean, power-off stall speed at gross weight (Vs, represented by the bottom of the green arc on the airspeed indicator). However, the multiplier we should apply differs among various airframe configuraitons, including single-engine fixed gear, single-engine retractable and light twins. So, we came up with three rules of thumb covering these three basic types of aircraft. Note that all of these rules are based on piston-powered airplanes-turbine operators have a whole different set of parameters with which to be concerned, including altitude.
For a single-engine fixed gear airplane, we want to find the airspeed corresponding to 1.6 times Vs. So, for example, if our Cessna 172 has a Vs of 50 KCAS, multiply 50 by 1.6 and we get a Vmr of 80 KCAS at gross weight.
For a single-engine retract, the multiplier is 2.0: Using our rule of thumb, a Beech Bonanza A36 with a Vs of 65 KCAS will have a Vmr of 130 KCAS.
Light twin drivers should use a multiplier of 1.7; a C55 Baron with a Vs of 74 KCAS sees its best range at 126 KCAS.
Of course, these multiples are based on an airspeed computed for the airplanes maximum gross weight. Strictly speaking, the only time the airplane is at its actual maximum gross weight is when the takeoff roll begins. So, we have one last thing to do: Establish a methodology to compute a reduction in airspeed as fuel is consumed and the aircrafts gross weight decreases.
Adjusting For Weight
This is actually fairly easy to do; just compute the percentages. In fact, we need to compute the airplanes initial weight when arriving at our cruise altitude. This is a fairly easy calculation based on our takeoff gross weight and the weight of the fuel burned while reaching our cruising altitude. Once we determine this number, we need to find its relationship to the airplanes maximum gross weight. For example, if our Beech Bonanza A36 has a maximum gross weight of 3600 lbs. and we depart at 3400 lbs., were at approximately 94.5 percent of gross. Multiply our Vmr of 130 KCAS by 94.5 percent and we get an initial target airspeed of approximately 123 KCAS.
Once weve been airborne for a while, the airplane will get lighter as fuel is burned. As discussed earlier, the ideal AOA-and with it our ideal Vmr-will change as the aircrafts weight changes. So, as fuel is burned and the airplane becomes lighter, we must slow down to maintain the airspeed for best range. There are any number of ways to apply the necessary weight-based reduction in our desired airspeed during our max-range flight. The easiest is to compute our reduced gross weight at the end of each hour and apply an appropriate reduction. An electronic fuel totalizer makes this about as easy as falling off a rock: Simply determine the amount of fuel still on board at the end of each hour, compute the resulting gross weight as a percentage of the airplanes maximum, and apply the correction.
At the end of our max-range trip, we will have burned off most of our fuel. Presuming our Bonanza departed with standard usable fuel of 74 gallons and we plan to land with 15, well have burned 59 gallons to get to our destination. Using another rule of thumb-avgas weighs six pounds per gallon-our gross weight will be reduced by 354 lbs. during our flight; our Bonanza will weigh 3046 lbs. when we land. That value, 3046, is roughly 85 percent of the airplanes maximum gross weight of 3600 lbs., which gives us a target cruise airspeed of approximately 111 KCAS.
Once we have decided the best airspeed to use in our maximum range operations, we also want to minimize our fuel consumption. The idea, of course, is to use the least amount of fuel necessary to maintain level flight at the correct airspeed.
Leaning the mixture is obviously a critical step in minimizing fuel consumption and, while we wont get up on our soapbox about lean-of-peak EGT settings, we will point out that the average engine will be making a relatively small percentage of its maximum power when the airplane is cruising at Vmr, on the order of 50 percent power or less.
Were not aware of any engine manufacturers admonitions on mixture settings at such low power. Teledyne Continental Motors basically says that, below 65 percent, the mixture can be anywhere: rich, lean or at peak EGT. We would set the mixture for the least fuel consumption necessary to maintain the power necessary to achieve Vmr while ensuring all engine parameters remained within their normal operating range.
But what about the power setting itself? The only time there is an easy answer to this question is when flying behind a fixed-pitch propeller: Set the throttle for the rpm producing the desired airspeed at the chosen mixture setting, as long as its allowed by the airplanes operating limitations.
When considering an engine spinning a constant-speed prop, it gets more complicated. Ideally, wed set the prop at its most efficient rpm. Unfortunately, that information is rarely published and can really only be obtained by conducting some detailed flight testing. However, and since weve already adopted one rule of thumb for our airplane, lets throw out another: Use the lowest rpm setting consistent with both the airplanes operating limitations and the amount of power necessary to maintain Vmr. This may require some trial and error testing to determine which rpm and throttle settings offer the best combination of smoothness and power. Generally speaking, however, the engine will be subject to less wear for each mile traveled, and fuel efficiency will improve at relatively low rpm and high manifold pressure settings. In any event, the idea is to find the most efficient power setting that produces the desired airspeed for the airplanes weight.
Choosing An Altitude
The last decision we have to make in planning and flying our max-range flight is at what altitude to fly. This will be determined by only one factor: the winds aloft.
The airspeed and power settings designed to give us maximum range are independent of altitude. We are going to be flying at an airspeed which is a multiple of the power-off stall speed as presented on our airspeed indicator. Since the indicated airspeed is independent of altitude, we dont really care how high we fly for purposes of setting power or speed. We do care, however, about the wind. Obviously, a tailwind is far preferable to a headwind when going for maximum range. Equally true, a headwinds effects are to be avoided. Basically, we want to shorten the length of time a headwind can work against us while lengthening the opportunity a tailwind can help. To keep things simple, we need to establish a threshold beyond which we take action when faced with stiff winds. Time again for two more rules of thumb, our last ones.
When flying with the wind at your back, reduce the target airspeed by 50 percent of the tailwind. For example, with a 10-knot tailwind, reduce Vmr by five knots. Conversely, when the wind is on the nose, increase Vmr by 40 percent of the headwind. In this instance, a 20-knot headwind would suggest increasing our target airspeed by eight knots.
Obviously, this can be carried to an extreme, so we probably should adopt a limit on our wind-based airspeed modifications of 20 percent of the gross-weight Vmr. In other words, when flying our Bonanza with a Vmr of 130 knots, dont apply a correction of more than 26 knots.
The Fine Print
Flying to achieve maximum range involves a host of complicated values requiring time and expertise to properly identify. Instead, what weve tried to do here is explain the different challenges and offer some ways to simplify the process.
Will you be able to fly your airplane to its absolute extreme range by using these rules of thumb? No way. You will, however, be able to fly longer legs with greater confidence than before and know how to compensate for the winds effects on range. Youll also be able to divert to an alternate destination while en route and know that youre using the minimum fuel necessary to get from Point A to Point B. And thats what maximum range is all about.