In general, pilots are a pretty smart bunch of people, so I’m always surprised reading aviation mailing lists and newsgroups to learn how many of them don’t seem to have the slightest understanding of how to control their planes’ airspeed and power. This ignorance will typically come out in a statement like “a Cessna 172p really only flies about 105 knots” (it can really fly around 120 knots true airspeed if the pilot knows how to operate it). I thought it would be interesting to look at how pilots actually set speed and power in planes with fixed-pitch propellers, and what they get for their trouble.
So the POH says that your plane will fly at 120 knots, but you never seem to get that — typically, pilots blame marketing writers for making up numbers so that their planes look better. In reality, the POH’s do usually try to put the plane’s speed in the best light, but the numbers are not made up. Normally, a plane with a normally-aspirated piston engine (like a 172 or Cherokee) will have its true airspeed will be calculated at between 7,000 and 8,000 feet density altitude, where the plane flies at its fastest: any lower, and the dense air slows you down; any higher, and there’s not enough oxygen for the engine to produce 75% power. If you normally fly below 7,000 feet or above 8,000 feet, you can expect to see a slower true airspeed. Some manufacturers will also test with the plane lightly loaded, providing a boost of a couple of extra knots (that’s the case for my Warrior) — if you put the whole family on board, you can expect to fly a bit slower.
One of the biggest problems, though, is the wind. It is a simple fact that you will spend longer (possibly much longer) flying with headwinds than tailwinds, because headwinds slow you down — for example, a trip might take three hours outbound against the wind and only two hours return with the wind, meaning that you spend 60% of your trip with headwinds. As a result, your average groundspeed will always be lower than your plane’s best true airspeed, possibly by 10% or more if you fly a lot with strong winds. That difference does not mean that the POH lied about the true airspeed, which should be as advertised (more or less), but just that wind is a big pain.
So how do pilots control true airspeed and fuel burn? It turns out that it’s easy to manage one or the other, but managing both can be a bit of a challenge.
The Constant-RPM Pilot
To start, consider the pilot who always flies with the same tachometer reading (say 2400 rpm), letting the indicated airspeed rise or fall as it will. Using a constant RPM will give a nearly constant true airspeed at any altitude, so this seems like a simple system: according to the POH, 2400 rpm will give a true airspeed of 109 knots at 2,000 feet density altitude, 107 knots at 6,000 feet density altitude, and 105 knots at 10,000 feet density altitude. That’s easily close enough, and makes flight planning simple: set the power to 2400 rpm and assume 105 knots true airspeed (to allow for old paint, chips in the propeller, draggy antennas, etc.), and everything will usually work out for any cruise altitude, plus or minus the wind.
Unfortunately, for this pilot fuel burn will vary. At 2,000 feet density altitude, the Skyhawk’s O-320 Lycoming engine spinning the propeller at 2400 rpm will be producing 69% power (110 hp) and burning 7.7 gallons of fuel per hour; at 12,000 feet density altitude, the engine will be producing 56% power (90 hp) and burning 6.3 gallons of fuel per hour — that’s an almost 20% difference in fuel consumption. Since fuel consumption seems unpredictable, the pilot has learned to fly short legs (so that there’s always lots of extra gas in the tank), making IFR flight difficult. The pilot also might decide to spend thousands on speed mods to get 3 or 4 extra knots, when simply using the right power setting at 7,000 or 8,000 feet density altitude would give an extra 12 knots without any modification to the plane. This is the pilot who goes on mailing lists and claims that his or her plane is much slower thant he POH says it should be.
The Constant-Indicated-Airspeed Pilot
Next, consider the pilot who always flies with the same indicated airspeed, varying the RPM as required. Let’s say that the pilot chooses 114 knots indicated (111 knots calibrated), which will give the maximum cruise performance for the Cessna 172p. At 2,000 feet density altitude, the pilot needs to set the engine to 2500 rpm to maintain this airspeed; at 8,000 feet, the pilot needs to set the engine to 2650 rpm (at 10,000 feet, the plane is no longer capable of this speed in level cruise). By using a fixed indicated airspeed, the pilot is actually using a fixed power setting, and that means a fixed fuel consumption: at either 2,000 feet or 8,000 feet density altitude, the fuel burn will be the same (about 8.5 gallons per hour according to the POH).
Unfortunately, for this pilot true airspeed will vary, making flight planning trickier. At 2,000 feet density altitude, the plane’s true airspeed will be 114 knots; at 8,000 feet density altitude, the plane’s true airspeed will be 121 knots. There’s also the problem that the plane might be draggier than the one used to calculate the POH numbers, so 114 knots might actually push the engine up to 80% power or higher, burning extra fuel and risking detonation.
So the constant-RPM pilots know how fast they will fly but not how much fuel they will burn, while the constant-indicated-airspeed pilots know how much fuel they will burn per hour, but not how fast they will fly.
Right about this point, a lot of people will argue that that’s not the case — after all, if a pilot always flies at about the same density altitude, he or she will find the fuel burn and true airspeed pretty predictable with either technique. The problem comes, however, when one of those pilots flies somewhere different than the normal summer cross-country at 3,000 feet (or whatever normal means for that pilot). For example, where I live, in Ottawa, it gets cold enough in the winter that the density altitude at 3,000 feet is sometimes still negative. A constant-RPM pilot who flies under these conditions will burn far more fuel than expected, and could end up landing with near-empty tanks when expecting a half-hour reserve; a constant-indicated-airspeed pilot who flies under these conditions will fly far slower than expected, and could end up landing with near-empty tanks (again) because of the extra travel time. I believe strongly that this is why some good, experienced pilots run out of fuel: something changes from their normal flying routine (colder weather, different cruise altitude, etc.), their normal technique produces abnormal results, and they do not understand how to compensate for it.
Knowing your power setting requires calculating your density altitude and then looking up your RPM in a table or graph, which is a big pain, but it does allow you to get maximum performance (true airspeed and range) out of your airplane safely. You do not have to do that for every flight, of course — once you know your indicated airspeed at any given power setting, and have confirmed your fuel burn, you can use a variation of the constant-indicated-airspeed approach, as long as you do the calculations to get your true airspeed. The alternative is believing that your plane is 10 knots slower than it really is, or never knowing quite how fast you’ll fly or how much fuel you’ll burn.