General Principles - Cruise — Page 256, Lesson 307

We're now into the cruise phase of the flight, and we've just finished looking at the jet aeroplane's speed for maximum range. Now I want to walk you through the propeller aeroplane, because the logic is the same, but the answer is different. For the propeller aeroplane, we're working with a power required curve, not a drag curve. And here's the key point: just like the jet, that power required curve is fairly flat at the bottom. That flatness is what gives us the freedom to trade a little extra power for a lot more speed. You can increase the airspeed significantly from VMP — that's the speed for minimum power — and the power required only creeps up a little. Now, a small increase in power required is bad for range, because you're burning a bit more fuel. But the airspeed has increased significantly, which is good for range, because you're covering more ground in the same time. The overall effect is that range actually increases. So where exactly is the best point? The speed at which the ratio of speed to power is at a maximum. And that point is found geometrically as the point of contact of the tangent drawn from the origin to the power required curve. That speed, you may recall, is VMD — the speed for minimum drag. So for a propeller aeroplane, it is VMD that is the speed for maximum range. That's the contrast with the jet, where the speed for maximum range was 1.32 times VMD. For the propeller, it's simply VMD itself. Now, there's one remaining item in the range formula we still need to resolve to maximize range even further, and that's specific fuel consumption. We want it as low as possible. And here's where the engine type really matters. For a piston aeroplane, specific fuel consumption is more or less best at low altitudes. But for a turbo-propeller aeroplane — and remember, a turbo-prop uses a jet engine — the specific fuel consumption decreases with altitude, up to a point about halfway up the troposphere. So the optimum altitude for range depends on which engine you're flying. Now let's move on to the factors that affect range, and the first one is weight. You'll recall that increasing the weight of the aeroplane increases induced drag. That induced drag increase moves both the total drag curve and the power required curve up and to the right. Let's look at what that does for a jet aeroplane in Figure 5.19. At higher weights, the aeroplane is subject to a higher drag force. Higher drag means the engine has to work harder, so it requires a higher rate of fuel flow. That higher fuel flow will decrease the specific range — the distance you get per unit of fuel. But here's the interesting part: even though range suffers, the speed for maximum range is now higher. For the jet, that speed, 1.32 times VMD, has shifted up. So a heavier aeroplane flies faster for best range, but it doesn't fly as far on the same fuel. That's the trade-off weight imposes on your cruise planning.