If the earthly atmosphere would just stand still, some of flight’s complications would not exist. But it won’t.
When people who are familiar with land vehicles first start to fly, they almost always have difficulty in understanding the concept of relative airspeed as it applies to their aircraft. In one form, the question might be:
In cruise flight with the aircraft in trim, does the wind blow against the aircraft? For example, an aircraft is traveling west at 100 kt but there is a wind from the west at 20 kt. Will the aircraft feel the air striking against its front proportional to 120 kt? Or will it not because it is moving backwards 20 kt with the wind?
The answer is that almost everything is relative. In this case, once an aircraft leaves the ground, the forces on it depend only on its relative speed with the surrounding air — that is, its airspeed. Its only connection with the ground is through the force of gravity acting toward the center of the earth. It might become clearer if you imagine yourself flying above a cloud layer that is moving along with the wind. Up there, you could circle a cloud top just as easily as you could circle a mountain top on a calm day no matter how fast the earth is turning under this peaceful scene.
It is the same with a fly buzzing around the cockpit; he doesn’t care how fast you are flying. You can even take this one step further by imagining the fly buzzing around in a model airplane flying up or down the aisle of a jet transport. He will do the same thing, no matter how fast the jet is flying, nor how fast the earth is turning below, nor how fast the earth is going around the sun, nor how fast our galaxy is rotating. He only cares about the speed of his wings through that little bit of air that’s his.
When the wind is crosswise to the flight path, it gives rise to a navigational problem not unique to aircraft. Seafarers have been faced with crosswinds, in addition to every-changing ocean currents, for centuries.
To illustrate the crosswind problem for helicopters, let’s follow a pilot delivering cargo to the summit of a mountain peak some 50 nm north. Figure 31-1 illustrates the scenario. The pilot plans to cruise at 100 kt, so the flight will take 30 minutes. On a calm day, he would set, and hold, a course directly to the peak. The navigation task is easy.
But let’s impose a 20-kt crosswind from the west. Now if he holds his initial, direct heading, he will end up 10 nm east of the summit. Since this is obviously unacceptable, he must select from several possible corrective measures.
An obvious solution is to set an initial course toward a point 10 nm east of the peak (11.5 deg west of true north) and then hold. He is assuming, of course, that the 20-kt wind will be constant throughout the flight. His ground track will be straight north but, both from the air and ground, the helicopter will appear going slightly sideways to the right, “crabbing” even though there is no sideslip with respect to the air mass. In fact, if the flight were above clouds moving with the wind and the pilot could not see the ground, no crabbing would be detected.
Another way of reaching the peak is to hold the aircraft’s nose on the peak while continually sideslipping 11.5 deg to the left. Again the ground track is straight up north a ground observer would say, “right on.” But the sideslip would cause higher drag, requiring more fuel for the mission.
A third possibility is to initially head due north with no sideslip and then “chase” the peak by changing heading toward the west whenever a correction appears required. The result is a fishhook-shaped ground course. It’s a likely solution for a pilot unaware of the crosswind.
Almost since the Wright Brothers, airplane pilots have been aware of the danger of downwind turns in which airspeed is lost. Unlike Wilbur, Orville and the rest of the fixed-wing crowd, helicopter pilots fly machines that are much less dependent on airspeed for lift and control. Nevertheless, several helicopter accidents have been attributed to such situations. These incidents occur largely because the pilot, flying low enough to perceive cures from the ground, tries to maintain constant groundspeed instead of constant airspeed.s
For airplane pilots, stall and loss of control can be fatal. For their rotary-wing brethren, the main trouble occurs when they find themselves behind the curves of power and collective-pitch requirement. Thus, without increased collective, a pilot may run out of power and fly into the ground.
The downwind-turn problem diminishes as the helicopter pilot gains altitude. He loses immediate cues of ground motion and turns his attention to the airspeed indicator, where it should have been all along.
Tracing perfect circles around a spot on the ground on a clam day is simple; it only requires constant airspeed and bank angle. It’s that simple even on a windy day, provided the center of the circle is a cloud of free balloon. This is — despite the fact that the ground track appears like a spiral or, more precisely, a cycloid — the same pattern traced by a rotor blade tip in forward flight.
When circling a free-floating cloud or balloon, the aircraft is only concerned with its relationship to air mass. It carries its inertial frame of reference with it and thus feels no varying inertial forces, even though its ground track is continually changing from upwind to downwind.
The situation is different, of course, when flying perfect circles at a constant ground velocity about a spot on the ground on a windy day. Now, both airspeed and bank angle must be constantly adjusted. Figure 31-2 shows the varying conditions that complicate the task.
By observing the helicopter’s motion with respect to the ground instead of the air, a pilot might be convinced that the turn in uncoordinated, since the rate of turn changes. Such misreading has caused accidents during a turning approach to land because the pilot tried to compensate to make the aircraft fly like his eyes told him it should.