Miscellaneous Drag

In addition to the major drag items discussed earlier, a modern helicopter has many minor sources of drag, individually small but significant in total. A partial list of these miscellaneous items includes:

Windshield wipers External store mounting points

Source: Pruyn and Miller, “Studies of Rotorcraft Aerodynamic Problems," WADD TR 61-124, 1961.

Overflow drain tubes Abrasive walkways

Lights: anticollision, formation, landing Fueling receptacles Ground electrical receptacles Skin gaps, steps, and mismatches

Most of these items have characteristic dimensions of less than 4 inches and thus operate at subcritical Reynolds numbers at normal flight speeds, with corre­spondingly high drag coefficients. The data and methods of reference 4.2 can be used to evaluate the drag of these items. Such an evaluation for the Lockheed AH 56A "Cheyenne” produced an estimate of slightly more than one square foot of flat plate area.

Less obvious sources of drag are those due to cooling and leakage. When air is taken aboard to cool or to ventilate and then dumped overboard with less velocity than the forward speed of the helicopter, its loss of momentum manifests itself as a drag force. Similar drag will be produced by air that simply leaks into the aircraft at one point and out another without serving any useful purpose.

Wheels alone:

Cq (Based on b x d)

L

-4 ^

Tubular

FIGURE 4.26 Landing Gear Drag

Source: Hoerner, “Fluid Dynamic Drag,” published by author, 1965; Sweet & Jenkins, “Wind – Tunnel Investigation of the Drag and Static Stability Characteristics of Four Helicopter Fuselage Models,” NASATN D 1363, 1962.

Compared to a solid wind tunnel model, the actual helicopter in flight has some of the characteristics of a sieve. Some indication of the magnitude of miscellaneous and leakage drag can be obtained from the wind tunnel tests of a Bell UH-1 fuselage, reported in reference 4.15. When all gaps were sealed and all protuberances removed, the drag was reduced by more than 2 square feet of equivalent flat plate area.

Cooling and leakage drag are difficult to estimate without a detailed thermodynamic and internal aerodynamic analysis. During preliminary design, they are usually accounted for by increasing the basic fuselage drag by 10-20%.

The fuselage is not the only component that can suffer from leakage. If a rotor blade has a passageway through its entire length, the rotor will act as a centrifugal pump, taking air in at the root and expelling it at the tip. Unless the air is turned at the tip to align it with the external velocity, losses can be substantial, increasing the power required by as much as 20% in extreme cases. For this reason, blades should be sealed, especially at the root. If a blade is open at the root but sealed at the tip, the centrifugal forces will compress the air inside the tip to a value equal to the dynamic pressure corresponding to the tip speed, about 3-5 psi for most rotors. These pressures were sufficient to collapse solid ribs in early built-up rotor blades. The solution to this problem was to open up a spanwise air passageway, a design feature that is no longer necessary since most modern blades are ribless.

The exhaust system of a turboshaft engine as installed on a helicopter can also be a source of drag. If the rearward speed of the exhaust gas relative to the aircraft is higher than the forward speed, the engine will produce positive residual thrust. The engine installations on turboprop airplanes are generally designed so that this is true even for their highest speeds. The exhaust system on a helicopter, however, is usually designed to produce relatively low exhaust velocities in order to optimize the hovering performance. Thus there is residual thrust only up to some forward speed; beyond that speed there is residual drag. This drag, which may amount to several hundred pounds, can be estimated by the engine manufacturer for a particular engine installation. If the exhaust stack is canted away from straight back, a further drag can be produced corresponding to the loss of the rearward momentum of the air passing through the engine. Thus the exhaust drag is:

Dex = m (V – Vtx cosx)

where m is the engine mass flow in slugs per second and % is the exhaust cant angle.