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Saturday, October 1, 2011

Leading Edge: On Relationships

By Frank Lombardi

Once again the next big helicopter expo will be upon us in short order. Highly polished aircraft will adorn the floors of a cavernous convention center. It may be your one and only chance to compare and contrast the various models of flying machines available to you side-by-side. Sure, you’d like to have one of each; we all would. But alas, we can’t. So the one you choose has got to do your bidding well. To make your selection task less daunting, you’ll collect a bunch of literature full of performance specifications in order to compare each aircraft. Soon you’ll be swimming in a sea of numbers trying to make sense of it all, trying to tease out the model that will be just right.

Designers begin a selection process similar to your own when they begin envisioning the type of aircraft you, the buyer, will need. They too look at a sea of numbers, mostly from previous designs as a means of comparison. It’s easy to get overwhelmed with such a broad set of parameters to look at. But engineers being, well, engineers, like to find relationships between numbers that establish trends, provide insight and lessen their hysteria.

As you’ll hear oftentimes in this column, performance is what sells aircraft. And in the simplest of any engineering terms, performance comes down to this: “power required vs. power available.” This relationship will define the entire operating envelope of your aircraft. Minimizing the first term while maximizing the second is therefore always a design goal.

In any helicopter, the combination of aircraft gross weight, rotor disk area and airframe shape will dictate how much power will be required to get things done. It is here that a particular relationship becomes meaningful to engineers during design. Disk loading is the weight that each square foot of the rotor disk carries. It is nothing more than aircraft gross weight divided by rotor disk area. Increase the aircraft weight, and disk loading goes up. Increase the rotor diameter, (i.e. disk area) and disk loading goes down. Of course, the opposite is true as well. Disk loading is important because it foretells many attributes of the final helicopter.

Engineers, being the typical statisticians they are, keep a long list of prior helicopters and design specs. With this, they can use the disk loading to make predictions about a new design. A high disk loading will produce a high rotor downwash velocity, quick RPM response in autorotation, and increase the helicopter’s resistance to wind turbulence. Expect a low disk loading to have the opposite effect.

While many other things like design cruise speed, physical sizing, and empty weight will determine the final rotor diameter in a design, probably the most important parameter that disk loading affects is hover performance. The lower the disk loading, the less hover power is required. It is a helicopter after all, and hovering is what it is intended to do best. To get it to do anything else well in addition to that involves tradeoffs. Less hover power required relates to cheaper operation, longer engine life and better fuel economy; but there is definitely a market for having a helicopter with faster legs. This requires more expensive, thirstier, higher horsepower engines to compensate for the increased drag at higher speeds. This will also drive disk loadings higher, as excess engine power will allow for a smaller rotor design.

Most turbine engines today provide great power while they remain light in weight. This describes another engineering relationship—the engine power-to-weight ratio. This is a ratio of engine power output compared to engine weight. It factors into disk loading and therefore into hover power calculations. Early helicopter designs needed very low disk loadings in order to have the rotor do the work their heavy, less powerful piston engines could not do. Today’s engines supply high power while remaining low in weight. This helps keep aircraft empty weights low and useful loads high. They allow disk loadings to be high, benefiting forward speed, and keeping rotor diameters small.

One more relationship is worth mentioning here. Power loading is a measure of the weight each unit of horsepower can lift (weight divided by horsepower required). Power loading can be thought of in the form of hover efficiency. Helicopters with high power loadings tend to hover more efficiently than those with lower ones, but are not as fast. Engineers are great at creating relationships between relationships, so to speak, until their pencils break. They’ll graph one against another, for many different models, and see where their new design fits in. This gives comparison that speeds the iterative design process, highlights potential problems, and provides instant focus. It might not be a bad thing to remember, as you survey the sea of new choppers. Using their methods could help to bring order to convention chaos.

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