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Understanding performance enhances your helicopter operations. Thorough performance
planning prior to your flight is still a must, however a good understanding of performance
will reduce your planning time and help in situations that might have been unforeseen.
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POWER AND AIRSPEED
Power and airspeed have a special relationship. As airspeed increases power requirements
change. From the relatively high power demand at a hover, as you increase airspeed,
power requirements decrease until a certain point, then increase again. (see chart
below)
Add to that the understanding that power is affected by both altitude and air temperature and
you have a basic understanding of their relationship.
If we add gross weight to the picture you can see how power requirements are
affected.
USING THE CHARTS
These charts become very valuable when understanding how critical airspeeds are
determined. Taking the low point in the power required curve you get your best rate of
climb and maximum endurance airspeed. That's because this is the lowest power
requirement for maintaining level flight. This also gives you the best fuel consumption.
Vh can also be determined with this chart. The intersection of the power required line with
the power available line will give you Vh airspeed.
Maximum range airspeed is determined by drawing a straight line from the point of origin
to the tangent intersection of the power required line. You can see how gross weight can
affect max. range airspeed.
????
So...Big deal. Some flashy charts with some big words. What does it all mean?
Well first, we can go back and dissect why there's that dip in the power required line.
One word...Drag.
I'm not talking two cars racing or how you might dress on Saturday night. I'm talking
about the combined drag on the aircraft. Lookyhere.. If we review our aerodynamics (Oh
no, I said that word) we know that in helicopter operations there are three types of drag;
induced, profile, and parasitic. Induced drag is the result of lift and steadily decreases as
airspeed increases. Parasitic drag are things that hang from the fuselage (or even the
fuselage itself) to slow the aircraft down. Parasitic drag rapidly increases as airspeed
increases. Profile drag is a type of parasitic drag that results from the friction of the blade
passing through the air. It gradually increases as airspeed increases. Putting them all
together, we'll find that as airspeed increases, overall drag decreases then increases.
The power to overcome the drag on the aircraft is the power required. Things that the pilot
has control over that might affect drag are: external loads, doors open operations, mirrors
etc.
So, if you are conducting external load operations and an emergency occurs where power
is desperately needed, releasing the load not only reduces weight it decreases
drag!
Going back to our power available vs. power required, we know that if we decrease our
altitude we increase our power available. So if you lose an engine..(if you have two)...
adjust your speed and decrease your altitude to get the most out of the good engine.
You can probably think of more examples if you gave it a little thought. The point is that
the reason for the development of these certain airspeeds that we learn is no mystery. The
power vs. airspeed chart is essential to the development of these critical airspeeds. By
understanding your aircraft's performance you become a more capable pilot.
Asta....Chuck
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