In an earlier Helicopter Ground School post we looked at how a helicopter creates lift. We can’t talk about lift without talking about drag. You can’t have one without the other. The more drag we have the more power we need to over come it. In the helicopter world we have three types of drag to talk about. Induced, parasite and profile drag.
In our ground school post on lift we discussed how a helicopter stays in the air. In its most basic form we are turning the airflow, that is, deflecting it down to hold us up. Another term for this is downwash or induced flow. Any time the helicopter is creating induced flow it is also creating induced drag.
If you recall one way to increase lift is to increase the airspeed. This also has the advantage of reducing the induced drag, not drag as a whole but we will get to that. The faster the helicopter flies the higher the flight velocity through the rotor disc. This means the induced portion of the airflow through the disc isn’t as high and therefore the induced drag is less.
There is no way to eliminate induced drag but the smart people that design helicopters do what they can to reduce it. There are a number of ways they can do this. The most common ways are reducing the pitch angle of the blade at the tip relative to the blade root (washout) or reducing the surface area of the blade at the tip (tapering).
To the right is a blade from the Sikorsky S 92 helicopter. One of the more complicated blade tip designs out there but it does have several advantages. The one we are concerned with today is a reduction in tip vorticies which make a large contribution to induced drag. We’ll discuss the other advantages another day.
So far we have talked about the drag associated with lift. If all airfoils (any surface designed to create lift) create drag then it stands to reason the rest of the helicopter being dragged through the air must also be subjected to drag. This is parasite drag.
Any none lifting surface on a helicopter creates parasite drag. What’s a non lifting surface? The body of the helicopter, skids, tail boom, wheels, even the rotor head creates a lot of parasite drag (note we are talking about the bit that holds the blades on, not the lift creating blades).
Right at the beginning we stated that the faster the helicopter flies the more drag it creates. Parasite drag is the reason why. It increases by the square of the helicopters velocity. In other words if you double your speed the parasite drag will increase by four! Again designers will work their magic and make any non-lifting surface as aerodynamic as possible to help reduce this drag. Below you can see the fairings Robinson uses on it’s skid tubes to help reduce parasite drag.
As our wings are moving through the air they create their own drag that isn’t covered by either of the two above. Whenever the blade is moved through the air the frontal surface of that blade gives some resistance to the air (form drag). Although a rotor blade may feel very smooth to us it isn’t for the air. This is what we call skin friction and it slows (or drags) the air flow in close proximity to the blade.
Profile drag does increase a little as airspeed increases but it isn’t enough to really concern us so we can assumme it remains constant. Below you can see a graph dipicting each type of drag we have disscussed and what happens to it as the helicopters airspeed increases. Note the sharp increase in parasite drag as airspeed climbs.