#heliseries-5

[rotorsexual]

Heliseries part 5 will cover empennage stabilizers. Check out previous parts for more information.

You may have noticed these fins (stabilizers) on the tail of helicopters, but have you ever wondered what they are for?

An airplane uses these stabilizers to balance aerodynamic forces in flight and manipulate the control surfaces on them to maneuver.

But a helicopter maneuvers using its main and tail rotors and has no need for control surfaces, right? This is correct - a helicopter doesn't need them to fly. In fact, the first production helicopters designed in the 1940s and early 1950s didn't have any stabilizers at all.

Sikorsky R-5

Sikorsky S-55/H-19

By the mid 1950s practically all helicopters were adding stabilizers. Early ones were designed without them because the engineers correctly figured they weren't necessary. If you dont want to go fast, that is.

A helicopter is basically suspended beneath its main rotor and will react accordingly. In forward flight the main rotor disc is tilted forward and the fuselage follows along in a nose-down attitude. Having the lift and thrust of the main rotor up high and the drag of the fuselage down low has an effect of pushing the nose down lower at higher airspeeds. This becomes an aerodynamic limit to high speed flight in helicopters. Engineers fixed this by adding a horizontal stabilizer.

A horizontal stabilizer works by using an airfoil shape to create lift just like a rotor blade or airplane wing. Except its upside down. This has an effect of pushing the tail down, and by extension, the nose up. Being out on the tail puts the stabilizer far from the center of gravity where it has a lot of leverage which reduces its required size.

Note the curved lower surface of this horizontal stabilizer indicating its designed for downward lift.

And just like how a higher airspeed will create more drag and lower the nose, it will also increase the downward lift of the stabilizer to lower the tail. This creates a self correcting mechanism to keep the helicopter stable in pitch. The fuselage will still be a bit nose down in cruise flight but at a much less extreme angle.

Ok so what about the vertical stabilizer?

A helicopter has a tail rotor to produce sideways thrust against the engine torque and prevent the helicopter from spinning around opposite of its main rotor. This torque exists in a hover and in forward flight so it must constant be resisted by the tail rotor.

The vertical stabilizer is there to assist the tail rotor in forward flight. It is just like the horizontal stabilizer in that it is an airfoil out on the tail. Only it is turned vertically to create sideways lift in the same direction as the tail rotor thrust.

Note the leading edge of this vertical stabilizer is angled to the right. This assists the tail rotor in pushing the tail to the right/nose left.

In a hover there is no airflow over it so the tail rotor is fully responsible for anti-torque. But the vertical stabilizer will begin to unload the tail rotor as the helicopter accelerates. This means there isnt as much pedal required to keep the helicopter flying straight which frees up a little power that can then be used to accelerate more or climb.

In the event of a tail rotor failure many vertical stabilizers are designed to permit limited controlled flight at a high enough airspeed and/or low enough power setting.

Pretty much every helicopter designed in the last 70 years has had horizontal and/or vertical stabilizers because the benefits greatly outweigh the small penalty to weight, drag, and cost. In a later post I may cover some moveable stabilizers.