Nov 17, 2012

Leading versus Trailing Edge Control: A Brief Foray into Rotor Head Mechanics

The rotor head of a Aerospatiale SA321 Super Frelon…and you thought ours were complicated?!

I get a lot of questions surrounding the the differences between leading and trailing edge control on flybarless systems…What’s the difference? Should I care? This one trips up a lot of people because of the complexity of the topic, but I think it’s worth explaining.

First and foremost, in today’s FBL systems where the delta angle is zero, leading and trailing edge control are exactly the same. In other words, there are no advantages or disadvantages. There’s the short answer to you question, but if you’re interested in some background, then read on…

When you have a rotor head system that has a non-zero delta angle, there are clear pros and cons.

To start off, what’s delta angle?

Look down on your heli’s head with the grips at zero pitch. Draw a line between the two grip balls and a line perpendicular to the blades and through the center of the head (like a virtual flybar). The angle between these two lines is a measure of the delta or delta-3 angle.

Who cares, right?

Think about a typical heli head. It’s what we refer to as semi-rigid because the feathering shaft and blade grips can actually teeter in the head block due to the compliance of the damper material. Helicopter rotor systems are designed to flap to compensate for the di-symmetry of lift in forward flight. 

Simply put, the advancing (left blade in a CW rotating system) blade sees a higher-speed air flow due to the blade speed plus the heli forward speed while the retreating (right) blade sees a lower-speed air flow. 

The result is higher lift on the left side of the rotor disk and a tendency for the heli to pitch up as a result (remember gyroscopic precession says that the resulting action on a rotating body occurs 90 degrees from the point of application of the force).

When the blades are allowed to flap, the advancing (left) blade sees more lift and flaps up, which reduces its relative angle of attack, thus reducing lift. The opposite occurs with the retreating (right) blade and the result is a more balanced lift distribution across the rotor disk.

Ok, back to delta angle…

When the blade flaps, the blade grip moves up or down with it, but the ball is constrained by the link that is rigidly attached to the swashplate, right? So, where that ball is located with respect to the teetering point or center of rotation of the feathering shaft dictates how the pitch of the blade varies during flapping.

If the ball is located at the teetering point, then the pitch does not vary with flapping. However, if the ball is shorter (i.e. non-zero delta angle), then when the blade flaps the pitch will increase or decrease depending on the flapping direction and whether it’s leading or trailing edge control.

Leading-edge control with non-zero delta angle will cause a decrease (increase) in blade pitch with an upward (downward) flap…this causes a natural restoring force that tends to reduce the lift on the blade. This is referred to as correcting or positive delta.

Trailing-edge control has the opposite effect…a non-zero delta angle will cause an increase (decrease) in blade pitch with an upward (downward) flap…this causes an increase in lift, which can cause an instability and lead to blade fluttering in extreme cases, but in general will only increase the stability of the heli. Of course, this effect can be partially reduced by stiffer dampers, but only to a certain extent. This is referred to as uncorrecting or negative delta.

In general, you want no delta in a 3D machine and you tend to want to run stiffer dampers to reduce the amount of flapping because this allows for a more direct transfer of disk forces to the heli airframe and results in a more responsive feel. Beginners would prefer more flapping (softer dampers) because it has a stabilizing effect. Finally, you may find some F3C pilots that use a negative delta system to further increase the stability of the heli in a windy hover.

Delta is fixed by the designer of the heli and the manufacturing tolerances, so it should not be “off” unless you’ve damaged the head in some way. The relative responsiveness and stability of the heli are, in fact, functions of the amount of positive or negative delta, among other things. That having been said, delta has an equal effect on both positive and negative pitch inputs for a given control type (leading = more responsive, trailing = more stable).

Remember that in the absence of delta, damper stiffness has a similar effect in that it determines how far up and down the blades can flap. Stiffer = more responsive, just like leading edge positive delta. Softer = more stable, just like trailing edge negative delta.

I hope that this provides you with a better understanding of some of the more complex topics in rotor head mechanics. Now get out there and fly!

– Justin